Road salts and our environment

It's hard to think of icy roads when here in Halifax we have recently been under a heat warning, but this week we had a speaker visit us at the Centre for Water Resources Studies who reminded us that seasonal human activities can impact aquatic environments all year long.

 Rock salt on an icy sidewalk, February 2015

Rock salt on an icy sidewalk, February 2015

To shed some light on this issue, ASPIRE investigator Dr. Barret Kurylyk invited Dr. Claire Oswald from Ryerson University to speak about "Fate and transport of road salt chloride in urbanizing watersheds".  Dr. Oswald discussed that the use of road salts (NaCl) in the wintertime to maintain safe, ice-free surfaces can have serious biological and physical impacts on nearby surface waters. Elevated chloride levels can disrupt the normal pattern of aquatic microorganisms and also create a density gradient that can prevent lake turnover. We might expect chloride concentrations to be high in the winter (during the salting season) and spring thaw, but what Dr. Oswald and a number of other researchers in the last few decades have discovered is that chloride can still be elevated above background levels in lakes and streams into the summer and even fall. This is called chloride retention.

 Dr. Oswald and Dr. Kurylyk after the talk on August 29, 2018.  Photo from Paula Zwicker.

Dr. Oswald and Dr. Kurylyk after the talk on August 29, 2018. Photo from Paula Zwicker.

Dr. Oswald and her multidisciplinary team looked at historical data from several watersheds in Southern Ontario to determine inputs, outputs, and storage (retention) of chloride. These watersheds represented a variety of land uses and could be categorized as urban, urbanizing, and non-urban. The results indicated that rural and urbanizing watersheds retained more salt than urban watersheds, possibly due to storm drains and stormwater conveyance. Essentially, these results prompted an adjustment of the simple mass balance (Retention = In - Out) to include more "ins and outs" and account for things like subsurface flow in the unsaturated zone and storage and export from stormwater retention ponds.

Dr. Oswald and her multidisciplinary team looked at historical data from several watersheds in Southern Ontario to determine inputs, outputs, and storage (retention) of chloride. These watersheds represented a variety of land uses and could be categorized as urban, urbanizing, and non-urban. The results indicated that rural and urbanizing watersheds retained more salt than urban watersheds, possibly due to storm drains and stormwater conveyance. Essentially, these results prompted an adjustment of the simple mass balance (Retention = In - Out) to include more "ins and outs" and account for things like subsurface flow in the unsaturated zone and storage and export from stormwater retention ponds.

 Road salt on concrete.  Photo from:  https://www.ryerson.ca/water/research/claire-oswald/

Road salt on concrete. Photo from: https://www.ryerson.ca/water/research/claire-oswald/

Salt use data are widely available for provincial and municipal roadways, which can help to estimate the total salt load in a watershed. However, the biggest question mark in this area of research is private road salt application, which is not regulated in the same way that provincial and municipal governments regulate their roads. Think of the crunch of rock salt under your boots as you walk through your local box store parking lot - these applications are not included in the government data. This makes it difficult for researchers to model chloride impacts on lakes and rivers. More work is needed in this area in order to optimize winter safety on our roadways and protection of aquatic environments and species.