Using Chemistry and Stable Isotopes to Characterize Ground Water Mixing in Eastern Nebraska's Saline Wetlands

The Lincoln, Nebraska area is home to regionally unique saline wetlands. Estimated to have once covered 16,000 acres in Lancaster County, these wetlands have been reduced by agriculture, construction and other human developments to a fragmented 1,200 acres. These saline wetlands are of great interest to wildlife and natural resource managers from the local to federal level as they provide habitat for endangered and threatened species such as the locally-endemic Salt Creek Tiger Beetle. Four sites adjacent to Little Salt Creek were chosen for the investigation of ground water mixing dynamics within the wetlands. In the Little Salt Creek valley, 60-80 feet of alluvium overlies the Cretaceous-age Dakota sandstone which in turn overlies Pennsylvanian-age limestone. At least three shallow monitoring wells were installed in the alluvium (20-30 feet) at each site. Deeper wells were also installed at the top and bottom of the Dakota sandstone. Analysis of water sampled from the shallow wells revealed variation in chemistry across the wetlands. Values for total dissolved solids ranged from < 1000 ppm to >22,000 ppm while d¹⁸O values varied between -7.05 per mil and -12.99 per mil, indicating a mixing of fresh meteoric water with saline water from the Dakota formation. Head measurements from the deeper wells indicate a vertical gradient, but the mechanism by which this water mixes near the surface is not yet fully understood. Further chemical and isotopic analysis of the deeper ground water will help to better characterize the flow and mixing dynamics within the wetlands.