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ECOSA Evaluation AdrianBrown <br />For this process to occur, three conditions are required at the base of the ECOSA: <br />1. Sufficient vertical permeability in the colluvium. For 6 inches per year of water to infiltrate <br />vertically under gravity through the colluvium, the colluvium needs a vertical hydraulic <br />conductivity of approximately 5x cm/sec or more. Three sets of measurements have been <br />made to check this: <br />a. The average hydraulic conductivity of the material excavated from the ECOSA footprint <br />is 1.1x10 " cm/s, after processing by removal of cobbles, remolding, moistening, and <br />compacting at optimum moisture content (Plate 9). Nearly 20% of these processed <br />samples exhibited hydraulic conductivity in excess of 5x10 cm/sec. In -situ hydraulic <br />conductivity is expected to be at least an order of magnitude higher. <br />b. A total of 22 in -situ hydraulic conductivity tests of colluvium were conducted as part of <br />the MLE -2 hydrology program, most of them within or close to the proposed footprint of <br />ECOSA. The results are presented in Plate 11. The geometric mean of the (horizontal) <br />hydraulic conductivity of the tests was 5.6x10 cm/s, two orders of magnitude higher <br />than the required hydraulic conductivity to allow infiltration, and the lowest value was <br />5x10 cm/sec, still high enough to allow infiltration. In -situ vertical hydraulic <br />conductivity is likely to be lower than these values, but the colluvium is expected to <br />conduct water vertically under gravitational drainage. <br />c. A total of 28 monitor wells have been completed in colluvium within or close to the <br />proposed footprint of ECOSA, and monitored for more than a year. Of these, 13 are dry, <br />and 6 more have only a few feet of water in the base of the screened interval of the well <br />(Plate 12). The only wells which show significant saturation in the colluvium all overly <br />Precambrian rocks, which suggests that even in those wells vertical flow is restricted by <br />the bedrock permeability, not by the colluvial permeability. <br />2. Sufficient vertical permeability in the bedrock beneath. For 6 inches per year of water to <br />infiltrate vertically under gravity through the bedrock, the bedrock also needs a vertical <br />hydraulic conductivity of approximately 5x10 cm/sec or more. A total of 48 tests of <br />(horizontal) hydraulic conductivity have been conducted in the bedrock in the vicinity of the <br />proposed footprint of the ECOSA. Of these tests, a total of 85% (41) exhibited hydraulic <br />conductivity values sufficient to allow vertical infiltration under gravity, and the geometric mean <br />hydraulic conductivity was 1x10 cm/s, more than a factor of 20 higher than is required to allow <br />vertical infiltration (Plate 11). In -situ vertical hydraulic conductivity would be expected to be <br />about the same as horizontal for Precambrian granite, schist, and gneiss; greater than horizontal <br />for the dominantly vertically jointed and faulted Tertiary volcanics. <br />3. Sufficient vertical downward hydraulic gradient in the bedrock beneath. For 6 inches per year of <br />water to infiltrate vertically under gravity through the bedrock, the bedrock needs to exhibit at <br />least some vertical downward hydraulic gradient. A total of 29 nests of monitor wells and <br />piezometer strings have been installed to depths up to 1,000 feet in the vicinity of the proposed <br />footprint of ECOSA to check the vertical head gradient, of which 5 are within the footprint. The <br />1385E.20120224 11 <br />