The American Tunnel, the Black Hawk mine, the Gold King mine, the Mogul mine, and the Red and Bonita mine are located in the Cement Creek watershed, tributary to the upper Animas River near Silverton, Colorado. All five sites have tunnels that drain groundwater from abandoned underground mine workings to the surface. This draining water has elevated concentrations of metals and degrades water quality in Cement Creek. Water quality (pH, and dissolved copper, manganese, and zinc concentrations) and discharge data were compiled from multiple sources to examine changes in these parameters through time. Copper, manganese, and zinc loads calculated from these data are included in the data files. Data are reported for the American Tunnel (November 1988 through July 2015), the Black Hawk mine (September 1991 through September 2005), the Gold King mine (August 1993 through July 2015), the Mogul mine (July 1992 through July 2015), and the Red and Bonita mine (June 1997 through July 2015).

Three synoptic sampling campaigns were conducted on upper Cement Creek, near Gladstone, Colorado, under low-flow conditions. The first campaign, conducted October 2012, was part of a larger campaign to characterize low-flow water quality in the entire Cement Creek watershed. The second campaign, conducted in September 2019, was designed to quantify metal loading and identify sources of contamination along a 2.5-kilometer study reach. The third campaign, conducted in September 2020, was designed to quantify loads and sources along the same 2.5-kilometer study reach during a test closure of a bulkhead on the Red and Bonita Mine, one of the primary sources of metals within the watershed. Streamflow measurements during the 2012 campaign were conducted using Acoustic Doppler Velocimetry. A continuous, instream injection of a sodium bromide tracer was initiated at the head of the study reach two days prior to the 2019 synoptic sampling campaign and maintained throughout the duration of the campaign. Bromide concentrations were subsequently used to determine streamflow using the tracer-dilution method. Streamflow estimates for the 2020 campaign were developed using a series of sodium chloride slug additions, wherein specific conductivity readings were used as a surrogate for chloride concentration. This data release includes concentration data (inorganic cations and anions), estimated streamflow, and calculated loads for the three sampling campaigns. Tracer data for the continuous tracer injection (2019) and slug additions (2020) are also included. Calculated loads may be used to compare low flow conditions with and without contributions from the Gold King Mine (2012 versus 2019) as well as before and after closure of the Red and Bonita bulkhead (2019 versus 2020). The data release consists of a kmz file showing site locations and the following 9 tables: Table 1, Locations of sampling sites Table 2, Synoptic sampling results, October 3, 2012 Table 3, Synoptic sampling results, September 5, 2019 Table 4, Synoptic sampling results, September 19, 2020 Table 5, Iron speciation results, September 7, 2019 Table 6, Bromide timeseries, September 3-5, 2019 Table 7, Slug addition chloride data, September 19, 2020 Table 8, Slug addition results, September 19, 2020 Table 9, Spatial profiles of streamflow and metal load

Three synoptic sampling campaigns were conducted on upper Cement Creek, near Gladstone, Colorado, under low-flow conditions. The first campaign, conducted October 2012, was part of a larger campaign to characterize low-flow water quality in the entire Cement Creek watershed. The second campaign, conducted in September 2019, was designed to quantify metal loading and identify sources of contamination along a 2.5-kilometer study reach. The third campaign, conducted in September 2020, was designed to quantify loads and sources along the same 2.5-kilometer study reach during a test closure of a bulkhead on the Red and Bonita Mine, one of the primary sources of metals within the watershed. Streamflow measurements during the 2012 campaign were conducted using Acoustic Doppler Velocimetry. A continuous, instream injection of a sodium bromide tracer was initiated at the head of the study reach two days prior to the 2019 synoptic sampling campaign and maintained throughout the duration of the campaign. Bromide concentrations were subsequently used to determine streamflow using the tracer-dilution method. Streamflow estimates for the 2020 campaign were developed using a series of sodium chloride slug additions, wherein specific conductivity readings were used as a surrogate for chloride concentration. This data release includes concentration data (inorganic cations and anions), estimated streamflow, and calculated loads for the three sampling campaigns. Tracer data for the continuous tracer injection (2019) and slug additions (2020) are also included. Calculated loads may be used to compare low flow conditions with and without contributions from the Gold King Mine (2012 versus 2019) as well as before and after closure of the Red and Bonita bulkhead (2019 versus 2020). The data release consists of a kmz file showing site locations and the following 9 tables: Table 1, Locations of sampling sites Table 2, Synoptic sampling results, October 3, 2012 Table 3, Synoptic sampling results, September 5, 2019 Table 4, Synoptic sampling results, September 19, 2020 Table 5, Iron speciation results, September 7, 2019 Table 6, Bromide timeseries, September 3-5, 2019 Table 7, Slug addition chloride data, September 19, 2020 Table 8, Slug addition results, September 19, 2020 Table 9, Spatial profiles of streamflow and metal load

