This research sought to establish soil metal toxicity thresholds for numerous plant species that are commonly used in reclamation activities in the Rocky Mountains.  This information is currently not available and, as a result, ecological risk assessments must rely on toxicity thresholds established for agronomic species.  These crop plants have very different physiological characteristics and sensitivity levels than native species used in the reclamation of sites contaminated with metals.  As a result, risk assessors may classify sites as phytotoxic to native species and call for intensive remediation activities that may not be necessary.  The objective of this work was to provide a better estimate of soil metal toxicity thresholds for four metals and a large number of native plant species (and a few commonly used introduced species).  These threshold values will be used by those in the reclamation industry (government regulators and private entities) to more accurately assess risks associated with soil metal contamination, and to better match revegetation plant species to site conditions

We have used a series of greenhouse screening studies where seedlings of reclamation species and common agricultural species were grown in sand culture and exposed to supplemental concentrations of soluble metals.  We then determined six measures of toxicity: The 60-day LC50, 60-day EC50-plant, 60-day EC50-shoot, 60-day EC50-root, PT50-shoot, and the PT50-root.  We have completed experiments screening a variety of grasses, forbs and shrubs using zinc, copper, manganese and arsenic.  Many of these studies have been published and several more are being prepared for publication.  Results from these studies suggest that restoration species generally have higher metal tolerance than agronomic species reported in the past.  Thresholds determined by these studies should be more useful for risk assessors than those currently used, which are based largely on agronomic crops.  Our research has provided phytotoxicity thresholds for plants commonly used in remediation of hazardous waste sites in the Western U.S.  This information will be extremely valuable to ecological risk assessors and restoration ecologists involved in the characterization and restoration of metal-contaminated sites by providing information that is critical for making decisions about the degree of contamination and the extent of restoration required.