Aachen, Germany "Mine Water- Managing the Challenges" IMWA 2011

Reactive Transport Modeling for the Proposed Dewey Burdock Uranium In-SituRecovery Mine, Edgemont, South Dakota, USA

Raymond H. Johnson

US. Geological Survey, PO Box 25o46, Denver Federal Center, Denver, CO 80225

Abstract In-situ recovery (ISR) mining extracts uranium by enhanced dissolution and mobilization of solid-phase uranium in sandstone aquifers. Geochemical changes that occur due to the ISR mining process are im-portant for local groundwater users, regulatory agencies, and other stakeholders to understand in order toevaluate the potential effects on surrounding groundwater quality during and after mining. Reactive trans-port modeling is being used at the proposed Dewey Burdock ISR mine to simulate the geochemistry of: i)uranium roll-front deposition; 2) current groundwater conditions; 3) mining processes; 4) post-miningrestoration; and, 5) long-term groundwater quality after restoration. This modeling uses groundwater flowcoupled with rock/water interations to understand geochemical changes during each stage. Conceptually,uranium roll-fronts are formed as oxygenated, uranium-rich groundwaters enter reducing zones where ura-nium minerals precipitate to form uranium ore. Through geologic time, the groundwater flow direction andincoming groundwater geochemistry can change, which may or may not alter the uranium roll-front deposit.During the mining process, oxygen and a complexing agent (such as carbon dioxide) are added to oxidize,solubilize, and remove the uranium. Post-mining, the mining solution is removed, and reducing agents maybe added to re-precipitate uranium. Longer term geochemistry depends upon the remaining solid-phaseminerals, their reactivity, and the composition of the incoming groundwater. All of these processes are high-lighted through the use of a simple three-dimensional reactive transport model (groundwater flow and geo-chemistry). While this research focuses specifically on the proposed Dewey Burdock uranium ISR site nearEdgemont, South Dakota, the procedures described are generally applicable to any proposed uranium ISRmine.

Key Words uranium, in-situ recovery, reactive transport modeling

IntroductionBackground information on the formation ofsandstone-hosted uranium roll-front depositsand predictive modeling strategies can be foundin Johnson et al. (2010). The proposed Dewey Bur-dock in-situ recovery (ISR) mine near Edgemont,South Dakota (USA) is being used for this study;however, the techniques discussed in this paperare applicable to similar sites. As such, the figuresin this paper focus on the techniques, approaches,and results, not site specific geology.

Reactive Transport ModelingReactive transport modeling for this paper usesPHAST (Parkhurst et al. 2010). PHAST uses a rela-tively simple groundwater flow code coupled withPHREEQC (Parkhurst and Appelo 1999) to calcu-late geochemical conditions at each time step. Forthis paper, the groundwater flow velocities andmass balances for the solid phase are still generic,as well as the time. Site specific data will be addedas the project progresses. Figures i through 8 arethe most relevant slides that show geochemicalprocesses and are a subset of a full twenty step an-imation. In these figures numerical dispersiondoes exist (actual dispersion is set to zero) due tothe large time steps. This dispersion may be rep-resentative of actual conditions, but still needs tobe evaluated further. Subsequent refinement of

flow velocities and solid phase concentrations willinclude improved time stepping to avoid artificialdispersion. The model domain size is also genericand has been run in three dimensions, but the fig-ures only represent two dimensions. Flow is al-ways from left to right and cooler colors (blue, teal,and green) are lower concentrations and warmercolors (yellow, orange, and red) are higher-concen-trations (figs. 1 - 8). Concentration units are alsogeneric.

SimulationsUranium roll-fronts are formed as groundwatercontaining oxygen and dissolved uranium moveinto a zone with solid phase reductants (organiccarbon and/or pyrite). This results in the precipi-tation of a reduced uranium mineral, such asuraninite (fig. 1). The initial geochemistry in thesesimulations start with pyrite, no uraninite, and nouranium or oxygen in the groundwater. Ground-water with uranium and dissolved oxygen areadded to the model domain, which progressivelyconsumes pyrite and forms a uraninite ore de-posit at the oxidized/reduced interface (fig. 1).Chloride has been added as a conservative tracer(fig. 1).

