The hyporheic zone, the transition region between groundwater and surface water, represents an important interface between terrestrial and aquatic ecosystems. When groundwater combines with surface water in this zone, the characteristics of each are blended and new gradients are established, especially for contaminants. Therefore, the hyporheic zone is important in considering the “big ecological picture” as the hydrologic continuum connecting groundwater and surface water. The importance is reflected by the current focus of this zone in ecological risk assessments conducted under the Resource Conservation and Recovery Act (RCRA), Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), and Clean Water Act (CWA) programs. A variety of tools can be used to measure, analyze, and predict the physical, chemical, and biological processes that occur within the hyporheic zone. Directly measuring the flux of water across the interface between groundwater and surface water determines whether surface water enters the streambed at downwelling zones or groundwater discharges from the streambed in upwelling zones. In addition to direct measurements of the flux of water, several states have developed models to characterize the interaction of groundwater and surface water. The variability in physical and chemical characteristics between upwelling and downwelling zones influences the local ecology within the zone. The study of the species within the hyporheic zone includes ecological surveys and ecotoxicological investigations. The evolving study of the hyporheic zone will necessitate an increase in basic research into hydraulic considerations, an identification of regional representative sites with contaminated hyporheic zones, and a better understanding of the ecology of the species within the zone. © 2001 John Wiley & Sons, Inc.

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