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Pit lakes present significant safety risks for boat-based water sampling crews. The Matrice-HydraSleeve method improves the safety of water sampling in pit lakes by eliminating the need for a boat-based sampling crew. The method connects an off-the-shelf unmanned aircraft system, or drone (DJI, Matrice 600) to an off-the-shelf water sampling device (GeoInsight, HydraSleeve). It is capable of collecting 1.75 L water samples from up to 122 m deep and is simpler than previous drone water sampling methods. To validate the method, water samples were collected from similar depths in Dillon Reservoir, Colorado, USA using the Matrice-HydraSleeve method and traditional boat sampling methods using a Van Dorn water sample bottle. Concentrations of Ca, Na, K, HCO3, SO4, Cl, and Zn showed less than 20% relative percent difference, and concentrations of Cd were within ± the detection limit, meaning variability between samples met the data quality objective for duplicate samples. The method was also used on two occasions to collect eight water samples from the 101 m deep pit lake at the Thompson Creek mine in Idaho, USA including a sample from 92 m deep. Calcium and sodium concentration profiles were nearly identical, indicating little change in water chemistry and providing confidence in the method. In situ profiles of temperature and electrical conductivity collected with a conductivity-temperature-depth probe (YSI, CastAway) suspended below the drone indicated the lake was meromictic during both sampling events. To date, the Matrice-HydraSleeve method has been used at 10 pit lakes in the USA to collect 81 samples. Use of this method at other pit lakes has the potential to improve safety while lowering sampling costs and increasing data acquisition, leading to better pit lake management.

Residual pit lakes from mining are often dangerous to sample for water quality. Thus, pit lakes may be rarely (or never) sampled. This study developed new technology in which water-sampling devices, mounted on an unmanned aerial vehicle (UAV), were used to sample three pit lakes in Nevada, USA, during 1 week in 2017. Water-quality datasets from two of the three pit lakes on public lands, Dexter and Clipper, are presented here. The current conditions of the Dexter pit lake were assessed by examining cation and anion concentration changes that have occurred over a 17-year period since the pit lake was last sampled in 2000. Data gathered during this sampling campaign assessed 2017 conditions of the Dexter and Clipper pit lakes by comparing constituent concentrations to the Nevada Division of Environmental Protection (NDEP) pit lake water-quality requirements, indicating that selenium concentrations exceeded regulatory standards. We compared our sampling data for Dexter lake to prior water-quality data from the Dexter pit lake collected in 1999 and 2000. This comparison for the Dexter pit lake indicates that evapoconcentration may have caused increasing cation and anion concentrations. This UAV sampling approach can potentially incorporate the use of additional multiparameter probes: pH, oxygen concentration, turbidity, or chlorophyll. Some limitations of this UAV water-sampling methodology are battery duration, weather conditions, and payload capacity.

Residual pit lakes from mining are often dangerous to sample for water quality. Thus, pit lakes may be rarely (or never) sampled. This study developed new technology in which water-sampling devices, mounted on an unmanned aerial vehicle (UAV), were used to sample three pit lakes in Nevada, USA, during 1 week in 2017. Water-quality datasets from two of the three pit lakes on public lands, Dexter and Clipper, are presented here. The current conditions of the Dexter pit lake were assessed by examining cation and anion concentration changes that have occurred over a 17-year period since the pit lake was last sampled in 2000. Data gathered during this sampling campaign assessed 2017 conditions of the Dexter and Clipper pit lakes by comparing constituent concentrations to the Nevada Division of Environmental Protection (NDEP) pit lake water-quality requirements, indicating that selenium concentrations exceeded regulatory standards. We compared our sampling data for Dexter lake to prior water-quality data from the Dexter pit lake collected in 1999 and 2000. This comparison for the Dexter pit lake indicates that evapoconcentration may have caused increasing cation and anion concentrations. This UAV sampling approach can potentially incorporate the use of additional multiparameter probes: pH, oxygen concentration, turbidity, or chlorophyll. Some limitations of this UAV water-sampling methodology are battery duration, weather conditions, and payload capacity.

Mine managers and regulators use geochemical predictions of pit lake chemistry to assess whether open pit mines will have positive or negative environmental effects after closure. Sensitivity analyses are a useful approach to explore uncertainties in these predictions. This study investigates the significance of subaqueous water–rock reactions and surface adsorption reactions on a geochemical model of a proposed pit lake at the Martha Au Mine, New Zealand. Our hypothesis was that subaqueous pyrite oxidation would lower surface water pH over time because pyrite will be present in over 1/3 of the submerged wall rock area, and that surface adsorption reactions would lower trace metal concentrations. An initial geochemistry prediction was created in PHREEQC based on site hydrology, representative input water chemistry, physical limnology, and the precipitation of ferrihydrite, manganite, amorphous gibbsite, and barite. Modeling water–rock reactions required the surface area of the submerged wall rocks, the concentration (volume %) of the dominant minerals found in the wall rock ( i.e. pyrite, adularia, albite, chlorite, illite, and kaolinite), reaction rates for each mineral, and the volumes of circulating lake layers. These variables determined the mass of each mineral that would react with lake water over a one-year period corresponding to annual turnover. To accommodate for uncertainty in the estimation of surface area, the surface area was increased by × 10, × 100, and × 1000 in three additional models. To model surface adsorption reactions, the mass of ferrihydrite precipitated annually was set equal to the mass of hydrous ferric oxide allowed to adsorb trace metals. Results of the baseline geochemical prediction without water–rock reactions showed the surface water pH dropped from 6.5 to 5.0 over 50 years. The sensitivity analysis on water–rock reactions produced nearly identical results to the initial model for surface areas of × 1, × 10, and × 100. Only the surface area × 1000 model lowered pH more than half a pH unit from the initial prediction. These data suggest that water–rock reactions will not have a significant effect on the pH of the proposed Martha lake owing to the surface area of submerged minerals. Surface adsorption reactions lowered the concentrations of some trace metals (As, Cu, and Pb), whereas other trace metals were unaffected (Cd, Ni, Zn), which emphasized the pH dependence of these reactions. In the absence of model calibration or observed water chemistry, sensitivity analyses are a useful tool to explore uncertainties in pit lake predictions.

