Extent of a mine contamination plume on the Willow Creek floodplain, Creede, Colorado, as determined by willow leaf analysis

Abstract

Ground and surface water in and along the broad floodplain of Willow Creek below Creede, Colorado, are contaminated by drainage from various mine adits and waste rock piles above the town and by leachates from a gravel-capped tailings pile below. These waters have been sampled through a set of 18 monitoring wells and found to have elevated in metal concentrations, especially of zinc (Zn) and cadmium (Cd). Zinc is of most concern because of its known toxicity to freshwater fish (e.g., Beregeri and Patil, 1986; Farag et. al., 1999; Hilmy et. al., 1987). Moreover, the mouth of Willow Creek spills into the Rio Grande River, a prime trout fishery. At issue, then, is the impact of the water quality of Willow Creek as it enters the Rio Grande River. In an attempt to find a simple and cost-effective method to monitor contamination of surface and ground water in areas impacted by mining, we measured the content of 37 elements in willows (sandbar willow, Salix exigua, and one blue willow, Salix drummondiana), which grow abundantly in this study area. We collected leaf samples at 14 sites, mostly on the Willow Creek floodplain below the town of Creede, Colorado. Willow functions as surrogate water well and a groundwater quality sampler because its roots usually extend into the ground water region (Robinson, 1956). Willows have also been shown to accumulate far more Cd than do other shrubs and trees in mineralized areas. Because Cd associates closely with Zn in plant tissue, and willow is fairly common at the project site, willow proved to be an ideal plant for our study. The washed and dried leaf samples were macerated in a Wiley mill and analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) for 37 elements. Monitoring wells were located close to the willow sample sites at 5 of the 14 locations. However, groundwater samples were not collected simultaneously from these monitoring wells and thus no comparisons could be made between the two media. Data from leaf analysis revealed clearly that the willows were highly enriched in Zn and Cd, more than any other of the 37 elements determined. A few sites on the shoreline of the Rio Grande River upstream from its confluence of Willow Creek provided values that can be considered background, which ran about two orders of magnitude less than the maximum concentrations found in samples at the base of the capped tailings. A few willow samples previously collected and analyzed from an anomalous seep seven miles below Willow Creek yielded elevated concentrations of both Zn and Cd, but not nearly to the extent as those sampled along the Willow Creek floodplain. This phytogeochemical study provided a cost-effective method for assessing the extent of a leachate plume from generally non-point sources. Such a method may be useful as a preliminary sampling tool to guide the design of hydrogeochemical and geophysical studies.