1996 Citizens Report: Slocums River, Dartmouth
Embayment: Slocums and Little River
Embayment and Watershed Characteristic
The Slocums River and Little River estuaries adjoin each other on the southwestern shore of the town of Dartmouth. Both are shallow embayments with little coastal development and contain extensive salt marshes, eelgrass beds, endangered species habitat, salt marshes, and freshwater wetlands. These two embayments also have some stark differences. The Slocums River estuary has a large watershed with many sources of nitrogen, with sizable flows from the Slocums and Paskamanset Rivers in a relatively confined area. Little River estuary on the other has a much smaller watershed, thus with much less fresh water input, and with considerably less nitrogen sources as well. The anadromous fish run up the Paskamanset River was once considerable, but has been seriously impaired suggesting that either stream obstructions or water quality degradation may need further investigation. Eelgrass is absent from the Slocums, except near to the mouth, implying that conditions are eutrophic.Earlier Buzzards Bay NEP estimates of nitrogen loading to the Slocums River suggest that residential land use accounts for 50% of the nitrogen load, followed by other commercial and industrial development (24%), then by cropland (14%). However, it has recently come to light that very high nitrogen concentrations (greater than 140 ppm) have been observed in groundwater at the now abandoned Dartmouth landfill at the banks of the Paskamanset on Russells Mills Road. Recent monitoring by the US Environmental Protection Agency (EPA; Ken Perez, pers. comm.) suggest that inorganic nitrogen concentrations along the Paskamanset triple in concentration as they pass by the landfill. The Buzzards Bay NEP has not yet calculated loadings based on this data but they could appreciably change the relative loadings of rain inputs The recharge area to this embayment is the fourth largest surrounding Buzzards Bay, and is heavily developed in its upper and eastern areas.
In contrast to the Slocums River watershed, 54% of the loadings to the Little River is from
agricultural lands followed by residential development (24%).
Water Quality monitoring
The Buzzards Bay NEP estimated that Slocums River is the second most heavily nitrogen loaded estuary in Buzzards Bay. It is not surprising then that some of the worst Eutrophication Index scores and highest levels of chlorophyll and total nitrogen were measured in this estuary. In particular, water transparency was poor, consistently around 60 cm (2 feet), and chlorophyll were consistently above 13 ppb in the three years this estuary was monitored. Station SR5, with salinities often below 5 ppt, was considered a river head station and not used for calculating the eutrophication index scores. Dissolved inorganic nitrogen, the most bioavailable form of nitrogen, was often between 15 and 40 µM (0.2-0.6 ppm) at this station, considerably higher than many Buzzards Bay streams. Because the Slocums/Paskamanset River system has about the third largest flow in the Buzzards Bay watershed, these levels represent a sizable load of nutrients to the estuary.In 1994, Total Nitrogen Concentrations showed a dramatic 30% rise in the Slocums River. This increase was consistent among the inorganic, dissolved, and particulate forms of nitrogen, suggesting that inorganic loads in 1994 were distinctly higher. Station SR5, a River station not included in the upper embayment eutrophication scores, shows the 1994 increase dramatically. In 1993, ammonia concentrations on three dates averaged around 0.8 micromolar, whereas in 1994, concentrations averaged 5 micromolar. Similarly, nitrate at this station was 1.5 and 1.7 micromolar on two dates in 1993, but over 30 micromolar in 1993, whereas in 1994, all 4 dates were between 17 and 37 micromolar. We are unsure if this was due to surface or groundwater sources in the watershed. The increases could be explained by a groundwater plume from the former landfill, but by other sources as well. The preliminary findings of EPA's groundwater and stream monitoring should be carefully reviewed to determine if the relative contribution of the former landfill can be calculated so it can be put into perspective compared to septic systems and other non-point and point sources. Depending upon how many sites were monitored in the EPA study, additional sites along the Slocums River could be added to the Citizen Monitoring Program to better track inputs in this complex watershed.
The results of monitoring in Little River were somewhat of a surprise, considering the low estimates of nitrogen sources in the watershed. Upon closer examination, it became apparent that water conditions and trends in the Little River often paralleled those in the Slocums River. This can be explained by the fact that the Little River and the Slocums River confluence at their mouths, and it is likely that some water leaving the Slocums River estuary, which has 4 times the mean tidal exchange volume of the Little River, returns on the incoming tide to the Little River. For example, in 1994, dissolved inorganic nitrogen and dissolved organic nitrogen showed dramatic increases in both the Slocums River and Little River although overall, concentrations are not as high in the Little River. Interestingly station SR3 tends to be slightly higher in inorganic nitrogen than SR2, the more northerly station. This suggests that more nitrogen may be entering the mouth of the Little River from the Slocums than coming from the watershed!
Oxygen was monitored at station SR1, SR2, SR3, and SR4 by Lloyd Center for Environmental
Studies personnel and volunteers. All four sites show wide swings in oxygen concentrations
during the summer. Such patterns are often observed in shallow bays where benthic algae and
seagrasses produce high levels of oxygen on sunny days, but organic matter in the sediments and
algae reduce oxygen concentrations on calm days. The four sites had consistent patterns of
oxygen from year to year, with the innermost stations showing lower oxygen. In most years the
innermost stations (SR1 and SR2) showed more extreme low and high values than outer stations
SR3 and SR4. Each of these sites showed marginally better oxygen concentrations in 1992, than
subsequent years, perhaps related to slightly cooler water temperatures and more rainfall, and
hence river flushing in 1992. In all 4 years, mean summer oxygen saturation concentrations
(lowest third of values) in the Little River were consistently lower (10% to 15% lower) than in
the Slocums River. The reason for these differences are not clear.
Eutrophication Index scores
Slocums River
Nitrogen Management needs
The Buzzards Bay NEP estimated that existing nitrogen loadings to the Slocums River are more than 200% over Project recommended limits for SA classified waters. The relatively poor water quality documented through the water quality monitoring program and limited distribution of eelgrass suggest the Slocums River is among the most eutrophic of Buzzards Bay embayments. The Buzzards Bay NEP estimated that this watershed also has considerable growth potential with 9,400 additional units possible from unbuilt or converted agricultural areas. However, the extensive wetlands in the watershed suggest this estimate of full buildout is too high. The extensive wetland coverage in the watershed could help assimilate some of the nitrogen headed to the estuary.In contrast to the Slocums River, the Buzzards Bay NEP estimated that existing nitrogen loadings to Little River are well below Project recommended limits for SA waters and this conclusion appears to be supported by the water quality monitoring program. The watershed is estimated to have considerable growth potential, especially from conversion of agricultural land to residential land. Despite this, Little River is not expected to exceed recommended SA limits even at build-out conditions. The town of Dartmouth could apply outstanding resource waters nitrogen loading limits in an effort to keep this embayment as pristine as possible. Based on this loading analysis, water quality in the Little River should be better than that observed in the monitoring program. One possible explanation is that the close proximity of the mouth of the Slocums River to Little River results in the higher nitrogen loaded Slocums River flow with the incoming tide.
Because the Slocums River watershed still has considerable growth potential and because part of
the watershed includes the City of New Bedford, nitrogen management in this watershed is
expected to be challenging. Like the Westport River, nitrogen management for this estuary will
require implementation of agricultural "best management practices", sewer extensions,
alternative septic systems, protection of open space, and agricultural protection restrictions are
important strategies in this watershed for managing future growth. Controls on the number or
performance of future septic systems and upgrade of cesspools to septic systems with advanced
nitrogen removal and sewering of dense areas are other management options for this estuary.
Back