TIME ESTIMATES FOR REACHING COMPLIANCE USING SOURCE REMEDIATION COMBINED WITH MONITORED NATURAL ATTENUATION

Source zone remediation combined with monitored natural attenuation (MNA) is one strategy for reaching regulatory closure at contaminated sites. A related remediation strategy is to couple partial source removal using an appropriate source zone remediation technology, including combined remedies, with a reduction in the size and concentration of the contaminant plume. Chapelle et al. (2004) present a framework for evaluating time of remediation (TOR) estimates for reaching regulatory compliance using source remediation combined with MNA that is independent of the method of solution. One component of the TOR problem, time of stabilization (TOS), involved quantifying the time required to reach compliance at a downgradient wells as an aqueous plume shrinks in size following mass and concentration reduction in the source area.

The objective of this research is to demonstrate this approach at a fractured-rock contaminated site. A large plume of chlorinated ethenes resulted from a former landfill that received ash from a solid waste incinerator. The plume is present in the upper-most aquifer which is located at the interface of glacial till and a weathered shale unit. Remediation consisted of source zone excavation and the installation of a permeable reactive barrier at the toe of the plume. Monitoring data over a 10-year period demonstrated reduction in plume concentrations. Natural Attenuation Software (NAS) was used to evaluate and interpret groundwater concentrations downgradient of the source. Results indicate the best match between the predicted and observed chloroethene concentrations was best achieved at wells closest to the source. Data gaps in the mid-1990s and time fluctuation in groundwater flow directions hinders the accurate estimation of the observed breakthrough time.