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Cresson Project Hvdroaeochemistry AdrianBrown <br />3. There is weak to no downward vertical hydraulic gradient in locations underlain by Precambrian <br />rocks that are distant from the Diatreme, or are adjacent to locations where the interface between <br />the slope of the interface is shallow (e.g. in lower Grassy Valley). <br />In all areas where there is moderate to strong downward vertical gradient, ground water flow is <br />predominantly vertically downward, and all infiltrating water is captured by the diatremal flow system <br />and flows via the regional ground water system to Carlton Tunnel and flows under permit to Fourmile <br />Creek. The area where infiltration capture occurs is shown on Plate 10. <br />2.4.3 Hydraulic Conductivity <br />The ground water flow system in the District is controlled by the hydraulic conductivity of the soil and <br />rockmass. The hydraulic conductivity of the District materials has been evaluated by the performance of <br />a total of 75 permeability tests in all available completed boreholes and monitor wells in the District <br />(Attachment 2). <br />The hydraulic conductivities of the wells tested in the District are presented by location and depth on <br />Plate 11, and by geologic material on Plate 12. The following observations are made with respect to the <br />hydraulic conductivity of District materials: <br />1. Soil. The "soil" refers to four tests in shallow wells located in the thalweg of Grassy Creek. The <br />(horizontal) hydraulic conductivity is approximately 10 -5 cm/sec, which is typical for soils. The <br />logs of the valley soils indicate that they are strongly layered, so the vertical hydraulic <br />conductivity of these materials would be expected to be approximately an order of magnitude <br />lower, or 10 " cm/sec. This is sufficiently low to significantly retard water seepage through these <br />materials to the underlying bedrock. <br />Soil horizontal hydraulic conductivity: 10 cm/sec <br />Soil vertical hydraulic conductivity: 10 cm/sec <br />2. Colluvium. The material described as "colluvium" ranges from boulder clay to sand and gravel <br />outwash, and the measured horizontal hydraulic conductivity range reflects this, ranging from <br />10 " cm/sec to 10 " cm/sec. The geometric mean of the hydraulic conductivity of the colluvium is <br />6x10 cm/sec. The vertical hydraulic conductivity of the material is likely significantly lower, <br />due to the presence in almost all locations of horizontally bedded clayey zones. This material is <br />being mined and processed in Grassy Valley to provide low- permeability bedding material for <br />VLF liner system construction, with a target hydraulic conductivity of less than 10 cm/sec, <br />which is reflective of the clayey zones in the in -place colluvium. <br />Colluvium horizontal hydraulic conductivity: <br />Colluvium vertical hydraulic conductivity: <br />10-5 cm/sec <br />10 -6 cm /sec <br />3. Tertiary. The Tertiary age materials comprise volcanic phonolites, welded breccias, and <br />occasional lamprophyre dikes and sills (Plate 3). The measured horizontal hydraulic conductivity <br />of this material has a geometric mean of 1.6x10 " cm/sec, with values ranging from 5x10 <br />cm/sec to 8x 10 cm/sec. This high variability reflects the fact that the hydraulic conductivity in <br />the material is largely secondary (derived from structures including joints, fractures, and faults). <br />1385L.20120125 9 <br />