Clay Mineralogy of Soils and Sediments from an Alluvial Aquifer, Rifle, Colorado

Abstract:

Alluvial aquifers along the Colorado River corridor in central to western Colorado contain legacy contamination including U, V, As and Se. These alluvial aquifers host important “hot spots” and “hot moments” for microbiological activity controlling organic carbon processing and fluxes in the subsurface that are both significant on their own, but also influence contaminant behavior. Mineral phases likely active in the sequestration of metal contaminants are chlorite, smectite-vermiculite, illite, and smectite. These minerals are also important biogeofacies markers. The Colorado alluvial sediments include lenses of silt and clay that are commonly more reduced than coarser grained materials. The clay minerals that make up the alluvial aquifer sediments include these mineral phases important for metal sequestration (chlorite, smectite, illite), as well as kaolinite and quartz. More specifically, the clay mineralogy of soils derived from these sediments at Rifle are composed of the same suite of minerals found in the alluvial sediments plus a vermiculite-smectite intergrade. The vermiculite-smectite intergrade is a weathering product of illite. The presence of illite and chlorite in both the sediments and the soils at Rifle reflect a mineralogically immature character of the source rocks. These assemblages are consistent with sediments and soils that formed in a moderately low rainfall climate, indicative of mixed provence of immature (chlorite, smectite, illite) and mature (kaolinite) minerals relative to their source areas.