Soil & groundwater investigation and hydrogeologic modeling for vulnerable drinking water aquifer

However, it was found that the investigations and remediation by the previous consultant were inadequate, and the drinking water supply beneath the site was threatened primarily by releases of the fuel oxygenate methyl tertiary butyl ether (MTBE) and secondarily by benzene. In response, Lindmark Engineering expeditiously further investigated the lateral and vertical extent of contamination in soil.

Unique site features included a great depth to groundwater (in excess of 120 feet) and contaminants that had migrated into young alluvium, older alluvium, and an underlying, variably consolidated sedimentary formation that contained major aquifers used for local drinking water supplies.

High concentrations of MTBE at depth and within close proximity of water-supply wells rendered this site a high priority in terms of regulatory oversight. The investigation was complicated due to the complex nature of the contact between alluvium and the underlying formation, with respect to shape and permeability, in addition to potential ephemeral saturation of the alluvium.

We generated site conceptual models using various sources for local geologic conditions, including water well and oil well electric logs that were included in our graphics to illustrate potential contaminant pathways.

We carried out the investigation in phases to allow for evaluating new data at each step before proceeding with the next phase. Work at the site included researching geology and hydrogeology, drilling soil borings, installing vapor extraction wells, and generating site conceptual models at several scales. We also developed and obtained approval from the California Regional Water Quality Control Board (Regional Board) of a remedial action plan that included vapor extraction and thermal treatment, as well as groundwater monitoring, including monitoring of shallow groundwater within 100 feet of the usable aquifer at depths below 200 feet.

We operated the vapor extraction and thermal treatment unit until the levels of volatile organic compounds subsided. At that point, we changed to a carbon treatment system for economic reasons. After removal of over 10,000 pounds of gasoline and MTBE, the Regional Board approved rebound testing, which we successfully completed. Following the rebound testing and verification soil sampling, we conducted one year of groundwater monitoring to confirm there had been no groundwater impact. We also performed a risk assessment and determined that the site met residential cleanup criteria. The Regional Board subsequently closed the site and it was developed for a higher use.

For more information about this project and others, please contact Lindmark Engineering.