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Trends in long-term water-quality and streamflow data from 14 water-quality monitoring sites in Connecticut were evaluated for water years 1974–2013 and 2001–13, coinciding with implementation of the Clean Water Act of 1972 and the Connecticut Nitrogen Credit Exchange program, as part of an assessment of nutrient and chloride concentrations and loads discharged to Long Island Sound. In this study, conducted by the U.S. Geological Survey in cooperation with the Connecticut Department of Energy and Environmental Protection, data were evaluated using a recently developed methodology of weighted regressions with time, streamflow, and season. Trends in streamflow were evaluated using a locally weighted scatterplot smoothing method. Annual mean streamflow increased at 12 of the 14 sites averaging 8 percent during the entire study period, primarily in the summer months, and increased by an average of 9 percent in water years 2001–13, primarily during summer and fall months. Downward trends in flow-normalized nutrient concentrations and loads were observed during both periods for most sites for total nitrogen, total Kjeldahl nitrogen, nitrite plus nitrate nitrogen, total phosphorus, and total organic carbon. Average flow-normalized loads of total nitrogen decreased by 23.9 percent for the entire period and 10.9 percent for the period of water years 2001‒13. Major factors contributing to decreases in flownormalized loads and concentrations of these nutrients include improvements in wastewater treatment practices, declining atmospheric wet deposition of nitrogen, and changes in land management and land use. Loads of dissolved silica (DSi; flow-normalized and nonflow-normalized) increased slightly at most stations during the study period and were positively correlated to urbanized land in the basin and negatively correlated to area of open water. Concentrations and loads of chloride increased at 12 of the 14 sites during both periods. Increases likely are the result of an increase in the use of salt for deicing, as well as other factors related to urbanization and population growth, such as increases in wastewater discharge and discharge from septic systems. Introduction The streamflows from rivers in Connecticut that drain to Long Island Sound (LIS) are a major source of nitrogen and other nutrients that fuel seasonal hypoxia in LIS (New York State Department of Environmental Conservation and Connecticut Department of Environmental Protection, 2000). For this study, the Connecticut Department of Energy and Environmental Protection (CTDEEP) defines hypoxia as dissolved oxygen of less than 3.0 milligrams per liter (mg/L) in the water column of LIS. The area of hypoxia during the summer in LIS has averaged 464 square kilometers (km2) since monitoring began in 1987, and the underlying recent trend in the areal extent of hypoxia is downward (Connecticut Department of Energy and Environmental Protection, 2013a). A number of changes related to land use and nutrient loading to LIS from these tributaries have occurred since the passage of the Federal Clean Water Act in 1972 (Public Law 92–500, 86 Stat. 816; 33 U.S.C. 1251 et seq.). Changes include increased development and population, as well as improvements to wastewater treatment, and decreased atmospheric wet deposition of nitrogen driven by decreases in oxidized nitrogen (Varekamp and others, 2014; Pinder and others, 2011). The Long Island Sound Basin (fig. 1, inset) covers an area of 43,564 km2. The population within the LIS Basin increased from 8.03 million in 1970 to 8.9 million in 2010 (Long Island Sound Study, 2010); population increases occurred primarily in the lower or southern part of the basin. The amount of forested and agricultural land decreased in the lower LIS Basin, whereas areas of developed land and turf and grass increased. (New England Interstate Water Pollution Control Commission, 2014). Although the amount of farmland generally decreased, the number of farms increased. However, the use of commercial fertilizers on agricultural land decreased by 38 percent in Connecticut, 25 percent in Vermont, and 32 percent in New Hampshire during 1987–2007. In addition, the number of farm animals has decreased (New England Interstate Water Pollution Control Commission, 2014). From 1973 to about 1983, nearly all wastewater treatment facilities (WWTFs) in Connecticut were upgraded to secondary 2 Concentrations and Loads in Long Island Sound Tributaries Following the Passage of the Federal Clean Water Act treatment (Connecticut Department of Energy and Environmental Protection, undated). In 1985, the Long Island Sound Study (LISS) was created to carry out a program to research, monitor, and assess the water quality of LIS in concert with the States of Connecticut and New York, forming a bistate partnership consisting of Federal and State agencies, user groups, nongovernmental organizations, and individuals dedicated to restoring and protecting LIS. In 2001, Connecticut and New York implemented a total maximum daily load (TMDL) to reduce the nitrogen