Groundwater Completion Strategies for Complex Sites

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. As a consequence, if the existing remedy will not achieve the groundwater remedial action objectives and as­so­ci­ated cleanup levels, either the remedial tech­nol­ogy or the com­pre­hen­sive remedy should be modified.

Especially on complex ground­wa­ter sites, such as those con­t­a­m­i­nated by dense non-aqueous phase liquids, or those ex­hibit­ing chal­leng­ing hy­dro­ge­o­logic con­di­tions with highly variable hy­draulic con­duc­tiv­i­ties, the con­ven­tional approach of se­lect­ing a remedial tech­nol­ogy and applying it until con­t­a­m­i­nant con­cen­tra­tions meet MCLs has proven inefficient or even futile.

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 mg/l concen­tra­tion in groundwater and a 5 µg/l 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 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

Scientists and prac­ti­tion­ers emphasise 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 stabil­i­sa­tion or reduction) make appropriate groundwater remedy performance metrics.