WINTER DEICING SALT CONTAMINATION OF TWO MUNICIPAL WELLFIELDS IN WILMINGTON, MASSACHUSETTS
Prior to the opening of I-93 in 1960, the background sodium and chloride concentrations at both wellfields averaged less than 7 mg/l. Since 1960, raw water concentrations at both supplies have steadily increased to as high as 200 and 320 mg/l, exceeding the secondary maximum contaminant levels of 20 and 250 mg/l for sodium and chloride, respectively.
During the winter of 2010-2011, the Browns Crossing SWPA received an estimated 594 tons of deicing salt. Of this amount, approximately 68% came from I-93, 21% from town roads and 11% from commercial parking lots. In the Barrows SWPA (which received about 214 tons of road salt), the breakdown is 63.5% from I-93 and 36.5% from town roads.
Beginning in 2010, the USEPA and the Town of Wilmington collected monthly grab samples of wellfield raw water and surface water for inorganic analyses. Dataloggers recording temperature and specific conductivity every 15 minutes at key locations in streams and in the raw water taps of the two wellfields show that saline runoff from I-93 and secondary roads flows past each wellfield in highly concentrated pulses after winter storm events. Saline pulses containing as much as 5,470 mg/l sodium and 8,200 mg/l chloride required average times of 3.7 and 9.2 hours to flow from the interstate highway to the Browns Crossing and Barrows Wellfields, respectively.
This paper illustrates how other Source Water Protection Areas impacted by dense road networks and rising salinities may be monitored to identify, track and quantify sources of road salt for future reductions through better management.