Underground Storage Tank (UST) Investigation and Characterization, UST Site 3, Naval Exchange (NEX) Gas Station, Naval Air Station (NAS) Lemoore, CA.

This task order including conducting an investigation and characterization of a fuel release from the UST at UST Site 3 NEX Gas Station. The objective of this investigation was to assess the extent of petroleum contamination in the groundwater at UST Site 3 because no work has been conducted in approximately 10 years. The field activities were conducted at NEX Gas Station with concurrence from the Central Valley Regional Water Quality Control Board. Five additional groundwater monitoring wells were installed, and three rounds of monitoring were conducted. A Site Characterization Report presented the site activities for assessing the extent of petroleum impacts in groundwater at UST 3.

The NEX gas station (UST 3) has been in operation since 1962 and has entailed several iterations of USTs containing gasoline and waste oil at UST 3. Following the discovery of an approximately 2,800-gallon gasoline release in May of 1988, there have been several environmental investigations and interim response actions including soil excavation and removal, floating free product removal, soil sampling events, installation and sampling of groundwater monitoring wells, installation and operation of a Soil Vapor Extraction/Air Sparge system, and a Cone Penetrometer and Membrane Interface Probe investigation.

The objective of this task order was to assess the extent of petroleum contamination in groundwater after a 10-year period of inactivity. A UST Site Characterization Report was prepared to discuss the activities conducted at the UST 3 and provide recommendations for future actions. The field activities were conducted in two phases. The first phase was an initial site characterization that included groundwater monitoring activities, and the second phase was an additional site characterization to address data gaps that were identified during the first phase. The initial site characterization included a site survey and two groundwater monitoring events. The site survey consisted of recording basic information (such as well ID, geographic coordinates, well diameter, top of casing elevation) of the previously existing groundwater monitoring well network at UST 3.  The two groundwater sampling events were conducted. All groundwater samples were analyzed for Total Petroleum Hydrocarbon as gasoline and diesel (U.S. Environmental Protection Agency [US EPA] Modified Test Method 8015B) and volatile organic compounds (US EPA Test Method 8260B).

After the evaluation of the groundwater sampling data collected during these events, it was determined that there were data gaps that needed to be addressed in order to fully characterize the current site conditions, which resulted in the second phase of the investigation. Identified data gaps were in the characterization of the lateral extent of free product, the characterization of the existing dissolved phase benzene plume, the extent of the downgradient dissolved phase methyl tertiary butyl ether (MTBE) plume and the extent of the source area dissolved phase volatile organic compounds (VOCs). To address the data gaps, the second phase of the investigation included installing five additional monitoring wells to be used in conjunction with the existing groundwater monitoring well network and conducting a third round of groundwater sampling. The groundwater sampling was conducted to characterize the groundwater impacted by the gasoline release at UST 3. Source area excavation and long-term monitoring were the recommended correction action.