U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Optimizing the Design of Vapor-Extraction Remediation Systems for Removal of Organic Contaminants From the Unsaturated Zone

Abstract

A combined field and laboratory study of low-molecular-weight organic acids was undertaken to improve an understanding of the distribution of organic acids and the geochemical parameters that influence the prevalence of these compounds in ground water of a shallow aquifer contaminated with gasoline, in Galloway Township, New Jersey. The degradation of aromatic hydrocarbons from gasoline occurred in situ by oxidation-reduction reactions mediated by bacteria. The important reactions were aerobic degradation and reduction of nitrate, sulfate, and Fe(III). Temporal and spatial shifts in the biogeochemical reactions occurred in response to changes in hydrogeochemical conditions in the aquifer.Low-molecular-weight aliphatic, aromatic, and alicyclic organic acids were associated with hydrocarbon degradation in oxygen-depleted zones of the aquifer. Laboratory microcosm experiments demonstrated that the biogeochemical fate of specific organic acids observed in ground water varied with the structure of the acid and the availability of electron acceptors. Benzoic and phenylacetic acid were degraded by the indigenous aquifer microorganisms when nitrate was supplied as an electron acceptor. Aromatic acids with two or more substituents on the benzene ring persisted under nitrate-reducing conditions. Organic acids produced during hydrocarbon degradation participate in processes such as metal complexation, sorption, and mineral dissolution; thus, understanding the biogeochemical fate of organic acids is essential to predicting the geochemical evolution of shallow aquifers containing degradable organic compounds.