References Cited

Purpose and Scope

This report describes a WQMS for collecting water-quality information in Mount Hope Bay and the Taunton River Estuary. The WQMS was specifically designed to identify monitoring needed to meet the following MassDEP management objectives that are each outlined and assessed in the five subsections of this report:

The study area for this report is the Taunton River Estuary and the Massachusetts part of Mount Hope Bay and the bordering Cole and Lee River embayments. MassDEP seeks to characterize current water-quality conditions (including the extent and duration of hypoxia) in this area as part of a review of coastal and marine DO criteria. The WQMS identifies core water quality and physical indicators used to support informed management strategies for marine DO and outlines sampling strategies needed to achieve these goals. The WQMS also reviews State monitoring objectives for Mount Hope Bay and the Taunton River Estuary and proposes a spatial and temporal monitoring design for selection of a mix of stations and sampling approaches that meet monitoring objectives. The WQMS includes an inventory of water-resource monitoring efforts in the region. The WQMS was developed for implementation by MassDEP and (or) one or more of its agents, contractors, or cooperating agencies. As a final planning effort, one or more quality-assurance project plans (QAPPs) would be needed before each of the five monitoring strategies is implemented.

Previous Studies

Water resources and water quality in the study area have been investigated over the past several decades by State and Federal agencies, academic institutions, and other organizations. MassDEP has several reports on assessments of water quality for the Taunton River, Taunton River Estuary, and Mount Hope Bay (Massachusetts Water Resources Commission [MWRC], 1971; Dorfman, 1988; MassDEP, 2001, 2009, 2010, 2011, 2012; Rojko and others, 2005). MassDEP’s Watershed Planning Program also maintains a database of discrete and 15-minute (hereinafter, “continuous”) measurements of water-quality data and metadata (https://www.mass.gov/guides/water-quality-monitoring-program-data, accessed June 23, 2022). Some MassDEP reports also provide water-quality data collected by other organizations. For example, MassDEP (2021a , b) provides water-quality data collected by the University of Massachusetts-Dartmouth School for Marine Science and Technology (SMAST) Coastal Systems Program from four stations in the Taunton River Estuary sampled in 2018, locations near the Somerset Power Facility sampled between 2004 and 2010, and 12 nearshore stations in Mount Hope Bay sampled by consultants for the Brayton Point Power Station (BPPS) in 2015–16.

Three resources that provide excellent summaries of water-quality and ecologic conditions in Narragansett Bay, including Mount Hope Bay and the Taunton River Estuary, are a report by the Narragansett Bay Estuary Program (NBEP) (NBEP, 2017), a book by Desbonnet and Costa-Pierce (2008), and a report by Raposa and Schwartz (2009). The NBEP report (NBEP, 2017) includes chapters on stressor indicators (landscape and chemical stressors, climate change) and condition indicators for Narragansett Bay (for the ecosystem, the drainage basin, and public health). The book by Desbonnet and Costa-Pierce (2008) arose from a 2004 symposium titled “State of Science Knowledge of Nutrients in Narragansett Bay” and includes chapters on nitrogen inputs and eutrophication in Narragansett Bay and chapters specific to water quality in Mount Hope Bay (Krahforst and Carullo, 2008) and circulation in Mount Hope Bay (Chen and others, 2008). The report by Raposa and Schwartz (2009) provides a comprehensive bay-wide overview of the ecology of Narragansett Bay, of which Mount Hope Bay and the Taunton River Estuary are important components. Additional water-quality data for Mount Hope Bay and the Taunton River Estuary are provided by Boucher (1991), Desbonnet and others (1992), Brayton Point Energy, LLC (Brayton Point Energy, LLC, written commun., 2016, as cited in Rhode Island Department of Environmental Management [RIDEM], 2021]), RIDEM (2021), Rountree and others (2003), Rountree and MacDonald (2006), Howes and Samimy (2007), and the Narragansett Bay Commission (NBC) (NBC, 2019) and by summarized information sources (GeoSyntec Consultants, 2006). Many journal articles on water quality in Narragansett Bay such as Codiga and others (2009), Hale and others (2016), Codiga (2020a, b), and Oviatt and others (2017, 2022) also provide data pertinent to Mount Hope Bay and the Taunton River Estuary. The NBEP maintains a resource library of scientific articles and research projects (https://www.nbep.org/technical-resources, accessed June 23, 2022).

Several USGS studies have documented hydrologic and water-quality data in the Taunton River Basin, including Williams (1968), Williams and Willey (1973), Williams and others (1973), Frimpter (1973), Wandle and Keezer (1984), and Lapham (1988). Barbaro and Sorenson (2013) compiled available data from 1996 to 2006 and developed a rainfall-runoff model for the Taunton River Basin that was calibrated for selected impaired reaches and used to evaluate nutrient loads to the Taunton River. More recently (January 2019 to December 2021), the USGS collected water-quality data at four USGS streamgages in the Taunton River Basin as part of a cooperative project with MassDEP, including Taunton River near Bridgewater, Mass. (station 01108000); Threemile (also known as Three Mile) River at North Dighton, Mass. (station 01109060); Segreganset River near Dighton, Mass. (station 01109070); and Mill River at Spring Street at Taunton, Mass. (station 01108410). Sampled water-quality constituents include major ions, nutrients, bacteria, and metals (quarterly), along with pH, water temperature, DO, specific conductance, turbidity, and alkalinity. These data are available in the USGS National Water Information System (NWIS) database (https://doi.org/10.5066/F7P55KJN, accessed June 25, 2023). An ongoing (2023) USGS study is using water-quality and discharge data to assess the nutrient loading at these four stations.

Mount Hope Bay

Mount Hope Bay is a shallow, tidally dominated embayment in the northeastern part of Narragansett Bay, on the border between Massachusetts and Rhode Island (figs. 1–3). The surface area of Mount Hope Bay is about 13.5 mi², about 70 percent of which is in Rhode Island. Mount Hope Bay has a complex coastline along its north shore, created by the mouth of the Taunton River (Fall River and Somerset, Mass.); embayments at the mouths of the Lee, Cole, and Kickamuit Rivers; and the adjoining peninsulas of Sewammock Neck (Somerset, Mass.), Gardners Neck (Swansea, Mass.), and Touisset (Warren, Rhode Island) (fig. 3). The western, southern, and eastern shores of Mount Hope Bay are made up by a peninsula (Bristol, R.I.), Aquidneck Island (Portsmouth, R.I.), and the Rhode Island-Massachusetts mainland (Tiverton, R.I., and Fall River, Mass.), respectively. The major connection between Mount Hope Bay and Narragansett Bay is through the East Passage, the waterway between the Bristol peninsula and Aquidneck Island that passes under the Mount Hope suspension bridge. Mount Hope Bay is also connected to the Sakonnet River, a tidal strait between Mount Hope Bay and Rhode Island Sound, through the Sakonnet River Narrows, the waterway between Aquidneck Island and the mainland (figs. 2 and 3). Bathymetrically, Mount Hope Bay consists of two regions, a shallow shelf in the north-central and northwestern regions of the bay (typical depths of 15 to 20 feet [ft]) and deeper waters in the East Passage (70 ft) and along the main shipping channels connecting the East Passage, Sakonnet River, and Taunton River (typical depths of 20 to 30 ft). The depth and orientation of these regions relative to seawater inputs from Narragansett Bay and the Sakonnet River and freshwater inputs from the Taunton River affect the circulation and water-quality conditions in Mount Hope Bay. The salinity in Narragansett Bay is about 32 parts per thousand (ppt). Surface water in Mount Hope Bay receives freshwater discharge from the Taunton River and is somewhat isolated from Narragansett Bay and so generally has lower salinities in the range of 20–27 ppt, depending on river flow. River discharge is generally highest in spring and lowest in summer; consequently, the average water salinity in the Mount Hope Bay system is typically lowest in spring and highest in summer. The southern part of Mount Hope Bay is generally well mixed to weakly stratified, whereas the shallower northern part of the bay is more likely to have instances of stratification, with the degree of stratification dependent upon freshwater input and the strength of tidal and wind-induced mixing (Howes and Samimy, 2007; Spaulding and Swanson, 2008).

Tides in Mount Hope Bay are approximately semidiurnal (two tides per day). Tidal fluctuations range between 3.3 ft for neap tides (tides with the smallest tidal range) and 5.5 ft for spring tides (tides with the largest tidal range) (Spaulding and White, 1990). The short length of the bay and narrow connections to Narragansett Bay and the Sakonnet River modify the tides, creating tides that have a double-peaked flood tide and single ebb tide. The timing of the tides is slightly asymmetrical; incoming tides take longer to reach high tide than outgoing tides take to reach low tide (Desbonnet and Costa-Pierce, 2008). Tidal currents are typically less than 1 foot per second in Mount Hope Bay but can reach 6.6 feet per second in the narrow Sakonnet Passage (Rountree and others, 2003). The estimated residence time of Mount Hope Bay is between 14 and 49 days (Kincaid, 2006), with about 80 percent of Mount Hope Bay drainage moving through the East Passage and the remaining 20 percent through the Sakonnet River Passage (Deleo, 2001; Kincaid, 2006).

