EASTERN NEBRASKA'S SALINE WETLANDS: SURFACE EXPRESSION OF THE REGIONAL GROUND-WATER FLOW SYSTEM

Eastern Nebraska’s saline wetlands are the most limited and endangered wetland type in the state and are considered to be critically imperiled. Historical impacts to the wetlands have resulted in 90% degradation of the system and the threat from urban and agricultural expansion continues. Saline wetlands support unique plant life and an endangered insect species found only in the open salt flats. State and federal agencies are interested in preserving the remainder of the saline wetlands around the Little Salt Creek and Rock Creek watersheds.

Research indicates the occurrence of salt is not from evaporative concentration of water at the surface but rather from a deeper hydrogeologic source. Saline wetlands are associated with the subcrop of the Maha aquifer, locally known as the Dakota aquifer. The Dakota consists of Cretaceous-age sedimentary rocks derived from both terrestrial and marine depositional environments. As a result, the formation is composed of a complex, three-dimensional mosaic of clay, silt and sand units. Measured concentrations of NaCl vary between monitoring sites from 5,900 to 15,700 mg/L. A significant upward vertical gradient, up to 0.04 in places, has been measured in multi-level monitoring wells within the study area. This vertical gradient component of the regional flow system forces salt water in the underlying Pennsylvanian marine deposits to readily discharge at the ground surface.

This regional flow theory is supported by stable isotope and tritium chemical analyses. Strontium isotope data suggest a carbonate source within underlying rocks, likely Pennsylvanian in age. Carbon-14 analyses indicate that the majority of ground-water recharge originated between 16,000 and 37,000 years ago. In addition, stable oxygen isotopes indicate that at the time of recharge, the mean annual air temperature was between 0.5 and 2.5 degrees Celsius, which is colder than modern conditions.

The locations and distribution pattern of saline wetlands depend on the interaction of the regional flow system and the geologically complex Dakota aquifer. Migration pathways of salt water are controlled by the interconnection of sand units within the aquifer and are expressed by salt concentration patterns found at the ground surface.