For this task order, GCE performed four semi-annual groundwater monitoring events at Underground Storage Tank (UST) Site 10 and investigated the cause of groundwater mounding in the vicinity of MW-03 at UST Site 1. Groundwater Monitoring Reports were prepared after each event.
UST Site 10 is the site of former Building 1077 at MCAGCC Twentynine Palms, CA. In March 2000, approximately 7,000 gallons of gasoline were released during filling of the UST system at Building 1077. A site assessment was conducted, and a soil vapor extraction (SVE) system was operated from 2001 to 2005. The SVE system removed 8,500 pounds of contaminant mass. The USTs were removed in 2010, and contaminated soil was excavated. Additional investigation characterized the groundwater contamination in the perched aquifer. Benzene and methyl tertiary butyl ether (MTBE) were present in the groundwater. A Corrective Action Plan was prepared that recommended monitored natural attenuation as the corrective action for the site based on results of groundwater monitoring.
The objective of this task order was to perform groundwater long-term monitoring activities at UST Site 10 at MCAGCC Twentynine Palms. Groundwater monitoring was conducted on a biannual basis at the site to determine if natural attenuation was occurring. Results indicated that natural attenuation was occurring. A Closure report was prepared at the conclusion of the project to request no further action status from the California Regional Water Quality Control Board - Colorado River Basin Region.
Additionally, this project included an investigation to determine the reason for groundwater mounding around monitoring well MW-03 at the site. The investigation to assess apparent groundwater mounding at monitoring well MW-03 included hourly synoptic water level elevation measurements over several working days, and a geophysical survey of the entire site to determine if any potential unknown subsurface utilities exist that may be leaking fluid into the subsurface. Old base utility drawings indicated the presence of abandoned sanitary sewer lines in the near vicinity of monitoring well MW-03. These potential sanitary sewer lines were excavated and were found to be intact but inactive water lines. It was concluded that the mounding was likely attributable to a localized perch water table at MW-03