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Lakewood, CO, United States

Meyerhoff S.B.,Colorado School of Mines | Meyerhoff S.B.,Itasca Denver Inc. | Maxwell R.M.,Colorado School of Mines | Revil A.,Colorado School of Mines | And 4 more authors.
Water Resources Research | Year: 2014

Groundwater flow in karst includes exchange of water between large fractures, conduits, and the surrounding porous matrix, which impacts both water quality and quantity. Electrical resistivity tomography combined with end-member mixing analysis (EMMA) and numerical flow and transport modeling was used to study mixing of karst conduit and matrix waters to understand spatial and temporal patterns of mixing during high flow and base flow conditions. To our knowledge, this is the first time EMMA and synthetic geophysical simulations have been combined. Here we interpret an 8 week time-lapse electrical resistivity data set to assess groundwater-surface mixing. We simulate flow between the karst conduits and the porous matrix to determine fractions of water recharged to conduits that has mixed with groundwater stored in the pore space of the matrix using a flow and transport model in a synthetic time-lapse resistivity inversion. Comparing the field and synthetic inversions, our results enable us to estimate exchange dynamics, spatial mixing, and flow conditions. Results showed that mixing occurred at a volumetric flux of 56 m3/d with a dispersivity around 1.69 m during the geophysical experiment. For these conditions, it was determined that conduit water composition ranged from 75% groundwater during base flow conditions to less than 50% groundwater in high flow conditions. Though subject to some uncertainties, the time-lapse inversion process provides a means to predict changing hydrologic conditions, leading to mixing of surface water and ground water and thus changes to water quantity and quality, as well as potential for water-rock reactions, in a semiconfined, sink-rise system. © 2014. American Geophysical Union. All Rights Reserved.

Godt J.W.,U.S. Geological Survey | Sener-Kaya B.,Colorado School of Mines | Sener-Kaya B.,Itasca Denver Inc. | Lu N.,Colorado School of Mines | Baum R.L.,U.S. Geological Survey
Water Resources Research | Year: 2012

Prediction of the location and timing of rainfall-induced shallow landslides is desired by organizations responsible for hazard management and warnings. However, hydrologic and mechanical processes in the vadose zone complicate such predictions. Infiltrating rainfall must typically pass through an unsaturated layer before reaching the irregular and usually discontinuous shallow water table. This process is dynamic and a function of precipitation intensity and duration, the initial moisture conditions and hydrologic properties of the hillside materials, and the geometry, stratigraphy, and vegetation of the hillslope. As a result, pore water pressures, volumetric water content, effective stress, and thus the propensity for landsliding vary over seasonal and shorter time scales. We apply a general framework for assessing the stability of infinite slopes under transient variably saturated conditions. The framework includes profiles of pressure head and volumetric water content combined with a general effective stress for slope stability analysis. The general effective stress, or suction stress, provides a means for rigorous quantification of stress changes due to rainfall and infiltration and thus the analysis of slope stability over the range of volumetric water contents and pressure heads relevant to shallow landslide initiation. We present results using an analytical solution for transient infiltration for a range of soil texture and hydrological properties typical of landslide-prone hillslopes and show the effect of these properties on the timing and depth of slope failure. We follow by analyzing field-monitoring data acquired prior to shallow landslide failure of a hillside near Seattle, Washington, and show that the timing of the slide was predictable using measured pressure head and volumetric water content and show how the approach can be used in a forward manner using a numerical model for transient infiltration. © Copyright 2012 by the American Geophysical Union.

Hanna B.,Itasca Denver Inc.
Tailings and Mine Waste'10 - Proceedings of the 14th International Conference on Tailings and Mine Waste | Year: 2011

Surface waters at the site of a former Minnesota taconite mine were reported to have solute concentrations elevated with respect to water-quality standards. Waste rock and ore generated from past mining were primarily from open pit mining of the Biwabik Iron Formation (BIF). The BIF is a variably bedded iron formation composed of inter-bedded cherty and slaty iron silicate and iron carbonate rich beds. A geochemical characterization was conducted to identify potential constituents of interest (COI), facilitate understanding of mechanisms controlling their environmental behavior at the site, and guide future site activities. Primary COI were determined to be SO 4, hardness (predominantly from Mg), alkalinity, Fe, Mn, and Al. BIF waste rock from the Lower Slaty member appears to be the primary source for the identified COI. Mechanisms of release are primarily attributed to pyrite oxidation and subsequent neutralization by dissolution of mixed-composition (Ca-, Mg-, Fe-, and Mn-bearing) siderite and ankerite. © 2011 Taylor & Francis Group, London.

Meyerhoff S.B.,Colorado School of Mines | Meyerhoff S.B.,Itasca Denver Inc. | Maxwell R.M.,Colorado School of Mines | Graham W.D.,University of Florida | And 2 more authors.
Hydrogeology Journal | Year: 2014

Subsurface heterogeneity is one of the largest sources of uncertainty associated with saturated hydraulic conductivity. Recent work has demonstrated that uncertainty in hydraulic conductivity can impart significant uncertainty in runoff generation processes and surface-water flow. Here, the role of site characterization in reducing hydrograph prediction bias and uncertainty is demonstrated. A fully integrated hydrologic model is used to conduct two sets of stochastic, transient simulation experiments comprising different overland flow mechanisms: Dunne and Hortonian. Conditioning hydraulic conductivity fields using values drawn from a simulated synthetic control case are shown to reduce both mean bias and variance in an ensemble of conditional hydrograph predictions when compared with the control case. The ensemble simulations show a greater reduction in uncertainty in the hydrographs for Hortonian flow. The conditional simulations predict surface ponding and surface pressure distributions with reduced mean error and reduced root mean square error compared with unconditional simulations. Uncertainty reduction in Hortonian and Dunne flow cases demonstrates different temporal signals, with more substantial reduction achieved for Hortonian flow. © 2014 Springer-Verlag Berlin Heidelberg.

Atkinson L.C.,Itasca Denver Inc. | Keeping P.G.,Victor Mine Hydrogeologist | Wright J.C.,Itasca Denver Inc. | Liu H.,Itasca Denver Inc.
Mine Water and the Environment | Year: 2010

The costs and efficiency of dewatering are particularly important at De Beers Canada's Victor diamond mine in northern Ontario, where the bottom of the water-bearing carbonate country rocks is near the bottom of the planned pit, which limits the available drawdown in the perimeter wells. Most of the inflow to the wells comes from a limited number of discrete zones in the carbonate rocks, resulting in low hydraulic efficiencies. The variable hydrogeologic conditions require efficient pumping over a wide range of yields and lifts, and there are logistical issues associated with the isolated setting and the extreme cold winter temperatures. The hydrogeology of the Victor mine area was characterised over three winter field seasons using packer tests, pumping tests, step-drawdown tests, and downhole logging to define the lateral and vertical variation in the hydraulic conductivity of the carbonate aquifer. Based on these data, wells were designed and submersible pumps with variable frequency drives were installed. Two 3-D numerical groundwater flow models were constructed, one a 'sub-regional' model to provide input to the mine feasibility study and permitting process and the other a near-pit 'window' model to simulate groundwater conditions in the immediate vicinity of the mine. These models are used in tandem to direct design of the dewatering system, evaluate its effectiveness, and to predict long-term environmental effects. © 2010 Springer-Verlag.

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