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't May 4, 1995 File No. 401.331 Ms. Wendy L. Cohen California Regional Water Quality Control Board Central Valley Region 3443 Routier Road, Suite A Sacramento, CA 95827-3000 Re: Supplement to Workplan, PG&E's Lathrop Gas Dehydrator Station Dear Ms. Cohen: The purpose of this letter is to present revisions and clarifications to the report entitled Workplan for Groundwater Investigation, Lathrop Gas Dehydrator Station, San Joaquin County, California as requested in a letter from your office, dated April 10, 1995. In the April 10th, 1995 letter from your office, the following issues were raised for revision and/or clarification: 1. Soil coring and sampling program protocols. In response to your request for continuous coring of the boreholes, all borings will be cored continuously from the ground surface to the total depth of each borehole using a 5-foot long core barrel which collects a relatively undisturbed sample as the hollow-stem augers are advanced. Samples from each 5-ft core collected from both the unsaturated and saturated zones will be screened in the field using a photoior ization detector(PID)at intervals from one to two feet to help determine the presence/absence and distribution of hydrocarbons in the soil column. The PID field measurements may be supplemented with results obtained from the Hanby TPH field test kit which is effective on wet soils. At the request of the Regional Board, soils in the saturated zone will be continuously cored and field tested to a minimum depth of 15 feet below the water table, unless an uncontaminated, substantial clay layer is identified at a shallower depth. The saturated soils will be cored to greater depths if the aquifer material at a depth of 15 feet below the water table appears to indicate the presence of petroleum hydrocarbons based on the field screening results. A soil sample(s)will be collected from each 5-ft core for subsequent chemical laboratory analyses by driving a brass tube into the core at the interval which yielded the highest PID readings and at significant lithologic changes. As stated on page 44 of the workplan, the results of all field tests, along with the lithologic descriptions of the subsurface soils, mtiill be recorded on the boring log form by the field geologist.