Building on a growing body of research, the U.S. Geological Survey recently reported that 40% of U.S. streams in and around northern cities have damaging concentrations of chloride -- a component of salt -- most likely from de-icing roads in the winter. Urban streams carried a staggering 88 tons of chloride per square mile of watershed land, compared to just six tons per square mile in a forested watershed.
That 14-fold difference matters. Elevated chloride can inhibit plant growth, impair reproduction and reduce the diversity of organisms in streams, according to the USGS. Chloride can also infiltrate drinking water supplies, particularly wells near highways, though USGS said that kind of contamination was less common, with fewer than 2% of drinking water wells sampled showing pollution. (Just as with streams, chloride in groundwater was much more common in urban areas -- with 16% greater contamination levels detected; urban areas are much less likely to draw drinking water from individual wells, though.) Salt in drinking water is of particular concern for those on low-sodium diets, due to cardiovascular disease and other health problems, because it can represent a hidden source of sodium in one's diet.
The study looked at 1,329 wells and 100 streams in 19 states.
Past research, in New York and elsewhere, has shown that it is not only urban streams that are being compromised by road salt. Over time, the burden of salt is degrading the quality of suburban streams and even pristine rural streams in such places as the Hudson Valley, according to research by the Cary Institute of Ecosystem Studies, among others. The new USGS study found that even in agricultural areas, 4% of streams exceeded federal clean water guidelines for salt.
The correlation of pollution to road de-icing is strong, with the highest levels of chloride contamination recorded in winter, when readings as high as 20-times the recommended clean water level were measured in some streams. Concentrations are also increasing over time, as salting continues year after year.
In addition to de-icing, though, other sources of chloride in water include: salt storage for deicing roads, septic systems, wastewater treatment facilities, water softening, animal waste, fertilizers, discharge from landfills, natural sources of salt and brine in geologic deposits, and from natural and human sources in precipitation.
The study is another reminder that salt is often overused, and that overuse can have a significant effect on the local environment. Remember these facts for yourself, if you're removing ice from a walkway or driveway, and talk to your local highway department about whether they are taking recommended steps to reduce over-salting; doing so not only helps preserve water quality, but saves taxpayers the expense of buying too much salt.
Salt is effective only before snow and ice falls. By creating a salty layer of moisture that resists freezing, it prevents snow and ice from bonding with pavement, walkways or sidewalks. Use only as much as needed (usually much less than you think). Throwing salt down after snow and ice has fallen does little. The best remedy at that point comes in the form of a good shovel and/or some sand to aid traction, like course sand.
This pollution is a classic example of "nonpoint source" pollution. The Clean Water Act has, over 35 years, been remarkably successful at cleaning up "point sources" of pollution sewage and industrial pipes that discharged contaminants directly to streams. "Nonpoint source" pollution, on the other hand, originates from multiple sources spread over a large area (another example includes the use of pesticides and fertilizers on residential lawns). It has proved a difficult problem to solve, and many formerly pristine streams have become degraded in recent years because of an increase in nonpoint source pollution, particularly in rural areas where suburbs are spreading. It is often referred to as "people pollution" since it is the actions of individuals that will make the difference between keeping streams clean or not.
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