environment magazine

Stabilization and Solidification of Contaminated Soil and Waste Part 5: Durability, Longevity, and Monitoring of S/S Systems

By Dr Colin Hills, Director of the Centre for Contaminated Land Remediation at the University of Greenwich

Scope

Contaminated soils and wastes may be toxic, carcinogenic or mutagenic and require remediation by an appropriate risk-management strategy. There is a successful track record for treating contaminants by stabilisation/solidification (S/S) spanning more than two decades and this is discussed in this article.

This fifth and final part of an occasional series on the treatment of waste and soil by Dr Colin Hills, Director of the Centre for Contaminated Land Remediation at the University of Greenwich, explores the durability and aging of S/S-treated material under various environmental loads operating in their environment of service and their long-term monitoring requirements. He is joined by Edward Bates, whom has recently retired from the US Environmental Protection Agency, and Dr Peter Gunning of Carbon8 Systems Ltd.

Introduction

The future use of an S/S-remediated site and the environmental conditions operating below ground may impact on the materials used to stabilize contaminants and their capacity to immobilize contaminants in the longer-term. Cement-based S/S stabilized wastes are vulnerable to the same physical and chemical degradation processes that affect concrete and other cement-bound materials.

Stabilised/solidified-treated material employing cement as part of the binder system will, however, differ from conventional concrete in significant ways. Concrete is normally used as a construction material and comprises a properly proportioned binding agent, aggregate and sand; selected strictly to optimise the durability and load-bearing properties of the hardened product.

During S/S treatment, mix designs are based on the properties of the contaminated media being treated so the selection of aggregate material is generally not an option. Concrete used in building materials typically have a minimum Unconfined Compressive Strength of 20MPa (3000 psi) or greater, whereas S/S-treated materials usually have UCS performance standards starting at 0.3MPa (50 psi).

Despite the very different design specifications, and nearly 60 years of the use of S/S in the USA, it is encouraging to note that there are no reported major failures of S/S waste-forms in that country. Thus, it can be assumed that S/S is a reliable, relatively easy to use management strategy that can mitigate the risks associated with contaminated soil and waste. This deduction has been supported by the findings of the PASSiFy project¹, which examined samples of S/S material taken directly from remedial operations in the USA, UK and France. The key conclusion from this work can be summarised as:

• The S/S materials examined passed their original acceptance criteria;
• S/S soil and waste behaved like cement-bound materials and contained mineral phases and microstructural features consistent with this observation;
• The leaching of contaminants from the S/S soils was generally below instrument detection levels;
• The microstructure of S/S materials was complex and involved interactions between all three system components; namely soil, contaminants and binder, and that this relationship was subject to modification due to aerial and sub-aerial environmental loads, experienced during treatment and after site closure;
• Potential risk-indicators were identified, but there was no indication and that these were impacting upon the efficacy of contaminant containment;
• Waste forms appeared to be subject to carbonation, but was not a cause of concern as a densification of waste form microstructure resulted through an infilling voids and micro-cracks; and,
• Some soils are naturally alkali sensitive and soil minerals were identified at two sites that were subject to this reaction when solidified by a cement-based binder.

The PASSiFy study highlighted that a number of risk-indicators were present in the samples examined, but these did not indicate the onset of deleterious reactions. As waste forms appeared to behave much like cement-bound materials, the interactions between the soil fraction, the waste and the binder system could be explained.

The waste forms examined were carbonated to a lesser or greater degree, and ettringite and other phases were formed within the pore space. This is an important observation, because as cementitious construction and other products are ubiquitous and are well-understood systems, the behaviour of S/S waste forms over long periods of time should be relatively easy to predict; providing the contaminated S/S soils treated and the binders used are well characterised, and the environmental loads impacting upon a waste form are known.

It can be seen that durability and longer-term performance could (in theory) be compromised by reactions that may be both intrinsic and extrinsic in nature. 

The need to monitor remedial operations involving S/S to protect the environment (and build a set of performance-related data) has been recognised in certain situations, and it is therefore useful to examine case studies involving S/S. 

In the USA, a number of superfund sites have been subject to post completion monitoring and performance reviews for a number of years. Similarly, in the UK S/S-remediated sites have also had longer-term monitoring strategies implemented. The main aspects of post-completion monitoring are discussed below.

resource for designing and managing post construction inspections, monitoring, and reporting, for any site. A number of completed 5 year review reports are available on the USEPA website¹⁸.

Additional guidance is available from the Environment Agency on the use of S/S¹⁹, especially in Section 5 and Appendix 4. The EA document (page 56) suggests that:  “Specific objectives for long-term monitoring for a re-use scenario may be:

• to demonstrate whether S/S remains effective:
• to provide a basis for implementing mitigation measures;
• to identify detrimental changes in the re-use scenario (e.g. water table rise); and
• to provide a basis for ceasing monitoring.

