An alternative to dig-and-dump or capping as a risk management choice for contaminated soil
December 10, 2012 by Theresa Phillips
The terms “remediation” and “dig-and-dump” are, for some, mutually exclusive. Just what are we “remedying” by excavating and dumping soil in a landfill?
Those familiar with site-specific risk assessment (SSRA) know that the bureaucratically acceptable options for dealing with a contaminated site often lack credibility, as real remedies.
The problem begins with how we define risk. Acceptance of a real risk assessment for soil is scarce; it’s even absent in some jurisdictions (especially those where probabilistic assessments are rejected). Governments tend to adopt an overly conservative precautionary approach: What if the only plant growing on a site is in the middle of the only square inch of contaminated soil? The solution? Remove all the soil so nothing gets harmed.
The Harvard School of Public Health course in Risk Analysis teaches that the precautionary principle rejects any action for which the consequences are uncertain. But this is sometimes an illusion wherein the solution that “appears” to pose the least risk may actually cause the most harm.
Ongoing debates attempt to define uncertainty and how much uncertainty in an action is acceptable. Regulators adopt the principle by enforcing removal of contaminants, even when the consequences of leaving them in place are not fully certain. We must ask, what about the uncertainties that go with removal? Moreover, who decides which consequences are acceptable and which are not?
Renowned risk analysis pioneer Richard Wilson, in reference to the evacuation of Japanese in Fukushima, reported that, “The risks of unnecessary evacuation exceeded the risk of radiation cancers hypothetically produced by staying in place.”
Simply put, as environmental risk assessors, our job should include an analysis of the consequences of dig-and-dump or capping solutions compared to leaving soil in place.
The risk assessment culture is becoming one of submission, where plausible remedies to soil contamination are rejected, and dig-and-dump or capping is favored. With costs rising and landfill space disappearing, the future of risk assessors themselves is itself at risk unless they change the status quo.
Progress in real cleanup is, unfortunately, further hindered by a tendency among developers to reject chemical or biological treatments because they take too long, cost too much, or may not always reduce contaminant concentrations to generic standards.
SSRA was meant to solve this problem, giving landowners a viable choice that wouldn’t grind the economy to a halt. But it’s not being used as often or as effectively as it could be.
The SSRA process was established in Ontario in the late 1990s with the purpose of giving landowners an alternative to simply disposing soil that doesn’t meet the environment ministry standards. (While a number of other provinces in Canada have set generic, or risk-based standards for soil and ground water contamination, few, with the exception of Alberta, have embraced the idea of generating site-specific remedial goals, or “Property-Specific Standards” (PSS), as they are known in Ontario.)
In the 20-plus years since, with regulatory changes, the process has become much less site specific and there is little remaining “wiggle room” for modeling exposure rates and deriving PSS values.
As a result, more land owners turn to the quick fixes to deal with contaminated soil, choosing either to dig-and-dump (under the guise of remediation) or perform a quick SSRA with capping as the obvious risk management measure.
But is dig-and-dump really remediation?
Perhaps at first, but it’s a short-sighted and expensive choice in terms of costs to the landowner, government, public and the environment, all of whom pay for the loss of useful land, landfill maintenance, trucking costs (fuel and environmental pollution), etc. At $50 to $80 per tonne, for disposal alone, the intersect point at which risk assessment becomes a more cost-effective means of handling contaminated soil is easy to figure out. (See Figure 1.)
Those that choose to do an SSRA and implement a hard cap mitigation measure (concrete, asphalt) aren’t really remedying anything either, since the hard cap does nothing to protect the on-site ecosystem.
We need to consider alternative approaches that leave on-site soil intact and protect any existing ecosystem. Consultants, landowners and developers need to collectively support and promote proper risk assessment methods based on the best new science, including statistical analyses and ecological surveys.
The US EPA accepts probabilistic risk assessment approaches that can include statistical analyses and the weighing of risks, based on the professional opinion of the qualified person. To do so, the equation “risk = probability x impact” applies. If the probability is low enough that ecological receptors will come into contact with a limited number of isolated soil exceedences on a property, and the predicted impact on the population is not exceedingly high, then the risk is considered acceptable and no further action is required.
By definition, a risk assessment considers not only the risks of an exposure, but the risks of mitigative actions. The pros and cons of taking action need to be considered. If the cost of the remedy is greater than the weighted risk, it’s not a viable solution to the problem. (And by cost, we include the above-noted environmental costs of over-utilizing landfills to dump marginally contaminated soil.)
Regulators are renowned for ignoring the cons of dig-and-dump. But recently, in Ontario, the government has recognized the problem and is offering an alternative: The new (2011) Tier 2 Modified Generic RA (MGRA) model allows for selection of a “modified ecological protection” (MEP) approach.
The User’s Guide to the Tier 2 model states that “current practice in Ontario is to remove ecological habitat to ensure no ecological species are present or exposed to contamination. This practice results in the removal of habitat which, although degraded, could and often does support a variety of ecological species.”
The MEP approach is intended to “provide another option that will allow for greater preservation of ecological habitat.”
By selecting the appropriate box in the model, ecological protection values for plants and invertebrates are pushed higher, and values for mammals and birds are eliminated. The intent is to support existing ecosystems of perhaps more tolerant species, while admitting that not all ecological receptors (i.e., the most sensitive) are protected by the less-stringent property-specific standards that are, thus, derived.
This appears to be a simple solution to any site where property-specific soil standards are driven by ecological risks. However, it’s unclear if this option is open to those that elect to conduct an SSRA.
