COMBINED HYDRAULIC, GEOPHYSICAL, AND GEOCHEMICAL INVESTIGATIONS TO CHARACTERIZE GROUNDWATER FLOW AND MASS TRANSPORT NEAR SALINE LAKES IN THE SEMI-ARID NEBRASKA SAND HILLS, USA

Shallow endorheic saline lakes supported by groundwater seepage and minimal surface runoff are common in semi-arid and arid environments around the world. The importance of understanding lake dynamics is increasing as social concerns over the consequences of environmental mismanagement and the effects of climate change increase. However, quantitative analysis of water and solute fluxes at the watershed scale remains a major challenge. In this work, we assess groundwater flow and mass transport in lakes with different subsurface flow regimes in the Nebraska Sand Hills.

The grass-stabilized dunes in the Nebraska Sand Hills attain heights up to 130 m. Within them are hundreds of interdunal lakes with surface areas >4 ha and depths <1 m. Annual lake evaporation exceeds precipitation by as much as 600 mm. Lake salinity ranges from fresh (~0.3 g/L) to hypersaline (>100 g/L), with pH values as high as 10.

A combination of hydrologic and geologic factors--namely regional groundwater flow, evaporation, precipitation, lake size, groundwater recharge, and geologic setting--may preclude outseepage from some of these lakes even in the presence of ambient regional flow. Solutes from generally fresh groundwater flow into these lakes, which become enriched by evaporation over time. Contrasts in the electrical resistivities of water in the lakes and groundwater in the aquifer serve as important indicators of solute mass fluxes that can be exploited using non-invasive geophysical techniques.

We studied groundwater flow and mass transport in clusters of saline and freshwater lakes with different subsurface flow regimes in a 40 km² area in the vicinity of Crescent Lake National Wildlife Refuge, Garden County, Nebraska. Investigating subsurface flow regimes with areas on the order of tens of square kilometers was made feasible by combining geophysical, hydraulic, and geochemical techniques. The integrated approach uses frequency-domain electromagnetics, land and marine electrical resistivity imaging, hydraulic test, aquifer coring, and hydrochemical analysis. Integrated interpretation of the results enabled classification of lakes in the study area as either discharge or flow-through, and the resultant lake classifications are consistent with theoretical lake salinity models.