A synoptic sampling campaign was conducted on the Animas River near Silverton, Colorado, under low flow conditions in September 2021. The sampling campaign was designed to quantify constituent loading and identify sources of contamination along a 3.8-kilometer study reach. The study reach began approximately 170 meters upstream of Arrastra Creek and extended downstream to U.S. Geological Survey (USGS) gage 09358000 within the city of Silverton, Colorado. A continuous, instream injection of a sodium bromide tracer was initiated at the head of the study reach three days prior to the start of the sampling campaign and maintained until the completion of main stem sampling. Bromide concentrations were subsequently used to determine streamflow using the tracer-dilution method. Water quality samples were collected at 23 sites along the Animas River main stem, and 28 inflow sites including springs, seeps, small tributaries, and ponded water. Main stem sites were sampled using three sampling approaches. Under the first approach, a subset of 8 main stem sites were sampled "simultaneously" (in less than 20 minutes) to assess the effects of diel variation in constituent concentration. Under the second approach, all main stem sites were sampled with the sampling team working in the downstream-to-upstream direction, a protocol typically used during synoptic sampling. A subset of 5 main stem sites were also sampled using an Equal Discharge Increment approach that was designed to indicate which side of the stream was responsible for the observed constituent loads. This data release includes field parameters (water temperature, pH, and specific conductivity), concentration data (inorganic cations and anions), estimated streamflow, and calculated loads for the sampling campaign. Calculated loads may be used to identify and rank sources of contamination to the Animas River. The data release consists of a kmz file showing site locations and the following 3 tables: Table 1, Locations of sampling sites Table 2, Synoptic sampling results, September 20-21, 2021 Table 3, Spatial profiles of streamflow and constituent load

A synoptic sampling campaign was conducted on the Animas River near Silverton, Colorado, under low flow conditions in September 2021. The sampling campaign was designed to quantify constituent loading and identify sources of contamination along a 3.8-kilometer study reach. The study reach began approximately 170 meters upstream of Arrastra Creek and extended downstream to U.S. Geological Survey (USGS) gage 09358000 within the city of Silverton, Colorado. A continuous, instream injection of a sodium bromide tracer was initiated at the head of the study reach three days prior to the start of the sampling campaign and maintained until the completion of main stem sampling. Bromide concentrations were subsequently used to determine streamflow using the tracer-dilution method. Water quality samples were collected at 23 sites along the Animas River main stem, and 28 inflow sites including springs, seeps, small tributaries, and ponded water. Main stem sites were sampled using three sampling approaches. Under the first approach, a subset of 8 main stem sites were sampled "simultaneously" (in less than 20 minutes) to assess the effects of diel variation in constituent concentration. Under the second approach, all main stem sites were sampled with the sampling team working in the downstream-to-upstream direction, a protocol typically used during synoptic sampling. A subset of 5 main stem sites were also sampled using an Equal Discharge Increment approach that was designed to indicate which side of the stream was responsible for the observed constituent loads. This data release includes field parameters (water temperature, pH, and specific conductivity), concentration data (inorganic cations and anions), estimated streamflow, and calculated loads for the sampling campaign. Calculated loads may be used to identify and rank sources of contamination to the Animas River. The data release consists of a kmz file showing site locations and the following 3 tables: Table 1, Locations of sampling sites Table 2, Synoptic sampling results, September 20-21, 2021 Table 3, Spatial profiles of streamflow and constituent load

