Soil Mixing for Remediation of Contaminated Sites

Abstract

Soil mixing involves mixing an additive into soil to change the physical or chemical properties of a soil. This geotechnical method is called solidification/stabilization (S/S) treatment when applied for remediation of contaminated soil or sediment. S/S is an established technology for contaminated site remediation. The effectiveness of S/S treatment for a broad variety of soil contaminants is demonstrated by the technology’s high selection rate for remedies at U.S. Superfund sites. According to U.S. Environmental Protection Agency’s 2017 report on Superfund remedies, S/S has been selected for 21% of in situ treatment source control remedies. Most Superfund projects involve contaminated soil. S/S is of increasing interest in in-place or in situ treatment of contaminated soil such as those found at former industrial properties. In situ S/S can prepare “brownfield” sites for redevelopment. S/S treatment protects human health and the environment by immobilizing hazardous constituents within the treated material. Successful treatment is accomplished through physical changes to the soil and often, chemical changes to the hazardous constituents themselves. S/S binding agents are mixed into contaminated soil through a variety of construction techniques including shallow and deep soil mixing. An appreciation of the versatility for the treatment technology can be gained by review of example projects. This paper discusses the principles and application of S/S treatment technology for soil and sediment.

1 Soil Mixing for Remediation of Contaminated Soil

1.1 Soil Mixing as Solidification/Stabilization Treatment

Soil mixing involves mixing an additive into soil to change the physical or chemical properties of a soil. This geotechnical method is often used in soil engineering to improve the bearing capacity and stability or to reduce the hydraulic conductivity of a soil. In environmental engineering, the same soil mixing methods are used to remediate contaminated soil. Use of soil mixing methods to address contaminated soil is termed solidification/stabilization (S/S) treatment.

Solidification/Stabilization (S/S) is a widely used treatment for the management/disposal of a broad range of contaminated media and wastes; particularly those contaminated with substances classified as hazardous in the United States. The treatment involves mixing a binding agent into the contaminated media (e.g. soil, sediment) or waste. Binding agents most commonly used include portland cement and slag cement. Other agents used include cement kiln dust (CKD), fly ash, bentonite clay, activated carbon, phosphate mixtures, and a variety of proprietary agents.

The treatment protects human health and the environment by immobilizing hazardous constituents within the treated material. Immobilization within the treated material prevents migration of the hazardous constituents to human, animal and plant receptors. S/S treatment can be performed on contaminated soil that remains in-place- in situ or on material that has been excavated- ex situ.

The terms solidification and stabilization sound similar, but they describe different effects that the binding agents create to immobilize hazardous constituents. Solidification refers to changes in the physical properties of a waste. The desired changes usually include an increase of the compressive strength, a decrease of permeability, and encapsulation of hazardous constituents. Stabilization refers to chemical changes of the hazardous constituents in a waste. The desired changes include converting the constituents into a less soluble, mobile, or toxic form.

1.2 The Case for S/S Treatment of Contaminated Soil

Established Treatment Technology. S/S treatment has been used to treat radioactive wastes since the 1950s and hazardous waste since the 1970s (Conner 1990). S/S continues as a cornerstone treatment technology for the management of radioactive waste, hazardous waste, contaminated site remediation and Brownfield redevelopment.

The U.S. Environmental Protection Agency (EPA) considers S/S an established treatment technology. S/S is a key treatment technology for the management of industrial hazardous wastes. These wastes are regulated in the United States under the Resource Conservation and Recovery Act (RCRA). RCRA hazardous wastes are grouped into two classes: RCRA-listed and RCRA-characteristic. RCRA-listed hazardous wastes are wastes produced by industry that are generally known by the EPA to be hazardous. These wastes are “listed” in RCRA regulations and must be treated, stored, and disposed according to RCRA hazardous waste management regulations. RCRA-listed wastes destined for land disposal are required to be treated to reduce hazards posed by the wastes after land disposal. EPA has identified S/S as Best Demonstrated Available Technology (BDAT) for 57 RCRA-listed hazardous wastes (United States Environmental Protection Agency 1993). RCRA-characteristic wastes are less routinely produced wastes that are found to be hazardous due to a characteristic of the waste. For RCRA-characteristic wastes, S/S can often be used to eliminate the hazardous characteristic. With the hazardous characteristic addressed the treated waste can be disposed at a lower cost or re-used.

