Releases of petroleum contaminants from a retail gasoline station in a medium-sized California Central Valley town threatened a municipal supply well immediately adjacent and approximately 200 ft downgradient to the station. Methyl tert-butyl ether (MTBE) was present in groundwater concentrations up to 30,000 micrograms per liter (µg/L) and GRO up to 33,000 µg/L in the first-encountered groundwater aquifer, between 30 to 45 feet below ground surface (BGS). The concern was that the MTBE would be pulled toward the supply well’s screened interval in the deeper zone, threatening the water and making site restoration more complex. Monitoring indicated that MTBE had been migrating off-site, requiring action to protect the water supply.
The source area was aggressively treated using soil vapor extraction for impacted vadose zone soils and a downgradient in situ treatment barrier – sparge curtain – at the property boundary. The boundary barrier included air/ozone sparge wells downgradient of the source area and upgradient of the municipal supply wells. Its intent was to reduce/destroy the MTBE and GRO contamination. The perimeter treatment barrier consisted of dual-completion air/ozone sparge points co-located in the deeper and shallow portions of the aquifer with overlapping radii of influence.
To alleviate Regulatory Agency concerns about possible harmful tertiary compound formation with use of a relatively new technology, URS designed a pilot test and shared the results with the agency, prior to full system operation. Test results showed MTBE concentrations that were initially 970 µg/L at the property line reduced to 50 µg/L after 8 days and 1.5 µg/L after 35 days. No tertiary compounds were generated. After completion of the pilot test, full-scale operations began. After six months of operation, concentrations were reduced to below cleanup goals. The system was turned off for four quarters of closure monitoring. GRO concentrations were reduced to below non-detect (<50 µg/L), and MTBE was detected in only two monitoring wells, both below regulatory standards (13 µg/L). Based on these results, site closure was expected in late 2007.
- Mass in Groundwater
- MTBE: 35 Pounds
- GRO:13.1 Pounds
- Benzene:0.4 Pound
- Plume Migrating Off-Site and Threatening a Municipal Supply Well