SITE INFORMATION AND CORRESPONDENCE FILE 1

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considered given the degraded condition of the contaminants and proximity to a large in-use irrigation pipeline. Given the degraded BTEX volatile component chemistry of the dissolved plume, Wright proposed that natural, biologic degradation .roemes would continue to decompose the gasoline. Biodegradation Approach/Discussion Summary ( To address the biologic degradation and at the suggestion of the PHS EHD representative, WRIGHT performed monitoring for biologic parameters to estimate biologic activity (i. e.,dissolved oxygen, pH, ferrous iron, sulfate and nitrate; Wiedemeier and others, 1995). This established a baseline to estimate previous biologic degradation activity inferred and gauge selected future biologic monitoring parameters for the site monitoring program (see Table 1 Soil Microbiologic Data). The most elevated TPHG and BTEX concentrations occurred at PB-8/MW-9, roughly in the center of the site. Again, these concentrations indicate a highly degraded volatile components in the gasoline product, with ? Tv4 concentrations declining one order of magnitude north between P13-8 and PB-7. Chemical analysis of.TOC indicates Q that roughly 0.049%of TOC occurs in the aquifer,some of which may be related to hydrocarbons since the TOC test C does not discriminate between indigenous carbon and possible hydrocarbon input. The low TOC value is consistent with young (10,000-100,000 years before present) semi-arid alluvial deposits and environments which contain only traces of organic carbon due to geologic degradation processes (Hunt, 1972 pp. 278-279). A calculation for a retardation factor is not considered feasible for this data since 0.049% TOC falls below 3% TOC (total in soil) threshold value considemd necessary for valid retardation computations (Dragon, 1988, pp. 242-251). These data were considered in the ongoing groundwater monitoring program in that TOC influence was,insignificant and contaminants were absorbed onto soil which tends to immobilize them. Outcome _ Wright considered using oxygen carrier to aid in-situ degradation of the fuel (by biostimulation of an oxygen sock f approach at MW-9), pending the approval of PHS-EHD. However the PHS-EHD representative did not approve; WRIGHT to move forward with this approach, and directed monitoring to continue pending a possible site closure following additional PHS-EHD review. Rationale for Not Recommending Further Cleanup at Well MW-9 I Review of Technical Approach Site cleanup technical approaches have always included the area of MW-9. A relatively slow groundwater movement is indicated from the groundwater monitoring and reconnaissance data. Overall site groundwater movement is consistently northerly. A low gradient, together with the clayey sand aquifer strata, appear to have limited migration of the plume. Some northeasterly flow movement at PB-7 may be due to a coarsening of sand which is surmised to have locally increased hydraulic conductivity. On the basis of this work, the plume has moved! j a total of about 275 feet north of the source and was not detected roughly 300 feet north(at MW-4): 1 Capillary Fringe Contaminants Excavation in the tank area has removed the contaminated soil, and excavation wall samples laboratory data indicate that only residual concentrations remain, mostly at the capillary fringe. Soil data from the capillary fringe sample collected for the installation of MW-9 revealed residual contaminants at that location (see Table 1), and dissolved contaminant trends have been routinely reported (see Table 2). Previous subsurface investigations have shown that there are TPHG contaminants have extended slightly onto the Tracy High School property, but the BTEX components were very low. However, groundwater sampled from well MW-4 showed that contaminants were not detected, indicating that contaminants are not mobile. However, rising and falling groundwater levels may cause desorbtion into groundwater causing limited "spikes" of elevated concentrations as observed in on-site wells, including MW-9 (see Table 2 Historic Groundwater Data). If these spikes were mobile, they should have been observed at MW-4. The net movement of these contaminant ends in the vicinity of the Tracy High property line. Contaminant Plume Configuration This plume configuration and length is, in our opinion typical for petroleum fuel dissolved plumes in shallow alluvial deposits. Sieve analysis and hydrometer data indicate that fine to very fine sand composes the aquifer. The hydraulic conductivity estimate for this material may range into the 0.001 to 0.00001.centimeter per second range, supporting this interpretation (Fetter, 1988). This infers relatively tow flow velocities. Data from PB-9 shows that contaminants were not detected on a northwesterly flowline direction from the former tank pit. Historic groundwater Mr Ed's Muffler Project No. 4090-P Page 3