By Sid Perkins
From San Francisco, at a meeting of the American Geophysical Union
In many arid regions, environmentally conscious gardeners who want to conserve water eschew lush lawns and instead grow indigenous drought-tolerant plants amid arrangements of ornamental rocks. Now, chemical analyses suggest that some such landscaping choices aren’t doing the environment any favors.
Landscaping with crushed rock is popular in Las Vegas and many other cities in the southwestern United States. After the plants in such tableaus are watered, evaporation of soil moisture sometimes creates colorful crusts of salts on the surface of the ground, says Stephanie A. Mrozek of the University of Nevada, Las Vegas.
Mrozek and her colleagues analyzed the minerals in these crusts at three sites in Las Vegas—one public park and two elementary schools—and discovered elevated concentrations of zinc, molybdenum, and copper. At one of the schools, the concentration of dissolved copper in a puddle measured more than 5.4 parts per million, or more than four times the concentration that the Environmental Protection Agency deems safe for drinking water. Although no one is expecting people to drink from such puddles, the measurements indicate that undesirable trace elements are getting into the environment, Mrozek says.
The copper and other trace elements found in the salt crusts don’t normally appear in the area’s soils. However, their relative concentrations in the crusts are about the same as the ratios found in the decorative rock itself, which for these three sites came from a quarry in a mining district of Arizona.
The trace metals in the crusts probably are leaching from the crushed rocks because of their pyrite minerals, the same compounds that generate stream-tainting acidic drainage from mines (SN: 11/15/03, p. 315: Attack of the Rock-Eating Microbes!). Because most of the rocks are gravel size or smaller, they collectively have a large surface area over which the acid-making and rock-dissolving reactions can take place, says Mrozek. Investigations at other sites in Las Vegas suggest that the salt crusts don’t form in landscaped areas if the crushed rock that’s used is free of pyrites.
Gardeners maintaining the sites that the researchers studied often unwittingly exacerbate environmental problems by using rakes or other tools to break up the unsightly salt crusts, says Mrozek. Those actions probably let loose mineral-tainted dust to be blown about by winds and inhaled by the gardeners and others. Copper chloride, one of the major components of the salt crusts, is known to be a respiratory irritant.
Currently, no state or federal environmental regulation governs the mineral composition of decorative rocks, says Mrozek.
