POINT VERSUS LINE SOURCE FOR ROAD SALT CONTAMINATION OF SURFACE WATERS

The Massachusetts Turnpike (I-90) passes through suburban areas west of Boston where small stream basins cross or are adjacent to the highway. Some of these basins contribute to ponds with nearby public water supply well fields drawing increasingly chloride and sodium enriched water. Sodium has increased from 15 mg/L to 85 mg/L in the last two decades, and some production wells have been turned off except during periods of high demand in the summer. Significant salt contamination has been traced by analysis of surface waters to the Turnpike.

Twelve sample locations were chosen in six pairs along streams up and downstream from the Turnpike. Recent sampling includes samples taken at low flow, approximating baseflow. To measure the salt contamination, chloride was chosen from the cation and anion analyses as a conservative element little affected by interaction with sediment. Enrichment of chloride was typically 2.5 times from upstream to downstream when choosing catchments crossing the Turnpike compared to those not crossing. Upstream chloride ranged from 115-160 mg/L, while downstream had 268 to 490 mg/L. Other potential factors including catchment size, total road length, road density, and distance from sampling point to the Turnpike had little if any correlation to salt content. As the Massachusetts drinking water secondary maximum contaminant level is 250 mg/L chloride, the drinking water for several public supplies is increasingly at risk.

One location showed an increase of more than 8 times (115 mg/L chloride to 949 mg/L for an isolated groundwater-fed pond on the down-gradient side of the Turnpike), which may be caused by either exit/entrance ramps adjacent to the site, or the salt storage facility for the Massachusetts Department of Transportation on the opposite side.