Hydrologic and Water Quality Modeling of the Pebble Mine Project Pit Lake and Downstream Environment after Mine Closure
2. Materials and Methods
2.1. Hydrologic Modeling
2.2. Geochemical Modeling, Inputs, and Assumptions
- Phase 1: Closure Years 1–15 (open pit backfilling of potentially acid generating (PAG) waste rock and pyritic tailings)
- Phase 2: Closure Years 16–20 (backfilling complete, pit lake filling, no water treatment)
- Phase 3: Closure Years 21–50 (treatment of pit waters; waters from bulk tailings storage facility (TSF), SCPs, treatment plant sludge and reject to pit)
- Phase 4: Closure Years 51–in-perpetuity (maintain pit water levels by pumping, pit water treatment, treatment sludge and reject water to pit).
- The pit lake model prepared for the EIS  assumes the pit lake will be stratified in perpetuity, with higher solute waters remaining at the bottom of the pit. As noted in Section 3.1.2., many geologic factors render this a highly unrealistic assumption. We assumed the pit lake will be well-mixed, and the removal of pit water to the treatment plant under managed conditions will not change pit water quality. See discussion in Section 3.1. for pit lake mixing. With a stratified pit and pumping to the treatment plant only from the surface (as proposed in  but not specifically proposed in the Project Description (Appendix N) of the FEIS), the lower-solute water would be preferentially removed from the pit over time, potentially increasing remaining concentrations when mixing occurs.
- Ongoing releases of metals, sulfate, and acidity from backfilled PAG waste rock and pyritic tailings are not included. Releases from submerged pit walls are also not included.
- Geochemical source terms (pit inflow concentrations) are taken from  (Appendix B1 and B2; 50th percentile values) and  (Appendix B1 and B2; 50th percentile values) (Table 1). Bulk TSF supernatant and Bulk TSF Main SCP pH values for Closure Phases 2 and 3 were taken from  (Table B1.1), because the DEIS  (Appendix K4.18) did not include pH values (“pH was not modeled”).
- Assumptions for water quality were the same as those used in  (Table B1.2), including that pit wall source terms were used for direct precipitation, additional snow blow, and pit wall runoff.
- Metals will behave conservatively (i.e., no precipitation or adsorption), as assumed in the DEIS [15,23] for chemical failure modes examined (spills of bulk and pyritic tailings and ore concentrate), which assume that the transport of spilled tailings and resulting downstream metal concentrations are only affected by dilution.
- The geologic map of the pit area was used to estimate the proportion of the pit walls that would generate acidic vs. non-acidic leachate. No estimates of the percentage of Pre-Tertiary and Tertiary rock on the completed pit walls are provided in the DEIS or associated documents, and the one geologic map of the pit rocks (see Section 3.1.) shows no Pre-Tertiary (non-acidic) material in the open pit area. Lorax Environmental  assumed all loadings from Pre-Tertiary wall rock would be acidic, and the water quality model for the FEIS assumed that all PAG wall rock would be 100% acidic after only 10 years. In this study, the pit walls were assumed to be 90% Pre-Tertiary acidic and 10% Tertiary non-acidic material. Lower percentages of Pre-Tertiary material were also modeled to evaluate uncertainty in wall rock leachate inputs.
