EXPERIMENTAL SIMULATION OF DEICER PATHWAYS

An imbalance in the road salt budgets between the total annual tonnage of applied road salt within two studied watersheds in Massachusetts and the amount of salt removed as total dissolved load by the streams suggests that approximately 50% of the applied road salt that is not accounted by this model may be retained within the watershed as the residual dissolved load. We present two lines of information that support a progressive accumulation of the heavier salt containing water near the aquifer base as the mechanism for road salt retention in the watersheds.

First data are from a suite of direct-push drill holes sampled at 10 ft regular intervals along a 80 ft vertical profile. The drill hole locations are a short distance down gradient from a Department of Public Works road salt storage and maintenance yard used for storage and loading during the winter months. Road salt is stored year round. Three of the most proximal wells have steep chloride gradients: from low levels (2 to 200 ppm) at the GW surface increasing sharply with depth to 760 and 1178 ppm, respectively, near the base, 70 to 90 ft below the groundwater surface. Second line of data comes from the bench top experiments designed to simulate similar conditions in the laboratory setting. Small container with clear sides is filled with fine sand and fresh water. Small amounts of dyed saline solutions (for visual contrast) are dispensed along the container sides to determine percolation rates of simulated highway conditions. Variably concentrated saline solutions predictably percolate rapidly into the saturated “aquifer” zone by a free convective process eventually reaching the container base creating a prominent vertical concentration gradient. Repeated experimental steps progressively displace more fresh water at the top gradually adding to saline solutions near the base. Estimated percolation rates at room temperature are of the order of inches to feet per hour. Mounding of saline solutions within the aquifers along highways near the aquifer base may explain the retention mechanism of road salt within the watershed systems.