INVESTIGATING GROUNDWATER CONNECTIVITY IN A TUNNEL CHANNEL LAKE

Water levels in lakes located in unconfined aquifers without surface water inflows or outflows are sensitive to variations in groundwater fluxes. Factors including landscape position, proximity of pumping wells, and land use change influence the magnitude, frequency and duration of lake level fluctuations. This study focuses on Long Lake, a tunnel channel lake located in an irrigated agricultural area in central Wisconsin. Long Lake dried in 2007 raising lakeside residents’ concerns about effects of high-capacity wells on lake levels (Kraft, 2012). However, local vegetable growers using groundwater for crop irrigation suggest that climate and land use change have greater influences on lake levels (High Capacity Well Fact Book, 2016).

This study investigates the connectivity between Long Lake and the regional aquifer over time. Methods include lakebed hydrostratigraphy characterization (via geoprobing, geophysical logging, and aquifer pumping tests), analysis of 2012 to 2017 lake and nearby groundwater levels, basic and isotopic water budget analyses, and construction of a sixty-four-year lake level time series using historic records and aerial photogrammetry.

Results suggest that Long Lake contains preferential groundwater flow paths, and alternates between drainage and groundwater flow-through lake regimes depending on regional groundwater levels. From 2012 to 2017, shallow (2 m) and medium (5 m)-depth nested piezometers on west and east lakebeds contained reoccurring upward gradients. Medium-depth piezometers were screened near moderately-sorted gravel layers, likely acting as preferential flow paths. In contrast, from August 2012 to March 2017, shallow and deep (14 m) nested piezometers showed downward vertical gradients on all lakebed sides. In April 2017, vertical gradients in these wells reversed and stable isotope signatures in lake and groundwater samples went from distinct (2016) to nearly uniform (2017) indicating that the regional water table rose above lake level. The sixty-four-year lake level time series showed that Long Lake routinely transitioned between drainage and flow-through regimes. These results along with precipitation, evapotranspiration, and high capacity pumping estimates will be used to quantify anthropogenic and climate factors affecting Long Lake.