SITE INFORMATION AND CORRESPONDENCE

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• 0 Bert E. Van Voris - 17 • 17 September 2004 Supervising Engineer velocity decreases once it reaches the low areas. Deeper groundwater flows northwest following the regional gradient. The RWD assumes that only the IDS portion of the discharge has the potential to impact groundwater at the site. The RWD estimates lateral groundwater velocity is about 73 feet per year, a value from Musco's recently submitted report on background groundwater quality. The estimated lateral groundwater velocity, according to data presented in this report,22 actually ranges from 73 to 393 feet per year. The RWD interprets the lateral velocity of 73 feet per year as `slow' and the residence time in localized areas as `long.' "In other words, constituents in groundwater are relatively immobile."23 The RWD presents a qualitative analysis of temporal and spatial trends for the groundwater quality data for individual wells in the shallow and intermediate zones. It identifies trends (increasing or decreasing concentrations) for selected constituents and assigns the trends one of three explanations: (1) occurrence of constituents in upgradient ambient groundwater conditions, (2) occurrence of constituents in localized and cross-gradient conditions, and (3) influences on shallow groundwater of recent site activity. The RWD calculates the average concentration data through time from various grouped wells to characterize ambient groundwater quality. For example, it uses MW-1 and MW-15 to characterize the upland ambient groundwater quality. Additionally, it uses MW-13, MW-14, MW-16, and MW-3, and MW-5 to characterize ambient quality of groundwater underlying the mid-gradient drainage swale area. The RWD indicates that the TDS levels in upland groundwater are lower than levels in groundwater underlying the drainage swale, and attributes the elevated concentrations to "accumulation of naturally occurring inorganic dissolved solids in groundwater in this area"24 due to essentially natural phenomena (e.g.,higher influx of groundwater, longer residence time once groundwater reaches the lower areas). The RWD notes the steady concentrations of TDS and sodium in MW-3 and MW-13. Regarding MW-16 downslope of the reservoir, the RWD explains that the increased concentration of TDS and sodium in groundwater passing through this well "cannot be attributed to seepage through the bottom of the reservoirs25 because of the tight permeability associated with native clay soil under the reservoir. The RWD cites the estimated permeability of these soils as 10-6 to 10-7 cm/sec. Comment: The RWD's use of average concentrations for various constituents to characterize groundwater quality is problematic, as it obscures the manner in which the discharge has impacted groundwater quality through time. The RWD should present an overall discharge history of the site—that is, when discharge was initiated at each field. This is particularly relevant for interpreting groundwater data from wells within and downgradient of historic discharge areas (e.g., the eucalyptus grove). For some wells (e.g., MW-3 and MW-13), the RWD appears to attribute steady- state conditions to the lack of discharge impact, whereas these conditions may mean past discharges have impacted groundwater in these wells and the impact itself is ongoing. Data from MW-15, near the reservoir, suggests there is little if any connectivity with water intercepted by this well and groundwater influenced by the discharge or storage of wastewater. However, data from MW-1 shows increasing concentrations of sodium and chloride since its installation in Spring 2002. Data trends for this well are similar in many respects to W-2, a shallow monitoring well situated at the bottom of the drainage that transects the site, as shown below: