A CONSTRUCTED POND AND ITS INTERACTION WITH GROUNDWATER

Paramecium Pond is a small (1950 m²) artificial pond on the campus of Wellesley College perched above the local water table. The depression was originally wetlands, and the pond was created by dredging in the 1920s, and again in 1971 and 1995 to remove organic peat and fine sediment. Over one meter of low permeability organic sediment covers much of the bottom of the pond and apparently is a barrier to flow. To keep it filled for aquatic plants and wildlife, the pond is fed by a completely artificial stream constructed at the same time and lined with stone and concrete. Water level is constrained by a drain and pipe connection to Lake Waban to the south. With time, the stream evolved by sedimentation to look normal to the casual observer. The water supply is currently from treated potable well water. Study of the pond system is used as an introductory geoscience lab research project. Plans are underway for conversion to raw well water, and we will be monitoring changes to the system as it occurs.

To the east and southeast at distances of 90 meters, two production wells pump approximately 830 cubic meters per day of water from the aquifer for campus sole source potable water. To the west and northwest, two observation wells extend through 2m of fill and 13m of glacial lake sand and gravel aquifer deposits into a thin basal till and end at bedrock, and provide an opportunity to study the interaction of the perched pond (currently about 1.16 meters higher hydraulic head than the water in the observation wells just 37 and 63 meters away in a high transmissivity aquifer). The water in the system has three chemically distinct sources: potable water with 20-30 mg/L potassium (added as KOH to raise the pH) and 120 mg/L sodium, normal groundwater with 120 mg/L sodium and negligible potassium, and precipitation with negligible amounts of these alkalis. Observation of elevated potassium levels in both observation wells suggests that there is some connection with the pond.