Road salt impact on lake stratification and water quality

Runoff from roadways on which road salt (NaCl) has been applied for driving safety in winter can form a saline water layer at the bottom of a lake, pond, reservoir, or river impoundment. Natural vertical mixing of lentic surface water bodies can be hindered by the presence of a benthic saline layer and thereby affect lake water quality and ecology. To study the formation and disappearance of the saline layer, temperature and specific conductance profiles were measured intermittently over two years (2007, 2008) and at high frequency during one year (2009) in an urban lake of the northern temperate region (Tanners Lake, Oakdale, Minnesota). Erosion of the saline layer in the spring occurred only in year 2007. In years 2008 and 2009, the saline layer persisted throughout the summer only to be removed during fall turnover when thermal stratification was at a minimum. In all three years, salinity dominated density stratification after ice-out, but was quickly overtaken by temperature stratification as the epilimnion warmed. The deterministic, unsteady dynamic one-dimensional (1D) lake temperature and dissolved oxygen (DO) model MINLAKE was modified by including vertical salinity gradients, and it was used to simulate summer stratification and mixing dynamics in Tanners Lake. The daily adjustment of the hypolimnetic eddy diffusion as a function of lake number was an important component in the developed model. This addition allowed mixing in the hypolimnion to be stronger in the fall and spring when the lake stratification was weaker than in the summer after thermal stratification formed. Model results of dissolved oxygen in the water column demonstrated that the saline benthic layer can prevent dissolved oxygen from reaching lake sediments. The adverse consequences of dissolved oxygen depletion on phosphorus recycling from the sediments, benthic microbial communities, and fish habitat are well known. Overall, the results show how salinity from road salt applications can influence water quality and natural mixing in urban lakes.