Potential effects evaluation of dewatering an underground mine on surface water and groundwater located in a rural area

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1 Potential effects evaluation of dewatering an underground mine on surface water and groundwater located in a rural area ITRODUCTIO Michel Mailloux* Eng. M.Sc, Vincent Boisvert, M.Sc, Denis Millette, Eng., Agr., Ph.D and M. Poulin, Geo., M.Sc. * mmailloux@golder.com Golder Associates Ltd., Montreal, Canada ABSTRACT: A few kilometers away from a major urban agglomeration, a mining promoter projected to develop an underground mine on a property located in a rural and touristic area. Many concerns rose from the potential impact of the mine dewatering on water resources availibility. The groundwater was used for drinking, and the surface water (irrigation ponds and creeks) was extensively used for irrigation. In 2006, a detailed hydrogeological study was conducted to address these concerns. The study consisted of the following: a water use quantification for drinking and irrigation; a hydrogeological characterization; a largescale groundwater flow model with FEFLOW to evaluate the pumping rate to dewater the mine and the extent of the drawdown cone; the impact on the groundwater resource and mitigation measures. The groundwater flow model considered the overburden and the bedrock aquifer. The parameterization of the groundwater flow model was facilitated by linking FEFLOW to a Geological Information System (GIS). For example, a detailed infiltration calculation was performed with the GIS. Runoff and evapotranspiration were calculated using a detailed grid in function of the ground slope and land use. Prior to the dewatering simulation of the underground mine, the model was calibrated against a static conditions piezometry, two pumping tests, baseflow to creeks and the previous dewatering rate of an abandoned mine located at 1km from the property. Once calibrated, the dewatering of the underground mine was simulated according to the mining plan provided by the engineers. The position of the irrigation ponds, drinking water wells and crops was compared to the position of the predicted drawdown extents obtained with FEFLOW. The results showed that the lowering of the water-table in the overburden would lead to a demand increase for irrigation water. Mitigation measures were therefore needed and FEFLOW was used to support the design of an injection well barrier to limit the extent of the drawdown cone. A few kilometers away from a major urban agglomeration, a mining promoter projected to develop an underground mine on a property located in a rural and touristic area. Many concerns rose from the potential impact of the mine dewatering on the water resources availibility. The groundwater was used for drinking, and the surface water (irrigation ponds and creeks) was extensively used for irrigation. The drawdown cone created by the mine dewatering could possibly reduce the residential well and irrigation ponds capacity, as well as the streams baseflow and create an increase for irrigation needs. In 2006, a detailed hydrogeological study was conducted to address these concerns. The study consisted of the following: a water use quantification for drinking and irrigation ; a hydrogeological characterization; a large-scale groundwater flow model with FEFLOW (Finite Element subsurface FLOW system) to evaluate the pumping rate to dewater the mine and the extent of the drawdown cone; the impact on the groundwater resource and mitigation measures. HYDROGEOLOGICAL COTEXT The mining site is located in the Western portion of a recent, fairly permeable, carbonate rock intrusion (Figure 1). Measuring 7 km x 2 km, this ovally shaped unit is bounded at orth, East and West by a low permeability metamorphic gneiss, and at South by permeable sedimentary rocks. To determine the bedrock s hydraulic properties, two long pumping tests and 15 short pumping tests were used. The hydraulic conductivity of the recent carbonate varies between 3.6x10-8 and 4.3x10-6 m/s. The hydraulic conductivity of the metamorphic gneiss varies between 6.9x10-9 and 5.9x10-8 m/s. o field data are available for the permeable sedimentary rocks. The overburden stratigraphy consists of a low permeability glacial till (K between 6.0x10-8 and 1.7x10-6 m/s) followed locally by a clay or littoral sand (K between 2.6x10-6 and 4.7x10-7 m/s). Overburden thickness varies between 1 and 100 m.

2 Two water-tables are present in the study area. Confined by the glacial till unit, the deep water table is located in the bedrock and shows local artesian conditions, especially in topographic lows. The surface water table is in the littoral sand unit, between 1 and 3 m from the surface. The groundwater flow is controlled by a stream located in the center of the valley and a major river at South (Figure 2). Underground mine Sedimentary rock Sedimentary rock Metamorphic gneiss Major river Major river Figure 1: Bedrock Geology Observation well Groundwater elevation contour Metamorphic gneiss Recent carbonate rock intrusion Major river Figure 2: Groundwater Elevation Map GROUDWATER AD SURFACE WATER USAGE Groundwater Users Inventory Private well owners located in the study area were interviewed to obtain information regarding their drinking water supply source (dug wells, wells in the bedrock, springs, etc..), their wells characteristics (diameter, depth, pump type, etc.) and their use of groundwater. The well owners were also asked about their type of harvest, and their water supply irrigation method (pond, stream,etc.). The data

