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U.S. Geological Survey Water-Resources Investigations Report 00-4262
By Marcus C. Waldron and Gardner C. Bent |
This report presents the results of a study conducted by the U.S. Geological Survey, in cooperation with the city of Cambridge, Massachusetts, Water Department, to assess reservoir and tributary-stream quality in the Cambridge drinking-water source area, and to use the information gained to help guide the design of a comprehensive water-quality monitoring program for the source area. Assessments of the quality and trophic state of the three primary storage reservoirs, Hobbs Brook Reservoir, Stony Brook Reservoir, and Fresh Pond, were conducted (September 1997November 1998) to provide baseline information on the state of these resources and to determine the vulnerability of the reservoirs to increased loads of nutrients and other contaminants. The effects of land use, land cover, and other drainage-basin characteristics on sources, transport, and fate of fecal-indicator bacteria, highway deicing chemicals, nutrients, selected metals, and naturally occurring organic compounds in 11 subbasins that contribute water to the reservoirs also was investigated, and the data used to select sampling stations for incorporation into a water-quality monitoring network for the source area.
All three reservoirs exhibited thermal and chemical stratification,
despite artificial mixing by air hoses in Stony Brook Reservoir
and Fresh Pond. The stratification produced anoxic or hypoxic
conditions in the deepest parts of the reservoirs and these conditions
resulted in the release of ammonia nitrogen orthophosphate phosphorus,
and dissolved iron and manganese from the reservoir bed sediments.
Concentrations of sodium and chloride in the reservoirs usually
were higher than the amounts recommended by the U.S. Environmental
Protection agency for drinking-water sources (20 milligrams per
liter for sodium and 250 milligrams per liter for chloride). Maximum
measured sodium concentrations were highest in Hobbs Brook Reservoir
(113 milligrams per liter), intermediate in Stony Brook Reservoir
(62 milligrams per liter), and lowest in Fresh Pond (54 milligrams
per liter). Bed sediments in Hobbs Brook and Stony Brook Reservoirs
were enriched in iron, manganese, and arsenic relative to those
in the impounded lower Charles River in Boston, Massachusetts.
Trophic state indices, calculated for each reservoir based
on nutrient concentrations, water-column transparency, and phytoplankton
abundances, indicated that the upper and middle basins of Hobbs
Brook Reservoir were moderately to highly productive and likely
to produce algal blooms; the lower basin of Hobbs Brook Reservoir
and Stony Brook Reservoir were similar and intermediate in productivity,
and Fresh Pond was relatively unproductive and unlikely to produce
algal blooms. This pattern is likely due to sedimentation of organic
and inorganic particles in the three basins of Hobbs Brook Reservoir
and in Stony Brook Reservoir. Molar ratios of nitrogen to phosphorus
ranged from 55 in Stony Brook Reservoir to 120 in Hobbs Brook
Reservoir, indicating that phytoplankton algae in these water
bodies may be phosphorus limited and therefore sensitive to small
increases in phosphorus loading from the drainage basin. Nitrogen
loads were found to be less important than phosphorus to the trophic
condition of the reservoirs.
Hobbs Brook and Stony Brook, the two principle streams draining
the Cambridge drinking-water source area, differed in their relative
contributions to many of the estimated constituent loads. The
estimated load of fecal coliform bacteria was more than seven
times larger for the mainly residential Stony Brook subbasin upstream
from Kendal Green, Mass., than it was for the more commercial
and industrial Hobbs Brook subbasin, though the drainage areas
of the two subbasins differ only by about 20 percent. The State
standard for fecal coliform bacteria in streams in the Cambridge
drinking-water source area (20 colony forming units per 100 milliliters)
was exceeded at all sampling stations.
Estimated subbasin yields for sodium and chloride were significantly correlated with the percentage of the subbasin area occupied by roads, indicating that the application of sodium chloride in road salt is a significant source of the high concentrations of sodium measured in the reservoirs. The estimated annual mean loads of sodium and chloride produced by the Hobbs Brook subbasin were about three times greater than those produced by the Stony Brook subbasin.
The Hobbs Brook and Stony Brook subbasins produced similar estimated loads for nitrate nitrogen and total nitrogen. Subbasin yields of the two nitrogen species also were similar. In contrast, the estimated total phosphorus load at the mouth of Hobbs Brook was nearly twice that at the Stony Brook station.
The Hobbs Brook and Stony Brook subbasins produced similar estimated annual mean loads for iron. However, the estimated annual mean manganese load from the Hobbs Brook subbasin was about three times greater than that from the Stony Brook subbasin. Estimated annual mean yields for iron were greatest at stations representing the upper Hobbs Brook subbasins; those for manganese were greatest at the two stations downstream from Hobbs Brook Reservoir.
Both concentrations and yields of dissolved organic carbon were correlated with percent areal coverage of forested wetland in the subbasins. Neither concentrations nor yields of trihalomethane formation potential could be correlated with subbasin features such as land use, land cover, slope, or surficial geology. Concentrations of trihalomethane formation potential were similar to those reported in the literature for surface-water supplies in other parts of the country. Estimated annual mean yields of dissolved organic compound and trihalomethane formation potential were uniform, suggesting that no subbasin was exporting a disproportionate amount of either constituent on an annual basis.
The mass balance for water in Hobbs Brook Reservoir indicated that the time required for complete flushing of the reservoir during water year 1998 was less than 6 months. Sodium accumulated during the water year as the reservoir refilled following an unusually dry summer. The reservoir retained much of the nitrogen and phosphorus contributed by tributary streams. Waterfowl and precipitation were insignificant as sources of nitrogen to the reservoir but may have been important as sources of phosphorus.
Based on the results obtained from these investigations, ten stream locations were selected for inclusion as primary tributary-monitoring stations in a source-area water-quality monitoring network developed jointly by the U.S. Geological Survey and the Cambridge Water Department. Criteria for inclusion in the network were the magnitudes of actual or potential contaminant loads and the proximity of the monitoring stations to the reservoirs. In addition, nine monitoring stations representative of water-quality and trophic conditions in Hobbs Brook Reservoir, Stony Brook Reservoir, and Fresh Pond were identified and incorporated into the network. Details of the monitoring network are included in an appendix to this report.
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