S/S treatment is used to treat contaminated media during remediation of contaminated properties. There are many federal and state programs that require or influence voluntary remediation of contaminated property. The best-known and best-documented remediation program in the United States is conducted under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). This remediation program is commonly called the “Superfund” program.

High Frequency of S/S Treatment Selection for Remediation. S/S is the second most frequently selected technology for treating the source of environmental contamination at Superfund program remedies. From 1982 to 2014, S/S has been selected for seventeen percent (17%) of Superfund ex situ source treatment remedies (290 of 1705 ex situ remedies). During this same period, S/S has been selected for twenty-one percent (21%) of Superfund in situ source treatment remedies (170 of 806 in situ remedies) (United States Environmental Protection Agency 2017).

Contaminated Soil Frequently Found at Superfund Sites. Since the beginning of the Superfund program in 1980, contaminated soil has been the most frequently encountered contaminated media at Superfund sites. Table 1 indicates that eighty-one percent (81%) of Superfund remedies involved contaminated soil.

Table 1 Media Addressed at Superfund Sites with Remedies FY 1982–2014.

Effective Treatment on Contaminants in Soil. There is a great variety of contaminants of concern (COCs) found in soil at Superfund sites. Figure 1 indicates the proportion of different general classes of chemical contaminants found in Superfund site soil.

Fig. 1

Detailed COCs in soil at superfund sites FY 1982–2014. Source EPA-542-R-17-001 (United States Environmental Protection Agency 2017). Abbreviations in this figure include VOCs for volatile organic compounds, PAHs for polycyclic aromatic hydrocarbons, BTEX for benzene, toluene, ethylbenzene and xylene, and SVOCs for semi-volatile organic compounds

As shown on the Table 2, S/S treatment has been used to treat all general chemical classes of contaminants typically found at Superfund sites.

Table 2 Contaminants Treated by Superfund Source Treatment Projects FY 1982–2005.

Versatility of S/S Treatment. The ability to effectively treat a wide variety of contaminants within the same media is a key reason why S/S is so frequently used in remediation. Adding to the versatility of S/S treatment is the fact that contaminated material can be treated in situ or ex situ as previously segregated waste or excavated material. S/S treatment contractors have devised a variety of methods to accomplish the mixing, as demonstrated in the example projects described further below.

Brownfields. A more recent development in U.S. remediation programs is the advent of “brownfield” initiatives. Brownfield sites are typically previously used industrial or urban properties that have not been re-used because of environmental contamination and the liabilities that attach to the properties because of the contamination. Recent initiatives in U.S. liability law and funding encourage the remediation and re-use of brownfield sites. S/S is increasingly being used to address contaminated brownfield sites. Developers quickly realized that S/S treatment not only addresses contamination, but can improve the physical properties of soil for subsequent building.

Examples of S/S use at brownfields include remediation of former Manufactured Gas Plant (MGP) sites. EPA estimates there are 5000 former MGP sites in the U.S (United States Environmental Protection Agency 1999). MGPs produced “town gas”. This flammable gas was used for lighting and cooking before being replaced by natural gas distribution systems. Originally built on the outskirts of a town, most former MGP sites are now located within urban areas due to sprawl and are often prime “brownfield” properties. Town gas was produced by heating coal in the absence of air. Coal tar—comprised of polycyclic aromatic hydrocarbons (PAHs), petroleum hydrocarbons, benzene, cyanide, metals and phenols were residues of this process (United States Environmental Protection Agency 1999). Cement-based In Situ Solidification/Stabilization (ISS) is now a common technology used to address coal tar contaminated soil remaining on former MGP sites (The Interstate Technology and Regulatory Council Solidification/Stabilization Team 2011).

Re-use of Dredged Material as Engineered Fill by Ex Situ S/S. Ex situ S/S treatment can be used to create an “engineered fill” from impacted or otherwise unsuitable material. This is common practice in the New York/New Jersey harbor system where dredged sediment is treated with cement and re-used as engineered fill upon land (Loest and Wilk 1998).

