Oregon Department of Geology and Mineral Industries - Mine Dewatering and Ground Water Protection

Mine Dewatering and Ground Water Protection

Mine or pit dewatering is an essential part of resource extraction at many of the aggregate operations throughout Oregon. DOGAMI classifies dewatering to be the withdrawal of ground water with a resultant decline in the water table or hydraulic head within an aquifer. As the number of ground water users increases and aquifers are designated as limited or critical, mine operators need to be informed about the potential for off-site impacts from dewatering. To ensure the protection of ground water it is necessary for permittees to consider certain issues prior to conducting this activity. These issues are both regulatory and technical in nature and include permitting, collection of baseline data, monitoring and/or modeling. This paper is a discussion of these issues and provides operators with an insight to the best management practices for conducting mine dewatering.

REGULATORY ISSUES

DOGAMI has a statutory directive to prevent or mitigate off-site impacts to natural resources from mining operations. Ground water is a natural resource that can be affected by mining and as a result dewatering is regulated by this department. Permittees should be aware that dewatering is generally allowed only if it is specified in their permit. Filing an amendment application and mine dewatering plan is required if dewatering is not authorized in the original permit. Consultation with DOGAMI will assist the permittee in identifying those concerns that need to be addressed in order for the dewatering plan to be considered complete. The technical aspects of a dewatering plan are to follow.

Sites permitted in the 1970’s and early 1980’s may have approved reclamation plans that may not specifically allow or prohibit dewatering nor was it evaluated. Permittees in this category are advised to contact DOGAMI for an evaluation of the likely impacts from dewatering. Data collection may be needed for protection against frivolous claims of well damage by adjacent landowners.

Discharging mine water is a component of dewatering that may also require permitting. If water is pumped from a gravel pit or rock quarry and is discharged to a surface water stream or off-site then a National Pollutant Discharge Elimination System (NPDES) 1200A permit is required. Operators should be aware that holding a 1200A permit alone does not qualify as an approval by DOGAMI to perform dewatering activities.

The Oregon Water Resources Department (WRD) is the issuing agency for water rights in the state. Permittees should determine if their dewatering operation constitutes a consumptive use such as gravel washing or dust suppression. If more than 5000 gallons per day of ground water is consumed for these 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 part of an application for an Operating Permit or when amending an existing Operating Permit. Basic elements 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 of 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 0.25-miles of the mine site and their current use (i.e., domestic, irrigation, abandoned). The well locations should be plotted on a site 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 in 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 to 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 the 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 parameters 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 on the basis of detecting declining water levels on-site prior to level declines off-site. Well depth should be 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 constructed in accordance with WRD rules.

Operators should also be aware of the potential for off-site sources of contamination that could be drawn 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 identify if there are any contaminated sites nearby. If a property with contaminated ground water is proximal to a mine site consult DOGAMI for guidance on how to proceed.

Mine Sequence and Development

Monitoring well selection is also based on mine sequence and development which is an important component of the dewatering plan. The permittee should provide a site map depicting the sequence for the life of mine development and its’ relationship to dewatering. For example, at many sand and gravel pits mining is performed by resource extraction from a number of cells and consists of dewatering from one cell and discharging the water to a mined out cell. This is significant in that filling an empty cell may provide a hydraulic barrier between the dewatered zone and any off-site wells. Under this scenario less monitoring wells may be required to detect off-site impacts. Conversely a mine development plan where ground water is extracted and discharged off-site to a surface water stream may require more monitoring wells due to the removal of ground water from the local hydrologic budget.

