1 Introduction and Importance

Surface water and groundwater resources have traditionally been treated as separate or weakly linked systems in some university courses, legal considerations, international cross boundary negotiations, and local and federal regulatory rules. In truth, groundwater and surface-water resources are a fully connected resource responding to changes in hydrologic conditions. Exchange of surface water and groundwater occurs at multiple scales, rates and time frames. Scientific literature clearly supports this model. However, in some jurisdictions, legal interpretations and government regulations isolate surface water and groundwater resources. In these situations, legal concepts need revision to bring them into line with current scientific knowledge.

The United States Geological Survey refers to surface water and groundwater as a single resource (Winter et al., 1998) and it is this framework that guides the material presented in this book. Even though this view is sound, Conant and others (2019) suggest that a gap exists between conceptualizing interactions and holistically integrating the role of groundwater-surface water exchange within physical, geochemical, biological and ecological frameworks. Clearly, interactions are complex and extend beyond solely identifying locations of exchange.

The groundwater system is an important component of the surface-water system and, conversely, surface-water features are linked, at multiple scales, to local and regional groundwater (e.g., Toth, 1963; Winter et al., 1998). Physically, the movement of water from one system to another is largely controlled by differences between surface-water stages (i.e., elevation of the water surface) and groundwater levels, as well as hydrologic conditions at the interfaces (Conant et al., 2019). These interconnections, referred to simply as exchange, result in the creation and sustainability of rivers, lakes, wetlands and coastal systems, support or limit the nature of aquatic and adjacent terrestrial ecosystems (e.g., Meyer, 1997), impact surface water and groundwater quality, and affect the fate of contaminants as they migrate within the hydrologic system (Conant et al., 2019). Exchange occurs under both natural and hydrologically modified conditions such as channelized stream reaches and at reservoirs (Winter et al., 1998). Exchange varies in time as hydrologic conditions naturally change and/or are purposely manipulated.

Investigations of natural or impacted groundwater-surface water exchanges and forecasting the consequences of proposed modifications and remediation efforts require the development of conceptual models describing exchange processes. In addition, methods to locate and quantify exchanges under natural, disturbed and remediated settings are requisite. For example, the design of a lake ecological study or plans to manage the lake stage requires information on how groundwater systems interface with the lakeshore and bottom. A stream restoration project with a goal of enhancing exchanges needs to be built on an appropriate conceptual model. The effort to protect a wetland calls for identification of the presence or absence of groundwater and surface water exchange. Resolution and quantification of natural or modified exchange processes necessitate well supported conceptual models, and the application of appropriate qualitative and quantitative methods and tools.

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Groundwater-Surface Water Exchange Copyright © 2020 by William W. Woessner. All Rights Reserved.