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Regional Groundwater Flow Models
The document discusses regional groundwater flow models presented to the U.S. Fish and Wildlife Service for the proposed Rosemont mine, focusing on modeling objectives, data availability, and hydrologic impacts. It details the methodology for predicting regional hydrologic changes, with considerations of groundwater-surface water interactions and the impact of mining activities over time. Conclusions indicate that different models yield similar predictions regarding drawdown propagation and impacts on sensitive ecosystems.
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Regional Groundwater Flow Models
- 1. Regional Groundwater Flow Models Presented to U.S. Fish and Wildlife Service Grady O’Brien March 8, 2012
- 2. Discussion Outline Introduction Modeling Objectives, Approaches, and Model Uses Data / Pumping / Fractures / GW-SW interactions Interpreting Results – What to Consider Model Construction Comparison Model Prediction Comparison Conclusions March 8, 2012 2
- 3. INTRODUCTION Rosemont Site Location 3 March 8, 2012
- 4. Regional Site Mapfor the Proposed Rosemont Mine
- 5. ROSEMONT PROJECT Hydrologic Highlights Open pit mine 2,000+ feet deep ~ 1 mile in diameter Pit dewatering for 22 year life of mine Major facilities Heap leach pads Dry stack tailings Waste rock March 8, 2012 5
- 6. MODELING OBJECTIVES Predict regional hydrologic impacts Sensitive areas Davidson Canyon Cienega Creek Las Cienegas National Conservation Area Sensitive features Springs Stream flow Riparian vegetation March 8, 2012 6 Aquatic life
- 7. IMPORTANT HYDROLOGIC PROCESSES Groundwater and surface-water interactions Nature of spring flow Nature of stream flow Fractured bedrock Hydraulic connections Recharge areas Backbone fault Mountain front Stream channels March 8, 2012 7
- 8. MODELING APPROACH Satisfy the objectives Appropriate scale Site versus Regional Fracture network versus porous media Simulate hydrogeologic features Data availability March 8, 2012 8
- 9. MODELING SCALE INFLUENCES APPROACH Regional impacts regional scale Regional scale regional resolution Data availability Fracture network versus porous media March 8, 2012 9
- 10. DATA AVAILABILITY Geology (site and regional scales) Wells 730+ water-level targets Lithology Water quality Springs and Streams Discharge Water quality Riparian vegetation March 8, 2012 10 Wells used for model calibration (Tetra Tech, 2010)
- 11. DATA AVAILABILITY Montgomery & Associates Geology / Hydrogeologic framework Aquifer tests / Hydraulic properties Short-term and Long-term tests Regional data Evapotranspiration (ET) Water levels Spring flows March 8, 2012 11
- 12. DATA AVAILABILITY Tetra Tech Site Water Management Infiltration analysis Infiltration, seepage, fate, and transport modeling Storm-water runoff March 8, 2012 12
- 13. DATA AVAILABILITY Arizona Geological Survey Geologic mapping Pima Association of Governments Cienega Creek Davidson Canyon U.S. Geological Survey Stream flows San Pedro flow model Tucson basin March 8, 2012 13 Various recharge analyses
- 14. TETRA TECH ANALYSES/ REVIEW Recharge Hydraulic properties Short-term, single well tests Long-term, multiple well tests Spring flow Field observations Water-level weighting 3D hydrogeologic framework model Davidson Canyon conceptual model March 8, 2012 14
- 15. DATA AVAILABILITY Extensive data available for model input Coverage across the region Best available data used March 8, 2012 15
- 16. GROUNDWATER PUMPING Significant pumping in Sonoita and Elgin areas Declining water levels Outside of model domain Exempt wells (<35 gpm) Hilton Ranch Singing Valley Pumping data unavailable Water-level data do not show pumping impact March 8, 2012 16
- 17. FRACTURE NETWORK versus POROUS MEDIA Fracture networks Highly data intensive Practical for very small areas Not used for regional models Research site specific processes Equivalent Porous Media (MODFLOW) Widely used and accepted Well developed functionality Appropriate based on objectives and hydrogeology March 8, 2012 17
- 18. FRACTURED BEDROCK Very low matrix permeability Low storage Types of fractures Interconnected, blind, diffuse Hydraulic connections Scale dependent - small versus large areas How far do connections extend? At what scale does it behave as a porous media? March 8, 2012 18
- 19. FRACTURED BEDROCK Intersecting Fractures March 8, 2012 19
- 20. FRACTURE CONNECTIVITY Influence ofPersistence of Discontinuity on the Degree of Fracturing and Interconnectivity March 8, 2012 Anderson, M.P. and Woessner, W.W., 2002, Applied Groundwater 20 Modeling: Academic Press, San Diego, CA, 381p.
