EVALUATION OF METHODS AND EVALUATION OF METHODS AND MODELS USED TO PREDICT WATER MODELS USED TO PREDICT WATER

QUALITY AT HARDROCK MINE SITES: QUALITY AT HARDROCK MINE SITES: SOURCES OF UNCERTAINTY AND SOURCES OF UNCERTAINTY AND

RECOMMENDATIONS FOR RECOMMENDATIONS FOR IMPROVEMENTIMPROVEMENT

Ann Maest, James Kuipers, Connie Travers, and David Ann Maest, James Kuipers, Connie Travers, and David AtkinsAtkins

Buka Environmental; Kuipers and Associates; Stratus Buka Environmental; Kuipers and Associates; Stratus Consulting, Inc.Consulting, Inc.

WMAN Conference, Worley, IDWMAN Conference, Worley, IDOctober 1, 2005October 1, 2005

Why Characterize and Predict?Why Characterize and Predict? Regulators use characterization and modeling Regulators use characterization and modeling

information to determine if a mine will be protective of information to determine if a mine will be protective of water resources during and after miningwater resources during and after mining

Will mine generate acid and contaminants?Will mine generate acid and contaminants? Future environmental liability – set bonds Future environmental liability – set bonds Cost of remediating mine sites on the National Priorities Cost of remediating mine sites on the National Priorities

List (NPL) ~$20 billionList (NPL) ~$20 billion Recent increases in the prices of precious and base Recent increases in the prices of precious and base

metals have triggered increase in new mines around the metals have triggered increase in new mines around the worldworld

~170 large hardrock mines in US in various stages of ~170 large hardrock mines in US in various stages of permitting, operation, closurepermitting, operation, closure

This StudyThis Study

Lays out framework for evaluating Lays out framework for evaluating methods and models used to predict water methods and models used to predict water quality at hardrock mine sitesquality at hardrock mine sites

Makes recommendations for improvementMakes recommendations for improvement Intended audience: regulators, citizens, Intended audience: regulators, citizens,

mine operators and managersmine operators and managers

Nature of PredictionsNature of Predictions

Forward modeling Forward modeling Timeframe of impacts Timeframe of impacts UncertaintiesUncertaintiesRegulatory authorities require predictionsRegulatory authorities require predictions

Study ApproachStudy Approach

Synthesize existing reviews Synthesize existing reviews Develop “toolboxes” Develop “toolboxes” Evaluate methods and modelsEvaluate methods and modelsRecommendations for improvementRecommendations for improvementOutside peer review (Logsdon, Nordstrom, Outside peer review (Logsdon, Nordstrom,

Lapakko)Lapakko)Case studies – NEPA/EIS StudyCase studies – NEPA/EIS Study

Characterization MethodsCharacterization Methods

Method descriptionMethod descriptionMethod referenceMethod referenceUse in water quality predictionsUse in water quality predictionsAdvantagesAdvantagesLimitationsLimitations

Characterization during different phases of Characterization during different phases of miningmining

Sources of Uncertainty - GeneralSources of Uncertainty - General

Extent/representativeness of Extent/representativeness of environmental samplingenvironmental samplingneed more environmental sampling; let need more environmental sampling; let

geologic/mineralogic variability dictate extent geologic/mineralogic variability dictate extent of sampling; define geochemical test units of sampling; define geochemical test units

Mass of Each Separate Rock Type (tonnes)

Minimum Number of Samples

<10,000 3

<100,000 8

<1,000,000 26

10,000,000 80

Recommended Minimum # Recommended Minimum # SamplesSamples

Sources of Uncertainty – StaticSources of Uncertainty – Static

Effect of mineralogy on NP and APP Effect of mineralogy on NP and APP Rely on mineralogy more than on Rely on mineralogy more than on

operationally defined lab testsoperationally defined lab tests Interpretation of static testing resultsInterpretation of static testing results

only use as initial screening technique to only use as initial screening technique to estimate total amount of AGP/ANPestimate total amount of AGP/ANP

Sources of Uncertainty – Leach Sources of Uncertainty – Leach TestsTests

Water:rock ratioWater:rock rationever known definitively; 20:1 too dilutenever known definitively; 20:1 too dilute

Use of unweathered materialsUse of unweathered materialsmust start with weathered materialsmust start with weathered materials

Interpretation of resultsInterpretation of resultsmay have limited use as scoping tool if use may have limited use as scoping tool if use

weathered rock and evaluate applicability of weathered rock and evaluate applicability of resultsresults

Sources of Uncertainty - KineticSources of Uncertainty - KineticParticle sizeParticle size

minimize amount of size reduction for minimize amount of size reduction for samples – field/lab discrepanciessamples – field/lab discrepancies

Length of testsLength of tests20 weeks is too short for kinetic tests, unless 20 weeks is too short for kinetic tests, unless

shown to be AG before then. NPshown to be AG before then. NP≥APP.≥APP. Interpretation of resultsInterpretation of results

analyze effluent for all COCs; use for short- analyze effluent for all COCs; use for short- and long-term AGP/leaching potentialand long-term AGP/leaching potential

Length of Kinetic TestsLength of Kinetic Tests

Source: Nicholson and Rinker, 2000 (ICARD).

