Research about Concord, Massachusetts

USGS2001

Geohydrology and limnology of Walden Pond, Concord, Massachusetts

The trophic ecology and ground-water contributing area of Walden Pond, in Concord and Lincoln, Mass., were investigated by the U.S. Geological Survey in cooperation with the Massachusetts Department of Environmental Management from April 1997 to July 2000. Bathymetric investigation indicated that Walden Pond (24.88 hectares), a glacial kettle-hole lake with no surface inlet or outlet, has three deep areas. The maximum depth (30.5 meters) essentially was unchanged from measurements made by Henry David Thoreau in 1846. The groundwater contributing area (621,000 square meters) to Walden Pond was determined from water-table contours in areas of stratified glacial deposits and from land-surface contours in areas of bedrock highs. Walden Pond is a flow-through lake: Walden Pond gains water from the aquifer along its eastern perimeter and loses water to the aquifer along its western perimeter. Walden Pond contributing area also includes Goose Pond and its contributing area. A water budget calculated for Walden Pond, expressed as depth of water over the lake surface, indicated that 45 percent of the inflow to the lake was from precipitation (1.215 meters per year) and 55 percent from ground water (1.47 meters per year). The groundwater inflow estimate was based on the average of two different approaches including an isotope mass-balance approach. Evaporation accounted for 26 percent of the outflow from the lake (0.71 meters per year) whereas lake-water seepage to the groundwater system contributed 74 percent of the outflow (1.97 meters per year). The water-residence time of Walden Pond is approximately 5 years. Potential point sources of nutrients to ground water, the Concord municipal landfill and a trailer park, were determined to be outside the Walden Pond groundwater contributing area. A third source, the septic leach field for the Walden Pond State Reservation facilities, was within the groundwater contributing area. Nutrient budgets for the lake indicated that nitrogen inputs (858 kilograms per year) were dominated (30 percent) by plume water from the septic leach field and, possibly, by swimmers (34 percent). Phosphorus inputs (32 kilograms per year) were dominated by atmospheric dry deposition, background ground water, and estimated swimmer inputs. Swimmer inputs may represent more than 50 percent of the phosphorus load during the summer. The septic-system plume did not contribute phosphorus, but increased the nitrogen to phosphorus ratio for inputs from 41 to 59, on an atom-to-atom basis. The ratio of nitrogen to phosphorus in input loads and within the lake indicated algal growth would be strongly phosphorus limited. Nitrogen supply in excess of plant requirements may mitigate against nitrogen fixing organisms including undesirable blooms of cyanobacteria. Based on areal nutrient loading, Walden Pond is a mesotrophic lake. Hypolimnetic oxygen demand of Walden Pond has increased since a profile was measured in 1939. Currently (1999), the entire hypolimnion of Walden Pond becomes devoid of dissolved oxygen before fall turnover in late November; whereas historical data indicated dissolved oxygen likely remained in the hypolimnion during 1939. The complete depletion of dissolved oxygen likely causes release of phosphorus from the sediments. Walden Pond contains a large population of the deep-growing benthic macro alga Nitella , which has been hypothesized to promote water clarity in other clear-water lakes by sequestering nutrients and keeping large areas of the sediment surface oxygenated. Loss of Nitella populations in other lakes has correlated with a decline in water quality. Although the Nitella standing crop is large in Walden Pond, Nitella still appears to be controlled by nutrient availability. Decreasing phosphorus inputs to Walden Pond, by amounts under anthropogenic control would likely contribute to the stability of the Nitella population in the metalimnion, may reverse oxygen depletion in the hypolimnion, and decrease recycling of phosphorus from the sediments.

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USGS2001

Hydrology and trophic ecology of Walden Pond, Concord, Massachusetts

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).

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