UPPER DELAWARE RIVER — An effort to better understand the magnitude and significance of groundwater discharge (GWD) to the hydrology and ecology of the upper reaches of the Delaware River is underway.
United States Geological Survey (USGS) staff have been conducting research this summer to evaluate the influence of groundwater influx on fisheries and aquatic resources at three locations on the Upper Delaware River. USGS provides support to federal, state and local public entities for better management of natural resources based on unbiased scientific data, according to USGS research hydrologist Donald Rosenberry.
The study focuses on three river reaches near Frisbie Island, Hankins, NY and Callicoon, NY. Geophysical methods are being used to map the riverbed to determine areas where it is likely that groundwater is discharging to the surface.
The GWD to surface water exchange is an active component of stream ecosystems that influences whole-system processes that may play an essential role in supporting aquatic life, providing habitat stability and mitigating some effects of land use on water quality.
Identifying and protecting such areas could help to conserve aquatic populations. It has been hypothesized that groundwater influx or upwelling may be related to species presence and abundance. The study will help to determine the influence of groundwater on habitat during low-flow warm-water conditions.
Several variables have been measured at each location, such as bed sediment temperature, which helps determine thermal conditions in the substrate during warm-water periods. Fiber optic cables were deployed at each reach. A laser pulse sent down the cable allows measurement of the temperature along every meter increment, allowing for two-dimensional rendering of the distribution of GWD.
“Groundwater provides a constant cold temperature year-round, so it offers a thermal refuge for fish and other animals when the river gets warm,” said Rosenberry. “What we’re doing is looking at the spatial distribution of the groundwater discharge and then actually trying to quantify the discharge.
Just how much water is flowing into the river? And how much does that vary over time? Does it decrease in late summer, or is it a really large reservoir that’s fairly constant? What drives the research is the ecological impact. Anyone who’s interested in the fishery here would benefit from these results.”
The project is designed to look at the distribution of groundwater as it discharges to the Delaware River. “The people who live here and fish here know to a large extent where the groundwater is focused,” said Rosenberry. “They know that groundwater is discharging to the river, but they probably don’t think about what it means. We do. This is the kind of work that USGS does all across the country.
“As a country, about 40% of the water we consume comes from groundwater. In rural areas, that percentage is much larger. The concern is that as we use more groundwater to supply our homes and infrastructure, there may be less groundwater discharging to the river. So at times when the river temperature is very warm, the fish and the animals that live in the substrate may become stressed.”
USGS staff have also been sampling the chemistry of several small hand-augured wells that they installed in the river bed and along the shore to determine the chemical inputs to the river. Several wells with sensors inside will continue to collect data every 20 minutes throughout the year.
Gathering such data now establishes a baseline against which changes can be measured in the future. “It’s not just people who make use of groundwater,” said Rosenberry. “It’s also the natural environment. Animals that live in the substrate, macroinvertebrates and plants — there are many microenvironments that rely on this. We don’t always think about that. This kind of work will set a baseline we can compare to years from now.”
The USGS produces numerous reports for the public in addition to those for its scientific colleagues. Two of particular relevance to this topic can be seen at http://pubs.er.usgs.gov/publication/cir1186  and http://pubs.er.usgs.gov/publication/cir1139 .