In its natural state, the east watershed of Mirror Lake was quite large, about 20 hectares in area, which was nearly as large as the west watershed (about 24 hectares) and 2/3 the size of the northwest watershed (about 30 hectares). As a result, in its natural state, runoff from the east watershed must have been a major contributor of water to the lake. However, as part of the construction of Interstate Highway I-93, a berm was constructed between the highway and Mirror Lake for the purpose of preventing highway runoff from entering the lake. The berm also diverted runoff from most of the east watershed of the lake uphill from the highway, reducing the size of the east watershed that contributes water to the lake from 20 hectares to 2.6 hectares. As a result of the diversion, a very small amount of surface water now enters Mirror Lake from its east watershed, draining only the part between the berm and the lake.

Highway I-93 opened in 1971. Chemical analyses of lake water, which had been sampled since 1968, indicated that concentrations of sodium and chloride began to increase several years after the highway opened. The increase in concentrations of these two chemical constituents has continued at a fairly steady rate since that time. Although it was fairly easy to implicate road salt used on highway I-93 as the source of the increased sodium and chloride in the lake, the transport path was not known. If the berm diverted all surface runoff from the vicinity of the highway, the transport of salt must be through the subsurface. To find the route the dissolved salt was taking from the highway to the lake, a special study was made. Wells to sample ground water for its chemical content, and to determine if ground water was entering the east stream, were drilled at several places on both sides of the stream in the reach between the berm and the lake. The study revealed that the stream contained dissolved road salt but the shallow ground water on both side of it did not. The conclusion was that, although the berm was constructed of fairly water-tight material, the dissolved road salt was seeping beneath the berm directly into the east stream, which, in turn, was the source of contamination to Mirror Lake.