ROLE OF REDOX GRADIENTS ON THE PERSISTENCE OF ORGANIC CONTAMINANTS IN THE SUBSURFACE ENVIRONMENT

A plume of contaminated groundwater on Cape Cod, Massachusetts, resulting from 30 years of rapid infiltration disposal of treated wastewater, was investigated. The conceptual model for the long-term fate of organic contaminants, such as surfactants, starts with removal of the most biodegradable compounds during wastewater treatment and additional removal during transport through the aerobic zone beneath the infiltration beds. Compounds that persist through this zone are recalcitrant and are transported along the regional groundwater flow path. Little additional attenuation occurs because 1) degradable and sorptive components have been removed, 2) the plume is anoxic, 3) nutrients are limited, and 4) microbial community structures change. Dynamic factors such as loading rates and wetting/drying cycles can result in contaminants being introduced as pulses. Research at the site has focused on linear alkylbenzene sulfonates (LAS), the major class of anionic surfactants in laundry detergents. Prior to 1964, branched alkylbenzene sulfonates (ABS) were used, but due to resistance to biodegradation, ABS was replaced with the more easily degradable linear analogs. Both LAS and ABS consist of oligomers with differing positional isomers and points of substitution on the phenyl ring, resulting in a mixture of structures with differing biodegradation and sorption characteristics that can be used as intrinsic tracers of environmental processes. The wastewater plume consists of distinct geochemical zones: 1) oxygenated recharge water with low specific conductance, 2) low dissolved oxygen concentrations with elevated specific conductance, and 3) the anoxic plume core with high specific conductance. Pulsed and continuous injection LAS tracer tests were conducted within the 3 zones using an array of multilevel sampling devices spaced at 1 meter intervals along the groundwater flow path. Results indicate: 1) biodegradation rates in the aquifer are low, spatially variable, and controlled by oxygen content, 2) biodegradation is self limiting as LAS and oxygen are consumed, 3) stable metabolites are formed, 4) biodegradation selectively removes isomers with the longest alkyl chains, and 5) sorption results in chromatographic separation that is inversely proportional to water solubility.