groundwater at each test site. Highway-drainage monitoring stations were installed within the drainage systems to measure continuous records of stage and specific conductance of the runoff. Samples of runoff are analyzed for concentrations of dissolved sodium, calcium, and chloride. Relations between stage and discharge and between specific conductance and chloride concentration are used to determine the amount of chloride discharged through each monitoring station.

MHD monitors the application of road salt to the highway. Department records indicate the amount of road salt applied to the entire 11-km section of Route 25 along which the test sites are located and are not specific to each test site. Therefore, it is assumed that road-salt application is equal at all test sites.

The four types of highway drainage system incorporated into the construction of Route 25 each represent a different method of control of runoff from the highway surfaces, shoulders, and median strip. Test sites designated A, B, C, and D, in order of increasing highway runoff control, are representative of each drainage system (Figure 1). Test Sites A and B represent standard drainage designs, whereas Test Sites C and D represent new, untested designs.

The roadway surface at all test sites is crowned to allow highway runoff to flow toward the highway shoulders and the median strip. The top 2.5 cm of the highway pavement is composed of an open-graded friction coarse bituminous concrete, referred to as “popcorn pavement, ” to limit ponding of water on the highway surface. This permeable layer is underlain by a 20-cm-thick layer of consolidated asphalt estimated to be at least 95 percent impervious (L.C. Stevens, Jr., MHD, personal communication, 1990). Rainfall and salt-laden water from melting snow and ice easily penetrate the 2.5-cm-thick popcorn pavement and then flow laterally to the edges of the roadways. The extent to which the water flows onto the shoulders or the median strip is controlled by the drainage systems, as described later. The quantity of salt-laden water percolating through the 20-cm-thick consolidated asphalt is assumed to be small and uniform at all test sites.

Site A represents an open drainage system, where local groundwater is unprotected from contamination by road salt (Figure 2). Runoff collected in catch basins on the pavement surface and drop inlets in the median strip is discharged lo-

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