Decorative landscaping rock as a source for heavy metal contamination, Las Vegas, Nevada

Recent drought coupled with population growth throughout the southwestern United States has increased the demand for water-efficient landscaping alternatives. As a result, xeriscaping has become a popular approach to landscaping in arid climates. Currently, no regulations control the mineralogy of decorative rocks that are used in these applications. Eight public sites were examined in Las Vegas, NV, where green and white salt crusts appeared on recently emplaced decorative rocks. The landscaping rocks, underlying soil, surface salt crusts, irrigation water runoff, and plants were analyzed for mineralogy and/or chemistry. Pyrite and high levels of copper were identified in the decorative rock. Acid-rock drainage caused by pyrite oxidation has concentrated copper in salt efflorescences, and As, Cu, Mo, Pb, and Cr in plant tissues. These metals are not known to occur naturally in Las Vegas Valley soils, and our results indicate that their source is the landscaping aggregate. These highly soluble salt crusts have the potential to contaminate the environment, and increase the risk of human exposure. Additionally, expenses resulting from plant mortality and infrastructure damage due to increased sulfate production are undesirable. This research supports the development of regulations preventing the use of sulfide-bearing rocks in future landscaping applications.