Substrates in Mount Hope Bay are predominantly mud, with smaller areas of sand mostly near the shoreline and in embayments (NBEP, 2017). Like Narragansett Bay, Mount Hope Bay is a phytoplankton-based ecosystem. Mount Hope Bay has few beds of Zostera marina L. (eelgrass) or macroalgae and little salt marsh (275 acres), primarily because of steep shoreline topography (Barrett and others, 2006; Raposa and Schwartz, 2007).

Taunton River Estuary

The Taunton River Estuary is defined as the area where freshwater from the Taunton River meets and mixes with saltwater from Mount Hope Bay. The estuary also includes tidal freshwater habitat in the upper part of the estuary. The Taunton River, which enters the northeastern corner of Mount Hope Bay adjacent to the city of Fall River, is the longest river unobstructed by dams in New England and provides the largest single freshwater discharge to Mount Hope Bay and Narragansett Bay. Tidal effects and the mixing between freshwater from the Taunton River and saltwater from Mount Hope Bay create complex and continually changing estuarine habitats. The lower Taunton River Estuary, from the Braga Bridge in Fall River upstream to the Berkley Bridge in Dighton (figs. 2–5), is a broad estuary strongly influenced by tides. This part of the estuary is wider, deeper, and more saline than the upper part of the Taunton River Estuary in Dighton, Berkley, and Taunton (fig. 6). The lower Taunton River Estuary and adjoining waters have a greater area of salt marsh (369 acres) than Mount Hope Bay or the upper Taunton River Estuary and most salt marsh is present in Broad Cove, the lower reaches of the Segreganset River, and Assonet Bay (fig. 5; Curley and others, 1974; NBEP, 2017). The narrowing of the river at Peters Point and the large rocks and ledge in the channel at this location mark the beginning of a natural transition between the lower and upper parts of the Taunton River Estuary (fig. 5). The boundary between the upper and lower parts of the estuary is generally considered to be near the Berkley Bridge in Dighton (station 15, figs. 5 and 6). The upper Taunton River Estuary from the Berkely Bridge in Dighton to the Route 24 bridge in Taunton/Raynham (fig. 6) is narrower, shallower, and more brackish than the lower Taunton River Estuary. Although the terms “upper” and “lower” are useful for describing different parts of the Taunton River Estuary, Mount Hope Bay and the upper and lower Taunton River Estuary can be described as a single estuary characterized by different levels of mixing of freshwater from the Taunton River and seawater from Narragansett Bay and the Sakonnet River (Desbonnet and others, 1992; U.S. Court of Appeals for the First Circuit, 2018).

Incoming tides carry saltwater about 12 miles (mi) into the Taunton River Estuary and past the confluence with the Threemile River (MWRC, 1971; Cantwell and others, 2007; figs. 2 and 6). The upstream extent of saltwater is not a fixed location but moves up and downriver about 2.5 mi in response to the magnitudes of the tide and freshwater flow of the Taunton River (Boucher, 1991). Because of the river’s low gradient, the flood tide entering the estuary backs up the freshwater flow in the Taunton River, creating freshwater tidal habitat that extends an additional 6 mi upstream from the farthest extent of saltwater. This backwater tidal effect extends to the head of tide in the reach of the Taunton River near the Raynham oxbow (upstream from Route 24 and downstream from the Cotley River; Taunton Wild & Scenic River Study Committee, 2005; figs. 2, 6). The head of tide, defined as a point where the mean tide range is less than 2.4 inches (Hicks, 2006), is not a fixed location but moves up and downriver in response to the magnitudes of the tide and freshwater flow of the Taunton River. The time taken for the propagation of tides upstream results in a delay of about 40 minutes between the time of high tide at the mouth of the estuary in Fall River and Somerset and the time of the high tide at the upstream end of the estuary in Taunton and Raynham and a lag of about 2 hours at low tide (Boucher, 1991). The estimated residence time of water in the Taunton River Estuary ranges between 1.3 hours during times of high freshwater input from the Taunton River to 2.7 days during periods of low flow (Boucher, 1991; Desbonnet and others, 1992).

The movement of saltwater in the Taunton River Estuary is complex and dynamic and varies with river discharge, tidal amplitude (neap and flood tides), baroclinic pressure gradients (arising from differing densities because of salinity and temperature), and other factors such as estuary geometry, channel curvature, bathymetry, bed roughness, and wind direction and strength. As saltwater from Mount Hope Bay enters the Taunton River Estuary, it mixes with freshwater from the Taunton River and the water column becomes partially mixed. As the flood tide progresses, partially mixed waters are transported up the river. This tidal influx of saltwater is referred to as a “saltwater intrusion” (Savenije, 2005). The transition from saltwater to freshwater (saltwater-freshwater interface, or salt front) is generally located in the reach upstream from the confluence of the Threemile River and downstream from the Plain St. Bridge in Taunton, Mass. The salt front is always moving with the tidal cycles; during the rising tide, saltwater pushes into the estuary, moving the salt front upstream, and during the ebb tide, freshwater flow in the river moves the salt front back toward the mouth of the river. The length of the saltwater intrusion is indirectly proportional to river discharge. High flows in the Taunton River limit the distance that the saltwater intrusion can move upstream, and low flows allow the salt intrusion to move farther upstream. Estuaries are hardly ever in steady state, and no tidal cycle is identical to the one preceding or after it (Fischer and others, 1979).

Nutrient Loading, Eutrophication, and Hypoxia

Mount Hope Bay receives wastewater discharges from 10 permitted WWTFs (NBEP, 2017; table 1). The WWTF with the largest discharge, the Fall River WWTF, discharges directly to Mount Hope Bay near the Massachusetts-Rhode Island border. The Taunton and Somerset WWTFs discharge to the Taunton River Estuary (fig. 2), and another four municipal WWTFs, in Brockton, Mansfield/Foxborough/Norton Wastewater District, Middleborough, and Bridgewater, and three smaller facilities discharge to freshwater reaches of the Taunton River and its tributaries (fig. 1). Estimates of total nitrogen loads from individual WWTFs during 2013–15 are listed in table 1 (NBEP, 2017). Together, the WWTFs discharging to the Taunton River, Taunton River Estuary, and Mount Hope Bay account for about 35 percent of the nitrogen loading to Narragansett Bay (fig. 1; NBEP, 2017; Codiga, 2020a, b). Codiga (2021) augmented the NBEP (2017) analysis of nitrogen loading to Narragansett Bay by generating time series for estimated loads of total nitrogen at a daily resolution and used the loading data to investigate relations between hypoxia and nutrient loads in Narragansett Bay (Codiga, 2020a). Using estimated daily resolution time series for total nitrogen load from 2013 to 2015, Codiga (2020a, b) estimated that the mass discharge and percentage contribution of nitrogen loading to Narragansett Bay from the Fall River WWTF (995,000 pounds per year [lb/yr]; 10.9 percent) and Somerset WWTF (137,000 lb/yr; 1.5 percent) were about one-half of the nitrogen loading from WWTFs that discharge upstream in the Taunton River (2,235,000 lb/yr; 24.5 percent).

Efforts to reduce eutrophication and algal blooms that contribute to low DO concentrations typically focus on reducing nitrogen loads. In Narragansett Bay, WWTF upgrades to tertiary treatment decreased nitrogen loading by more than 50 percent to the Rhode Island part of Narragansett Bay over the last decade (Oviatt and others, 2017; Oczkowski and others, 2018). Although nutrient loading to the upper and mid-Bay regions of Narragansett Bay is lower than in years past, Mount Hope Bay has not yet demonstrated as much improvement. This may be due to the continued wastewater inputs to Mount Hope Bay from WWTFs in Massachusetts and from septic systems, stormwater runoff, and other sources related to urban sprawl in combination with inputs of freshwater from the Taunton River into Mount Hope Bay (Oviatt and others, 2017; see also Coastal State Discussion: A New Narragansett Bay https://www.youtube.com/watch?v=ZheLIR4bW90, accessed June 21, 2023). Implementation schedules for nitrogen reduction have been included in the wastewater discharge permits issued to five WWTFs upstream from the Somerset WWTF that discharge to waters tributary to the Taunton River Estuary or directly into the estuary (Mansfield/Foxborough/Norton, Middleborough, Bridgewater, Taunton, and Brockton; https://www.epa.gov/npdes-permits/region-1-final-medium-wastewater-treatment-facilities-general-permit-massachusetts, accessed June 25, 2023). These ongoing and future upgrades to WWTFs and improved management of stormwater are expected to provide future nutrient load reductions to Mount Hope Bay and improve future water quality in the bay.

