Item A: Public Hearing onMetro Plan Amendment (Delta Sand and Gravel)

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Oregon Department of Geology and Mineral Industries - Mine Dewatering and Ground W... Page 2 of 4 purposes a water right permit from WRD may be required. TECHNICAL ISSUES To protect off-site natural resources DOGAMI requires the preparation of a mine dewatering plan as pa of an application for an Operating Permit or when amending an existing Operating Permit. Basic eleme: of a dewatering plan may include collection of baseline data, mine sequence and development, ground water modeling and on-site or off-site monitoring. Baseline Data When an application for an Operating Permit is submitted for a new site where dewatering will be performed, a fundamental part of the mine dewatering plan is the collection of baseline data. Examples important baseline data are cited here and can also be found in DOGAMI'S Application Guidelines for Additional Information Requirements for sites that are in Hydrologically Sensitive Areas (9/98). Baseline data should include the location of all off-site wells located within O.25-miles of the mine site and their current use (i.e., domestic, irrigation, abandoned). The well locations should be plotted on a si vicinity map or aerial photograph and the water well reports if available should be obtained from the WRD. In addition, if there are water wells on the subject property their locations should also be plotted and the respective well reports should be obtained. Additional data that can be acquired from the well reports includes aquifer depth, type (i.e., confined or unconfined) and the local hydrogeologic system ir which the mine is located. Other relevant baseline data that is important includes the pre-mine static water levels in on-site and off site wells and water quality data. Collection of water level data documents the baseline conditions prior mining or dewatering, which may be used for later comparative purposes if there are apparent off-site impacts. Operators should note that measuring water levels in wells with an active pump will produce erroneous data and should be performed after the pump is shut off for a minimum of 1-2 days to allow t water levels to equilibrate. Baseline water chemistry data is important for establishing pre-mine conditions if there is a potential concern for impacting water quality from mining. In addition, baseline water chemistry data is important if it is necessary to monitor mine discharge water for certain parametc such as turbidity, hydrocarbons, coliform, metals, nitrate/nitrite, salinity or hardness. After identifying off-site wells that could be affected by mine dewatering the operator should assess location(s) for one or more ground water monitoring wells. Proposed well locations should be selected << the basis of detecting declining water levels on-site prior to level declines off-site. Well depth should b~ sufficient to access the same aquifer that off-site wells are set in and be no shallower than the final elevation of the pit floor. Proposed well locations are subject to approval by DOGAMI and may be altered in location or number if data gaps exist between the dewatering area and off-site wells. Monitoring well installation should be performed by an Oregon licensed well installer and be constructt in accordance with WRD rules. Operators should also be aware of the potential for off-site sources of contamination that could be draw into the mine area by pumping ground water. These off-site sources may originate from leaky underground storage tanks, surface spills or farm practices that have contaminated local aquifers. A review of the Oregon Department of Environmental Quality's spill data base will help permittees identi if there are any contaminated sites nearby. If a property with contaminated ground water is proximal to mine site consult DOGAMI for guidance on how to proceed. file://X:\0290 Delta Sand & Gravel\04-0020 Pit Expansion\Hydrualic Barrier\Oregon Deoa... 11/1/2006