Laserfiche WebLink
Cresson Pro'ect H dro • eochemist AdrianBrown <br />precipitation events on the surface. Since the substantial modification of the surface that occurred in the <br />early 2000s as a result of surface mining and overburden storage (Plate 8), a significant proportion of the <br />infiltrating water passes through generally disturbed areas, lengthening and attenuating the precipitation <br />peaks by seepage through the surface materials. <br />2.4.2 Infiltration <br />The rate of infiltration of precipitation in the District prior to 2002 represented a seepage rate of 5.9 <br />inches per year, or 30% of the average precipitation at that time of 19.47 inches per year. This is a <br />relatively high infiltration rate; the expected value would be no more than 25 %, based on infiltration <br />evaluations of mountain basins in Nevada (Maxey and Eakin, 1949). <br />The reason for the high infiltration rate is the reduction of the natural water table in the Diatreme by the <br />drainage tunnels. This results in depressurization of the diatremal rockmass down to the elevation of the <br />Carlton Tunnel (approximately elevation 7,000 ft msl, 3,500 feet below ground level). This creates a <br />high downward vertical hydraulic gradient in the diatremal rockmass, and in the adjacent Precambrian <br />rocks. <br />The infiltration behavior in the Diatreme and the Precambrian rockmass immediately adjacent to the <br />Diatreme has been established by installation and monitoring of a total of 23 nested well sets and 13 <br />piezometer strings in single boreholes. The piezometric pressure has been monitored continuously in <br />these wells for periods ranging from 1 year to 3 years. These data have been processed to produce <br />piezometric elevations, and then plotted to produce average vertical hydraulic gradients (Attachment 1). <br />Examples of this processing for two well nests completed in Tertiary volcanic rocks overlain by <br />alluvium are shown in Plate 9. These wells show vertical head conditions that cause essentially complete <br />capture of infiltrating ground water by the depressurized diatremal ground water system: <br />1. A strong downward vertical hydraulic gradient in the upper sections of the ground water system <br />(to a depth in excess of 750 feet at the WLMW -01 location, and to approximately 100 feet in the <br />GVPW -05 location) where the ground water system is depressurized and flow is dominantly <br />vertically downward. <br />2. A weak downward vertical hydraulic gradient in the lower sections of the ground water system <br />(below 750 feet at the WLMW -01 location and below 100 feet in the GVPW -05 location), where <br />the ground water system is pressurized, and flow becomes dominantly horizontal as the water <br />moves laterally towards collection by the Carlton Tunnel drainage system. <br />The downward vertical gradient results for all well clusters analyzed are summarized in Table 4, and <br />presented for the District in Plate 10. The following observations are made: <br />1. There is a strong downward vertical hydraulic gradient in all locations where the dominant <br />geology is Tertiary volcanic phonolites and welded breccia (i.e. within the Diatreme). <br />2. There is a strong downward vertical hydraulic gradient in areas of the Precambrian rocks close to <br />the Diatreme where the slope of the interface is steep (e.g. in upper Grassy Valley). <br />1385L.20120125 8 <br />