Abstract
This study presents an effective approach to tackle the challenge of long-term monitoring of contaminated groundwater sites where remediation leaves residual contamination in the subsurface. Traditional long-term monitoring of contaminated groundwater sites focuses on measuring contaminant concentrations and is applicable to sites where contaminant mass is removed or degraded to a level below the regulatory standard. The traditional approach is less effective at sites where risk from metals or radionuclides continues to exist in the subsurface after remedial goals are achieved. We propose a long-term monitoring strategy for this type of waste site that focuses on measuring the hydrological and geochemical parameters that control attenuation or remobilization of contaminants while de-emphasizing contaminant-concentration measurements. We demonstrate how this approach would be more effective than traditional long-term monitoring, using a site in South Carolina, USA, where groundwater is contaminated by several radionuclides. A comprehensive enhanced attenuation remedy has been implemented at the site to minimize discharge of contamination to surface water. The immobilization of contaminants occurs in three locations by manipulation of hydrological and geochemical parameters, as well as by natural attenuation processes. Deployment of our proposed long-term monitoring strategy will combine subsurface and surface measurements using spectroscopic tools, geophysical tools, and sensors to monitor the parameters controlling contaminant attenuation. The advantage of this approach is that it will detect the potential for contaminant remobilization from engineered and natural attenuation zones, allowing potential adverse changes to be mitigated before contaminant attenuation is reversed.
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Introduction
Post-closure long-term monitoring is required at waste sites where residual contamination is left in the subsurface to inform site owners and regulators of any unexpected contaminant behavior that could be a potential threat to human health and the environment. Sites with residual contamination are those where contaminant source zones have been contained but not eliminated, as well as those in which remediation of contaminated groundwater is designed to immobilize contaminants in the subsurface, rather than remove them. These include many types of sites, such as landfills, coal- and metal-mining sites, industrial sites, and locations of accidental spills of toxic substances. The objective of long-term monitoring at these sites is to ensure that risk to humans and the environment from residual contaminants remains low long after operational and remedial activities have ceased.
Here we focus on an approach to improve long-term monitoring at sites where contaminants remain in the subsurface after remediation of contaminated groundwater. Regulatory agencies have become more accepting of groundwater-remediation strategies that rely on immobilizing, or attenuating, contaminants in the subsurface, referred to here as attenuation-based remedies (USEPA 1999, 2007a, 2015; IAEA 2004, 2006; ITRC 2010; NRC 2013). Formal guidance has been issued by federal and state agencies in the United States for use of monitored natural attenuation (MNA) to remediate metal- and radionuclide-contaminated groundwater (USEPA 2007a, 2015; ITRC 2010). MNA is an accepted remedy in Europe and elsewhere for contamination by organic compounds, but attenuation-based remedies are less prevalent for metals and radionuclides (Rügner et al. 2006; Usher et al. 2008; Declercq et al. 2012; Majone et al. 2015). There were 16 years between the time the United States Environmental Protection Agency (USEPA) issued a formal directive on MNA for organic compounds and the issuance of a similar directive for metals and radionuclides. In the interim, extensive research was done to demonstrate that MNA was a safe and effective remediation strategy for metals and radionuclides under the right circumstances. The abundance of research in other countries on natural and engineered attenuation of metals and radionuclides (e.g., Blowes et al. 2000; Lee and Hartwig 2005; LaPointe et al. 2005; Fytas 2010; Krishna et al. 2010; Vázquez et al. 2011; Jurkovič et al. 2019; Wang et al. 2019; Sakala et al. 2020) suggests that these will become accepted remedies in other countries as well, increasing the need for efficient long-term monitoring.
Attenuation-based remedies for metals and radionuclides in groundwater have become common in the United States because they are effective and efficient at sites where contaminant removal is impractical. In many cases, natural processes, such as precipitation, sorption, and dilution, limit the migration of contaminants sufficiently to minimize any hazard. In other cases, subsurface zones are engineered to produce conditions conducive to contaminant attenuation (Evanko and Dzombak 1997; Hashim et al. 2011; Obiri-Nyarko et al. 2014). Long-term monitoring is particularly important where attenuation-based remedies are deployed to ensure that attenuation capacity remains high enough to affect the entire mass of contamination and ensure that conditions do not change in a way that promotes remobilization of contaminants.
The time span of long-term monitoring depends, in part, on how long contaminants will persist. Long-term monitoring may be required for only years to decades for organic contaminants because the attenuation of these contaminants generally results in their transformation or degradation to less-harmful chemicals. Long-term monitoring at these sites is done until the risk posed by the residual mass of contaminants is acceptable. Thus, for organic contaminants, there is a finite time period for long-term monitoring based on degradation rate and mass of contaminants. In contrast, toxic metals and long-lived radionuclides will persist for long time periods in attenuation zones, if hydrologic and geochemical conditions continue to limit their migration. Thus, the period required for long-term monitoring is indefinite and can span decades to centuries, depending on the toxicity of the contaminants, their concentrations, and their proximity to potential receptors.
The original definitions of long-term monitoring included the objective of evaluating the effectiveness of remedial activities in addition to ensuring that future risk at a site remains acceptable. Within the framework of this definition, long-term monitoring is the last stage of a continuously evolving program and sampling network that primarily measures contaminant concentrations at point locations (e.g., monitoring wells or springs). For the long-term monitoring stage, statistical methods and numerical modeling are used to optimize the preexisting network of sampling points and sampling frequency to maintain collection of key information while reducing the costs (Reed et al. 2000, 2001; Reed and Minsker 2004; USEPA 2004, 2005a; Harre et al. 2009; Hunter et al. 2011). This approach is appropriate for sites at which contaminant mass is removed from the subsurface by extraction or in situ degradation because the objective of long-term monitoring remains similar to that of remedy-effectiveness monitoring, ensuring that remedial processes are sustained until the remaining contaminant mass is no longer a risk.
Such a traditional approach to long-term monitoring is less appropriate for attenuation-based remedies of metal and radionuclide contaminants. First, the objective of long-term monitoring of attenuated metals and radionuclides should be different than the objective of remedy-effectiveness monitoring. These contaminants will remain in the subsurface in perpetuity, and the objective of long-term monitoring should be to ensure that the contaminants stay attenuated. Measuring contaminant concentrations can only indicate when attenuated contaminants are remobilized. In other words, measurements of contaminant concentrations downgradient from an attenuation zone are lagging indicators. It is preferable to measure leading indicators that warn of changing conditions that could potentially remobilize attenuated contaminants before they are affected. Second, for sites that require long-term monitoring for decades to centuries, the cumulative cost of obtaining samples and analyzing them for contaminant concentrations becomes prohibitive (NRC 1999, 2013). Given these reasons and the proliferation of attenuation-based remedies for metals and radionuclides, alternative approaches to long-term monitoring should be considered.
In this paper, we present an improved approach to long-term monitoring at sites where attenuation-based remediation of metal and radionuclide contamination has been deployed. The approach de-emphasizes measurement of contaminant concentrations in favor of measuring the variables that control attenuation of the contaminants. While previous guidance on monitoring of attenuation-based remedies recommends measuring controlling variables, these measurements have always been secondary to measurements of contaminant concentrations. We show that putting primary focus on measuring controlling variables is a more effective and efficient approach to long-term monitoring at these sites. We then present an example of how improved long-term monitoring would be implemented at the F-area Seepage Basins on the US Department of Energy’s Savannah River Site using a structured approach and new-generation tools. Recently, a multiyear project began at this site to assess the improved long-term monitoring approach contemporaneous with the current more traditional monitoring program. We present preliminary data here, yet because of the long-term nature of the project it will be many years before there is definitive data demonstrating the effectiveness of this new approach. Nevertheless, we believe that it is important to present this now for consideration by waste-site owners, regulatory agencies, and stakeholders because acceptance of new remediation and monitoring strategies is, by necessity, a slow and careful process.
Framework for a New Approach to Long-term Monitoring
The framework for the new long-term monitoring approach proposed here is built upon USEPA (1999, 2007a, 2015) and Bunn et al. (2012). The USEPA documents provide guidance on the use of MNA, including the monitoring objectives necessary to demonstrate that residual contamination is not a risk. The overall monitoring objective is summarized by the statement in USEPA (2015):
“Ultimately, monitoring programs should be designed to demonstrate continued stability of the plume over time and to identify changes in groundwater chemistry that may lead to decreases in rates or capacity of the aquifer to attenuate the contaminant of concern or changes that may lead to re-mobilization of attenuated compounds.”
Bunn et al. (2012) provide a systematic approach to monitoring a contaminated site throughout the remediation lifecycle. They defined monitoring in four phases, shown in Fig. 1, proceeding in the sequence “characterization monitoring → process monitoring → performance monitoring → long-term monitoring.” In the first three monitoring phases, plume stability is established, and understanding of the processes controlling contaminant attenuation is developed. In the long-term monitoring phase, contaminant concentrations and controlling variables are measured to warn of remobilization of contaminants. Bunn et al. (2012) suggested that long-term monitoring networks should be passive, robust, low maintenance, low cost, and able to use leading indicators.
The primary difference between the proposed long-term monitoring approach and more traditional approaches is the way in which potential receptors are protected from contaminants. Traditional long-term monitoring protects receptors by emphasizing the monitoring of pathways by which contaminants reach receptors and detecting the presence of contaminants in those pathways. The emphasis of the proposed approach is on protecting receptors by protecting the stability of attenuated contaminants. Guarding the pathways to receptors is still a part of the proposed long-term monitoring strategy, but more as a “defense in depth” against contaminants reaching receptors. The basis for this is that if contaminants are prevented from being remobilized, the need to guard the pathways to receptors is less urgent. The operational result is a difference in what to monitor, where to monitor, and the methods for monitoring.
Monitoring Controlling Variables
The parameters measured in the proposed new long-term monitoring strategy affect attenuation of contaminants and provide information necessary to keep the monitoring network relevant over long time periods. These parameters, or controlling variables, reflect the physical forces that drive contaminant movement and the chemical forces that control contaminant attenuation. Table 1 defines important hydrologic, morphologic, and geochemical controlling variables and their influence on contaminant migration and attenuation. An assumption for beginning long-term monitoring is that the site is hydrologically, morphologically, and geochemically stable, and contaminant behavior is well understood following the first three phases of monitoring. Parameters can vary spatially and temporally at stable sites, but their variations are understood and consistent with maintaining attenuation of contaminants. Interpretation of long-term monitoring data consists of recognizing abnormal spatiotemporal trends in controlling variables that may affect contaminant behavior or the efficacy of the monitoring network. Action levels will be established for parameters that strongly influence contaminant attenuation. These will be specified in a long-term monitoring plan that will also describe the response to adverse trends and the measurement of an action level for each pertinent parameter.
