UNLV researchers have found that some decorative landscaping rock sold in Las Vegas contain high levels of sulfide minerals, which could lead to heavy metal contamination of the environment and possibly become a health hazard for humans.
A UNLV study directed by Brenda Buck, associate professor of geosciences, determined that high levels of heavy metals were concentrated in surface salt crusts, plant tissues, and water runoff that emanated from a particular variety of newly emplaced landscaping rock at three public sites in Las Vegas. Additionally, the salt crust contained copper sulfate, a known hazardous substance. However, this initial UNLV study did not include a risk analysis to measure to what degree this problem could impact human or environmental health.
"As xeriscaping becomes an increasingly popular and important way to conserve water in our arid environment, we need to ensure that the decorative rock we use does not endanger our environment or public health," said Buck. "Prior to its sale, decorative landscaping rock should undergo analyses by trained geologists to determine if any potential hazards to the public exist."
Additionally, Buck says the research strongly supports the development of regulations that would prevent the use of sulfide-bearing rock for landscaping purposes. Currently, there are no regulations in Nevada that control sulfide concentration in landscaping rock.
According to Buck, the metals present in most landscaping rock are harmless in our region's highly alkaline soil; however, landscaping rock that contains sulfide minerals - namely pyrite - creates strong acids, causing those previously inert metals to become mobile.
In arid climates such as Las Vegas, the only visual indication is a green, yellow, or white salt crust after the rock is emplaced .The study also found that the acidic conditions created by the minerals in the rock were almost completely neutralized after three months.
In other parts of the United States, extensive drainage of such metals - including copper, lead, arsenic and zinc - has lead to severe environmental contamination and numerous Superfund site listings. Previous research has also shown that surface salt crusts with high concentrations of metals could also pose a human health hazard through direct skin contact, dust inhalation and drinking water contamination.
Other problems associated with sulfide-bearing landscaping rock include increased plant mortality and infrastructure damage, both of which could result in significant economic costs.
The research samples were collected in 2004 from three public sites in Las Vegas, including Del Sol High School, Canyon Spring High School, and Ed Fountain Park. As a result of the research, the Clark County School District established specifications that limit sulfide concentration in landscaping projects.
The landscaping rock that was studied was quarried from an area near former copper and molybdenum mines in Chloride, Ariz. However, this type of mineral assemblage is not exclusive to quarries sited near former mines. In fact, many rock formations contain sulfide minerals and metals at levels that are not economically feasible for mining, yet are high enough to cause significant environmental contamination. It is not known how many such quarries are producing landscaping aggregate with a similar mineral composition.
The research was funded by a National Science Foundation EPSCoR grant, a UNLV undergraduate research award, the Clark County School District, and Kalamazoo Incorporated. The study will be published in the September issue of the peer-reviewed international journal "Soil and Sediment Contamination.
