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�.. /99nEE <br /> GEOMATRIX <br /> Mr. Don O. Culbertson <br /> Chevron Pipe Line Company <br /> 8 March 1995 <br /> Page 6 <br /> materials). The alkanes are not measurably water soluble, some of the oxidized <br /> hydrocarbons may be soluble. <br /> How does non-dissolved material get into the water sample? <br /> Non-dissolved constituents are incorporated into water samples via two primary <br /> avenues. The most obvious is when a bailer or other sampling device passes <br /> through a sheen on top of the water column during sampling. The bailer may pick <br /> up separate-phase PHCs that are then extracted by the laboratory into the sample <br /> aliquot as pure product, not dissolved material. Another avenue is that non- <br /> dissolved PHCs could be sorbed onto solid particulates suspended in the water <br /> column inside the well. This is a particularly prevalent problem when collecting <br /> samples from wells screened across PHC-affected soil (such as MW-23 at <br /> Dividend). During purging and sampling activities, some amount of silt to clay-size <br /> material (and sometimes emulsified product) is drawn into the well and entrained or <br /> suspended within the "water" samples. As shown by our examples (Figures 4a and <br /> 5), non-dissolved PHCs entrained in the sample will be extracted along with the <br /> water at the laboratory when using standard analytical procedures. Consequently, <br /> the "TPH" or "TRPH" result for the groundwater sample will include these non- <br /> dissolved constituents. Samples affected in this manner do not provide an accurate <br /> assessment of dissolved-phase concentrations of PHCs in groundwater. <br /> How can we address this problem and obtain analytical results that better reflect <br /> dissolved-phase PHC concentrations? <br /> Our experience suggests that TPH by EPA Method 8015 and TRPH by EPA <br /> Method 418.1 are problematic for groundwater samples because the results may be <br /> difficult to interpret, are subject to multiple positive interferences, and typically <br /> yield poor information about dissolved PHC constituents on which to base <br /> regulatory decisions. Rather, constituent-specific analyses using EPA Method 8020 <br /> and/or EPA Method 8240/8270/8310 are far superior in assessing the presence and <br /> concentration of dissolved-phase PHC constituents that are of regulatory concern in <br /> groundwater. Geomatrix is currently working with the petroleum task force of <br /> RWQCB-SF to develop an analytical suite for groundwater samples that adequately <br /> evaluates water quality with the use of EPA Method 8015 or Method 418.1. <br /> Recommendation for groundwater sampling and analysis at Dividend: <br /> We are currently required to analyze groundwater samples for TPH by EPA Method <br /> 8015 and TRPH by EPA Method 418.1, along with BTEX (EPA Method 8020), and <br /> PNAs (EPA Method 8310). We recommend that we mitigate the effects of non- <br /> dissolved constituents influencing the groundwater analytical results by field filtering <br /> (0.45 micron filter) selected samples collected for TPH and TRPH analysis. We <br /> propose that we collect filtered and unfiltered sample pairs to be analyzed for TPH <br />