At the Dewey Burdock site, the current ground-water in the roll-front area does not contain anydissolved oxygen. Because of this, these roll fronts

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IMWA 2011 "Mine Water- Managing the Challenges" Aachen, Germany

Figure i Initial uranium roll-front formation. Flow Figure 2 Uranium rollfront with current ground-is from left to right, water conditions. Flow is from left to right.

Figure3 Five-spot ISR mining with oxygen and Figure4 End of mining with lower uranium con-carbon dioxide. Flow is from left to right. centrations in solution. Flow is from left to right.

are probably not being formed at the present. Tosimulate pre-mining conditions, groundwaterwith no dissolved oxygen was transportedthrough the model domain. The result is a solidphase uraninite roll front associated with pyriteon the solid-phase reduced side and uranium anddissolved oxygen are not found in solution (fig. 2).

Uranium in-situ mining is simulated using aleach solution with oxygen and carbon dioxide.The resulting oxidation makes uranium solubleand the carbon dioxide creates a complexingagent. A five-spot well pattern is simulated with acenter pumping well and four surrounding injec-tion wells (fig. 3). In the ore zone, the result is ele-vated concentrations of uranium in thegroundwater where the ore zone is being minedand uraninite and pyrite are completely removed(fig. 3). Once mining is complete (fig. 4), uraniumin solution is much lower and the mined area hashigh dissolved oxygen and chloride.

During the restoration phase, the existing wellfield is used to flush out the mining solutionsfrom the groundwater. This process was simu-

lated as water with low dissolved consitutent con-centrations, but with 50 ppb residual uranium(figs. 5 and 6). In one simulation, oxygen was leftin the restoration fluids (fig. 5) and in another sim-ulation, oxygen was kept at zero (fig. 6). At thisstage, reductant addition to help precipitate ura-nium could be simulated, but was not completedfor this paper.

An important issue in uranium in-situ recov-ery mining is the groundwater quality post-min-ing and the potential for long-term naturalattenuation. For longer-term groundwater quality,inflowing water of similar composition to thegroundwater found at the Dewey Burdock site wasadded (calcium sulfate water with no dissolvedoxygen). For restoration that left behind dissolvedoxygen, the solid phase uraninite and pyritezones continue to move downgradient and someuranium and dissolved oxygen are found in thegroundwater (fig. 7). For restoration with no dis-solved oxygen left behind, the movement of theuraninite and pyrite is much less and uranium isnot found in solution (fig. 8).

222 Rude, Freund E.; Wolkersdorfer (Editors)

Aachen, Germany "Mine Water - Managing the Challenges" IMWA 2011

Figure 5 Restoration with dilute water (no chlo-ride) but some dissolved oxygen. Flow isfrom left

to right.

Figure 6 Restoration with dilute water (no chlo-ride) and no dissolved oxygen. Flow is from left to

right.

Figure 7 Return to ambient flow after restorationthat had dissolved oxygen. Flow is from left to

right.

SummaryThis paper provides initial reactive transport sim-ulations that support our conceptual understand-ing of uranium roll-front formation, currentgroundwater conditions, mining geochemistry,restoration geochemistry, and long-term ground-water quality at a uranium ISR site. These simula-tions are a starting point for additionalrefinement that will reflect detailed site condi-tions and can provide stakeholders with a betterunderstanding of groundwater quality during andafter uranium ISR mining.

AcknowledgementsThe author thanks the USGS Uranium Resources andthe Environment Project and the U.S. EPA RegionalApplied Research Effort grant for funding this proj-ect.

Figure 8 Return to ambientflow after restorationwith no dissolved oxygen. Flow is from left to

right.

ReferencesJohnson RH, Yoshino ME, Hall, SM, Shea, VR, (2010)

Predictive modeling strategies for operations andclosure at uranium in-situ recovery mines. Inter-national Mine Water Association 2010, Mine Waterand Innovative Thinking, Proceedings, September5-9, 2010, Sydney, Nova Scotia, p 475-478

Parkhurst DL and Appelo CAl (1999) User's guide toPHREEQC (Version2)-A computer program for-speciation, batch-reaction, one-dimensionaltransport, and inverse geochemical calculations.U.S. Geological Survey Water-Resources Investiga-tions Report 99-4259,310 pp

Parkhurst DL, Kipp KL, and Charlton SR (2010) PHASTVersion 2-A program for simulating groundwa-ter flow, solute transport, and multicomponentgeochemical reactions. U.S. Geological SurveyTechniques and Methods 6-A35, 235 pp

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