U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12, 1999--Volume 3 of 3--Subsurface Contamination From Point Sources, Water-Resources Investigations Report 99-4018C

SELECTING REMEDIATION GOALS BY ASSESSING THE NATURAL ATTENUATION CAPACITY OF GROUND-WATER SYSTEMS

By Francis H. Chapelle and Paul M. Bradley

ABSTRACT

Remediation goals for the source areas of a chlorinated ethene-contaminated ground-water plume in Kings Bay, GA were identified by assessing the natural attenuation capacity of the aquifer system. The redox chemistry of the site indicates that sulfate-reducing (H₂ ~ 2 nanomoles per liter, (nM)) conditions near the contaminant source grades to Fe(III)-reducing conditions (H₂ ~ 0.5 nM) downgradient of the source. Sulfate-reducing conditions facilitate the initial reduction of perchloroethene (PCE) to trichloroethene (TCE), cis-dichloroethene (cis-DCE), and vinyl chloride (VC). Subsequently, the Fe(III)-reducing conditions drive the oxidation of cis-DCE and VC to carbon dioxide and chloride. This sequence of redox conditions gives the aquifer a substantial capacity for biodegrading chlorinated ethenes.

Natural attenuation capacity, defined as the slope of the steady-state contaminant concentration profile along a ground-water flowpath, is a function of biodegradation rates, aquifer dispersive characteristics, and ground-water flow velocity. Natural attenuation capacity at the Kings Bay site was assessed by estimating ground-water flow rates (~0.23 ± 0.12 m/d) and aquifer dispersivity (~1 m) from hydrologic and scale considerations. Apparent biodegradation rate constants (PCE and TCE ~ 0.01d^-1; cis-DCE and VC ~ 0.025 d^-1) were estimated from observed contaminant concentration changes along aquifer flowpaths. Given estimates for these parameters, a boundary-value problem approach was used to estimate levels to which contaminant concentrations in the source-areas must be lowered (by engineered removal), or ground-water flow velocities lowered (by pumping) in order for the natural attenuation capacity to achieve maximum concentration limits (MCLs) prior to reaching a predetermined regulatory point of compliance.