Heavy Metal and Radionuclide Contaminant Migration in the Vicinity of Thermal Power Plants: Monitoring, Remediation, and Utilization

Abstract

Environmental pollution has been an important issue particularly in a period which is witnessing a rapid industrial growth. The major sources of heavy metal and radionuclide pollution in urban areas are anthropogenic which includes the fossil, coal combustion, etc. Coal, one of the most abundantly found materials in nature, has trace quantities of primordial radionuclides, ²³⁸U, ²³²Th, and ⁴⁰K. Thus, combustion of coals in thermal power plants (TPPs) is one of the major sources of environmental pollution enhancing the natural radiation in the vicinity of TPPs. Fly ash, the main residue from the combustion of pulverized bituminous or sub-bituminous coal (lignite), is directly disposed of in large ponds in the nearby areas of TPPs. This problem is of particular significance in India, which utilizes coals of very high ash content (approximately 40–55 %). Background knowledge, i.e., monitoring of the origin, chemistry, and risks of toxic heavy metals (Pb, Cr, As, Zn, Cd, Cu, Hg, and Ni) and radionuclides in contaminated soils, is necessary for the selection of appropriate remedial options. For this purpose, soil, ash, and coal samples were collected from the nearly areas of TPPs. In order to validate the fact of heavy metal and radionuclide contaminant migration in the vicinity of TPPs, several geophysical, radiochemical, and trace element analysis tests were carried out. Lack of proper insulation at the bottom of the ash disposal ponds enhances the chances of groundwater contamination. The direct current (dc) resistivity survey, employing Schlumberger configuration, was undertaken to identify the local subsurface and to estimate the depth of contamination around ash ponds near a thermal power plant in Eastern India. A continuous conductive zone with resistivity <5 Ωm was identified throughout the studied region at a depth of about 2–10 m indicating the presence of water with higher ionic concentration. The study was also supported by the radiometric geophysical measurements indicating the presence of radionuclides in the vicinity of ash ponds. Groundwater samples have also been collected from the tube wells located near the ash ponds and analyzed for pH, TDS and trace elements. Results of the chemical analysis show high values of TDS and high concentration of the toxics. The studies on the geochemistry of the ash ponds have shown that the ashes are characterized by high concentration of As, Pb, Cu, Ni, Fe, Zn, Cr, Co, and Mn all of which exceeded the crustal abundance by a factor of 3–5. The high concentration of the toxics (As, Al, Li, Zn, Ag, Sb, Si, Mo, Ba, Rb, Se, Pb) in the water samples implies significant input from the ash pile due to leaching. This increases the TDS values of the water. Radiochemical characterization of coal, ash, and soil samples was done based on Radium equivalent activity, dose rate, and hazard index. The activity concentration variations infer to the high radioactivity in the vicinity of the ash ponds up to a radius of 500 m and indicate a decreasing trend beyond 500 m. In general, the radioactivity of the nearby area showed three to four times higher values than the prescribed limit, and the hazard index was estimated as unity. In order to mitigate the associated risks, make the land resource available for agricultural production, enhance food security, and scale down land tenure problems, remediation of contaminated soil is necessary. Immobilization, soil washing, and phytoremediation are one of the best technologies for cleaning up of contaminated soils but have been mostly demonstrated in developed countries. Such technologies need high recommendation for field applicability and commercialization in the countries (developing) where agriculture, urbanization, and industrialization are causing environmental degradation. The utilization of contaminated soil is also discussed. Rare earth elements, heavy metal, and radionuclides, which are of economical importance, can be extracted from the contaminated soil and the remainder can be utilized in filling voids and reclamation of the mining lands.

Introduction

One of the most significant fuel sources for steam and energy production is coal. Coal combustion in thermal power plants increases the three aspects of atmospheric pollution. Huge quantities of gaseous and particulate pollutants (SO₂, NO_x, CO₂, hydrocarbons, and fly ash) are discharged into the local environment. Additionally, small amounts of toxic trace elements (As, Hg, and Cd) and minute amounts of radionuclides (U, Th, and their radioactive daughters) are also released, which may have a radiological impact on the environment.