Fall River is the largest urban area on Mount Hope Bay and the Taunton River Estuary, and its location immediately adjacent to the bay increases the potential for stormwater runoff and combined sewer overflows (CSOs) from the city to degrade water quality in Mount Hope Bay (RIDEM, 2010). In 2005, Fall River completed construction of a deep-rock tunnel 20 ft in diameter and 3 mi long that has a 38-million-gallon capacity for holding storm water and combined sewage until treatment (City of Fall River, 2017). The tunnel, together with expansion of the regional wastewater treatment plant, increased the wet-weather treatment capacity at the Fall River WWTF from 50 to 106 million gallons per day. Fall River has also implemented an integrated wastewater and stormwater plan that includes additional stormwater management such as constructing CSO screening and disinfection facilities and developing green infrastructure, including tree filter retrofits, rain barrels, and stormwater credits (EPA, 2016). More recently (2021), the EPA and City of Fall River began a 5-year plan to reduce discharges from CSOs into local waterways.

Historically, thermal loading from the former BPPS likely contributed to warmer temperatures and low DO conditions in Mount Hope Bay (fig. 3). The BPPS, at the confluence of the Lee and Taunton Rivers on the shallow north shore of Mount Hope Bay, was once the largest fossil-fueled generating station in the northeast. Thermal discharges and water withdrawals by the BPPS likely degraded aquatic habitat quality and contributed to violations of applicable DO criteria for designated uses in Mount Hope Bay and were implicated in the collapse of the bay’s finfish populations (Gibson, 1996; Mustard and others, 1999; EPA, 2003b; RIDEM, 2021). Concerns over degradation of the Mount Hope Bay ecosystem led to the construction of two cooling towers in 2011. The towers were part of a closed-cycle water system that recycled cooling water and ended the discharge of heated water to the bay. Analysis of temperature data collected in Mount Hope Bay indicated a cooling trend for near-surface waters in Mount Hope Bay after the discharge of heated water ceased (Codiga, 2020a, Benoit and Fox-Kemper, 2021). The BPPS went offline in June 2017, and the towers were demolished in April 2019. The thermal discharges from BPPS affected water quality, water temperature, and hypoxic conditions in the bay and must be considered when making comparisons between past and current DO conditions in Mount Hope Bay.

A study of metals and toxics in Mount Hope Bay is provided in Desbonnet and others (1992). More recently (2019–22), public demands increased for water-quality monitoring to assess the potential effects of stormwater runoff and atmospheric deposition of dust from a Somerset scrap metal facility on water quality and aquatic life in Mount Hope Bay (https://www.epa.gov/newsreleases/settlement-leads-halt-scrap-metal-releases-mt-hope-bay, accessed June 25, 2023). The scrap metal facility ceased operation in March 2022 (https://www.mass.gov/news/ags-office-settles-claims-of-illegal-dust-and-noise-pollution-and-stormwater-discharge-against-owner-and-operators-of-somers ets-brayton-point-scrap-facility, accessed January 4, 2024). Any future assessment of the environmental effects of water quality in Mount Hope Bay in relation to the scrap metal facility or other facilities proposed for the site of the former BPPS would be separate from the WQMS developed by this study.

DO may be the single most important indicator of environmental conditions in estuaries, and oxygen depletion poses a serious threat to estuarine and coastal ecosystems (Diaz and Rosenberg, 2008). The primary mechanism for depletion of DO is through the eutrophication cycle where nutrient overenrichment stimulates algal blooms and the subsequent die off and decomposition of organic matter after blooms increases oxygen consumption and leads to bottom-water hypoxia. Hypoxia and anoxia can have a wide range of negative effects on estuarine and marine ecosystems depending on the spatial extent, duration, frequency, and severity of hypoxic conditions. Hypoxia, defined as a DO concentration less than 2 milligrams per liter (mg/L), can cause physiologic stress in aquatic organisms (but see Vaquer-Sunyer and Duarte, 2008). Anoxia, when there is little to no oxygen in the water, leads to mortality of aquatic and marine organisms and is defined as a condition in which the DO concentration is less than 0.5 mg/L. Moderate hypoxia can disrupt life cycles, reduce growth rates of marine organisms, compress habitats, cause shifts in the benthic and pelagic community structure, and alter predator-prey relations (Codiga and others, 2009). Severe hypoxia and anoxia are associated with fish kills and can reduce species richness and biomass for benthic communities (Hale and others, 2016). Various taxa demonstrate a range of sensitivity to low concentrations of DO, and biological stress for organisms has been documented at DO concentrations between 2 and 5 mg/L (Diaz and Rosenberg, 1995; EPA, 2000; Vaquer-Sunyer and Duarte, 2008). The frequency and duration of the depletion of DO in bottom waters is critical to the structuring of habitat (Howes and others, 2003). Mobile organisms such as fish and crabs can move to avoid hypoxic and anoxic conditions but more sessile species such as benthic invertebrates cannot. Larvae are generally more acutely sensitive to hypoxia than juveniles and adults (EPA, 2000). Where hypoxia is a recurrent problem, benthic and pelagic communities tend to shift dominance from large, long-lived species to more tolerant or opportunistic short-lived species (Deacutis and others, 2006).

The processes or sequences of events that lead to hypoxic conditions may differ for different wind, tide, and current conditions. Vertical stratification, which inhibits the reoxygenation of bottom waters, often plays an important role in the development of hypoxic conditions. Vertical stratification is commonly affected by nonbiological variables, including freshwater discharge and meteorological conditions such as specific wind conditions and warmer air temperatures (Pinckney and others, 2001). Freshwater and saltwater have different densities and do not mix readily. Water with lower salinity or higher temperature is less dense and will float on top of water with higher salinity or lower temperature. This density difference can cause stratification that limits the vertical mixing of oxygen-rich surface water with oxygen-poor bottom water and can lead to bottom-water hypoxia. Increased water temperatures also can affect the vulnerability of organisms to hypoxia because the solubility of oxygen decreases as water temperature increases.

Hypoxic conditions in Mount Hope Bay generally develop over relatively short periods (days or weeks) and are infrequent (Hale and others, 2016). The intermittency of hypoxic events in Mount Hope Bay may be related to the relatively rapid tidal flushing of the bay (Codiga and others, 2009). Historically, areas in Mount Hope Bay that have demonstrated hypoxic or near hypoxic conditions include the mouths of the Lee and Cole River embayments and the Taunton River and along the eastern shore of Mount Hope Bay, south of the Fall River WWTF outfall (Deacutis and others, 2006).

Some of the low DO events that were detected historically in the upper reaches of Mount Hope Bay were attributed to stratification resulting from freshwater input from the Taunton River (Krahforst and Carullo, 2008). Several factors likely combine to create conditions that reduce mixing and allow stratification and hypoxia to develop. The shallow bays and sheltered coves that characterize the northern shore of Mount Hope Bay have poor circulation and flushing, making these areas vulnerable to the effects of nutrient loading such as algal blooms and prolonged periods of oxygen depletion from biodegradation processes. The critical season for low DO concentrations in Mount Hope Bay tends to be during late summer when water is the warmest. Stratification events have been detected during the mid-to-latter part of weak neap tides when tidal mixing is reduced in combination with periods of hot weather and low winds from directions that act to stall currents or gyres and increase the residence time in bottom waters. When stratification events are developing, DO concentrations in bottom waters tend to decrease over a period of several days. Once established, hypoxic events typically persist for periods ranging from hours to weeks until events such as strong tidal action from spring tides and (or) steady winds cause mixing and break up stratification (Desbonnet and Costa-Pierce, 2008; Codiga and others, 2009; Oviatt and others, 2017).

Conditions favoring hypoxia change within and between years. Hypoxia may be episodic (less than one event per year), periodic (lasting from days to weeks), or seasonal (Diaz and Rosenberg, 2008). Summers with large freshwater flows in June have been associated with more severe hypoxia in July and August (Nixon and others, 1995, Stoffel and Kiernan, 2009). In Narragansett Bay, the number of days (duration) and intensity of hypoxia also tend to be higher in wet summers than during dry summers (Stoffel and Kiernan, 2009; Oviatt and others, 2017). During wet summers, rivers discharge more freshwater into the bay, creating density differences that can lead to stratification. Heavy rains can also contribute to stratification.

Although stratification and hypoxia have been documented in Mount Hope Bay, little information is available to determine whether hypoxic events existed in the Taunton River Estuary. Historical discrete DO data in the Taunton River Estuary are available from MassDEP from 1970, 1981, and 1986 (MWRC, 1971; Massachusetts Department of Environmental Quality Engineering [MDEQE], 1982; and Dorfman, 1988, respectively); Boucher, 1991; and from SMAST in 2005–6 and 2018 (Howes and Samimy, 2007; MassDEP, 2021a, respectively). These reports indicate DO conditions in the lower Taunton River Estuary may be as much as 3 mg/L lower in bottom waters than at the surface; however, the short residence time of water in the Taunton River Estuary likely provides less opportunity for tidal waters to stratify and hypoxic conditions to develop.

