Comment pubs.acs.org/est
Salt: The Final Frontier
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Valley of California proved that natural dissolved ions of selenium and arsenic from irrigation supplies could be increased dramatically by evapotranspiration of water which drained to the Reservoir. It is a grizzly death for wading birds and fish that die from chronic arsenic or selenium poisoning. Irrigated soils may also accumulate the toxic salts through decades of evapotranspiration. Too much of world’s arable land is already ruined by too much salt. A related problem exists for reverse osmosis treatment processes which generate brine as drinking water is purified or desalinated. At high pressures, these units operate much like the kidneys of humans, purifying the treated water while generating a small(er) volume of concentrated saltwater. What should we do with this salt? If your community is land-locked like my native Iowa, it is nearly impossible to obtain a permit to discharge the brine into a freshwater receiving stream. You could inject it to a deep underground geologic unit that does not serve as an aquifer, but it is expensive and it is difficult to obtain the permits as well. I’ve heard it said that when humans are gone from the earth, cockroaches will remain and they will be faced with an awful lot of salt. Perhaps the “T” in the title of ES&T should be renamed with another “S”S for Solutions. Please send us your research papers to solve these problems with salt− one of our greatest long-term challenges on the final frontier.
hen you live life well, you leave behind a difficult legacy... salt. It comes from municipal and industrial discharges, road salt, water softeners, landfill leachate, runoff, and virtually every activity we undertake. For an average person in the U.S. it sums to about 1 kg per day, or 28 t in a lifetime. When Lot’s wife looked over her shoulder to Sodom and Gomorrah in the Bible, she was transformed into a pillar of salt. Based on modern-day emission factors, it must have been a very big pillar. But is not salt a vital nutrient required by all? It is true that we have a physiological requirement for both sodium and chloride. We need about 5 g of salt per day, but 100 g taken suddenly might kill you. Like Paracelsus remarked in the 16th century, “It’s the dose that determines the poison”. A little salt is beneficiala lot can be overkill. My advisor Donald J. O’Connor, a renowned mathematical modeler and one of the most intuitive people I have ever known, taught me a terse lesson about salt back in the day (1970s). “See how the chloride concentration is building in the Great Lakes?” he asked. I distinctly remember thinking, “Why in the world is he working on (boring) chloride? Shouldn’t he devote his considerable talents to something really important, like modeling pesticides in the aquatic food chain that threaten to kill us?” Of course, O’Connor was schooling me (with some professorial care) that chloride ions are extremely important mobile tracers of human activity, and they have been accumulating in the environment ever since human settlement began. They are conservative and resistant to degradation they do not go away. His model demonstrated precise contributions in the Great Lakes Basin from municipal wastewater treatment plants, industries, and road salt generously distributed on icy roads. O’Connor’s human salt emission factors remain fairly accurate to this day. There you have it: HUMANS CAN BE TRACKED QUITE ACCURATELY BY THE AMOUNT OF SALT THEY ADD TO RECEIVING WATERS EACH DAY. We do not need a census to count the number of people in a communitysimply measure the chloride concentration in the water. So what? Big deal. Sodium chloride is not toxic at concentrations normally found in fresh water; there is no maximum contaminant level (MCL) determined by EPA, and we are not (in general) required to remove it from either wastewater or drinking water. It is not hurting us, so why worry? First of all, limnologists know that even small changes in total dissolved solids (TDS) may cause shifts in species composition of algae, zooplankton, and benthic communities. And, eventually, surface waters and groundwater aquifers could become too salty to use for drinking supplies. But society’s “salt problem” also stems from a difficulty in treating “blow-downs” such as agricultural irrigation return water and reverse osmosis (RO) reject waters. In both cases, it is nearly impossible to dispose responsibly of the “left-over” salts from these practices. For example, the infamous Kesterson Reservoir in the Central © 2013 American Chemical Society
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Jerald L. Schnoor, Editor-in-Chief AUTHOR INFORMATION
Corresponding Author
[email protected]. Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS. The authors declare no competing financial interest.
Published: February 13, 2013 2152
dx.doi.org/10.1021/es4004312 | Environ. Sci. Technol. 2013, 47, 2152−2152
