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The daily and annual loads of nitrate plus nitrite and total nitrogen for the Connecticut River at Middle Haddam, Connecticut, were determined for water years 2009 to 2014. The analysis was done with a combination of methods, which included a predefined rating curve method for nitrate plus nitrite and total nitrogen for water years 2009 to 2011 and a custom rating curve method that included sensor measurements of nitrate plus nitrite nitrogen concentration and turbidity along with mean daily flow to determine total nitrogen loads for water years 2011 to 2014. Instantaneous concentrations of total nitrogen were estimated through the use of a regression model based on sensor measurements at 15-minute intervals of nitrate plus nitrite nitrogen and turbidity for water years 2011 to 2014. Annual total nitrogen loads at the Connecticut River at Middle Haddam ranged from 12,900 to 19,200 metric tons, of which about 42 to 49 percent was in the form of nitrate plus nitrite. The mean 95-percent prediction intervals on daily total nitrogen load estimates were smaller from the custom model, which used sensor data, than those calculated by the predefined model. Annual total nitrogen load estimates at the Connecticut River at Middle Haddam were compared with the upstream load estimates at the Connecticut River at Thompsonville, Conn. Annual gains in total nitrogen loads between the two stations ranged from 3,430 to 6,660 metric tons. These increases between the two stations were attributed to the effects of increased urbanization and to combined annual discharges of 1,540 to 2,090 metric tons of nitrogen from 24 wastewater treatment facilities in the drainage area between the two stations. The contribution of total nitrogen from wastewater discharge between the two stations had declined substantially before the beginning of this study and accounted for from 31 to 52 percent of the gain in nitrogen load between the Thompsonville and Middle Haddam sites. Introduction The Long Island Sound Study began in 1985 when Congress appropriated funds for the U.S. Environmental Protection Agency (EPA) to carry out a program to research, monitor, and assess the water quality of Long Island Sound as part of the National Estuary Program. This work has been undertaken in concert with the States of Connecticut and New York, forming a partnership consisting of Federal and State agencies, user groups, concerned organizations, and individuals dedicated to restoring and protecting Long Island Sound. In 1994, the Long Island Sound Study completed a comprehensive conservation and management plan that identified problems that merit special attention, including low levels of dissolved oxygen (hypoxia), toxic contamination, pathogen contamination, the effects of habitat degradation and loss on the health of living resources, land use and development resulting in the habitat loss, and overall degradation of water quality (Long Island Sound Study, 1994). Hypoxia was identified as the highest priority water-quality problem for Long Island Sound. The Long Island Sound Study defined hypoxia as concentrations of dissolved oxygen of 3 milligrams per liter (mg/L) or less in the water column (Long Island Sound Study, 1994). Hypoxia occurs during the summer months in the bottom waters of western Long Island Sound and is believed to be caused by overenrichment of nitrogen. A total maximum daily load (TMDL) was implemented in 2000 with the goal of reducing the point and nonpoint source nitrogen loads to Long Island Sound from New York and Connecticut by 58.5 percent by 2014 (New York State Department of Environmental Conservation and Connecticut Department of Environmental Protection, 2000). In 2010, the Long Island Sound Study formed a five-State committee (including representatives from the States of Connecticut, New York, Massachusetts, New Hampshire, and Vermont) to evaluate progress towards meeting the TMDL goals and to determine revised nitrogen loading targets. In addition, the comprehensive conservation and management plan was updated 2 Nitrogen Concentrations and Loads for the Connecticut River at Middle Haddam, Connecticut, Water Years 2009 to 2014 and revised in 2014 (Long Island Sound Study, 2014), which involved reviewing program progress, soliciting stakeholder and public input, and developing a framework for updating this document. These efforts required information on tributary nitrogen loads to Long Island Sound in order to track progress toward management goals. In 2016, the EPA created a new nitrogen strategy, organized by three customized watershed groups: embayments, large riverine systems, and priority western Long Island Sound point-source discharges to open water. For each group, there is the need to develop nitrogen thresholds that will translate the narrative water-quality standard into a numeric target, identify where watershed nitrogen loading results in threshold exceedances, and assess options for the load reductions from point and nonpoint sources that would be needed to remain below thresholds. An