During the performance of an ecological risk assessment, it is often unclear whether observed impairments are due to the contaminants of concern, to other contaminants or to other factors such as habitat disruption. In 2000, the U.S. Environmental Protection Agency developed a methodology for determining the causes of biological impairments termed Stressor Identification (SI), and the utility of the methodology is being demonstrated in the context of case studies for the Clean Water Act. The Agency has not applied this methodology to a waste site assessment. The purpose of this project was to apply the SI process at a terrestrial contaminated site to shed light on its utility in such an environment. The site chosen is in a highly mineralized area of the Colorado Rocky Mountains, consisting of the 500-year floodplain and adjacent irrigated lands of the Upper Arkansas River from the confluence of California Gulch to approximately 11 miles downstream. Impairments evaluated were barren areas in the floodplain (reduced vegetation), and reduced plant growth and plant species richness in meadows irrigated with water from the Upper Arkansas River. After a number of candidate causes were considered, the various lines of evidence support the interaction of elevated levels of extrinsic metal with decreased pH as the cause of the barren areas in the floodplain.