Coal is serving as a major power source in our country by fulfilling the 70 % electricity demand. Remaining part of power generation comes from the hydroelectricity and nuclear power plants. Coal ash is the residual after coal combustion, which was not fully burnt. The volume of the end residue generated after coal combustion is dependent on the quality of coal used in thermal power stations. In Indian thermal power stations, bituminous or sub-bituminous type of coal is used which is generating about 55–60 % ash content. Consequently, 160 million tons of ash is produced yearly and is projected to reach 300 million tons per annum by 2016–2017 (Singh et al. 2011). Because of the enrichment of radionuclides in coal ash, the management of coal ash is a major environmental problem.

Fly ash and bottom ash are mainly two types of solid wastes, which are generated as a byproduct after coal combustion. Trace elements, which are smaller in size and larger in surface area, got absorbed on the ash particles (Culec et al. 2001). Bottom ash is the coarse-grained fraction of ash, which precipitates due to its weight and settles at the bottom of the furnace. This course-grained fraction is directly thrown into the closer ash ponds. Due to the light weight of fly ash particles, it spreads and distributes in the vicinity by air, henceforth deposits on the soil surface, which increases the radioactivity level of the local environment.

The toxicity of the soil and groundwater gets enhanced due to continuous disposal of bottom ash into the ash ponds. The solid wastes generated after coal combustion are released either directly through the stack releases or indirectly from waste storage areas. Most of the power stations in India operate close to coal fields, which are mainly located in the eastern region of India, i.e., Bihar, Jharkhand, and West Bengal. Due to dense population density in these areas, the environmental aspect is of major concern. According to Lalit et al. (1986), the radioactivity level of coals from these coal fields is on the higher side compared to the other coal fields. Consequently, population residing in this particular region is facing more environmental hazard.

Out of 31 coal-based thermal power plants of eastern India, 14 of these are located in West Bengal. Kolaghat, Durgapur, and Bandel are one of the biggest thermal power plants of eastern India with a capacity of 1260, 350, and 530 MW of electricity, respectively. The Kolaghat thermal power plant is located in Midnapur district, West Bengal (22°27´–22°25´ N latitude and 87°50´–87°55´ E longitude). Coal used in this thermal power plant is of bituminous type and 1260 MW power is generated in six units of 210 MW. The Durgapur thermal power plant is situated in Burdwan district, West Bengal (22°56´–23°53´ N latitude and 86°48´–88°25´ E longitude), and Bandel thermal power plant in Hooghly district, West Bengal (23°39´32´´–23°01´20´´ N latitude and 87°30´15´´–88°30´20´´ E longitude). Due to the use of low-grade coal (sub-bituminous), huge amount of ash is produced, which is thrown away in ash ponds. These ash ponds are open and exposed to the sun, and therefore the slurry starts drying and becomes hard and compact, which later becomes a source of radiation hazard.

Recently, the use of fly ash is increased many folds as building material in the form of ash bricks and as filling material for underground cavities. Thus it is evident to monitor the radiation risk to the population.

Here, we are going to discuss the three power plants (Bandel, Durgapur, and Kolaghat) in detail. For this purpose, these power plants were investigated (Mandal and Sengupta 2006; Maharana et al. 2010; Mondal et al. 2006; Parial et al. 2015, 2016). The general objectives were as follows:

• i. The determination of the presence of radionuclides (U, Th, and K) in coal and ash, and their enrichments in ash after coal combustion. It gives an idea about the environmental hazard of the area.

• ii. Because of the improper management of ash, it is directly dumped into the open ash ponds and thereby severely affecting the surrounding population. Therefore, the radiological impact is determined by computation of the dose rate.

• iii. The determination of the subsurface contamination was done using noninvasive geophysical methods.

• iv. A comparison between the radiometric data in the vicinity of Kolaghat, Durgapur, and Bandel thermal power plants is required in order to estimate the radiation potential in these areas.

• v. Hazard index estimation for the fly ash admixture for cement, concrete, and fly ash bricks.