Measuring such physical and chemical parameters that indicate change to a system has multiple advantages over measuring contaminant concentrations at specific locations. First and foremost, they are “leading” indicators of potential unexpected contaminant behavior that promote proactive decisions that can prevent remobilization of attenuated contaminants. Second, most of them are easily and inexpensively measured by robust in situ sensors that transmit data wirelessly to a central location (Schmidt et al. 2018).
Monitoring Hydrologic Stability
The purpose of monitoring hydrologic stability is to detect events or conditions outside the norm of the known stable state and identify whether they will potentially affect contaminant attenuation or the relevance of elements of the monitoring network. Water is the physical force that drives contaminant migration by leaching contaminants from near-surface sources, vadose-zone sources, or locations where contaminants are attenuated in the saturated zone. Therefore, flux of water passing through contaminant sources at the surface or deeper in the vadose zone must be monitored. Furthermore, as long-term monitoring begins at many sites, contaminants in the saturated zone will still be migrating into zones where they will be immobilized. A change in groundwater-flow direction can alter the path of the contamination plume, resulting in contaminants bypassing the attenuation zone. Similarly, a change in groundwater-flow direction can render monitoring locations irrelevant because the groundwater they monitor no longer passes through an attenuation zone. Infiltration, evapotranspiration, and hydraulic head are key parameters for monitoring long-term hydraulic stability.
Infiltration is a measure of the flux of water entering subsurface soils, and the balance of infiltration and evapotranspiration determines the flux of water through the vadose zone into the saturated zone. This recharge drives groundwater flow and can dilute the top of a contaminant plume in a water-table aquifer and push it deeper into the aquifer (Weaver and Wilson 2000). Temporally averaged regional or site-wide values of infiltration are normally used for groundwater modeling. Yet, some sites may require more frequent or localized measurement of infiltration, such as the sites that have a significant mass of contaminant at the surface or deeper in the vadose zone. If these contaminant sources are not covered by impermeable barriers, then flux of water through the source is an important parameter for understanding the rate of contaminant leaching from the source. If sources are protected by physical barriers to water, measuring infiltration into, and through, the barrier, is important for understanding barrier degradation.
Evapotranspiration is a measure of water removed from infiltration by evaporation or uptake by plant roots. It is important because of its role in controlling water flux through the vadose zone. Evapotranspiration increases significantly where there is dense vegetation, and its effect may vary across a waste site, depending on the vegetation density. In arid regions, evapotranspiration may equal infiltration for most of the year and severely limit the flux of water into the vadose zone. Furthermore, in arid regions where the water table is near the surface, such as near-discharge points, removal of water by evapotranspiration can result in precipitation of contaminant-bearing evaporite minerals (Looney et al. 2014; Dam et al. 2015). Dissolution of these minerals occurs when infiltration exceeds evapotranspiration releasing the contaminants to groundwater or surface water.
Hydraulic head is the elevation to which water rises in a well that is screened within an aquifer. It is the most important parameter in long-term monitoring of hydrologic stability because the potentiometric surface delineated by interpolation between numerous hydraulic head measurements is how groundwater-flow velocity and direction are determined. Groundwater flows from a higher to a lower hydraulic head, and flow velocity depends on the gradient between hydraulic head measurements. Measurement of hydraulic head in wells screened at different depths allows determination of vertical groundwater flow. Changes in groundwater-flow directions with time will be reflected by the potentiometric surfaces periodically constructed from hydraulic head measurements. Thus, it is important to long-term monitoring to collect hydraulic head measurements on a regular schedule and on a fine-enough spatial scale to be able to determine any significant changes to groundwater-flow direction near contaminant-attenuation zones.
There are other parameters that are easy to measure that influence hydrologic stability. Infiltration and evapotranspiration depend on precipitation (rainfall and snowfall), relative humidity, and temperature. These vary on timescales ranging from hourly to yearly, yet the importance to long-term monitoring is when systematic changes occur over long periods that may indicate irreversible changes to climate.
Monitoring Geomorphic Stability
The important geomorphic features for long-term monitoring of a waste site are topography and the density and type of vegetation. Changes in these characteristics can affect both the hydrological and geochemical stability of a waste site, and therefore are important for long-term monitoring.
Topography encompasses the structural features at the surface, including surface elevations, drainage patterns, stream courses, and architecture of groundwater-discharge areas such as wetlands. Changes to these features can have profound effects on groundwater flow and can also result in geochemical changes to sediments where contaminants have been naturally attenuated. For example, changes to topography can remobilize contaminants attenuated in naturally reduced zones (Janot et al. 2015) by exposing them to rapid oxidation. Erosion can also release contaminants from anoxic wetland soils by exposing them to oxygen. Long-term monitoring should measure changes to topography at different spatial and temporal scales. There is the measure of large features such as drainage patterns that change slowly, barring intervention from man or catastrophic events like extreme storms, floods, or seismic activity. Yet, on covers protecting surface-contaminant sources, topographic changes at the centimeter scale can be important indications of cover degradation. Fine-scale topographic measurements can detect subsidence of the surface that indicates degradation of the underlying layers of the cover, as well as detecting animal burrows and fissures that can be pathways for increased infiltration. These small-scale changes can occur rapidly relative to changes in the large-scale features.
The nature and density of vegetation affects evapotranspiration and can also affect contaminant behavior by the uptake of contaminants and replenishing organic matter that adsorbs contaminants in sediments. Natural vegetative cover changes slowly, usually in response to topographic changes, unless human intervention by removal of vegetation or introduction of invasive species occurs. In contrast, deep-rooted vegetation can become established on caps and covers of contaminant sources relatively rapidly and contributes to degradation of these structures. Therefore, caps and covers over surface sources should be surveyed for vegetation more frequently than the overall site.
Monitoring Geochemical Stability
Quantifying changes in parameters that most influence chemical and biological processes that attenuate contaminants is a critical function of long-term monitoring. A wealth of literature is available that discusses the behavior of metals and radionuclides in natural waters (e.g., Stumm and Morgan 1996; Drever 1997; USEPA 2007a). Basic discussions of the behavior of specific metal and radionuclide contaminants in groundwater are presented in Evanko and Dzombak (1997), IAEA (2006), and USEPA (2007b, c).
In summary, pH and oxidation-reduction potential (ORP) are of primary importance in controlling the behavior of metals and radionuclides in soils and groundwater. Acidity of groundwater, measured as pH (negative log of hydrogen-ion activity), is a fundamental control on metal and radionuclide attenuation. It controls surface charge on most minerals, and hence, adsorption of ions to mineral surfaces (Stumm and Morgan 1996). Likewise, pH influences aqueous speciation of most metals and radionuclides, which, in turn, influences their adsorption and precipitation (USEPA 2007a; Dafflon et al. 2013). The redox state of an aquifer system, measured in groundwater as ORP, also influences aqueous speciation and solubility of contaminants that can exist in multiple oxidation states. Under sulfate-reducing conditions, even metals that are not normally sensitive to ORP, such as lead, cadmium, and zinc, can be strongly attenuated by precipitation of sulfide minerals (USEPA 2007a). Measured ORPs are not a direct indication of the redox state of any contaminant, but changes in such measurements with time do generally indicate changes to the system that may be important to the stability of an attenuated contaminant. Table 2 lists common metal and radionuclide contaminants and whether their behavior in groundwater is sensitive to pH and ORP.
At some sites, parameters other than ORP and pH may be important indicators of system changes that can lead to contaminant remobilization. For example, carbonate alkalinity or dissolved carbon dioxide coupled with pH is a critical parameter for mobilization of uranium (Langmuir 1978; Hsi and Langmuir 1985; Waite et al. 1994) and other metals readily complexed by carbonate ions (USEPA 2007b, c). Ionic strength is also an important influence on adsorption of many metals and radionuclides, and increases in ionic strength could portend desorption of these contaminants. The conceptual site model must dictate the important chemical parameters to measure for each site, or even for individual areas within a site.
Monitoring Vulnerabilities
Monitoring locations are defined by the objective of the new long-term monitoring approach, guarding the stability of residual contaminants. Thus, monitoring focuses on locations where contaminants are attenuated, referred to here as vulnerable zones, to detect changes in conditions that could potentially remobilize contaminants. Vulnerable zones can be at the surface, in the vadose zone, or in the saturated zone. Vulnerable zones at the surface should be monitored for infiltration through the zones or barriers covering the zones. If barriers to infiltration are in place, then monitoring should include methods to assess their degradation. Monitoring a vulnerable vadose zone should consist of measuring changes in water flux that could increase mobilization of contaminants. In the saturated zone, monitoring should be done upgradient of vulnerable zones for adverse changes in hydrologic and chemical conditions that could mobilize attenuated contaminants. Conditions that reverse attenuation of contaminants in the saturated zone can originate from within the waste site or from outside the site boundaries because waste sites are rarely hydrologically isolated. Therefore, events that occur beyond the waste-site boundaries, such as clearcutting timber or dewatering of a construction site, that may not be known by waste-site managers, can affect groundwater-flow direction at the waste site. Likewise, altered groundwater chemical conditions caused by off-site events or practices that are unknown to site managers can migrate onto a waste site and affect vulnerable zones. An extra layer of protection is gained by monitoring the perimeter of the site to provide early warning of adverse conditions originating from off-site events.
Wetlands that have received groundwater contaminated with metals and radionuclides present a unique long-term monitoring challenge because wetlands are a hydrologically and geochemically dynamic environment. Wetlands become a vulnerable zone because natural processes result in attenuation of metals and radionuclides in wetland soil (Gambrell 1994; Du Laing et al. 2009; Frohne et al. 2011). Yet, attenuation can also be reversed by natural conditions, releasing contaminants to surface water. The storage and periodic release of contaminants from wetlands means that long after remedial goals have been met, contaminants may continue to be released from wetland soils to surface water. The challenge to long-term monitoring of wetlands is to detect spatiotemporal changes in wetland conditions, both natural and man-made, that can potentially cause release of contaminants to exceed remedial goals. This requires monitoring a large heavily vegetated area for physical and chemical conditions that change naturally over various timescales.
