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at Boulder, 4 BBA, 5 Hatch 1. Background Mine pit lakes are becoming more common as metallurgical processing techniques allow for ore bodies of lower grade to be economically processed. Once mining ends and a pit lake is formed, water-quality samples may need to be collected for a variety of reasons including scientific research and regulatory compliance (Castendyk et al., 2015). In the State of Nevada, which hosts approximately 40 pit lakes (Newman, 2016) the Nevada Division of Environmental Protection (NDEP) is responsible for evaluating the water quality of pit lakes. Although regular monitoring is important, monitoring actions at pit lakes may pose a risk to human health and safety. In summer 2017, a sampling campaign was conducted in Nevada to apply an unmanned aircraft system (UAS) for pit-lake sampling (Figure 1).

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A multi-component geochemical dataset was collected from groundwater and surface-water bodies associated with the urban Fountain Creek alluvial aquifer, Colorado, USA, to facilitate analysis of recharge sources, geochemical interactions, and groundwater-residence times. Results indicate that groundwater can be separated into three distinct geochemical zones based on location within the flow system and proximity to surface water, and these zones can be used to infer sources of recharge and groundwater movement through the aquifer. Rare-earth-element concentrations and detections of wastewater-indicator compounds indicate the presence of effluent from wastewater-treatment plants in both groundwater and surface water. Effluent presence in groundwater indicates that streams in the area lose to groundwater in some seasons and are a source of focused groundwater recharge. Distributions of pharmaceuticals and wastewater-indicator compounds also inform an understanding of groundwater–surface-water interactions. Noble-gas isotopes corroborate rare-earth-element data in indicating geochemical evolution within the aquifer from recharge area to discharge area and qualitatively indicate variable groundwater-residence times and mixing with pre-modern groundwater. Quantitative groundwater-residence times calculated from 3H/3He, SF6, and lumped-parameter modeling generally are less than 20 years, but the presence of mixing with older groundwater of an unknown age is also indicated at selected locations. Future investigations would benefit by including groundwater-age tracers suited to quantification of mixing for both young (years to decades) and old (centuries and millennia) groundwater. This multi-faceted analysis facilitated development of a conceptual model for the investigated groundwater-flow system and illustrates the application of an encompassing suite of analytes in exploring hydrologic and geochemical interactions in complex systems.