Paula L. Sturdevant-Rees
The University of Massachusetts Blackstone River Water Quality Study was conducted in August of 2005 through November of 2006. Water quality was monitored throughout two dry weather events and five wet weather events along the Blackstone at over 20 sites from the headwaters downstream to Woonsocket, Rhode Island. Special emphasis was placed on the analysis of the nutrient loadings crossing the Massachusetts/Rhode Island state border. Continuous nitrate and phosphate readings were collected at the state line using MicroLABs. The MicroLAB results required a high degree of data adjustment; duplicate samples are recommended to ensure future data quality. Data from events and daily samples showed higher nitrate and phosphate concentrations during low flows than high flows. As a contribution to the total observed nitrate flux, wastewater treatment plants account for an estimated 58% during dry weather and 18-27% during wet weather. Tributaries are estimated to contribute 42% of the total nitrate flux during dry weather and 25-50% during wet weather. Nitrate mass balances show along-stream losses during dry weather and unidentified contributions accounting for an estimated 29-55% of the total nitrate flux during wet weather. As a contribution to the total observed phosphate flux, wastewater treatment plants account for an estimated 80% of the total flux during dry weather and 45-47% during wet weather. Tributaries are estimated to contribute only 2% of the total observed phosphate flux during dry weather and 6-13% during wet weather. Mass balances show an along-stream loss in phosphate under dry weather conditions and an estimated 41-47% contribution to the total phosphate flux by unidentified sources. At the point of discharge, UBWPAD is estimated to contribute 17-38% of the wet weather flow and 54-90% of the dry weather flow. The data suggest that the Blackstone River is nitrogen limited near the point of discharge and phosphorous limited at the Massachusetts-Rhode Island state border. Assuming zero along-stream losses or attenuation, the absolute maximum possible contribution from UBWPAD to the total nitrate flux at the state border was estimated at 70%(370 kg/day) during dry weather and 24% (6,990 kg/day) during wet weather. With the same assumptions, the absolute maximum possible contribution from UBWPAD to the total phosphate flux at the state border was estimated at 100% (56 kg/day) during dry weather and 54% (910 kg/day) during wet weather.