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the permeability, and the pore-water pressure all change in response to the passage of the <br /> water through the overburden. As a result, while there is some seasonal pressure change in the <br /> shallowest piezometer,there is none at depth.Theattenuation is also reflected in the very low <br /> variability of the Carlton Tunnel flow since 2002, by which time a substantial portion of the <br /> diatremal surface was covered with overburden (Plate 1). It is also observable in nature in the <br /> piezometric monitoring, where the shallowest piezometer generally shows some variability, <br /> while deeper piezometers do not (see for example the results in the shallow piezometer at <br /> WXGW-04, shown Plate 3). <br /> 4. Infiltration through soil underlying the OSA. The constant flow of water from the base of the <br /> OSA then passes vertically into the material at the base of the OSA.In general,there is some soil <br /> and/or colluvium at this horizon, although the soil is normally removed and stockpiled for later <br /> reclamation use prior to placement of the overburden. The soil and colluvium have average <br /> saturated hydraulic conductivities of 10-5 cm/s (soils) and 6 x 10-5 cm/sec (colluvium)where <br /> measured (Plate 4), which is significantly more than the 5 x 10-'cm/sec hydraulic conductivity <br /> needed to conduct the steady 6" of infiltrating water. <br /> 5. Seepage through the Diatreme. The water exiting the basal soil/colluvium horizon will proceed <br /> by vertical unsaturated flow into the diatremal rockmass.This will occur as a result of the <br /> hydraulic conductivity of the Tertiary volcanic rocks in the diatreme having an average hydraulic <br /> conductivity of 1.6 x 10-5 cm/sec, 30 times higher than the 5 x 10-'cm/sec hydraulic conductivity <br /> necessary to transport the 6" of seepage vertically through the diatreme. <br /> 6. Lateral flow to Carlton Tunnel. The infiltrating water will proceed vertically through the <br /> unsaturated diatremal rock until it reaches the natural water table, which is expected to be in <br /> the range of elevation 7,000 ft msl to 7,200 ft msl, based on water level observations made in <br /> deep drilling in the district and the elevation of the Carlton Tunnel (7,000 ft msl).At that point <br /> the water will move in the natural saturated groundwater flow system in the diatreme, towards <br /> the Carlton Tunnel and the mine workings connected to it.This will be the first lateral flow that <br /> the infiltrating water will undergo since entering the ground at the OSA surface. <br /> 7. Flow through Carlton Tunnel to Permitted Discharge. Finally,the OSA seepage mixes with all <br /> other infiltration on and near the diatreme, and flows through the Carlton Tunnel to the <br /> sediment control ponds at the tunnel portal, and then discharges under permit to Four Mile <br /> Creek. <br /> This flow and transport system for infiltration through OSAs has been verified as follows: <br /> 1. Observation. No seepage water has been observed issuing from the toe of any CC&V OSA during <br /> the period of operation to date, and there is no baseflow in the streams in which the OSAs are <br /> located. <br /> 2. Modeling. A detailed regional model of the system has been constructed using the topography <br /> and geology of the district and observed surface infiltration rates, and was calibrated against the <br /> observed stream baseflow and three dimensional head conditions measured at the site11. It <br /> confirms that all seepage from OSAs (and from overburden stored in backfilled mines) enters <br /> 11Hydrogeochemistry Report,Attachment 3. <br /> Adrian Brown Consultants, Inc. Page 6 of 8 <br />