A condition of using this approach at the Tier 2 level is that the Record of Site Condition (RSC) must indicate that it’s been done, thereby acknowledging the possibility of adverse effects on some ecological receptors. The SSRA process is designed to demonstrate absence of adverse effects. However, we propose that the MEP approach could be applied if one were to: (a) demonstrate that higher concentrations do not result in adverse effects (e.g., because of low bioavailability, or higher tolerance/adaptive ability of species on-site); or (b) if the risk management plan, Certificate of Property Use (CPU) and RSC recognized the possibility adverse effects.
As part of this new approach to SSRA, we might not only utilize statistical analyses, but also perform ecological surveys onsite and compare them to nearby wilderness areas, parks, etc. as a means of quantifying and characterizing the existing plant and soil invertebrate species on a piece of land, establishing the viability of organisms that may be robust enough to survive in a contaminated environment, and gaining insight as to how that differs from a relatively uncontaminated area.
This approach is, as yet, untried by our risk assessment team, but conversations with other consultants suggest it’s not a novel idea.
With a few willing landowners and participation by teams of risk assessors both on the submission and review sides, perhaps we can steer the SSRA process in the “right” direction, and work on some true “remedies” to brownfield development. HMM
Theresa Phillips, Ph.D. is a Senior Toxicologist and Risk Assessment Specialist with exp Services Inc. in Markham, Ontario. Contact Theresa at email@example.com
Biopiles are a form of composting or bioremediation in which soil is treated ex situ to reduce concentrations of biodegradable contaminants. Excavated soil is arranged in piles or rows, and microbial degradation of contaminants is stimulated via the application of nutrient amendments and/or moisture, aeration, adjustment of pH, or other treatments, as required. The enhanced microbial activity promotes biodegradation at a faster rate than would be achieved by in situ natural attenuation. Degradation is generally via respiration; therefore, aerobic processes dominate. The biopiles are usually oxygenated by pumping air through perforated pipes; however, sometimes the piles are physically turned to promote mixing and aeration.
A site-specific risk assessment (SSRA) is a scientific evaluation of potential hazards posed by the presence of chemicals of concern on a given property. Predicted risks posed by each chemical are evaluated for each pathway of contaminant migration, plus known and potential receptors both on and off site, using prescribed toxicological methods. The SSRA takes into account site-specific parameters which may influence either the exposure pathways, or the nature and susceptibility of human or ecological receptors impacted by the contaminants. In doing so, remediation criteria that are protective of both human health and the ecological environment, and apply to that site alone, are derived for each chemical of concern.
As an example of the application of SSRA, the exp Services Inc. team from Markham, Ontario (formally Barenco Inc.) was retained by a chemical manufacturing client to design and conduct a remediation plan for a five-acre former chemical products packaging facility located in northeast Toronto. Environmental investigations had indicated the presence of petroleum hydrocarbons, chlorinated solvents and freon 11 and 12 impacts.
The remediation strategy was two-fold. First, about 18,000 tonnes of petroleum hydrocarbon impacted soil would be excavated and bioremediated to meet the appropriate industrial/commercial generic criteria, thereby allowing reuse of the soil on site as backfill. Second, an air sparging and vapour collection system designed to remove chlorinated solvents and their degradation products from ground water would be installed. The objective was to reduce the concentrations of chlorinated solvents to meet criteria derived using SSRA.
Approximately 18,000 tonnes of soil were successfully bioremediated to meet the appropriate criteria. Environment ministry-approved site-specific criteria for six chemical parameters (PCE, TCE, 1,1,1-TCA, 1,1-DCE and cis-1,1-DCE) were derived using the SSRA process and comparison to these criteria, rather than to the generic criteria, indicated that only PCE and TCE required remediation in two areas of the property. These were successfully treated through removal of impacted soil and DNAPL, where present, and the use of an air sparging/soil vapour extraction system. Finally, all surface buildings were removed, excavations filled and compacted, and the site was returned to its original grade.
Phytoremediation is a form bioremediation which applies to all chemical or physical processes that involve plants for degrading or immobilizing contaminants in soil and ground water. This is one of the only remedial alternatives to dig-and-dump, for metal contamination in soil.
There are several types of phytoremediation, including phytosequestration and phytoextraction.
Phytosequestration, also called phytostabilization, involves a number of different processes by which metals may be absorbed to roots, adsorbed to the surface of roots, or sequestered, precipitated or otherwise immobilized by biochemicals released by the plant.
Phytoextraction, also known as phytoaccumulation, is a process wherein plants are used to take up or hyperaccumulate contaminants through their roots and store them in the tissues of the stem or leaves. The contaminants are not necessarily degraded, but are removed from the environment when the plants are harvested. This is particularly useful for removing metals from soil and, in some cases, the metals can be recovered for reuse (by incinerating the plants) in a process called phytomining.
Air sparging (AS), or air stripping, is a method of in situ soil remediation for reducing concentrations of volatile organic compounds (VOCs), particularly those sorbed to soil in the saturated zone, or present in groundwater. The technology is applied by injecting air into the subsurface. The uncontaminated air induces the phase transfer of VOCs, which volatilize and move upwards into the unsaturated zone. A soil vapour extraction (SVE) system is often used in conjunction with AS.
Soil vapour extraction (SVE) is an in situ method of removing volatile organic contaminants (VOCs) from the unsaturated zone. Also known as soil venting or vacuum extraction, this method can remove VOCs that have sorbed to the soil or are present in the soil gas. Extraction wells are installed near the source of soil contamination and pumps are used to create a vacuum or negative pressure gradient. Contaminant vapourus migrate toward the extraction wells and are removed and collected for treatment.