by Kim Kastens, Brewster Conant, Jr., and Peter Severance
Summary
The Green Acton Water Committee collaborated with the Organization for the Assabet, Sudbury and Concord Rivers (OARS) to survey the specific conductance (SC) of the water in Nashoba Brook and its tributaries in Acton and Westford. SC is an indicator of salt concentration in the water, most often from road salt pollution. We learned that SC is highest in the heavily developed headwaters of the stream system, near Rt. 495 in Westford. Throughout the watershed, SC was lower in the high-rainfall year of 2023 than in the drought year of 2024, which we interpret as a result of dilution by fresh water during rainy spells. SC was still elevated many months after the last road salt application, consistent with an interpretation that salt reaches the brooks via groundwater. The highest SC values measured were above the Environmental Protection Agency’s national recommended water quality guideline for protection of aquatic life, and the Massachusetts Secondary Maximum Contaminant Level for drinking water. Reducing road salt use without increasing motor vehicle accidents is tricky, but several recent reports discuss strategies that other Massachusetts towns have used successfully. Surveys such as ours can help municipalities prioritize how to focus their road salt remediation efforts.
Salting the Land — Salting the Waters
In ancient times, salt was sometimes spread on the land of traitors or defeated enemies to harm the soil’s ability to support life. In modern times, society is salting roads and adjacent land and — albeit inadvertently — harming the ability of freshwater brooks and ponds to support aquatic life. And, if too much of this salt gets into the groundwater, it can contaminate the wells from which towns like Acton draw their drinking water.
As explained on the website of the Organization for the Assabet, Sudbury and Concord Rivers (OARS), the most common source of elevated saltiness in brooks and rivers of the SuAsCo (Sudbury-Assabet-Concord) watershed is the de-icer applied to roads in winter, commonly called road salt. Road salt gets into streams and rivers by dissolving into rainwater or melting snow. Some of the resulting salty water flows overland, reaching brooks and rivers directly. Some travels through storm drains, which also discharge into brooks. The remainder of the salty water infiltrates through soils and becomes groundwater. Groundwater can flow underground and enter brooks, ponds, or rivers through a process called “baseflow.” In New England, baseflow is the main source of water flowing in brooks and rivers throughout the year, so if groundwater is salty, the brook water will become salty, too. Septic systems, landfills, and wastewater treatment plants can also contribute to elevated salt in groundwater.
Green Acton’s Program to Understand Salt in Acton’s Waters
In 2021 and 2022, Green Acton’s Water Committee collaborated with OARS to survey the saltiness of the main stem of Fort Pond Brook and its tributaries, which drain southern and western parts of Acton. As reported in a previous webpost, such a spatial survey can identify stream sections with elevated saltiness, which can point to potential pollution sources. That survey found unusually salty stream sections in the Coles Brook tributary and along Rt. 2. The Water Committee also analyzed archival data from the U.S. Geological Survey (USGS) and OARS, which showed that Nashoba Brook at Wheeler Lane has been growing saltier for the past 50 years.
Now, building on what we had learned from these earlier investigations, we have surveyed Nashoba Brook and its tributaries. The headwaters of the Nashoba Brook watershed are in Westford, near Rt. 495. Flowing generally southward, Nashoba Brook passes through the Nashoba Brook Conservation Land, then runs alongside the Bruce Freeman Rail Trail, enters and then exits Ice House Pond, merges with the waters of Fort Pond Brook just south of Rt. 2, passes behind West Concord’s Nashoba Brook Bakery, and finally, enters the Assabet River in West Concord.
Together, Nashoba Brook, Fort Pond Brook, and their tributaries drain 93% of Acton — the entire area of the town except for a small sliver in the northeastern corner (which is in the Spencer Brook watershed), and another small sliver in the southern corner (which drains directly into the Assabet River).
We conducted two comprehensive surveys of Nashoba Brook and its tributaries in Westford and Acton: one in September 2023 and the other in October 2024. As in our previous work, we submerged sensors into the stream water to measure its electrical conductivity. Electrical conductivity measures the ability of water to conduct a current and is an indicator of the concentration of charged ions (e.g., sodium or chloride) in the water. When the measurement is corrected for water temperature effects, it is called “specific conductance” (SC); this is the measurement we report throughout this webpost. In the SuASCo watershed, SC is tightly correlated to the concentration of chloride ions in the water; higher SC means higher amounts of chloride from salt. Colloquially, then, we can use the SC of a body of water as a measure of its “saltiness.”
