Hydraulic Characterization of Aquifers near Long Canyon, Nevada Active

Accurate subsurface hydrologic characterization is essential for understanding groundwater-flow paths and evaluating pumping effects on hydrologic systems. Long-term pumping can alter hydraulic gradients in a groundwater-flow system, which, in turn, can affect spring discharge rates, surface water flow, other well users, and the overall groundwater budget in a hydrographic area. The Long Canyon Mine Project in northwestern Goshute Valley, northeastern Nev., requires groundwater pumping from the carbonate-rock aquifer to sustain proposed open-pit gold mining and processing activities during the life of the mine, which currently is projected at 8 years.

Groundwater beneath northwestern Goshute Valley flows through basin-fill and carbonate rocks that have been offset along the eastern flank of the Pequop Mountains by range front faults¹. The Johnson Springs system and Big Springs occur within the Long Canyon Project area immediately downgradient from proposed pumping wells open to the carbonate-rock aquifer. Multiple-well aquifer tests^1,2 and isotopic data³ indicate that the carbonate-rock aquifer is a major source of water discharged by these springs.  Big Springs and the Johnson Springs system provide habitat for the Relict Dace (Relictus solitaries), an endemic species in east-central and northeastern Nevada. Outflow from the Johnson Springs system and Big Springs, and potential gaining reaches from the basin-fill aquifer contribute to perennial surface water flow in Hardy Creek¹. An improved hydraulic understanding of the study area is necessary to evaluate potential effects of groundwater development on this interconnected hydrologic system.

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Long-term pumping effects on groundwater systems are largely controlled by bulk hydraulic properties of aquifers. Aquifer testing can provide an integrated assessment of hydraulic properties within complex geologic systems, where the responses to pumping can be measured at distant observation wells and springs⁴.  However, detecting pumping-induced drawdowns in wells and reduced discharges from springs can be problematic because the responses can be small and obscured by natural environmental fluctuations. Previous studies showed that small pumping responses in distant observation wells can be isolated by removing environmental fluctuations from recorded water-level measurements. Analytic water-level model simulations can be used to remove environmental fluctuations. These models use measurements of background water levels and barometric pressure, and theoretical earth-tidal potentials^5,6 to account for the non-pumping related responses.  Background measurements of water levels and spring discharges include regional-scale temporal trends related to variable natural groundwater recharge and discharge and are critical for removing environmental fluctuations because at time scales relevant to the pumping tests they are generally affected by the same imperfect barometric coupling and earth tides that are superimposed on the pumping responses in observation wells and springs. 

AQUIFER TESTS

Several single-well and multiple-well aquifer tests have been performed by the Long Canyon Mine Project within carbonate-rock and basin-fill aquifers in Goshute Valley and near vicinity^1,2. Pumping responses (signals) in the carbonate-rock aquifer have propagated more than a mile from the pumping well and have been observed in overlying basin-fill wells. During late-summer 2016, a large-scale multiple-well aquifer test was performed in the carbonate-rock aquifer by the Long Canyon Mine Project. Two test wells were pumped at a combined rate of about 3,700 gallons per minute (gpm) for over a month.

The pumping signal from the summer 2016 pumping test propagated several miles beyond the pumping well through both carbonate-rock and basin-fill aquifers. To facilitate water-level drawdown and spring capture analyses, mine consultants collected the following data:

• Water levels in northern Goshute Valley monitoring wells screened in carbonate-rock and basin-fill aquifers, and spring stage and discharge in Big Spring and the Johnson Springs system were monitored hourly from early April 2016 through December 2016.
• Distant observation wells and grouted-in piezometers screened in carbonate-rock and alluvial aquifers were monitored continuously from early April through December 2016.

