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Nutrients, suspended sediment, and trace element loads in the Blackstone River and selected tributaries were estimated from composite water-quality samples in order to better understand the distribution and sources of these constituents in the river basin. The flow-proportional composite waterquality samples were collected during sequential 2-week periods at six stations along the river’s main stem, at three stations on tributaries, and at four wastewater treatment plants in the Massachusetts segment of the basin from June 2007 to September 2009. Samples were collected at an additional station on the Blackstone River near the mouth in Pawtucket, Rhode Island, from September 2008 to September 2009. The flow-proportional composite samples were used to estimate average daily loads during the sampling periods; annual loads for water years 2008 and 2009 also were estimated for the monitoring station on the Blackstone River near the Massachusetts-Rhode Island border. The effects of hydrologic conditions and net attenuation of nitrogen were investigated for loads in the Massachusetts segment of the basin. Sediment resuspension and contaminant loading dynamics were evaluated in two Blackstone River impoundments, the former Rockdale Pond (a breached impoundment) and Rice City Pond. Total nitrogen and phosphorus loads along the Blackstone River in Massachusetts showed similar general patterns during the sampling periods monitored in this study. Total nitrogen loads were relatively low at the farthest upstream monitoring station in Millbury, Massachusetts (typically less than 430 kilograms per day (kg/d) for total nitrogen and 37 kg/d for total phosphorus). Loads typically increased (5to 10-fold for nitrogen and 6to 15-fold for phosphorus) downstream from the first, large wastewater treatment plant along the river, the Upper Blackstone Water Pollution Control Abatement District in Millbury. Further downstream, total nitrogen and phosphorus loads remained elevated but variable (typically about 1,000 to 3,000 kg/d for nitrogen and about 100 to 370 kg/d for phosphorus) from Millbury to the Massachusetts-Rhode Island border near Millville, Mass. Monitored tributaries of the Blackstone River and wastewater treatment plants other than the Upper Blackstone Water Pollution Control Abatement District rarely contributed more than a small fraction of the total nitrogen and phosphorus loads observed at the main stem monitoring stations. Loads of suspended sediment also were substantially larger along the river’s main stem than in tributaries during most sampling periods. Very large loads of suspended sediment from the West River tributary during several sampling periods may have been associated with flood-control operations. The estimated annual load of total nitrogen in the Blackstone River at Millville, about 1.3 miles upstream from the Massachusetts-Rhode Island border, was 936,000 kilograms (kg) (2,600 kg/d) in water year 2008 and 878,000 kg (2,400 kg/d) in water year 2009. The estimated annual load of total phosphorus at Millville was 81,400 kg in water year 2008 (223 kg/d) and 80,900 kg (222 kg/d) in water year 2009. The estimated annual load of suspended sediment in was 4,940,000 kg (13,600 kg/d) in water year 2008 and 7,040,000 kg (19,300 kg/d) in water year 2009. The higher load in water year 2009 likely reflects several large storms in summer 2009, which resulted in streamflows in the Blackstone River that were 10 times the typical July flows. Loads of total nitrogen, total phosphorus, and trace elements were almost always lower in the Blackstone River at Millville than in the river near its mouth at the Pawtucket monitoring station, when loads were monitored at both stations in the latter part of water year 2008 and in water year 2009. Loads of suspended sediment at Millville and Pawtucket varied by about the same range, but were usually lower at Pawtucket than at Millville. Total nitrogen loads were higher during sampling periods when the base-flow contribution to streamflow was substantially less than the runoff contribution than in sampling periods when the base-flow dominated. During these sampling periods when the runoff component of streamflow was relatively large, loads of total nitrogen in wastewater discharge from Upper Blackstone Water Pollution Control Abatement District also were high but also constituted smaller fractions of the total nitrogen loads in the river. Nitrogen attenuation may have occurred during some sampling periods, based on net changes in total nitrogen load between consecutive monitoring stations, especially in the Blackstone River reach between the South Grafton and Uxbridge monitoring stations. 