A FINITE-ELEMENT APPROACH FOR VISUALIZING GROUNDWATER FLOW VARIATIONS AT A WETLAND IN THE NEBRASKA SAND HILLS

As part of a study led by Dr. Tom Winter of the U.S. Geological Survey on lake-groundwater interaction in the Nebraska Sand Hills, surprising variations of the water-table configuration were observed at a dune/wetland site in the project area. The site consists of a long (>100 m) narrow wetland depression, with a very shallow water table (<0.5 m). This wetland lies adjacent to an elongate parabolic dune. Axes of both features trend northwest to southeast. Monitoring wells were installed in the wetland, dune ridges, and along the dune axial trough, and depth to the water table varied significantly from well to well.

Groundwater movement adjacent to the wetland varied by season and essentially reversed its flow direction over the course of a year. On the basis of continuous monitoring (using daily average water-table elevations), three general patterns of flow were observed during 1987. Groundwater flowed south-southeast with low gradients (<0.0010) during late-fall and winter. In the spring, flow shifted to east-northeast away from the wetland with much higher gradients (to ~0.0040), and then in summer, flow was toward the wetland (southwest) with moderate gradients (~0.0024).

A finite-element approach was developed to calculate the gradient’s direction and magnitude (Keen, 1992), and is updated here using Excel. This method is easy to utilize. Accurate x,y coordinates of 3 wells (in a triangle) and water-table elevations at those wells are the only data required. Once the coordinates are entered, the determinant of the coordinate matrix is calculated. Three shape functions are then used to derive the partial derivatives of the gradient in the x and y directions. From these the magnitude of the gradient vector is determined and the azimuth is found from the arctangent of the gradient components. Temporal variations of the gradient are best shown on polar plots.

The results of this study show that wetlands in this stabilized dune area can act as sites of focused recharge during spring and then change to be sites of focused discharge (via evapotranspiration) in summer. The continuous gradient calculations for 1987 suggest that recharge exceeded discharge by roughly 12%. Because interdunal wetlands are abundant in the Sand Hills, recharge, discharge, and flow patterns are expected to be dynamic in those areas.