Turnover events in pit lakes influence the water quality of pit lakes by homogenizing the chemistry of mixing layers and distributing dissolved oxygen, hence limnologic models that predict the depth of annual turnover are required for predictive studies of post-mining pit lake water quality. This study demonstrates a limnologic prediction of a proposed pit lake at the Martha Au–Ag Mine, New Zealand, created with the hydrodynamic program DYRESM. The model considered 5 years of lake-filling conditions where the pit will receive river water, groundwater, pit wall runoff, and direct rainwater followed by 1 year of steady-state conditions where the lake will receive only groundwater, runoff, and rainwater. Sensitivity analyses showed that input groundwater temperatures ≤ 17 °C produced meromictic conditions, whereas groundwater temperatures ≥ 17 °C produced holomictic conditions. Changes in the chemistry of runoff inputs had no effect on turnover because the quantity of runoff is significantly less than the quantity of other inputs. These results suggest that understanding the density of major lake inputs is critical toward predicting the limnologic evolution of a pit lake as small changes in the water chemistry or temperature between inputs may affect input density and the resulting turnover depth. Lakes filled with water from one principle source, such as groundwater, are more likely to be holomictic due to the relatively homogeneous character of the resulting lake water, whereas lakes filled with water from more than one source, such river water and groundwater, may develop meromictic conditions due to slight density differences between input waters. It may be possible for mine managers to engineer holomictic or meromictic conditions in future pit lakes to improve lake water quality.

Upon closure in 2007, the Martha gold mine, New Zealand, will be flooded with river water creating a 192-m-deep pit lake. The mine owners intend to rehabilitate the lake into a public recreation area for boating and swimming, which will have additional value as a habitat for waterfowl and fish. This study demonstrates how a geochemical prediction of pit lake water quality can identify modifications to closure plans that will potentially improve post-mining water quality and increase the value of a proposed pit lake resource. The geochemical model PHREEQC was used to construct three, 55-year predictions of epilimnion and mixolimnion water quality in the proposed Martha pit lake, based on recent mineralogic and limnologic investigations. Model 1 considered the current closure strategy, whereas Model 2 covered all wall rocks that produced highlyacidic runoff (HAR) before lake filling (21 ha covered), and Model 3 covered only HAR rocks exposed above the steady-state lake surface (3 ha covered). After 50 years of steady-state conditions, Model 1 predicted that surface water will have a pH < 5.0 and will not comply with New Zealand water quality guidelines for recreational use. The epilimnion will also contain Cu and Zn concentrations that exceed aquatic life protection guidelines. By covering all HAR producing areas, Model 2 showed the epilimnion pH increased to 7.0 and Cu concentrations dropped significantly. Most significantly, Model 3 showed that covering only HAR producing areas exposed above final lake surface produced similar results to Model 2 after 50 years of steady-state conditions, increasing pH to 6.8 and lowering Cu concentrations. Zinc concentrations remained above aquatic life protection guidelines in each modeling scenario, and may limit the number of sensitive waterfowl and fish species utilizing the lake. Because only 11% of the wall rock area exposed above the steady-state lake surface will produce HAR, covering these wall rock areas is a feasible remediation option for the mine owners to consider, which is likely to improve future water quality and allow the pit lake to be used as a recreational resource. Additional

Pit lakes that result from open pit mining are potential water resources or potential environmental problems, depending on lake water quality. Wall rock mineralogy can affect lake chemistry if surface water inputs and/or groundwater inputs and/or lake water in contact with submerged wall rocks react with the wall rock minerals. This study presents a mineral quantification method to measure the distribution and concentration of wall rock minerals in open pit mines, and applies the method to the Martha epithermal Au–Ag mine, Waihi, New Zealand. Heterogeneous ore deposits, like Martha, require a large number of wall rock samples to accurately define mineral distributions. X-ray diffraction analyses of 125 wall rock samples identified the most abundant minerals in the wall rocks as quartz, adularia, albite, illite, chlorite, kaolinite, pyrite and calcite. Distribution maps of these minerals defined 8 relatively homogenous areas of wall rock referred to as “mineral associations”: weakly-altered, propylitic, fresh-argillic, weathered-argillic, oxidized, potassic, quartz veins, and post-mineralization deposits. X-ray fluorescence, Leco furnace, and neutron activation analyses of 46 representative samples produced the geochemical dataset used to assign quantities of elements to observed minerals, and to calculate average mineral concentrations in each association. Thin-section petrography and calcite concentrations from Sobek acid-digestions confirm the calculated mineralogy, providing validation for the method. Calcite and pyrite concentrations allowed advanced acid–base accounting for each mineral association, identifying 3 potential acid-producing associations and one potential acid-neutralizing association. The results target areas, where detailed hydrologic and kinetic tests would be valuable in the next stage of pit lake evaluation. Detailed understanding of wall rock mineralogy will help strengthen predictions of pit lake water quality.