Mechanisms of Trace Metal Mobilization and Retention in Weathering Sulfidic Shale

Exposure of rocks to surface conditions can result in weathering reactions that transform minerals and alter pore fluid chemistry. Redox sensitive minerals, such as sulfides, play important early roles in weathering and can release metals into the surface and ground water. These metals can reach freshwater reservoirs used in irrigation or drinking water. Sulfide weathering in the Mancos Shale affects numerous watersheds in the Upper Colorado River basin. This sedimentary formation contains variable concentrations of sulfide minerals generated both by primary precipitation with in the ancient depositional environment and by subsequent hydrothermal alteration.

We used metric and micrometric scales field, lab and synchrotron-base studies of the mineralogical and geochemical profiles of soil and rock in a weathering hillslope to elucidate the weathering pathways of the Mancos Shale at East River, Colorado. This work seeks to establish the mineralogical and geochemical controls on the release of trace metals into the watershed. All weathering reactions proceed from hydrated fracture surfaces and generate clear fronts of sulfide, carbonate and clay mineral transformations. The consequence of weathering on downstream water quality is controlled by the abundance of carbonate and sulfide minerals that regulate the local pH and Eh conditions during weathering. These conditions variably control mobilization of trace elements within weathered shale through the following mechanisms: 1) precipitation on fracture surface at a geochemical gradient, 2) sorption onto fine-grained iron oxyhydroxide minerals, and 3) structural incorporation into clay minerals. Reactive transport modeling has verified the conceptual model for trace metal mobility during weathering and will be use to predict weathering impact on water quality at the watershed scale.