Monitoring Methods and Technologies
Bunn et al. (2012) suggest that the extended time span requires different qualities for long-term monitoring than the first three stages of site monitoring where toxic metals or radionuclides remain in the subsurface. The list of long-term monitoring qualities includes that it should be predominantly passive, robust, low maintenance, low cost, and measure parameters that predict potential threats to remobilization of contaminants. This guides the selection of monitoring methods and tools used to measure parameters important to insuring the stability of attenuated contaminants.
In-well in situ sensors
In recent years, an increasing number of configurable multiparameter sensor platforms equipped with wireless data-transmission technology have become commercially available for the long term in-well measuring of water-quality parameters such as pH, ORP, dissolved oxygen, temperature, and specific conductance, as well as water levels. These sensor packages continue to improve as more sophisticated sensors, such as optical sensors for nitrate and organic matter (Pellerin et al. 2012), are developed. Downhole multiparameter probes were not particularly advantageous for long-term monitoring of groundwater, until the advent of wireless technology, because they required either a person present to record the measurement or download dataloggers that stored measurements. Now, systems are available that continuously and autonomously measure parameters and upload measurements wirelessly to cloud storage so that the data are available any time to designated people via web-based services. Diagnostics are also available so that faulty sensors can be identified and replaced, minimizing gaps in time-series data. The above developments enable the deployment and use of wireless sensor networks involving scores of sensor platforms across a waste site to characterize spatially heterogeneous properties (e.g., Kerkez et al. 2012).
There are multiple factors that determine which sensors are suitable for a particular site and condition. Danielson (2020) evaluated commercially available point source in situ single- and multiparameter sensor platforms for deployment in wells at the US Department of Energy’s Savannah River Site. A semiquantitative optimization function was used that rated sensor packages by parameters measured, diameter, the required calibration frequency, data logging/wireless communication platform, detection sensitivity, and price. The rating system was optimized for long-term monitoring needs at a specific site and could easily be tailored to the needs of other sites.
Distributed fiber-optic sensors
Significant advances have been made in the past two decades in use of optical fibers for measuring environmentally relevant parameters, including temperature (Selker et al. 2006; Suárez et al. 2011), strain (Schenato 2017), soil moisture (Sayde et al. 2010), acoustic waves (Bao and Chen 2012; Cox et al. 2012), gamma radiation (Gaebler 1984), and limited chemical parameters (e.g., Potyrailo and Hieftje 1998). For distributed sensing approaches, a pulsed laser source at one end of the fiber interrogates the state of the fiber at all locations through measurement of back-scattered light, yielding property measurements along the entire length at a fine lateral resolution, often at up to submeter resolution depending on the technique. Coherent optical time or frequency-domain reflectometry methods are typically utilized to measure localized differences in optical scattering (Rayleigh, Brillouin, or Ramen) along fibers caused by changes in the environment (e.g., Hartog 2017). Secondary codeployed cables may be used to actively probe the area around the cable with perturbations measured using fiber-optic methods; an example is active heat-pulse-distributed temperature sensing, which can be utilized for soil moisture quantification (Sayde et al. 2010). Fiber-optic measurement approaches can function effectively over a broad range of temperatures since the cables contain no electronics and can be packaged to withstand harsh conditions in either near-surface or borehole environments. Significantly, distributed fiber-optic sensing (DFOS) systems only require power and environmental control at the location of the interrogator unit (IU); since a single IU can conduct measurements of up to 50 km of fiber depending on measurement approach, DFOS methods can replace massive arrays of point sensors and provide for large-scale site measurements.
To date, distributed temperature and acoustic sensing have found a wide variety of applications in monitoring infrastructure, including leak detection in buried pipelines (Frings 2011), the structural health of buildings (Lopez-Higuera et al. 2011), and road conditions in tunnels (Krohn and Nicholls 2009). Distributed fiber-optic systems have also been deployed for deep-borehole (Daley et al. 2013) and integrated monitoring of shallow arctic test sites (e.g., Ajo-Franklin et al. 2017).
Distributed fiber-optic sensors have yet to be used for long-term monitoring of waste sites, but they have enormous potential use for passive monitoring. Moisture flux in the vadose zone is a key parameter controlling contaminant attenuation in the vadose zone (Truex et al. 2013), and measuring changes in moisture content with depth over time in the vadose zone can be done passively with distributed fiber-optic sensors, as well as measuring infiltration rates. Strain sensors can be used to monitor degradation of physical barriers installed to contain contaminants. Seismic measurements, acquired using distributed acoustic sensing, could be used to monitor the impacts of strong ground motion from natural seismicity (e.g., Lindsey et al. 2017) on disposal sites in tectonically active regions. Gamma-radiation sensors can be used to monitor temporal changes in radionuclide distributions in dynamic environments, such as wetlands, streams, and estuaries. They could also be installed on caps over uranium-tailing piles or NORM-disposal sites for spatiotemporal monitoring of radon emanation. Sensors that measure pH or other chemical parameters could be deployed in boreholes for depth-discrete monitoring or placed along groundwater-discharge zones such as wetlands and streams. During long-term monitoring, it is not known where or when a contaminant-controlling variable will change, and point sensors can miss the change because they are not in the right location. The spatially integrative property of distributed fiber-optic sensors and substantial linear extent makes them valuable for monitoring controlling variables over time along continuous profiles. Likewise, 2D networks of fiber-optic cables can be installed to provide higher- density measurements over the areas of interest.
Autonomous geophysical monitoring
Geophysical methods are spatially integrative, some allowing imaging of properties in large volumes of the subsurface over time. They have been increasingly used in environmental studies to characterize the subsurface in a noninvasive manner (e.g., Binley et al. 2015). Geophysical methods can image subsurface structure and physical properties (e.g., Wainwright et al. 2016), contaminant plumes (e.g., Johnson et al. 2010, 2012; Dafflon et al. 2013; Liao et al. 2018), and hydro-biogeochemical properties that are important for long-term monitoring (e.g., Johnson et al. 2010, 2012; Dafflon et al. 2011; Wainwright et al. 2014). High-performance computing capabilities allow high-resolution imaging (submeter) in a large spatial extent (up to several kilometers).
Autonomous electrical resistivity and phase tomography (ERT) is a geophysical method that is particularly sensitive to the spatiotemporal distribution of hydrological and geochemical properties relevant for short- to long-term monitoring, including soil-water content and fluid ionic strength. ERT provides spatiotemporally resolved information using an array of electrodes installed at the surface or in wellbores to measure the subsurface electrical resistance. The data are computationally reduced to three-dimensional images, or four-dimensional if resistance is measured as time lapses. ERT has been used to monitor degradation of low-permeability caps (Genelle et al. 2012; Sirieix et al. 2015), measure hydrologic recharge (Gasperikova et al. 2013), and soil moisture (Uhlemann et al. 2017) at field scales, and track the movement of fluids with contrasting specific conductance, such as contaminant plumes or reagent injections (Johnson et al. 2015; Dafflon et al. 2013). This approach can bridge the gap in sparse well-bore locations, by providing high-resolution and spatially extensive information in a minimally invasive manner. While electrodes are generally installed at the ground surface or buried using trenching or horizontal drilling technology, they can also be deployed in wellbores to enhance vertical resolution in strongly stratified plumes and related heterogeneity (Dafflon et al. 2013). The autonomous ERT system is robust and can be left in place for years with minimal maintenance while collecting data. Most of the cost of ERT is in the installation of the electrodes and cables, which can be combined with deployment of fiber-optic sensing methods. Once those are in place, measurements are inexpensive with most of the cost going to data analysis.
Likewise, seismic methods can be automated to allow autonomous active-source monitoring; to date, deployment of such systems has been more limited due to the complexity of active-source construction and automation, but they have recently been deployed for permafrost (Ajo-Franklin et al. 2017) and deep-borehole monitoring (Dou et al. 2017). In this case, their application to contaminant-site monitoring would be to probe perturbations in seismic measurables that could be inverted for elastic moduli and other mechanical properties, particularly at depth where ERT measurements might prove more difficult to obtain.
Unmanned aerial vehicle (UAV)-based surface mapping
UAVs are promising new tools for aerially mapping features and parameters of interest to long-term monitoring of waste sites. A number of review articles have covered the rapid development of various aspects of this technology (Whitehead and Hugenholtz 2014; Whitehead et al. 2014; Pádua et al. 2017; Aasen et al. 2018; Manfreda et al. 2018; Singh and Frazier 2018; Ren et al. 2019; Tmušić et al. 2020). While these articles mostly highlight UAV uses in ecological and agricultural sciences, many of the advances are relevant to the proposed new approach to long-term monitoring. The advantages are that they can survey large areas of terrain at high resolution (up to cm scale) with an array of sensors to gather data, including photogrammetry, topography, fluvial morphology, gamma spectroscopy, surficial soil moisture, magnetic field anomalies, surface temperature, and health and type of vegetation. The key to UAV use in long-term monitoring is detection of changes in features or parameters between surveys. Therefore, accurate georeferencing and high-quality data collection and processing are critical, as discussed in the review papers cited above.
Periodic surveys by UAVs can be used in long-term monitoring to detect changes in surface characteristics that are important to contaminant attenuation and cannot be measured at high spatial resolutions by walk-over surveys or satellite platforms. For example, changes to topography and vegetative cover over time can have significant effects on infiltration rate (Christensen et al. 2018) and groundwater flow and contaminant transport, as well as indicate degradation of covers and caps installed over contaminant sources, or changes in surface-water-drainage paths and accumulation zones. Thermal maps can indicate where springs and other groundwater discharges are located, and track changes in the morphology of stream beds.
In addition, UAV capabilities have been coupled with radiation detectors for mapping gamma-radiation sources on the surface (e.g., Vetter et al. 2019). Machine-learning methods have been used to reconstruct complex environments (e.g., topography and buildings) fusing them with radiological information in three dimensions (Haefner et al. 2015; Barnowski et al. 2015). This so-called Scene-Data Fusion capability enables the effective and accurate detection and mapping of radiological and nuclear materials and its visualization in indoor and outdoor domains. Hand-portable and unmanned aerial system platforms have been deployed effectively in the contaminated area in the Fukushima Prefecture and in several locations in the United States (Pavlovsky et al. 2018). Further, periodic gamma-radiation surveys can indicate shifts in the distribution of contaminated windblown dust near uranium-mining and milling sites, as well as changes in the distribution of gamma-emitting radionuclides in soils of wetlands and estuaries.
Data analytics
The new long-term monitoring strategy integrates data analytics, predictive modeling, and machine learning to develop an optimal initial program and provide the basis for the long-term monitoring program to evolve as needed. These tools allow systemic assessment of the status of contaminant attenuation at the site through understanding of the complex relationships between variables and threats to contaminant attenuation. In addition, the integration of these tools into long-term monitoring promotes flexibility in the program by warning when a part of the network is functioning improperly or becoming less relevant and predicting where new monitoring locations should be established.
