INCREASED CHEMICAL FLUXES TO THE CRITICAL ZONE DUE TO ROAD SALT (HALITE) ADDITION

Previous research shows that road salt deicers significantly increase concentrations of Na⁺ and Cl^- in groundwater and streams. Cl^- concentrations currently exceed EPA secondary limits in some streams and are projected to climb in coming decades with detrimental effects on some drinking water sources. Elevated concentrations of Na⁺ and Cl^- can impact organisms in contact with surface water and groundwater. Additionally, infiltration of road salt runoff containing high concentrations of Na⁺ can drive cation exchange reactions in soils and aquifers. These exchange reactions may temporarily increase concentrations of Ca, K, and Mg – important micronutrients for terrestrial and aquatic organisms – in groundwater and streams but ultimately result in depletion of these micronutrients from the exchangeable complex. However, little work has been done to quantify road salt-driven exchange reactions. Another understudied aspect of road salt impacts is the role of storm-water detention basins (SDBs) in altering the timing and magnitude of road salt loading to urban and suburban groundwater and streams. We are investigating road salt impacts on two SDBs in Owings Mills, MD. The SDBs discharge into a shallow floodplain aquifer as well as first-order streams and a second-order stream. Our study is focused on mapping the size and seasonal movement of the “salted” water plume from the SDBs, discharge and rates of movement to the floodplain aquifer, and subsequent discharge to streams. We find a sustained groundwater plume of water with high Na⁺ and Cl^- concentrations in the shallow aquifer below the SDBs and also downgradient in the shallow floodplain aquifer. Concentrations vary seasonally with the highest concentrations in winter (post salting) and early spring but remain much higher than the background throughout the year. Na⁺ and Cl^- from road salt appear to be altering the background cation composition of the aquifer through cation exchange reactions. Preliminary model results of the CEC show that the Ca:Na ratio decreases by a factor of 2 - 5 in soil and aquifer materials affected by road salt inputs. Our study provides needed data on the role of storm water detention basins in salinizing groundwater and streams through cation exchange reactions and fast halite weathering.