Water-Quality Assessment in Mount Hope Bay and the Taunton River Estuary

The CWA requires MassDEP to classify surface waters in Massachusetts, complete water-quality assessments, and report to the EPA and the public whether the water resources meet Massachusetts SWQS 314 Code of Massachusetts Regulations 4.00 (314 CMR 4.00) (MassDEP, 2021b). Information on the Massachusetts Water Quality Management Program is available from MassDEP (https://www.mass.gov/guides/watershed-planning-program, accessed January 5, 2024) in reports entitled “A Strategy for Monitoring and Assessing the Quality of Massachusetts’ Waters to Support Multiple Water Resource Management Objectives” (MassDEP, 2018) and “Massachusetts Consolidated Assessment and Listing Methodology (CALM) Guidance Manual for the 2022 Reporting Cycle” (MassDEP, 2022) and in the Final Massachusetts Integrated List of Waters for the Clean Water Act 2022 Reporting Cycle (MassDEP, 2023).

Massachusetts rivers, lakes, and coastal waters are classified into discrete segments called AUs, each of which represents a waterbody segment considered to have similar water quality as classified in the Massachusetts SWQS. The AUs are differentiated to account for changes in water-quality conditions that may be expected in relation to factors such as bathymetry, geographic extent, tidal effect, water residence time, the confluence of major tributaries, or point-source discharges (MassDEP, 2022). The study area in Mount Hope Bay and the Taunton River Estuary has nine AUs: two large AUs within Mount Hope Bay, three smaller AUs in the Lee and Cole River embayments, three AUs in the Taunton River estuary, and one AU in Assonet Bay (table 2; fig. 2). A few adjoining waterbodies to the Taunton River Estuary, such as Broad Cove (MA62–50) and the lower reaches of the Segreganset and Threemile Rivers (MA62–55 and MA62–57, respectively), are tidal waterbodies and classified as separate AUs but are not included in this report at MassDEP’s request. The nontidal freshwater part of the Taunton River upstream from the Taunton River Estuary has a separate AU classification (MA62–01). A map of AUs in the Taunton River Basin can be viewed in appendix 24 of the Final 2022 Integrated List of Waters (MassDEP, 2023).

The Massachusetts SWQS assign all inland and coastal and marine waters to classes according to the intended beneficial uses of those waters. There are three class designations for coastal and marine waters in Massachusetts: classes SA, SB, and SC (MassDEP, 2022). These classes distinguish shellfish harvesting and recreational uses while providing suitable habitat for wildlife, fish, and other aquatic life. Minimum water-quality criteria (such as for DO and temperature) are specified for each class based on the most sensitive designated use for that class.

Water-quality conditions in each AU are evaluated under CWA Sections 305(b) and 303(d) to determine if surface waters meet numeric and narrative water-quality criteria to support designated uses, as defined in the Massachusetts SWQS (MassDEP, 2021b). These designated uses include aquatic life, fish consumption, shellfish harvesting, primary (for example, swimming) and secondary (for example, boating) contact recreation, and aesthetics. Public water supply is also a designated use but is assessed separately from the CWA 305(b) and 303(d) evaluations. MassDEP completes surface water-quality monitoring and uses ambient water-quality data (water quality of a waterbody measured immediately upstream from [or outside of] the effect of a particular source of contaminants) to assess whether waters are meeting their designated uses. Assessed waterbodies are assigned a use-attainment status (either fully supporting, not supporting, insufficient information, or not assessed) that characterizes whether each designated use is being supported, is impaired, or is not assessed.

Aquatic-life use assessments for estuaries are based primarily on DO and nutrient data relevant to cultural eutrophication. In addition to physical and chemical water-quality data, MassDEP collects quantitative information for public health, including fecal coliform bacterial indicators (Escherichia coli [E. coli] and enterococci), along with qualitative information on ecological health, including biological indicators, physical indicators, sediment characteristics, and aesthetic and bottom contaminant and alteration indicators. The nutrient-related ecological health of an estuary can be assessed by the nutrient (organic and inorganic nitrogen, phosphorus, silicon), chlorophyll, and DO concentrations of its waters, but because these can be highly variable, ecological health is also generally assessed using biological indicators, such as eelgrass versus macroalgal distribution, and benthic animal community presence and diversity. The rationale for using biological indicators is that they integrate environmental conditions over seasonal and annual intervals (Howes and others, 2003). MassDEP uses a weight-of-evidence approach to make aquatic life use assessment decisions that consider biological (including habitat) data first, followed by toxicological data, followed by chemical (physicochemical, sediment chemistry, whole-fish tissue residue) data (MassDEP, 2022, 2023).

Water-quality surveys are typically completed during the summer months (June through September) because this reflects the time of year when primary productivity is high and low marine DO concentrations are most likely to be detected. The timing of water-quality surveys is particularly important for sampling constituents where concentrations vary with location within or among coastal waters and with the time of sampling (day, season, year) and for constituents such as DO that have large temporal variability. To document worst-case conditions, profiling for DO and temperature is typically completed later in the sampling period, during mid-July through September, when the estuary is expected to demonstrate the greatest response to nutrient overenrichment. The sampling period for other constituents related to primary contact recreation such as E. coli and enterococci bacterial indicators may be broader, for example, from April 1 to October 15 (MassDEP, 2022).

The EPA, under Section 303(d) of the CWA, requires States to submit reports on the status of their waterbodies every two years. The most recent listings of impairments are reported in the Massachusetts 2022 Integrated List of Waters (MassDEP, 2023), which lists the two AUs in Mount Hope Bay (MA61–06 and MA61–07) as impaired for their designated uses because of multiple water-quality stressors. Aquatic-life impairments are listed in these AUs for chlorophyll a, DO, Fecal Coliform, fish bioassessments, and total nitrogen (MassDEP, 2023). In the Taunton River Estuary, MassDEP (2023) lists aquatic-life impairments for DO, enterococcus, and E. coli in MA62–01, for chlorophyll a, enterococcus, fecal coliform, total nitrogen, and total phosphorous in MA062–02, for DO, enterococcus, fecal coliform, and total nitrogen in MA62–03, and for DO, enterococcus, fecal coliform, fisheries bioassessments and total nitrogen in MA62–04. Additional impairments are listed for assessment units in the Cole, Kickamuit, and Lee Rivers (MassDEP, 2023).

Water-Quality Monitoring in Mount Hope Bay and the Taunton River Estuary

Water-quality data are collected throughout Mount Hope Bay and the Taunton River Estuary to help address resource-management questions and to identify trends in water quality, water quantity, or ecological conditions. MassDEP relies mostly on data collected by its ambient water-quality monitoring program. Historical water-quality assessments for Mount Hope Bay and the Taunton River Estuary are available in MassDEP publications such as the 1970 Taunton River study (MWRC, 1971), the 1986 Taunton River Basin water quality survey (Dorfman, 1988), the 2001 Taunton River watershed water-quality assessment report (Rojko and others, 2005), and the Narragansett and Mount Hope Bay watersheds 2004–8 water-quality assessment report (MassDEP, 2009). MassDEP also can use credible data from other State and Federal agencies, local governments, drinking water utilities, volunteer organizations, and other sources for making assessment decisions, contingent on quality-assurance review and approval (MassDEP, 2018, 2022).

Coordinating Federal, State, and local water resources monitoring efforts has been demonstrated to be an effective strategy for improving and maintaining water quality in estuaries (Fisher and others, 2018). Several organizations have programs that are engaged in monitoring water-quality conditions in the Mount Hope Bay and Taunton River Estuary study area (figs. 1 and 2). These programs are summarized in appendix 1 (table 1.1). This table was compiled with input from stakeholders to identify potential overlap of sampling sites and areas where efforts can be coordinated.

One of the major water-quality monitoring efforts in the region is the RIDEM Narragansett Bay Fixed Site Monitoring Network (NBFSMN) (https://dem.ri.gov/environmental-protection-bureau/water-resources/research-monitoring/narraganset-bay-assessment-1, accessed June 25, 2023). The NBFSMN collects water-quality data throughout Narragansett Bay including at three fixed monitoring-buoy stations in Mount Hope Bay. The monitoring buoys are operated seasonally (May–November) to monitor DO and nutrient conditions. The stations include two buoys in the Massachusetts part of Mount Hope Bay and one buoy in the Rhode Island part of the bay (figs. 2 and 3). The buoy in the Rhode Island part of Mount Hope Bay (station B12) is southeast of Spar Island near the Rhode Island-Massachusetts border. Data collection at this buoy began in 2005. Yearly data downloads of water-quality data for station B12 are available online from the NBFSMN (see https://dem.ri.gov/environmental-protection-bureau/water-resources/research-monitoring/narraganset-bay-assessment-0, accessed December 9, 2022).

