DEICING SALTS, SALT-TOLERANT VEGETATION AND CALCIUM SULFATE

A study of deicing salts, salt-tolerant vegetation, and calcium sulfate was undertaken as part of the Massachusetts Highway Department Research Program. The objectives of this research were (1) to conduct a literature search to characterize chemical processes and subsequent damage to vegetation from airborne and soil-borne deicing salts, (2) to characterize roadside conditions in Massachusetts by assessment of damage to trees, shrubs, and grasses along highways, (3) to conduct a survey of cold-region highway departments for methods and specifications of mitigating salt damage along highways, (4) to prepare specifications and methodology for ameliorative practices and recommendations for evaluation of salt-tolerant plants. The research process began in January 2000 with research continuing in the spring, summer, and fall of 2000. The literature search and survey of highway departments showed that sodium chloride (NaCl) is the most common deicing agent used. In Massachusetts, an average of 240 lb of NaCl is used per lane mile in multiple applications per year, an amount that is common with other agencies (280 lb per lane mile). The total amount of NaCl used in Massachusetts is about 290,000 tons per year. Highway departments reported that salt damage occurred commonly to roadside vegetation within about 50 ft of the pavement. Dieback, defoliation, and abnormal branching were identified in the literature and in the survey as symptoms of salt injury to roadside plants. Aerial spray was a major means of transmission of salts to plants. Evergreen, coniferous trees were reported to suffer more damage than deciduous trees and damage was on the side of trees facing the road. Analysis for sodium (Na) indicated that the evergreen trees accumulated more foliar Na than deciduous trees and that the accumulation diminished with distance from the highway. Na concentration in soil also diminished with distance from the highway. It was not determined whether airborne delivery or soil-borne delivery of salt was the more ruinous process. The review of literature identified salt-tolerant grasses and woody ornamentals for roadside planting. Nurseries in New England were surveyed for the availability of the woody plant materials, and a list of available plants by nursery was prepared along with a list of vendors for grasses. Experiments were designed to test ameliorative practices and salt-tolerant plants along medians and at intersections of highways as considerations for future research. The survey suggested that ameliorative practices were not used commonly by other agencies in cold regions and hence were untested. Highway departments in cold-weather regions generally do not monitor salt damage to vegetation and have not evaluated practices to ameliorate damage. Vegetation differs considerably in tolerance to salt, but much plant materials have not been tested in roadside conditions. Future research could address landscape design in planting patterns and use of salt-tolerant plants and, in severely affected areas, the use of ameliorating agents to lessen salt damage to roadside plants.