The Salty Truth

August 29, 2024

OARS River Log | By Ben Wetherill, OARS Water Quality Scientist | Published August 29, 2024

Figure 1: A standard-sized dump truck with a carrying capacity of about 20 tons of salt.

How much salt do we put into our rivers when we salt our roads? Imagine dumping 130 dump trucks of salt into the Assabet or Sudbury rivers in one day.

In the winter and spring months, whenever there is a big thaw or rain event, all of the salt that has accumulated along roadways, parking lots, and sidewalks is washed into the rivers and streams in a sudden flush. Luckily, the high flows carry most of this salt downstream to the ocean, but the amount of salt that is washed into rivers is indicative of a corresponding amount of salt that also percolates into the ground and mixes with groundwater. OARS’ long-term river sampling data show that salt levels in our rivers during the summer months have been steadily increasing as a result of increasing amounts of accumulated salt in groundwater and soils. Some of our smaller streams have chloride concentrations (a road salt component) close to or above chloride toxicity levels for aquatic organisms.

In March 2024, OARS had a unique opportunity to measure the total chloride load in our rivers following a major end-of-winter rain event. With the help of our intrepid volunteers, every March, we collect river water samples at 15 locations around the watershed. This year, it just so happened that our sample collection was scheduled for the day after a very large rainstorm that dropped over 2 inches of rain in 24 hours (see Figure 2).

Figure 2: Graph illustrating daily precipitation for the SuAsCo region and river flow for the Assabet and Sudbury rivers. The blue vertical line depicts the timing of the March 24th sampling event.

Samples were collected on March 24^th and analyzed for chloride at two sampling locations in the Sudbury and Assabet rivers. These two locations in Maynard and Saxonville also have real-time flow gauges (maintained by the US Geological Survey, USGS) that provide very accurate estimates of river flow volumes at the time of our sampling. With this information about chloride concentration and river flow volume, we calculated the total mass of salt being carried in dissolved form downstream. The total mass carried downstream in 24 hours can be referred to as the daily salt load. Based on our calculations, we estimate that the 24-hour salt load on March 24^th at the Assabet River Maynard site was equivalent to 135 20-ton trucks of salt. At the Sudbury River Saxonville site, it was equivalent to 131 trucks of salt (see Figure 3). OARS also documented a very reliable correlation between the electrical conductivity of the water (which we measure at all sites) and chloride concentration, so we can estimate chloride concentration at the other sites we sampled even without analyzing the samples for chloride. Two other sampling sites have USGS flow gauges, allowing us to calculate load in the Concord River in Lowell and Nashoba Brook in Acton. Based on our conductivity-chloride model, we estimate that the salt load at the Concord River Lowell site was equivalent to 288 trucks of salt and 15 trucks of salt at the Nashoba Brook Acton site (see Figure 3).

Figure 3: Map of the SuAsCo watershed with calculated salt volumes at four sampling locations. Each truck icon represents ten 20-ton trucks of salt in 24 hours of river flow.

Is all of this salt really necessary? How can each of us help reduce the salt we are distributing?

River Log

~30%

of the SuAsCo rivers watershed is wetlands, crucial for wildlife habitat and water quality

~30%

of water samples in the SuAsCo rivers watershed contain PFAS, leading to increased monitoring and remediation by local authorities and environmental groups

~70%

of dams in the SuAsCo watershed are over a century old, requiring extensive upkeep and presenting obstacles to river restoration and fish passage

>40%

of the land within the watershed is publicly accessible, offering recreational opportunities such as hiking, boating, and birdwatching to the community

~15%

of the land in the SuAsCo rivers watershed is covered by impervious surfaces such as roads, rooftops, and parking lots. This contributes to stormwater runoff and water pollution issues

The Salty Truth

OARS River Log | By Ben Wetherill, OARS Water Quality Scientist | Published August 29, 2024

Measuring the March 2024 Salt Flow in Our Watershed

Is all of this salt really necessary? How can each of us help reduce the salt we are distributing?