The buoys in the Massachusetts part of Mount Hope Bay are in the shallow northern shelf of Mount Hope Bay. The Cole buoy (station B10) is south of the mouth of the Cole and Lee River embayments, and the Taunton buoy (station B11) is south of the mouth of the Taunton River. The Massachusetts buoys are an extension of the NBFSMN and are funded by MassDEP, in partnership with RIDEM, the University of Rhode Island Marine Ecosystem Research Laboratory, and the EPA Southeast New England Program for Coastal Watershed Restoration. Data collection at these buoys began in 2016. The Cole and Taunton buoys were deployed at these locations to represent water-quality conditions in AUs MA61–06 and MA61–07 in Mount Hope Bay (figs. 2 and 3, table 2).

The MassDEP buoys in Mount Hope Bay are equipped with multiparameter water-quality sondes (hereinafter referred to as “sondes”) that record continuous water-quality measurements at near-surface (1 meter [m] below surface) and near-bottom (0.5 m above bottom) locations in the water column. Constituents measured by the sondes include depth, water temperature, specific conductance, salinity, pH, DO, chlorophyll a, and phycoerythrin (a pigment present in blue-green algae). The near-surface sonde also measures nitrate using a Sea-Bird Scientific Submersible Ultraviolet Nitrate Analyzer that makes measurements hourly via ultraviolet light-absorbance measurement. To verify the accuracy of sensor measurements, water samples are collected for chemical analysis every 2 weeks at each buoy location for comparison of water quality to corresponding sensor data. Sondes at the stations are maintained and calibrated using recommended methods using an EPA-approved QAPP (https://www.mass.gov/guides/water-quality-monitoring-quality-management-program, accessed June 25, 2023) and NBFSMN standard operating procedures (Stoffel and Kiernan, 2009). The water-quality data collected at the Cole and Taunton buoys are published by MassDEP (MassDEP, 2020). Continuous water-quality data from the buoys are available online at https://www.mass.gov/info-details/mount-hope-bay-marine-buoy-continuous-probe-data (accessed December 9, 2022).

Organizations that collect water-quality monitoring data in the Taunton River Estuary include the Taunton River Watershed Alliance (TRWA), the Narragansett Bay Commission (NBC), and the Fall River, Somerset, and Taunton WWTFs. The TRWA, through a citizen monitoring effort, collects instantaneous water-quality data monthly (April to October) at 20 stations in the Taunton River Basin, including 3 stations on the Taunton River Estuary (Berkley Bridge, Plain Street, and the Assonet River) and 4 stations on tributaries to the estuary (Segreganset River, Threemile River, Mill River, and Forge River, see https://savethetaunton.org/water-quality-monitoring/, accessed June 25, 2023). These monitoring data include monthly measurements of water temperature, DO, pH, total suspended solids, nitrate, total phosphorus, and enterococci bacteria. The TRWA was founded in 1988 and has water-quality data dating back to 2003. Individual WWTFs in Fall River, Taunton, and Somerset also regularly complete water-quality monitoring in the Taunton River and Mount Hope Bay upstream and downstream from the permitted discharge mixing zone for their wastewater discharges. The NBC Environmental Monitoring and Data Analysis Section collects samples for nutrients from the Taunton River at Berkley Bridge, as well as sites on the Cole and Kickamuit Rivers that discharge to Mount Hope Bay (NBC, 2019).

Several agencies provide the physical data needed to support water-quality data monitoring. The National Oceanic and Atmospheric Administration (NOAA) operates three stations in Mount Hope Bay: the Fall River tide station (8447386) records tidal water level, air and water temperature, and conductivity; the Borden Flats light station (8447387) provides wind speed and direction and air temperature; and the Fall River visibility station (8447412) provides air temperature, visibility, and relative humidity (https://tidesandcurrents.noaa.gov/ports/index.html?port=nb, accessed June 25, 2023). NOAA also produces nautical charts that portray water depths, coastlines, aids to navigation, bottom characteristics, and other features in Mount Hope Bay and the Taunton River Estuary (NOAA, 2023). Maps of the Federal navigation channels are published by the U.S. Army Corps of Engineers (USACE) (https://www.mvr.usace.army.mil/Missions/Navigation/Hydrographic-Surveys/HydrographicSurveysMap/, accessed June 25, 2023). The USGS monitors streamflow in the Taunton River (station 01108000) and four tributaries in the Taunton River Basin including the Mill River (station 01108410), Segreganset River (station 01109070), Threemile River (station 01109060), and Wading River (station 01109000) (fig. 2). Streamflow data are reported in the USGS NWIS database (https://doi.org/10.5066/F7P55KJN).

Review of Coastal and Marine Dissolved Oxygen Criteria

The existing DO criteria in the Massachusetts SWQS prescribe the minimum DO necessary to sustain designated uses in coastal and marine waters (table 3; MassDEP, 2021b). Exceedances of DO criteria are determined by comparing coastal and marine DO measurements to the criteria, which are dependent on the waterbody classification (class SA or SB) in the Massachusetts SWQS.

In 2017, MassDEP began completing a multiyear review of existing coastal and marine DO criteria in the Massachusetts SWQS 314 CMR 4.00 (MassDEP, 2021b). To provide expert guidance during the review of coastal and marine DO criteria, MassDEP worked with Tetra Tech, Inc., and Normandeau and Associates to convene a TAC. The TAC includes representatives from the EPA, Massachusetts Coastal Zone Management, Massachusetts Department of Fish and Game, RIDEM, University of Rhode Island, USGS, Footprints in the Water LLC, and Great Bay National Estuarine Research Reserve. MassDEP’s criteria review considered several approaches as the basis for developing DO criteria, including the reference condition approach (EPA, 2001), the Virginian Province approach (EPA, 2000), and the Chesapeake Bay approach (EPA, 2003a). The Virginian Province approach was the basis for DO criteria developed and adopted in the mid-2000s by other States on the East Coast, including Connecticut, New York, and Rhode Island. MassDEP is evaluating approaches that build on the Virginian Province approach while considering information from the National Marine Fisheries Service and other literature sources. Additional factors that may be considered include whether there is the occurrence of threatened or endangered species in a waterbody, such as Atlantic or shortnose sturgeon, and whether and whether sufficient data exist to indicate that higher DO concentrations are needed to support the growth and reproduction of aggregate fish communities.

Recent EPA guidance for marine DO (EPA, 2000, 2003a) recommends that in addition to establishing the magnitude of DO criteria, revised aquatic-life criteria for DO should also specify the frequency and duration of allowable exceedances. Here, the term “frequency” refers to the allowable number of exceedances of each criterion and the term “duration” refers to the time frame used to evaluate an exceedance of each criterion. To adequately protect aquatic life, including endangered coastal and marine species in Massachusetts, potential revisions to the marine DO criteria in the Massachusetts SWQS 314 CMR 4.00, may include acute and chronic DO criteria magnitudes, as well as frequency and duration components. Continuous DO data will be necessary to provide the time-series data needed for evaluating the frequency and duration of DO conditions.

MassDEP convened several meetings of stakeholders during 2018–19 (https://www.mass.gov/guides/watershed-planning-program-surface-water-quality-standards-review-and-development-activities, accessed June 25, 2023). The MassDEP meetings were held to keep stakeholders informed during the ongoing review of existing coastal and marine DO criteria. The stakeholder meetings also provided important input to the WQMS for Mount Hope Bay and the Taunton River Estuary. The stakeholders and TAC helped MassDEP to identify five management topics, described in the next section, that are related to water quality and environmental health within the study area and that were important to the WQMS for Mount Hope Bay and the Taunton River Estuary.

Monitoring to Support Review of Coastal and Marine Dissolved Oxygen Criteria

Monitoring stations collecting continuous DO data will be needed to provide the time-series data needed for evaluating the frequency and duration of DO conditions in Mount Hope Bay and the Taunton River Estuary to support MassDEP’s review of coastal and marine DO criteria. As part of the review, MassDEP is interested in evaluating spatial and temporal variations in DO concentrations.

Continuous data for monitoring DO conditions in the Massachusetts part of Mount Hope Bay are provided by two NBFSMN long-term monitoring stations (B10 and B11), as previously discussed. Currently (2023), no monitoring programs collect continuous DO data in the Taunton River Estuary, and information on stratification and DO conditions in the Taunton River Estuary is limited. Available resources are limited, however, and the addition of long-term fixed-site monitoring stations in the Taunton River Estuary collecting a full suite of water-quality data like the NBFSMN buoys may not be possible because of the expense of these stations. The need for additional long-term fixed stations could be reevaluated, however, once data have been collected at short-term monitoring stations. Short-term (month to years), long-term (decades), and targeted sampling stations are discussed in appendix 3.

