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Framework Geochemical Analysis, Lithogeochemical Characterization, and Geoenvironmental Considerations Affecting Land Use Decisions of the Mancos Shale Landscape

Task Objectives

Rocks of the Mancos Shale, a black (organic-enriched) shale that occurs in western Colorado and nearby areas, contribute salinity and toxic trace elements, most notably Se, to the landscape. These contaminants leach out of the rocks and travel throughout the environment where they can build up to and persist at harmful levels. Salinity effects from the Mancos Shale on the Colorado River have been estimated as costing 300 million dollars, and the toxic and lethal effects of Se on fauna have been noted in this and in other similar black shale-hosted landscapes.

1. Lithogeochemical analysis: Collect and analyze samples of the Mancos Shale that are collected in a way to minimize number but maximize understanding of the contaminant element and mineralogical composition of the shale.

Provide an analysis of the variance of this compositional data such that the expected variability of rocks collected from nearby at various spatial scales for a given composition can be predicted. Coordinate this analysis with several other Tasks of the Project that will benefit from having the compositional data.

2. Conduct field and laboratory experiments that measure the concentrations of contaminant elements that can be leached from various types of Mancos Shale and its derivative soils. Test the effects of particle size, leaching time, pH, and extent of oxygenation in the leachate water.
3. Initiate collection of atmospheric vapor and dust samples.
4. Initiate a collection of plants from the Mancos Landscape that indicate the extent of mobility of various contaminant elements derived from the Mancos into the environment.
5. Establish linkages between this Task and other Tasks in the Mancos Landscape Project. Coordinate scientific objectives and methodology to maximize information return from all Tasks.
6. Coordinate interaction between the Task and outreach clients and collaborators. Establish mechanisms that accommodate any derived OFA funding and allow for the preparation of products to satisfy these fund sources.

Highlights & Key Findings

Preliminary analysis of soil on southeastern and northwestern aspect soil reveals only minor differences; mineralogy has only subtle differences in the amount of clay and carbonate. Soils developed on the Mancos Shale consist of a surficial Crust, 0-5 cm thick, underlying Subcrust, 10-40 cm, and underlying Subsoil 30-50 cm. Soil leachate solutions (20:1 water:rock; 24 hours) have slightly alkaline pH, varying from 7.4 to 8.7, and are slightly oxidizing, with oxidation-reduction potentials (ORP) of 142 to 181 µvolts. Total Se is greater in the southeastern aspect samples and increases in depth from 1.2 ppm in the crust to 2.8 ppm in the subsoil. Crust, subcrust, and subsoil of the northwestern aspect locality have Se concentrations of about 1.5 ppm, with no concentration trend with depth.

Most early release of dissolved solids into leachate solutions happens quickly; there is little difference in conductivity measurements between 15 minute and 12 hour leachate times. South-facing soil releases more salinity into water leachates than North-facing localities. Between 1/4 and 1/2 of the total Se in the solids dissolves into the leachate solutions.

Subsoil releases more salinity than Subcrust, and Subcrust releases more than Crust. The soils developed on the Shale release more than the Shale. Concentrations of contaminant elements are mostly low in these well-oxidized soil samples from Elephant Skin Wash. Se concentrations in the solids are generally <3 ppm.

Rainfall simulation studies show that slope aspect greatly influences the amount of runoff and of the dissolved solids in the runoff, with south-facing slopes producing the greatest runoff and highest concentrations of dissolved salt and Se in the runoff. For these runoff samples, conductivity is a reliable predictor of salinity, sulfate and, with somewhat greater scatter, selenium. Sites in the Candy Lane area generally have higher runoff volumes and release amounts of salinity and selenium compared to those in the Elephant Skin Wash area. Intentional fresh disturbance at the south- and north-facing sites decreased the runoff amounts to 20 and 4 percent, respectively, of the applied rainfall, suggesting that this disturbance initially increases the water-holding capacity of the soil. The largest releases of selenium in the runoff occur on south-facing slopes in the CL area, with concentrations ranging from 10 to 30 µg/L. Released selenium was 400 µg in a volume of 20 Liters. The consistent relationship between runoff selenium and the major, highly soluble elements indicates that there is a minor but readily soluble phase of selenium in the soil.

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