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~ i <br />Memo to Wallace Erickson 3 May 3. 2001 <br />pumping wells in unconfined, or water table aquifers tend to decline at a slower rate than [hat predicted <br />by the Theis solution. Under water table conditions, there are three distinct segments recognized in the <br />time-drawdown curves. For a short period of time following the commencement of pumping, an <br />unconfined aquifer responds the same as a confined aquifer. Water is released instantaneously from <br />storage in the aquifer and into the well though compaction of the aquifer and by expansion of the water. <br />This is followed by a decrease in the slope of the time-drawdown curve relative to the Theis curve for <br />confined aquifers due to the effects of gravity drainage. Water delivered to the well by the dewatering <br />accompanying the falling water table is greater than that delivered to a well by an equal decline in the <br />potentiometric surface of a confined aquifer. The result is that well interference during this stage of <br />drawdown is less for a water table aquifer, such as that at the Line Camp Pit, than it would be for a <br />confined aquifer. The third stage of drawdown in an unconfined aquifer occurs after prolonged pumping <br />and time-drawdown data tend to once again conform to a Theis type curve. Since the Line Camp Pit <br />dewatering plan is for seasonal rather than year round dewatering, the third stage of drawdown may <br />never be reached, thus lessening potential well interference. However, to be conservative in potential <br />drawdown predictions, and because the permit application does not present an enforcable limitation to <br />the length of the dewatering season, the DMG's analysis will assume that [he third stage of dewatering <br />takes place resulting in the maximum theoretical drawdown. <br />The third stage of drawdown of an unconfined aquifer can be predicted by the solution of Neuman <br />(1972, 1973, 1975) as follows: <br />4nT <br />where <br />r'S~ <br />r~B = - <br />4Tt <br />and <br />_ r'K. <br />~ b~K <br />h„ - h is the magnitude of drawdown in meters at a given distance from a pumping well <br />U is the flow rate from the pumping well in cubic meters per second <br />Tis the transmissivity of the aquifer in square meters per second <br />W(7rg,r~) is called the unconfined well function <br />r is the distance from the pumping well in meters <br />h is the saturated thickness of the unconfined aquifer <br />K. is the vertical hydraulic conductivity <br />K, is the horizontal hydraulic conductivity <br />