Project Description

The City of Calistoga’s Public Works Department established a maintenance yard in the southeast part of the small city in the late 1970’s including the installation of one gasoline underground storage tank (UST) and one diesel fuel UST.  The two USTs were decommissioned in January 1999.

Results from soil and groundwater samples obtained from the UST decommissioning indicated that gasoline constituents, including dissolved methyl tertiary-butyl ether (MTBE) and petroleum hydrocarbons (PHCs) had been released from the gasoline UST into the soil and groundwater.  Additional soil and groundwater testing indicated that dissolved MTBE and PHCs had migrated in groundwater beyond the tank excavation to the adjacent property and were threatening a nearby potable groundwater supply to a local beverage company.

A remedial action plan (RAP) involving groundwater pump and treat technologies and monitored natural attenuation (MNA) was designed and executed with regulatory agency approval to affect source zone remediation and containment/remediation of off-site migration of the dissolved MTBE and PHC contaminant plume.  The RAP included approximately nine groundwater extraction wells, 19 monitoring wells, and 14 multi-level monitoring wells into deep and shallow aquifers.

The treatment system included suspended solids filtration and granular activated carbon treatment of extracted groundwater which was effective in reducing contaminant concentrations to acceptably low levels allowing for the approved discharge of the closely monitored treated effluent to the nearby Napa River.