SITE INFORMATION AND CORRESPONDENCE_FILE 1

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Wendy Cohen,SLIC Unit -4- 1 February 19% chemist., May Hoe, who advised that filtration for samples containing oily constituents is problematic, since oily constituents tend to partition onto the solid phase of the filter, causing clogging and a negative bias in analytical results. The large impact of filtration on TRPH results is consistent with filter clogging caused by solid-phase partitioning. She recommended that sediment removal from oily samples always be accomplished by centrifugation rather than filtration. Solid-phase partitioning and negative bias can also be a problem with silica gel cleanup of oily samples, if care is not taken to minimize this effect. To more clearly demonstrate the contribution of biogenic hydrocarbons, May recommended GC-MS fingerprint analysis, rather than reliance on silica gel cleanup. My Conclusions Given the above water quality criteria and data limitations, the following conclusions can be drawn from the recent 3 to 4 quarter monitoring program: 1) Monitoring wells MW-13 and MW-23 have shown significant concentrations of diesel-range. and gasoline-range hydrocarbons (TEPH and TPPH,respectively). Other constituents are below beneficial use-protective thresholds. Concentrations of diesel-range hydrocarbons have increased with time in both of these wells. 2) Cross gradient wells from MW-13 (MW 14,MW-15, and MW-20) did not show significant water quality impacts. Downgradient wells from MW-13 (MW-16,MW 17, MW-18, and MW-19) were not sampled during the 1994/1995 monitoring period. However,MW-17 did show significant gasoline-range hydrocarbons (TPPH) in an August 1992 sample. Therefore, these data do not demonstrate plume stability in the area of MW-13. 3) Downgradient wells from MW-23 (MW-4A,MW-5, and MW-6) do not show significant waiter quality impacts, although higher analytical quantitation limits could have masked these impacts. 4) Filtration and centrifugation showed little change in diesel-range and gasoline range by drocarbon concentrations, respectively. Therefore, it appears that suspended sediment is not a significant contributor to these analytical results. Limited reductions in diesel-range concentrations may be due to solid-phase partitioning onto the filter. 5) Silica gel cleanup with or without filtration produced diesel-range hydrocarbon results in excess of both toxicity- and organoleptic-based criteria. While it does appear that biodegradation of petroleum hydrocarbons is occurring at this site, silica gel results still show significant water quality impacts from diesel-range hydrocarbons at the MW 13 and MW-23 locations. A portion of the concentration reductions shown by these two procedures may be the result of solid-phase partitioning. 6) Monitoring well MW-24 is directly adjacent to impacted well MW-23. However, MW-24 does not show measurable hydrocarbons during the past four monitoring periods. Sampling of MW-24 in 1992 showed significant diesel-range and gasoline-range hydrocarbon