Post-Closure Monitoring Report for Corrective Action Unit 329: Area 22 Desert Rock Airstrip Fuel Spill, Nevada Test Site, Nevada (Rev. No.: 0, June 2002)

Abstract

There was an increase in total petroleum hydrocarbons (TPH) concentrations at all three depths within Borehole DRA-0. The oxygen concentration at 40 ft below ground surface (bgs) decreased. There was also an increase in carbon dioxide concentration at that depth. The decrease in oxygen concentrations and the increase in carbon dioxide concentration at the 40 ft bgs level could be possible indicators of natural attenuation. It is not possible to determine trends or biodegradation rates with the limited amount of data collected from the site. The sample results from this first monitoring period did not correlate with the baseline results collected in August 2000. Additional samples will be collected and the results will be compared to previously collected samples to determine if the site was at equilibrium in August 2000. Continued annual monitoring will be conducted as specified in the Closure Report to determine trends at the site. As natural attenuation occurs, the TPH concentrations should decrease. The TPH concentrations will be compared over successive monitoring events to determine trends and approximate rates. As natural attenuation occurs, oxygen will be consumed and carbon dioxide will be produced. The oxygen, nitrogen, and carbon dioxide concentrations will also be evaluated to determine if biodegradation is indicated. When all available oxygen has been consumed, methane-producing bacteria may continue the natural attenuation process so methane levels will be monitored as an additional possible indicator of natural attenuation. The rate of decrease will be determined on the microbial populations, contaminant concentrations, available nutrients, and other environmental factors. Samples were collected and submitted for microbial analysis during closure activities. The results indicated that the microbial populations and nutrients were adequate for limited bioremediation (DOE/NV, 2000). Additional sampling for microbial analysis are not planned. The site is currently inactive and the source of additional contamination was removed. It was determined during closure activities that the wetting front has stabilized. Monitoring of Borehole DRA-3 has not shown any indications of contamination. Contamination migration to the water table is not expected based on current site conditions.