Materials and Methods

Characterization of the nearby areas of thermal power plants, in terms of heavy metals and radionuclide contaminant migration, is done by conducting field-based and lab-based analyses. Mostly field-based studies include noninvasive methods; however, lab-based analyses are done by conducting several geochemical tests. In this section, a brief summary of all the methods utilized is presented, which are used for the characterization of the vicinity of the TPPs.

• 1. Noninvasive Geophysical investigations:

  • i. In situ radiometric study

    A reconnaissance survey was conducted in and around Kolaghat thermal power plant (KTPP) based on in situ radiometric analysis. The ambient terrestrial dose rate was measured using a handheld scintillation counter. The counter is commonly known as micro-R survey meter. The model used was UR-709, manufactured by Nucleonix Systems Private Limited, Hyderabad, India. Soil samples from high ambient dose rate areas were collected for lab-based radiometric analysis. The locations of the sampling points were marked using GPS (manufactured by Trimble, Juno ST Handheld, Trimble Juno Series).

  • ii. Subsurface characterization using direct current (dc) resistivity method

    1D vertical electrical sounding (VES) measurements were conducted using Terra Science resistivity meter. VES were measured at twelve locations around KTPP and at one reference site outside the contaminated area. These twelve soundings were conducted in the vicinity of the two ash ponds and within a radius of 1 km from the ash disposal sites using Schlumberger electrode configuration. To achieve greater depth of penetration, longest possible profile at every location was selected. The measurements were carried out during winter and spring season to avoid the interferences due to rain.

    In dc resistivity sounding, current (I) is injected into ground through a pair of electrodes and the potential difference (ΔV) is measured by another pair of electrodes (potential electrodes). The placement of these two pairs of electrodes determines the array configuration. According to the Ohm’s law, the ratio of the potential difference to the current injected estimates the resistance. The apparent resistivity is estimated using Ohm’s law (Eq. 1):

    $$\rho_{a} = G.\left( {\frac{\Delta V}{I}} \right)$$

    (1)

    where G is the geometric factor due to the array configuration, $\Delta V$ is the potential difference between the two potential electrodes, and I is the current injected. A detailed discussion can be found in Telford et al. (1990). The greater the current electrode separation, the greater is the depth of penetration.

• 2. Laboratory (Geochemical) investigations

  • i. Sample collection and preparation

    The feed coal samples were collected from inside of the power plants. The coals from four different coal fields (Jharia, Talchir, Giridih, and Raniganj) of lower Gondwana age were mixed together and pulverized before combustion. The entire mass of generated ash in the power plants was disposed off in large ponds located near the power plant. Ash samples were collected from the ash ponds. Soil samples were collected from the villages in the vicinity of the active ash ponds. The collected samples were stored in polyethylene bags. The samples were homogenized and then dried for 24 h in an air circulation oven at 110 °C. About 100 g of the dried samples were weighed and sealed in gas-tight, radon impermeable, cylindrical polyethylene containers (7.0 cm height and 6.5 cm diameter). These samples were then left for 15 days in order to allow for radium, thorium, and their short-lived progenies to reach secular equilibrium before radioactive determination of uranium and thorium. Subsequently, gamma-ray spectrometric analyses of the samples were conducted for the estimation of the ²³⁸U, ²³²Th, and ⁴⁰K in soil, coal, and ash samples. The analysis was performed at the Radiochemistry Division, Variable Energy Cyclotron Centre, BARC, Kolkata.

  • ii. Radioactivity measurements

    Low-level gamma-ray spectrometry was used to determine the activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K in fly ash samples. The present system is an intrinsic p-type coaxial HPGe detector with a relative efficiency of 50 % and energy resolution of 1.9 keV at 1332.5 keV for ⁶⁰Co. It is placed vertically in a lead cylindrical shield of 12 cm thickness and 65 cm height to reduce the background radiation. A low ambient temperature is maintained by electric refrigeration system. A solid radionuclide mixture of gamma reference materials, sealed in standard cylindrical polyethylene containers (7.0 cm in height and 6.5 cm diameter), was used to estimate the absolute efficiency of the gamma-ray spectrometer.

