GROUNDWATER MONITORING IN AN AQUIFER CONTAMINATED BY ROAD SALT

The town of Norwell in SE Massachusetts provides public water from groundwater serving over 10,000 residents. The main public water supply well field consists of two wells (NOR-1 and NOR-6) that can pump of up to 2 million gallons per day. An on-going water quality monitoring since the 1980's shows a rather consistent and steady increases in Na⁺ and Cl^- concentrations attributed to road salt applications during the winter months. The underlying aquifer, drained by Third Herring Brook (THB), contains a well travelled two-lane highway (four lanes total in both directions) with a partial clover leaf interchange, a large shopping mall, secondary roads with residential buildings, and a road salt storage shed. The objective of this study is to determine zones of the most influence on the deterioration of water quality at the public water well NOR-1. NOR-1 well is the most impacted well by road salt contamination that currently limits its drinking water supply potential.

Previous studies point to a highly heterogeneous aquifer structure, strong variation in road salt concentration in groundwater, as well as interdependence between annual seasons and the levels of dissolved road salt in groundwater. During the year of 2013 a set of 10 fully screened 2 inch monitoring wells was installed between NOR-1 well and the proximal THB stream to identify in 3D-time the mode of transport of elevated chloride plumes between the source zones and the well. Preliminary data shows a substantial range in Cl^-concentrations (variation range of over 300 mg/L) in both vertical and lateral directions. One source of interest is the pathway of elevated saline plume radiating from the road salt storage shed. Understanding the sources and transfer mechanism of contaminants to the pumps is important in future decision making and assuring good water quality for the residents.