Abstract

Rapid development, population growth, and the changes in land and water use accompanying development are placing increasing stress on water resources in the Taunton River Basin. An assessment by the Massachusetts Department of Environmental Protection determined that a number of tributary streams to the Taunton River are impaired for a variety of beneficial uses because of nutrient enrichment. Most of the impaired reaches are in the Matfield River drainage area in the vicinity of the City of Brockton. In addition to impairments of stream reaches in the basin, discharge of nutrient-rich water from the Taunton River contributes to eutrophication of Mount Hope and Narragansett Bays. To assess water quality and loading in the impaired tributary stream reaches in the basin, the U.S. Geological Survey, in cooperation with the Massachusetts Department of Environmental Protection compiled existing water-quality data from previous studies for the period 1997–2006, developed and calibrated a Hydrological Simulation Program–FORTRAN (HSPF) precipitation-runoff model to simulate streamflow in areas of the basin that contain the impaired reaches for the same time period, and collected additional streamflow and water-quality data from sites on the Matfield and Taunton Rivers in 2008. A majority of the waterquality samples used in the study were collected between 1999 and 2006. Overall, the concentration, yield, and load data presented in this report represent water-quality conditions in the basin for the period 1997–2008.

Water-quality data from 52 unique sites were used in the study. Most of the samples from previous studies were collected between June and September under dry weather conditions. Simulated or measured daily mean streamflow and water-quality data were used to estimate constituent yields and loads in the impaired tributary stream reaches and the main stem of the Taunton River and to develop yield-duration plots for reaches with sufficient water-quality data. Total phosphorus concentrations in the impaired-reach areas ranged from 0.0046 to 0.91 milligrams per liter (mg/L) in individual samples (number of samples (n)=331), with a median of 0.090 mg/L; total nitrogen concentrations ranged from 0.34 to 14 mg/L in individual samples (n=139), with a median of 1.35 mg/L; and total suspended solids concentrations ranged from <1 to 69 mg/L in individual samples (n=155), with a median of 5.3 mg/L. Concentrations of total phosphorus and total nitrogen in most of the samples collected from the impaired-reach areas were higher than various nutrient guidelines for reference streams in the northeast. Among the impaired reaches, median total phosphorus concentrations ranged from 0.016 mg/L in the Rumford River (impaired reach MA62–39) to 0.22 mg/L in the lower part of the Salisbury Plain River (MA62–06); median total nitrogen concentrations ranged from 0.61 mg/L in the Rumford River (MA62–39) to 6.2 mg/L in the lower Salisbury Plain River (MA62–06), and median total suspended solids concentrations ranged from 2 mg/L in the Rumford River (MA62–39) to 8 mg/L in Trout Brook (MA62–07). Median concentrations of nutrients were highest in the lower Salisbury Plain and Matfield Rivers, the reaches downstream from the Brockton Advanced Water Reclamation Facility outfall. High concentrations indicate that the outfall was a major point source for nutrients for the period 1999–2006 when these data were collected. In the remaining impaired reaches affected only by diffuse (nonpoint) sources, median concentrations were substantially lower than in reaches downstream from the outfall, and generally increased with the percentage of developed land in the drainage area to the reach. A Spearman rho rank correlation test was used to evaluate relations between median concentration and the percentage of developed land in the drainage area to the reach. Correlation coefficients for reaches affected by diffuse sources only were 0.667 (p=0.07) for total phosphorus (n=8), and 0.750 (p=0.05) for both total nitrogen and total suspended solids (n=7).

Yield-duration plots also were used to compare measured daily yields to threshold-yield curves. Threshold-yield curves are developed using water-quality guidelines, which are defined as allowable or threshold concentrations expected to limit eutrophication in streams, for a wide range in flows. Results showed that measured total phosphorus yields in the impaired reaches typically were greater than threshold yields based on the 75th percentile total phosphorus concentration (0.021 mg/L) in two minimally affected U.S. Geological Survey reference streams in New England, but less than threshold yields based on a commonly used 1986 U.S. Environmental Protection Agency water-quality criterion concentration (0.1 mg/L). Measured total nitrogen yields in the impaired reaches typically were greater than threshold yields based on the 75th percentile total nitrogen concentration (0.48 mg/L) in the minimally affected reference streams and the 25th percentile concentration (0.57 mg/L) for U.S. Environmental Protection Agency subecoregion 59 streams. Measured total suspended solids yields in the impaired reaches typically were greater than threshold yields based on a tenfold dilution of the Brockton Advanced Water Reclamation Facility winter daily maximum effluent limit of 30 mg/L (3 mg/L), but considerably less than an informal Massachusetts Department of Environmental Protection guideline (80 mg/L). Similar to concentrations, the highest total phosphorus and total nitrogen yields were observed in the reaches downstream from the Brockton Advanced Water Reclamation Facility outfall (lower Salisbury Plain River, MA62–06; and upper and lower Matfield River, MA62–32). Yields for lower flows were up to 7 pounds/square mile/day (lb/mi²/d) for total phosphorus and 100 lb/mi²/d for total nitrogen in these reaches. In most of the impaired reaches not affected by the Brockton Advanced Water Reclamation Facility outfall, yields were lower than in reaches downstream from the outfall, and the difference between measured and threshold yields was fairly uniform over a wide range of flows, suggesting that multiple processes contribute to nonpoint loading in these reaches.

The Northeast and Mid-Atlantic SPAtially-Referenced Regression On Watershed (SPARROW) models for total phosphorus and total nitrogen also were used to estimate annual nutrient loads in the impaired tributary stream reaches and main stem of the Taunton River and predict the distribution of these loads among point and diffuse sources in reach drainage areas. SPARROW is a regional, statistical model that relates nutrient loads in streams to upstream sources and land-use characteristics and can be used to make predictions for streams that do not have nutrient-load data. The model predicts mean annual loads based on longterm streamflow and water-quality data and nutrient source conditions for the year 2002. Predicted mean annual nutrient loads from the SPARROW models were consistent with the measured yield and load data from sampling sites in the basin. For conditions in 2002, the Brockton Advanced Water Reclamation Facility outfall accounted for over 75 percent of the total nitrogen load and over 93 percent of the total phosphorus load in the Salisbury Plain and Matfield Rivers downstream from the outfall. Municipal point sources also accounted for most of the load in the main stem of the Taunton River. Multiple municipal wastewater discharges in the basin accounted for about 76 and 46 percent of the delivered loads of total phosphorus and total nitrogen, respectively, to Mount Hope Bay. For similarly sized watersheds, total delivered loads were lower in watersheds without point sources compared to those with point sources, and sources associated with developed land accounted for most of the delivered phosphorus and nitrogen loads to the impaired reaches.

The concentration, yield, and load data evaluated in this study may not be representative of current (2012) point-source loading in the basin; in particular, most of the water-quality data used in the study (1999–2006) were collected prior to completion of upgrades to the Brockton Advanced Water Reclamation Facility that reduced total phosphorus and nitrogen concentrations in treated effluent. Effluent concentration data indicate that, for a given flow rate, effluent loads of total phosphorus and total nitrogen declined by about 80 and 30 percent, respectively, between the late 1990s and 2008 in response to plant upgrades. Consequently, current (2012) water-quality conditions in the impaired reaches downstream from the facility likely have improved compared to conditions described in the report.

First posted January 17, 2013