Monitoring the natural attenuation of petroleum in ground water at the former naval complex, Operable Unit A, Adak Island, Alaska, May and June 2003
During May and June 2003, the U.S. Geological Survey installed monitoring wells and collected data to characterize the effectiveness of natural attenuation processes for remediating petroleum-contaminated ground water at Operable Unit A of the former Naval complex on Adak Island, Alaska. In addition, the evidence for petroleum biodegradation in ground water was evaluated at selected petroleum sites, plans for future natural attenuation monitoring were suggested for the selected petroleum sites, and the natural attenuation monitoring strategy for the Downtown area of Adak Island was reviewed and refinements were suggested. U.S. Geological Survey personnel measured water levels and collected ground-water samples from about 100 temporary boreholes and 50 monitoring wells. Most samples were analyzed on-site for concentrations of selected petroleum compounds and natural attenuation parameters such as dissolved oxygen, ferrous iron, and carbon dioxide. The U.S. Geological Survey evaluated the data on-site, selected new monitoring well locations, and installed, developed, and sampled 10 monitoring wells. The review and suggestions for the natural attenuation monitoring strategy focused on how to better achieve monitoring objectives specified in the Record of Decision for Adak Island petroleum sites. To achieve the monitoring objective of verifying that natural attenuation is occurring, the monitoring plans for each monitored natural attenuation site need to include sampling of at least one strategically placed well at the downgradient margin of the contaminant plume margin, preferably where contaminant concentrations are detectable but less than the cleanup level. Collection of natural attenuation parameter data and sampling background wells is no longer needed to achieve the monitoring objective of demonstrating the occurrence of natural attenuation. To achieve the objective of monitoring locations where chemical concentrations exceed specified cleanup levels, at least one natural attenuation well within or immediately downgradient from the contaminant source area at each site needs to be monitored. Achieving the Record of Decision-specified final monitoring objective of estimating the rate of natural attenuation to demonstrate achievement of cleanup levels within 75 years will be problematic. Demonstrating (predicting) achievement of cleanup levels within any timeframe in a technically defensible manner will be difficult to achieve using any type of short-term monitoring and evaluation, and will be particularly difficult to achieve through monitoring and evaluation of dissolved-phase petroleum only. Overall, natural attenuation processes appear to have greatly limited the extent of ground-water contamination at most sites investigated and have limited the risk that petroleum contaminants pose to downgradient receptors. Clarification or refinement of the monitoring objective to demonstrate cleanup within 75 years would be a reasonable prelude to developing a monitoring and data evaluation strategy to meet the objective.