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<br />Memo to Wallace Erickson 4 Mav 3 2001 <br />If the aquifer is isotropic, K,-K„ and r~=r'/b'. The transmissivity T is defined as T= /~',b. The prediction <br />of the average drawdown at any radial distance r from a pumping well at any time [can be obtained <br />using the above equations given O, 5,., /.'„ K_, and b. The specific yield S,. is the storage term for <br />ttncontined aquifers. [t is analogous to storativity in a confined aquifer and is sometimes called the <br />unconfined storativity. Specific yield is defined as the amount of water that an unconfined aquifer <br />releases from storage per unit surface area of aquifer per unit decline in the water table. The specific <br />yields of unconfined aquifers are much higher than the storativities of confined aquifers. The usual range <br />ofS:,. is 0.01-0.30. The higher values reflect the tact that releases from storage in unconfined aquifers <br />represent an actual dewatering of the soil pores, whereas releases from storage in confined aquifers <br />represent only the secondary effects of water expansion and aquifer compaction caused by changes in the <br />fluid pressure. The favorable storage properties of unconfined aquifers make them more efficient for <br />exploitation by wells. These properties also mean that gravel pit dewatering causes less well interference <br />than might be expected by someone with confined aquifer experience. When compared to confined <br />aquifers, water table aquifers can produce the same yield with smaller head changes over less extensive <br />areas. 5~=0.16 is a typical value for a sand and gravel alluvial aquifer like the one at the Line Camp Pit <br />location. This is the value of S~. used in the analysis provided by the applicant and will be used in the <br />example produced in this memo. DMG also ran a number of simulations using a value of Si=0.20 in <br />order to test the sensitivity of this parameter on the analytical results, but as discussed below, DMG is <br />not including all of the parametric sensitivity data with this memo for reasons of space. <br />The values assigned to the remaining parameters input to the example produced in this memo and <br />required for Neuman's solution were determined as follows: <br />•:• The flow rate O is the pumping rate needed to dewater the proposed Line Camp Pit. The Applicant <br />used a flow rate of 20.4 cubic feet per second (cfs) for the simulation provided in the revised Exhibit <br />G to the application. Based on the DMG's experience with gravel pits of this size, and considering <br />physical and hydrogeological constraints to the flow of ground water into the proposed pit, DMG <br />has determined that the 20.4 cfs flow rate used by the Applicant is unrealistically high. A realistic, <br />but still conservatively high flow rate for the Line Camp Pit would be 0.1224 cfs which is <br />approximately 80,000 gallons per day pumped from the pit or 0.0035 cubic meters per second. <br />DMG's justification for selection of this flow rate is based on the anticipated hydraulic conductivity <br />of the alluvial aquifer materials. As discussed previously, small amounts of silt and clay within the <br />alluvial materials will substantially reduce hydraulic conductivity. In estimating flow rate for [heir <br />simulation, the Applicant assumed a hydraulic conductivity of 10,000 gallons per day per square <br />foot. This is a very high value that might be expected for clean gravel with sand, for example a <br />product such as sized and washed aggregate. Alluvial sand and gravel in place is likely to have a <br />hydraulic conductivity on the order of six (6) gallons per day per square foot (Davis, 1969). <br />Conversion from the Applicants assumed flow rate to the flow rate selected by the DMG is in <br />