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water treatment facility be conducted. This evaluation will follow the guidelines within the MW-4 Investigation Workplan for Task 1 (see Appendix B). The data collected for the Harney Lane Sanitary Landfill will be compared to other sites that have proceeded with and completed field investigations of the horizontal and vertical extent of contamination. A preliminary review of the data from well -2 indicates that the PCE concentrations from four sampling events (0.8, 0.5, 0.7, and 0.6 µg/1) at the point of compliance are only slightly elevated above the MDL (0.5 µ ). In addition, a preliminary review of the groundwater flow velocities over the past 2 years indicates that a low velocity of approximately 91 feet per year is typical for the site, which would give rise to a relatively slow-moving plume. A more definitive review of the site-specific data will be compared to data at other VOC release sites in the Central Valley to bracket the potential extent of the plume. Extensive studies have been performed that indicate VOC concentrations decrease by orders of magnitude at unit distances from their source. For example, if the VOC concentration (e.g., PCE) is 100 µg/1 at the point of compliance, the concentration a unit distance away would be 10 µg/1, and at a second unit distance away would be 1 µg/l. The unit distance is variable between sites and is, for the most part, based on subsurface geologic conditions (permeability, gradient, soil organic content, porosity, etc.). No two sites are identical; however, by comparing Harney Lane Sanitary Landfill to other well studied sites, it may be possible to place a bracket on the extent of the release. A preliminary assessment might indicate that the areal extent of the plume would be limited. With a PCE concentration of less than 1 µg/1 at the point of compliance (well MW-2) and a groundwater velocity of less than 100 feet per year, the extent of the plume would likely be within 300 to 500 feet from the landfill. The first phase bracketing assessment described above will be refined and will be useful in determining future exploratory procedures if they are necessary. At this time, it is proposed that the second phase of assessment (risk assessment and/or field investigation) be instituted if one of the following conditions are present: • VOC concentrations are significantly high (exceeding MCLs) or are statistically increasing. • An assessment of geologic data indicates that the plume could be significantly beyond 500 feet. • Groundwater users within the potential areal extent of the plume are identified. SAC/pjM39\09390011.8a1-94/1fc:9 Rev.0, 10/17/94 0939-001.18 7