Mass Flux Performance Metrics for Support of Less Aggressive Groundwater Remedies under the Federal CCR Rule

摘要

Some coal combustion residual(CCR)disposal units regulated by the Federal CCR Rule will require groundwater remedies.Groundwater pump and treat(P&T)technology is often considered to be a presumptive remedial technology by regulatory agencies and,while it can perform well as a hydraulic barrier to protect nearby receptors,it is usually very slow,costly,and inefficient in restoring groundwater quality.More efficient and sustainable technologies such as in situ redox controls,permeable reactive barriers,and monitored natural attenuation(MNA)should be considered at sites with less potential for short-term exposure to receptors.The wide range of inorganic constituents in CCR leachate with differing mobility in various geochemical environments cannot currently be treated by a single in situ technology.For units regulated by the CCR Rule,the point of compliance(POC)is at the edge of waste and provides no space for sequential in situ treatment barriers that could otherwise address this range of constituent mobility.Therefore,in situ treatment approaches face a serious space constraint at these sites.Although the CCR Rule is very prescriptive regarding the POC and the numerical water quality cleanup standards,it provides some flexibility regarding the time needed to achieve cleanup.Furthermore,the time variable provides an avenue for incorporating risk assessment into the remedy selection process.The planned introduction of a permit program and regulator under the Infrastructure Act should enhance the opportunity to justify less aggressive groundwater remedies.Mass flux and mass discharge metrics are now being used at groundwater remediation sites.Several recent CERCLA remedies embrace these concepts as interim remedy performance metrics for source control measures and as metrics for the transition from more aggressive/active remedial systems to less aggressive/passive remedial technologies.Interim mass-based performance metrics can help to take the short-term focus away from achieving drinking water standards.