Aquifer Test Analysis

Hydraulic connectivity and hydraulic properties of carbonate rocks and basin fill in Goshute Valley, NV will be estimated from an integrated analysis of the late-summer 2016 aquifer test and previously completed aquifer tests. Specific tasks include

• establish sites in a database for pumping, observation, and background wells and springs monitored by the Long Canyon Mine Project and provided by the Nevada State Engineer (cooperator);
• evaluate existing and planned water-level and spring discharge datasets provided by the cooperator for consistency and erroneous data,
• estimate water-level drawdown in observation wells and the capture or reduction of spring discharge in response to pumping from the 2016 test well using water-level models,  
• review previous aquifer test datasets collected by the Long Canyon Mine Project and provided by the cooperator and reinterpret if necessary, and
• estimate distributions of aquifer transmissivities and storage coefficients with a groundwater-flow model by simultaneously interpreting water-level changes and spring-flow capture that resulted from the previous and planned aquifer tests.

Drawdown and Spring Capture Estimation

Drawdowns at pumping and observation wells, and reductions in spring stage and discharge will be used to evaluate hydraulic connectivity and estimate hydraulic properties of carbonate-rock and basin-fill aquifers. For purposes of drawdown and spring capture estimation, drawdowns are defined as pumping-induced changes in measured water levels and spring capture is defined as pumping-induced changes in measured spring discharge.

• Precise, continuous water-levels and spring stage and discharge measurements can be used to reliably detect small pumping-related changes (less than 0.1 ft of water-level change, for example).
• Drawdowns at the pumping well primarily will define hydraulic property estimates near the pumping well.
• Drawdowns in distant observation wells and capture of spring discharge will define the bulk hydraulic diffusivity (the ratio of the transmissivity to storage coefficient) of carbonate-rock and basin-fill aquifers between pumping and observation wells or springs. 
• Water-level drawdown in observation wells and spring capture from aquifer testing will be estimated using water-level models.

Hydraulic Property Estimation

Hydraulic properties will be estimated by analyzing drawdowns, spring-flow capture, and pumping records with analytical and numerical methods. Three-dimensional drawdowns from each multiple-well aquifer test will be simulated with a MODFLOW model.  Multiple aquifer-test models will be integrated and analyzed simultaneously where areas investigated by aquifer tests overlap. Integration of multiple groundwater-flow models allows for

• consistent and optimum hydraulic-property estimates from all aquifer tests, 
• grid refinement near each pumping well and different pumping schedules specific to each aquifer test,
• facilitation of independent aquifer test assessments and provides assurance that simulated drawdowns and sensitivities are computed and interpreted correctly, and
• ensurance that a single, more consistent set of hydraulic properties is estimated for the study area.

DATA

Water-level and spring stage and discharge data used during the study will be archived online through the USGS ScienceBase web site. Aquifer tests will be archived and made available online from the USGS Nevada Water Science Center aquifer test web page. Models will be archived on ScienceBase.

¹ Golder Associates, 2012, Hydrogeologic Characterization, Long Canyon, Newmont USA Limited. Report 113-81813. March 8, 2012.

² Barnett Intermountain Water Consulting, Global Hydrologic Services Inc., and Aqua Engineering, 2011, Report on the Long Canyon bedrock well aquifer test, Goshute Valley, Elko County, Nevada. Prepared for Fronteer Development (USA) Inc. March 2011.

³ Mayo and Associates, LC, 2013, Isotopic Characterization of groundwaters in the Long Canyon Mine Area, Nevada. October 19, 2013.

⁴ Halford, K.J. and D.K. Yobbi, 2006, Estimating Hydraulic Properties Using a Moving-Model Approach and Multiple Aquifer Tests: Ground Water, v. 44, no. 2, 284-291

⁵ Halford, K., Garcia, C.A., Fenelon, J., and Mirus, B., 2012, Advanced methods for modeling water-levels and estimating drawdowns with SeriesSEE, an Excel add-In, U.S. Geological Survey Techniques and Methods  4–F4, 28 p. 

⁶ Garcia, C.A., Halford, K.J., and Fenelon, J.M., 2013, Detecting drawdowns masked by environmental stresses using Theis transforms: Groundwater, v.51, no. 3, p. 322-332.