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ABSTRACT: Vadose Zone Enhanced Biodegradation of Hydrocarbon Contamination

Robert E. Hinchee, Douglas C. Downey

Over the past two decades, the practice of in-situ enhanced biodegradation has increased, particularly for treating the soluble fuel components in groundwater; and up until the last 2-5 yr, less emphasis was given to enhancing biodegradation in the vadose zone. In most cases, oxygen is the limiting factor for aerobic biodegradation. The current conventional enhanced bioreclamation process utilizes water to carry oxygen or an alternative electron acceptor to the contamination whether it occurs in the groundwater or unsaturated zone. If pure oxygen is utilized and 40 mg/L of dissolved oxygen is achieved, approximately 75,000 lb of water must be delivered to the formation to degrade a single pound of hydrocarbon. If 500 mg/L of hydrogen peroxide is successfully delivered, th n 12,000 lb of water is necessary. The use of pumped air to deliver oxygen to the vadose zone results in a minimum ratio of 13 lb of air pumped per pound of hydrocarbon degraded. An additional advantage of utilizing an airborne process is the greater diffusivity of gases than liquids, lessening problems associated with geological heterogeneities.

Data will be presented from two sites at which air pumping has been used to stimulate in-situ biodegradation. One site is located at Hill Air Force Base, Utah, and the other at Tyndall Air Force Base, Florida. Even in the arid environment of Utah's high desert, significant biodegradation can be induced by pumping air into the vadose zone. Biodegradation rates, however, may be substantially increased by the addition of nutrients and increasing soil moisture content. At Tyndall, with very shallow groundwater, the site was dewatered to increase the vadose zone depth and stimulate in-situ biodegradation of petroleum fuels in the vadose zone. This paper will illustrate the effectiveness of the technology in two very different environments, and show it to be a cost-effective alternative as pposed to more conventional groundwater-based enhanced biodegradation technologies.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990