GROUNDWATER MIXING DYNAMICS WITHIN THE SALINE WETLANDS OF THE LITTLE SALT CREEK WATERSHED, LANCASTER COUNTY, NEBRASKA

The saline wetlands of eastern Nebraska are a unique and highly endangered ecosystem. Saline groundwater discharging into streams and adjacent floodplain areas of the Little Salt Creek and Rock Creek watersheds in Lancaster County has created saline habitats that host regionally rare halophytes and the federally endangered Salt Creek tiger beetle. Landscape modifications associated with agriculture, flood control, and urban expansion have led to the disappearance, degradation, or fragmentation of many saline wetlands.

In an attempt to learn how better to improve and preserve the remaining saline wetlands, research has been conducted to examine the role of groundwater in sustaining these wetlands under the current hydrologic regime of the Little Salt Creek watershed. As part of that larger, more comprehensive work, this study focused on describing the mixing of fresh modern meteoric water with deeper saline groundwater and how that mixing affects the distribution of salinity in shallow wetland groundwater.

Stratigraphy was interpreted from borehole logs while hydraulic head and groundwater chemistry data were obtained from piezometers installed at three sites along Little Salt Creek. An assessment and synthesis of stratigraphy from alluvial valley boreholes showed significant clay layers at the top of the Dakota formation and in the lower alluvium. The hydraulic head data from this study indicated that, while the presence of the Dakota clay affects hydraulic gradients to some degree, clays in the alluvium are likely a more significant control on near-surface groundwater flow. Most groundwaters sampled were of the Na-Cl type but varied in TDS and chloride concentration, with chloride concentrations increasing with depth and distance down the watershed. Variations in chloride and δ¹⁸O values were used to estimate the portion of modern meteoric water sampled at each piezometer using a basic two end-member mixing model. Results showed that two of the wetland sites had significant modern meteoric fractions in shallow groundwater. Specifically, the meteoric component exceeded 50% at 7 of the 10 shallow piezometers installed at the two southern-most sites. In contrast, the third site was relatively unaffected by meteoric waters with all piezometers producing water having a meteoric fraction of less than 30%.