SODIUM CHLORIDE AS AN ENVIRONMENTAL TRACER IN A GLACIAL AQUIFER SYSTEM, EASTERN MASSACHUSETTS

A rapid and continuing increase over two decades in sodium and chloride levels in Natick and Wellesley Massachusetts drinking water supplies has been traced to a road salt storage facility near the headwaters of Bogle Brook located along the Massachusetts turnpike. We delineated the path of the road salt contaminated water from the salt storage facility, through surface wetlands continuing as a surface stream through two separate ponds, gradually infiltrating ground water and the drinking water supply wells 6.5 km downstream. Geochemical analyses of frequently collected water samples from several locations along the contaminant path show variation patterns on a detailed enough time scale that allows estimates of streamflow rates in the surface system, and may yield improved understanding of infiltration rates and ground water flow velocities and better estimates of remediation time scales.

Bogle Brook headwaters are about 4 kilometers above Morses Pond, in an area of wetlands and small ephemeral streams. November headwater stream measurements at low flow (base level) show 41-49 μg/mL sodium, increased to 187 μg/mL within hours after de-icing salt was applied on nearby streets, (some with outfalls into the stream) then dropped back to 52 μg/mL by the next day. Headwater tributaries varied from 12 to 69 μg/mL sodium, except for one at 436 μg/mL associated with the salt storage. Wetlands associated with this salt storage plume can reach up to 2300 μg/mL of sodium.

Two adjoining subbasins provide useful comparisons 4 km downstream; they show peak sodium within two days associated with the street deicing event, while Bogle Brook peaks eight days later. We postulate that storm flow taps the salt reservoir in the wetlands, generating a pulse of sodium, suggesting a mean streamflow rate of 0.6 cm/sec (520 m/day).