Deicing Road Salts May Contribute to Impairment of Streambeds through Alterations to Sedimentation Processes

Abstract

Deicing road salts have been increasingly used in the United States over the past 80 years. Previous studies have shown that these salts can have deteriorating effects on freshwater organisms. Here, we hypothesize that the introduction of road salts to streamwater can also boost aggregation of mud particles suspended in the water column. Such aggregation, also known as flocculation, enhances deposition rates and may lead to increased accumulation of fine sediment on streambeds, thereby contributing to the degradation of benthic ecosystems. In this laboratory study, we specifically investigate how road salts impact (1) the size distribution of mud aggregates in a turbulent water column and (2) the overall settling rate of the mud. The results showed that adding road salts to water samples collected from a stream in southwest Virginia, USA changed the distribution of suspended particle sizes. The addition of salt led to more of the mud existing in large flocs and an overall increase in average size by about 40%. As a result, the settling rate of the mud increased relative to the suspensions without salt. Our results suggest that potential negative effects of road salts on mud deposition should be investigated further.

Stormwater best management practices: Experimental evaluation of chemical cocktails mobilized by freshwater salinization syndrome

“…Our results suggest that FSS can alter organic matter in stormwater BMPs similar to other studies (Kaushal et al, 2022a;Kaushal et al, 2022b); this may have implications for changes in soil aggregate structure, erosion potential, and infiltration rates. Our study found that concentrations of DOC increased in response to initial stages of salinization, significantly from salinization with Na + , consistent with changes in solubility and destruction of soil aggregates due to Na + dispersion and loss of cation bridging from Ca 2+ and Mg 2+ in sodic soils (Figure 3; Table 4) (Green et al, 2008a, c;Abolfazli and Strom 2022;Norrstrom and Bergstedt, 2020). In sodic or saline-sodic soils with exchangeable sodium percentages >15 and an exchange of Na + , replacing Ca 2+ or Mg 2+ soil aggregates break up, hydraulic conductivity decreases and colloid mobility increases.…”

“…Our results also showed some rapid declines in DOC concentrations at higher levels of salinization at Bioretention #1 and Wetland #1, before reaching a threshold (between 2.5 and 5 g Cl − /L) and once again increasing. This may indicate flocculation of soil particles at higher salinities (Figure 3) (Abolfazli and Strom 2022). At high concentrations ≥947 μS/cm flocculation is thought to occur due to a decrease in the thickness of the double layer.…”

“…FSS has increasingly affected temperate areas of the United States and Europe over the past century corresponding to increased road salt use (Corsi et al, 2010;Dugan et al, 2017;Kaushal et al, 2019;Stets et al, 2020;Kaushal et al, 2020Kaushal et al, , 2022b. For example, road salt sales in the US have increased from 164,000 tons in 1940 to over 22 million tons in 2019 (Abolfazli and Strom 2022). In temperate regions, extremely high salinity is sometimes caused by road salting during winter precipitation events (Kaushal et al, 2005;Moore et al, 2020;Galella et al, 2021).…”

“…Through the mobile anion effect (this is when negatively charged ions move through the soil profile), bonded base cations can be mobilized along with anions, exhausting buffering capacity) (Green et al, 2008a, c). Elevated concentrations of NaCl, CaCl 2 , and MgCl 2 in streams in response to road salting can also increase flocculation of suspended sediments and soils (Abolfazli and Strom 2022), leading to larger soil aggregates, increased deposition, and compromising benthic ecosystems (Abolfazli and Strom 2022). Flocculation increases with ion concentrations and is caused by cations (particularly divalent and multivalent cations) decreasing the thickness of the double layer (the two layers of charge surrounding an object), allowing for the attraction force to overcome the repulsion force, forming aggregates (Abolfazli and Strom 2022).…”

The flocculation state of mud in the lowermost freshwater reaches of the Mississippi River: spatial distribution of sizes, seasonal changes, and their impact on vertical concentration profiles

“…Specific conductance and ion composition and concentration are fairly consistent between the summer and winter, though specific conductance and the calcium and magnesium levels are all slightly higher during summer relative to winter (Tables 2 and 5). Abolfazli and Strom (2022) have shown that the presence of calcium chloride and magnesium chloride both can have a stronger influence on the flocculation potential of a suspension of natural mud than sodium chloride, and data from the Belle Chasse station do indicate that these ions were present at a slightly higher concentration dur- ing summer. However, the overall specific conductance values, while larger than those of headwater creeks (0 to 100 µS/cm), are nowhere near levels significant enough to produce 1 PSU, and it is unclear if the variation between summer and winter in terms of specific conductance (Table 2) is sufficient to account for the 40 µm change in the floc d 50 .…”