    • a. Estimation of Radiation dose and Hazard index

      Natural radioactivity is a consequence of the radioactive emission of mainly ²³⁸U, ²³²Th, and ⁴⁰K. Potassium can be measured with the help of its own peak (1460 keV); however, uranium and thorium are measured with the help of their daughter products. For uranium the peaks 295 keV, 352 keV (²¹⁴Pb), and 609 keV (²¹⁴Bi), and for thorium the peaks of 338 keV, 911 keV (²²⁸Ac), 239 keV (²¹²Pb), 727 keV (²¹²Bi), and 583 keV (²⁰⁸Tl) are considered (Abd El-mageed et al. 2011; Khan et al. 2002). Efficiency of these respective peaks is calculated using a standard Eu¹⁵² sample, with known half-life, with the help of Eq. (2). Each sample is measured for 40,000 s to obtain a reasonable ϒ-ray peak area. From this peak area the activity is measured with the help of a multi-channel analyser Canberra DSA 1000, which is connected to computer through Genie 2 K software:

      $$\varepsilon = \frac{CPS \times 100}{{A_{t} \times I}},$$

      (2)

      where $\varepsilon$ = Efficiency; CPS = Counts per second; $A_{t}$ = Activity of ¹⁵²Eu; and I = Intensity of ¹⁵²Eu.

    • b. Absorbed Dose Rate

      It is basically the radiation received by any person per kilogram of its mass. The conversion factor required to calculate absorbed dose rate at 1 m above the ground surface, due to uniform distribution of gamma radiations from ²³⁸U, ²³²Th, and ⁴⁰K, is calculated by the method proposed by the UNSCEAR 2000. The equation is mentioned below:

      $${\text{D}}\,\left( {\text{nGyh}} \right) = 0. 4 6 1 {\text{A}}_{\text{U}} + 0. 6 2 3 {\text{A}}_{\text{Th}} + 0.0 4 1 4 {\text{A}}_{\text{K}} ,$$

      (3)

      where A_U, A_Th, and A_K are the specific activities of ²³⁸U, ²³²Th, and ⁴⁰K in Bq kg⁻¹.

    • c. Annual Effective Dose Rate

      It basically gives us an idea about the internal and external radiation exposures. It is calculated using the conversion coefficients (0.7 SvGy⁻¹) and 0.2 as the outdoor occupancy factor, as proposed by UNSCEAR 2000:

      $${\text{AED}}\,\left( {{\text{mSvy}}^{ - 1} } \right) = {\text{D}}\,\left( {{\text{nGyh}}^{ - 1} } \right) \times 8 7 60\,\left( {{\text{hy}}^{ - 1} } \right) \times 0. 2\times 0. 7 {\text{Sv}}\,{\text{Gy}}^{ - 1} \times 10^{ - 6} .$$

      (4)

    • d. Radium Equivalent

      It describes the gamma output from different mixtures of U, Th, and K in samples and helps in comparison of radiation exposure due to different radioisotopes:

      $${\text{Ra}}_{\text{eq}} = {\text{A}}_{\text{U}} + 1. 4 3 {\text{ A}}_{\text{Th}} + 0.0 7 7\,{\text{A}}_{\text{K}} .$$

      (5)

      The equation is proposed by UNSCEAR 2000, based on the assumption that contributions from other radionuclides are insignificant.

    • e. Hazard Index

      It is the index that indicates the external exposure due to the radioisotopes in the environment:

      $${\text{H}}_{\text{ex}} = {\text{A}}_{\text{U}} / 3 70 + {\text{A}}_{\text{Th}} / 2 5 9+ {\text{A}}_{\text{K}} / 4 8 10.$$

      (6)

      H_ex < 1 is considered to be within limit, and H_ex > 1 is unsafe and equivalent to 370 Bq kg⁻¹.

