2008-09-17_REVISION - M1980244 (28)

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• PZ-4 1 41581 1 63707 1 10135 30 9 39 Qal Riparian area (alluvium Note. Qal = Alluvium, X = Precambrian granite, schist, and gneiss; T = tertiary volcanics Surface Water Surface water would be monitored in Grassy Valley at four locations, as shown in Table 3, and illustrated in Figure 4 included as Attachment 8. GV-04 is a new monitor station, located at approximately the point where Grassy Creek crosses the diatreme boundary. It is the downstream-most monitoring point. All monitoring locations would be recorded by transducers linked to flumes, and these data will be downloaded quarterly for analysis. Surface water quality would be monitored by quarterly sampling at any of the four monitoring points at which water is observed to be flowing. Samples will be analyzed for the standard suite of parameters for CC&V's current surface water monitoring program. Table 3 - Surface Water Monitoring of Gracev vallPv Station East (ft) North (ft) Device Monitoring Frequency GV-01 43132 62877 Parshall Flume Transducer Continuous GV-02 45666 61572 Parshall Flume Transducer Continuous GV-03 47048 59607 Parshall Flume Transducer Continuous GV-04 50031 59187 Parshall Flume Transducer Continuous The hydraulic behavior of the shallow alluvial aquifer in the riparian area of Grassy Valley has been studied in detail by monitoring of the water level in four piezometers installed in the • riparian area for a period of three years. The water level results of one of the piezometers are presented in Figure 5 included as Attachment 8. The water level in the alluvial aquifer at PZ-1 remained between 12 and 30 feet below ground surface in the three years that were monitored. The results of the other three piezometers were similar. It is clear from these results that the alluvial aquifer in Grassy Valley is not the hydrologic element that "feeds the wetlands" in the riparian areas. The riparian areas are in fact fed by incident precipitation and surface runoff to the riparian areas. The riparian areas grow in (and in fact form) a clayey organic loam, which holds the water that provides moisture for the plants during dry periods, and is of low enough permeability to prevent all the held water from draining through the alluvium into the underlying (unsaturated) bedrock. This process sustains the riparian vegetation. During periods of high incident moisture in the riparian areas, water recharges the surface organic loam of the riparian areas, infiltrates into the alluvial material beneath, and recharges the shallow alluvial aquifer. This is shown by the water level increases in the alluvial aquifer, typically in May (due to snowmelt and spring precipitation) and August (due to monsoonal thunderstorms). During the entire year water drains out of the alluvial aquifer, through the surficial weathered clayey bedrock, and into the underlying (unsaturated) lozv permeability bedrock, which is generally Precambrian granite and schist. The outflow is sufficient to drain the entire alluvial aquifer in most years at PZ-1 (Figure 5 included as Attachment 8). In May • and August the infiltration to the shallow alluvial aquifer exceeds the drainage to the bedrock, and the water level rises in the aquifer. ?n