Data analytics has multiple uses in the new long-term monitoring approach. It allows decisions to be made in designing groundwater-monitoring networks when multiple diverse objectives must be considered (Kollat et al. 2011; Reed and Kollat 2013; Luo et al. 2016), ascertains complex correlations between variables and contaminant concentrations (Schmidt et al. 2018), and optimizes selection of variables to observe and at which sampling frequency (Schmidt et al. 2018). Data analytics will also be important to identifying action values for the rate of trends in critical variables and absolute values that present potential threats to contaminant attenuation.
A variety of machine-learning methods are available to integrate the disparate forms of data and assist decision-making in the new long-term monitoring strategy. Data from well-bore sensors, distributed fiber-optic sensors, UAV surveys, contaminant concentrations, and groundwater-flow directions must be integrated into actionable intelligence. Multiview machine learning can extract critical information from these data sets (e.g., Sun 2013) and integrate it to enable decisions on mitigation of threats to attenuated contaminants or the need for network maintenance. Importantly, machine learning can identify abnormal data and continually adapt the definition of abnormal over time (Liu et al. 2020). In addition, supervised learning can evaluate the accuracy of estimating contaminant concentrations based on in situ-measurable parameters (Schmidt et al. 2018), as well as the effectiveness of a monitoring configuration to capture contaminant-concentration distributions (Sun et al. 2020). The extracted information can support key decisions such as site-closure strategies and well/sensor placement.
Predictive modeling
Predictive contaminant transport modeling has long been used in all stages of groundwater monitoring (e.g., Mangold and Tsang 1991; Loaiciga et al. 1992; Reed et al. 2000), and it will be an integral aspect of the new long-term monitoring strategy. Contaminant transport modeling has been improved dramatically in recent years, thanks to high-performance computing (e.g., Bea et al. 2013; Wainwright et al. 2019). Transport modeling will also benefit from the increasing amount of observations that will improve their parameterization, validation, and/or calibration. Predictive modeling, validated and verified by existing monitoring data, will be used to calculate the spatial and temporal distribution of key properties and fluxes, as well as to refine the deployment and frequency of data collection of the downhole and surface-water sensor network. Stress-testing the monitoring design will be done by simulating potentially adverse events, such as chemical spills or changes to water balance, to be certain that the monitoring system provides sufficient warning of possible threats to attenuated contaminants. Climate-change effects on contaminant attenuation will also be simulated to build flexibility into the monitoring program (Libera et al. 2019), and provide input into programmatic projections of long-term monitoring maintenance needs and costs.
Case Study: F-area Seepage Basins—Savannah River Site
Site Background
The F-area Seepage Basins at the United States Department of Energy Savannah River Site in South Carolina is used here as an example of how the proposed approach to long-term monitoring can be implemented. It was chosen for evaluation of long-term monitoring strategies and tools because it is a mature well-characterized site with 30 years of monitoring data and in the latter stages of remediation. The site has complex soil and groundwater contamination with multiple contaminants having disparate behavior. It has faced the same basic challenges of many other sites where groundwater is contaminated by metals and radionuclides. The groundwater contamination is widespread and deep (20 m below the surface source); thus, excavation would have created enormous quantities of solid-radioactive waste. A pump-and-treat system operated briefly, but ultimately proved ineffective, and more innovative solutions were sought. Since 2005, a comprehensive attenuation-based remedy has been employed that will leave contaminants beneath a capped source, in an engineered attenuation zone, and attenuated in wetland soils. Hence, a robust long-term monitoring program will be important to achieving closure of the F-area Seepage Basins. The new long-term monitoring strategy presented in this paper has not been proposed formally, but has been discussed with state and federal regulators who are open to the possibility of using this strategy in some form, if it proves to be effective. The USDOE has funded a multiyear project (Advanced Long-term Monitoring Systems) to test the new long-term monitoring strategy contemporaneous with traditional monitoring of contaminant concentrations.
The F-area Seepage Basins consisted of three man-made unlined ponds into which low-level radioactive liquid waste was disposed (Killian et al. 1987). Disposal began in 1955 and ended in 1988, with ~7 billion liters disposed during this time. The waste was acidic with sodium and nitrate as the dominant constituents and contained various radionuclides associated with plutonium processing. The mobile radionuclides, tritium, uranium, iodine-129 (I-129), and strontium-90 (Sr-90), migrated through the vadose zone, contaminating groundwater in the saturated zone at concentrations of environmental concern. The resulting plume contaminates groundwater beneath ~1 square kilometer of the surface and discharges into wetlands and a local stream called Fourmile Branch. The map in Fig. 2 shows the location of the three basins relative to the wetlands and Fourmile Branch, with an arrow showing the general direction of groundwater flow in the upper aquifer.
The contamination plume occupies only a portion of the vertical extent of the upper-saturated aquifer zone—~3 m of a total thickness of 10 m. Recharge downgradient of the basins and a downward hydraulic gradient cause the contamination to migrate downward from the basins toward a discontinuous clay layer at the bottom of the aquifer. Two lower aquifers are also contaminated, but hydrogeological factors make the risk to human health and the environment posed by contamination in these aquifers substantially less than the risk posed by contamination in the water-table aquifer. Thus, remediation has focused on the uppermost aquifer, with the first objective being to minimize discharge of contaminants to surface water followed by reducing concentrations in upgradient groundwater to the maximum concentration levels allowable.
Remediation of the F-area Seepage Basins began with installation of a low-permeability cap in 1990 on chemically stabilized contaminated soils at the bottoms of the basins, and evolved to a comprehensive attenuation-based remedy. Installation of a subsurface barrier system that funnels contaminated groundwater into gaps in the barrier, into which alkaline fluids are periodically injected, creates in situ-treatment zones for Sr-90 and uranium. The injections are intended to keep pH of groundwater downgradient of the treatment zones >5.5. The subsurface barriers were placed across “troughs” in the top of the local confining zone that were preferential flow paths for contaminant migration to the wetlands and Fourmile Branch. This effectively blocked tritium migration, facilitating attainment of the remedial goal of a 70% reduction in tritium flux to Fourmile Branch. The elevated pH in the treatment zones within the gates causes attenuation of uranium and Sr-90 by enhanced adsorption to mineral surfaces. I-129 is treated by injection of silver chloride particles upgradient of the base-injection zone at the center gate. The silver chloride particles react with natural iodine and I-129 to form sparingly soluble silver iodide, causing partitioning of I-129 from groundwater to solid silver iodide.
Remediation of F-area Seepage Basins groundwater is also affected by natural attenuation of contaminants in the organic-rich soils of the wetlands associated with Fourmile Branch. Uranium is attenuated by binding with organic matter and plant roots as U(VI), with some fraction attenuated by reduction to U(IV) (Li et al. 2014, 2015). I-129 is also primarily attenuated by binding with organic matter (Xu et al. 2011). Less is known about Sr-90 attenuation, though based on its known behavior elsewhere, Sr-90 is likely bound to the relatively abundant clay-size particles in wetland soils. The controlling variables for attenuation of uranium, Sr-90, and I-129 in the wetlands are not fully understood and are the subject of ongoing research.
Currently, monitoring at the F-area Seepage Basins is focused on remediation performance. Substantial progress has been made toward remedial goals, and development of a long-term monitoring strategy is underway. As stated earlier, the traditional approach would be to reduce the number of monitoring points by eliminating those that provide redundant or irrelevant data, but most of the monitoring data would still consist of contaminant concentrations. There are 93 monitoring wells, 16 wetland groundwater locations, and 6 surface-water stations in the current monitoring plan. Traditional optimization of the monitoring network by reducing the number of monitoring locations and frequency of sampling would reduce costs. Nevertheless, while reduction in long-term monitoring cost is a goal, the more important objective is to be sure that funding allocated for long-term monitoring is spent wisely and efficiently. The long-term monitoring strategy proposed here would achieve both goals by reducing sampling and analytical costs, and measuring leading indicators that warn of potential threats to attenuated contaminants in time to implement mitigation actions.
Vulnerable Zones
The current remediation of the F-area Seepage Basins relies on attenuation of contaminants in basin soils beneath impermeable caps, in the treatment zones in the gaps in the subsurface barrier, and in the wetlands. A summary of the vulnerable zones, the threats to attenuated contaminants, and parameters to be monitored for each zone is shown in Fig. 3.
Basin soils and vadose zone
The bottoms of the basins were not excavated prior to capping and a significant mass of contaminants remains in the basin’s soil (Corbo et al. 1985). In addition, contaminants remain in the vadose zone, either adsorbed to mineral surfaces or in dissolved pore water held in place by capillary pressure. The attenuation of these contaminants depends on the integrity of the low- permeability caps preventing infiltration of water through these sources. The two measures of this threat are evidence of cap degradation and moisture increases within the cap and upper vadose zone.
Evidence of cap degradation will be detectable from near-surface measurements made with distributed fiber-optic sensors and multisensor UAV-based surveys. Erosion, animal burrows, plant roots, and damage from trespassers can lead to pathways for water to breach the protective clay layer of the cap. Damage can also come from natural settling and seismic activity that fractures the clay layer. Currently, subsidence and surface disturbances of the basin caps are monitored once a year by walk-over assessments and surveys of subsidence markers. UAVs equipped with photogrammetry and Lidar sensors will be used to make high-resolution topographic maps and identify any erosion, deep-rooting vegetation, burrows, ant mounds, or other damage to the surface of the caps. Overlays of georeferenced images from subsequent flights will identify areas of subsidence. The periodic flights will initially be done annually to compare with the manual surveys of the caps. Over time, the frequency of flights may be varied, corresponding to the rate of change in cap conditions. Distributed fiber-optic strain sensors will be installed over the surface of the cap to detect subsidence in the interim between flights.
Monitoring moisture content in and below the impermeable clay layer of the basin caps is a direct measurement of cap degradation. This can be done with ERT by installing an array of electrodes on the cap surface and along the sides of the caps. If a moisture increase is detected, the vadose-zone thickness of ~20 m allows sufficient time for cap repair before contaminants mobilized from the upper vadose zone become a threat to the saturated zone. This is preferable to waiting until increases in contaminant concentrations are detected in monitoring wells downgradient of the basins. Nevertheless, monitoring wells will be maintained adjacent to downgradient edges of the basins that will be sampled periodically and instrumented with sensor packages measuring pH, specific conductance, ORP, and water level.
Currently, the low-permeability caps and vadose zone beneath are in a steady-state condition with minimal drainage, and a transition to long-term monitoring of this zone of vulnerability can begin. Modeling and experiments to evaluate the effectiveness of ERT for cap monitoring are underway at the site. Additionally, testing the effectiveness of UAV-deployed imagery of the cap surfaces is in the planning stages.
In situ-treatment zones and groundwater
Three contaminants of interest are attenuated by the remedial actions within the treatment zones between segments of the subsurface barrier. Sr-90 and uranium are attenuated by enhanced adsorption caused by increasing the pH of the acidic plume with periodic alkaline-fluid injections. I-129 is attenuated by precipitation of silver iodide on silver chloride particles. Maintaining attenuation of these contaminants depends on preventing specific chemical changes from occurring within this zone of vulnerability. If pH returns to pretreatment acidic levels, then Sr-90 and uranium will be remobilized. Changes in redox potential will not remobilize Sr-90 and uranium, but solubility calculations indicate that decreasing redox potential will cause dissolution of silver iodide and remobilization of I-129 (Fig. 4).
Natural aquifer conditions are protective of the attenuated contaminants in the treatment zones. ORPs in groundwater in water-table aquifers at the Savannah River Site are oxidizing and close to saturation with dissolved oxygen (Strom and Kaback 1992). The median pH of uncontaminated groundwater at the F-area Seepage Basins site is 6.0. As remediation proceeds, the pH of groundwater entering the treatment zones will approach 6.0 when all acidic groundwater has flushed through the portion of the system upgradient of the subsurface barrier. Once this occurs, there are no natural mechanisms that will change pH and ORP in the subsurface-treatment zones in a way that would remobilize contaminants. Only artificial influences, such as a chemical spill, can cause conditions that threaten contaminant attenuation in the treatment zones. Therefore, long-term monitoring of the in situ-treatment zones will focus on monitoring ORP and pH of groundwater entering this zone of vulnerability. Sensor platforms measuring these parameters, specific conductance, and water level will be deployed in wells upgradient of and within the in situ-treatment zones. Existing monitoring wells downgradient of the in situ-treatment zones will remain in place for periodic sampling and measurement of contaminant concentrations.
An initial test deployment of sensor packages in three wells at the F-area Seepage Basins site was conducted from 2015 until 2019 to assess their ability to provide inexpensive long-term data on attenuation-controlling variables. The sensors collected measurements, and data were sent to an off-site database, via a wireless telecommunication system, and users were able to evaluate the data in real time. Figure 5 compares data acquired by a sensor platform to measurements made during routine monitoring at the sentry well for an in situ-treatment zone. During the sensor trial period, there were 43,000 measurements recorded by the sensors and 87 samples collected and analyzed for contaminants. The sensors effectively recorded the changes in pH and specific conductance induced by base-injection events, and the subsequent influx of acidic groundwater into the treatment zone. The pH measurements from late 2017 to 2019 indicate some issues with the pH sensor, possibly compromise of the electrolyte solution within the sensor or calibration issues. Newer pH sensors under development (Khan et al. 2017; Lu et al. 2019) should have fewer problems with maintenance and calibration than the galvanic cell electrode used in the sensor platforms initially deployed at the F-area Seepage Basins.
The sensor measurement of specific conductance and pH also shows the effect of purging the wells prior to sampling. For each sampling event, the sensor measurements are consistent with measurements made by the sampling crew, but the sensors record increasing specific conductance and decreasing pH during well purging. This likely reflects heterogeneity in hydraulic conductivity across the length of the well screen. When the well is purged and sampled, a higher proportion of groundwater reaching the well is from zones of the highest hydraulic conductivity. In the in situ-treatment zone, these preferential flow paths are likely to contain more highly contaminated groundwater. Thus, as purging proceeds, specific conductance increases and pH decreases. The sensor measurements between sampling periods resemble no purge or passive sampling methods (e.g., ITRC 2007). As with other no purge or passive sampling, vertical placement of the sensors within a well screen is important. Vertical flow within wellbores should also be considered when installing sensor platforms in wells (McMillan et al. 2014). In locations with substantial vertical heterogeneity, the long-term monitoring may benefit from deployment of sensors at multiple depths, or from depth-resolved geophysical methods such as ERT or fiber-optic sensing.
Values of ORP and pH that are potential threats to contaminant attenuation, action values, will be determined from experimental results, groundwater modeling, and existing monitoring data. An example of the usefulness of existing monitoring data is presented in Fig. 6. It shows monitoring data for pH and U-238 concentrations from 2005 to 2015 from the sentry well in a treatment zone used to indicate when injections of alkaline fluids are needed. The monitoring data suggest that the pH-action value for uranium is between 4.3 and 5.3. This is consistent with adsorption experiments using cored aquifer solids from the F-area Seepage Basins site reported in Dong et al. (2012). In their experiments, the adsorption edge for uranium was between pH values of 4 and 5.3, with >70% of uranium adsorbed at pH > 5. The monitoring data do not provide a clear indication of a range of action values for Sr-90 and I-129, so these values will be determined experimentally.
Groundwater modeling, data analytics, and machine learning will be used to refine criteria for decisions on mitigation actions. For example, a single exceedance of an action value will not initiate a mitigation action because of sensor imprecision and natural variation in controlling variables. Data analytics and machine learning will discern patterns in parameter variation, recognize abnormal patterns, and inform decisions on mitigation actions. In addition to aiding the optimization of the initial sensor locations, groundwater modeling will be used to examine various scenarios of contaminant remobilization to design a suite of responses to abnormal and potentially threatening parameter variations (Wainwright et al. 2019).
Wetlands
Contaminants have interacted with organic matter and clays in wetland soils associated with the F-area Seepage Basins since the contaminants initially reached the wetlands, at least 20 years prior to capping of the basins. The mass of contaminants in the wetlands and the controls on the mechanisms of attenuation and release are poorly understood and are currently the focus of ongoing studies. For example, monitoring data show seasonal effects on attenuation of I-129 in wetland soils with concentrations in surface water varying with quarterly rainfall (Fig. 7). Ongoing studies are focused on determining whether this is predominantly a hydrodynamic or a biogeochemical process. This is important to long-term monitoring because it will determine the parameters to be measured and the range of values that are acceptable. This is just one example of the dynamic processes in the wetlands that must be monitored.
Long-term monitoring of the wetlands will be done with a combination of UAV-deployed imagery and sensors on the surface or in the shallow subsurface. UAVs will be deployed periodically to survey changes in topography, vegetation, the locations of groundwater seeps, the course of Fourmile Branch, and the distribution of gamma-emitting radionuclides in wetland soils. Distributed fiber-optic sensors can be used to measure pH (Shao et al. 2013; Lu et al. 2019), flow rates in the area of rapid fluid movement (using heat pulse or thermal tracer methods, e.g., Somogyvári, Bayer 2017), dissolved gas concentrations, temperature (Selker et al. 2006), and potentially other parameters that indicate changing patterns of groundwater discharges along stretches of wetland soils and Fourmile Branch.
There will be a network of monitoring wells instrumented with sensor platforms to measure pH, ORP, and water level upgradient and within the wetlands. The existing surface-water-monitoring locations will also be monitored for the same parameters using sensors. Nevertheless, samples from the surface-water-monitoring stations will continue to be collected and analyzed for contaminant concentrations, until attenuation mechanisms for Sr-90, uranium, and I-129 are better understood. The integration of these monitoring tools will provide information on systemic changes to the wetlands across all pertinent time frames. Long-term monitoring in the wetlands highlights the benefit of its systemic view because contaminant measurements alone cannot provide insight into the dynamic processes affecting contaminant attenuation.
Implementing the New Long-term Monitoring Approach
The new long-term monitoring approach will be implemented in phases. Sensors will be deployed, and baseline surveillance by UAV imagery will initially be done contemporaneously with the current monitoring of contaminant concentrations. ERT or other geophysical techniques will also be field-tested during this time. When all interested parties are comfortable with these alternative methods, select monitoring wells will be removed from scheduled contaminant monitoring and another period of evaluating the new approach will begin. Assuming that the new approach is assessed as valid, this process will continue until 70–80% of the contaminant-monitoring wells are removed. During these implementation phases, the frequency of well sampling will also be reduced as appropriate. Data analytics will be used to decide the optimum frequency of contaminant monitoring in the remaining wells (Schmidt et al. 2018).
Location, type of sensor platform, and nature of installation will be determined based on function. The function will be defined by the zones shown in Fig. 8. The perimeter, shown as the purple zone, will have an array of sensor platforms to detect the effects of off-site events that could influence conditions at the waste site. The sensor platforms will include probes for measuring water level, pH, ORP, and specific conductance. They can be installed in wells pushed into the subsurface by direct push technology as exposed screen installations (USEPA 2005b) with no infrastructure other than the well screen and casing. If samples are ever needed, a pump can be lowered into the well to the correct depth. The vulnerable zones are shown in green. The same type of sensor platforms used in the perimeter zone will be used at the vulnerable zones, except that they will be installed in monitoring wells constructed according to the specifications in the South Carolina Department of Health and Environmental Control Regulation 61-71 (SCDHEC 2016). Sensor platforms deployed in existing monitoring wells will, by definition, adhere to this regulation. Those installed in new locations will adhere to the regulation in case the location needs to be converted to a standard monitoring well in the future. Sensor platforms will be installed upgradient and within the in situ-treatment zones and the wetlands. They will be installed at the downgradient edges of the basins. Sensor platforms installed in the yellow zone will be used solely to monitor water levels, so that high-precision water-table maps can be made. These can be installed in exposed screen wells placed by direct push technology. The locations of each type of sensor and the frequency of measurement will be determined by groundwater modeling and data analytics.
Conclusion
The proposed new strategy for long-term monitoring of waste sites at which attenuation-based remedies have been used to treat groundwater contaminated with metals and/or radionuclides will provide early warning of systemic changes that may remobilize attenuated contaminants. Attenuation-based strategies leave contaminants in the subsurface by design, and thus will require monitoring for long periods of time after site closure to ensure that contaminants remain in place. Traditional long-term monitoring, with emphasis on measurement of contaminant concentrations, is inefficient at sites where attenuation-based remedies have been used. It is more efficient to monitor changes to the system that threaten to remobilize attenuated contaminants. This will lead to more proactive responses in the event such changes occur. Furthermore, the methods used in the new strategy are less costly than traditional long-term monitoring strategies. This is important given the long monitoring periods that will be required for sites where attenuation-based remedies are used for treatment of metal- and radionuclide-contaminated groundwater.
