USING MODFLOW TO UNDERSTAND THE FACTORS CONTROLLING LAKE SEEPAGE PATTERNS

Understanding groundwater-lake interactions increasingly requires knowledge of detailed seepage patterns to determine key physical, chemical, and biological processes in lakes. Seepage patterns vary spatially, and the patterns of variation differ from lake to lake. Research from both modeling and field measurements has shown that lakes are commonly characterized by an exponential decrease of seepage with increasing distance offshore. However, field data has also revealed deviations from this decay pattern. Although groundwater models have been used to evaluate the factors that influence rates and patterns of recharge and discharge, little attention has been focused on re-creating such deviations using modeling.

We are using the USGS modeling code MODFLOW to show how various geologic factors, including the distribution of hydraulic conductivity and thickness of lake bed sediment, bedrock fracture zoning, and lake morphology can influence lake bed seepage patterns. The modeling is hypothetical, but many of the geologic and hydrologic factors will be based on a small lake and lake basin (1000 m long basin).

For example, simulating a lake with 5 m thick lake sediments of hydraulic conductivity two orders of magnitude less than the surrounding geology reduced the slope of exponential decay. Instead of more than 90% of the discharge focusing within 25 m offshore, discharge was spread throughout the lake bottom. The exponential fall off was nearly flat with less than an order of magnitude variation in discharge. Doubling the thickness of the lake sediments reduced the slope slightly further. Thus, addition of low conductivity lake bed sediments has a strong influence on whether discharge is focused along the shore. Thinning a zone of sediment to half as thick as the overall lake sediment caused a slight peak in seepage (less than half an order of magnitude), creating point deviations from the exponential decline. However, complete absence of sediment for a lake cell creates an order of magnitude more seepage. Thus, localized thinning of lake sediments does not have as much influence in causing offshore seepage peaks compared to localized absence of sediments. Modeling can help guide field studies by providing geologic examples of what creates deviations from exponential decline in seepage away from a lake shore.