4 Wetlands and Groundwater Exchange

Groundwater exchange with wetlands includes sites where water is continually or intermittently present and a local groundwater system is effluent, influent, flow-through or a combination of exchange conditions (mixed). Wetlands occur in depressions, low areas of the landscape, and, in contrast to lakes, on slopes. Wetlands can be dominated by groundwater, precipitation/runoff, or a combination of conditions that vary over time. Commonly, a wetland is viewed as wet area created by either precipitation or by a water table that is at or near land surface some or all of the year. Wetlands have specific vegetation types (hydrophytes) and underlying soils that reflect the constant or periodic saturation of the soils (hydric) (Mitsch and Gosselink, 2000; Tiner, 1996).

The literature defines wetlands using a wide range of terms based on location, topography, biological composition, and the influence of runoff or groundwater (e.g., bog, fen, marsh, mire, moor, carr, oxbow, peatland, pothole swamp, slough, vernal pool, wet meadow) as discussed by Mitsch and Gosselink (2000). Wetland definitions vary depending on whether a physical science or ecological science approach is applied. A broadly used definition was developed by the U.S. Fish and Wildlife Service (Cowardin et al., 1979).

“Wetlands are lands transitional between terrestrial and aquatic systems where the water table is usually at or near the surface or the land is covered by shallow water. Wetlands must have one or more of the following three attributes: 1) at least periodically, the land supports predominantly hydrophytes; 2) the substrate is predominately undrained hydric soil; and 3) the substrate is nonsoil and is saturated with water or covered by shallow water at some time during the growing season each year.”

Cowardin and others (1979) suggest wetlands can occur with some or all the listed criteria. Legal and regulatory definitions most often require all three attributes be present for a feature to be classified as a wetland. Wetlands are valued for the role they play in temporally storing runoff, impacting water quality, and supporting diverse ecological systems (Mitsch and Gosselink, 2000; Fretwell et al., 1996).

Wetlands often contain fine-grained sediments and decomposed organic sediments both at the perimeter and bottom. When such conditions are present, exchange rates with groundwater systems can be constrained. In contrast, lakes, with fine-grained bottom sediments, are often shallow at the shoreline where wave action limits fine sediment deposition resulting in less restricted groundwater-surface water exchange. Groundwater exchange with a wetland can be conceptualized using the cross sections presented in Figures 4 through 8 and map views representing lakes (Figures 33 through 36). Wetlands also can occur without a connection to the local groundwater system and be dominated by surface runoff.