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

Preliminary Assessment of Jet Fuel Contamination of an Atoll Aquifer

Abstract

Jet fuel spilled in 1991 on the atoll island of Diego Garcia (Chagos Archipelago, Indian Ocean) has contaminated one of the fresh ground-water lenses on the island. Freshwater lenses on the island are unconfined and strongly influenced by tidal oscillations in sea level. The U.S. Geological Survey is using a multidisciplinary approach to (1) evaluate contaminant migration and, (2) determine the geochemical processes that affect contaminant degradation. A network of water-level recorders is being used to determine the effect of a hydraulic control scheme on the water-table configuration and contaminant transport. The water table near the spill is 1 to 2 feet above mean sea level and undergoes diurnal oscillations of as much as 4 feet in response to ocean tides.

Geochemical characterization of the aquifer will help to resolve ambient and fuel-affected levels and types of microbial oxidation. This information is useful to determine the need for, and type of, remediation of residual contamination after efforts to remove the fuel by pumping are completed. Initial data from the first of three planned trips indicate that natural levels of microbial oxidation are sufficient to deplete O₂ in all aquifer waters. Concentrations of CO₂, CH₄, and H₂S indicate that the natural levels of microbial oxidation are enhanced by the presence of hydrocarbon contaminants. An evaluation of the rates, mechanisms, and limits of microbial oxidation in the aquifer will require additional sampling and physical measurements to assess hydraulic processes such as aquifer properties and the response of ground-water flow to different recharge and withdrawal conditions.