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GEOMATRiX 3.0 DISCUSSION—PETROLEUM HYDROCARBONS IN GROUNDWATER ' Concentrations of total petroleum hydrocarbons (TPH) dissolved in groundwater typically are derived from laboratory analysis using a modified version of EPA Method 8015 described in the California Department of Health Services' Leaking Underground Fuel Tank Manual The TPH-D concentrations ' reported for the samples described above were derived using this method However, extraneous material may interfere with this analysis and result in reported concentrations of TPH that overstate the true ' amount of petroleum hydrocarbon dissolved in groundwater Concentrations of dissolved petroleum hydrocarbon in groundwater may be overstated due to interference ' from (1) non-dissolved petroleum hydrocarbons that are adsorbed onto particulate matter that is entrained in the sample,(2) separate-phase petroleum hydrocarbon that is entrained in the sample, and/or ' (3) dissolved and non-dissolved by-products (non-petroleum hydrocarbons) of biodegradation of petro- leum hydrocarbons that are present in the groundwater These materials are extracted along with the dissolved petroleum hydrocarbon during laboratory analysis by modified EPA Method 18015 and, if tpresent, consequently are included in the reported dissolved TPH concentration As a result, the true amount of dissolved petroleum hydrocarbon in groundwater may be overstated if these materials are ' present In order to evaluate if one or all of these interferences are affecting TPH concentrations, accepted methods can be used that remove these materials prior to analysis of the sample rFiltering will remove particulate matter with adsorbed petroleum hydrocarbon and separate-phase petro- leum hydrocarbon that is entrained in the sample A silica gel cleanup performed by the laboratory will ' remove 90 to 95 percent of the non-petroleum hydrocarbon by-products of biodegradation that may be present in a sample (Bruya, 1995) The results of the sample analysis following filtering and/or silica ' gel cleanup are then compared to the non-filtered sample and the filtered sample without silica gel clean- up Based on this comparison, an evaluation can be made with respect to the relative contribution of non-dissolved petroleum hydrocarbon and non-dissolved by-products to the analytical result expressed ' as TPH ' 4.0 GROUNDWATER MONITORING Groundwater monitoring was performed at the site by Geomatnx on 12 May 1995 using the three ' existing monitoring wells (Figure 2) Prior to collecting groundwater samples, the water level was measured, and at least four casing volumes of water were removed from each well Field parameters ' (electrical conductivity, pH, and temperature) and the clarity of the water were monitored during purging Purging and groundwater sampling were performed using an electrical submersible pump Groundwater samples were placed in laboratory-prepared containers, placed in an ice-cooled chest, and ' transported under chain-of-custody procedures to the laboratory for chemical analysis Selected samples were filtered in the field using an in-line, positive-pressure filtration device with a 0 45 micron filter 2