3 obtained during this inventory were plotted in a Geographical Information System (GIS) to compare their location with the predicted drawdown cone position s created by the mine dewatering. The private well inventory is summarized in Table 1 and Figure 3. As shown in the study area, there are 93 private wells in the bedrock and 22 dug wells in deposits. The depth of the private wells in the bedrock varies from 18 to 146 m with a diameter of 0.15 cm. For the dug wells, the depth varies from 3.6 to 25.2 m with a diameter of 0.15 m to 3 m. In addition to the residential use, groundwater is also occasionnally used for farming and/or commercial purposes. Groundwater is rarely used for irrigation. Type of usage umber of well installed in bedrock umber of dug wells Residential only Residential/Farming (fruit and 10 1 vegetable crops) Residential/Farming (greenhouses) 10 4 Residential/Farming (cattle breeding) 7 2 Total Table 1: Summary of the Private Well Inventory Radial distance to the underground mine Irrigation pond Private well Major River Figure 3: Location of Private Wells and Irrigation Ponds

4 Surface water user s inventory In the study area, surface water from streams, drainage ditches or irrigation ponds is mainly used for farming purposes. Distance from the underground mine (km) umber of Ponds Total Storage Capacity (m³) , , , , , ,104 Table 2: Summary of the Total Storage Capacity of Irrigation Ponds The irrigation ponds depth was measured with a probe from a kayak. The basins lateral dimensions were derived from digital aerial photographs with an accuracy of about 1 m. The data obtained during this inventory were plotted in a Geographical Information System (GIS) to compare their location with the predicted drawdown cone s position created by the mine dewatering and to estimate the water storage capacity loss. In the study area, there are 47 irrigation ponds (Figure 3 and Table 2). The irrigation ponds water columns average thickness is 2 meters for an average of 1,876 m³. The total storage capacity is 93,104 m 3. GROUDWATER FLOW MODELIG A FEFLOW numerical model was used to assess the extent and magnitude of the drawdown cone created by the underground mine dewatering. The same model was used to estimate the groundwater inflow rate into the mine. Definition of boundary conditions The numerical model included overburden and bedrock. The modeling domain was within an area covered by the carbonate footprint and a buffer zone of 500 to 1500 m (Figure 4). The domain covers an area of 8.1 km by 3.7 km A 22.8 m specified head boundary conditions were assigned along the major river over the entire thickness of the model (Figure 4).. The specified head assigned to this boundary corresponds to the major river s elevation. A 65 m specified head boundary conditions were assigned to the orthern boundary of the model. Specified head boundary conditions were also applied to the model s first layer along the various streams within the area covered by the model. Hydraulic head values specified along these boundary conditions are the observed water level measured along the streams. o flow boundary conditions were assigned along the West and East boundaries. A detailed infiltration calculation was performed with GIS. Runoff and evapotranspiration were calculated using a detailed grid in function of the ground slope and land use. The spatial distribution of infiltration is shown in Figure 5. The recharge rate varies between 50 and 527 mm/year for an average of 272 mm/year.

5 Major River Specified head = 65 m o flow boundary Figure 4: Boundary Conditions Assigned to the Groundwater Flow Model Grid Design and Hydraulic Parameters Estimation The construction of a three-dimensional mesh requires that a two-dimensional mesh has been previously constructed. The latter is then extended vertically through the various model s layers to form the three-dimensional mesh. The two-dimensional mesh used is composed of 14,807 nodes forming 29,009 triangular elements. The size of the elements is minimal (5 m) within a radius of 325 m around the underground mine. In order to have a discretization fine enough to adequately simulate the underground mine dewatering. Outside the radius, the size of the elements is 70 m. Recharge (mm/yr) Major River m 000 m Figure 5: Recharge Distribution The three-dimensional mesh was generated by extending the two-dimensional mesh between the 10 layers of the model. The three-dimensional mesh consists of 148,070 nodes forming 261,081 triangular elements. The first layer starting from the top of the model was assigned to the units shown

6 on the surficial geology map. The littoral sand, which forms 50% of the surface deposits, has been assigned to this layer only. Layers 2 to 4 represent the glacial till unit while layers 5 to 10 represent the bedrock. A summary of the hydraulic parameters assigned to the model is presented at Table 3. Unit K (m/s) Specific storage Drainage (1/m) porosity Metamorphic gneiss 2.0x x x10-7 to intrusion 1.3x x Sedimentary rock 2.0x x Glacial till 6.0x10-8 to 2.0x x Marine clay 1.0x x Littoral sand 1.3x10-5 to 5.0x x o Table 3: Summary of the Hydraulic Parameters Assigned to the Groundwater Flow Model Calibration The first step was to calibrate the model in steady-state using piezometric data recorded on April 19 and 20, The latter was done by performing several simulations, each with a set of different hydraulic parameters until the difference between the simulated and observed hydraulic heads was minimized. The model was then calibrated until the root mean square (RMS) error was less than 5% of the water levels total variation observed in the modeled area. The difference between the minimum and maximum hydraulic head observed within the model domain is about 89 m. Therefore, the acceptable RMS target error is 4.45 m. The RMS obtained in the calibrated model is 3.83 m, which is 4% difference between the hydraulic minimum and maximum observed within the modeled area. The simulated baseflow to streams located within the model domain is 3,590 m 3 /d (41.6 l/s). The baseflow value is within the range of value (25.5 to 46.9 l/s) for a stream located downgradient of the model domain. Once the model is calibrated in steady state, the two pumping tests conducted in March 2006 have been simulated to verify if the model can reproduce the drawdown observed during these tests. Tables 4 and 5 show the difference between simulated and observed drawdown at the pumping tests. The predicted drawdown is, in general, consistent with the measured data. Well Observed drawdown after Simulated drawdown after 48 hours (m) 48 hours (m) Difference (m) PO PO PP-1 (test well) Table 4: Comparison Between Simulated and Observed Drawdown at the End of the 48 Hours Pumping Test Well Observed drawdown after Simulated drawdown after 120 hours (m) 120 hours (m) Difference (m) PO PO-5 (test well) PO PP Table 5: Comparison Between Simulated and Observed Drawdown at the End of the 120 Hours Pumping Test

