GROUNDWATER FLOW AND SALT TRANSPORT IN SALINE WETLANDS OF EASTERN NEBRASKA
Four major units comprise the hydrostratigraphy beneath the Rock Creek saline wetland. From the uppermost unit to the lowest unit, these are: (1) a clay-silt layer with a relatively uniform thickness; (2) a fine- to medium-grained sand; (3) shale and mudstone members of the Dakota formation; and, (4) fine- to coarse-grained poorly cemented sandstone and fine-grained gravel beds, also of the Dakota formation. Dakota shale and mudstone appear to be interbedded with sandstone and form lenticular bodies and discontinuous layers. Dakota units rest on Paleozoic-age shale and limestone.
Major ion concentrations are relatively constant within water samples collected from several monitoring wells. The average chloride concentration is about 15% and the average sodium concentration is about 20% of that for seawater. Consistency of constituent concentrations with depth suggests that very little mixing of surface water and groundwater occurs, at least within the local flow system.
The general groundwater-flow pattern beneath the wetland is atypical of Nebraska stream valley systems. Water-level observations indicate a strong upward hydraulic gradient between the Dakota and the land surface. This steep gradient produces an upward flow component that causes saline groundwater deep within the Dakota to rise to the surface across large parts of the wetland area, especially in the absence of semi-confining shale and mudstone units.
Corrected carbon-14 ages determined from water samples suggest that the groundwater is about 22,000 years old. In addition, Oxygen-18 indicates that the flow system received relatively cold recharge water, near a temperature of 1.24º C. Thus, it appears that groundwater within the Dakota was replenished under conditions unlike those of the present time.