Percent of NEPA Mines Conducting Different Types of Geochemical Characterization

None10%

Static only13%

Short-term leachonly6%

Static+short-termleach18%

Static+kinetic16%

Static+short-termleach+kinetic

37%

N = 69

Characterize geology,

alteration, mineralogy,

liberation

Define geochemical

test units; estimate volumes

Determine # samples/

unit

Bench-scale testing

Whole rock analysis of

each test unit

Static testing for each test

unit

Modify APP and ANP based on

mineralogyTailings?

Mineralogy

Yes

Aerially exposed: humidity cell

testsSubmerged: batch tests

Aerially exposed: aerobic column tests/

minewall washingSubmerged:

Continuous-flow column tests

Site-specific scaling factors

Kinetic testing for

each test unit

No

Mineralogy

Results for total amt AG+NP

material, block model, waste management

Potential COCs

Results for short/long-

term AGP and contaminant-

leaching potential

Inputs for geochemical

models

Modeling ToolboxModeling Toolbox

Category/subcategory of codeCategory/subcategory of codeHydrogeologic, geochemical, unit-specificHydrogeologic, geochemical, unit-specific

Available codesAvailable codesSpecial characteristics of codesSpecial characteristics of codes Inputs requiredInputs requiredModeled processes/outputsModeled processes/outputsStep-by-step procedures for modeling Step-by-step procedures for modeling

water quality at mine facilitieswater quality at mine facilities

Modeling OpportunitiesModeling Opportunities

water table(approximate)

Pit Outline

Waste Rock PileHeap/Dump Leach Pile

Precipitation

Evapotranspiration Precipitation

EvapotranspirationInfiltration

Runoff

Vadose Zone/Geochemical Models

Groundwater Flow and Geochemical Speciation/ Reaction Path Models

Limnologic ModelsGeochemical Speciation/Reaction Path Models

Stream/RiverModels

Precipitation

Evapotranspiration

Near SurfaceHydrologic Models

Sediment GenerationModels

Pyrite Oxidation andGeochemical Speciation/Reaction Path Models

Site-Wide Conceptual

Model

Baseline Conditions

Sources

Pathways

ProcessesMitigations

Receptors

SourcesSources

PathwaysPathways

ProcessesProcesses

Sources of Uncertainty - ModelingSources of Uncertainty - Modeling Conceptual modelConceptual model

Conceptual models are not unique and can change over timeConceptual models are not unique and can change over time Revisit conceptual models and modify mining plans and Revisit conceptual models and modify mining plans and

predictive models based on new site-specific information predictive models based on new site-specific information Use of proprietary codesUse of proprietary codes

need testable, transparent models – difficult to evaluate, should need testable, transparent models – difficult to evaluate, should be avoided. Need efforts to expand publicly available pit lake be avoided. Need efforts to expand publicly available pit lake models (chemistry).models (chemistry).

Modeling inputsModeling inputs large variability in hydrologic parameters; seasonal variability in large variability in hydrologic parameters; seasonal variability in

flow and chemistry; sensitivity analyses (ranges) rather than flow and chemistry; sensitivity analyses (ranges) rather than averages/mediansaverages/medians

Estimation of uncertaintyEstimation of uncertainty Acknowledge and evaluate effect on model outputs; test multiple Acknowledge and evaluate effect on model outputs; test multiple

conceptual modelsconceptual models “…“…there is considerable uncertainty associated with long-term there is considerable uncertainty associated with long-term

predictions of potential impacts to groundwater quality from predictions of potential impacts to groundwater quality from infiltration through waste rock...for these reasons, predictions should infiltration through waste rock...for these reasons, predictions should be viewed as indicators of long-term trends rather than absolute be viewed as indicators of long-term trends rather than absolute values.”values.”

Develop Site Conceptual

Model

Gather input data for

geochemical test units and receptors

Select appropriate model(s) for

predicting water quality

Conduct modeling to determine

concentrations at receptors/other

locations

Conduct sensitivity/uncertainty

analysis using range of input

values

Evaluate effect of

mitigations

Concentrations at receptors > standards?

Yes

EndNo

Concentrations at receptors > standards?

No

Redesign Mine Plan

Yes

Percent of NEPA Mines Using General Types of Predictive Models

No Models44%

Water Quantity26%

Water Quality1%

Water Quantity+Quality

29%

N = 69

SummarySummary Characterization methods need major re-Characterization methods need major re-

evaluation, especially static and short-term leach evaluation, especially static and short-term leach teststests

Increased use of mineralogy in characterization Increased use of mineralogy in characterization – make less expensive, easier to use/interpret– make less expensive, easier to use/interpret

Modeling uncertainty needs to be stated and Modeling uncertainty needs to be stated and defineddefined

Limits to reliability of modeling – use ranges Limits to reliability of modeling – use ranges rather than absolute valuesrather than absolute values

Increased efforts on long-term studies and Increased efforts on long-term studies and collection of site-specific data over modelingcollection of site-specific data over modeling

ConclusionConclusion

Predictive modeling is an evolving science Predictive modeling is an evolving science with inherent uncertaintieswith inherent uncertainties

Using the approaches described in this Using the approaches described in this report, predictive water quality modeling report, predictive water quality modeling and site characterization information can and site characterization information can be reliably used to design protective be reliably used to design protective mitigation measures and to estimate the mitigation measures and to estimate the costs of future remediation of hardrock costs of future remediation of hardrock mine sites.mine sites.