Stratigraphic, geochemical, and hydrologic data for the Boston Peak wetland, Larimer County, CO, USA

Comprehensive sampling of peat, underlying lakebed sediments, and coexisting waters of a naturally uraniferous montane wetland are combined with hydrologic measurements to define the important controls on uranium (U) supply and uptake. The major source of U to the wetland is groundwater flowing through locally fractured and faulted granite gneiss of Proterozoic age. Dissolved U concentrations in four springs and one seep ranged from 20 to 83 ppb (g/l). Maximum U concentrations are ~300 ppm (mg/kg) in lakebed sediments and >3000 ppm in peat. This study documents the conditions and processes controlling the efficient uptake of U in a relatively remote, natural wetland that is absent of reported U occurrences, mining impacts, or other obvious sources of pollution. Unlike previous studies of U-rich wetlands, this study is distinguished because it provides an exceptionally detailed three-dimensional view of the distribution of uranium in as much as 3.7 m of Holocene peat, underlain by organic-rich lacustrine silt and clay (gyttja) and organic-poor clay and silt of a precursor post-glacial lake, with a combined thickness of as much as 8.5 m. The hydrologic characteristics of the entire sedimentary package from surface to bedrock were investigated with an extensive array of installed piezometers and water-table monitoring wells. Mechanisms of U uptake were investigated by a variety of techniques that utilize analyses of waters and of core and auger samples collected from depths as great as 11 m.