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Lnrenciln Caneon Mine - Grnundwmer Technical Reoon <br />• Interburden <br />The Interburden in the Lorencito Canyon Permit area has a total estimated thickness of <br />approximately 570 feet. Recharge to the Interburden is via precipitation on outcrop surfaces, and <br />from inflow from fractures or overlying units. Infiltration rates for this unit are anticipated as <br />low. Discharge from this unit is anticipated as being low. <br />Groundwater in the Interburden is characterized as being under confined conditions. Water was <br />not apparent in two of the Ciruela borings into the Interburden; however, the third well (MW-lI) <br />had water present. Wells MW-2I and MW-3I both have been dry during the baseline period. <br />These wells were set in the sandstone directly below the Ciruela. Water was present in all three <br />wells installed in the Interburden above the Primero. Water was encountered in the borings for <br />these wells; however, this water was encountered during the drilling through alluvium (MW~ and <br />MW-5) and at 100 feet in the boring for site MW-6. All Interburden Primero wells were set in the <br />sandstone immediately above the Primero coal seam. <br />The potentiometric surface of the Interburden is to the east-southeast, based on the data from the <br />wells MW-4, MW-5, and MW-6 (Map 2.04.7-4). The gradient of this surface is 0.0055 ft/ft or <br />29 ft/mi. This is a relatively shallow gradient. Comparison of this gradient to the regional <br />gradient of Howard (1982) (Figure GW-ti), shows that this gradient is much more easterly. The <br />• gradient accepted as the regional gradient is generally to the north or northeast in the permit area. <br />Characteristics of the Interburden were determined via slug testing. A slug test was performed <br />in well MW-6I, and recovery of the well was monitored for approximately 17 hours. Within this <br />time period the water level recovery was extremely limited. Extrapolation of the slug test curve <br />data indicates complete recovery would take approximately 695 days. This indicates an extremely <br />low hydraulic conductivity for the Interburden. Estimated hydraulic conductivity as determined <br />via this testing was 1.28 x 10"' ft/day. This value can be translated to transmissivity (T) by the <br />equation T = Kb, where T is transmissivity, K is hydraulic conductivity, and b is the Interburden <br />thickness. Therefore: <br />T=Kb <br />T = (1.28 x 10-' ft/day) (5 ft) <br />T = 6.40 x 10-' fiz/day or 4.78 x 10-Z g/d/ft <br />Comparing this to the regional values: <br />T=Kb <br />T = (0.0408 ft/day) (5 ft) <br />T = 0.204 ftz/day or 1.53 g/d/ft <br />L~ <br />GrountlwattrA75\Dtttmber 9. 1996 40 <br />