Regional Groundwater Flow Models

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A presentation on the groundwater models (both Tetra Tech and EL Montgomery) associated with the Rosemont Copper Project Operations. This presentation was given by Engineering Analytics to the Forest Service, Bureau of Land Management, Fish and Wildlife Service, Arizona Game and Fish and their contractors during a meeting in March 2012.

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Regional Groundwater Flow Models

  1. 1. Regional Groundwater Flow Models Presented to U.S. Fish and Wildlife Service Grady O’Brien March 8, 2012
  2. 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 ConclusionsMarch 8, 2012 2
  3. 3. INTRODUCTION Rosemont Site Location 3March 8, 2012
  4. 4. Regional Site Map for the Proposed Rosemont Mine
  5. 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 rockMarch 8, 2012 5
  6. 6. MODELING OBJECTIVES Predict regional hydrologic impacts Sensitive areas  Davidson Canyon  Cienega Creek  Las Cienegas National Conservation Area Sensitive features  Springs  Stream flow  Riparian vegetationMarch 8, 2012 6  Aquatic life
  7. 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 channelsMarch 8, 2012 7
  8. 8. MODELING APPROACH Satisfy the objectives Appropriate scale  Site versus Regional Fracture network versus porous media Simulate hydrogeologic features Data availabilityMarch 8, 2012 8
  9. 9. MODELING SCALE INFLUENCES APPROACH Regional impacts  regional scale Regional scale  regional resolution Data availability Fracture network versus porous mediaMarch 8, 2012 9
  10. 10. DATA AVAILABILITY Geology (site and regional scales) Wells  730+ water-level targets  Lithology  Water quality Springs and Streams  Discharge  Water quality Riparian vegetationMarch 8, 2012 10 Wells used for model calibration (Tetra Tech, 2010)
  11. 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 flowsMarch 8, 2012 11
  12. 12. DATA AVAILABILITY Tetra Tech  Site Water Management  Infiltration analysis  Infiltration, seepage, fate, and transport modeling  Storm-water runoffMarch 8, 2012 12
  13. 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 basinMarch 8, 2012 13 Various recharge analyses
  14. 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 modelMarch 8, 2012 14
  15. 15. DATA AVAILABILITY Extensive data available for model input Coverage across the region Best available data usedMarch 8, 2012 15
  16. 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 impactMarch 8, 2012 16
  17. 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 hydrogeologyMarch 8, 2012 17
  18. 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. 19. FRACTURED BEDROCK Intersecting FracturesMarch 8, 2012 19
  20. 20. FRACTURE CONNECTIVITYInfluence of Persistence of Discontinuity on the Degree ofFracturing and Interconnectivity March 8, 2012 Anderson, M.P. and Woessner, W.W., 2002, Applied Groundwater 20 Modeling: Academic Press, San Diego, CA, 381p.
  21. 21. GROUNDWATER FLOW THROUGH FRACTURESType 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. 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 connectionMarch 8, 2012 22
  23. 23. INTERACTION OF STREAMS and GROUNDWATER March 8, 2012 23Alley, 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. 24. INTERACTION OF STREAMS and GROUNDWATERMarch 8, 2012 24
  25. 25. MODELING APPROACH AND USES 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. 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. 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. 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. 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. 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. 31. MODEL CONSTRUCTION COMPARISON External model boundaries Geology Recharge Streams ETMarch 8, 2012 31
