Road Salt Threatens Aquatic Ecosystems in Acton

Contributed by Kim Kastens and the Green Acton Water Committee

As part of a study by OARS of road salt pollution across the Sudbury/Assabet/Concord (SuAsCo) watershed, the Green Acton Water Committee measured electrical conductivity in brooks and rivers at 29 sites throughout the Fort Pond Brook tributary system. High conductivity is typically, although not always, caused by road salt runoff. Conductivity values high enough to indicate potential harm to aquatic life were found at five sites, with the highest values found adjacent to the Town of Acton Transfer Station and near the Route 2 / Route 111 intersection.

Once a month, from March through November, just after dawn on a Sunday morning, volunteers from OARS fan out across the SuAsCo watershed to monitor the quality of the water flowing in the Sudbury, Assabet, and Concord Rivers and their tributaries. The volunteers lower an instrument into the brook or river to measure water temperature, conductivity, dissolved oxygen concentration, and pH (a measure of alkalinity/acidity). They then wade out into the river to capture water samples, which are sent to a laboratory for analysis. The OARS network of sampling stations includes two in Acton and one just across the town line into West Concord.

OARS has been monitoring SuAsCo streams since 1992. One concerning trend OARS has noticed over the last 10–20 years has been an increase in dissolved salts in rivers and brooks throughout the watershed. As documented on the OARS website, this trend is attributed to road de-icer, also called “road salt,” running off of roads and parking lots. Road salt can have several harmful impacts on aquatic ecosystems, including killing the zooplankton at the base of the food chain and stunting the growth of fish.

In OARS data, the impact of road salt is inferred from two types of measurements. The first indicator is the concentration of chloride (Cl^–). Chloride is an ion formed by dissolving the various salts used in de-icing compounds, including NaCl, CaCl₂, KCl, and MgCl₂. This is a laboratory measurement done on samples captured in bottles.

The second indicator is electrical conductivity, which is measured in the field by lowering an instrument package (a “sonde”) into the river or brook. Elevated conductivity can indicate a variety of kinds of pollution, but OARS measurements show that in our region conductivity and chloride concentration are closely correlated. Thus, conductivity measurements — which are fast and cheap to make — can be used as a proxy for the more expensive and more laborious chloride measurements.

The graph below shows the data from OARS’ NSH-002 site in West Concord. This site samples water draining from the Fort Pond Brook and Nashoba Brook watersheds, which together cover most of the town of Acton. The data show quite a bit of variation within any given year, influenced by dilution or lack of dilution by rainfall in the days prior to sampling. However, looking across the 24-year span of the data, there has been an unambiguous rise in conductivity. Samples taken more than 10 years ago averaged 359 µS/cm (MicroSiemens per centimeter), while samples taken since 2013 averaged 535 µS/cm. OARS considers that water with conductivity below 500 uS/cm is sufficiently healthy to support good mixed fisheries. OARS’ two other sampling sites in Acton also showed a step up in conductivity about 10 years ago.

Although OARS’ monthly sampling has provided a good window into the temporal trend in saltiness of SuAsCo brooks and rivers, OARS standard sampling network is too widely spaced to pinpoint specific sources of this type of pollution. Of the many tributaries and sub-tributaries that feed into the main stems of the Sudbury, Assabet, and Concord Rivers, are there some that are especially high in salt? To find out, OARS offered its cadre of trained water quality monitors the opportunity to use one of OARS’ in situ instruments to conduct a spatially dense survey throughout a tributary system of interest to them. Two members of the Green Acton Water Committee are also OARS volunteers, so the committee stepped up to conduct a survey of the Fort Pond Brook tributary system. The Fort Pond Brook system has its headwaters in Littleton, Boxborough, and Stow, then meanders west to east across the southern third of Acton, picking up several sub-tributaries along the way. Fort Pond Brook merges with Nashoba Brook near the Rt. 2 rotary, and finally enters the Assabet River in West Concord.

In June of 2021, the Water Committee field party surveyed 23 sites. They covered the main stem of Fort Pond Brook from the Boxborough town line to the brook’s confluence with the Assabet River; they also took a few measurements in the Grassy Brook and Coles Brook sub-tributaries. The data are summarized on the map below.

The main stem of Fort Pond Brook and the Grassy Brook tributary all yielded conductivity values below 500 µS/cm, within the healthy range for aquatic life. The lowest value in the entire survey was at the confluence of Heath Hen Meadow Brook with Fort Pond Brook, where a measurement of 241µS/cm was recorded. Heath Hen Meadow Brook is crossed by very few roads, and flows through conservation lands in both Acton and Stow.

