Surface-water, water-quality, and meteorological data for the Cambridge, Massachusetts, drinking-water source area, water years 2007-08

Abstract

Records of water quantity, water quality, and meteorological parameters were continuously collected from three reservoirs, two primary streams, and five subbasin tributaries in the Cambridge, Massachusetts, drinking-water source area during water years 2007-08 (October 2006 through September 2008). Water samples were collected during base-flow conditions and storms in the Cambridge Reservoir and Stony Brook Reservoir drainage areas and analyzed for dissolved calcium, sodium, chloride, and sulfate; total nitrogen and phosphorus; and polar pesticides and metabolites. Composite samples of stormwater also were analyzed for concentrations of total petroleum hydrocarbons and suspended sediment in one subbasin in the Stony Brook Reservoir drainage basin. These data were collected to assist watershed administrators in managing the drinking-water source area and to identify potential sources of contaminants and trends in contaminant loading to the water supply. Monthly reservoir contents for the Cambridge Reservoir ranged from about 30 to 95 percent of capacity during water years 2007-08. Monthly reservoir contents for the Stony Brook Reservoir ranged from about 47 to 91 percent of capacity during water years 2007-08, while the monthly reservoir storage values for Fresh Pond Reservoir were maintained at greater than 92 percent of capacity. If the average water demand by the city of Cambridge is assumed to be 15 million gallons per day, the volume of water released from the Stony Brook Reservoir to the Charles River during water years 2007-08 represents an annual surplus of about 107 and 94 percent, respectively. The annual precipitation total of about 47 in (inches) recorded at the Cambridge reservoir during water year 2007 was about 5 to 21 percent lower than recorded totals for the previous four water years, whereas the annual precipitation total of about 62 in. during water year 2008 was about 5 to 32 percent higher than recorded totals for water years 2002-07. In general, most monthly mean specific-conductance values for water year 2007 for U.S. Geological Survey (USGS) stations on the two primary streams and four subbasin tributaries in the Cambridge, Massachusetts, drinking-water source area were below the previous median monthly values and often were below the previous minimum monthly values for available data since water year 1997. The annual mean specific-conductance value for Fresh Pond Reservoir during water year 2007 was 483 (u or mu)S/cm (microsiemens per centimeter), which was lower than the prior three water years. The monthly mean specific-conductance values for streamflow for Hobbs Brook below the Cambridge Reservoir for December through July 2008 were greater than the 75th percentile for historical data since water year 1997. These relatively high values were caused by the inflow of high specific conductance water from the tributaries when the reservoir water level was low at the onset of winter. Increased rainfall in the watershed beginning in February 2008 caused monthly mean specific-conductance values for Hobbs Brook to decrease to about 700 (u or mu)S/cm by the end of the water year. Monthly mean specific-conductance values for many of the other USGS stations were higher than historical values for several months during the winter of water year 2008. The large amount of rainfall in the watershed also caused the monthly mean specific conductance at these stations to decline to near-median values or to values within the interquartile range for available historical data. The annual mean specific conductance for Fresh Pond Reservoir during water year 2008 was 497 (u or mu)S/cm, slightly greater than the corresponding value for the prior year. Water samples were collected in nearly all of the subbasins in the Cambridge drinking-water source area and from Fresh Pond during the study period. Discrete water samples were collected during base-flow conditions with an antecedent dry period of at least 3 days. Composite sampl