History matching and predictive modelling of TMA operations at a postash mine site in Saskatchewan
Date
2004
Authors
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Degree Level
Masters
Abstract
Development of decommissioning plans by the potash mine sites in
Saskatchewan require the assessment of long-term brine impact to the
environment and to the aquifer systems in the vicinity of potash tailings
management areas (TMA). Three-dimensional numerical simulations provide the
only means of providing credible predictions of groundwater flow and
contaminant transport for TMAs.
The objective of this study was to use a 3D groundwater flow and transport
model to match the operational history of a selected mine site and estimate the
extent of long-term brine migration from the TMA within the groundwater flow
system. Geological, hydrogeological, and physiographic data were collected,
compiled, and simplified to formulate a conceptual model of the site. This
conceptual model was used to construct a 3D mesh that incorporated this
information and could be used to simulate groundwater and contaminant
transport.
Conceptualization of the groundwater flow system and brine migration from the
site necessitated the utilization of a 3D saturated-unsaturated, variable-density,
flow and transport code. FEMWATER, a finite element method code, developed
for the U.S. Environmental Protection Agency (EPA) and maintained by U.S.
Army Engineer Waterways Experiment Station (WES), was selected to simulate
groundwater flow and contaminant transport.
The steady-state model was developed using data that pre-dated mining activity
and quantifies the natural flow system in the modeled area. Three aquifers are
important in the natural system at the selected mine site: 1) a surficial sand
aquifer; 2) the Floral lntertill Aquifer; and, 3) the Hatfield Valley Aquifer. The
surficial sand aquifer is an important shallow aquifer in the immediate vicinity of
the TMA, receiving an estimated 1,200 m3/d recharge from natural infiltration
over the entire model region. Most of the flow in this shallow system discharges
locally to surface streams and sloughs. The Floral lntertill Aquifer is a confined local aquifer at considerable depth (> 50 m) below the TMA with a relatively low
natural inflow/outflow estimated at 300 m3/d (for the modelled area). The aquifer
heads are near ground surface in the Floral lntertill Aquifer. Wells in this unit can
be flowing artesian, particularly to the south of the TMA. The Hatfield Valley
Aquifer is a major regional aquifer and is located 10 to 20 m below the Floral
lntertill Aquifer beneath the TMA. The Hatfield Valley Aquifer has much larger
inflow/outflows than the other aquifers in the modelled region, estimated at
3,300 m3/d. Flow directions in the deep aquifers are generally to the south and
southeast. There is a predominantly upward gradient from the deep aquifers
over most of the modelled domain.
History matching and predictive results must be regarded as first or second
estimates rather than "fits" or calibrated transient solutions. A major factor
considered at the TMA was porewater pressure, generated by the loading
applied to the system by the tailings pile. The transient calibration process is
ongoing and future improvements are expected, nevertheless the preliminary
results are in reasonable agreement with observational data and the model is
regarded as an effective tool for comparative evaluation of alternatives.
Brine migration depends primarily on the advective velocity field in the more
permeable aquifer units. In the thick tills beneath the site, porewater pressure
responses to loading by the tailings pile are large, but density effects and
diffusion (rather than advection) are likely the controlling factors on brine
migration. Overall, lateral gradients generated by porewater pressure dissipation
in the aquifers appear to have more influence on directing brine migration than
the larger excess porewater pressures in the aquitards.
In the history-match and predictive simulations, brine advances laterally in the
surficial sands as a shallow advective plume and predictions suggest that the
slurry wall and drainage ditch containment systems are effective in intercepting
such movements. The thick till sequence inhibits downward movement of brine
and the deeper aquifers are unaffected by brine in the 100-year model-timeframe.
Large changes in the natural flow system have been induced by the mining
operation as a result of pile loading and freshwater pumping from the Hatfield
Valley Aquifer. In respect to groundwater flow, the impacts of the mine site
extend from the surficial sediments to the deep aquifers and aquitards. Despite
these large and widespread changes in flow patterns, contaminant migration
impacts approximately 380 hectares outside the footprint of the TMA to an
approximate depth of 20 m after 100 years.
Note:Missing: Appendix D title page
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Degree
Master of Science (M.Sc.)
Department
Environmental Engineering
Program
Environmental Engineering