SITE INFORMATION AND CORRESPONDENCE

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Bioremediation 5 - 10 September 1993 Aerobic in-situ bioremediation of the subsurface saturated and unsaturated zones generally involves establishing a hydrostatic gradient through the area of contamination. For saturated zone remediation, typically, nutrients and oxygen are added to the contaminated areas by infiltration trenches, injection wells, or air sparging wells. Ground water flow is controlled and maintained by dowtngradient interceptor wells (recovery wells) or trenches. The extracted ground water can be further treated above-ground by activated carbon, aeration, bioremediation, etc. Finally the ground water is inoculated with oxygen and nutrients and reinjected. This continuous recirculation is carried on until the site has been cleaned up. Recovery of the percolating water and ensuring adequate distribution of oxygen are the most difficult aspects of this method. Hydraulic control of injection and recovery water must he achieved in order to ensure that hydrocarbons are not spread and are effectively recovered. The bioreclamation system must achieve an even distribution of water flow through the treatment area. Aerobic in-situ bioremediation of the unsaturated zone is affected by many factors, including dissolved oxygen levels, soil moisture content, soil permeability, soil porosity, oxidation-reduction potential, temperature, pH, compound availability and concentration, nutrient availability, and the natural microbial community, Oxygen and nutrients are added by infiltration trenches or injection wells with water table interceptor wells. Water and nutrients are injected and recirculated by extracting and reinjecting, as in ground water bioremediation. Aeration depends on the total amount of air filled pore space. Elimination of air-filled pore space by waterlogging or compaction reduces oxygen transfer. Many monitoring wells may be needed to determine the effectiveness of the system. Due to poor mixing with these types of systems it may be necessary to treat for a long time to ensure that pockets of contaminated soil ate treated. Soils containing silts or clays are not favorable for bioremediation due to their relative impermeabilities and adsorptive capacities. An alternative to recirculating water through the unsaturated zone involves manipulation of the soil air by adding oxygen, humidity, and gaseous nutrients by air irgection wells_ Soil gas flow is controlled with vacuum extraction wells. This method requires careful monitoring of soil moisture to prevent desiccation. For in-situ biodegradation, about 3 lbs of oxygen are required for every pound of petroleum hydrocarbons degraded. There are several methods to introduce oxygen to the suhtlrrFtce: 1, Sparging into the saturated zone (biosparging) can deliver about 4 ppm at the injection point; 2, Injection of air or oxygen into the vadose zone (bioventing) can deliver up to 10 ppm; 3. Hydrogen peroxide (FIzO2) can be dissolved and injected at above 500 ppm and will break down to oxygen and water during transport through the contaminated area AOI in high concentrations can be toxic to microbes); 4. Injection of aerated water; To operate in-situ bioremediation systems, extensive knowledge of the subsurface is required. Knowledge of soil characteristics is necessary to track hydrocarbon migration and adsorption onto soils. Hydraulic relationships between multiple aquifers is necessary to evaluate potential migration of hydrocarbons between aquifers. It is also necessary to understated the horizontal and vertical components and rate of flow of ground water and seasonal and daily water table fluctuations. OT ' d 800' nN £E=6 26' 9T daS ON I31