LOCATING THE SOURCE OF SUSPENDED SEDIMENT IN THE UPPER BLACKSTONE WATERSHED, WORCESTER, MA

The Blackstone River, which flows 46 miles from Worcester, MA to Narragansett Bay in Rhode Island, has experienced continued urbanization of its 540 mi² basin area since the start of the Industrial Revolution. The 70 mi² upper Blackstone watershed consists of the uppermost tributaries located entirely within the Worcester metropolitan area. The population of Worcester, located in central Massachusetts, is approximately 180,000 people making it the second largest city in New England. Many of the structures and systems needed for urbanization, such as pavement and storm drains, add impervious area and otherwise alter the hydrology of the watershed, which can then negatively impact the ecological health of the river. In this study we investigate the relationship between suspended sediment concentration (SSC)(mg/L) and discharge (ft³/s) and attempt to determine the primary source tributary or tributaries of suspended sediment in the main stem Blackstone River. We sampled at 8 sites, 3 on the main stem and 5 on 4 major tributaries from June 1^st to July 15^th, 2015. The headwater tributary watersheds range in area from 2.88 mi² to 18.3 mi² and the contributing area for the main stem sites range from 49.9 mi² to 70.4 mi². During this sampling period there was a total of 7 rain events. We collected water samples at a range of discharges, from 60 cfs to 1570 cfs, using a depth integrated sampler and then used standard vacuum filtration and oven techniques to measure the SSC. The SSCs in this study range from 0 to 126 mg/L. Suspended sediment concentrations in the main stem increase during storm events as the discharge of the river increases. We constructed a suspended sediment rating curve for the USGS 01109730 Blackstone River gage at Millbury. We calculate that 308.7 metric tons of sediment came through the system during the course of the 45-day study, 87% of which came through during the 7 storm events. We determined that once discharge is about 415 cfs, SSC will most likely be about 25mg/L and thus impaired. For 3.6% of the 45-day study, the water in the main stem Blackstone River was likely impaired due to frequent but brief precipitation events. We also conclude that two heavily urbanized tributaries, Mill Brook and Beaver Brook, contribute a disproportionally higher amount of sediment to the main stem Blackstone River.