Geochemistry and Environmental Tracer Data for Groundwater, Stream Water, and Sediment from the East Mancos River, Colorado, 2023

This U.S. Geological Survey (USGS) data release contains data from stream water, groundwater, and stream sediment samples collected in August 2023 in the East Mancos River watershed in southwestern Colorado. Data were collected as part of a study aimed at identifying sources of elevated metal concentrations in the stream. Eighteen streambank wells were installed in pairs at nine locations to capture the emerging groundwater from the left and right banks (relative to downstream-facing direction) and a synoptic sampling campaign was conducted to quantify stream metal loads. A continuous, instream injection of sodium bromide (NaBr) was initiated at the head of the ~17 km study reach prior to the synoptic sampling campaign and maintained throughout the duration of the study. Bromide concentrations were subsequently used to determine streamflow in the upper ~2.7 km of the stream using the tracer-dilution method. Streamflow was quantified for tributaries and locations farther downstream using data from a series of sodium chloride slug additions wherein specific conductivity readings were used as a surrogate for the tracer concentration. Stream and groundwater samples, including the wells and inflows from the left (LBI) and right (RBI) banks, were collected mainly in the upper ~0.9 km of the stream course (primary study reach). Environmental tracer data were collected to better understand groundwater flowpaths to the stream. Stream sediment samples, including some ferricrete samples, were also collected to help identify potential instream and near-stream metal sources.