SPATIAL AND TEMPORAL VARIABILITY OF CHLORIDE AND OTHER WATER QUALITY PARAMETERS ON TOXICITY IN AN URBAN LAKE, MINNESOTA, USA

Shallow lakes imbedded in urban environments are subject to runoff that often contains chemicals that can negatively impact water chemistry and food web dynamics. Salts used to deice roads can result in high chloride concentrations in snowmelt and subsequent runoff. Our study, investigated water quality over four years in Medicine Lake, an urban lake in Plymouth, MN, USA, adjacent to highways and neighborhoods. Seasonal runoff causes road salts to enter Medicine Lake through inlets and non-point source inputs, annually raising chloride concentrations in the water due to a lack of removal mechanisms. Our study focused on temporal changes in stratification and water quality in 8 zones within the lake based on dominant water inputs, bathometry, and aquatic life. We found that chloride concentrates in the deepest stratified portions of the lake (below 6 meters depth), coincident with the lake’s thermocline. Specific conductance also increases at the thermocline. Interestingly, pH and oxidation-reduction potential had an inverse-linear correlation with depth. Dissolved oxygen was prevalent in shallow water but below detection limit deeper than four meters. In 2018, we investigated a zone of the lake where an inlet fed water directly to the deepest area of the lake and found an average chloride concentration of 127 mg/L in shallow water (above 6 meters depth) and 507 mg/L deeper than 6m (maximum 883mg/L), a concentration where aquatic life and food web dynamics are at risk. According to the Minnesota Pollution Control Agency, chronic (defined as 4 days) and acute (1 day) exposure to chloride concentrations of 230 and 860 mg/L respectively, is toxic to aquatic life. Water samples from Medicine lake were also analyzed for baseline, acute toxicity using a DeltaTox II analyzer. Up to 72% of water samples analyzed in high chloride zones exhibited some evidence of toxicity compared to none in zones with lower chloride. Concentration of chloride increases in lakes due to inputs impacted by winter road salting and a lack of removal mechanisms for chloride ions. Considering potential toxicity to aquatic life, alternatives to road salt and mechanisms to protect urban lakes from runoff should be further investigated.