  32. 32. MODEL GRID AND EXTERNALBOUNDARYCONDITIONS Montgomery & Associates (2010)
  33. 33. MODEL LAYERS Tetra Tech (2010)March 8, 2012 33
  34. 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. 35. GEOLOGY(Tetra Tech, 2010)
  36. 36. GEOLOGY(Montgomery and Assoc., 2010)
  37. 37. HYDRAULICCONDUCTIVITY(Tetra Tech, 2010)
  38. 38. HYDRAULICCONDUCTIVITY (Montgomery and Assoc., 2010)
  39. 39. GEOLOGY TO FLOW MODEL
  40. 40. GEOLOGYMarch 8, 2012 40
  41. 41. RECHARGEPrecipitation – runoff approach bysurface-water basin (Tetra Tech, 2010)
  42. 42. RECHARGE Simulatedpre-mining steady- state recharge (Tetra Tech, 2010)
  43. 43. RECHARGESimulated recharge - before and during 22-year mining period (Montgomery & Associates, 2010)
  44. 44. RECHARGE Post-closurerecharge in project facility area (Tetra Tech, 2010)
  45. 45. RECHARGESimulated recharge for mining facilitiesfor 1,000-year post- mining period (Montgomery & Associates, 2010)
  46. 46. Evapotranspiration (ET) (Montgomery & Associates, 2010)
  47. 47. MODEL PREDICTION COMPARISONS Pit-lake formation and inflows Drawdown propagation Stream flow changes ET changes March 8, 2012 47
  48. 48. MODEL PREDICTION COMPARISON Pit-Lake Formation and InflowsPit Lake Conceptual Model (Tetra Tech, 2010b) March 8, 2012 48
  49. 49. MODEL PREDICTION COMPARISON Pit-Lake Formation and InflowsSimulated Pit Lake Water Balance (Tetra Tech, 2010) March 8, 2012 49
  50. 50. MODEL PREDICTION COMPARISON Pit-Lake Formation and InflowsSimulated Pit Lake Water Balance (Montgomery and Associates,2010) March 8, 2012 50
  51. 51. DRAWDOWNSimulated drawdown at end of mining (Tetra Tech, 2010)
  52. 52. DRAWDOWNSimulated drawdown at end of mining (Montgomery & Associates, 2010)
  53. 53. DRAWDOWNSimulated drawdown 20 years after closure (Tetra Tech, 2010)
  54. 54. DRAWDOWNSimulated drawdown 20 years after closure (Montgomery & Associates, 2010)
  55. 55. DRAWDOWNSimulated drawdown 150 years after closure (Tetra Tech, 2010)
  56. 56. DRAWDOWNSimulated drawdown 150 years after closure (Montgomery & Associates, 2010)
  57. 57. DRAWDOWNSimulated drawdown 1,000 years after closure (Tetra Tech, 2010)
  58. 58. DRAWDOWNSimulated drawdown 1,000 years after closure (Montgomery & Associates, 2010)
  59. 59. STREAMS Stream gage locations andsimulated flows (Tetra Tech, 2010)
  60. 60. STREAMSSimulated Stream Flow(Montgomery & Associates, 2010)
  61. 61. STREAM FLOW CHANGESSimulated change instream flows at end of operations (Tetra Tech, 2010)
  62. 62. STREAM FLOW CHANGESSimulated change in stream flows 1,000 years post closure (Tetra Tech, 2010)
  63. 63. MODEL PREDICTION COMPARISON Stream Flow ChangesImpact of Dewatering at Cienega Creek(Montgomery & Associates, 2010) March 8, 2012 63
  64. 64. MODEL PREDICTION COMPARISON Stream Flow ChangesImpact of Dewatering at Davidson Canyon(Montgomery & Associates, 2010) March 8, 2012 64
  65. 65. MEASURED STREAM FLOW CHANGESMarch 8, 2012 65
  66. 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. 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. 68. SENSITIVITY ANALYSIS Tetra TechBedrock specific yield decrease (at 150 years)
  69. 69. SENSITIVITY ANALYSIS Tetra TechBasin fill specific yield decrease (at 150 years)
  70. 70. SENSITIVITY ANALYSIS Tetra TechSteady State Recharge near pit (no infiltration due to facilities)
  71. 71. SENSITIVITY ANALYSIS Tetra TechSteady State Recharge near pit (no infiltration due to facilities)
  72. 72. SENSITIVITY ANALYSIS Tetra Tech 20-percent pit lakeevaporation increase
  73. 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

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