A monitoring approach that includes a combination of short-term and targeted monitoring stations with accompanying discrete water-quality sampling and spatial surveys collecting vertical water-quality profiles to assess DO conditions during periods when stratification may be a concern would provide the flexibility needed to characterize DO conditions in Mount Hope Bay and the Taunton River Estuary. One or more short-term monitoring stations could be selected to monitor conditions at surface and bottom depths. Sensors could be installed on a range of platforms including buoys, moorings with bottom sensors, piers, pilings, or floats (shore-based platforms or marine docks). Telemetered data would be needed at a few stations to help inform targeted monitoring. These stations could be complemented by additional monitoring stations equipped with instruments that collect a limited set of constituents such as water temperature, DO, and salinity from near-bottom waters—the depth most at risk for hypoxia. These stations could use lower cost instrumentation that is not telemetered and internally log data that can be downloaded upon retrieval.

Continuous water-quality data collection for DO should, at a minimum, include a DO sensor equipped with a mechanical wiper and accompanying sensors to measure specific conductance, water temperature, and water depth. The specific conductance, water temperature, and depth data provide the means to correct DO concentration data, calculate DO saturation, and relate DO conditions to water levels (tidal stage). A separate sensor is needed to measure atmospheric barometric pressure. One or two barometric pressure sensors would likely be sufficient to correct depth sensors (during postprocessing of data) for all sites on Mount Hope Bay and the Taunton River Estuary. Before deployment, temperature sensors should be checked against a National Institute of Standards and Technology-certified thermometer. Any new continuous DO data should be collected at a frequency and duration that supports potential revisions to the existing coastal and marine DO criteria in the Massachusetts SWQS, and raw data must be evaluated and meet data-quality objectives before final approval.

Monitoring to Evaluate Water-Quality Conditions in Assessment Units in Mount Hope Bay and the Taunton River Estuary for the Clean Water Act

Water-quality data collected at monitoring stations can support multiple monitoring objectives. MassDEP is obligated by the CWA to evaluate water-quality conditions in waterbodies of the State, including the Massachusetts part of Mount Hope Bay and the Taunton River Estuary. Section 305(b) of the CWA requires each State to monitor, assess, and report on the quality of its waters relative to designated uses established in accordance with the Massachusetts water-quality standards. Section 303(d) of the CWA requires each State to list waters not meeting water-quality standards and prioritize those waters for development of total maximum daily load (commonly referred to as “TMDL”) or other management plans for restoration or protection. The 19 stations discussed previously for monitoring DO conditions (fig. 2, table 4) could also be used to evaluate water-quality conditions in the AUs of Mount Hope Bay and the Taunton River Estuary to support monitoring required by the CWA. Constituents used to evaluate water-quality conditions in the AUs include in situ continuous (15-minute) measurements of water-quality properties (water temperature, DO, percentage saturation, pH, specific conductance) and collection of discrete water-quality samples for analysis of nutrients (nitrate, ammonia, total nitrogen, and total phosphorus) and physicochemical constituents (total dissolved solids, total suspended solids, total alkalinity, total hardness, and chlorides) (MassDEP, 2001). In addition to water-quality data, monitoring to assess the compliance of waters with applicable criteria in the Massachusetts SWQS typically requires use of a multi-indicator approach that also considers physical and microbiological data (MassDEP, 2001, 2018, 2022; EPA, 2002a, b). Where possible, stations selected for use in the WQMS are monitoring stations that had been sampled previously so that comparisons could be made to historic water-quality data (MWRC, 1971; MDEQE, 1982; Dorfman, 1988; Rojko and others, 2005).

When evaluating the water-quality conditions in the AUs, MassDEP is obligated by the CWA to list all impairments and subsequently develop TMDLs for those impairments that are defined by the CWA as pollutants. Consequently, when reported in the CWA Section 303(d) list, a segment may in some cases be listed as impaired when only a small part of an AU demonstrates the impairment (EPA, 2020). More detailed monitoring of AUs may be necessary in some locations to determine localized impairments.

Water-quality monitoring stations can be configured in several different ways to collect necessary data and still be cost effective. Although continuous water-quality monitoring data at surface and bottom depths provide the best data for evaluation of stratification, to reduce costs, the measurement of a limited number of constituents, such as DO, conductivity, and water temperature, could be limited to bottom-depth locations at selected stations. If necessary, a cost-effective approach for monitoring water-quality conditions could include moving (or rotating) instrumentation for the collection of continuous water-quality data between stations during alternating months (June and August or July and September; table 5), as previously discussed. If equipment is moved or rotated between stations, however, one or more stations in Mount Hope Bay and the Taunton River Estuary should be selected to collect data throughout the sampling period.

Water-quality monitoring is needed at several locations within the large AUs in Mount Hope Bay. Stations 5 and 6 in AU MA61–07 were prioritized for locations near the boundaries between AUs in areas near the mouths of the Cole and Lee River embayments that have demonstrated low DO conditions in the past (Deacutis and others, 2006; fig. 3). Two potential monitoring locations were identified for MA61–06 (fig. 3, table 4). The station near the Massachusetts-Rhode Island border (station 7) is in deeper water (about 30 ft) near the outfall for the Fall River WWTF. The station would need to be outside the mixing zone of the Fall River WWTF discharge and outside the shipping navigation channel. A second station (station 8) was near the mouth of the Taunton River. This station could potentially be onshore or on a dock structure such as a breakwater barge at Borden Light Marina. One station was used to represent water-quality conditions in the smaller AUs in the Cole and Lee Rivers (stations MA61–04 and MA61–01, respectively). Two stations (stations 3 and 4) were identified in the Lee River (MA61–02) because of the long open-water boundary between AUs at the mouth of the Lee River embayment. Water depths at these locations are shallow, and conditions could potentially be characterized using bottom sensors only.

Monitoring stations in the three AUs in the Taunton River Estuary (MA62–04, MA62–03, and MA2–02) were selected to be near the upstream and downstream boundaries of the AUs (figs. 2–6) to allow comparison of the quality of water coming into the AU during outgoing tides with water leaving the reach at the downstream boundary of the segment, and vice versa for incoming tides. An additional station was near the center of each AU. One station was selected in the Taunton River upstream of State Route 24 (MA62–01; figs. 2, 6) to document conditions upstream of tidal effect.

Data collection at monitoring stations could be supplemented by spatial surveys of DO conditions in the Taunton River Estuary that would collect vertical water-column profiles at selected locations throughout the estuary. Potential areas to assess include deep reaches of the channel and areas near embayments off the main channel that may have less mixing, such as Assonet Bay or reaches of the Taunton River adjacent to Broad Cove (fig. 5). If low DO conditions are detected in a reach of the Taunton River during the first year of data collection and more detailed information is desired, monitoring equipment collecting continuous DO data could potentially be installed in subsequent years.

Monitoring to Characterize Salt Intrusion in the Taunton River Estuary

The intrusion of saltwater into an estuary is referred to as a “salt wedge” because of the profile shape assumed by the leading edge of saltwater, which is denser than freshwater and pushes under overlying river water as the tide moves upstream. The leading edge of a salt intrusion into the Taunton River Estuary is diluted seawater and is more of a diffuse boundary than a sharp boundary. To allow comparisons among water-quality surveys that do not reach the head of the salt or where the leading edge is hard to identify, the salt front is often defined by the location where the salinity is 5 practical salinity units or the chloride concentration is 100 mg/L. Water movement along the salt wedge can be complex, with saltwater moving upstream along the river bottom and less saline water flowing downstream at the surface.

Turbulence following the leading edge of a salt intrusion typically causes resuspension of sediments and other particulate matter and elevated concentrations of flocculent material and suspended solids. This zone of high turbidity in a salt wedge is referred to as the “estuary turbidity maximum.” If present, this feature can increase particle residence time in estuaries and affect water-quality conditions and estuarine biogeochemical and biological processes (Uncles, 2002; Ralston and others, 2017). However, Boucher (1991) found little evidence of a permanent turbidity maximum in the Taunton River Estuary.

