Salt Pollution in Acton’s Waters

The Green Acton Water Committee has been collaborating with OARS (Watershed Organization for the Sudbury Assabet and Concord Rivers) and with environmentalists in neighboring towns to better understand the excessive and increasing levels of salt in Acton’s natural waters (brooks, river, ponds, and groundwater). Excessive levels of salt can be harmful to aquatic organisms that evolved to live in fresh waters, and to humans if the salt gets into drinking water sources. Salt levels can be monitored by measuring the water’s electrical conductivity with a sensor immersed into the water or by taking a sample and sending it to a lab for chemical analysis.

Here is a summary of our posts about this work:

• Road Salt Threatens Aquatic Ecosystems in Acton (Feb. 23, 2023): This post reports data from our 2021 and 2022 surveys of specific conductance (SC) and chloride concentration in the Fort Pond Brook tributary system, including Coles Brook. Most of the main stem of Fort Pond Brook had low SC (<400 µS/cm, or microsiemens per centimeter), indicative of good water quality for aquatic ecosystems. However, Coles Brook tributaries had very high SC, in excess of 1,000 µS/cm. The highest readings were found in the vicinity of Route 2 and the Department of Public Works (DPW) facility on Forest Road. Potential sources at the DPW facility include the capped landfill, the current or previous salt storage shed, and the area where excess plowed snow is dumped.

• Nashoba Brook Has Been Growing Saltier for 50 Years (Sept. 7, 2024): This post reports on specific conductance data from a site on Nashoba Brook near Wheeler Lane, which is a stream monitoring site for both OARS and the U.S. Geological Survey (USGS). Between USGS and OARS, we have an SC data record going back to 1973, as well as a continuous record of stream discharge (cubic feet of water per second flowing in the stream). These data show that SC has increased dramatically over this 50+ year time span, from an average of 150 µS/cm in the 1970s to an average of 570 µS/cm since 2010. In this post we also compare SC with stream discharge, and document that SC tends to be lower during and following spikes in stream discharge. The long-term increase in SC (on the scale of decades) is attributed to increased use of road salt on area roads and parking lots. The short-term decreases in SC (on the scale of days) are attributed to dilution by rainfall.

• Where Does the Salt in Nashoba Brook Come From? (May 17, 2024): This post reports on data from our 2023 and 2024 surveys of specific conductance in the Nashoba Brook tributary system, in both Acton and Westford. The two years differed dramatically in rainfall and thus, streamflow: 2023 was exceptionally wet and 2024 was exceptionally dry. During the wet year (2023), SC throughout the watershed was lower than during the dry year (2024), which we attribute to dilution by fresher water input. In each of the two years, the highest SC was found in the headwaters at the north end of the tributary system, in the built-up area near Routes 495 and 110, which we attribute to input from road salt from the highways and nearby parking lots. This post also reviews strategies that have been used in other towns to reduce input of road salt into waterways.

• Tons of Salt Transported by Nashoba Brook? Insights from Continuous Measurement of Specific Conductance (coming soon): This post reports on data from a sensor that captures specific conductance every 15 minutes. We deployed this sensor in Nashoba Brook at Wheeler Lane for three months during the high-flow period of winter/spring 2024, and three months during the low-flow period of summer/fall 2024. By comparing the continuously recorded SC data with the stream discharge data collected by the USGS at the same site, we reinforced our earlier inference that during rainfall-driven stream flow spikes, SC decreases due to dilution. Specific conductance is elevated in the fall, months after the last road salt deployment, so we infer that salt is traveling slowly from road salt application areas into Nashoba Brook via groundwater. By combining specific conductance data with discharge data, we calculated that more than 1,000 tons of salt was transported in the brook during the months of the survey.