Incorporating Ecological Risk Considerations into a Mine Design

Transcription

1 Visionary Environmental Consulting Incorporating Ecological Risk Considerations into a Mine Design Brigitte Howe 1/29/2013

2 Presentation Overview Describes uses for screening-level ecological risk assessment (SLERAs) at mine sites Describes how a SLERA was used at a confidential site to: Evaluate potential ecological risks associated with a planned expansion Use the SLERA results to modify the design elements Evaluate additional mitigation measures

3 Uses for SLERAs at Mine Sites Pit lakes Regulation NAC 445A.429 requires that pit lake water quality does not adversely affect ecological health Triggered if Bureau of Land Management (BLM) National Environmental Policy Act (NEPA) analyses indicate water quality issues Often done pro-actively Other surface water features (e.g., evaporation ponds) Evaluation in the Environmental Impact Statement (EIS) or Plan of Operations (PoO) may be required by the BLM Associated soils may also need to be evaluated Sludge and waste water Determine land application suitability

4 Initial Expansion Plan Tailings Storage Facility Heap leach pad

5 SLERA Overview SLERA Scope Screening-level evaluation based on conservative assumptions (e.g., maximum chemical concentrations) Not intended to provide definitive estimates of risks Screen out unimportant exposures and focus subsequent evaluations (e.g., BERA) SLERA Guidance Nevada Bureau of Land Management (BLM) US Environmental Protection Agency (EPA)

6 SLERA Approach Data Evaluation Estimate chemical concentrations for each pond For operating and closure conditions Exposure Assessment Identify key receptor populations and exposure pathways Develop site conceptual exposure model Identify toxicity reference values (TRVs) Calculate hazard quotients (HQs)

7 Data Evaluation Challenge: predict concentrations in the planned ponds Available data: Supernatant from tailings - untreated Supernatant - bottle rolled and rinsed (BTR) Supernatant - BTR and meteoric water mobility procedure (MWMP) Supernatant - humidity cell test (HCT) MWMP data from existing heap leach pad Evaluated potential representativeness for each pond Based on planned processing methods Intent was to be conservative

8 Data selection for each pond Operating Conditions Rougher Pond Use untreated drum supernatant samples Tailings pond/tailings draindown pond Inputs will be cyanide leached Use HCT data Heap leach pond MWMP data from existing heap leach pad Post Closure Conditions Evapoconcentrate to 80 % using EQ3/6. Example Concentrations (mg/l) Rougher Talings and Draindown Heap Leach As Se

9 SLERA Exposure Assessment SLERA indicator species Key exposure pathway: ingestion of pond water

10 Risk Characterization Risk estimation method Ecological risks are described as hazard quotients (HQs) Formula: HQs estimated for each receptor/chemical combination If the HQ is < 1, a potential for ecological risk is not expected If the HQ is > 1, ecological risk is possible and further evaluation is warranted 1 TRVs in this SLERA were based on no observed adverse effects levels (NOAELs).

11 SLERA Results - Summary of HQs > 1 Rougher pond Aluminum and thallium white-tailed deer, coyote, white-footed mouse Tailings and tailings draindown ponds Aluminum and thallium Same receptors as above plus cottontail rabbit, little brown bat More HQs > one for post-closure conditions Heap leach pond Aluminum, thallium, antimony, arsenic, mercury, selenium Same receptors as for tailings pond, one avian receptor (swallow) More HQs > one for post-closure conditions

12 SLERA Results (continued) HQs were < 10 for all ponds not very high Estimated pond concentration also compared to Nevada beneficial use criteria (NBUC) for life stock Criteria exceeded for several chemicals ph typically < 4, whereas NBUC = Overall SLERA results indicate that the ponds could pose risks to wildlife Nevada criteria would be exceeded

13 Management Considerations Could perform a refined SLERA to further evaluate risks However, the NBUC still may apply Future concentrations estimates are uncertain Based on limited data could theoretically be worse than estimated Physical measures to deter wildlife impractical for some ponds due to size Management decision: change controllable elements of the expansion to proactively prevent future liability

14 Redesigned Tailings Storage Facility Replaces rougher pond Replaces cyanide leach Replaces tailings draindown pond

15 Additional Measures Keep tailings under water for the first 2 years of operations Reduce reaction potential Change Tailings Storage Facility design to minimize water Reduces size and contact potential for ecological receptors Makes it easier to use physical deterrents No design changes for the heap leach pad and pond Use measures to minimize contact (e.g., bird netting)

16 Conclusions SLERAs may be required for permitting of mine expansions and closures Can be used to demonstrate absence of risks identify possible mitigation measures Avoid undesired liabilities Advantageous to perform SLERA early in the planning process for increased management flexibility Collect data to limit uncertainties Perform more realistic risk assessments Re-design to avoid liability

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