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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INTRODUCTIONThis literature review of the risk assessment process addresses both ecological and human receptors. The review covers the risk assessment literature including methodology, analysis, interpretation, uncertainty, and regulatory guidance.Companion reviews in this volume that address the effect of pollutants and their fate and transport processes in the aquatic environment overlap the discipline of risk assessment and, therefore, the reader is urged to review these sections as they will not be covered in this review. The focus of the review is on the risk assessment process as it is applied to sediment and water quality, site remediation, and natural resources. ECOLOGICAL Problem FormulationMoore (2001) applied the Anna Karenina principle, "Happy families are all alike; every unhappy family is unhappy in its own way" to ecological risk assessments of multiple stressors. Moore states that there are few pathways to success and many ways to ruin an ecological risk assessment, and all successful assessments must achieve the following: (1) societal and political buy-in, (2) consideration of a broad range of stressors and management options, (3) focusing, models and data to characterize risk, and tools for characterizing uncertainty, (4) adoption of an adaptive management strategy, where system responses are observed, assessed and revised as necessary.As our environmental and regulatory focus shifts from managing point sources to large Literature Review 2002 23 methodology for deriving the RBC for vinyl chloride. Unlike typical RBCs, individual RBCs were derived for childhood and adult exposures to vinyl chloride due to age-based cancer slope factors that are known for vinyl chloride.References Abdel-Megeed, M.A.; Suh, D.H.; and Abdel-Rahman, M.S. (2001) Intra-species Extrapolation in Risk Assessment of Different Classes of Antimicrobials. Hum. Ecol. Risk Assess., 7, 15. Anderson, B.S.; Hunt, J.W.; Phillips, B.M.; Fairey, R.; Roberts, C.A.; Oakden, J.M.; Puckett, H.M.; Stephenson, M.; Tjeerdema, R.S.; Long. E.R.; Wilson, C.Boring, C.S.; Brisbin, I.L. Jr.; Snodgrass, J.; Peles, J.D.; Bryan, A.L.; Smith, M.H.; and Gochfeld, M. (2001b) Radiocesium in Fish from the Savannah River and Steel Creek: Potential Food Chain Exposure to the Public.

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