We think that the “natural” level of SC in Nashoba Brook, before the extensive use of road salt, would have been about 150 micro siemens per centimeter (µS/cm) or even lower, based on measurements taken in the 1970s. OARS uses 500 µS/cm as the threshold SC value above which freshwater streams are not healthy for fish and aquatic ecosystems. For drinking water, Massachusetts sets a Secondary Maximum Contaminant Level (MCL) of 250 µS/cm of chloride content, which equates to approximately 977 µS/cm; above this level, drinking water starts to taste salty and can corrode water pipes. (See the Appendix for more about standards and thresholds.)
Nashoba Brook 2023 Survey
For our September 29, 2023 survey, we assembled two survey teams, with volunteers from Acton, Chelmsford, and Westford. As for our previous surveys, OARS provided the equipment and technical assistance. Together, the two teams visited 31 sites. The first map below shows the SC measurements we collected.
The highest SC values were at the north end of the watershed, in the headwaters of Nashoba Brook, with readings of 660 and 712 µS/cm. Going downstream along the main stem of Nashoba Brook, SC decreased, reaching 379 µS/cm at the downstream end of our 2023 survey. All but one of the tributaries feeding into Nashoba Brook carried relatively low-SC water compared to the main stem, including (from north to south), Nonset Brook, Butter Brook, Nagog Brook, Conant Brook, two unnamed tributaries along the Bruce Freeman Trail, Baptist Brook, and Fort Pond Brook.
Our interpretation of this pattern is that the most problematic input of salt into Nashoba Brook comes from the built-up area around Rt. 495 and Rt. 110, where there are large, busy roads and many commercial establishments and apartment complexes with large parking lots. Extensive use of de-icers on large areas of pavement could lead to the high values of SC that we recorded at the upstream end of our survey area. As the surface water flows southward, the salt content is diluted by the inflow of relatively fresh water from multiple tributaries, lower-SC groundwater, and rainfall. These processes account for the major pattern in the data: a trend of higher SC in the north to lower SC in the south along the Nashoba Brook main stem.
Within the Acton portion of the Nashoba Brook watershed, the only area of higher SC we identified was in the tributary that flows down from Wills Hole in central Acton, where we measured 546 µS/cm at Skyline Drive. This tributary runs alongside a built-up section of Great Road (north of its intersection with Rt. 27) that has numerous parking lots. We also sampled one site on Coles Brook in the Fort Pond Brook system, where prior work had documented exceptionally high SC; in 2023, this site was still rather high (840 µS/cm), as discussed below under “Comparison of Nashoba Brook with the Fort Pond Brook System.”
Note that our measurements were taken in September, months after the most recent snow or ice in the region and any possible de-icer applications. And yet the entire survey area — with main stem readings between 379 and 660 µS/cm — had SC higher than what we think is the “natural” level of 150 µS/cm. Thus, if de-icer is, indeed, the ultimate source of much or most of the salt in Nashoba Brook, it must be coming in via groundwater rather than by being washed in by overland flow as the snow melts or when rain falls. Overland flow would likely result in a short-lived peak of high salt and SC, and would not persist months later. So even one of the rainiest summers in recent memory (2023) did not flush the salt out of the system.
Although the 2023 survey was informative as far as identifying stream sections of concern, we suspect that 2023 is likely not very representative of the year-in and year-out situation experienced by the aquatic organisms in Nashoba Brook. To put the 2023 year in context, we looked at the 50-year time series of SC data at the Wheeler Lane site, where the USGS collects streamflow data continuously and OARS measures SC monthly. At that site, our September 2023 survey team measured SC of 452 µS/cm. During the last 10 years, only 10% of the measurements taken at this site have been this low; most of the time, Nashoba Brook is subject to higher SC levels.
So what mechanisms could have caused SC levels to be unusually low during 2023? We know from OARS’s measurements across the SuAsCo watershed, and from Green Acton’s and USGS’s work in Nashoba Brook, that measurements taken in the days following rainy spells tend to record high stream flow and low SC levels. This is because the freshwater input from rain and surface water runoff dilutes the salt dissolved in the brook, pond, or river. But most of the stream water in the brook is thought to be from groundwater, so what happens with that? On a longer time scale, a season of heavy rain can create larger amounts of clean groundwater, diluting the shallow groundwater prior to it discharging to the stream, thus causing lower SC in the stream water.
The summer of 2023 was exceptionally rainy. You might recall weekend after weekend of rain on your parade or other planned outdoor activity. That rainy weather resulted in exceptionally high streamflow, as shown on the graph below. In fact, we had to postpone our 2023 Nashoba Brook sampling campaign because of torrential rain on the day before we had planned to go out. Even on Sept. 29, 2023, when we finally were able to do our survey, the streamflow was approximate 10 times as high as the average for that date. So our interpretation was that the low SC values across the 2023 survey area had been due to high rainfall, and we resolved to resurvey the same network of sites during a drier season.
