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Groundwater Completion Strategies for Complex Sites

Ernest AShley, CPG, LEP, LSP, CHMM
A new paradigm for cleanup of complex groundwater sites is emerging that includes reconsidering how cleanup goals are selected and structured, combining technologies in space or in time, and using improved performance metrics to evaluate real progress toward risk reduction during the lifetime of the remedy.

Complex ground­wa­ter sites, such as those con­t­a­m­i­nated by dense non-aqueous phase liquids (DNAPLs), or those ex­hibit­ing chal­leng­ing hy­dro­ge­o­logic con­di­tions with highly variable hy­draulic con­duc­tiv­i­ties, continue to confound cleanup efforts, in some cases for decades. The con­ven­tional approach of se­lect­ing a remedial tech­nol­ogy and applying it until con­t­a­m­i­nant con­cen­tra­tions meet maximum con­t­a­m­i­nant levels (MCLs) in ground­wa­ter are achieved has proven inefficient at best, and in many cases for complex sites, futile.

Establishing Cleanup Goals
Maximum con­t­a­m­i­nant levels (MCLs) and the restora­tion of beneficial uses have long formed the expected stan­dards for cleanup of ground­wa­ter con­t­a­m­i­na­tion. Over time, however, reg­u­la­tors and re­spon­si­ble parties have found that the time­frames for re­me­di­a­tion to these ultimate cleanup goals are often much longer at complex sites than orig­i­nally es­ti­mated, and that the selected tech­nol­ogy might be limited in its ef­fec­tive­ness beyond a certain point. The National Con­tin­gency Plan states: “When restora­tion of ground­wa­ter to beneficial uses is not practicable, EPA expects to prevent further mi­gra­tion of the plume, prevent exposure to the con­t­a­m­i­nated ground­wa­ter, and evaluate further risk re­duc­tion.” This language provides some flexibility to establish remedial goals other than at­tain­ment of MCLs in some cases, but at a minimum paves the way to es­tab­lish­ing interim goals related to halting mi­gra­tion, pre­vent­ing exposure, and reducing risk on the way to the ultimate cleanup goal of MCLs.

In May of 2014, EPA issued a Ground­wa­ter Remedy Com­ple­tion Strategy rec­om­mend­ing site-specific actions and decision making processes to achieve groundwater remedial action objectives (RAOs) and as­so­ci­ated cleanup levels using an updated con­cep­tual site model, per­for­mance metrics and data derived from site-specific remedy evaluations. This document  encourages critical review of the per­for­mance of ground­wa­ter remedies at some regular fre­quency relative to cleanup goals and, if nec­es­sary, re­assess­ment of the remedy. Ac­cord­ing to this strategy, “If the existing remedy will not achieve RAOs and as­so­ci­ated cleanup levels, either the remedial tech­nol­ogy or the com­pre­hen­sive remedy should be modified.”

We need to set realistic remedial goals to reduce risk and establish a measureable path toward remedial action objectives.

Complex Sites Require Multiple Remedies
A major challenge for remediation of groundwater at many complex sites is that the difference between the initial or residual contaminant concentrations and the ultimate concentration-based cleanup objectives are many orders of magnitude apart. For example, the difference between 50 ppm concentration in groundwater and a 5 ppb MCL is four orders of magnitude, requiring a 99.99% reduction in concentrations! In addition, large quantities of contaminant mass are often trapped in soil or bedrock horizons of very low hydraulic conductivity, making them very difficult to access.

In the past, feasibility studies and records of decision (RODs) often focused on a single remedial technology. However, each technology has a “sweet spot” or range of concentrations and site-specific factors where it can provide maximum concentration reduction, and our “tool box” of treatment technologies contains no technologies that alone can cost-effectively achieve the final objectives for all concentration ranges across the full range of geologic conditions at a complex site. Therefore, multiple-technology remedies are often needed to achieve these objectives.

Realistic Goal Setting for Combined Remedies
A recent EPA workshop, titled Combined Remedies – Their Time Has Come, emphasized that different technologies are typically needed for different areas of a site (e.g., high concentration source area or dilute plume) AND for different phases of the remediation. Encouraging the use of multiple technologies to achieve real remedial progress requires setting realistic, functional goals – those that demonstrate risk reduction while measuring progress toward absolute objectives. These interim, functional goals measure performance of a given technology throughout its useful life, and provide indicators for when the technology performance has reached a point that transition to a more appropriate technology for the new condition should be made.

Contaminant Mass Discharge as a Performance Metric
A key challenge for the environmental professional is to set realistic remedial goals that are necessary and appropriate to reduce risk and establish a measurable path toward remedial action objectives. Cleanup time frames are especially difficult to estimate, and contaminant mass in the subsurface is very difficult to quantify. However, contaminant mass is often not directly related to risk of exposure or harm. Rather, contaminant mass discharge is directly related to migration, potential exposure and risk. Contaminant discharge can be assessed and monitored in a variety of ways and can form the basis for realistic risk reduction and continued assessment of progress of the remedial systems. Accordingly, mass discharge and the associated dynamics of the plume (i.e., plume stabilization or reduction) make appropriate groundwater remedy performance metrics.

CDM Smith was directly involved in the establishment of the first Record of Decision for a Superfund site that involved mass discharge as a key performance metric. At the Well 12A site, the combined remedy for the source area (excavation, thermal remediation, and bioremediation) will be considered “operational and functional” when mass discharge from the source area to the plume has been reduced by 90 percent. This goal was based primarily on a detailed evaluation of the reduction of mass discharge that would be required to shrink the plume to prevent exposure to receptors, and to allow natural attenuation processes in the plume to reduce concentrations to the ultimate cleanup goal. This exemplifies the paradigm of incentivizing remediation progress and risk reduction at complex groundwater sites, while recognizing the challenges posed at such sites and still maintaining the ultimate goal of restoration.

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