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Road Salt Transport at Two Municipal Wellfields in Wilmington, MA
Citation:
Road Salt Transport at Two Municipal Wellfields in Wilmington, MA. JOURNAL OF THE NEW ENGLAND WATER WORKS ASSOCIATION. NEWWA, 127(1):1-23, (2013).
Impact/Purpose:
Quantify road salt loading and transport at two municipal drinking water wellfields
Description:
Chloride concentrations in raw water at the Browns Crossing and Barrows Wellfields in Wilmington, Massachusetts have increased from an average of 6 mg/l in 1961 to 242 mg/l today. Average sodium concentrations since 2009 are six to seven times the state’s drinking water guideline of 20 mg/l for those individuals restricted to a total sodium intake of 500 mg/day. These increases are assumed to be largely due to the use of thousands of tons of deicing chemicals (primarily sodium chloride) on I-93, secondary roads and parking lots over the last five decades. During winter storms, highly saline stormwater is discharged into streams and wetlands through approximately 87 outfalls in the wellfields’ source water protection areas (SWPAs) In order of tons applied, major sources of sodium and chloride are: 1) seasonal loadings of deicing chemicals applied on 22.3 lane-miles of roads by the Massachusetts Department of Transportation (MassDOT); 2) seasonal loadings on 12.5 lane-miles of local roads by the Town of Wilmington; and 3) seasonal loadings to 40 acres of commercial parking lots. During the winter of 2010-2011, the salt load apportionment to Browns Crossing among these three sources was 69%, 21% and 10%, respectively, and totaled approximately 364 tons of chloride. For the winter of 2011-2012, which was less severe, the allocations were 53%, 24% and 23%, respectively, and totaled about 160 tons of chloride. Allocations for Barrows Wellfield (which has no commercial parking lots in its recharge area) were 64% and 36% and 131 tons chloride in 2010-2011 and 56% and 44% and 52 tons chloride in 2011-2012. These totals demonstrate how loads may vary from year to year due to variations in winter severity. Sodium and chloride concentrations in both wellfields are likely to increase over time unless significant reductions are implemented by these sources. Stormwater travel times in surface water flowing to the wellfields were determined from continuous monitoring of specific conductivity during the winter months. Stormwater transport from I-93 to the wellfields required an average 3.6 hours at Browns Crossing and 8.9 hours at the Barrows Wellfield.