- 21. GROUNDWATER FLOW THROUGH FRACTURES Type 1: Flow and storage only in fractures (single porosity) March 8, 2012 Nelson, R.A.,2001, Geologic Analysis of Naturally 21 Fractured Reservoirs (2nd Edition): Elsevier.
- 22. HYDRAULIC CONNECTION BETWEEN PIT AND SENSITIVE AREAS “What if there is a fracture between the pit and…” Evidence? Davidson Canyon fault zone High water-levels in pit area No large, perennial springs in Davidson Canyon Water quality and isotopes Weak connection March 8, 2012 22
- 23. INTERACTION OF STREAMS and GROUNDWATER March 8, 2012 23 Alley, W.M., Reilly, T.E., and Franke, O.L., 2007, Sustainability of Ground-Water Resources: U.S. Geological Survey Circular 1186, available at http://pubs.usgs.gov/circ/circ1186/index.html.
- 24. INTERACTION OF STREAMS and GROUNDWATER March 8, 2012 24
- 25. MODELING APPROACH ANDUSES Conditions to simulate? Average Annual Average Seasonal Non-Average – response to specific changes Input data consistent with conditions Recharge (precipitation) Evapotranspiration Water levels Stream flow March 8, 2012 25
- 26. AVERAGE ANNUAL CONDITIONS Reference point for relating changes Identify important processes Identify sensitive areas Relative changes – not absolute values In context of natural variability March 8, 2012 26
- 27. INTERPRETING RESULTS What To Consider – How to Use Representation of the conceptual model – major features Model boundaries Proximity to stresses Inflows and outflows Reasonable and conservative parameter values Water budget Calibration targets and statistics March 8, 2012 27
- 28. INTERPRETING RESULTS What To Consider – How to Use Conditions being simulated – average annual No natural seasonal or annual variations Predictions – best estimate with best parameter values Sensitivity analysis – vary parameter values Compare predicted impacts to natural variability March 8, 2012 28
- 29. INTERPRETING RESULTS What To Consider – How to Use Identify most important features and conditions Identify where to monitor Identify where and how to mitigate impacts March 8, 2012 29
- 30. MODEL CONSTRUCTION COMPARISON Tetra Tech (2010) and Montgomery & Associates (2010) MODFLOW-SURFACT code Steady-state conditions (current / pre-mining) Calibrated to stable, observed water levels M&A calibration to 30-day test Mining phase (22 years) Simulates pit deepening in 2 year steps Pit dewatering simulated with drains Post-closure phase (1,000 years) Pit-lake formation simulated with LAK2 package March 8, 2012 30
- 31. MODEL CONSTRUCTION COMPARISON External model boundaries Geology Recharge Streams ET March 8, 2012 31
- 32. MODEL GRID AND EXTERNAL BOUNDARY CONDITIONS Montgomery & Associates (2010)
- 33. MODEL LAYERS Tetra Tech (2010) March 8, 2012 33
- 34. CONCEPTUAL MODELS Tetra Tech (2010) Davidson Canyon Dike Backbone Fault Montgomery & Associates (2010) Davidson Canyon Fault Zone Backbone Fault Flat Fault March 8, 2012 34
- 35.
- 36. GEOLOGY (Montgomery and Assoc., 2010)
- 37.
- 38. HYDRAULIC CONDUCTIVITY (Montgomery and Assoc., 2010)
- 39. GEOLOGY TO FLOWMODEL
- 40.
