Geochemistry and Environmental Tracer Data for Groundwater and Stream Water from the Perigo Mine Site, Colorado, 2024

This U.S. Geological Survey (USGS) data release contains data from stream water and groundwater samples collected in July through October 2024 at the Perigo Mine site in Gilpin County, northern Colorado. Data were collected as part of a study aimed at characterizing the local groundwater flow system in the hillslope hosting the mine’s draining crosscut tunnel, and identifying sources of elevated metal concentrations in the adjacent Gamble Gulch stream. In September and October 2023, two bedrock boreholes were drilled on the hillslope to a depth of 77 m and each completed with four nested monitoring wells. Five shallow wells were installed along the left (from the perspective of an observer looking downstream) streambank to a depth of 1-2 m to capture emerging groundwater from the side of the stream where the mine workings are located. The newly installed wells were sampled in July and August 2024 along with two other existing wells, five springs, discharge from the collapsed crosscut portal, and six stream sites within a ~1-km long study reach near the crosscut portal. All samples were analyzed for major ion and trace element concentrations and stable water isotope ratios (oxygen-18 and deuterium), with a subset of samples analyzed for tritium, noble gases (He, Ne, Ar, Kr, and Xe), 3H/4He ratio, and sulfur hexafluoride for determining recharge parameters and groundwater age. Streamflow was measured concurrently with sample collection for four of the stream sites using data from sodium chloride slug additions wherein specific conductivity readings were used as a surrogate for the tracer concentration. In October 2024, a synoptic sampling campaign was conducted to quantify stream metal loads within the study reach. A continuous, instream injection of sodium bromide (NaBr) was initiated at the head of the reach prior to the synoptic sampling campaign, and Br concentrations were subsequently used to determine streamflow using the tracer-dilution method.