MAPPING ROAD-SALT CONTAMINATED GROUND-WATER DISCHARGE IN A WATERSHED IN SOUTHERN NEW HAMPSHIRE

The use of salt for road deicing has caused increases in chloride concentrations in ground- and surface-water resources. Concentrations of chloride in excess of water-quality standards have been measured in watersheds adjacent to interstate highway 93 (I-93) in southern New Hampshire (NH). A proposed widening plan for I-93 has raised concerns over potential increases in chloride. To formulate a load reduction plan, the NH Departments of Environmental Services and Transportation are working to identify sources of chloride and develop an understanding of chloride fate and transport in the hydrologic system, as part of a total maximum daily load study.

As part of this effort, road-salt contaminated ground-water discharge was mapped in the fall of 2006 to identify potential sources of chloride during base-flow conditions in Policy Brook. This brook drains a small watershed (10.2 square miles) of mixed land use (wooded, residential, commercial, and highway) with multiple sources of salt contamination. The area is underlain by a thin (20 feet) surficial sand layer and basal till, which are underlain by fractured crystalline bedrock.

Streamflow in Policy Brook ranged from 0.59 to 515 cubic feet per second from February to November of 2006. Measurements of specific conductance showed high levels (1.3 millimhos/centimeter (mmhos/cm)), about 5 times background, during periods of base flow, indicating a ground-water source of road salt.

Electromagnetic (EM) terrain induction conductivity surveys were performed along the longitudinal axis of Policy Brook to facilitate mapping of road-salt contaminated ground-water discharge. Three different EM tools were used that probed slightly different depths below the streambed (ranging from 0 to 12 feet). Results from the three tools showed good agreement and identified several reaches where high conductivity ground water may have been discharging. EM conductivity values ranged from 10 (typical background value for sand) to 300 mmhos/meter in the streambed of Policy Brook. Based on the delineation of high EM values, seven streambed piezometers were installed to sample streambed pore water. Specific conductance in the pore water ranged from 0.64 to 26 mmhos/cm. Locations with high specific conductance in streambed pore water matched well with locations with high EM-conductivity values.