Pick up a shovel & put away the road salt
Many studies during the past 35 years have linked increased salt concentrations in drinking water with highway salting operations. New chlorides sprayed on roads to reduce the need for rock salt also are being questioned. Taste is one objection to salt in drinking water; a more serious concern is its impact on health. Recently, considerable attention has been focused on the effects of salt on hypertension, or high blood pressure. The U.S. EPA, World Health Organization and private healthcare organizations calls for moderate salt restriction to prevent hypertension have made people more aware of sources of salt in the diet, including drinking water. Accordingly, salt’s relevance to health and road salt’s impact on drinking water have led to increased measures being taken to mitigate drinking water contamination by road salt.
A Variable Contaminant
Sodium concentrations in drinking water are variable. For example, a study conducted by the EPA in the 1970s found that concentrations in tap water varied from 1 to 90 mg/L. Likewise, in a survey of 354 public water systems conducted by the New York State Department of Public Health, sodium concentrations ranged from 5 to 20 mg/L in 265 communities to as high as 220 mg/L in two communities. A 1977 analysis of nationwide tap water reported a mean sodium concentration of 28 mg/L (Craun et al. 1977).
Among the reasons for the wide variability is that sodium can reach water supplies through a number of sources, including seawater, natural salt deposits, oil field brines, sewage, industrial waste and agricultural chemicals, as well as road salt. Water in arid and semi-arid regions naturally is high in soluble salts compared with regions where rainfall is abundant. Also, water treatment frequently is a source of sodium in drinking water. Sodium-based compounds often added for treatment include sodium fluoride during fluoridation, sodium hypochlorite during chlorination, sodium carbonate for corrosion control, and sodium carbonate or sodium hydroxide for pH adjustment.
Although there is not a standard for primary drinking water, EPA has listed 500 ppm as a secondary standard. In diet booklets for patients restricted to 500 to 1,000 mg of sodium per day, the American Heart Assn. (AHA) recommends that distilled water be used if drinking water contains more than 20 mg/L. This recommendation is based on the assumption that people use about 2 liters of water daily for drinking and cooking, and that water does not contribute more than 10% of sodium consumed.
The federal Safe Drinking Water Act requires EPA to periodically update drinking water quality criteria. The criteria is announced through Federal Register notices for explanation of rationale and for public comment. EPA does not regulate sodium because of the normally minor contribution of drinking water to daily sodium intake. As a general guidance level, EPA recommends that sodium concentrations in drinking water not exceed 20 mg/L for the higher-risk population, on the basis of dietary recommendations by AHA. Currently, EPA requires that all public water systems monitor sodium levels and report levels greater than 20 mg/L to local health authorities so that physicians treating people on sodium-restricted diets can advise patients accordingly. Concentrations more than 20 ppm often can be tasted.
Road salt can enter water supplies by percolation through soil into groundwater, or runoff and drain directly into private wells and reservoirs. Upon entering fast-flowing streams and larger rivers and lakes, salt runoff usually has little effect because concentrations quickly are diluted. Road salt infiltration is more common for groundwater-based supplies, such as wells, springs and reservoirs that are recharged primarily by groundwater. Road salt can reach groundwater in several ways. For example, many highways, especially in rural areas, use open drainage systems, where highway runoff is not collected and diverted by gutters and catch basins, but instead flows off the pavement into unlined ditches and roadside soil. In addition, road salt applied during snowstorms often mixes with snow that is completely plowed off the roadway and paved shoulder. When the snowbanks melt, the meltwater and dissolved salt can migrate through the soil and move to the water table.
In colder regions where large snowbanks accumulate during the winter, spring thaws can produce high concentrations of salt. For example, meltwater samples near several large snowbanks in Wisconsin contained sodium concentrations exceeding 1,000 mg/L. Road salt’s ability to reach groundwater depends on a number of factors, including the frequency of precipitation and the texture and drainage characteristics of roadside soil. Only a portion of the sodium ever reaches the groundwater. Moreover, whether the saltier groundwater beneath the highway eventually will migrate into nearby wells depends on several factors, including the depth of the well and its distance from the highway, the permeability of the aquifer material, and the direction and rate of groundwater flow. Wells most likely to be affected are those within 100 ft from a roadway in the direction of groundwater movement.
In response to resident complaints, the state of Wisconsin has instituted a reduced-salting program along this section of highway. Straight salt was replaced by sand and a mixture of salt and calcium chloride. The state believes that this program has succeeded in both identifying and diminishing the effect of salt runoff on roadside wells. Four years after the program began, average sodium concentrations in the wells dropped by more than 50%, from 79 to 36 mg/L. The experience in Wisconsin indicates that highway runoff can elevate sodium levels in water supplies.
Because so few states routinely monitor the effect of road salt on water supplies, it is difficult to characterize the impact on nationwide salt runoff. In the survey of state highway agencies conducted for this study, seven other states reported that they periodically investigate and monitor runoff in water supplies. Of these states, the most active are New Hampshire, Connecticut and New York. Altogether, 16 states reported receiving complaints related to road salt contamination in drinking water during the past 10 years. The number ranged from one or two per year in Maryland, West Virginia and Ohio to several dozen per year in the New England states. As might be expected, most states that reported complaints are located in the Northeast and Great Lakes regions, where population densities are highest and road salt use is heaviest.
Mitigation efforts have reduced the issues. However, there are still high numbers of private and public wells with increased chlorides. This results in a need for a guaranteed solution, which is remediation at the point of use.