The location and movement of the salt intrusion in the Taunton River Estuary have received little attention over the past three decades. Historic water-quality data for the salt wedge in the Taunton River Estuary, collected during 1970, 1981, and 1986 (table 6; MWRC, 1971; MDEQE, 1982; Dorfman, 1988), determined that the leading edge of the salt intrusion (the salt front) tended to exist in the reach between the Berkley Bridge and a location just upstream from the Taunton WWTF where salinities ranged from 15 to 0.1 ppt, respectively (fig. 6, table 6). An additional resource documenting the salt front in the reach between the Berkley Bridge and the Taunton WWTF is provided by the draft National Pollutant Discharge Elimination System permit for the Taunton Municipal Lighting Plant, which indicated that the salinities in the Taunton River near the Taunton Municipal Lighting Plant (about 0.9 mi downstream of the Taunton WWTF) consistently have low values in the range of 0–2 ppt (EPA, 2006). Additional water-quality data indicating the extent of the salt wedge may be available from Marine Research, Inc. (1983), Boucher (1991), the Taunton WWTF, and the Aquaria Taunton River Desalination Plant. The Taunton River Estuary is freshwater upstream from the salt front, although backwater conditions brought about by the incoming tide create a long reach of tidal freshwater habitat that extends upstream to State Route 24 and the Raynham Oxbow (fig. 6). Salinity data collected weekly at low tide in 2004 by SMAST (Howes and Samimy, 2007) from the Taunton River Estuary at the Weir Village Bridge (station 17) indicated that location to be above the effect of saltwater at low tide (average low tide salinity of 0.2 ppt), and salinity data indicated no stratification of the water column at the site (Howes and Samimy, 2007). Moving downstream from the Berkley Bridge, salinities increase steadily to the former Old Colony-New Haven Railroad jetty in Somerset (figs. 4 and 5), then increase more slowly approaching the river’s mouth until the average salinity at all depths is about 30 ppt at the Brightman Street Bridge (figs. 3 and 4).

A new effort to collect water-quality data in the Taunton River Estuary would update the location and movement of the salt intrusion in the estuary; improve the understanding of the relations between river discharge, stratification, and hypoxic conditions in the Taunton River Estuary and Mount Hope Bay; and provide a baseline to inform future investigations of salt-wedge excursion distance resulting from hurricane storm surges or sea-level rise. Estuary tidal conditions are continually changing, and stratification patterns may differ considerably between spring and neap tides and between seasons with high or low river flow conditions. To fully assess the movement and extent of the salt wedge in the Taunton River Estuary, a monitoring program was designed to collect longitudinal and depth profiles over a monthly tidal cycle (table 7). Data should be collected from near-surface and near-bottom depths in the river and should include continuous (5- to 15-minute) measurements of specific conductance, water temperature, DO, and water level. The monitoring survey should be completed several times a year (such as May and August) during different flow conditions. Periodic measurement of vertical water-quality profiles collected from bridges or boats at these stations would provide more detailed information about water-quality conditions and stratification within the wedge. The monitoring program includes data collection at many of the same locations used historically by MassDEP and SMAST to allow a comparison of current water-quality conditions to water quality during these historic surveys.

Two areas of interest along the Taunton River Estuary may warrant a higher density and frequency of data collection. The first of these is the reach that includes the farthest extent of the salt front, and the second is the reach that includes the head of tide. Collection of continuous water-quality data at stations 16 and 17 and of continuous water-level data at station 18 would help to evaluate the variability of the salt wedge in these reaches (fig. 6, table 7). One or more additional stations between stations 16 and 17 and between stations 18 and 19 would help define the movement of the salt front and the head of tide in these reaches of the river, respectively.

The structure and variability of the salinity and DO distribution in estuaries also can be estimated using hydrodynamic models, which are discussed in the “Monitoring to Develop Hydrodynamic and Water-Quality Models in Narragansett Bay” section. The use of these models typically requires collection of data such as water-surface elevations, water temperature, and salinity at high temporal resolution (5- or 6-minute frequency) to simulate short-term processes whose concentrations can have wide spatial or temporal variations such as chlorophyll a and DO cycles. Collection of data that could be used to develop a model of the salt wedge in the Taunton River Estuary would require measurement of additional information such as channel bathymetry, river discharge, velocity, and turbidity (Ralston and others, 2008, 2010, 2017).

Monitoring to Assess Nutrient Loading to Mount Hope Bay and the Taunton River Estuary

Sources of nutrients to the Taunton River Estuary and Mount Hope Bay include effluent from wastewater treatment plants, agricultural runoff, stormwater runoff, groundwater seepage from septic systems, and atmospheric deposition. Excess nutrients in estuaries, particularly nitrogen, contribute to algal blooms, stimulate bacteria growth, and can lead to reduction in DO concentrations through increased respiration and decomposition of organic matter. Additional implications of excess nutrients include loss of critical habitats (aquatic vegetation such as eelgrass), macroalgae blooms, toxic algal blooms, alteration of benthic substrates and macroinvertebrate community structure, and loss or depletion of shellfisheries (Fisher and others, 2018). The Massachusetts SWQS include site-specific numeric criteria for nitrogen and phosphorus for certain waterbodies. Although generally applicable numeric nutrient criteria are not included in the Massachusetts SWQS, narrative criteria specify that waters “shall be free from nutrients in concentrations that would cause or contribute to impairment of existing or designated uses” (MassDEP, 2021b, p. CMR 80). MassDEP analysts rely on a variety of indicators of nutrient enrichment in a weight-of-evidence approach when assessing water-quality conditions (MassDEP, 2022).

Estimates of nutrient loading can be developed through direct sampling of surface waters coupled with flow data from streamgages. In a fixed-frequency sampling program, discrete samples are manually collected, and given sufficient years of sampling, a wide range of flow and water-quality conditions at a location can be collected. The USGS, in cooperation with MassDEP, collected water-quality and streamflow data during 2019–21 from the Taunton River Basin to provide a dataset of selected constituents that could be used for the general assessment of water-quality conditions and for the estimation of constituent loads from specific subdrainage basins (MassDEP, 2018). Data were collected from monitoring stations at or near four existing USGS streamgages (Taunton River near Bridgewater [station 01108000], Mill River at Spring Street at Taunton [station 01108410], Threemile River at North Dighton [station 01109060], and Segreganset River near Dighton [station 01109070]; fig. 2). The USGS collected discrete samples at these sites from January 2019 through December 2021. Discrete samples were collected monthly for phosphorus (including total phosphorus and orthophosphate) and nitrogen (including particulate nitrogen, total dissolved nitrogen, ammonia plus organic nitrogen, and nitrate plus nitrite). Continuous (15-minute) data were also collected at the Taunton River (01108000) and Mill River (01108410) stations from October 2019 through September 2020. Continuous water-quality constituents included water temperature, specific conductance, pH, and DO. Water-quality data for these stations are available in the USGS NWIS database (https://doi.org/10.5066/F7P55KJN). Data collected to support nitrogen loading estimates at these USGS streamgages in the Taunton River Basin provide the means to estimate the freshwater loads from about 75.5 percent of the total drainage area of the Taunton River Basin. The stations are above tidal effect, however, and loading from areas downstream from the streamgages to the estuary would be needed to estimate nutrient loads for the entire Taunton River basin (Codiga, 2021). Estimates of total nitrogen loads at a daily resolution for 2001–17 were derived by Codiga (2021) and used to investigate hypoxia in Narragansett Bay (Codiga, 2020b).

Another approach for estimating nutrient loading is through flow-proportional sampling or a combination of manual sampling and flow-proportional composite sampling. Composite samples are more cost effective and are less sensitive to environmental extremes (such as high-flow and low-flow conditions) than those collected at a single discrete point in time. Composite samples collected using an automated flow-proportional sampling approach capture daily variations and provide the high resolution needed to develop accurate concentration and loading information for different periods (such as a 4-day, biweekly, monthly, annual, or tidal period[s]). A combination of traditional manual discrete sampling and flow-proportional composite sampling has been determined to result in robust monthly and annual loading values that are less susceptible to extreme conditions and reduce uncertainties (Sorenson and others, 2018). Automated flow-proportional sampling at the Taunton River near Bridgewater (01108000; fig. 2) and at an additional streamgage in the Taunton River Estuary would be needed to provide estimates of monthly and annual nutrient loads from the freshwater part of the main stem and for the Taunton River Basin draining into Mount Hope Bay. This sampling could also potentially provide a better understanding of the circulation of nutrients between Mount Hope Bay and the Taunton River Estuary. A station equipped with automated flow-proportional sampling capability could be configured to collect two composite samples, with one composite representing the inflowing tide and the other the outflowing tide. Flow and water-quality data collected at a streamgage in the Taunton River Estuary could also be used to facilitate future water-quality modeling or to verify the performance of hydrodynamic models of Mount Hope Bay, as discussed in the next section.

Total nitrogen yield-duration plots, developed by Barbaro and Sorenson (2013), indicated that measured yields, in pounds per square mile per day, collected between 1997 and 2008 at lower flows from the Taunton River near Bridgewater (station 01108000) were substantially higher than threshold yields at higher flows (based on the 25th percentile concentration of 0.57 mg/L for EPA subecoregion 59). Furthermore, differences in total nitrogen yields were minor at the Taunton River near the Berkley Bridge (TR41), about 19 mi downstream in the middle of the Taunton River Estuary (figs. 2, 5, and 6; table 4). This pattern in total nitrogen yields indicates that point sources in surface water and diffuse sources in groundwater were the primary contributors of nitrogen during this 11-year period. Updating the water-use data in Massachusetts for freshwater and saline water withdrawals from groundwater and surface water and return flows from wastewater discharges would allow the Hydrological Simulation Program–FORTRAN (HSPF) model used in Barbaro and Sorenson (2013) to be updated and fully calibrated for the basin. Estimated streamflow output by the HSPF model could be used to estimate nitrogen loads at ungaged stations where sufficient water quality data have been collected.

