EFFECT OF TUNNEL CHANNEL STRATIGRAPHY ON SURFACE-WATER AND GROUNDWATER INTERACTIONS: A CASE STUDY OF LONG LAKE, WI

Understanding the hydrostratigraphy beneath lakes is critical to understanding and quantifying groundwater-lake interactions. Consecutive years of low water levels in Long Lake – Oasis, Waushara County, WI, have fueled speculation about the causes of the low water levels. Many people blame nearby high-capacity irrigation wells for the reduction in water levels, while others cite the possible effects of climate change. One popular theory frequently discussed by local citizens describes the lake as a “clay bowl” isolated from the underlying aquifer by low-permeability sediment. This study investigated the hydrostratigraphy around and beneath Long Lake and its effect on the connection between the lake and the regional aquifer. Research methods included sediment core sampling, well construction report analysis, geophysical investigations, and surface-water- and groundwater-level monitoring. The lake sits in the Late Wisconsin Plainfield-Huron tunnel channel. Near the lake, sediment cores and well construction reports reveal that fine sediments are present beneath Long Lake and are uncommon outside the tunnel channel. However, sediment and geophysical analyses showed that these fine sediments are not continuous, and that instead, sands, gravels and fine sediments were deposited in layers. Fine sediment layers beneath Long Lake range from a few millimeters to several meters in thickness and are discontinuous across the lakebed. Groundwater-level monitoring from July 2012 to July 2014 revealed downward vertical hydraulic gradients between deep (<40 ft) and shallow (<10 ft) piezometers on three sides of the lakebed, showing that the lake loses water to the groundwater system along its west, north, and eastern sides. However, during the same period, vertical gradients on the south side of the lake were negligible. These results show that Long Lake is connected to the regional aquifer and is affected by changes in regional groundwater elevation. Long-term surface water levels in Long Lake will likely continue to fluctuate given their sensitivity to groundwater pumping and close proximity to the groundwater divide.