7 Simulation of the dewatering of the underground mine The underground mine dewatering has been simulated using specified head boundary nodes, whose values were assigned to the mine levels base s altitude. A constraint has been assigned to these nodes to prevent water injection. For each mine level, these boundary conditions have been assigned according to the mining sequence defined by the engineers. According to the modeling results, the estimated groundwater inflow into the mine should be around 8,000 m 3 /d. The extent of the 3 m drawdown iso-contour in the bedrock should be at about 1.9 km from the underground mine (Figure 6). The extent of the 1 m drawdown iso-contour in the overburden should be more important. Considering the significant extent of the drawdown cone, a mitigation measure was necessary to reduce the impact on the water ressource availibility. The proposed measure was a reinjection of a portion of the groundwater inflow into the mine through a barrier of injection wells. The site s hydrogeological context is favourable for this application as the carbonate is a low storativity formation. This means that the volume of water injected to limit the extent of the drawdown cone would be relatively low. The mitigation measure s efficiency has been simulated using the numerical model. The simulation was carried out by introducing a series of injection wells in the model. The injection barrier should be constructed using 10 wells and the total injection be at 2,300 m 3 /d. Figure 7 shows the extent of the simulated drawdown cone considering the operation of the injection well barrier. IMPACT ASSESMET O THE WATER RESOURCE An impact assessment was conducted on the water resource over the area located within the drawdown cone reduced by the operation of the injection well barrier. Four aspects were considered during the impact assessment: reduction of stream baseflow, reduction of private well capacity, reduction irrigation ponds storage capacity and augmentation of irrigation water demands. The impact assessment was made by comparing in the GIS the position of private wells, irrigation ponds and irrigated culture with the position of the simulated drawdown cone. Drawdown isocontour (m) Major river Figure 6: Maximal Extents of the Simulated Drawdown Cone

8 Impact on the Private Wells, Irrigation Ponds and s Baseflow A total of 18 private wells would be affected with the drawdown cone (Figure 7). The implementation of an aqueduct network is considered as a mitigation measure. Six irrigation ponds are located within the drawdown cone. On average, the surface water table drawdown may induce a 2.1 m lowering on the water table near these six irrigation ponds. If one considers a linear effect between the lowering of the water table and the lowering of the water in ponds, a potential average loss of 92% of the current water storage capacity is expected. The deepening of the irrigation ponds is considered as a mitigation measure. Baseflow to streams should be reduced by 30%. Irrigation pond Injection well barrier Private well Drawdown isocontour (1 m) Figure 7: Maximal Extents of the Simulated Drawdown Cone with the Operation of the Injection Well Barrie

9 Impact on Irrigation Water Demand and Water Balance Currently, the water balance in the impacted area is positive and varies between 78,000 and 92,000 m³ for the full season of growth. During peak periods, the water balance is negative and the deficit ranges between 7,000 and 14,000 m³. Based on predicted extension of the water table drawdown cone and the interpretation of the effects arising from this drawdown, the future water balance for a full season of farming should be positive and varying between 29,000 and 53,000 m³ (Table 6). During the dry periods of four weeks, the future water balance would be negative and vary between 34,000 m³ and 47,000 m³ (Table 7). The construction of a 50,000 m³ irrigation water reservoir is considered as a mitigation measure. Water balance (m³) Difference Irrigatin water demand -89, ,872 Avalaible water from irrigation ponds - Based on 5 recharges 22,755 22,755 Available water from streams (may to sept) 118, ,881 Total 52,595 28,764 Table 6: Future Water Balance for the Area within the Predicted Drawdown Cone Complete Season Water balance (m³) Difference Irrigatin water demand -54,071-66,964 Avalaible water from irrigation ponds - Based on 1 recharge 4,551 4,551 Available water from streams (july) 15,578 15,578 Total -33,942-46,835 Table 7: Future Water Balance for the Area within the Predicted Drawdown Cone Four Weeks Peak Season