Nashoba Brook 2024 Survey
As it turned out, the summer of 2024 was an exceptionally dry summer. The Massachusetts Drought Management Task Force declared our region to be in Level 2 significant drought and then in Level 3 critical drought. So the resurvey of SC in the Nashoba Brook watershed occurred during unusually dry conditions with much lower than average stream flows — much different from those in wet, rainy 2023.
We completed the resurvey on October 5, 2024. On that date, the streamflow was about one-third of the long-term average for that day of the year. At six of the sites that we had sampled in 2023, there was not even enough water in the brook to submerge the water quality sensor.
What we observed is shown on the next map.
Once again, we saw an upstream–downstream trend along the main stem of Nashoba Brook, from higher SC at the upstream end of the survey to lower SC toward the downstream end. However, overall, the values across the survey were significantly higher than what was seen in 2023. Although some tributary sites remained below the 500 µS/cm threshold for healthy aquatic ecosystems, every site along the Nashoba Brook main stem was above that threshold.
Three measurements in Westford, at the upstream end of the survey, are of particular concern: measurements of 1,682 and 2,148 µS/cm in Vine Brook and 1,276 µS/cm in Nashoba Brook. These are among the highest values that OARS has ever recorded anywhere in the SuAsCo watershed, and far above the level that OARS and the U.S. Environmental Protection Agency (EPA) consider unhealthy for aquatic ecosystems. These values are also well above the Secondary Maximum Contaminant Level (MCL) standard for Massachusetts drinking water (see the Appendix). Since the stream flow during low flow conditions is almost all baseflow from groundwater, these observations alert us to the possibility that at some times and places the local groundwater is salty enough to harm water supply wells.
Our observation of higher SC values in dry year 2024 than in wet year 2023 is consistent with our understanding that during times of low rainfall and low streamflow, the level of salt is higher in both groundwater and surface water, as it has not been diluted by input of freshwater. From the point of view of aquatic organisms, low flow seasons are doubly stressful, in that they may experience not enough water and too much salt.
Comparison of Nashoba Brook with the Fort Pond Brook System
Although our 2023 and 2024 surveys were focused on the Nashoba Brook watershed, we took a few minutes at the end of each survey day to resample a few sites in the Fort Pond Brook watershed, in order to compare the two watersheds. Prior to our latest surveys, we suspected that the Nashoba Brook system as a whole would have higher SC than the Fort Pond Brook system. One clue came from June 2021, when OARS volunteers measured SC of 531 µS/cm in Nashoba Brook at Wheeler Lane, and Green Acton measured values in the 300s and 400s for most of the Fort Pond Brook system.
A dramatic exception was Coles Brook, a tributary of Fort Pond Brook that drains lands around Rt. 2, the Acton Department of Public Works (DPW) facility, and the capped landfill at the Transfer Station site. SC was higher than 1,000 µS/cm in Coles Brook in 2021 and 2022.
The new data confirmed our prior expectations. In our 2023 dataset, the last Nashoba Brook measurement before the Nashoba/Fort Pond confluence read 449 µS/cm, whereas the last Fort Pond Brook measurement before the confluence was considerably lower, at 311 µS/cm. The sole Coles Brook site that we had time to resample that day read 840 µS/cm, the highest value recorded anywhere in our 2023 dataset.
In dry year 2024, we measured 1,477 µS/cm in Coles Brook. That salty input could be traced all the way downstream to Warner’s Pond in West Concord. However, above its confluence with Coles Brook, Fort Pond Brook came in at 408 µS/cm, less salty than any 2024 measurement in the main stem of Nashoba Brook.
Conclusions
Overall, there is cause for concern about how salty the stream water can get throughout all of Nashoba Brook — especially in drought conditions such as in 2024, when the entire stream had SC higher than the 500 µS/cm threshold meant to protect aquatic ecosystems. The biggest problem areas and highest SC measurements are in the northern headwaters in Westford, near Rts. 495 and 110. Conditions improve (i.e., SC levels decline) as Nashoba Brook flows through Acton, but remain far saltier than the inferred natural level at all sites.
Actions to improve the surface water quality in the headwaters in Westford will be the key to improving the concentrations of salt in Nashoba Brook within Acton. The only tributary of Nashoba Brook found to be of concern within Acton is the one that flows down from Wills Hole along Rt. 2A just north of the intersection of Rts. 27 and 2A. There, SC was measured at 546 µS/cm in the wet year of 2023 (but was too dry to measure in 2024).
