Geophysics for USGS Groundwater/Surface Water Exchange Studies

Overview

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Understanding exchanges of groundwater and surface water is essential to water managers and hydrologists for the development of effective water-resources policy, protection, and management. Surface water (including streams, lakes, wetlands, and estuaries) “gains” groundwater discharge via seeps and springs, while surface water also infiltrates into adjacent groundwater under “losing” hydraulic conditions. Groundwater discharge is the main component of stream baseflow, or the channel water flowing in between storm events and snowmelt. Many streams, lakes, and wetlands are primarily sourced by groundwater discharge during dry conditions, while coastal water quality can be strongly influenced by submarine groundwater discharge. Groundwater recharge occurs when surface water is exchanged into aquifers below, impacting groundwater chemistry and water supply. The sediment interface between groundwater and surface water, such as a streambed, is often highly reactive due to diverse chemical and microbial conditions, further modifying water quality over short transport distances (e.g., centimeters).

Physical methods of monitoring groundwater/surface-water exchange are often labor intensive and limited in spatial scale. The effects of groundwater/surface-water exchange can occur on a variety of time scales and distances. The dynamics of groundwater/surface water exchange at the stream reach to regional scale are often characterized based on measurements made at a few individual points, though such extrapolation can be highly uncertain do to inherent spatial and temporal variability. The hydrogeophysics toolkit produces data that span scales and helps put point-based measurements into hydrogeological context, often leading to improved understanding of groundwater/surface water exchange processes and associated management concerns.

Using Geophysics to Study Groundwater/Surface-Water Exchange

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The USGS Water Resources Mission Area conducts applied research to evaluate the use of new or emerging hydrogeophysical tools and methods to improve our understanding of groundwater/surface-water exchange. Geophysical methods based on measuring the electrical, thermal, and (or) physical properties of surface water, groundwater, and the shallow subsurface can enable scientists to efficiently locate and quantify groundwater and surface-water related processes. Such spatially comprehensive and spatially distributed information can tie point measurements to larger geologic structures controlling flow and transport at local and regional scales. Similar data types collected over time (i.e., time-lapse data) allow researchers to track highly dynamic processes such as the movement of contaminant plumes, soil moisture, and saltwater intrusion. As a result, we are better able to understand and forecast movement of water between groundwater and surface-water bodies and associated changes in water quality and quantity.

USGS has been a leader in advancing the use of hydrogeophysics to study groundwater/surface-water exchange for decades via methods and software development and pioneering research. Current efforts continue to foster innovation and development of hydrogeophysical technologies and methodologies to answer important questions about our water resources. This work is also part of the USGS Next Generation Water Observing Systems state-of-the-art monitoring technology and methods to increase the spatial and temporal coverage of USGS water data and to make data more affordable and more rapidly available. The USGS Water Resources Mission Area recently released a groundwater/surface water exchange related methods selection tool to aid in the discovery of complimentary tools that may be well suited for specific applications, and to increase the general awareness of the diverse existing toolkit. 

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