SALINIZATION OF WATER SUPPLY AQUIFERS: A CASE STUDY OF FISH BROOK CATCHMENT BASIN IN NE MASSACHUSETTS

Fish Brook catchment basin covers an area of 3.8 sq mi entirely enclosed within the perimeter of Town of Andover. Its drainage water is intercepted at the merging point with Merrimack River by a dam and the collected water is pumped back into Haggertts Pond for treatment and distributed for public water supply serving Towns of Andover and North Reading. The basin consists of large percentage of wetlands, protected lands, lesser amount of residential developments, and minor services. It is however criss-crossed by two major interstate highways along with two major cloverleaf interchanges.

Water quality data show a near constant value of 30 ppm of Na in finished water between 1975 and 1998 and a steady increase toward 60 ppm of Na in more recent years. The rise in Na is likely due to (1) general increase of road salt application rates during the last decade; and (2) the 1998 installation of a road salt storage shed constructed inside a cloverleaf loop at the interstate highway interchange. Water chemistry data for samples collected from within the basin on November 20, 2007 (over 6 months of no salt interval since the previous winter season) show a wide spread of specific conductance values: from 144 to 1320 microSiemens/cm. Water sample with 144 uS/cm was collected from seepage at the base of a drumlin with no maintained road in its proximity. By contrast seepage less than 50 ft from a maintained road yielded 1320 uS/cm. Specific conductance data strongly correlate with Na, Cl, Ca, and Mg, indicating that the principal source of contamination is from deicing chemicals applied during the previous five to ten winter seasons. .

Chloride concentrations exceed 250 mg/L secondary MCL for chloride in the lower third of the Fish Brook channel with corresponding Na values in the 120 to 130 ppm range. Stoichiometric ratios of Cl vs Na, Ca, and Mg suggest that chloride concentrations are supported by sodium in the 65 to 80% range, by calcium in the 20 to 30% range, and the rest by magnesium. Potassium concentrations are in the 2 to 4.5 ppm range, independent of all the other cations. Sodium chloride and calcium chloride are used on the interstate highways as deicing and anti-icing chemicals and their presence in the downstream waters shows a road salt build up within the lower half of the aquifer.