The highest values in the 2021 survey, exceeding 1,300 µS/cm, were in the Coles Brook tributary, which joins the Fort Pond Brook main stem just east of Hosmer Street. This conductivity “hotspot” was suspected from a previous measurement taken by MassDEP in 2010. The network of small brooks that feed into this area drains many potential road salt sources, including several large commercial parking lots, a section of Rt. 2, Kelley’s Corner, and the Acton Department of Public Works (DPW) Transfer Station.

In 2022, the Green Acton Water Committee went back into the field to follow up on the hotspot discovered in 2021 with additional conductivity measurements plus some bottle sampling. The team reoccupied three sites on the main stem of Fort Pond Brook plus the two Coles Brook sites at which high conductivity had been discovered in 2021. Then, they pushed further upstream and occupied sites on four sub-tributaries feeding into the Coles Brook hotspot.

The 2022 sampling campaign ruled out the two southernmost sub-tributaries of Coles Brook as potential salt pollution sources. Volunteers found the highest conductivity value at a site adjacent to, and downhill from, the DPW Transfer Station. When that brook exits the Acton Arboretum, its conductivity value is low: 343µS/cm. Adjacent to the Transfer Station, a value of 1,632µS/cm was recorded. This is one of the highest values that OARS has found so far in its SuAsCo conductivity mapping. High values were also found in the tributary that drains the Kelley’s Corner area, where Rt. 2, Rt. 111, and Rt. 27 (Main Street) are all potential road salt sources.

A Green Acton Water Committee representative presented our conductivity 2021 data at the July 18, 2022 meeting of the Town’s Green Advisory Committee during that Committee’s discussion of Weston & Sampson’s Architecture and Engineering Evaluation for the DPW and its Transfer Station & Recycling Facility. During that discussion and subsequent conversations, the following potential sources of pollution entering the brook from the Transfer Station were considered:

• the “Salt Shed,” where road salt is stored for use by the Town de-icing crews
• the former “Salt Shed,” which had much less protection against salt dissolving and entering the environment
• the storage area for the sand and salt mixture provided for residents
• washing of vehicles after they have been used for plowing
• plowed snow from around town, which contains de-icer and is disposed of along the west edge of the DPW site
• pollution leaching from the capped but unlined landfill that is proximate to the Transfer Station

The Weston & Sampson engineers at that July 18 meeting encouraged the Town to include a vehicle washing station with wash water captured in a tight tank in its planning for DPW renovation. When the Department of Public Works Facility Committee submitted its recommendations to the Select Board, the committee noted “the need to investigate possible off-site contamination issues, such as in Coles Brook,” and the need to “address present-day requirements for stormwater management, as such standards were not in existence in 1969” (when the landfill became inactive, approximately). The DPW Facility Committee recommended a phased approach to address the many defects and needs identified in the Weston & Sampson report; the vehicle washing station and tight tank did not make it into the recommended first phase.

In addition to contaminating brooks and rivers, road salt can get into drinking water wells, potentially affecting human health. In response to an email query, Acton Water District (AWD) District Manager Matt Mostoller wrote, “As for road salt concerns, we have spotty data for sodium and chloride in our raw water sources. The chloride data set is more comprehensive and certainly shows an upward trend in concentrations. This is most pronounced since 2012.” Sodium and chloride in drinking water are unregulated substances with no established MCL (Maximum Contaminant Level); however the AWD now posts sodium concentration data on its website for the benefit of people on a salt-restricted diet.

The Green Acton Water Committee will continue to advocate for measures to minimize the amount of road salt entering Acton’s streams, rivers, and groundwater — without jeopardizing the safety of motorists. In addition, the committee plans to expand the conductivity survey to cover the Nashoba Brook watershed, which covers the northern two-thirds of Acton.

Acknowledgments: Many thanks to the Green Acton Water Committee field parties: Lucy Kirshner, Alissa Nicol, Norm Strahle, and Carolyn Kiely. Thanks also to Jim Snyder-Grant, who obtained the necessary permissions and braved briars and mud to establish the sampling site adjacent to the Transfer Station. Additional thanks to OARS for the equipment loan, and to OARS staff scientist Ben Wetherill, who supported the program logistically, helped to interpret the data, and created the maps used in this post.

Terminology note: “Citizen scientist” is the traditional term used for people who are not science professionals, but who collect data following protocols designed to result in data of sufficient quality to be used for decision making or research. Green Acton and some other organizations have moved away from this term because it leaves aside people who happen not to be citizens of the jurisdiction in which the research is being conducted. In this post, we simply used the term “volunteers”; other terms in use are “community science,” “participatory science,” and “crowdsourced science.”