Over the past 15 years Douglas County, NV has removed production wells in northern Carson Valley from use due to relatively high arsenic concentrations (Carl Ruschmeyer, January 2013, Douglas County Public Works Director, verbal communication). To maintain the supply of water to the public, the town of Minden has been providing water to Douglas County and Carson City. Due to the projected increases in municipal demand, water resource managers are concerned that increasing pumping rates from wells in Minden may change groundwater chemistry and degrade the resource by potentially drawing in arsenic enriched water. Long-term exposure to arsenic can cause illnesses ranging from skin discoloration to various cancers including those of the bladder, skin, and kidney (U.S. Environmental Protection Agency, 2001). Naturally occurring arsenic is one of the most common contaminants in groundwater in the western United States. Arsenic found in basin-fill aquifers is oftentimes associated with alluvial/lacustrine sedimentary deposits derived from the weathering of volcanic rocks and geothermal waters (Welch and others, 1988). The primary aquifers beneath Carson Valley are comprised of quaternary aged basin-fill deposits of weathered granitic and volcanic material (Welch, 1994). Factors contributing to increasing arsenic concentrations in groundwater include, but are not limited to, proximity to arsenic bearing rocks, relatively long groundwater flow paths, the application of phosphate containing fertilizers, and leaching from soils in irrigated areas (Busbee and others, 2009; Anning and others, 2012). The vulnerability of groundwater resources to contamination is influenced by the physical properties of the aquifer, pumping rates, locations of wells and screened intervals relative to the groundwater flow system, and geochemical environment (Focazio and others, 2006). Arsenic mobility and transport through the subsurface is largely controlled by the interaction of groundwater with aquifer sediments. Arsenite (As(III)), the reduced form of inorganic arsenic, usually exhibits greater mobility in groundwater than the oxidized form, arsenate (As(V)) largely due to the greater attraction of As(V) to aquifer sediments relative to that of As(III) at pH values exceeding 8.5 (Smedley and Kinniburgh, 2002). Arsenic speciation (form) is influenced by the relative redox condition of the aquifer environment. For example, in the vicinity of the Douglas County Airport, where arsenic speciation has been characterized, arsenic in groundwater collected at depths greater than 250 feet below land surface was found to be primarily As(III); however, in the upper 150 feet of the aquifer As(V) predominated (Paul and others, 2010). This data set provides a spatial and temporal assessment of available chemical and physical data from local, county, state, and federal databases for the Carson Valley, near Minden, Nevada. Critical data gaps will be identified and, if necessary, additional sample collection and monitoring under conditions of routine groundwater pumping from both municipal and agricultural supply wells will be suggested. Data included as part of this data set, are data provided by the USGS and Carson Valley water purveyors with the support of the Carson Water Subconservancy District and Nevada Division of Environmental Protection to evaluate arsenic mobility and transport in Carson Valley. The data available and described in this release are groundwater water level observations and water chemistry for selected wells in the Carson Valley, Nevada. Cited reference information are available in the supplemental information field in the metadata file associated with this data release.

