ARCHIVED REPORTS_XR0008789 - Laserfiche WebLink

Home Browse Search

Former Beacon Station No 12419 January 29,2003 T, Stockton, California Groundwater Groundwater Monitoring Report Davis, California The samples were analyzed for total petroleum hydrocarbons as gasoline 40 (TPHg), the volatile aromatic compounds benzene, toluene, ethylbenzene and total xylenes (BTEX), and the fuel oxygenate methyl-t-butyl ether (MTBE) by Environmental Protection Agency (EPA) Method 8260B Groundwater Monitoring Results Depth-to-groundwater measurements were subtracted from the surveyed TOC elevations to calculate groundwater elevations Groundwater elevation data were used to construct a groundwater contour map as shown on Figure 2 The groundwater flow direction was variable across the site and generally northwest in the underground storage tank area Groundwater gradients ranged from approximately 0 002 to less than 0 001 Grounc water analytical results are summarized in Table 1 Benzene concentration data from this sampling event were used to construct a benzene isoconcentration map as shown on Figure 3 Copies of the laboratory data sheets and chain-of-custody documents are included as Attachment C Recommendations Horizon recommends continued quarterly monitoring be performed to evaluate trends in the groundwater gradient and concentrations of dissolved hydrocarbons over time The Problem Assessment Report / Corrective Action Plan (PAR/CAP) for the site submitted in March 2002 has been approved by the San Joaquin Country Environmental Health Department (SJCEHD) Horizon has submitted a Remedial Action Plan to the SJCEHD which describes the proposed remedial system installations At the request of Ultramar, Horizon proposes to modify the quarterly monitoring program The proposed modified monitoring program will continue to provide adequate data to monitor the plume at the site and improve cost effectiveness of the monitoring program Results of previous quarterly monitoring events have confirmed or ruled out the presence of the petroleum hydrocarbons analytes in each monitoring well Annual analyses for the full complement of analytes is adequate to monitor their occurrence The groundwater monitoring frequency may be modified to consider site requirements depending on the investigative or remedial status of the site Groundwater samples from future well installations will initially be evaluated for all site analytes to establish baseline concentrations for those particular wells It is proposed that we decrease the sampling frequency of monitoring well MW-7,MW-11, air sparge well AS-2 and AS-3 to a semi-annual (first and third quarters) schedule, and monitoring wells MW-1 through MW-5 and air sparge AS-1 to an annual schedule (first quarter) The remaining monitoring wells MW-6 and MW-8 through MW-10 shall continue to be sampled on a quarterly schedule The proposed quarterly monitoring schedule would take effect starting the first quarter of 2003 419gmr02Q4 doc Project No 1419 4i 2 HORIZON ENVIRONMENTAL INC