Mining operations can affect the groundwater system. Groundwater levels decline in response to active and passive dewatering (Henton 1981; Timms et al. 2018). Conversely, groundwater levels rise in response to pumping-recovery (Henton 1981; Timms et al. 2018); mine-induced infiltration at rapid infiltration basins (Nimmer et al. 2009; Davis et al. 2022); and stream infiltration following mine-water discharge to streams or unlined ditches.
Mine operators are required to understand the spatial extent of mine-induced water-level declines and rises for permitting and regulatory purposes. Because mining and non-mining (e.g., agricultural) water users can affect community water supplies or groundwater-dependent ecosystems, mine operators also need to understand where their mining operations are not affecting the groundwater system.
Water-level monitoring networks are used to understand historic and current effects of mine-related pumping and infiltration on the groundwater system. At wells adjacent to mining facilities, large water-level rises and declines can be readily attributed to mining activities. However, farther from the mine site, mine-related pumping and infiltration stresses are attenuated by hydraulic properties of the groundwater system and these stresses can be masked by natural water-level trends.
Understanding natural trends is important because groundwater levels can rise and decline naturally in response to precipitation-derived recharge and groundwater discharge to streams, springs, and phreatophyte areas. An observed water-level decline can result from natural or pumping stresses. Differentiating the source of the decline is more difficult when water-level declines are small, because correct attribution of the decline is based on an understanding of long-term, natural, water-level trends in the groundwater system and the likely distant propagation of drawdown from the pumping center.
Once the long-term natural trend is understood, then a water-level trend analysis can be done to attribute all natural and anthropogenic stresses affecting water levels in wells within a monitoring network. Typical water-level trend analysis methods use statistical-based nonparametric approaches (Helsel and Hirsch 2002), such as the Kendall-Theil robust line method (Fenelon 2000), Mann-Kendall test (Hamed and Rao 1998; Yue et al. 2002; Ribeiro et al. 2015; Kumar et al. 1998; Kumar and Rathnam 2019; Lou et al. 2019; Minea et al. 2022; Vinushree et al. 2022; Sinha 2023), Sen’s Slope (Kumar et al. 1998; Ribeiro et al. 2015; Minea et al. 2022; Vinushree et al. 2022; Sinha 2023), or Spearman’s Rho test (Yue et al. 2002; Minea et al. 2022). However, these statistical approaches are constrained by sample size and only quantify significant upward or downward monotonic trends. Multivariate statistical approaches, such as principal-component analysis (Winter et al. 2000; Jung et al. 2021), have been used to quantify variables affecting water-level trends; however, results of these type of analyses are not always intuitive when presented to regulators.
The three objectives of this study are to: (1) present a method for estimating a baseline water-level trend that represents long-term, natural water-level fluctuations in the groundwater system prior to mine operations; (2) present a curve-matching trend analysis approach to determine all natural, mining, and non-mining stresses affecting water levels in wells; and (3) use trend-analysis results to delineate the approximate extent of mining and non-mining effects on a groundwater system. The water-level monitoring network at the Twin Creeks (TC) Mine in north-central Nevada, USA, is used as a case-study example for the trend analysis and baseline water-level estimation.
Site Description
The TC Mine is about 55 km northeast of Winnemucca, in north-central Nevada. Active mine features at TC Mine include North Mega, South Mega, and Vista Pits; and an underground mine beneath Vista Pit (Fig. 1). The TC Mine monitoring network includes well sites that are measured quarterly or semi-annually, and these data are reported annually to the Nevada Division of Water Resources. The wells are in two surface-water hydrographic areas (HAs): Kelly Creek Valley and Little Humboldt Valley (Fig. 1).
The TC Mine is within the Basin and Range Province (Harrill and Prudic 1998), which is characterized by extensional tectonic forces that cause down-dropped fault blocks to form valleys and upthrown fault blocks to form intervening mountain ranges. The TC Mine is on Kelly Creek valley floor, which is bounded on the west and east by the Osgood and Snowstorm Mountains, respectively (Fig. 1). Land-surface altitudes range from 1,310–1,520 meters above mean sea level (m amsl) on the Kelly Creek valley floor. Land-surface altitudes are up to 2,645 m amsl in the Osgood Mountains and up to 2,561 m amsl in the Snowstorm Mountains.