The long-term monitoring strategy proposed here emphasizes monitoring of the physical and chemical processes responsible for contaminant attenuation. In groundwater, these are the direction and rate of the flow of groundwater, as well as the chemical parameters controlling contaminant behavior. These can be monitored with sensor platforms deployed in monitoring wells and equipped with wireless data-transmission technology. For low-permeability caps over contaminant sources, evidence of cap degradation can be detected using imagery from UAVs. Additionally, geophysical methods, including ERT and fiber-optic methods, can be used to measure moisture increases within the caps or in the upper vadose zone. Wetland and surface-water monitoring can be done using a combination of sensor technology and imagery from UAV flights.
Measuring contaminant concentrations at specific locations will remain a part of the new strategy, but the number of locations and frequency of sampling will be significantly reduced. This will be accomplished by phasing in the new strategy in a way that meets regulator and stakeholder needs for demonstrating the efficacy of the new strategy. Part of the design of the new strategy will be establishing thresholds of change in the indicator parameters that will trigger sampling and analysis of groundwater for contaminant concentrations. This will be done using historical monitoring data and data collected during the phase-in period to determine natural variation in parameters and experimental results to determine action values of the chemical parameters.
The new long-term monitoring approach will allow conditions that threaten to remobilize attenuated contaminants to be mitigated before remobilization occurs. In contrast, traditional long-term monitoring, relying on contaminant-concentration measurements, can only alert that remobilization has occurred.
References
Aasen H, Honkavaara E, Lucieer A, Zarco-Tejada PJ (2018) Quantitative remote sensing at ultra-high resolution with UAV spectroscopy: a review of sensor technology, measurement procedures, and data correction workflows. Remote Sens 10:1091. https://doi.org/10.3390/rs10071091
Ajo-Franklin J, Dou S, Daley T, Freifeld B, Robertson M, Ulrich C, Wood T, Eckblaw I, Lindsey N, Martin E, Wagner A (2017) Time-lapse surface wave monitoring of permafrost thaw using distributed acoustic sensing and a permanent automated seismic source. SEG Tech Program Expanded Abstr 2017:5223–5227
Bao X, Chen L (2012) Recent progress in distributed fiber optic sensor. Sensor 12:8601–8639
Barnowski R, Haefner A, Mihailescu L, Vetter K (2015) Scene data fusion: real-time standoff volumetric gamma-ray imaging. Nucl Instrum Methods Phys Res A 800:65–69
Bea BA, Wainwright HM, Spycher N, Faybishenko B, Hubbard SS, Denham M (2013) Identifying key controls on acidic-U(VI) plume behavior at the Savannah River Site, using reactive transport modeling. J Contam Hydrol 151:34–54. https://doi.org/10.1016/j.jconhyd.2013.04.005
Bethke CM, Farrell B, Sharifi M (2020) The Geochemist’s Workbench®, release 14: GWB essentials. Aqueous Solutions, LLC, Champaign, IL
Binley A, Hubbard SS, Huisman JA, Revil A, Robinson DA, Singha K, Slater LD (2015) The emergence of hydrogeophysics for improved understanding of subsurface processes over multiple scales. Water Resour Res 51:3837–3866. https://doi.org/10.1002/2015WR017016
Blowes DW, Ptacek CJ, Benner SG, McRae CWT, Bennett TA, Puls RW (2000) Treatment of inorganic contaminants using permeable reactive barriers. J Contam Hydrol 45:123–137
Bunn AL, Wellman DM, Deeb RA, Hawley EL, Truex MJ, Peterson M, Freshley MD, Pierce EM, McCord J, Young MH, Gilmore TJ, Miller R, Miracle AL, Kaback D, Eddy-Dilek C, Rossabi J, Lee H, Bush RP, Beam P, Chamberlain GM, Whitehurst L, Gerdes KD, Collazo Y (2012) Scientific opportunities for monitoring of environmental remediation sites—(SOMERS). DOE/PNNL-21379, Pacific Northwest National Laboratory, Richland WA USA
Corbo P, Kantelo MV, Fliermans CB (1985) Analytical results, database management and quality assurance for analyses of soil cores from the F- and H-area seepage basins. DPST-85-921. Du Pont de Nemours (E.I.) and Co., Aiken, SC
Cox B, Wills P, Kiyaschenko D, Mestayer J, Lopez J, Bourne S, Lupton R, Solano G, Henderson N, Hill D, Roy J (2012) Distributed acoustic sensing for geophysical measurement, monitoring, and verification. Canadian Society of Exploration Geophysics Recorder, February 2012:7–13
Christensen JN, Dafflon B, Shiel AE, Tokunaga TK, Wan JM, Faybishenko B, Dong WM, Williams KH, Hobson C, Brown ST, Hubbard SS (2018) Using strontium isotopes to evaluate the spatial variation of groundwater recharge. Sci Total Environ 637:672–685
Dafflon B, Irving J, Barrash W (2011) Inversion of multiple intersecting high-resolution crosshole GPR profiles for hydrological characterization at the Boise hydrogeophysical research site. J Appl Geophys 73:305–314
Dafflon B, Wu Y, Hubbard SS, Birkholzer JT, Daley TM, Pugh JD, Trautz RC (2013) Monitoring CO2 intrusion and associated geochemical transformations in a shallow groundwater system using complex electrical methods. Environ Sci Technol 47(1):314–321
Daley TM, Freifeld BM, Ajo-Franklin JB, Dou S, Pevzner R, Shulakova V, Kashikar S, Miller DE, Goetz J, Henninges J, Lueth S (2013) Field testing of fiber-optic distributed acoustic sensing (DAS) for subsurface seismic monitoring. Lead Edge 32(6):699–706
Dam WL, Campbell S, Johnson RH, Looney BB, Denham ME, Eddy-Dilek CA, Babits SJ (2015) Refining the site conceptual model at a former uranium mill site in Riverton, Wyoming, USA. Environmental Earth Sciences 74(10):7255–7265
Danielson TL (2020) Sensor recommendation for long-term monitoring of the F-area seepage basins. SRNL-STI-2020-00208. Savannah River National Laboratory, Aiken SC
Declercq I, Cappuyns V, Duclos Y (2012) Monitored natural attenuation (MNA) of contaminated soils: State of the art in Europe—a critical evaluation. Sci Total Environ 426:393–405
Denham M, Vangelas K (2008) Biogeochemical gradients as a framework for understanding waste-site evolution. Remediation 19:5–17
Dong W, Tokunaga TK, Davis JA, Wan J (2012) Uranium(VI) adsorption and surface complexation modeling onto background sediments from the F-area Savannah River Site. Environ Sci Technol 46:1565–1571
Dou S, Wood T, Ajo-Franklin J, Robertson M, Daley T, Freifeld B, Pevzner R, Gurevich B (2017) Surface orbital vibrator for permanent seismic monitoring: A signal contents and repeatability appraisal. SEG Tech Program Expanded Abstr 2017:5906–5910.
Drever JI (1997) Geochemistry of natural waters: the surface and groundwater environments. Prentice-Hall, Upper Saddle River, NJ, USA
Du Laing G, Rinklebe J, Vandecasteele B, Meers E, Tack FMG (2009) Trace metal behavior in estuarine and riverine floodplain soils and sediments: a review. Sci Total Environ 407:3972–3985
Evanko CR, Dzombak DA (1997) Remediation of metals-contaminated soils and groundwater. Technical Evaluation Report TE-97-01, Ground-Water Remediation Technologies Analysis Center, Pittsburgh, PA USA
Frohne T, Rinklebe J, Diaz-Bone RA, Du Laing G (2011) Controlled variation of redox conditions in a floodplain soil: impact on metal mobilization and biomethylation of arsenic and antimony. Geoderma 160:414–424
Frings J (2011) Enhanced pipeline monitoring with fiber optic sensors. In: Proceedings of the 16 Pipeline Technology Conference, Hannover, Germnay
Fytas K (2010) Use of permeable reactive barriers to treat acid mine effluents. Int J Min Reclam Environ 24:206–215. https://doi.org/10.1080/17480930903305929
Gaebler W (1984) “Characteristics of fiber optic radiation detectors”, Proceedings SPIE Vo. 403: Optical fibers in broadband networks, instrumentation and urban and industrial environments, p 142–145, Paris France
Gambrell RP (1994) Trace and toxic metals in wetlands—a review. J Environ Qual 23:883–891
Gasperikova E, Hubbard S, Watson D, Bake G, Peterson J, Kowalsky M, Smith M, Brooks S (2013) Long-term electrical resistivity monitoring of recharge-induced contaminant plume behavior. J Contam Hydrol 142-143:33–49
Genelle F, Sirieix C, Riss J, Naudet V (2012) Monitoring landfill cover by electrical resistivity tomography on an experimental site. Eng Geol 145-146:18–29
Haefner A, Gunter D, Barnowski R, Vetter K (2015) A filtered back-projection algorithm for 4π compton camera data. IEEE Trans Nucl Sci 62(4):1911–1917
Harre K, Chaundry T, Greenwald R, Weiwei J, Davis C, Zavislak M Minsker B (2009) Adaptive long-term monitoring at environmental restoration sites. ESTCP Project ER-0629 Final Report. US Department of Defense, Environmental Security Technology Certification Program, Port Hueneme CA USA
Hartog AH (2017) An introduction to distributed optical fibre sensors. CRC Press, Boca Raton FL USA
Hashim MA, Mukhopadhyay S, Sahu JN, Sengupta B (2011) Remediation technologies for heavy metal contaminated groundwater. J Environ Manag 92:2355–2388
Hsi C-KD, Langmuir D (1985) Adsorption of uranyl onto ferric oxyhydroxides: application of the surface complexation site-binding model. Geochim Cosmochim Acta 49:1931–1941
Hunter P, Cameron K, Stewart R (2011) Demonstration and validation of GTS long-term monitoring optimization software at military and government sites. ESTCP Project ER-200714. US Department of Defense, Environmental Security Technology Certification Program, Arlington, VA USA
IAEA (2004) Remediation of sites with dispersed radioactive contamination. Technical Report Series No. 424. International Atomic Energy Agency, Vienna
IAEA (2006) Applicability of monitored natural attenuation at radioactively contaminated sites. Technical Report Series No. 445. International Atomic Energy Agency, Vienna
