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
by
Claire Welty (Drexel University, Department of Civil and Architectural
Engineering, Philadelphia, Pa. 19104), Craig J. Joss (Applied Environmental
Sciences, Ltd., P.O. Box 943, Bala Cynwyd, Pa. 19004), and Arthur L. Baehr
(U.S. Geological Survey, West Trenton, N.J. 08628)
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.
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