DEICING CHEMICAL PATHWAYS IN NEW ENGLAND AQUIFERS

Our previously reported data show that more than 50% of road salt applied annually on impervious surfaces in Massachusetts is retained within the impacted aquifers whereas the remainder is removed as dissolved load into the oceans. Road salt retention is further supported by datasets obtained from direct-push drill holes from two unrelated aquifer systems and from a set of three continually recording (every 15 min) sensors (depth, sp.cond., and water depth) positioned at three different vertical positions (shallow aquifer, medium depth, and deep aquifer) at one of the two aquifers. Two important features can be observed: (1) there is in general increase in road salt concentration with depth, except (2) the medium depth tend to show in some places apparently stable mid-aquifer zones which can have higher concentration of road salt than the deeper zone. This stratification is not universal. At some locations the deeper zone is more concentrated with expected gradual increase in road salt concentration. Furthermore the road salt concentration in the more concentrated mid zone never exceeds 300 to 400 ppm NaCl.

Road salt runoff brines can be 4% more dense than water and should readily set convection through the aquifer thickness. While traveling as discrete cells the more concentrated pockets/fingers will mix with fresh aquifer waters polluting the unimpacted aquifer zones. Our objective is to experimentally study static and dynamic properties of the mixing processes, advective flow in equilibrium and non-equilibrium scenarios for different fraction sizes and for polar and non-polar solutions. Field evidence suggests that aquifer material heterogeneity may partially account for some of the anomalous observations.