A buried bedrock valley is present on the south side of Mirror Lake. This valley in the bedrock surface is filled with sand and gravel that was deposited by Hubbard Brook when the stream was much higher and it carried water and sediments from melting glaciers following the ice age. Because this sand and gravel deposit is very permeable, a substantial amount of water seeps out of the lake on its south side. The amount of lake water that seeps out is calculated in the same way as for the inseepage, described at Stop 6. The only difference is that the gradient on the outseepage side is away from the lake.

To determine the ground-water flow paths that the water takes after it seeps out of Mirror Lake, a number of water-table wells and three piezometer nests were constructed between Mirror Lake and Hubbard Brook. Actually, water-table wells and piezometer nests were constructed at other places within the Mirror Lake watershed to determine the flow paths that ground water takes as it moves from recharge areas on the hillside to the lake on its inflow sides. Before discussing the flow paths, however, we need to explain what water-table wells and piezometer nests are, and how they are constructed.

A water-table well is used to determine the position and elevation of the upper surface of the ground-water system, which is termed the water table. To construct such an observation well, a hole is drilled to about half a meter below where the water table is encountered. Next, a well casing (steel or plastic pipe) with a well screen attached to the bottom is lowered into the hole. Sand is then poured down the annular space between the bore hole wall and the outside of the screen is. This is termed a sand pack. Finally, drill cuttings are placed in the annular space above the sand pack and the land surface to prevent water from flowing down the outside of the casing to the screened interval. If the well was drilled to the proper depth, ground water will flow in through the screen and rise to the level of the water table, which ideally should be within the screened interval.

A piezometer is used to measure the hydraulic head at a point within the ground-water system below the water table. A piezometer is constructed by drilling a hole to a desired depth. Then, on the land surface, an assembly is made of well casing and a screen, similar to a water-table well, but in this case a device called a grout basket is placed around the outside of the casing between the screen and the casing. This assembly is then lowered into the hole until the screen reaches the bottom. Finally, an impermeable seal, such as cement, is pumped into the hole so it fills the annular space between the drill-hole wall and the casing in the interval above the grout basket. The grout basket keeps the cement from entering the screened interval. By constructing piezometers in this way, the screened interval is isolated within the ground-water system, and the altitude of the water level in the piezometer is a measure of the hydraulic head at that point in the ground-water system where the screen is located. A number of piezometers completed at different depths at one location is called a piezometer nest. In nearly all settings, each piezometer in the nest will have a different water level. This information is used to determine the vertical gradients within the ground-water system at the site of the piezometer nest. For example, if a deeper piezometer has a lower water level than a shallower piezometer, there is a downward movement of ground water. Conversely, if a deeper piezometer has a higher water level than a shallower piezometer, there is an upward movement of ground water.

By using water level data from water-table wells and piezometer nests, it is possible to discuss the ground-water flow systems that interact with Mirror Lake. Perhaps the simplest way to visualize this ground-water movement is by using a cross-sectional view of a flow system. Such diagrams show the subsurface geology and the flow paths of ground water through part of the Mirror Lake watershed. The profile of the water table is determined from water-table wells and the flow paths within the ground-water system are determined from the altitude of water levels in piezometers. In the Mirror Lake area, the movement of ground water is downward in the higher part of the watershed, mostly lateral in the mid part of the hill slope, and has an upward component as it approaches Mirror Lake.

The ground-water flow system is far more complicated on the south side of the lake. Offshore on the south side, some of the ground water that was recharged on the north side discharges into the lake. However, nearer shore the lake loses water to the ground-water system. At this location flow is from the lake to ground water in the shallower piezometers, but there is upward flow from the bedrock into the deeper part of the glacial deposits. Following the flow path toward Hubbard Brook, the flow is upward out of the bedrock all along that part of the area. Upward components of flow, even near the land surface, are responsible for the presence of a fen wetland between Mirror Lake and Hubbard Brook. The source of water that maintains that wetland is water that seeps out of Mirror Lake, which then moves as ground water before discharging to the wetland.