Ground Water Modeling

Ground water modeling is a tool that may be used at sites where extensive testing and characterization of the underlying aquifer(s) is required to predict future drawdowns in off-site wells or adjacent surface water bodies. Predicting future water declines through modeling may be necessary where the hydrogeology is complex (i.e. multiple aquifer or fracture flow systems) or in ground water limited/critical areas that are sensitive to the withdrawal of ground water. Modeling consists of performing an aquifer pump test and inputting the data into a software program (i.e., Visual MODFLOW) that generates a graphical representation of the ground water surface and the resultant changes due to mine dewatering over time. This information can be used to determine the appropriate depth, lateral extent of the mine and the mining sequence. In addition, the model results will indicate the potential impacts to off-site wells or surface streams. Operators can then formulate mitigation strategies prior to the occurrence of these off-site impacts. Modeling is a predictive tool that can benefit the operator by allowing the maximum resource extraction to occur without creating off-site impacts. In addition, it can be used in the public hearing process to address concerns about the potential effects from dewatering operations. To conduct aquifer testing and modeling for a mine site DOGAMI recommends operators retain a qualified hydrogeological or engineering consultant for these services. DOGAMI maintains a list of consultants that can be provided to operators.

Monitoring and Mitigation

Monitoring and mitigation are closely related elements of the mine dewatering plan that allow tracking of ground water conditions during the life of mine and implementation of measures to alleviate off-site impacts should they occur. As previously stated water level measurements and water quality analyses should be initiated prior to mining if possible. An ongoing ground water monitoring program should be implemented during mine dewatering and may include quarterly or more frequent measurements of water levels in on and off-site domestic and monitoring wells. Water level and/or chemistry data should be compiled into a report format and submitted to DOGAMI on a semi-annual basis.

If during the life of mine the monitoring data provides evidence of off-site impacts to ground water quantity or quality then DOGAMI may require the operator to implement corrective action to offset the impacts. Such measures may include modifying mine development or sequence, deepening affected wells or installation of new wells, supplying water to affected users or ceasing dewatering activities. This assumes the off-site well owner(s) have senior water rights to the quarry or pit. Conversely if the mine is deeper in the same aquifer as adjacent wells and the aquifer is not fully penetrated by the wells then it may be incumbent upon the well owners to access the full extent of the aquifer. In these cases a ruling on the senior water right holder will be determined by the WRD.

In certain cases altering the mine plan may suffice in mitigating off-site impacts. For example, if dewatering is causing unacceptable off-site declines during seasonal low water levels it may be possible to mine in other portions of the site until water levels recover during the wet season. Extraction could then resume in those sensitive areas once ground water levels have recovered. This scenario assumes the mine plan affords this flexibility and the hydrogeology is compatible with such a plan. If modifying the mine plan is not feasible then deepening or drilling new wells to provide water to the affected users may be required. This method of mitigation has been implemented at mine sites where DOGAMI and WRD determined that resultant water level declines from dewatering could be offset by accessing deeper portions of the affected aquifer. Corrective action consists of selecting a new well location and installing the well to a sufficient depth that it has the capacity to produce ample water and tolerate simultaneous dewatering activities.

At those sites where the hydrogeologic conditions preclude the option of drilling deeper wells due to poor ground water quality (i.e., high salinity) or limited aquifer thickness it may be necessary to provide water to affected users by other means. This may include connection to a municipal water supply if available. Connection to a municipal water supply may also be preferable if the costs are less than drilling new wells.

The final option for mitigation is ceasing dewatering activities, which is typically undesirable for the operator unless resource extraction through wet mining can be performed. In some cases total cessation of dewatering can be avoided if site conditions are such that dewatering and mining can be performed during the winter and spring without causing unacceptable off-site declines in the water table. Dewatering during the winter and spring may also be restricted in the DOGAMI permit if the site is located within a floodplain due to concerns about potential pit capture during flood events.

In addition to impacts to quantity operators need to follow practices to prevent degradation to ground water quality. This includes proper storage and handling of hazardous or regulated materials such as petroleum hydrocarbons or solvents which if introduced into an aquifer via spills can prove very costly (>$10,000.00) to remediate. Other sources for water quality degradation include backfilling of contaminated fill material into a pit or quarry. Permittees should be able to certify that fill imported to a site is not contaminated with any of the substances regulated by DEQ such as metals or organic compounds.

Finally, operators should know that DOGAMI’s role is to provide them with assistance and advice on the regulatory and technical issues associated with mine dewatering. In doing so DOGAMI can help permittees protect their operations while ensuring compliance, preventing potential off-site impacts and prepare mitigation strategies when corrective action is required.

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