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<br />Oregon Department of Geology and Mineral Industries - Mine Dewatering and Ground W... Page 3 of 4 <br /> <br />Mine Sequence and Development <br /> <br /> <br />Ground Water Modeling <br /> <br />Ground water modeling is a tool that may be used at sites where extensive testing and characterization ( <br />the underlying aquiferes) is required to predict future drawdowns in off-site wells or adjacent surface <br />water bodies. Predicting future water declines through modeling may be necessary where the <br />hydrogeology is complex (i.e. multiple aquifer or fracture flow systems) or in ground water <br />limited/critical areas that are sensitive to the withdrawal of ground water. Modeling consists of <br />performing an aquifer pump test and inputting the data into a software program (i.e., Visual MODFLO' <br />that generates a graphical representation of the ground water surface and the resultant changes due to <br />mine dewatering over time. This information can be used to determine the appropriate depth, lateral <br />extent of the mine and the mining sequence. In addition, the model results will indicate the potential <br />impacts to off-site wells or surface streams. Operators can then formulate mitigation strategies prior to 1 <br />occurrence of these off-site impacts. Modeling is a predictive tool that can benefit the operator by <br />allowing the maximum resource extraction to occur without creating off-site impacts. In addition, it can <br />be used in the public hearing process to address concerns about the potential effects from dewatering <br />operations. To conduct aquifer testing and modeling for a mine site DOGAMI recommends operators <br />retain a qualified hydrogeological or engineering consultant for these services. DOGAMI maintains a Ii <br />of consultants that can be provided to op'erators. <br /> <br />Monitoring and Mitigation <br /> <br />Monitoring and mitigation are closely related elements of the mine dewatering plan that allow tracking <br />ground water conditions during the life of mine and implementation of measures to alleviate off-site <br />impacts should they occur. As previously stated water level measurements a~d water quality analyses <br />should be initiated prior to mining if possible. An ongoing ground water monitoring program should be <br />implemented during mine dewatering and may include quarterly or more frequent measurements ofwa1 <br />levels in on and off-site domestic and monitoring wells. Water level and/or chemistry data should be <br />compiled into a report format and submitted to DOGAMI on a semi-annual basis. <br /> <br />If during the life of mine the monitoring data provides evidence of off-site impacts to ground water <br />quantity or quality then DOGAMI may require the operator to implement corrective action to offset the <br />impacts. Such measures may include modifying mine development or sequence, deepening affected we <br />or installation of new wells, supplying water to affected users or ceasing dewatering activities. This <br />assumes the off-site well owner(s) have senior water rights to the quarry or pit. Conversely if the mine i <br />deeper in the same aquifer as adjacent wells and the aquifer is not fully penetrated by the wells then it <br />may be incumbent upon the well owners to access the full extent of the aquifer. In these cases a ruling ( <br />the senior water right holder will be determined by the WRD. <br /> <br />file://X:\0290 Delta Sand & Gravel\04-0020 Pit Exoansion\Hvdrual1c Rarrler\nr~o-()n T)pn~ 11 /l/~n()h <br />