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The water balance of wetlands affects their structure and function because nutrient inputs and outputs are altered (LaBaugh 1986). Precipitation, surface runoff, and ground-water discharge are water sources that can contribute to the water and chemical budgets of wetland basins (Figure 7). Water losses occur through evaporation and transpiration, surface outflow, and ground-water recharge. The flow patterns that dominate a wetland basin dictate the hydrologic functions of the basin, water chemistry, and ultimately, the biota that will dominate the basin (Figures 8-10).

Figure 7. Dominant sources of water in prairie wetlands

Figure 8. Dominant flow patterns of a flow-through type of prairie wetland containing pooled water low in dissolved salts.

Figure 9. Dominant flow pattern of a closed-system type of prairie wetland containing pooled water intermediate in dissolved salts.

Figure 10. Dominant flow pattern of a hydrologic-sump type of prairie wetland containing pooled water that accumulates dissolved salts.

The dominant salts carried by ground water are ultimately controlled by mineral composition and their solubility characteristics. Salts are dissolved and then redeposited as concentrations increase and more highly soluble salts are encountered. Once dissolved salts enter a wetland basin they can be concentrated by evaporation and transpiration or removed through surface outflow, ground-water recharge, or both. Undissolved salts can also be removed from dry basins or the dry shorelines of wet basins through wind erosion. Wetlands that lose water to surface outflow remove salts at rates that are a function of annual turnover rates (volume lost versus wetland storage capacity). As turnover rates increase, the salt content of the wetland approaches that of the dominant water source. Water loss to ground water also removes salts from the wetland. The rate of loss is influenced by the permeability of the till that controls the rate of export. Wetland basins that do not lose water to surface outflow, but are ground-water through-flow systems, tend, because of low permeability, to be higher in dissolved salts. Wetlands that lose water primarily to the atmosphere by evapotranspiration tend to concentrate salts to high levels (Swanson et al. 1988).

Wetlands differ from one another in their chemical characteristics based on their location in groundwater flow systems (LaBaugh 1989). Topography and the geologic characteristics of glacial tills provide the framework within which ground water flow systems develop.

Till in south-central North Dakota has a high percentage of silt and clay, is poorly sorted, and has low permeability; outwash is largely sand and gravel, is well sorted, and has high permeability (Swanson et al. 1988). Significantly higher average values of specific conductance and concentrations of sodium, potassium, sulfate, and alkalinity were found in wetlands located in outwash, whereas significantly higher average concentrations of calcium occurred in wetlands located in till (Swanson et al. 1988). Most wetlands that had values of specific conductance greater than 10 mS/cm in southcentral North Dakota were located in outwash, and nearly all wetlands located in till had values less than 10 mS/cm. Saline wetlands located in outwash are topographically low, nonintegrated ground water discharge areas that function as hydrologic sumps and consequently concentrate salts.

Seven different chemical types were reported by Swanson et al. (1988) among wetlands studied in south-central North Dakota: calcium bicarbonate, magnesium bicarbonate, sodium bicarbonate, magnesium sulfate, sodium sulfate, sodium chloride, and magnesium chloride. The percentage of wetlands having the various chemical types of water were as follows: calcium bicarbonate-l, magnesium bicarbonate-22, magnesium sulfate-17, sodium bicarbonate-10, sodium sulfate-45, sodium chloride-4, and magnesium chloride-1 (Swanson et al. 1988). Specific conductivity of most of the wetlands (65%) was between 0.8 and <8 mS/cm. This range is within the oligosaline salinity category of the Cowardin et al. (1979) wetland classification system. Only 6% of the wetlands fell within the fresh category (<0.8 mS/cm). Twenty percent fell within the mesosaline category (8-<30 mS/cm), 7% were within the polysaline category (30<45 mS/cm), 1% were within the eusaline category (45-60 mS/cm), and 1% were within the hypersaline category (>60 mS/cm) (Swanson et al. 1988).

A combination of hydrogeologic factors, including topography and geology, affect the water chemistry of wetlands (Driver and Peden 1977). LaBaugh et al.'s (1987) study of the Cottonwood Lake area, Stutsman County, ND, reveals that wetland chemistry is dependent on the position of individual wetlands with respect to ground-water flow systems. Ground-water flow systems integrate topography and geology that are important physical boundaries of flow systems. The basic chemical type is further modified seasonally by the excess of evaporation over precipitation.

In the Cottonwood Lake area, potassium and calcium bicarbonatedominated waters low in specific conductivity occurred in wetlands that recharge ground water. The wetland with the largest value of specific conductance was in an area of ground-water discharge and was characterized by the magnesium sulfate water type, similar to that of the adjacent ground water. Concentration by evaporation is also an important factor affecting seasonal differences in chemistry of prairie wetlands. Different wetlands have different hydrologic relationships to ground water: some recharge groundwater, others discharge it, and still others do both.

Knowledge of the plant communities that occupy the different zones of a wetland basin (Stewart and Kantrud 1971) and the ionic composition of surface water can be used to draw inferences concerning the hydrologic functions of prairie wetlands (LaBaugh 1987; LaBaugh et al. 1987). Phosphorus and nitrogen differences were more closely related to the effect of hydroperiods on aquatic macrophytes (LaBaugh et al. 1987). The retention of phosphorus, nitrogen, and major ions by wetland basins is a function of their hydrologic settings as demonstrated in Figures 7-10 and described by Swanson et al. (1988). While phosphorus tends to be restricted to surface flow, nitrogen compounds that are water soluble can follow ground-water flow systems.