DE-ICER PATHWAYS FROM IMPERVIOUS SURFACES TO STREAMS AND PUBLIC WATER SUPPLIES

Trends of aquifer degradation and ensuing salinization of streams by de-icer chemicals require an improved understanding of subsurface hydrologic processes of brine migrations from the points of infiltration to the points of water withdrawal for public water supply or points of stream discharge as baseflow. In both of these instances multiyear water quality records indicate continuing water quality degradation and the need to evaluate the subsurface processes on a multiyear platform. A study of annual budgets of de-icer chemicals stored within the impacted watersheds and in particular their annual changes from year-to-year permits a direct assessment of the overall watershed dynamics between de-icer inputs and de-icer removal. In our field experiment we deployed several In-Situ Inc. AquaTroll 200 probes along a headwater stretch to monitor water level and specific conductance as proxies for streamflow and dissolved chlorides, recording both variables at 15 minute intervals. Total annual removal of dissolved chloride compared to the amount of chloride added during the November 2013 to November 2014 period is estimated between 59% and 78% of the amount added. The total annual net watershed chloride storage has increased by annual retention of 22% to 41% of the total annual de-icer application load.

The brine infiltration and percolation rates however depend on the brine concentration and the type of aquifer earth material. Benchtop experiments with various brine concentrations and a variety of earth materials provide records of the percolation mechanisms, dispersion of the brines, and the rates of brine advancement. Progress of these experiments in transparent 21x12x12 cm tanks is recorded in time lapse photography over the course of 2 to 5 days. The de-icer brine advances by two separate mechanisms: one slower and one faster. The slower advancing pathway creates a diffuse and a strongly diluted zone that tends to stay near the groundwater surface predictably following a shallow hydrologic transport pathways. The faster mechanism is by smaller inch-size and finger-like convective cells that quickly transport more concentrated brine solutions to deeper aquifer zones and consequently longer aquifer migration pathways. Field observations are in agreement with both of these types of transport pathways.