Over the past 15 years Douglas County, NV has removed production wells in northern Carson Valley from use due to relatively high arsenic concentrations (Carl Ruschmeyer, January 2013, Douglas County Public Works Director, verbal communication). To maintain the supply of water to the public, the town of Minden has been providing water to Douglas County and Carson City. Due to the projected increases in municipal demand, water resource managers are concerned that increasing pumping rates from wells in Minden may change groundwater chemistry and degrade the resource by potentially drawing in arsenic enriched water. Long-term exposure to arsenic can cause illnesses ranging from skin discoloration to various cancers including those of the bladder, skin, and kidney (U.S. Environmental Protection Agency, 2001). Naturally occurring arsenic is one of the most common contaminants in groundwater in the western United States. Arsenic found in basin-fill aquifers is oftentimes associated with alluvial/lacustrine sedimentary deposits derived from the weathering of volcanic rocks and geothermal waters (Welch and others, 1988). The primary aquifers beneath Carson Valley are comprised of quaternary aged basin-fill deposits of weathered granitic and volcanic material (Welch, 1994). Factors contributing to increasing arsenic concentrations in groundwater include, but are not limited to, proximity to arsenic bearing rocks, relatively long groundwater flow paths, the application of phosphate containing fertilizers, and leaching from soils in irrigated areas (Busbee and others, 2009; Anning and others, 2012). The vulnerability of groundwater resources to contamination is influenced by the physical properties of the aquifer, pumping rates, locations of wells and screened intervals relative to the groundwater flow system, and geochemical environment (Focazio and others, 2006). Arsenic mobility and transport through the subsurface is largely controlled by the interaction of groundwater with aquifer sediments. Arsenite (As(III)), the reduced form of inorganic arsenic, usually exhibits greater mobility in groundwater than the oxidized form, arsenate (As(V)) largely due to the greater attraction of As(V) to aquifer sediments relative to that of As(III) at pH values exceeding 8.5 (Smedley and Kinniburgh, 2002). Arsenic speciation (form) is influenced by the relative redox condition of the aquifer environment. For example, in the vicinity of the Douglas County Airport, where arsenic speciation has been characterized, arsenic in groundwater collected at depths greater than 250 feet below land surface was found to be primarily As(III); however, in the upper 150 feet of the aquifer As(V) predominated (Paul and others, 2010). This data set provides a spatial and temporal assessment of available chemical and physical data from local, county, state, and federal databases for the Carson Valley, near Minden, Nevada. Critical data gaps will be identified and, if necessary, additional sample collection and monitoring under conditions of routine groundwater pumping from both municipal and agricultural supply wells will be suggested. Data included as part of this data set, are data provided by the USGS and Carson Valley water purveyors with the support of the Carson Water Subconservancy District and Nevada Division of Environmental Protection to evaluate arsenic mobility and transport in Carson Valley. The data available and described in this release are groundwater water level observations and water chemistry for selected wells in the Carson Valley, Nevada. Cited reference information are available in the supplemental information field in the metadata file associated with this data release.

This dataset includes magnetotelluric (MT) sounding data collected in July 2019 in the Silverton Caldera complex, Colorado, in the Southern Rocky Mountain Volcanic Field, by the U.S. Geological Survey (USGS). Along with geologic mapping, airborne magnetics, airborne electromagnetics, and audiomagnetotellurics, the USGS collected MT data at 24 sites along five profiles ranging from 2 to 5 kilometers in length: across Red Mountain of the Silverton caldera, within the caldera in Eureka Graben, across the south-eastern margin of the caldera along Arrastra Gulch, across the southern margin of the caldera along the western margin of Kendall Mountain, and across the south-western margin of the caldera along South Fork Mineral Creek. The data included here are for MT station MTMB09 located across Red Mountain.

This dataset includes magnetotelluric (MT) sounding data collected in July 2019 in the Silverton Caldera complex, Colorado, in the Southern Rocky Mountain Volcanic Field, by the U.S. Geological Survey (USGS). Along with geologic mapping, airborne magnetics, airborne electromagnetics, and audiomagnetotellurics, the USGS collected MT data at 24 sites along five profiles ranging from 2 to 5 kilometers in length: across Red Mountain of the Silverton caldera, within the caldera in Eureka Graben, across the south-eastern margin of the caldera along Arrastra Gulch, across the southern margin of the caldera along the western margin of Kendall Mountain, and across the south-western margin of the caldera along South Fork Mineral Creek. The data included here are for MT station MTMB08 located across Red Mountain.

This dataset includes magnetotelluric (MT) sounding data collected in July 2019 in the Silverton Caldera complex, Colorado, in the Southern Rocky Mountain Volcanic Field, by the U.S. Geological Survey (USGS). Along with geologic mapping, airborne magnetics, airborne electromagnetics, and audiomagnetotellurics, the USGS collected MT data at 24 sites along five profiles ranging from 2 to 5 kilometers in length: across Red Mountain of the Silverton caldera, within the caldera in Eureka Graben, across the south-eastern margin of the caldera along Arrastra Gulch, across the southern margin of the caldera along the western margin of Kendall Mountain, and across the south-western margin of the caldera along South Fork Mineral Creek. The data included here are for MT station MTMB08 located across Red Mountain.