load to LIS by 58.5 percent by 2014 (New York Department of Environmental Conservation and Connecticut Department of Environmental Protection, 2000), including a 64-percent reduction in nitrogen from WWTFs (Connecticut Department of Energy and Environmental Protection, 2013a). The LIS TMDL targeted reductions in nitrogen from point sources, such as WWTFs, and nonpoint sources, such as stormwater. The established wasteload allocation for nitrogen from WWTFs formed the basis for Connecticut’s Nitrogen Credit Exchange (NCE), which was initiated in 2002 (Connecticut Department of Energy and Environmental Protection, 2013a). As a result of the wasteload allocation and consequent upgrades at WWTFs, the goals for nitrogen reductions from WWTFs in Connecticut have been nearly achieved as of 2014. Planning and construction of upgrades at 12 additional WWTFs through 2018 are underway (Connecticut Department of Energy and Environmental Protection, 2014b). As part of the reduction described above, the load allocation is based on achieving a 10-percent reduction in the total nonpoint source of nitrogen from urban and agricultural land. In Connecticut, a number of regulatory and nonregulatory programs have been established to directly or indirectly reduce nonpoint sources of nitrogen. These projects include stormwater permits, agricultural partnerships, combined-sewer overflow control, onsite wastewater permits, sanitary sewer extensions, and improved standards for development and redevelopment. The efficacy of these programs is difficult to measure; however, declines in stormwater nitrogen concentrations, which are recorded in the Connecticut Department of Energy and Environmental Protection (CTDEEP) industrial stormwater database, indicate improvements in stormwater quality during 1995–2011 (Connecticut Department of Energy and Environmental Protection, 2013c). In this study, the method Weighted Regressions on Time, Discharge, and Season (WRTDS), developed by Hirsch and others (2010), was used to analyze trends in concentration and flux (load) of nutrients, organic carbon, and chloride (Cl) at selected tributaries in the LIS Basin using long-term streamflow and water-quality records for 14 sites (fig. 1; table 1). The main focus of the research, which was conducted by the U.S. Geological Survey (USGS) in cooperation with the Connecticut Department of Energy and Environmental Protection, was to understand the changes to nitrogen load as a result of management action or land-use change since 1973, independent of random climate variability. The WRTDS method also was applied to other constituents of concern for either the freshwater tributaries or LIS, including total phosphorus (TP), total organic carbon (TOC), dissolved silica (DSi) and chloride (Cl). Phosphorus is not considered to be a limiting nutrient in LIS but is implicated in the impairment of at least 21 fresh waterbody segments in Connecticut, and Connecticut is mandated to establish limitations on phosphorus in all wastewater discharge permits where the discharge may contribute to impairments through eutrophication (Connecticut Department of Energy and Environmental Protection, 2014a). TOC and DSi were included in the analysis, as they are considered important constituents in LIS. Modeling associated with the TMDL development for LIS indicated that there are short time periods when silica limits algal growth. The oxidation of organic carbon is considered to be responsible for as much as 25 percent of the oxygen consumption in LIS; therefore, it was also considered in this study (New York State Department of Environmental Conservation and Connecticut Department of Environmental Protection, 2000). Chloride concentrations were analyzed using the WRTDS method in order to identify changes in concentrations and loads, an indicator of the effects of increasing urbanization with time. The period of this study coincides with the passage of the Federal Clean Water Act of 1972 (Public Law 92–500, 86 Stat. 816; 33 U.S.C. 1251 et seq.), which established the basic structure for regulating discharges of pollutants into the waters of the United States and regulating quality standards for surface waters. Trend analyses in this report are provided for water years1 1974–2013 and 2001–13, the latter period following the beginning of the NCE in Connecticut. General trends in streamflow were evaluated for this investigation because annual streamflow is the major factor in the magnitude of constituent load. Water-quality data collected by the U.S. Geological Survey in Connecticut has been previously analyzed for trends in concentrations and loads of nitrogen and phosphorus (Trench and others, 2012; Sprague and others, 2009; and Trench and Vecchia, 2002). Nitrogen loads were previously estimated for these stations for varying periods of record by Mullaney and Schwarz (2013), Varekamp and others (2014)

scientific investigations report

Nutrient, organic carbon, and chloride concentrations and loads in selected Long Island Sound tributaries—Four decades of change following the passage of the Federal Clean Water Act