Hydrology and Trophic Ecology of Walden Pond, Concord, Massachusetts
Water-Resources Investigations Report 01-4153
By Paul J. Friesz and John A. Colman
Walden Pond and the underlying and surrounding aquifer are bordered by the Sudbury and Concord Rivers (fig. 1). Walden Pond, a kettle-hole lake with no surface water inlet or outlet, formed at the end of the last glaciation about15,000 years ago by the melting of a large block of ice that broke off into glacial Lake Sudbury from the retreating glacier. The ice block eventually was surrounded by sorted, stratified sediments ranging mainly from fine sand to coarse gravel deposited by glacial meltwater (fig. 2), (Koteff, 1963). Seismic reflection and fathometer data indicate that the lake and its associated fine-grained bottom sediments extend to the till and bedrock surface in the deepest areas. Three deep areas are defined in the lake (fig. 3); the middle deep area was unmapped before this investigation. The maximum measured depth of 100.1 feet (ft) was within 2 ft of that measured by Thoreau in the winter of 1846.
Knowledge of the water balance and the land-surface area contributing water to Walden Pond is needed to understand the hydrology of the lake and to derive a nutrient budget for the lake. Sources of water to Walden Pond include precipitation on the lake surface and ground water (fig. 2). Water from precipitation infiltrates the permeable surficial deposits, recharges the aquifer, then flows in the direction of decreasing water levels. Thus, ground-water flow does not necessarily follow land surface topography. Along steep shoreline areas sloping towards Walden Pond, small quantities of overland flow may occur after intense precipitation events, thereby adding small amounts of water to the lake. Water leaves Walden Pond by evaporation from the lake surface and from lake-water seepage to the aquifer (fig. 2).
Walden Pond, the deepest lake in Massachusetts, has great historical, naturalistic, and limnological significance as the subject of Henry David Thoreau's well-known essay "Walden: or, Life in the Woods" (Thoreau, 1854). Only 15 miles (mi) northwest of Boston, Mass. (fig. 1), Walden Pond potentially is threatened by environmental stresses common to urban lakes: a municipal landfill, septic leachate, high visitor-use rates, acid and other contaminants from atmospheric deposition, and invasion of exotic species. Walden Pond retains clear, undegraded water because of conservation efforts that protect the shore and woods surrounding the lake. Questions remain, however, regarding the extent of ecological changes that may already have occurred and the degree to which conservation efforts will preserve water quality in the future. Hydrologic and limnologic results from a cooperative investigation of Walden Pond between the U.S. Geological Survey and the Massachusetts Department of Environmental Management are summarized in this plate report, which shows Walden Pond in relation to the ground-water system and land use. Details of the investigation from data collected from 1997 to 1999 (Colman and Friesz, 2001) and basic information about limnology and Walden Pond (Colman and Waldron, 1998) also are available.
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