PETER E. CHURCH AND PAUL J. FRIESZ
A study to determine the relative effectiveness of four highway drainage designs in preventing the contamination of groundwater by road salt is being conducted by the U.S. Geological Survey in cooperation with the Massachusetts Highway Department and FHWA. Four test sites, each representing a specific highway drainage design, are located along a 5-km section of Route 25 in southeastern Massachusetts. The drainage designs being tested are open drainage, closed drainage, closed drainage with snow berm, and full-snow-berm drainage. Preliminary comparisons of the effectiveness of the highway drainage systems are based on computations of chloride loads from road salt in groundwater and chloride loads from road salt discharged through the highway-drainage monitoring stations at each test site. A comparison of monthly chloride loads from November 1990 through May 1992 shows that chloride loads in groundwater at the closed drainage site, the closed drainage site with snow berm, and the full-snowberm site are about 40, 50, and 20 percent, respectively, of the chloride load in groundwater 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 site with snow berm. Evaluation of the effectiveness of these drainage systems will be refined as additional data are collected and analyzed. Results from this study should also be applicable to the transport of other conservative chemical constituents in highway runoff.
Road-salt contamination of public and private water supplies has become a serious and costly problem, particularly in the northeast and midwest United States. For example, the Massachusetts Highway Department (MHD) received complaints of road-salt contamination from 100 of the 341 municipalities in the state from 1983 through 1990. MHD spent about $1.2 million to investigate and remediate road-salt contamination complaints during this period (1). Nationally, state and local governments spend about $10 million each year to prevent and remediate problems of road-salt contamination (2).
One method that state highway agencies use to reduce road-salt contamination of public water supplies is that of diverting highway runoff from sections of highway that pass near public supplies to less sensitive areas. Four types of highway drainage systems were incorporated into the design of an 11-km, sixlane section of Route 25 in southeastern Massachusetts completed in 1987. Three of these drainage systems —two of which are new, untested designs—divert highway runoff away from adjacent public water supplies. The methods by which the diverted highway runoff is collected (and, correspondingly, the cost of highway construction) differ between drainage systems. The most expensive, and potentially the most effective, drainage system added about $1.6 million/km to construction costs for that section of highway (2). However, the relative effectiveness of the individual drainage systems in preventing road-salt contamination of the public water supplies is not yet known.
An investigation of the relative effectiveness of these highway drainage designs in preventing groundwater from being contaminated by road salt is being conducted by the Water Resources Division of the U.S. Geological Survey (USGS) in cooperation with MHD and FHWA, U.S. Department of Transportation. Four test sites, each representing one of the highway drainage designs, are located along a 5-km section of Route 25 (Figure 1). Data collection for analysis and comparison of the effectiveness of the drainage designs began in November 1990 and is planned to continue through December 1995.
The purpose of this paper is to describe the highway drainage systems being tested, discuss the general hydrogeology of the study area, define the methodologies by which the drainage systems are being evaluated, and present preliminary findings of the effectiveness of the highway drainage systems in preventing road-salt contamination of groundwater. These findings are based on computations of chloride loads from road salt in groundwater and chloride loads from road salt discharged through the highway drainage systems from November 1990 through May 1992.
Networks of observation wells were installed at each test site to enable the comparison of water samples collected from wells upgradient (background) and downgradient (potentially contaminated) from Route 25. Water samples from the wells are analyzed for concentrations of dissolved sodium, calcium, and chloride to determine the amount of road salt entering
U.S. Geological Survey, Water Resources Division, Massachusetts– Rhode Island District, 28 Lord Road, Suite 280, Marlborough, Mass. 01752.