  • iii. Radon exhalation rate measurements

    Radon exhalation measurements were undertaken using Cup dosimeters. The fly ash samples were dried and sieved through a 100-mesh sieve and placed in cups/cans, up to the sensitive volume (diameter 7.0 cm and height 7.5 cm). An LR-115 type-II solid-state nuclear track detector (2 cm × 2 cm) was fixed at the top, within the cup. The sensitive, lower surface of the detector is freely exposed to the emergent radon and was capable of recording the alpha particles, due to the decay of radon in the cup. After a week or more, radon and its daughters attain equilibrium. Thus, the equilibrium activity of emergent radon could be obtained from the geometry of the cup and time of exposure. After exposure for 100 days, the detectors from all the cups were retrieved. For the revelation of tracks, the detectors were etched in 2.5 N NaOH at 60 °C for a period of 90 min, in a constant-temperature water bath. Alpha tracks on the exposed face of the detector foils were scanned using an optical microscope at magnification of 400 times. From the track density, the radon activity was obtained using a calibration factor of 0.056 Tr cm⁻² d⁻¹ obtained from an earlier calibration experiment, for an LR-115 type-II detector (Singh et al. 1997).

    An experiment was performed in order to study the effect on radon exhalation rates by adding varying amounts of fly ash. The measurement of the exhalation rate of radon from a homogeneous mixture of (i) fly ash and soil and (ii) fly ash and cement was obtained. For this purpose, different percentages of fly ash were introduced in the soil and cement, keeping the total weight of the sample constant. All the samples were dried and sieved through a 100-mesh sieve before mixing, for homogeneity.

    The exhalation rate was estimated from the relationship provided by Fleischer and Morgo-campero (1978) and Khan et al. (1992):

    $$E_{x} = \frac{CV\lambda }{{A\left[ {T + 1/\lambda \left\{ {e^{ - \lambda T} - 1} \right\}} \right]}},$$

    (7)

    where E_x is the radon exhalation rate (Bq m⁻² h⁻¹), C is the integrated radon exposure measured by an LR-115 type-II plastic track detector (Bq m⁻³ h), V is the effective volume of Can (m³), $\lambda$ is the decay constant for radon (h⁻¹), T is the exposure time (h), and A is the area covered by the Can (m²), respectively.

Results and Discussion

Environmental pollution measurements in and around TPPs were done by adopting two-step methodology. At first, the sites were investigated using noninvasive geophysical measurements and thereafter based on the preliminary noninvasive geophysical survey results, locations were selected for sample collection. As a second-step methodology, geochemical tests of soil/ash samples were conducted in laboratory.

• i. Noninvasive Geophysical investigation results:

  The isorad map, based on surface gamma radiation, from the vicinity of Kolaghat thermal power plant is shown in Fig. 1. The surface radiation varies from 4.5 to 9.2 μRhr⁻¹ showing an increasing trend toward northwest. The maximum surface radiation dose was observed toward the northern edge of the area, which coincided with the ash ponds present in the area. This indicated the presence of a high background radiation in the vicinity of the thermal power plant as well as the ash ponds (Parial 2016).

  Fig. 1

  

  Isorad map based on surface radiation in the vicinity of Kolaghat thermal power plant (after Parial 2016)

  These observations emphasize that the contamination is maximum close to the ash ponds. The leachate from the ash ponds consists of water-soluble metallic species, which are posing maximum risk to groundwater. Generally, clay acts as a natural barrier to the leachates; however, due to high porosity sand not only acts as a potential aquifer but also allows an easy passage to the leachates from the waste dumps. Extreme precipitation events during monsoon can allow higher infiltration of the contaminants from the ash dumps to the deeper levels. Therefore, to assess the status of groundwater aquifers noninvasive vertical electrical sounding measurements were conducted in and around the Kolaghat thermal power plant. VES measurements using Terra Science resistivity meter (DDR2) were conducted at 12 locations and at one reference site. The reference site was located ~3 km northwest of the thermal power plant; five soundings, VES-S4 to VES-S8, were in the vicinity of the two ash ponds; VES-S1 to VES-S3, VES-S9, VES-S10 were located within a radius of 1 km from the ash disposal sites. VES-S11 was located to the east of the thermal power plant, downstream of the River Rupnarayan, and VES-S12 was present south of the thermal power plant within 1 km (Fig. 2). The maximum profile length in the thirteen soundings varied between 100 and 800 m. The maximum profile length measured was at VES-S12 and the reference site.