1.3 Cements Are Common Additives for S/S Treatment and Soil Mixing

Portland Cement. Portland cement is a commonly available, generic material principally used in concrete for construction. This material is also a versatile S/S binding agent with the ability to both solidify and stabilize a wide variety of wastes. Portland cement-based mix designs have been applied to a greater variety of wastes than any other S/S binding agent (Conner 1990). Cement is frequently selected for the agent’s ability to (a) chemically bind free liquids, (b) reduce the permeability of the waste form, (c) encapsulate waste particles surrounding them with an impermeable coating, (d) chemically fix hazardous constituents by reducing their solubility, and (e) facilitate the reduction of the toxicity of some contaminants. This is accomplished by physical changes to the waste form and, often, chemical changes to the hazardous constituents themselves. Cement-based S/S has been used to treat wastes that have either or both inorganic and organic hazardous constituents. Mix designs often include byproducts or additives in addition to portland cement that may improve effectiveness in treatment of specific hazardous constituents.

Slag Cement. Slag cement is increasingly being used in soil mixing. Soil mixing mix designs often include slag cement. Slag cement is almost entirely comprised of the industrial byproduct from manufacture of steel known as ground granulated blast furnace slag (GGBFS). Reuse of GGBFS in soil mixing and S/S can contribute to the “sustainability” rating of a project (Wilk and Tiefenthaler 2017).

2 Example Soil Mixing and S/S Treatment Projects

2.1 Former Wood Treating Facility, Port Newark, New Jersey

Two types of mixing techniques (Fig. 2) were used to treat soils contaminated by wood preserving operations at a former wood treating facility in Port Newark, New Jersey (Delisio 2002).

Approximately 3.2 ha (8 acres) of soil at the site were contaminated with arsenic, chromium, and PAHs. In situ soil mixing was used to treat 17,000 m³ (22,000 cu yd) of soil from 0.6 m (2 ft) to 3.7 m (12 ft). This treatment involved (1) pre-excavation of contaminated material, (2) placement of the stockpiled material back into the excavated area in lifts, and (3) portland cement-based S/S treatment of each lift with an excavator-mounted soil mixer. Performance standards set for the treatment of the soil included attaining a minimum of 0.17 MPa (25 psi) unconfined compressive strength (UCS). S/S-treated soils exceeded this requirement.

Fig. 2

Left photo: excavator mounted soil mixer, Right photo: Pugmill mixing

20,000 m³ (26,000 cu yd) of contaminated soil were treated ex situ using a pugmill to mix portland cement into contaminated soil. Contaminated soil mixed with the pugmill was placed on top of the in situ treated soils in a 0.6 m (2 ft) layer. This layer was carefully compacted to have the similar structural properties as that of soil-cement. This soil-cement-like layer achieved UCS of greater than 1.7 MPa (250 psi), providing an excellent base for pavement placed over the entire site. The mix design for both mixing techniques called for an addition rate of 8% portland cement by wet weight of the soil. Future use of the site is a paved shipping container storage area.

2.2 Former Manufactured Gas Plant Site, Augusta, Georgia

Cement-based solidification/stabilization treatment was completed at a former MGP site in Augusta, Georgia (Wilk 2003). The site is adjacent to a residential area near downtown Augusta. The 0.73 ha (1.8-acre) parcel of land treated by S/S was the former location of the MGP operating facilities and gas holders, which were in use from 1852 to 1955.

Byproducts from the manufacture of town gas impacted soil at this site. The depth of impacted soil ranged from just under the surface to 9 m (30 ft) below ground surface. The impacted soil is considered a source of groundwater contamination for the surrounding area. The groundwater table at the site was approximately 3 m (10 ft) below ground level. The layer of impacted soil above the groundwater table was excavated and transported off site for disposal at an industrial landfill. Approximately 44,000 metric tons (48,000 tons) of site soil was excavated and disposed.

Fig. 3

Right photo: Crane-mounted auger ISS, Left photo: Note change in grade for ISS beginning at water table

Cleanup plans for soil within the groundwater saturated zone included S/S. As an established treatment technology protective of human health and the environment, S/S treatment was approved for the Augusta site by the Georgia Environmental Protection Division. S/S had already been successfully used in the cleanup at other former MGP sites in Georgia. At the Augusta site, S/S was selected for the shallow groundwater saturated soil to enable remediation to be completed in a shorter time and to minimize heavy truck traffic, with its risks and inconvenience, through the community.

