In the year 2000 4.1% of the world’s production of phosphate came from the ore mined in the Idaho and Utah region.  Over 15% of the current US supply is from Idaho; this amount is expected to increase as the Florida deposits decrease and the one-hundred year supply in Idaho gains in importance.  Idaho is the Nation’s only producer of elemental phosphorus, and is thus designated a strategic National resource.  The primary current mining activities occur on public lands in the Caribou National Forest; however, historical mining has occurred in nearby BLM, Shoshone-Bannock tribal land, State of Idaho land, and on private land.

            Since the discovery of selenosis in nearby livestock in 1996, site investigations have revealed widespread selenium contamination from the reclaimed mine areas into the contiguous Blackfoot River watershed, threatening wildlife that inhabit the region.  Due to the complex biogeochemistry of Se the best management plan (BMP) to minimize threats to the ecosystem are unclear.  In this project we will use field monitoring and laboratory studies, including bulk and microscopic XAFS, to gain a better understanding of Se biogeochemistry in the soils and plants on the remediated mine sites, and how various plants and amendments effect Se cycling.  The data will be used to develop BMPs that will reduce Se exposure to wildlife, livestock, and aquatic organisms.

            The plant-soil system can have a significant impact on the potential for Se toxicity to occur in the ecosystem.  Selenium can exist in natural settings as both organic and inorganic forms, with oxidation states –2, 0, +4, and +6.  Each of these species has different toxicities and behavior in the environment.  The reaction processes controlling Se biogeochemical cycling include biotic and abiotic mechanisms.  Due to the multiple forms and the importance of biotic and abiotic factors in determining Se availability and toxicity, it is critical that an intensive study of reaction processes be done so that successful, long-term management of these remediated landscapes can be implemented.

            The goal of this study is to gain a better understanding of the influence of vegetation and amendments on Se speciation, availability for transport to surface water sources, and uptake into the plant shoots.  Knowledge of Se speciation and availability as affected by remediation and management will lead to improved strategies for preventing Se toxicosis in wildlife and domestic animals.  To achieve this goal, we have established study plots using a completely randomized block design.  The treatments include a standard method (no soil treatment), soil plus CaSO4, and soil plus an organic amendment.  Subplots will test the ability of different plant species to accumulate Se and their influence on the speciation of Se within the soil.  To gain a complete understanding of the biogeochemical cycling of Se in the plots we have established the following experimental objectives:

1.         Measure in-situ Se availability by sampling the soil water

2.         Measure total plant uptake of Se.

3.         Speciate Se in the plants, soil water, and the soil solid phase.

Objective 1 will provide quantitative information on the effect of the remediation strategies and plant variety on aqueous Se that is available for plant uptake and leaching.  Objective 2 will provide quantitative information on the total Se available in the plants for uptake by foraging animals, as well as to allow for important correlations between plant uptake and soil solution Se concentrations (objective 1).  Objective 3 will provide information on how the remediation strategy impacts Se speciation in the plants, the solid phase, and the soil solution.  Since soil processes are dynamic, information about the Se species present, and how environmental conditions impact these species, will allow for improved modeling of Se availability, remediation strategies, and land use management and planning.

            One of the areas in this projects that is of particular interest to me is the speciation experiments.  We are using XAFS and micro-XAS to determine in what phases the Se exist in the soils, plants, and soil solution.  Micro-XAFS is ideally suited for analysis of Se speciation in natural materials since the K-edge absorption produces a strong edge jump in the hard X-ray region (12,658 eV), and the X-rays can be focused on individual mineral aggregates in heterogeneous samples.  There have been only a limited number of studies that have combined XAFS and microscopy for determining metal speciation in natural samples.  To our knowledge, this is one of the first studies to correlate Se speciation in soils with uptake and speciation in plants under field settings.

            In the current study we will use both bulk and μ-XAFS spectroscopy to analyze oxidation state and molecular environment of Se in the soils and plants from the Western Phosphate Resource Area.  In addition we will use XAFS to determine the iron mineralogy of the aggregates in which Se is associated.  An underlying goal of the micro XAFS work is to investigate how best to prepare samples for micro-XAS analysis and preserve the redox status of the sample.