ABSTRACT

 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. 

 

## Contents

 

Abstract

Introduction

Geologic Setting and Bathymetry

Hydrology

Data-Collection Methods

Water-Table Configuration and Extent of the Ground-Water Contributing Area

Ground-Water and Lake-Stage Fluctuations

Water Balance

Precipitation and Evaporation 

Ground-Water Inflow

Contributing-Area Approach

Isotope Mass-Balance Approach

Average Ground-Water Inflow

Water-Balance Results

Limnology

External Inputs

Ground-Water Nutrient Inputs

Sampling and Analytical Methods for Ground Water

Geochemical Conditions in the Aquifer 

Ground-Water Point Sources

Ground-Water Background Source

Atmospheric Deposition Source

Swimmer Source

Other Nutrient Sources

Nutrient Limitation, Ratios, Source Size, Timing, and Disposal   Strategies

Nutrient Loading Trophic Index

Plant Growth and Internal Cycling of Chemical Constituents 

Methods of Sampling and Analysis of the Water Column 

Temperature Stratification

Phytoplankton, *Nitella*, and Light

Dissolved Oxygen

Dissolved Oxygen in the Epilimnion 

Dissolved Oxygen in the Metalimnion

Dissolved Oxygen in the Hypolimnion 

Conductance, pH, Phosphorus, Nitrogen, Iron, Manganese, and   Internal Nutrient Recycling

Historical, and Between-Lake Trophic Assessments 

Trophic State from Water-Column Assessment 

Historical Dissolved-Oxygen Profiles

Comparisons of Dissolved Oxygen Among Lakes

Trophic Stability

Trophic Ecology and Management Options

Summary and Conclusions

References Cited

Appendix A. Delta deuterium and delta oxygen-18 of lake water,   ground-water inflow, and precipitation,Walden Pond

 

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