Approach

Chemical patterns and thermal regimes in headwaters, unlike many higher order streams, can show substantial spatial variability. Groundwater gains and losses exert a spatially discontinuous influence on streams that reflects, in part, the preferential nature of groundwater discharge as controlled by recharge, topography, and geology. Such variability in discharge patterns presents a challenge for USGS monitoring and interpretation. New thermal infrared field surveying techniques combined with localized point measurements allow discrete discharge locations such as streambank springs to be assessed in detail, but extrapolation of such measurements to the larger catchment can be difficult. Conversely, streamflow-based methods of physically or chemically distinguishing base flow from rapid runoff in high-order streams can be applied to broad catchment areas, but spatial detail is lost.

The wide range of current methods that can be used to evaluate groundwater discharge to streams across scales can be explored with the USGS Groundwater/Surface-Water Method Selection Tool (Hammett and others, 2022).When applied to the known characteristics of the Neversink Reservoir watershed, this software tool indicated that several chemical hydrograph separation and geophysical methods were unlikely to be effective in identifying discharge sources because of the widespread similarity in groundwater and surface-water electrical conductivity, with values generally less than 50 microsiemens per centimeter at 25 degrees Celsius. However, several heat tracing methods were deemed applicable, in part because of strong contrasts between surface and groundwater temperature during summer and winter. Ambient tracer methods based on stable water isotopes and dissolved radon gas measured in streamwater were also expected to be useful because of the local bedrock geology, elevation gradients, and snowmelt processes in the Catskill Mountains.

As a first step in 2019, NGWOS updated and (or) activated streamgage sites at six locations in the headwater subbasin with sensors for water temperature and electrical conductivity, in addition to establishing two sites for intensive monitoring of groundwater discharge at existing streamgages along the West Branch Neversink River and mainstem Neversink River. These two sites were complemented at the watershed scale by establishing 51 multiyear water and air temperature monitoring stations across headwater streams with a range of sizes and elevations. Geophysical and drone data were also collected at the stream monitoring sites and paired with emerging analytical methods to refine the mapping of near-surface geology across the subbasin. This multiscale and multiparameter approach represents a transferable example of groundwater discharge monitoring to support the improvement of water quality, temperature, flow permanence, and habitat forecasts in headwater stream systems.

Streambank-Scale Monitoring of Deep and Shallow Groundwater Discharge

The USGS has monitored lateral groundwater-to-stream hydraulic gradients at some Super Gage sites (for example, Eddy-Miller and others, 2012). NGWOS built upon these earlier efforts by identifying varied locations of groundwater discharge using heat as a tracer at the West Branch Neversink River (USGS site 01434498) and mainstem Neversink River (USGS site 01435000) streamgage sites and instrumenting nearby seeps and springs directly. By targeting areas of discharge, the approach focused on sensor deployment to active surface-water and groundwater hydrologic connections, rather than relying on connectivity inferred from well water levels. A critical component of streambank-scale instrumentation at each site was initial site characterization, primarily using thermal infrared cameras and fiber-optic distributed temperature sensing to quickly identify discharge locations, and passive seismic geophysical methods for measuring local bedrock depth (fig. 2).

Small uncrewed aviation systems (drones) were deployed to map the terrain and collect additional thermal infrared data (Briggs and others, 2020). A subset of discharges were selected at the two streamgage sites to represent primary discharge types throughout the watershed, such as deep groundwater seeping from thick alluvial sediments and shallow groundwater emerging in riparian springs along the toe of both wooded and exposed hillslopes. Shallow piezometers and vertical temperature profilers were installed directly into discharge locations, soil moisture profilers were placed in nondischarge bank sediments, and local meteorological variables were monitored (fig. 2). Each general data type is described briefly below.

Fixed-Mount Thermal Infrared Imaging

To enhance the spatial context of thermal variations in surface waters resulting from groundwater discharge, continuous, fixed-mount, thermal infrared (TIR) imagery sensors were installed at the mainstem Neversink River streamgage site (USGS site 01435000). River water and streambank surface temperature images as interpreted from TIR data help comprehensively visualize where along the riverbank discharge is occurring, expanding on point measurements from temperature probes (fig. 3). The TIR data are currently collected hourly at this site, paired with visible light imagery, and displayed as still frames or time lapses on the USGS National Water Information System page (https://waterdata.usgs.gov/monitoring-location/01435000), where the data are also available for download (U.S Geological Survey, 2022).

Headwater Network-Scale Monitoring of Groundwater Discharge Influence on Streamwater

Paired streambank air and streamwater temperature data were collected at 51 temporary monitoring stations installed in fall 2020. Stream water samples were also collected in spring and fall seasons and analyzed for stable water isotopes and dissolved radon gas.

Paired Local Air and Streamwater Temperatures

In the absence of groundwater influence, stream temperature patterns are generally expected to track local air temperature changes at weekly to annual timescales. However, accumulated groundwater discharge can disrupt that coupling. Therefore, time series analysis of paired, local air and streamwater temperatures is used to indicate relative groundwater influence along small and large streams at the watershed scale. Metrics related to the timing and magnitude of annual stream temperature signals compared to local air signals can also be used to infer groundwater influence and potentially indicate stream sections dominated by shallow groundwater discharge (fig. 4). Paired air and water temperature data can be analyzed by using locally measured data or the free web application PASTA (https://cuahsi.shinyapps.io/pasta/), which provides estimated air temperature data from a large-scale database.

Geophysical Mapping of Bedrock Depth

The thickness of surface soils influences the routing of groundwater flow paths to streams, which in turn affects groundwater discharge temperature and water quality. From 2019 to 2022, the USGS collected passive seismic geophysical data at over 80 stream corridor and hillslope locations that were interpreted to determine depth to bedrock (Glas and others, 2021), which varied between the East Branch Neversink River and West Branch Neversink River, and data were used to improve near-surface geologic maps.

Enhanced Geologic Mapping

Mapping surficial sediments and bedrock exposures across the Neversink Reservoir watershed at high spatial resolution (1:24,000 scale or smaller) has provided an improved geospatial dataset in support of groundwater modeling efforts. A deep-learning, image segmentation approach was developed to aid in the mapping of exposed and covered bedrock surfaces using a light detection and ranging (lidar)-derived digital elevation model at 1-meter spatial resolution (Odom and Doctor, 2022). In addition, a map of estimated minimum sediment thickness was derived by using an inverse-distance weighting interpolation between measurable heights of the banks of streams that cut through surficial sediments and the nearest bedrock exposures as well as local geophysical measurements of sediment thickness.