  Fig. 2

  

  Geoelectric section from the vicinity of Kolaghat thermal power plant derived from interpreted resistivity and borehole log (modified after Parial et al. 2015)

  These investigations help in understanding the subsurface in the region close to the thermal power plant. The 1D interpreted models infer the presence of a thick conductive zone (resistivity < 5 Ωm) throughout the region (Fig. 2). This conductive layer starts at a depth ranging from 2 to 10 m from the top and its thickness ranges between ~27 and 39 m (Parial et al. 2015). Based on the available borehole data and the earlier observations (Mandal et al. 2007), this zone is identified as saturated clay with high concentration of various ionic species. From the borehole information, the presence of sandy layers at the top facilitates the percolation of the leachate into the deeper clay layer. Although clay is considered as a repository of trace/heavy metals, continuous loading of these species at higher concentration results in enhanced percolation. Ultimately, it percolates up to the deeper aquifer at a depth of ~50 m (Fig. 2). As groundwater is the major source of potable water in this region, an extension with the previous studies by Mandal et al. (2007), Parial et al. (2015) emphasizes the importance of meticulous monitoring of the groundwater quality and the temporal variation in the nature of the contaminant(s).

• ii. Geochemical investigation results:

  The geochemical studies of water samples, collected from a 50-m-deep tube well in the vicinity, by Mandal et al. (2007), further strengthened the presence of the mentioned contaminated layer (<5 Ωm). They confirmed the presence of high TDS count (550–760 ppm), abundance of Ca²⁺, Na⁺, presence of anions (HCO₃ ⁻, SO₄ ²⁻, and Cl⁻), and high concentration of trace elements (like Li, As, Zn, Ag, Pb, Cr, Mn, Se, Cd, Cu) indicating the high concentration of various ionic species within the clay layer.

  We analyzed the ash samples from three power stations located in West Bengal (Kolaghat, Durgapur, and Bandel). Tables 1, 2, and 3 show the results of radiometric analysis from these power stations (Mondal et al. 2006). Kolaghat ash showed the activity concentration variation of ²³²Th, ²³⁸U, and ⁴⁰K in the range from 126 to 146 Bq kg⁻¹ (average 140 Bq kg⁻¹), 98–119 Bq kg⁻¹ (average 111 Bq kg⁻¹), and 266–415 Bq kg⁻¹ (average 351 Bq kg⁻¹), respectively. Similarly, the activity concentration variations of ²³²Th on Durgapur and Bandel Thermal Power Stations indicate an average value of 107 and 106 Bq kg⁻¹ (ranged from 62 to 139 Bq kg⁻¹), respectively.

  Table 1 Radioactivity measurements in fly ash samples from Kolaghat thermal power station, West Bengal (after Mondal et al. 2006)

  Table 2 Radioactivity measurements in fly ash of Durgapur thermal power plant (after Mondal et al. 2006). The annual external effective dose rate (mSv y⁻¹), radium equivalent activity (Ra_eq), and external hazard index (H_ex) have also been provided

  Table 3 Radioactivity measurements on fly ash samples of Bandel thermal power plant (after Mondal et al. 2006). The Ra equivalent activity (Ra_eq) and external Hazard index (H_ex) are also provided

On comparing the variation in radionuclides activity concentration measured at these power stations, it indicated that the enrichment of a particular radionuclide in the ashes is different at different power stations. Here, Kolaghat ashes indicate maximum enrichment of ²³²Th compared to Durgapur and Bandel ashes. However, Bandel ashes are enriched in ²³⁸U. Durgapur ashes show lesser activity concentration for ⁴⁰K compared to Kolaghat and Bandel ashes. Ashes from these power stations show more activity concentration for ²³²Th compared to ²³⁸U activity concentration. The feed coal analysis from these power plants also indicates the maximum activity concentration of ²³²Th compared to ²³⁸U (Mandal and Sengupta 2003). These power plants infer to the high radionuclides activity concentration compared to the power plants located in other parts of India. Table 4 shows the mean activity coefficients of radionuclides (U, Th, and K) and absorbed dose rates of fly ashes from other thermal power plants (Mondal et al. 2006).