Overall, the Fort Pond Brook system is less salty than Nashoba Brook. A notable exception is Coles Brook, where salt levels are above the level that OARS considers healthy for aquatic ecosystems, and higher than EPA’s guideline for protecting aquatic life from long-term exposure (see the Appendix). Coles Brook feeds into Fort Pond Brook near School Street and elevates the SC in Fort Pond Brook downstream of the confluence. Based on all the SC survey data collected to date (2021 to 2024), Coles Brook, between the DPW site and Fort Pond Brook, is currently the most salt-impacted section of stream within Acton.
What Next?
Increasing saltiness is a trend that is seen in surface waters across New England. Some areas have much higher levels of SC and chloride concentration than even the highest readings we have found in Nashoba Brook and Fort Pond Brook watersheds. Along the Rt. 128 corridor, for example, readings in excess of 5,000 µS/cm have been recorded repeatedly. Acton’s waters are not there yet, but are certainly on a troubling trajectory.
As the salt concentration in surface and groundwater rises toward levels that pose a threat to aquatic ecosystems and drinking water quality, the quest for solutions has intensified across the region.
In 2023, the nearby Town of Lincoln produced a detailed Salt Reduction Plan. In December 2024, the Massachusetts Department of Environmental Protection (MassDEP) released a report on Chloride Concentration in Massachusetts Towns and Rivers, which provides links to salt mitigation efforts by the Massachusetts Department of Transportation and the towns of Cambridge, Andover, and Westfield (see their Appendix A, beginning on page 25). All these documents emphasize the challenge of balancing the safety of motorists and pedestrians against the need to protect water quality.
Among the ideas put forward in these documents that Acton and surrounding towns could consider are:
- pretreat roadways with brine before precipitation begins, which can reduce overall salt use during a storm
- monitor roadway surface temperatures, and cease application of de-icer when temperatures are too low for de-icer to melt snow
- pre-wet solid de-icer to produce quicker melting action
- use a control system on salting trucks to dispense de-icer in proportion to ground speed
- designate zones near water supply sources or sensitive ecosystems for reduced salt application and post warning signage for motorists
- situate snow dumps for plowed snow away from sensitive resource areas
- educate the public and businesses about MassDEP’s guidelines for road salt storage and guidance for snow disposal, and advocate for MassDEP to promulgate guidance for road salt application
Areas which could be prioritized for attention include:
- roads in the headwaters of Nashoba Brook in Westford, especially Rt. 495, Rt. 110, and large parking lots, such as those serving retail plazas, office parks, and apartment complexes
- the upper reaches of Coles Brook, especially around the Acton DPW/capped landfill site and Rt. 2
- groundwater protection zones 1 and 2, especially for supply wells near heavily traveled and salted roads
The fact that the saltiness of Nashoba Brook has been steadily increasing over the past several decades means that patience will be required to remediate the impacts of salt on our brooks and ponds. It has taken decades to get to where we are now, so it is likely to be a long, slow process to reverse this trend. However, knowing there is a problem means we can be strategic in our efforts and start making changes where it will have the greatest benefit.
Acknowledgments
We thank OARS and especially its Water Quality Scientist Ben Wetherill, Acton field volunteer Shankara Rao, Westford field volunteers Pete Ewing and Jason Cunha, and Cucurbit Farm for permission to sample on its land.
Appendix: How Salty Is Too Salty?
This post (and others) about salt in Acton’s waters contain lots of numbers. These represent either chloride concentration (expressed in milligrams of chloride per liter of water, mg Cl/L), or specific conductance (SC, expressed in microSiemens per centimeter, µS/cm). Because there is a well-established linear correlation between these two values as measured in streams throughout New England and in the SuAsCo watershed, we can convert back and forth between the two and retain enough accuracy for the types of comparisons we are making in this series of Acton water studies.
The table below shows various standards and threshold values that you might want to compare with the numbers in the text and maps above. In each row, the yellow highlighted cell is the primary value and the non-highlighted cell is the converted number. All conversions were made using OARS’s correlation relationship: chloride (mg/L) = (0.2753*SC) – 18.987.
Links to sources for table above:
- Green Acton’s analysis of 50 year time series
- OARS Water Quality final report
- Canadian Water Quality Guidelines for Aquatic Life
- U.S. EPA Recommended Water Quality Guidelines for Aquatic Life
- MassDEP Drinking Water Standards and Guidelines