Uptake of Selenium and Other Contaminant Elements into Plants and Implications for Grazing Animals in Southeast Idaho

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Exploration and Environmental Geochemestry




James R. Hein



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As part of a series of geoenvironmental studies on the mobilization and fate of selenium (Se) and other potentially toxic trace elements in southeast Idaho phosphate mining areas, trace element concentrations (mg kg1 dry mass) in plant samples collected along transects at the Wooley Valley Unit 1,3, and 4 waste-rock dumps were compared with samples collected from undisturbed sites at Dairy Syncline, Deer Creek, Dry Valley, Maybe Canyon, and Rasmussen Ridge. Additionally, trace-element concentrations in veg- etation samples collected from wetlands associated with mine waste-rock piles were compared with samples collected from a single reference wetland. In undisturbed areas, Se in vegetation growing in soils overlying and derived from Phosphoria Formation phosphatic rocks tended to be higher than vegetation in undisturbed Wells Limestone or Rex Chert soils. Vegetation growing in highly disturbed soils, such as those comprising waste-rock dumps, had the highest tissue Se. Vegetation in a wetland at the base of Wooley Valley Unit 4 waste-rock dump accumulated decreasing concentrations of Se with increasing distance away from the waste-rock dump along the wetland flow path. Iron oxides were observed coating wetland sediment surfaces and helped control Se bioavailability. Plant uptake, as well as coprecipitation and sorption of Se by iron oxides, were key processes in the natural attenuation of Se in this wetland. Legumes at the rock dumps contained higher Se (mean = 80 mg kg-1) than trees (mean = 52 mgkg1), grasses (mean = 18 mgkg1), shrubs (mean = 6 mgkg1), and forbs (mean = 3 mg kg-1). However, grasses were among the highest Se accumulators among plant lifeforms in contaminated wetlands, with a mean value of 53 mg kg1 Se. In most places, uptake of Cd, Cr, Cu, Mn, Mo, Ni, and Zn was below critical high levels for plants. However, Se, Cd, Cr, and Zn uptake by some plants may have been large enough to affect their growth. Several plant lifeforms had Se concentrations that surpassed the acute and chronic toxicity thresholds for grazing livestock and wildlife, posing a lethal risk to these animals. Forages, particularly legumes, sampled from waste-rock dumps had increased Mo concentrations, resulting in Cu/Mo ratios below 1. High Mo (above 10 mg kg1) and Cu/Mo ratios below 2 may cause molybdenosis in ruminants. There were instances where tree Zn content exceeded upper chronic intake for livestock/wildlife. This may pose some concern for browsing animals feeding upon trees, particularly in winter months. Based on the vegetation survey, possible remediation strategies via physical, chemical, and biologi- cal manipulations of the contaminated sites include removal of the most contaminated soils, capping contaminated soils and revegetating capping materials, application of selec- tive herbicides to remove legumes from reclaimed waste-rock dumps, and fencing some contaminated areas to better manage grazing.