Climate
The study-area climate is a semi-arid steppe, with hot summers and cold winters. The climate is characterized by low relative humidity, low annual precipitation, and high potential evapotranspiration. Average maximum temperatures range from 14–26°C in summer, and from − 12 to 6°C in winter (NOAA 2023a). Average annual precipitation ranges from 0.6 m in the Osgood and Snowstorm Mountains to 0.2 m on the Kelly Creek valley floor (Climate Engine 2023). Average annual pan evaporation and open-water evaporation rates are 1.7 m and 1.2 m, respectively (Itasca 2022).
Hydrogeology
Quaternary alluvium and Paleozoic fractured rocks form the primary aquifers in the study area, and Tertiary volcanics form a secondary fractured-rock aquifer (Itasca 2022). Quaternary alluvium composes unconsolidated sediments that occur on the valley floors, along surface-water channels, and in alluvial fans (Fig. 1). The alluvial aquifer is recharged mostly by surface-water flows along losing reaches of Jake Creek, Kelly Creek, Rabbit Creek, and the Humboldt River. Undifferentiated Paleozoic rocks underlie much of the study area and consist of fractured carbonate and siliciclastic rocks, predominantly limestone, chert, greenstone, quartzite, shale, siltstone, and sandstone. Paleozoic rocks form a fractured bedrock aquifer, which receives groundwater recharge directly at outcrops within highland areas (Fig. 1), or indirectly from overlying alluvial or volcanic aquifers. Tertiary volcanics consist of fractured rhyolitic, andesitic, and basaltic lava flows, which occur throughout the Snowstorm Mountains. Tertiary volcanics receive groundwater recharge from outcrops in the Snowstorm Mountains.
In Kelly Creek Valley HA, groundwater flows from highland recharge areas in the Osgood and Snowstorm Mountains toward the Kelly Creek valley floor (Fig. 1). Groundwater beneath Kelly Creek valley flows south-southwest toward the Humboldt River. In Little Humboldt Valley HA, groundwater derived from the northern parts of the Osgood and Snowstorm Mountains flows northward to discharge along the Little Humboldt River.
Groundwater – Surface Water Interactions
The headwaters of Jake Creek occur in the Snowstorm Mountains, and streamflow from the mountain front traverses low-permeability volcanic rock that maintains surface-water flow in the creek (Itasca 2022). Farther downgradient, the creek crosses into alluvium and is a losing reach that recharges the alluvial groundwater system (Fig. 1).
The headwaters of Kelly and Rabbit Creeks also occur in the Snowstorm Mountains. Only the upper parts of the creeks maintain perennial flows. Flows in Rabbit Creek are artificially supplemented near South Mega Pit by discharge of treated water from mine dewatering operations (Fig. 1). Surface water from the two creeks reaches the Humboldt River only during exceptionally wet periods (Itasca 2022). Most of the time, water in the channels recharges the alluvial groundwater system and then discharges downgradient from phreatophytes near the Humboldt River.
The Humboldt River is a losing reach along the southern boundary of Kelly Creek Valley HA, whereas the Little Humboldt River is a gaining reach near Chimney Creek Reservoir (Itasca 2022). Water levels in Chimney Creek Reservoir change in response to variations in precipitation, reservoir water storage, and irrigation usage.
Mine Dewatering and Rabbit Creek Discharge
Gold was discovered at the location of the Mega Pits in 1984, but mining operations did not begin until 1987. From 1987–2022, dewatering has occurred during mining of the Mega Pits, where annual pumping rates were between 0.2 and 9 million cubic meters per year (Mm³/yr). Open pit and underground mining began at Visita Pit in 2012, with dewatering rates ranging from 9.2 to 17.3 Mm³/yr between 2012 and 2022.
Excess groundwater withdrawals, which are not used for mine-water consumptive use, undergo water treatment prior to discharge to Rabbit Creek. Water discharged to Rabbit Creek was from Mega Pit dewatering wells prior to 2012, and mostly from Vista Pit dewatering wells after 2012. Discharged water to Rabbit Creek infiltrates into the alluvium upstream of the Humboldt River (Itasca 2022). Estimated Rabbit Creek discharges have ranged from 0.9 to 3.6 Mm³/yr prior to 2012 and from 3 to 6.8 Mm³/yr between 2012 and 2022 (Itasca 2022).