ITRC (2007) Protocol for use of five passive samplers to sample for a variety of contaminants in groundwater. DSP-5, Interstate Technology & Regulatory Council, Diffusion/Passive Sampler Team. www.itrcweb.org
ITRC (2010) A decision framework for applying monitored natural attenuation processes to metals and radionuclides in groundwater. APMR-1. Interstate Technology & Regulatory Council, Attenuation Processes for Metals and Radionuclides Team, Washington DC. www.itrcweb.org
Janot N, Lezama Pacheco JS, Pham DQ, O’Brien TM, Hausladen D, Noël V, Lallier F, Maher K, Fendorf S, Williams KH, Long PE, Bargar JR (2015) Physico-chemical heterogeneity of organic-rich sediments in the Rifle Aquifer, CO: impact on uranium biogeochemistry. Environ Sci Technol 50:46–53. https://doi.org/10.1021/acs.est.5b03208
Jurkovič L, Majzlan J, Hiller E, Klimko T, Voleková-Lalinská B, Méres S, Göttlicher J, Steininger R (2019) Natural attenuation of antimony and arsenic in soils at the abandoned Sb-deposit Poproč, Slovakia. Environ Earth Sci 78(24):1–13
Johnson T, Versteeg R, Thomle J, Hammond G, Chen X, Zachara J (2015) Four‐dimensional electrical conductivity monitoring of stage‐driven river water intrusion: accounting for water table effects using a transient mesh boundary and conditional inversion constraints. Water Resour Res 51:6177–6196
Johnson TC, Versteeg RJ, Rockhold M, Slater LD, Ntarlagiannis D, Greenwood WJ, Zachara J (2012) Characterization of a contaminated wellfield using 3D electrical resistivity tomography implemented with geostatistical, discontinuous boundary, and known conductivity constraints. Geophysics 77(6):EN85–EN96
Johnson TC, Versteeg RJ, Ward A, Day-Lewis FD, Revil A (2010) Improved hydrogeophysical characterization and monitoring through parallel modeling and inversion of time-domain resistivity and induced-polarization data. Geophysics 75:WA27–WA41
Kerkez B, Glaser SD, Bales RC, Meadows MW (2012) Design and performance of a wireless sensor network for catchment‐scale snow and soil moisture measurements. Water Resour Res 48(9). https://doi.org/10.1029/2011WR011214
Khan M, Mukherjee K, Shoukat R, Dong H (2017) A review on pH sensitive materials for sensors and detection methods. Microsyst Technol 23:4391–4404
Killian TH, Kolb NL, Corbo P, Marine IW (1987) Environmental information document: F-area seepage basins. DPST-85-704. E.I. du Pont de Nemours & Co., Savannah River Laboratory, Aiken, SC
Kollat JB, Reed PM, Maxwell RM (2011) Many-objective groundwater monitoring network design using bias-aware ensemble Kalman filtering, evolutionary optimization, and visual analytics. Water Resour Res 47:W0259. https://doi.org/10.1029/2010WR009194
Krishna K, Kurakalva RM, Murthy N (2010) Monitored natural attenuation as a remediation tool for heavy metal contamination in soils in an abandoned gold mine area. Curr Sci 99:628–635
Krohn D, Nicholls P (2009) Fiber optic sensor applications in transportation infrastructure protection. In: Photonics in the transportation industry: auto to aerospace 2, proceedings of SPIE, vol 7314, SPIE Defense, Security, and Sensing, Orlando FL USA
LaPointe F, Fytas K, McConchie D (2005) Using permeable reactive barriers for the treatment of acid rock drainage. Int J Surf Min Reclam Environ 19:57–65. https://doi.org/10.1080/13895260500045241
Langmuir D (1978) Uranium solution-mineral equilibria at low temperatures with applications to sedimentary ore deposits. Geochim Cosmochim Acta 42:547–569
Lee DR, Hartwig DS (2005) Zeolite prevents discharge of strontium-90-contaminated groundwater. Canadian Nuclear Society, Toronto, Ontario, ISBN 0-919784-81-X
Liao Q, Deng Y, Shi X, Sun Y, Duan W, Wu J (2018) Delineation of contaminant plume for an inorganic contaminated site using electrical resistivity tomography: comparison with direct-push technique. Environ Monit Assess 190(4):1–17
Li D, Kaplan DI, Chang H-S, Seaman JC, Jaffe PR, Koster van Groos P, Scheckel KG, Segre CU, Chen N, Jiang D-T, Newville M, Lanzirotti A (2015) Spectroscopic evidence of uranium immobilization in acidic wetlands by natural organic matter and plant roots. Environ Sci Technol 49:2823–2832
Li D, Seaman JC, Chang H-S, Jaffe PR, Koster van Groos P, Jiang D-T, Chen N, Lin J, Arthur Z, Pan Y, Scheckel KG, Newville M, Lanzirotti A, Kaplan DI (2014) Retention and chemical speciation of uranium in an oxidized wetland sediment from the Savannah River Site. J Environ Radioact 131:40–46
Libera A, de Barros F, Faybishenko B, Eddy-Dilek C, Denham M, Lipnikov K, Moulton D, Maco B, Wainwright H (2019) Climate change impact on residual contaminants under sustainable remediation. J Contam Hydrol. https://doi.org/10.1016/j.jconhyd.2019.103518
Lindsey NJ, Martin ER, Dreger DS, Freifeld B, Cole S, James SR, Biondi BL, Ajo‐Franklin JB (2017) Fiber‐optic network observations of earthquake wavefields. Geophys Res Lett 44(23):11–792
Liu J, Gu J, Li H, Carlson KH (2020) Machine learning and transport simulations for groundwater anomaly detection. J Comput Appl Math 380:112982. https://doi.org/10.1016/j.cam.2020.112982
Loaiciga HA, Charbeneau RJ, Everett LG, Fogg GE, Hobbs BF, Rouhani S (1992) Review of ground-water quality monitoring network design. J Hydraulic Eng 118:11–37
Looney BB, Denham ME, Eddy-Dilek CA (2014) Independent technical evaluation and recommendations for contaminated groundwater at the Department of Energy Office of Legacy Management Riverton Processing Site. Report No. STI-2014-00163. Savannah River National Laboratory, Aiken, SC
Lopez-Higuera JM, Cobo LR, Incera AQ, Cobo A (2011) Fiber optic sensors in structural health monitoring. J Lightwave Technol 29(4):587–608
Lu F, Wright RF, Lu P, Ohodnicki PR Jr (2019) Distributed fiber optic pH sensor for the application in the well bore. The Electrochemical Society Abstracts, Pennington, NJ USA
Luo Q, Wu J, Yang Y, Qian J, Wu J (2016) Multi-objective optimization of long-term groundwater monitoring network design using a probabilistic Pareto genetic algorithm under uncertainty. J Hydrol 534:352–363. https://doi.org/10.1016/j.jhydrol.2016.01.009
Majone M, Verdini R, Aulenta F, Rossetti S, Tandoi V, Kalogerakis N, Agathos S, Puig S, Zanaroli G, Fava F (2015) In situ groundwater and sediment bioremediation: barriers and perspectives at European contaminated sites. N Biotechnol 32(1):133–146
Manfreda S, McCabe MF, Miller PE, Lucas R, Madrigal VP, Mallinis G, Dor EB, Helman D, Estes L, Ciraolo G, Müllerová J, Tauro F, de Lima MI, de Lima JLMP, Maltese A, Frances F, Caylor K, Kohv M, Perks M, Ruiz-Pérez G, Su Z, Vico G, Toth B (2018) On the use of unmanned aerial systems for environmental monitoring. Remote Sens 10:641. https://doi.org/10.3390/rs10040641
Mangold D, Tsang C (1991) A summary of subsurface hydrological and hydrochemical models. Rev Geophys 29(1):51–79
McMillan L, Rivett M, Tellam J, Dumble P, Sharp H (2014) Influence of vertical flows in wells on groundwater sampling. J Contam Hydrol 169:50–61
NRC (2013) Alternatives for managing the nation’s complex contaminated groundwater sites. National Research Council, The National Academies Press, Washington, DC
NRC (1999) Environmental cleanup at naval facilities: risk-based methods. National Research Council, The National Academies Press, Washington, DC
Obiri-Nyarko F, Grajales-Mesa SJ, Malina G (2014) An overview of permeable reactive barriers for in situ sustainable groundwater remediation. Chemosphere 111:243–259
Pádua L, Vanko J, Hruška J, Adão T, Sousa JJ, Peres E, Morais R (2017) UAS, sensors, and data processing in agroforestry: a review towards practical applications. Int J Remote Sens 38. https://doi.org/10.1080/01431161.2017.1297548
Pavlovsky R, Haefner A, Joshi T, Negut V, McManus K, Suzuki E, Vetter K(2018) 3-D radiation mapping in real-time with the localization and mapping platform LAMP from unmanned aerial systems and man-portable configurations arXiv Prepr 1901:05038. https://arxiv.org/abs/1901.05038
Pellerin BA, Bergamaschi BA, Horsburgh JS (2012) In situ optical water quality sensor networks—Workshop Summary Report. USGS Open-File Report 2012-1044, vol 13, Reston VA USA
Potyrailo RA, Hieftje GM (1998) Advanced strategies for spatially resolved analyte mapping with distributed fiber-optic sensors for environmental and process applications. In: Proceedings on the conference for environmental and remediation technologies, SPIE, vol 3534
Reed MR, Kollat JB (2013) Visual analytics clarify the scalability and effectiveness of massively parallel many-objective optimization: a groundwater monitoring design example. Adv Water Resour 56:1–13. https://doi.org/10.1016/j.advwatres.2013.01.011
Reed P, Minsker B (2004) Striking the balance: long-term groundwater monitoring design for conflicting objectives. J Water Resour Plan Manag 130:140–149
Reed P, Minsker B, Goldberg D (2001) A multiobjective approach to cost effective long-term groundwater monitoring using an elitist nondominated sorted genetic algorithm with historical data. J Hydroinformatics 3:71–89
Reed P, Minsker B, Valocchi AJ (2000) Cost effective long-term groundwater monitoring design using a genetic algorithm and global mass interpolation. Water Resour Res 36:3731–3741
Ren H, Zhao Y, Wu X, Hu Z (2019) A review of UAV monitoring in mining areas: current status and future perspectives. Int J Coal Sci Technol 6(3):320–333. https://doi.org/10.1007/s40789-019-00264-5
Rügner H, Finkel M, Kaschl A, Bittens M (2006) Application of monitored natural attenuation in contaminated land management—a review and recommended approach for Europe. Environ Sci Policy 9:568–576. https://doi.org/10.1016/j.envsci.2006.06.001