additional recommendation was to continue to pursue opportunities to monitor, model, and research the link between nitrogen load and bottom-water dissolved oxygen conditions in the open water of Long Island Sound (U.S. Environmental Protection Agency, 2015). To address these issues, the U.S. Geological Survey (USGS), in cooperation with the Connecticut Department of Energy and Environmental Protection, has been quantifying nitrogen loads to Long Island Sound through analysis of streamflow and water-quality data collected in the Connecticut part of the watershed (Mullaney, 2016a,b). Historically, however, nitrogen loads from the Connecticut River have been calculated at Thompsonville, Conn., near the Massachusetts border, because this is the first location upstream from Long Island Sound where the river flows are not tidally affected. In addition, there are major nitrogen sources in the lower reaches of the Connecticut River below the Thompsonville monitoring station, including an additional 3,204 square kilometers (km2) of drainage area (fig. 1) that includes 24 wastewater treatment facilities. As a result of management actions and wastewater facility upgrades, nitrogen loads from wastewater treatment plants in the lower Connecticut River Basin declined from about 2,600 metric tons (t) to about 1,540 t from 2002 to 2014 (fig. 2). Because the Connecticut River is the largest tributary of Long Island Sound, detailed characterization of nitrogen load from the lower Connecticut River is important for tracking of the overall nitrogen budget and understanding of hypoxia in Long Island Sound. This report describes streamflow and the concentrations and loads of nitrate plus nitrite nitrogen and of total nitrogen at the Connecticut River at Middle Haddam and for comparison, those values for the upstream streamgage on the Connecticut River at Thompsonville from October 2008 to September 2014; the methods used to collect and process the waterquality and streamflow data at the Middle Haddam streamgage; and the uncertainties in the estimates of concentrations and loads. The study area encompasses an area of 28,223 km2, which covers about 97 percent of the Connecticut River Basin, and includes those parts of the basin where the river drains in Vermont, New Hampshire, Massachusetts, and Connecticut, representing about 96.5 percent of the drainage area of the Connecticut River. The streamgaging station at Middle Haddam (USGS station 01193050) is approximately 39 kilometers (km) upstream from Long Island Sound and is characterized by freshwater tidal conditions. The major land uses and land covers of the upstream basin, based on a land use and land cover analysis for 2011 (Homer and others, 2015), are forested (73 percent), developed (9.4 percent), cultivated crops (6.4 percent), and wetlands (5.3 percent). The drainage basin above the Middle Haddam streamgage contains many small cities and towns, and the larger cities of Springfield, Mass., and Hartford, Conn. The headwaters of the Connecticut River begin in the mountainous regions of northern Vermont and northern New Hampshire, with elevations as high as 1,914 meters in the White Mountains of New Hampshire. The western side of the Connecticut River forms the border between Vermont and New Hampshire. As the river enters Massachusetts, it enters the Connecticut Valley Lowland, a broad lowland underlain by Mesozoic sedimentary bedrock, which it follows until Middletown, Conn., where the river enters the eastern uplands of Connecticut, just upstream from the streamgage at Middle Haddam. Wastewater discharged to streams and rivers in the study area from 109 municipal facilities totals about 280 million gallons per day (Mgal/d; 1.06 million cubic meters per day [Mm3/d]; Mullaney and others, 2002). The average annual air temperature ranges from less than 4 degrees Celsius (°C) in the northern mountainous areas to about 10 °C at Long Island Sound. Precipitation ranges from 86 centimeters per year (cm/yr) in the Connecticut Valley Lowland to more than 165 cm/yr in the northern mountainous areas (Bjerklie and others, 2011). The Connecticut River is tidally influenced from the mouth of the river at Long Island Sound upstream to Windsor Locks, Conn. The streamgaging station at Thompsonville, Conn., (USGS station 01184000; drainage area 24,864 km2) has typically been the farthest downstream water-quality monitoring site for which nitrogen loads have been estimated because of difficulties in the past with continuously monitoring the flow in the tidal reach of the river. The flow at the Middle Haddam site is tidally affected at all but the highest discharges. The water itself, however, is fresh at this site, and the salt wedge from the waters of Long Island Sound has never been observed upstream from Haddam, Conn., about 14 km downstream from the Middle Haddam streamgage (Meade, 1966).

scientific investigations report

Nitrogen concentrations and loads for the Connecticut River at Middle Haddam, Connecticut, computed with the use of autosampling and continuous measurements of water quality for water years 2009 to 2014