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 water-quality 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 (5- to 10-fold for nitrogen and 6- to 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.

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 high-flow events monitored during this study.