SITE INFORMATION AND CORRESPONDENCE

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Comment: 4. Accurate water levels cannot be obtained from monitoring wells within the floating product plume. Immiscible, lighter than water liquids actually float on the capillary zone, not the free water table. Wells screened across the floating product zone will fill with product, displacing the water level inside the well downward to a degree dependent upon the thickness of the capillary zone. The end results are a thicker than reality product zone and a lower than reality water level in monitoring wells. To further confuse the situation, the water table is, in reality, depressed relative to the local water table, due to the floating product. These two factors prevent computation of "corrected" water levels, based on density differences. Accordingly, on-site gradients must be determined by a minimum of three monitoring wells outside the area of floating product. Response: 4. Based on the results of the soil vapor survey, it is anticipated that observation wells OW-1, OW-3 and OW-4 as shown on Figure 1, will be located beyond any floating product plume that may exist. Comment: 5. Sample analyses can be reduced to the indicator parameters BTX and total fuel hydrocarbons in soils (EPA Methods 8020 and 8015 modified) , and volatile aromatic and chlorinated hydrocarbons in water (EPA Methods 601 and 602) . Selected samples should be analyzed for the entire suite proposed for confirmation of all constituents. Response: 5. Analysis for ethylene dibromide by EPA Methods 8010 (soil ) and 601 (water) were proposed in the work plan. It was not intended that analyses for other chlorinated hydrocarbons be performed. In accordance with the draft LUFT (Leaking Underground Fuel Tank) Field Manual issued by the California State Water Resources Control Board (July 1987), we propose to omit EDB from our proposed list of analytes. The manual states that "EDB has been so widely used in rural areas that its detection may not be due to a gasoline leak. When it has been found affiliated with a gasoline leak, its levels often have been so low as to be of questionable value". Therefore, PGandE proposes the following analyses: Soil : BTX (EPA Method 8020) Total Fuel Hydrocarbons (DHS Procedure) Total Lead (selected samples, EPA Method 7421) Water: BTX (EPA Method 602) Total Fuel Hydrocarbons (DHS Procedure) Total Lead (EPA Method 7421)