AUTOMATED PROCESS FOR MONITORING GROUND - WATER QUALITY USING ESTABLISHED MANUAL SAMPLING PROTOCOLS: APPLICATIONS

Automated Monitoring for the USGS Cultural Eutrophication project at Walden Pond, Concord Massachusetts

by, John Colman, Marcus Waldron, and Gregory Granato, Hydrologists

In June 1997, the U.S. Geological Survey (USGS) began a cooperative investigation with the Massachusetts Department of Environmental Management (MA-DEM) to (1) assess the impact of nutrient loading on Walden Pond using methods that will permit monitoring of changes in response to nutrient-source remediation and (2) refine existing estimates of nutrient loading due to seasonal recreational use. The study is designed to establish the status of ecological features of the pond: to determine whether it is in a stable or changing tropic state, to determine the dynamics of nutrient cycles in the pond, and to quantify inputs from nutrient sources. A common problem for ponds in developed areas is excessive loading of nutrients (especially phosphorus and nitrogen), which stimulates algal growth.

Excessive algal growth decreases water clarity, and periodic algal die-off at the surface and sinking of dead algal material leads to loss of fish habitat because of oxygen consumed by decaying algae in the deep water. Increased nutrient loading to ponds, called eutrophication, occurs naturally as lakes fill in with nutrient-rich sediment derived from their banks and upstream soils. The term "cultural eutrophication" refers to an accelerated form associated with people's use of fertilizer, rerouting of surface drainage,and disposal of domestic waste.

Nutrient Sources. Refinements to the estimates made for sources of nutrients from atmospheric deposition, swimmers, and groundwater would be especially useful in managing Walden Pond. Swimmers represent a potentially large but controllable source, and groundwater might be affected by point sources of nutrients from the nearby Concord landfill, the trailer park, and the septage leach field for the pond bath house. Swimmer input will be determined by measuring the increase of urine constituents in the epilimnion during the swimming season when the pond is stratified. Atmospheric input will be determined from samples obtained from a wetfall-dryfall collector. Nutrient contamination in groundwater will be determined by defining exactly the groundwater contributing area for the pond and by sampling water from monitoring wells in background locations and locations that intercept any plumes originating from known point sources.

The automated groundwater monitoring system was installed in a well downgradient from the septage leach field for the pond bath house. It is hoped that the real-time data produced by the demonstration unit will be helpful in determining the short-term variability, seasonality, and (potentially) the long term variability of nutrient sources to groundwater in this area.