Clay mineral transformations in anthropic organic matter-rich sediments under saline water environment. Effect on the detrital mineral assemblages in the Upper Chicamocha River Basin, Colombia

Abstract The results of this study showed how clay mineral assemblages from alluvial sediments were used to deduce the environmental impact of anthropogenic modifications in the Upper Chicamocha River Basin (UCRB) in Colombia. Two dams were investigated: La Playa dam (with anthropogenic input from farm activities and wastewater) and Sochagota Lake dam (with input from natural hydrothermal waters) and found to contain organic-rich deposits modified under highly saline, eutrophic conditions. Results from X-ray diffraction (XRD), scanning electron microscope (SEM) and analytical high resolution electron microscope AEM-HRTEM analyses indicated that the mineral assemblage in the sediments trapped in La Playa dam in the upper part of the basin is characterized by the presence of Fe-bearing smectite (up to 0.4 atoms per formula unit (a.p.f.u.)), whereas sediments downstream the Sochagota Lake dam are characterized by the presence of illite and dispersed fragments of pyrite framboids. Intense eutrophication due to organic compounds and nutrient loads from urban wastewater and agricultural activities in the area above La Playa dam created a reducing environment in which sediments promoted Fe-uptake in authigenic smectite and efficiently trapped it in the sediments. Periodic release of water from Sochagota Lake into the Chicamocha River allowed the incorporation of detrital illite grains and fragments of pyrite framboids (formed by low temperature illitization and Fe-uptake in the lake sediments) into the alluvial sediments in the lower section of the UCRB.