Test sites are on a coastal outwash plain bounded to the north and west by till and bedrock hills and to the south and east by the saltwater bays of the Atlantic Ocean. The study area is directly underlain by a layer of fine to coarse sand with gravel that varies in thickness from 9 to 27 m. Fine to coarse sand with silt is present below this layer. Depth to the water table below the highway ranges from about 6 m at Site B to 18 m at Site A, and annual water-table fluctuations are less than 1.5 m. Estimated hydraulic conductivity of the aquifer ranges from 30.5 m/day at Site C to 67 m/day at Site A. Background concentrations of sodium, calcium, and chloride—the primary constituents of road salt—generally range from 5 to 10 mg/L, 1 to 5 mg/L, and 5 to 20 mg/L, respectively, in groundwater.

Groundwater samples are collected from clusters of wells with 1.5-m-long screens upgradient and downgradient from the highway. The monthly mass flux of road-salt chloride is computed by use of chloride concentrations, water-table gradients, and hydraulic conductivities at each test site. Highway-drainage monitoring stations at Sites B, C, and D consist of Palmer-Bowlus flumes within trunkline drainage pipes from which stage and specific conductance are continuously monitored and recorded. Monthly chloride loads discharged through the highway drainage systems are computed by use of relations between stage and discharge and between specific conductance and chloride concentration.

A comparison of accumulated monthly chloride loads in groundwater from November 1990 through May 1992 shows that chloride loads at the closed drainage site, the closed drainage with snow berm site, and the full-snow-berm drainage site are about 40, 50, and 20 percent, respectively, of the chloride load at the open drainage site. The chloride load discharged through the full-snow-berm drainage site, and thus prevented from entering groundwater, is twice that discharged from the closed drainage site and from the closed drainage with snow berm site.

These preliminary findings show that the effectiveness of the highway drainage systems in preventing road-salt contamination of groundwater varies widely. However, it is premature to use these data for quantitative evaluation of the effectiveness of the drainage systems. The 1992 chloride load data through May at each site do not represent the entire road-salt plumes developed from salt applied during winter 1991–1992, and they do not represent the total discharge of road salt through the highway-drainage monitoring stations. Additionally, difficulties in matching the leading and trailing edges of annual road-salt plumes in groundwater with road salt applied in the same winter at each test site introduces some uncertainties in the analysis that can be reduced by collection of more data. Data collection is planned to continue through 1995 to reduce uncertainties that might occur in the analysis of the effectiveness of the highway drainage systems.

This paper was prepared in cooperation with MHD and FHWA. The authors thank the Research and Materials Section and the District 5, Wareham Office of MHD for providing ongoing logistical support during this study, including drilling and well installation, surveying, and recording of road-salt application. The authors also thank those in the MHD Research and Materials Section and FHWA for their comments and suggestions in the development of the study approach.

1. Pollock, S. J.Remediating Highway Deicing Salt Contamination of Public and Private Water Supplies in Massachusetts. Proc., Focus Conference on Eastern Regional Groundwater Issues , National Water Well Association, Portland, Maine, 1991, pp. 347–368.

2. Special Report 235: Highway Deicing: Comparing Salt and Calcium Magnesium Acetate. TRB, National Research Council, Washington D.C., 1991, pp. 108–110.

3. Kilpatrick, F. A., W. R. Kaehrle, J. Hardee, E. H. Cordes, and M. N. Landers. Development and Testing of Highway Storm-Sewer Flow Measurement and Recording System Water-Resources Investigation Report 85-4111. U.S. Geological Survey, U.S. Department of the Interior, 1985.

4. Palmer, H. K., and F. D. Bowlus. Adaptation of Venturi Flumes to Flow Measurements in Conduits. Transactions, American Society of Civil Engineers , Vol. 101, 1936, pp. 1195–1216.

5. Kilpatrick, F. A, and V. R. Schneider. Use of Flumes in Measuring Discharge. Techniques of Water-Resources Investigations, Book 3 . U.S. Geological Survey, U.S. Department of the Interior, 1983, Ch. A14.

Effectiveness of Highway Drainage Systems in Preventing Road-Salt Contamination of Groundwater: Preliminary Findings (1-12)