Ground water is present in the glacial deposits in the Mirror Lake watershed. One type of glacial deposit, glacial till is found throughout the Mirror Lake watershed except on the south side of the lake. Glacial till consists of a heterogeneous mixture of clay through boulder-size rocks. In the Mirror Lake area the matrix of the till is mostly silty and sandy, and it is not very permeable. The till is thickest on the north side of the lake, where it reaches a thickness of 55 meters near the TR1 piezometer site. The till is relatively thin on the east side of the lake, only about 1 to 3 meters thick.

The ground-water system is referred to as the saturated zone because all of the pore spaces between the rock particles are filled with water. The water table is the upper surface of the ground-water system. Above the water table the pore spaces are filled with water and air and is therefore referred to as the unsaturated zone. The direction of ground-water movement along its upper surface is indicated by the configuration of the water table because, in general, ground-water flow direction is approximately perpendicular to water-table contours. There are also vertical components of flow within the ground-water system, but we will discuss them at Stop 9.

The amount of ground water that discharges to Mirror Lake can be calculated using Darcy’s Law; Q = KIA, where Q is the ground-water discharge, K is hydraulic conductivity, I is hydraulic gradient, and A is the cross sectional area if the ground-water system at the shoreline of the lake. Hydraulic conductivity, K, of the geologic deposits can be determined by measuring the recovery time of the water level in a well after a known volume of water is added or removed from the well. Hydraulic gradient, I, can be determined by knowing the altitude of the water level in a well, the altitude of the lake surface, and the distance of the well from the lake. The vertical component of the cross-sectional area of the aquifer can be determined by test drilling or geophysical measurements. To calculate the ground-water inflow to Mirror Lake, the watershed was divided into segments that were determined by the presence of “index wells” that could be used for calculations of hydraulic gradient for different parts of the watershed. Total ground-water inflow is determined by summing the discharge to the lake for each segment.