SITE INFORMATION AND CORRESPONDENCE

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Mr. Gordon Boggs • December 11, 1996 Page ? many cases more conservative, estimate of petroleum hydrocarbon concentrations in ground water. In a small population of wells/sites, an especially large discrepancy between no-purge and post-purge concentrations was found. This effect appears to be due to site-specific conditions/mechanisms, is reproducible, and such wells/sites can be identified using the sampling methodology presented in this Study. The second publication, titled "Ground Water Sampling - A Pilot Study of the Effects of Well Purging," was prepared by Mr. Ken Williams (Regional Water Quality Control Board-Santa Ana Region), Mr. Anthony Martinez (Orange County Health Care Agency (OCHCA}), Mr. Seth Daugherty (OCHCA) and Mr. Paul Lundegard (Unocal Corporation). The publication was presented in November 1996 at the 1996 Petroleum Hydrocarbons and Organic Chemicals in Ground Water Conference in Houston, Texas. This study analyzed data collected from 69 different sites and included samples from approximately 164 wells. The conclusions of this publication include: ...Qualitatively, there appears to be no general trend for measured concentrations to be greater following purging. To the contrary, the mean value of all constituents measured was greater for the samples collected prior to well purging. Paired-sample statistical tests provide strong evidence that the concentrations of these fuel constituents are, in fact, greater on average in samples collected prior to well purging. The conventional belief that the concentration of dissolved fuel constituents is systematically lowered in undisturbed well water as a result of volatilization or other processes appears unwarranted. Under most circumstances, ground water samples from unpurged monitoring wells should be adequate for tracking concentration of TPH and BTEX at contaminated UST sites, especially for the purposes of routine monitoring. On the basis of these data, sampling without purging would not only yield concentrations at least as high as with conventional sampling protocol, but it would have the added benefit of reducing the volume of contaminated purge water that needs to be treated and disposed.... Since both studies conclude that no-purge samples generally produce similar TPH and BTEX concentrations as compared to post-purge samples, ARCO will start obtaining only no-purge samples from all existing ground water monitoring wells completed in unconfined ground water aquifers. ARCO will continue obtaining post-purge samples for two sampling events after installation of each new ground water monitoring well and during the final sampling event prior to closure. The rationale for obtaining post-purge samples for two sampling events e:\h m d\an:n\m g i u n 5.d m