Sakala E, Fourie F, Gomo M, Madzivire G (2020) Natural attenuation of acid mine drainage by various rocks in the Witbank, Ermelo and Highveld Coalfields, South Africa. Nonrenewable Resources 14:1–14
Sayde C, Gregory C, Gil‐Rodriguez M, Tufillaro N, Tyler S, van de Giesen N, English M, Cuenca R, Selker JS (2010) Feasibility of soil moisture monitoring with heated fiber optics. Water Resour Res https://doi.org/10.1029/2009WR007846
SCDHEC (2016) Reguation 61-71: well standards. South Carolina Department of Health and Environmental Control, Columbia SC. https://www.scdhec.gov/environment/bureau-water/water-regulations-standards/water-regulations-standards-well-standards
Schenato L (2017) A review of distributed fiber optic sensors for geo-hydrological applications. Appl Sci 7:896. https://doi.org/10.3390/app7090896
Schmidt F, Wainwright HM, Faybishenko B, Denham M, Eddy-Dilek C (2018) In situ monitoring of groundwater contamination using the Kalman filter. Environ Sci Technol 52(13):7418–7425
Selker JS, Thévenaz L, Huwald H, Mallet A, Luxemburg W, van de Giesen N, Stejskal M, Zeman J, Westhoff M, Parlange MB (2006) Distributed fiber-optic temperature sensing for hydrologic systems. Water Resour Res. https://doi.org/10.1029/2006WR005326
Shao L-Y, Yin M-J, Tam H-Y, Albert J (2013) Fiber optic pH sensor with self-assembled polymer multilayer nanocoatings. Sensors 13:1425–1434
Singh KK, Frazier AE (2018) A meta-analysis and review of unmanned aircraft systems (UAS) imagery for terrestrial applications. Int J Remote Sens https://doi.org/10.1080/01431161.2017.1420941
Sirieix C, Genelle F, Barral C, Touze-Foltz N, Riss J, Bégassat B (2015) Characterizing the aging of a geosynthetic clay liner through electrical resistivity. Can Geotech J 52:1–8
Somogyvári M, Bayer P (2017) Field validation of thermal tracer tomography for reconstruction of aquifer heterogeneity. Water Resour Res 53(6):5070–5084
Strom RN, Kaback DS (1992) Groundwater geochemistry of the Savannah River Site and vicinity. WSRC-RP-92-450, Westinghouse Savannah River Company, Aiken SC. https://doi.org/10.2172/10103229
Stumm W, Morgan JJ (1996) Aquatic chemistry: chemical equilibria and rates in natural waters. John Wiley & Sons, New York NY USA
Suárez F, Hausner MB, Dozier J, Selker JS, Tyler SW (2011) Heat transfer in the environment: development and use of fiber-optic distributed temperature sensing. In: Bernardes MADS (ed) Developments in heat transfer, Chapter 31, IntechOpen. https://doi.org/10.5772/822
Sun S (2013) A survey of multi-view machine learning. Neural Comput Appl 23(8):2031–2038
Sun D, Wainwright HM, Oroza CA, Seki A, Mikami S, Takemiya H, Saito K (2020) Optimizing long-term monitoring of radiation air-dose rates after the Fukushima Daiichi Nuclear Power Plant. J Environ Radioact 220:106281
Tmušić G, Manfreda S, Aasen H, James MR, Gonҫalves G, Ben-Dor E, Brook A, Ploinova M, Arranz JJ, Mészáros J, Zhuang R, Johansen K, Malbeteau Y, de Lima IP, Davids C, Herban S, McCabe MF (2020) Current practices in UAS-based environmental monitoring. Remote Sens 12:1001. https://doi.org/10.3390/rs12061001
Truex M, Oostrom M, Carroll KC, Bunn A, Wellman D (2013) Integrated systems-based approach to monitoring environmental remediation. In: Proceedings of the annual waste management symposium 2013. Phoenix AZ, Waste Management Symposium, Inc. Tempe AZ USA
Uhlemann S, Chambers J, Wilkinson P, Maurer H, Merritt A, Meldrum P, Kuras O, Gunn D, Smith T, Dijkstra T (2017) Four-dimensional imaging of moisture dynamics during landslide reactivation. J Geophys Res Earth Surf 122:398–418
USEPA (1999) Use of monitored natural attenuation at Superfund, RCRA corrective action and underground storage tanks. Office of Solid Waste and Emergency Response Directive 9200.4–17P, Washington DC USA
USEPA (2004) Demonstration of two long-term groundwater monitoring optimization approaches. United States Environmental Protection Agency: EPA-542-R-04-001b, Washington DC USA
USEPA (2005a) Roadmap to long-term monitoring optimization. United States Environmental Protection Agency: EPA-542-R-05-003, Washington DC USA
USEPA (2005b) Groundwater sampling and monitoring with direct push technologies. United States Environmental Protection Agency: EPA 540/R-04/005, Washington DC USA
USEPA (2007a) In: Ford RG, Wilkin RT, Puls RW (eds) Monitored natural attenuation of inorganic contaminants in ground water: volume 1—technical basis. United States Environmental Protection Agency: EPA/600/R-07/139, Ada OK USA
USEPA (2007b) In: Ford RG, Wilkin RT, Puls RW (eds) Monitored natural attenuation of inorganic contaminants in groundwater, volume 2—assessment for non-radionulcides including arsenic, cadmium, chromium, copper, lead, nickel, nitrate, perchlorate, and selenium. US Environmental Protection Agency, EPA/600/R-07/140, ADA OK USA
USEPA (2007c) In: Ford RG, Wilkin RT (eds) Monitored natural attenuation of inorganic contaminants in groundwater, volume 3 assessment for radionuclides including tritium, radon, strontium, technetium, uranium, iodine, radium, thorium, cesium, and plutonium-americium. US Environmental Protection Agency, EPA/600/R-10/093, ADA OK, USA
USEPA (2015) Use of monitored natural attenuation for inorganic contaminants in groundwater at superfund sites. Office of Solid Waste and Emergency Response Directive 9283.1–36
Usher BH, Pretorius JA, Gebrekristos RA (2008) Guidelines for the acceptance of monitored natural attenuation processes in South Africa. Report to Water Research Commission, WRC 1501/4/08, ISBN 978-1-77005-666-4
Vázquez S, Hevia A, Moreno E, Esteban E, Peñalosa JM, Carpena RO (2011) Natural attenuation of residual heavy metal contamination in soils affected by the Aznalcóllar mine spill, SW Spain. J Environ Manag 92:2069–2075
Vetter K, Barnowski R, Cates JW, Haefner A, Joshi TH, Pavlovsky R, Quiter BJ (2019) Advances in nuclear radiation sensing: enabling 3-D gamma-ray vision. Sensors 19(11):2541
Wainwright H, Arora B, Hubbard SS, Lipnikov K, Moulton D, Flach G, Denham M (2019) Sustainable remediation in complex geologic systems. In: Evans WJ, Hanusa TP (eds.) The heaviest metals: science and technology of the actinides and beyond, Wiley, New York, pp. 415–426
Wainwright HM, Chen J, Sassen DS, Hubbard SS (2014) Bayesian hierarchical approach and geophysical data sets for estimation of reactive facies over plume scales. Water Resour Res 50:4564–4584. https://doi.org/10.1002/2013WR013842
Wainwright HM, Faybishenko B, Molins S, Davis JA, Arora B, Pau G, Johnson J, Flach G, Denham M, Eddy-Dilek C, Moulton JD, Lipnikov K, Gable C, Miller T, Freshley M (2016) Effective long-term monitoring strategies by integrating reactive transport models with in situ geochemical measurements. In: Proceedings of the annual waste management symposium 2016. Phoenix AZ, Waste Management, Inc. Tempe AZ USA
Waite TD, Davis JA, Payne TE, Waychunas GA, Xu N (1994) Uranium(VI) adsorption to ferrihydrite: application of a surface complexation model. Geochim Cosmochim Acta 58:5465–5478
Wang T, Liu Y, Wang J, Wang X, Liu B, Wang Y (2019) In-situ remediation of hexavalent chromium contaminated groundwater and saturated soil using stabilized iron sulfide nanoparticles. J Environ Manag 231:679–686
Weaver W, Wilson JT (2000) Diving plumes and vertical migration at petroleum hydrocarbon release sites. LUSTLine Bulletin 36, New England Interstate Water Pollution Control Commission, Lowell, MA
Whitehead K, Hugenholtz CH (2014) Remote sensing of the environment with small unmanned aircraft systems (UASs), part 1: a review of progress and challenges. J Unmanned Veh Syst 2:69–85
Whitehead K, Hugenholtz CH, Myshak S, Brown O, LeClair A, Tamminga A, Barchyn TE, Moorman B, Eaton B (2014) Remote sensing of the environment with small unmanned aircraft systems (UASs), part 2: scientific and commercial applications. J Unmanned Veh Syst 2:86–102
Xu C, Zhang S, Ho Y-F, Miller EJ, Roberts KA, Li H-P, Schwehr KA, Otosaka S, Kaplan DI, Brinkmeyer R, Yeager CM, Santschi PH (2011) Is soil natural organic matter a sink or source for mobile radioiodine (129I) at the Savannah River Site? Geochim Cosmochim Acta 75:5716–5735
Acknowledgements
The corresponding author, MED, was funded through subcontract agreement with Inspection Experts, Inc. by the United States Department of Energy, Office of Environmental Management (USDOE-EM). The contributions of MBA and CAE-D were funded by USDOE-EM through contract with Savannah River Nuclear Solutions, operator of Savannah River National Laboratory. Contributions by HMW, BD, and JA-F were funded by the USDOE-EM, and as part of the Lawrence Berkeley National Laboratory Science Focus Area, which is funded by the US Department of Energy, Office of Science, and Office of Biological and Environmental Research, both under Award Number DE-AC02-05CH11231 to Lawrence Berkeley National Laboratory.
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Funding sources for this work are provided in the Acknowledgements section. For the past 3 years all authors have received the majority of their funding from the U.S. Department of Energy. The authors have not received any funding from commercial sources that stand to benefit from publication of this work, and therefore claim no conflicts of interest.
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Denham, M.E., Amidon, M.B., Wainwright, H.M. et al. Improving Long-term Monitoring of Contaminated Groundwater at Sites where Attenuation-based Remedies are Deployed. Environmental Management 66, 1142–1161 (2020). https://doi.org/10.1007/s00267-020-01376-4
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DOI: https://doi.org/10.1007/s00267-020-01376-4