2 Nutrient, Suspended Sediment, and Trace Element Loads in the Blackstone River Basin in Mass. and R.I., 2007 to 2009 Analysis of the representative constituents (total phosphorus, total chromium, and suspended sediment) upstream and downstream of impoundments indicated that the existing impoundments, such as Rice City Pond, can be sources of particulate contaminant loads in the Blackstone River. Loads of particulate phosphorus, particulate chromium, and suspended sediment were consistently higher downstream from Rice City Pond than upstream during high-flow events, and there was a positive, linear relation between streamflow and changes in these constituents from upstream to downstream of the impoundment. Thus, particulate contaminants were mobilized from Rice City Pond during high-flow events and transported downstream. In contrast, downstream loads of particulate phosphorus, particulate chromium, and suspended sediment were generally lower than or equal to upstream loads for the former Rockdale Pond impoundment. Sediments associated with the former impoundment at Rockdale Pond, breached in the late 1960s, did not appear to be mobilized during the highflow events monitored during this study. Introduction The Blackstone River Basin in central Massachusetts and Rhode Island comprises an area of 475 square miles (mi2) and includes one of New England’s largest cities, Worcester, Massachusetts (fig. 1). From its origin near Worcester, the river flows 48 miles (mi) through Woonsocket and Pawtucket, Rhode Island, before discharging into Narragansett Bay. The river basin has a long history of contamination, dating back to the industrial revolution. Improved infrastructure and wastewater treatment have resulted in improved water quality, but the river’s water quality is still categorized as impaired along its entire length (Massachusetts Department of Environmental Protection, 2010; Massachusetts Executive Office of Energy and Environmental Affairs, 2015). English settlers colonized the Blackstone River Valley in the 17th century (Kerr, 1990). The landscape and hydrology of the river basin were conducive hydropower development and, as a consequence, to industrialization; cotton mills came first, followed by factories that processed cotton cloth and that built supporting machinery or that just needed readily accessible hydropower. In addition to providing the power that mills and other factories required, the river also served as an open sewer for the wastes that were created by industry and the burgeoning population. The disposal of cotton dyes, industrial wastes, and sewage in the river left behind contaminated sediments containing toxic organic chemicals and metals. Improvements to infrastructure and to wastewater treatment, especially after the passage of the Clean Water Act (33 U.S.C. §1251 et seq.) in 1972, have reduced pointand nonpoint-source contaminant loadings that are eventually transported by the river to Narragansett Bay (Rhode Island Department of Environmental Management, 1998). Loads are currently being reduced through wastewater treatment plant upgrades, but reductions in contaminant loadings are still needed to achieve water-quality goals (U.S. Environmental Protection Agency, 2007; Upper Blackstone Water Pollution Abatement District, 2012, 2013; Massachusetts Executive Office of Energy and Environmental Affairs, 2015). Nutrients may contribute to eutrophication in the Blackstone River and Narragansett Bay, and trace elements may have toxic effects on aquatic organisms in the Blackstone River. In order to better focus reduction efforts, more detailed information is needed about loadings to the Blackstone River and its major tributaries. To address this need, the Massachusetts Department of Environmental Protection (MassDEP) and the U.S. Geological Survey (USGS) initiated a cooperative study of nutrient, suspended sediment, and trace element loads in the Blackstone River in 2006. The overall objectives of the study were to (1) determine the magnitude and spatial and temporal patterns of nutrient, suspended sediment, and trace element loads in the Massachusetts and Rhode Island segments of the Blackstone River and (2) quantify the transport of nutrients and trace elements from Massachusetts to Rhode Island and to Narragansett Bay. Within the context of those objectives, expanded investigations were undertaken to evaluate nitrogen-attenuation throughout the Blackstone River Basin and to assess resuspension and transport of nutrients, suspended sediment, and trace elements from two main stem impoundments. Previous Studies Numerous studies of historical contamination in the Blackstone River have been undertaken. McGinn (1981) outlined plans for dredging sediments and testing samples for toxic trace elements and organic compounds. Izbicki (1993) described the results of a 3-year study of the Blackstone’s stratified-drift aquifers, surface-water and groundwater quality, and the effect of induced infiltration from the river on the quality of well-water supplies. Surface-water-quality data were tabulated by Izbicki (1993) but not discussed in detail. Wright and others (2001) reported the results of detailed investigations of water quality during the summer, dry-season, low-flow months of July and

Nutrient, suspended sediment, and trace element loads in the Blackstone River Basin in Massachusetts and Rhode Island, 2007 to 2009