- 41. RECHARGE Precipitation – runoff approach by surface-water basin (Tetra Tech, 2010)
- 42. RECHARGE Simulated pre-mining steady- state recharge (Tetra Tech, 2010)
- 43. RECHARGE Simulated recharge - before and during 22-year mining period (Montgomery & Associates, 2010)
- 44. RECHARGE Post-closure recharge in project facility area (Tetra Tech, 2010)
- 45. RECHARGE Simulated recharge formining facilities for 1,000-year post- mining period (Montgomery & Associates, 2010)
- 46. Evapotranspiration (ET) (Montgomery & Associates, 2010)
- 47. MODEL PREDICTION COMPARISONS Pit-lake formation and inflows Drawdown propagation Stream flow changes ET changes March 8, 2012 47
- 48. MODEL PREDICTION COMPARISON Pit-Lake Formation and Inflows Pit Lake Conceptual Model (Tetra Tech, 2010b) March 8, 2012 48
- 49. MODEL PREDICTION COMPARISON Pit-Lake Formation and Inflows Simulated Pit Lake Water Balance (Tetra Tech, 2010) March 8, 2012 49
- 50. MODEL PREDICTION COMPARISON Pit-Lake Formation and Inflows Simulated Pit Lake Water Balance (Montgomery and Associates, 2010) March 8, 2012 50
- 51. DRAWDOWN Simulated drawdown at end of mining (Tetra Tech, 2010)
- 52. DRAWDOWN Simulated drawdown at end of mining (Montgomery & Associates, 2010)
- 53. DRAWDOWN Simulated drawdown 20 years after closure (Tetra Tech, 2010)
- 54. DRAWDOWN Simulated drawdown 20 years after closure (Montgomery & Associates, 2010)
- 55. DRAWDOWN Simulated drawdown 150 years after closure (Tetra Tech, 2010)
- 56. DRAWDOWN Simulated drawdown 150 years after closure (Montgomery & Associates, 2010)
- 57. DRAWDOWN Simulated drawdown 1,000 years after closure (Tetra Tech, 2010)
- 58. DRAWDOWN Simulated drawdown 1,000 years after closure (Montgomery & Associates, 2010)
- 59. STREAMS Stream gage locations and simulated flows (Tetra Tech, 2010)
- 60. STREAMS Simulated Stream Flow (Montgomery & Associates, 2010)
- 61. STREAM FLOW CHANGES Simulated change in stream flows at end of operations (Tetra Tech, 2010)
- 62. STREAM FLOW CHANGES Simulated change in stream flows 1,000 years post closure (Tetra Tech, 2010)
- 63. MODEL PREDICTION COMPARISON Stream Flow Changes Impact of Dewatering at Cienega Creek (Montgomery & Associates, 2010) March 8, 2012 63
- 64. MODEL PREDICTION COMPARISON Stream Flow Changes Impact of Dewatering at Davidson Canyon (Montgomery & Associates, 2010) March 8, 2012 64
- 65. MEASURED STREAM FLOWCHANGES March 8, 2012 65
- 66. FLUCTUATION IN GROUNDWATER LEVELS Short Term Long Term Time Period 3 years 37 to 55 years (2007-2009) No. of Wells 14 52 Minimum Fluctuation (ft) 0.7 0.7 Maximum Fluctuation (ft) 33.1 69.0 Average Fluctuation (ft) 7.1 19.7 (Montgomery & Associates, 2010b) March 8, 2012 66
- 67. SENSITIVITY ANALYSIS Tetra Tech Most sensitive parameters Bedrock specific yield decrease (at 150 years) Basin fill specific yield increase (at 150 years) Recharge near pit (no infiltration due to facilities) 20-percent pit lake evaporation increase March 8, 2012 67
- 68. SENSITIVITY ANALYSIS Tetra Tech Bedrock specific yield decrease (at 150 years)
- 69. SENSITIVITY ANALYSIS Tetra Tech Basin fill specific yield decrease (at 150 years)
- 70. SENSITIVITY ANALYSIS Tetra Tech Steady State Recharge near pit (no infiltration due to facilities)
- 71. SENSITIVITY ANALYSIS Tetra Tech Steady State Recharge near pit (no infiltration due to facilities)
- 72. SENSITIVITY ANALYSIS Tetra Tech 20-percent pit lake evaporation increase
- 73. CONCLUSIONS Different conceptual models provide similar predictions Rate and direction of drawdown propagation varies Impacts are generally within the range of natural fluctuations at distant locations Stream and riparian vegetation impacts depend on groundwater and surface-water interactions Groundwater disconnected from Davidson Canyon stream channel March 8, 2012 73