2.3. Surface Water Mixing
3.1. Conceptual Hydrogeologic Model Assumptions and Alternatives
3.1.1. Potential Pit Inflows and Outflows
3.1.2. Pit Lake Stratification, Pit Wall Stability, and Water Quality Implications
3.1.3. Pit Wall Leaching and Pit Water Quality
3.1.4. Failure Modes and Abandonment Scenario
3.2. Fate and Transport of Pit Lake Overflow and Outflow
3.2.1. Prediction of Pit Lake Abandonment and Effects on Water Flows
3.2.2. Prediction of Pit Lake Water Quality Using DEIS Inputs and PHREEQC Modeling
3.2.3. Prediction of Water Quality Effects to the South Fork Koktuli River from Management Failure or Abandonment
3.2.4. Discussion of Uncertainty
Conflicts of Interest
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|Pit Lake Inflow Descriptions||Water Quality Source Terms and Assumptions ||Solution Number for Mixing in PHREEQC||Mixing Percentages Used in PHREEQC||Water Quality Source Terms Used in PHREEQC|
|Direct Precipitation||Pit wall source term||1 (Pre-Tertiary acidic, 90%); 2 (Tertiary non-acidic, 10%)||1: 17.5%|
|Pit wall source term 1; (subtracted total evaporation)|
|Undisturbed Surface Runoff||SFK 100F||4||4: 8.60%||SFK 100F 2|
|Diversion Channel Leakage||SFK 100F||4||SFK 100F|
|Groundwater||Pit area groundwater||3||3: 16.0%||Pit area groundwater 2|
|Additional Snow Blow||Pit wall source term||1 and 2||Pit wall source term 1|
|Pyritic Tailings Re-Slurry Water to Open Pit||Calculated concentrations in PTSF (assuming full mixing)||5||5: 20.1%||Open Pit-Closure, Backfilled Waste Rock, high pyritic tailings 3|
|Pit Wall Runoff||Pit wall source term||1||Pit wall source term|
|Reject Flows and Sludge Flows from WTP #2||WTPs—sludge and reject concentrations||6 (sludge) and 7 (reject brine)||6: 2.47%|
|WTP sludge and|
WTP reject 4
|Reject Flows and Sludge Flows from WTP #3||WTPs—sludge and reject concentrations||6 (sludge) and 7 (reject brine)||WTP sludge and|
|Surplus from Bulk TSF-Phase 2||Bulk TSF||8||8: 3.2%||Bulk TSF, Closure|
Phase 2 5
|Surplus from Bulk TSF-Phase 3||Bulk TSF||9||9: 27.9%||Bulk TSF, Closure|
Phase 3 6
|Surplus from Bulk TSF Main SCP||Calculated concentrations in Main Embankment SCP||10||10: 2.2%||Bulk TSF Main Embankment SCP, Closure Phase 2 5|
|pH (field, Standard Units)||3.54–8.85||6.63|
|Water Temperature (°C)||−0.33–23.4||4.33|
|Specific Conductance (field, μS/cm)||20–133||52.3|
|Calcium (mg/L, dissolved)||2.28–13.4||6.18|
|Magnesium (mg/L, dissolved)||0.35–3.9||1.4|
|Sodium (mg/L, dissolved)||1.09–4.67||2.33|
|Potassium (mg/L, dissolved)||0.12–1.07||0.36|
|Alkalinity (total; mg/L as CaCO3)||3.1–40||18|
|Hardness (mg/L as CaCO3)||7.91–52.9||20.5|
|Cadmium (μg/L, dissolved)||0.0062–0.074||0.019|
|Copper (μg/L, dissolved)||0.15–4.9||1.1|
|Lead (μg/L, dissolved)||0.022–0.42||0.072|
|Zinc (μg/L, dissolved)||0.47–11||2.8|
|20-Year Pit||78-Year Pit|
|SW outflow 1||0.0||2.4||118||36.0|
(mg/L or SU)
|Lorax Environmental (2018), Closure Year 105 a||PHREEQC Predicted Pit Lake, Closure Year 105 b||SFK WQC (mg/L) c||Lorax: Conc/WQC||PHREEQC: Conc/WQC||Berkeley Pit, MT, USA 10/16/87 d|
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Maest, A.; Prucha, R.; Wobus, C. Hydrologic and Water Quality Modeling of the Pebble Mine Project Pit Lake and Downstream Environment after Mine Closure. Minerals 2020, 10, 727. https://doi.org/10.3390/min10080727
Maest A, Prucha R, Wobus C. Hydrologic and Water Quality Modeling of the Pebble Mine Project Pit Lake and Downstream Environment after Mine Closure. Minerals. 2020; 10(8):727. https://doi.org/10.3390/min10080727Chicago/Turabian Style
Maest, Ann, Robert Prucha, and Cameron Wobus. 2020. "Hydrologic and Water Quality Modeling of the Pebble Mine Project Pit Lake and Downstream Environment after Mine Closure" Minerals 10, no. 8: 727. https://doi.org/10.3390/min10080727