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Soil Development, Trace Element Mobility, and Salinity in the Mancos Shale

Task Objectives

Irrigation and off-road vehicle use has led to mobilization of salts and potentially toxic metals such as selenium within soil derived from the Mancos Shale. Although the Mancos Shale is indisputably a major source of the salt and metal contamination, little is known about the residence of constituents in the shale and derived soils, the extent of chemical weathering of minerals during soil formation, and the mechanisms that mobilize and transport solutes. In addition, land managers have no quantitative measure to evaluate the impact of land use on Mancos soil because few geochemical baselines exist. Long-term mitigation strategies require knowledge of

1. chemical weathering and soil formation,
2. mobility and transport mechanisms of metals and salt in both undisturbed and disturbed Mancos soils, and
3. rates of metal and salt loading related to natural weathering reactions in undisturbed and disturbed Mancos soil.

The goal of this task were two-fold: 1) Provide quantitative data for potentially toxic elements and salinity on disturbed and undisturbed Mancos soils, and 2) Construct a conceptual model of weathering and response of soil to surface disturbance. Accomplishment of these goals facilitated scientifically based decisions by land managers; established background information on Mancos weathering processes; and provided chemical data for USGS national soil databases and comparative data for shale weathering studies influenced by different climates (USGS Geochemical Landscapes and Basin Brines projects).

There were three task objectives.

1. Chemistry, mineralogy, and physical properties of Mancos soil. Geochemical baselines for element (major, minor, and trace metal) concentrations, mineralogical residence, and physical properties in undisturbed Mancos soil horizons were developed for the small selected areas chosen for soil evolution studies.
   
   Chemostratigraphy of elements and salinity in the soil from undisturbed and disturbed sites provided the framework to correlate soil horizons with enrichment or depletion of trace elements and salt. Mineralogy of horizons were used to identify minerals enriched or depleted in trace metals. Physical properties (bulk density and porosity) were used in mass-balance weathering calculations.
   
   Visualization of chemical and mineralogical data, were on small-scale maps with thematic overlies of existing topography and hydrology, clay mineralogy (Eberl task), soil type (D. Dearstyne, USDA), organic-matter characteristics (M. Lewan, USGS) and remote sensing spectral data (Livo task). Collectively, these maps characterized the Mancos soil at each study site and were available for incorporation into the broader Mancos geochemical framework (Herring task).
2. Geochemical reactions and mobility/transport mechanisms during soil evolution. Objective 1 data were used to formulate hypotheses regarding the geochemical reactions that concentrate, mobilize, and/or transport trace elements and salinity within the soil. These hypotheses were tested with geochemical models and laboratory experiments simulating reactions within the soil. Comparison of trace-element profiles between disturbed and undisturbed sites were used to create a conceptual weathering model for the chemical response of the soil surface to disturbance. Data such as the date of disturbance, change in mineralogy, density, and trace-element content of the soil profiles were used to evaluate the nature and extent of chemical and physical weathering processes and develop weathering rates for undisturbed and disturbed landscapes. Element cycling models provided critical data to researchers pursuing bioavailability studies.
   
   The type of laboratory experiments designed depended on those processes identified as being important based on chemical, mineralogical, and modeling results. Possibilities included: 1) Determination of clay cation exchange and its effect on trace metals (collaboration with Eberl task), 2) Evaluation of the change in geochemistry of undisturbed Mancos soil when irrigation water was applied (variables include time of exposure, wet/dry cycling, water composition) (collaboration with Eberl task), and 3) Simulation of chemical reactions occurring at the water /shale interface in Mancos shale directly beneath soil formation.
3. Impact of land use on Mancos soils. Geochemical data for soils from undisturbed and disturbed sites were compared in order to assess the impact of anthropogenic activities on soil evolution. Differences in data, together with modeling and experimental results, were correlated with erosion (Elliot task), salt loading (Herring task), and metal contamination (Herring task). Correlative results provided a valuable link between specific environmental problems resulting from land-use practices and their impact on soil geochemical processes.

Highlights & Key Findings

Weathering of Mancos shale extends for at least 6 meters, surprisingly deep considering the arid climate in the Gunnison Gorge National Conservation Area (GGNCA). All the joints and fractures in deeper weathered shale from some of the trenches are filled with gypsum and thenardite Na₂SO₄. In addition, thenardite is the dominant salt on the soil surface. In one trench, profiles of pH and salinity of saturation-paste extracts ran at Bureau of Reclamation are highest in soil horizons 3 to 25 cm below the land surface. This zone coincides with the depth of maximum moisture penetration. This accumulation of salt results from recycling of sulfate, sodium, and, to a lesser extent, calcium within the soil profile. If undisturbed, the salt is likely to remain in situ with only small amounts being transported off site. Selenium on the other hand is not recycled and is removed from the soil as it weathers at the surface. Extrapolation of our trench data suggests that one foot of soil across the 81 km² of the Mancos Shale-covered GGNCA contains 100, 000 - 180,000 tons of readily soluble salt and 4 - 20 tons soluble Se. This inventory was refined with data from our ’05 regional sampling. Nitrate and chloride are not uniformly distributed in the GGCNA Mancos Shale and their concentrations are dependent on weathering rates (the higher the rates, the less of these salts). Chloride is atmospherically deposited and nitrate is derived from weathering of organic matter in the shale. The spatial variability of soil geochemistry occurs across the GGCNA and, preliminary results suggest that erosion rates play a role in salt and Se content of the soil.

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