by Kim Kastens, Katarina Spasojevic, and Brewster Conant, Jr.
Over the last 50 years, Nashoba Brook in Acton has grown saltier and saltier, according to data collected by the US Geological Survey and OARS (the Organization for the Assabet, Sudbury & Concord Rivers). This trend aligns with a global pattern, called Freshwater Salinization Syndrome, which parallels the increasing use of road salt. Increasing salt levels are cause for concern because they can adversely affect the health of aquatic ecosystems and humans on salt-restricted diets.
The US Geological Survey (USGS) maintains a nationwide network of stream monitoring sites, including one site in Acton on Nashoba Brook near Wheeler Lane. When OARS began its water quality monitoring program on the Assabet River and its tributaries, the organization adopted the USGS site as one of its stations. One of the parameters OARS monitors is specific conductance (SC), an indicator of the concentration of charged ions in the water. In colloquial terms, SC can be thought of, essentially, as the “saltiness” of the water – although stream water contains many ions besides the familiar sodium and chloride of table salt. In the SuAsCo watershed (that of the Sudbury, Assabet, and Concord rivers), the dominant driver of SC is road salt used in winter to deice roads and parking lots. Other potential sources include wastewater from sanitary septic systems, fertilizers, and leachate from landfills.
At present, USGS uses an automated gage to measure the stream height of Nashoba Brook, from which metric it calculates stream discharge (the amount of water flowing in the stream in cubic feet per second). But the Green Acton Water Committee recently discovered that before the automated gage was installed, USGS sent a field technician out to Nashoba Brook multiple times per year, and monitored specific conductance. By combining the USGS data and the OARS data, we now have a 50-year time series of specific conductance in Nashoba Brook!
Combining the two data sets shows a trend of rising specific conductance over time. In the 1970s and early 1980s, SC hovered between 100 and 200 µS/cm (MicroSiemens per centimeter). In the last decade, most measurements of SC have been between 500 and 800 µS/cm. OARS considers that 500 µS/cm is the upper limit that can support a healthy mixed fishery. So, over the 50-year span of the data set, Nashoba Brook has changed from a healthy aquatic environment to a marginal environment.
There is a lot of scatter in the data (meaning that within any year, the SC values can jump around quite a bit). This scatter makes the trend harder to interpret and explain. We hypothesized that this scatter could be due (at least in part) to some samples being diluted by freshwater runoff following storm events. Green Acton volunteer Katarina Spasojevic (Acton-Boxborough Regional High School class of 2024) tested this hypothesis with the help of local hydrogeologist Brewster Conant, Jr.
Stream discharge in Nashoba Brook and many other small streams has a “flashy” character, which means it rises to a sharp peak as water runs off the land and into the stream following a storm event, and then gradually tapers off to pre-storm values. Snow melt events in winter and spring also cause peaks in discharge rate. In between storm and melting events, most of the water in the stream comes from “base flow,” which is a steady source of water that seeps into the stream from groundwater. Base flow carries ions dissolved from geological materials and potentially from road salt or other pollution that has percolated into the groundwater. In contrast, water from precipitation contains almost no dissolved ions and so, has very low SC.
With this in mind, Katarina made numerous graphs with which to compare stream discharge and SC, as shown in the example below. In keeping with our dilution hypothesis, we observed that many of the lowest SC values coincided with large peaks in discharge caused by precipitation events.
The next step was to develop a methodology that would allow each SC measurement to be categorized as either falling within a storm-runoff-impacted event (when dilution would be expected) or within a base-flow-dominated interval (when dilution would not be expected). Initially, Katarina categorized each of the approximately 300 monitoring dates as either falling within a discharge peak or within a base-flow-dominated interval. Later, she refined her technique to eliminate tiny peaks and focus only on events during which the peak discharge was at least 250% as high as the discharge preceding the onset of the peak. In the graph below, the measurements that were collected on dates that were impacted by large peaks in discharge (orange dots) are distinguished from measurements collected on dates that were not so impacted (blue dots).
Across the data set, measurements associated with large discharge peaks (orange dots) tend to show lower SC than measurements taken on days that were not impacted by such events (blue dots). This pattern is consistent with our hypothesis that dilution by freshwater pulls down the SC values during high discharge events (during and following rain storms), and that most of the SC comes from discharging groundwater.
Focusing only on the blue dots in the graph above gives an indication of how the stream water and groundwater salinity have been changing over the decades, absent the impact of temporary dilution from rain. Without the effect of dilution, the upward trend is steeper and the data are less scattered than in the first graph above. After this new analysis, the overall trend is clearer: it appears that groundwater contributions to the stream are making it saltier at a rate even faster than it might appear at first glance.
Nashoba Brook is not unusual in recording an upward trend in salinity over time. Across the country and around the globe, freshwater lakes and streams are experiencing an increase in salinity, as measured by electrical conductivity or by laboratory measurement of water samples. According to a review by Heinz et al. (2022), use of road deicing salts has tripled over the last 45 years. On the positive side, this has been credited with a 78–87% reduction in snow- and ice-related vehicular accident rates. On the negative side, the same trend has degraded water quality in lakes, streams, and wetlands — to the detriment of freshwater ecosystems.
Potential contamination of drinking water is another emerging concern in regions with heavy road salt use. Deicer components are not regulated contaminants in drinking water. However, the Massachusetts Department of Environmental Protection (MassDEP) sets a guideline of 20 milligrams per liter (mg/L) of sodium for individuals on very-salt-restricted diets. The Acton Water District measures sodium concentration annually, and publishes this data on its website; recent measurements ranged from 30.6 to 61.3 mg/L. Sodium can also come from erosion of earth materials, and some deicers do not contain sodium. High chloride concentrations (above 250 mg/L or about 800 µS/cm) are considered by MassDEP and the US Environmental Protection Agency (EPA) as an “aesthetic” issue in drinking water, because the water will taste salty. Salt is difficult to remove from drinking water by treatment.
OARS and its community volunteers are working to document how stream conductivity changes on time scales from minutes to decades in the SuAsCo watershed, and where SC hotspots are located. The Green Acton Water Committee has taken responsibility for the Acton component of this regional effort. These data will be used to better understand where stream pollution is coming from, and eventually to advocate for measures to remediate such sources while still protecting the safety of motorists on the (sometimes) snowy roads of Massachusetts winters.