Table 4 Mean activity of uranium, thorium, and potassium concentration along with absorbed dose rates of fly ash samples from some thermal power plants in India

Due to the presence of uranium in the organic material in sedimentary rocks, it accumulates in coal, during the formation of coal. Uranium is mainly retained in the carbonaceous part, due to sorptive uptake during the initial stages of peat accumulation and subsequent burial (Zielinski et al. 1986). In contrast, the inorganic phases retain most of the thorium. The geological formation around the ash ponds of these power plants is mainly laterites overlained by clay. Clays are considered as potential trace elemental repositories. Therefore, the mixture of dry ashes and clay forms the top soil around these three thermal power plants. This facilitates the adsorption of radionuclides on the top soil (clayey layer). Due to the volatile nature of uranium, it migrates deeper by the percolating rain water and subsurface run-off; however, thorium remains adsorbed on the clayey formations. This is the reason that the ash samples show high activity concentration for ²³²Th compared to ²³⁸U. Hence, the dose rates of the measured ash samples are following the trend of thorium activity concentration, which mean high concentration thorium contributes more in the dose rate. The activity concentration of the natural radionuclides in feed coal samples is 2–3 times lesser than the ash samples (Fig. 3).

Fig. 3

Radioactivity enhancement from coal to ash in Kolaghat, Durgapur, and Bandel thermal power plants (after Mondal et al. 2006)

To estimate the external gamma dose and internal dose due to radon and its daughters, Ra_eq is calculated (Tables 1, 2, and 3) for the ash samples. The maximum permissible value for Ra_eq is 370 Bq kg⁻¹ and for external hazard index (H_ex) is unity. These values are calculated using Eqs. (5) and (6).

In order to monitor the migration of contaminants laterally, several soil samples were collected and analyzed. High radioactivity was observed within a radius of 500 m and it decreases as we move further away from the ash ponds (Parial et al. 2016).

Nowadays, fly ash is being utilized in civil constructions in several forms; therefore, the radiation potential estimation becomes necessary (Eisenbud and Gesell 1997; MacKenzie 2000; UNSCEAR 1993, 2000). On comparison with coal, enhanced levels of uranium are present in fly ash (Jojo et al. 1993). Therefore, it is not easy to calculate the radon exhalation rate by uranium’s or its daughter product’s concentration due to its dependence on the texture and grain size composition (Tufail et al. 1991). Interestingly, the radon exhalation values are more in coal samples compared to fly ash samples. This may be due to the difference in the texture of coal and fly ash. In addition, the finer fly ash particles fill the small voids in the material and check the radon escape.

Easy availability and low cost of fly ash makes it an attractive substitute as building materials. In order to limit the radiation potential, it can be used as an additive in building materials. Several studies are reported which show the influence of fly ash as an additive on radiation level. However, different results are reported by different groups. Some authors reported a decreasing trend in radon exhalation in the mixture of concrete and fly ash additives (Stranden 1983; Maraziotis 1985). On the contrary, others have reported opposite trend when fly ash was used as an additive in cement (Siotis and Wrixon 1984). Some case studies reported no significant change in radon exhalation in cement and soil samples after mixing of fly ash in it (Ulbak et al. 1984; Karmadoost et al. 1988).

