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I' quarterly data is needed to verify the temporal extent of this gradient directional change. Further analysis of the groundwater contours reveals that groundwater may be slightly mounded near the AS/SVE system. Vertical Groundwater Movement Comparison of the groundwater elevation measurements in well pairs MW-DIS-7S/7D and MW-DIS-8S/8D reveal a 0.012 ft/ft vertical gradient. Previous measurements at the site have shown a vertical gradient of between 0.006 and 0.02 ft/ft. Vertical migration is expected to be at least an order of magnitude less than the horizontal migration, despite the proportion of vertical to horizontal gradients. The slowness of vertical migration can be attributed-to the typical settling properties of irregular colloidal materials. Pore spacing and flow pathways preferentially align in the horizontal plane. Analytical Results Analyses of groundwater samples from monitoring wells are presented in Table 3-3. Gasoline I constituent concentrations show a favorable declining trend over the period of record (1991 - 1995). The following points describe how the declining trends noted.at the site exhibit i evidence of natural microbial degradation. • Gasoline constituents are preferentially degraded in a natural in-situ groundwater setting. The most readily degradable constituent under anoxic conditions is toluene, followed by the xylene species. Benzene and ethyl benzene are 'typically consumed more slowly. However, BTEX concentrations approaching solubility limits can be toxic to in-situ microbes. } • MW-DIS-2 groundwater has lower BTEX constituent concentrations than in MW-DIS-1, where a floating NAPL layer has occasionally been observed. Toluene concentrations have decreased in MW-DIS-2 by two orders of magnitude, xylenes have decreased by one order of magnitude, and the l ; concentration of benzene is unchanged over time. • Historically, there is little evidence of constituent concentration reduction in MW-DIS-1 until the AS/SVE system started up in February 1995. MW-DIS-1 is in the heart of the plume, where concentrations are greatest. Concentrations in MW-DIS-I are at the solubility limit. • Areas of a plume with the highest concentrations are typically in a state of negative oxidation-reduction potential, meaning they are electron-acceptor limited. • The initial contamination in MW-DIS-3,likely to have come from cross I contamination during well construction,declined'to non-detect levels over a ., `' l SFO10030982.DOC 3-21 i