Isn’t there a large outflow on the west side of West Twin Lakes?
A public hearing was held at the Village of Roberts Park Building on June 22, 2004. A presentation of wastewater treament plant improvements was made to the nine people in attendance. Craig Smith, Warren Township landowner was concerned about any rise in lake level that would impact his land being flooded by the treatment plant discharge. Jim Lund, P.E., consultant with Ayres responded that in the worst case, there would be a 1′-8″ rise without any outflow. Jim Lund stated that there is a large outflow on the west side of West Twin Lakes (1). The elevation of West Twin was approx 969 ft in August of 2017 (2). This is a 5 ft increase in elevation from 2004 (approx 964 ft).
As noted by the 2008 Twin Lakes Groundwater/Surface Water Interaction Study (3), soil boring of West Twin Lake reveals sand to a depth of 26 ft, followed by a 4 ft thick layer of clayey sand. The rate of seepage through this restrictive clay layer is slow relative to an expected seepage rate from the lake, suggesting that groundwater seepage from the lakes is in a predominately lateral direction. The monitoring results imply that lateral flow of the local groundwater aquifer is primarily from east to west and follows the same tendency of the regional groundwater aquifer as shown in the USGS Hydrological Investigations Atlas.
In sum, water quickly seeps vertically through the sand in the surface aquifer. When it reaches the restrictive clay layer at 26 ft, it moves laterally (and very slowly) to the west. Assuming a seepage rate of 1976 in/yr for sand, Darcy’s Law predicts a seepage rate of 11.2 in/yr to the St. Croix River, 10 miles to the west and approximately 300 feet below the level of Twin Lakes. Based on the 43-yr historic average, seepage of Twin Lakes is likely about 10 in/yr (2).