To obtain a statistically reliable correlation on the application of fly ash as an additive, the ‘t-test’ was used on the measurements obtained. The results for a homogeneous mixture of (i) cement and fly ash and (ii) soil and fly ash, in varying amounts, are summarized in Table 5. The samples with fly ash as additive in cement samples have shown a gradual increase, whereas a decrease is seen in soil samples, subsequent to the addition of fly ash. The experimental values for mixtures of ash and cement are higher than the values expected from the computations based on relative proportions of ash and cement, respectively. An opposite trend was observed for mixtures of ash and soil. This indicates that the values for the admixture increase, if the additive has higher exhalation rate than the base material and decrease if the value for additives is less (Kumar et al. 2005). This has been attributed to the emanating powers of different materials and also due to the difference in the grain size. The results obtained are significant, in terms of appropriate suppression of radon.

Table 5 Radon exhalation rate in cement and soil with different proportions of fly ash (expressed in weight percent) added in the samples of equal total weight (Kumar et al. 2005)

Fly Ash Remediation and Utilization

Both India and China have the highest fly ash production per year. However, the amount utilized is about 38–45 % only. This is compared to about 65 % fly ash utilization in U.S.A. (Basu et al. 2009; other references cited there-in).

The use of fly ash as agricultural amendment, fly ash bricks, cement, and concrete and in pavements and highways is of considerable significance. In addition, they are being used in production of glasses and glass-ceramics (Barbieri et al. 1999), as well as utilized to construct fills and embankments, grouts for pavements, structural fills, and retaining walls (Lamb 1974). More recently coal ashes have also been suggested as potential sources for rare earth extraction including yttrium (Kashiwakura et al. 2013; Seredin and Dai 2012).

The Organization for Economic Cooperation and Development (OECD) has classified the coal ash byproducts as Green List waste. Under Basel Convention it is not considered as a waste. The coal ash byproducts have not been utilized in many countries; however, they have been neglected like a waste material. Lack in the availability of appropriate cost-effective technologies prevents the optimum utilization of fly ash in India (Bhattacharjee and Kandpal 2002).

Geologically, various toxic elements and natural radionuclides are present in coal. Therefore, after burning the coal, the residues either gaseous emission or solid wastes (fly and bottom ash) or liquid discharge (leachates from ash ponds) are having these elements. According to Gabbard (1993), burning of the coal reduces the coal volume by 85 %, which mean that the radionuclides and trace elements are more concentrated in the residues (Technologically Enhanced Naturally Occurring Radioactive Material, TENORM). The partitioning behavior and fate are decided by combustion environment and the association of these elements with different phases/matrixes in coal (Papastefanou 2010; Karangelos et al. 2004). Due to the presence of ²³²Th in inorganic matrix and ²³⁸U in organic matrix, ²³⁸U and ²³²Th partition differently during combustion (Menon et al. 2011). ²³⁸U shows a preference for fine to ultrafine ash particles and are mostly released in the ambient environment through the stack (Papastefanou 2010; Coles et al. 1979). However, ²³²Th shows no preferential partitioning (Papastefanou 2010). The overall impact due to the radioactivity of the ash thus is mainly dependent on the type and amount of coal burnt for the energy generation. Therefore, the type and amount of coal burnt in the power production will be the deciding factor for radiation impact due to the radioactivity of the end products.

Conclusions

The radioactivity analysis presented in this chapter shows considerable amount of contamination of the surface soil in the vicinity of the ash ponds. It has also been observed that the surface soil contamination is more prominent in the predominant wind direction. The background radioactivity, calculated from the emitted dose, of the ambient environments of Kolaghat, Durgapur, and Bandel thermal power plants is approximately three times higher than the world average of 43 nGyh⁻¹. The ashes from these power plants have 2–3 times more concentration of natural radionuclides than feed coal. Consequently, the populations living in the nearby areas are at higher health risk. The radium equivalent activity (Ra_eq) and external hazards index (H _ex) values are close to 370 Bq kg⁻¹ and unity, respectively. Therefore, the amount of ash used as additives in building materials should be examined cautiously in order to keep a check on the concentration of natural radionuclides.

Among the leading available technologies for the remediation of heavy metal contaminated soils are immobilization, soil washing, and phytoremediation but have been frequently practiced in developed countries. These technologies are suggested for field applicability and commercialization in the developing countries, where environmental degradation is the rancorous legacy of the urbanization, industrialization, and agricultural activities.