 The EA document also discusses what topics should be included in a monitoring report and the decision basis for discontinuing monitoring.

The ITRC (Interstate Technology & Regulatory Council) has also recently published guidance on Development of Performance Specifications for S/S²⁰. Included in Section 7 of this document is post construction monitoring, which is referred to as “stewardship”. The ITRC states (page 56) “Long-term stewardship of a completed S/S remedy may include monitoring of environmental media in contact with and potentially affected by the remedy, monitoring of institutional controls, monitoring and maintenance of engineering controls, financial assurances, and periodic review(s) by the controlling environmental agency “. These important issues are discussed further below.

Specific features common in monitoring programs

Monitoring programs at sites treated by S/S often include monitoring for the following purposes:

• To assure that the S/S treated material continues to meet its original design performance property of reducing the release of contaminant load to groundwater or surface water bodies through low permeability and low leachability. This determination is usually made by locating groundwater wells immediately adjacent to the monolith and sampling for a selected list of Contaminants of Concern (COCs) along with field measurements for pH, Eh, and other selected indicator parameters. Immediately adjacent is relative as it is influenced by the rate of groundwater movement and physical access, including avoidance of compromising the cap. For example a distance of 3 m (10ft) to 30m (100ft) from the edge of the monolith would be normal. Often a subset of the COCs present in the S/S treated material is selected with preference for those most likely to be detected (most soluble) should the S/S material fail to maintain its control over release. Although sampling/monitoring is often done quarterly (or seasonally) for the first couple years, the frequency may be reduced thereafter to for example, annually if no issues are detected. It should be noted that immediately after S/S treatment; especially if S/S was carried out in-situ, there will be a slightly elevated pH in the groundwater contacting the treated material. This is normal, should dissipate in a few months, and does not indicate a failure of the treated material. 

• To document that groundwater quality is improving after remediation. Often on sites where S/S is implemented as the ‘source control technology’ to treat contaminated soils, groundwater has also been impacted through the release of COCs. Thus, once the source has been successfully managed by S/S, it is reasonable to expect the groundwater quality to improve. For some sites, this has led to selection of MNA (monitored natural attenuation) as either the primary, or secondary, method for remediating impacted groundwater. Thus, monitoring wells are installed and a monitoring plan developed to assess the magnitude of groundwater improvement over time and the eventual achievement of groundwater quality objectives.

• To document that engineering controls are functioning properly and maintenance is being conducted, on for example, fencing, the cap, surface water and runoff controls, and may include the installation of impermeable barriers or walls. Caps, surface water and other surface engineering controls are inspected during the periodically and after unusual precipitation events. Subsurface engineering controls, e.g. impermeable barriers, are monitored indirectly (via groundwater monitoring) in the same manner as the S/S treated material. If the site has a vegetative cap, this is inspected and repaired where necessary. It is important that invasive trees and shrubs are removed from caps as their roots can be damaging and may even invade the treated S/S material.

• To document accordance with national or local institutional controls, including deed restrictions and other instruments to prohibit for example, excavation or the construction of wells within the S/S waste without specific prior approval from the regulatory agencies. There may also restrictions on how the general site can be used, and any inappropriate activity is noted during periodic site inspections. 

• To document the exit strategy, that may be specific to the nature of regulatory oversight, and the site-specific characteristics. Under the USEPA Superfund program, S/S-treated soils are considered as containment cells, and 5-yearly reviews are required in perpetuity. Sites remediated under State programs in the USA follow the requirements of individual states, which are highly variable. The EA guidance on S/S, as previously mentioned, provides information on the potential decision basis for discontinuing monitoring. However, in either case the site owner, regulatory agencies, and other stakeholders will need to agree on what aspects of monitoring can be discontinued and on what basis.

Summary

Stabilisation/Solidification technology has been routinely used in the USA for the treatment of soil and waste for nearly 60 years. During this time there have been no reported major failures and S/S remains a best available demonstrable technology. Stabilised/Solidified-treated materials employing cementitious binders have potential to behave in the same way as other commonly employed, cement-bound systems. Waste forms are predicted to perform in their environment of service for up to thousands of years. The environmental loads operating on S/S waste forms will vary from site to site and it is known that risk indicators have been identified in treated materials extracted from historical remedial operations; nevertheless they continuing to meet their original performance specifications. Post-completion monitoring of S/S sites may be a requirement after S/S and adequate guidance is available in the UK and elsewhere to ensure the maintenance of waste form performance and the continued protection of the environment. 

References

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