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Ms. Anne P. Conner 2 10/9/2007 Pacific Gas&Electric Company WORK PLAN FOR SOIL AND GROUNDWATER CHARACTERIZATION UNDER POND 3 PG&E MCDONALD ISLAND GAS STORAGE FACILITY MCDONALD ISLAND, CALIFORNIA 1.0 INTRODUCTION This Work Plan for Soil and Groundwater Characterization under Pond 3 (work plan) has been prepared by MSE Group (MSE) on behalf of Pacific Gas and Electric Company (PG&E) for the PG&E McDonald Island Gas Storage Facility (Facility) near Holt in San Joaquin County, California, approximately eight miles west of Stockton, California (Figure 1-1). This work activity will be conducted to supplement Regional Water Quality Control Board (RWQCB) Waste Discharge Requirements Order (WDR) No. 96-176 (RWQCB, 1996). WDR 96-176 was issued after the closure of three Class II surface impoundments located at the site, one of which (Pond 3) was historically utilized for the disposal of drilling fluids and produced formation fluids generated from the installation of underground gas storage facilities at McDonald Island. 1.1 Background The three surface impoundments were constructed in 1974 for the disposal of material associated with natural gas well drilling and production byproducts at the Facility. The closed impoundments are located adjacent to Whiskey Slough and were originally constructed as agricultural silage pits. They are referred to as Pond 1, Pond 2 and Pond 3. From 1974 to 1986 the majority of well-drilling muds, well completion fluids, natural gas byproducts, and industrial wastewater from production platforms were disposed in Pond 3 (MSE, 2007a). Drilling muds and well production fluids were not disposed in Ponds 1 and 2. Discharge to the ponds, specifically Pond 3, was terminated in 1986, and all production fluids and drilling muds were subsequently disposed offsite (PG&E, 2006). Drilling fluids or muds are made up of a base fluid (water, diesel or mineral oil, or a synthetic compound), weighting agents (most frequently barium sulfate [barite] is used), bentonite clay (to help remove cuttings from the boreholes and to form a filter cake on the walls of the hole), lignosulfonates and lignites (to keep the mud in a fluid state), and various additives that serve specific functions. At present, the exact type of drilling mud disposed in Pond 3 has not been established; however, based on early post-closure sampling results, barite is considered to have been a significant component of the drilling mud. In 1994 closure activities were conducted for the three surface impoundments. These activities included the removal and offsite disposal of approximately 20,000 cubic yards of non-hazardous drilling mud and native soil from within, and beneath, Pond 3. Soil sampling beneath Ponds 1 and 2 verified that no chemicals exceeded background levels; therefore no excavation was necessary. Neither soil nor groundwater sampling was