2008-03-05_REVISION - M1997054 (24)

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I GROUNDWATER MANAGEMENT DURING MINING ' During mining the operator will excavate trenches below the pit floor. The operator will then pump and discharge ground water collected in the trenches to the Arkansas River and/or Tallahassee Creek. This process is intended to lower the water table immediately at and under the pit floor to facilitate dry mining of the aggregate material contained within. The collection and discharge of ground water at the site will result in a decrease in ground water levels in the alluvial aquifer surrounding the site, resulting in a 'cone of depression' in the immediate vicinity of the site. The extent of the cone of depression will be limited by adjacent hydraulic boundaries and recharge of the aquifer from surface water flow from Tallahassee Creek, the Arkansas River, and surrounding surface irrigation. Pumping from the site to discharge collected ground water will not be continuous. The average rate of discharge to the river will depend on seasonal fluctuations of ground water flow to the pit, but is not expected to exceed 4,100 gallons per minute (gpm) at any given time. On average the rate of discharge should be considerably less. ' The discharge of ground water from the pit may also at times affect Tallahassee Creek by depleting its flow. However, between the point of depletion and the creek's natural discharge point to the Arkansas River there are no structures or water rights that would be adversely affected by the depletion. PARKDALE MINE ANALYTICAL GROUND WATER MODEL Of particular concern during mining operations at the Parkdale Sand and Gravel Pit is the possibility of adverse impacts on the surrounding ground water flow due to the dewatering operation. To evaluate the anticipated extent of expected effects caused by the dewatering, an analytical model was developed using standard steady-state ground water well hydrology methods. The analytical approach taken in the analysis utilizes the best currently available information, which is limited in terms of actual ground water flow data and geologic data outside of borehole data within the pit and quarry boundaries. ' The basic analytical approach was based on well established methodologies for determining draw- down surrounding a single pumping well. The equation for estimating drawdown is derived from Darcy's Law, which states simply that the rate of flow is directly related to the hydraulic conductivity of the aquifer and the hydraulic gradient of the resultant water surface. Applying this principle to a single pumping well results in the following equation relating pumping rate from the well to observed drawdown at locations surrounding the well (sometimes referred to as the Dupuit equation): Q = (hZ - hi ) K? ' r2 In (r1) r ' Parkdale Aggregate Mine I Parkdale Mine Analytical Ground Water Model