Investigation into freezing point depression in stormwater ponds caused by road salt

Abstract

Variable and unpredictable ice cover conditions present a serious safety concern for winter recreational activities on urban stormwater ponds. Ice cover formation and decay are largely controlled by thermal and hydrodynamic factors; however, stormwater ponds receiving salt-laden runoff can have altered chemical properties, which may also affect ice processes. To identify the causal factors of variable ice cover conditions, water sampling and continuous water quality monitoring were conducted over two winters on four stormwater ponds located in Edmonton, Alberta, Canada. Laboratory experiments were conducted to determine the freezing point temperatures of the water samples. It was found that the freezing point temperature correlates strongly with specific conductivity and closely to the chloride concentration in the ponds. The magnitude of freezing point depression varied from pond to pond with a maximum magnitude of 0.353°C. The freezing point depression was also determined for the pond ice by measuring the specific conductivity of thawed ice cores, and was found to be significantly smaller than that of the pond water. A calculation of the heat transfer in a layer of ice showed that the ice growth and melting processes were not significantly affected by freezing point depression of this magnitude. Therefore, it was concluded that salt-laden runoff from nearby roads does not have a significant influence on ice formation or thawing in stormwater ponds.