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G ound Water Flow Recharge and Aquifer Characteristics. The recharge, discharge and <br /> movement of groundwater in the alluvial aquifer is controlled by 1) the gradient of the <br /> rock channel into which the alluvium has been deposited, 2) the width of the alluvial <br /> deposit, 3) the thickness of the alluvium, 9) the permeability of the alluvium, 5) the <br /> recharge or discharge of underlying rock strata, and 6) the flow characteristics of the <br /> associated stream (Dry Creek). <br />In the region, only very limited ground water yields have been obtained from the Lewis <br />shale. The unit is predominantly impermeable and acts as a confining layer to any <br />underlying sandstone units. Limitetl amounts of recharge to the Lewis shale occur from <br />direct precipitation on portions of the exposed unit and from discharge from the saturated <br />portion of the alluvium to those portions of the shale in contact with the alluvium. <br />Recharge to the alluvial aquifer occurs primarily in the spring and early summer when <br />snowmelt and surface water runoff is abundant and ET is minimal. Aa noted above, <br />potentiometric surface data indicate groundwater flow in the alluvial aquifer in the <br />vicinity of the Loadout is to the north and northeast. <br />. Aquifer characteristics of the alluvium and Lewis ahale can be inferred from the <br />lithologic properties of the two units, responses to sampling stresses and aquifer tests <br />conducted upgradient from and below the Loadout in the Dry Creek drainage. The shallow <br />Lewis shale is relatively impermeable (acts as an aquitard) and would be expected to have <br />hydraulic conductivities in the range of 10-6 to 10-g ft./day. The alluvial aquifer has a <br />considerable percentage of clay and silt size particles and the different grain sizes are <br />poorly sorted. During sampling in December, 1991, well HGDALI with a saturated bore <br />volume of four gallons was bailed dry with the removal of only 9.5 gallons. Well HGDAL2 <br />had a saturated bore volume of 11.2 gallons and was bailed dry with the removal of 22 <br />gallons of water. An aquifer test performed at Well HGDAL3 utilizing the Aron-Scott <br />analysis method yielded a transmissivity value of 280 gpd/ft and a hydraulic conductivity <br />value of 4.6 ft/day. Alluvial properties at and immediately below the Loadout exhibit a <br />range of hydraulic conductivities which is probably a reflection of grain size and sorting <br />heterogeneities. Aquifer test information for Well HGDAL3 is presented in Attachment 7-2. <br />• <br /> TR-06 14a Revised OS/OS <br />