Monitoring to Develop Hydrodynamic and Water-Quality Models in Mount Hope Bay and Taunton River Estuary

EPA guidelines specify the kinds of data and information that should be used when making decisions to list waters as impaired, and this information is not restricted to direct observations, such as water-quality monitoring data. Rather, waters can be listed if evaluations such as dilution calculations or predictive simulation models forecast nonattainment of water-quality standards. Because of the complex and site-specific nature of water-quality impairments (for example, excessive nutrients or low DO), predictive models have limited value in identifying impaired waters. Most modeling is carried out for waters where impairments have already been confirmed by actual water-quality or fecal indicator data; however, predictive models can be used in combination with field data to fully assess water-quality conditions and to derive acceptable contaminant loadings from point and nonpoint sources (Chen and others, 2008). Model results are considered in the CWA Section 303(d) listing process in cases where verified or quality-assured modeling forecast unconfirmed water-quality conditions.

Accurate simulation of water-quality conditions in estuaries typically requires a water-quality model coupled with a hydrodynamic model to evaluate the effects of physical characteristics such as tidal forcing, variable river discharge, and wind to simulate estuarine flushing, residence time, and other factors that affect the flow, transport, mixing, and dispersion of constituents (Warner and others, 2005; Ralston and others, 2010; Ganju and others, 2016). In many estuaries, stratification caused by differences in salinity and water temperature modifies the rates of vertical mixing, and thus, it is important to use models that can incorporate the effects of stratification on transport (Ganju and others, 2016). A three-dimensional hydrodynamic model is needed to simulate estuaries that are stratified or that have complex flows where vertical variations of variables are important.

Hydrodynamic models are driven by tidal, discharge, wave, and meteorological forcing and require considerable data to apply, calibrate, and validate simulations of estuaries. Input data to models include channel geometry and bathymetry data to describe the shape and depth of the estuary and substrate composition to estimate bed roughness. Boundary conditions for the seaward extent of the estuary must also be established to characterize semidiurnal tides, water temperature, salinity conditions, current velocities, and tide elevations (Abdelrhman, 2015). Hydrodynamic models for estuaries open to offshore waves such as Narragansett Bay also require wave simulation. Water-level and discharge information are needed at model boundaries and along the length of the estuary. Freshwater inflows to the estuary include discharge from rivers, riparian areas, groundwater, and discharges from WWTFs. Meteorological data are needed for variables such as precipitation, solar radiation, wind speed and direction, air temperature, atmospheric pressure, evaporation rate, and relative humidity. To model the structure and variability of the salinity distribution in estuaries, data are needed from continuous water-quality records together with data for water discharge, velocity, and turbidity. One or more stations would need to be equipped with an upward looking acoustic Doppler current profiler (ADCP) to collect data on current speed, direction, water temperature, and pressure. Velocity measurements at a fixed site should be supplemented by velocity profiles measured in a transect across the channel or estuary using a second ADCP operated at the water surface (Ralston and others, 2008, 2010).

Water-quality models coupled with hydrodynamic models are needed to assess ecosystem processes and interactions in estuaries and the dynamic feedbacks between biotic and physical processes that are important within estuarine systems such as the effect of nutrients on phytoplankton growth and oxygen production, sediment-water interactions (settling, resuspension, sediment nutrient release), and denitrification (Ganju and others, 2016; Dettmann and others, 2020). Coupled hydrodynamic and water-quality models can be used to simulate future scenarios and to evaluate alternatives for remedial action in response to nutrient loading, eutrophication, habitat loss, and freshwater diversion (Ganju and others, 2016).

Simulations generated by an updated hydrodynamic model coupled with a water-quality model would increase understanding of (1) the exchange of flow and, therefore, nutrients and algal blooms between Mount Hope Bay and the Taunton River Estuary and between Mount Hope Bay and Narragansett Bay, including the Sakonnet River; (2) the circulation patterns in Mount Hope Bay and in the Taunton River Estuary; (3) the residence time of water/particles in the Taunton River Estuary and Mount Hope Bay; and (4) the effect of freshwater inputs from the Taunton River Estuary on Mount Hope Bay water quality.

Several hydrodynamic models and water-quality models have been developed to investigate DO and nutrient issues in Narragansett Bay, and some of these models could be updated to improve model performance in the Mount Hope Bay region. The existing Regional Ocean Modeling System (ROMS) hydrodynamic model of Narragansett Bay (Kincaid and Rogers, 2009; Kincaid, 2012) could be refined to develop a more accurate model of Mount Hope Bay and the Taunton River Estuary. ROMS is a three-dimensional free surface terrain-following numerical model that solves finite-difference approximations of the Reynolds-averaged Navier-Stokes equations on a curvilinear orthogonal grid with vertical stretched terrain-following coordinates (Warner and others, 2005). The ROMS model requires continuous records of sea-level surface, water temperature, and salinity to parameterize the model; these data are available at selected locations and could be applied to model refinement. ROMS models also require further information regarding boundary conditions, advection schemes, turbulence parameterizations, and submodels (Fall and others, 2014). Sea-level elevation and velocity must be defined at open boundaries of the model, such as at the East Passage at the southwestern end of Mount Hope Bay and at the Sakonnet River narrows.

Data for development of ROMS models are typically collected at buoy stations or mooring arrays equipped with instruments such as bottom-mounted ADCP to measure velocity; vertical profiling units fitted with conductivity, water temperature, and depth sensors; optical backscatter sensors for measuring near-bottom and near-surface salinity and sediment concentrations; and pressure sensors for water level. Additional data may be collected by tilt current meters or from moving boat ADCP surveys. Although the ROMS models can be used to inform analysis of the movement of phytoplankton, zooplankton, and nitrogen, they do not have the capability to model water quality or ecological processes, and other models would be needed to provide these capabilities. The communication process between ROMS and other models is challenging and labor intensive.

An example of a linked hydrodynamic and water-quality model is the model of Narragansett Bay completed for the year 2009 by the EPA Atlantic Coastal Environmental Sciences Division (Abdelrhman, 2015, 2017; Rashleigh and others, 2015). The hydrodynamic model used for this modeling effort was the Environmental Fluid Dynamics Code (EFDC) model linked to a Water Quality Analysis Simulation Program (WASP) model, which was then used to simulate chlorophyll a, DO, and nitrogen in Narragansett Bay (Abdelrhman, 2015, 2017; Rashleigh and others, 2015; Dettmann and others, 2020; Knightes, 2023).

The ability of hydrodynamic models to represent physical processes depends on the spatial resolution. To account for the complex water movement and rapidly changing conditions within an estuary, hydrodynamic models typically require a fine grid (on the scale of tens of meters) and run on a short time step. For example, the model grid for the Narragansett Bay hydrodynamic model included 754 horizontal segments and 8 vertical layers (a total of 6,032 segments) with a typical segment size of 642 m in width and 1,218 m in length (Abdelrhman, 2015). The model was run for 365 days using a 120-second (2-minute) time step. This model grid for the Narragansett Bay hydrodynamic model is considered too coarse to be used to accurately simulate water quality in Mount Hope Bay and the Taunton River Estuary, but the model could be used to generate boundary conditions if a more refined model were to be developed (Brenda Rashleigh, Assistant Director for Water, U.S. Environmental Protection Agency, written commun., 2018). Predictions for a full-year simulation for the Narragansett hydrodynamic EFDC model took 13 hours on a desktop computer (Rashleigh and others, 2015). Once calibrated and validated, an EFDC model could be used to predict water-surface elevation at 2-hour increments, velocity distributions of surface and bottom velocities at 2-hour increments, bimonthly distributions of surface and bottom temperature, top and bottom salinity distribution at 2-hour increments, density gradient and stratification strength, and flushing time for the estuary. Other models that potentially could be used for simulating the Taunton River Estuary and Mount Hope Bay include Delft3D, and the Finite-Volume Community Ocean Model (FVCOM) (Zhao and others, 2006). These hydrodynamic models differ in many ways, including computational schemes (finite difference, finite element, finite volume) and grid resolution.

The continuous water-quality data collection in Mount Hope Bay and the Taunton River Estuary discussed in other sections of this report could be used to update and refine hydrodynamic and water-quality models. Note that the time step typically required for hydrodynamic models (2 minutes) or water-quality models (5–6 minutes) is more frequent than that used for water-quality data collection at buoys or streamgages, which is generally 15-minute data but could be configured to collect at higher frequencies with additional power sources. Water-quality data collected at buoy stations or mooring arrays would be more valuable for modeling efforts if the sites also are equipped with instruments such as a bottom-mounted ADCP to monitor water velocity and water-level data.