USING CONCEPTS OF GRADIENT RATIO AND TOPOHYDROLOGIC OFFSET FOR LAKE SALINTY DELINEATION: THE NEBRASKA SAND HILLS

Emergence of groundwater-dominated lakes in eolian semi-arid environments commonly follows the climate transition from dry to wet conditions and rise of the water table. Groundwater discharge to topographic depressions, coupled with extensive evaporation results in elevated lake salinity. Salinity variations between lakes in similar geographic and climatic conditions can be significant, and attempts to delineate these variations are scarce.

Although it is understood that lake concentrations and solute balance are strongly related to the lake water balance (Anderson and Bowser, 1986, Townley and Trefry, 2000), quantitative assessments require extensive data sets that are rarely available even for a single lake (Winter, 1988, Turner and Townley, 2006).

To investigate leading factors affecting salinity variations between groundwater-dominated lakes, we start with steady-state relationship between the groundwater concentration (C_g) and the lake water concentration (C_l): C_l =C_g (Q_in / Q_out), where Q_in is the groundwater inflow and Q_out is outflow. Dimensional hydrodynamic analysis and numerical modeling indicate that evaporation factor (Q_in / Q_out) is largely a function of dimensionless gradient ratio G=i/(H/a), where a is the lake radius, i is the regional water table slope, and H, or topohydrologic offset, is the local deviation of the actual lake level from the regional water table. For large G values, lakes have flow-through regimes; with G reduction, a critical value G_c can be achieved and lakes become fully gaining, which leads to high lake salinity.

Importance of gradient ratio G was assessed for a group of lakes in Crescent Lake National Wildlife Refuge area, which is a part of the Sand Hills, Nebraska, the largest sand dune field in the western hemisphere (58,000 km2). Numerous shallow lakes (mostly not deeper than 1 m) in topographic depressions under west-east regional groundwater flow have TDS ranging from 0.3 g/L to more than 100 g/L, and pH from 8.4 to more than 10. The groundwater concentrations are typically low. Using data on aquifer properties from various databases, field investigations, and numerical modeling, we demonstrate merits of our gradient ratio-based approach within the GIS framework.