After impacted surface soil was removed for off-site disposal, In Situ S/S treatment began. The shallow groundwater saturated zone was located approximately 3–9 m (10–30 ft) below the original ground surface. A soil mixing auger was used to inject and mix portland cement into the soil. The 3 m (10 ft) diameter auger was advanced through the soil. The auger had a hollow stem with auger flights equipped with nozzles. Cement-based grout was injected into the soil. The depth of auger mixing continued through the groundwater saturated zone and a few feet into the soft fractured rock zone beneath. An overlapping pattern of mixing “columns” was used to insure complete mixing and treatment of the area (Fig. 3).

Within the treated area, the tar-like source material in the impacted soil was solidified in place. S/S changed the physical properties of the treated soil, creating a mass impervious to infiltrating precipitation and groundwater while further inhibiting leaching and transport of source material.

Goals for S/S-treated soil at the site included: (a) UCS of at least 0.34 MPa (50 psi), (b) reduction of hydraulic conductivity by 2–4 orders of magnitude compared to untreated soil surrounding the site, and (c) durability of the treated mass based on wet-dry cycling and compressive strength tests.

2.3 Re-use of New York Harbor Sediments

Federal regulations restrict the ocean disposal of sediment dredged from the harbors of New York and Newark, NJ. The Port Authority of New York and New Jersey is faced with a critical situation: find land-based disposal/uses for tens of millions of cubic meters of sediment or lose its standing as a primary commercial port for ocean-going ships. One of the technologies routinely employed to manage the sediment is portland cement-based S/S treatment (Loest and Wilk 1998). Millions of cubic meters of the sediment have undergone cement-based S/S treatment. This treatment immobilizes heavy metals, dioxins, PCBs, and other organic contaminants in the sediment.

The treatment changes the sediment from an environmental liability into a valuable structural fill. Dredged sediment was transported by barge to a pier. At the pier, cement was mixed into the sediment while it remained in the barge. The mixing method used an excavator-mounted soil mixing head. The treated material was removed from the barge and used as structural fill (Fig. 4). This structural fill has already been used at two properties. The first property is an old municipal landfill in Port Newark, NJ. The treated sediment was used as structural fill to cover about 8 ha (20 acres) of the landfill. Covering the landfill with competent structural fill allowed redevelopment of the landfill property into a shopping mall.

Fig. 4

Right photo: Mixing cement into barge of sediment, Left photo: Port-side treatment

The second property called the Seaboard site was the location of a former coal gasification facility and later a wood preservation facility. This 65 ha (160-acre) property has been designated for brownfield redevelopment. More than 1.1 million m³ (1.5 million cu yd) of treated sediment already covers this site.

Sediment dredged from New York and Newark harbors was processed through a large-scale stationary pugmill in Bayonne, NJ. Approximately 2,300,000 m³ (3,000,000 cu yd) were treated and re-used as structural fill to create a golf course. This S/S treatment used portland cement as the binding agent added at a rate of 8% per wet weight of the dredged sediment. Additional properties owned by the land developer hold capacity for another 2,300,000 m³ (3,000,000 cu yd) of S/S treated sediment when the dredged material becomes available. Due to the ban on ocean disposal of some New York and Newark harbor dredge material, cement-based treatment of the material to produce an engineered fill continues to be considered for other properties near the harbor area.

3 Conclusion

Soil mixing methods are used to treat contaminated soil and sediment in a treatment technology called Solidification/Stabilization. S/S treatment protects human health and the environment by safely immobilizing contaminants within the treated material.

Superfund program sites comprise some of our nation’s worst contaminated soil sites. The latest data from EPA indicate that S/S has been selected for a high proportion of in situ and ex situ treatment remedies within the Superfund program. S/S has been demonstrated to effectively treat a broad variety of contaminants of concern found in soils. Adding to the popularity of the treatment are the common availability of generic binding agents (cements) used in the treatment and the variety of mixing methods devised by remediation contractors. An appreciation of the versatility for the treatment technology can be gained by review of example projects. S/S is expected to continue to be an indispensable tool in waste management, remediation, and redevelopment.