Isotopic and geochemical evidence of regional-scale anisotropy and interconnectivity of an alluvial fan system, Cochabamba Valley, Bolivia

Abstract

A study of groundwater chemistry and isotopic composition was conducted in Cochabamba Valley, Bolivia of an alluvial fan aquifer system located along the northern edge of the valley. In total, 75 wells were sampled for major and minor (F −, Br −, Si 4+, NO 3 −, Fe total, H 2S, PO 3 4−) ion chemistry, 18O and 2H, and 3H. In the alluvial fan aquifer, groundwater flow patterns appear to be controlled by the unique hydrostratigraphy of the fan sediments. The radial morphology of alluvial fans and the entrenchment of channels in the apex of the fan are two probable causes of deposition of coarser, more permeable material along the axis of the fan. Geochemical and isotopic parameters give indirect evidence of these more permeable zones. Ratios of Na + Ca ++ are higher off the axis of the fan, which is most likely due to longer residence times so that diffusion of saline residual pore water or ion exchange reactions create more Na +-rich groundwater. Also, 3H concentrations down the flow system are higher along the axis of the alluvial fan, a direct indication of younger groundwater age along the axis. The distribution of other minor ions, such as F − and Si 4+, and the redox parameters NO 3 −, Fe total, SO 4 2− and H 2S suggest a shorter residence times along the axes. The even pattern of Na + Ca ++ and Na + K + ratios, and Si 4+, NO 3 −, Fe total, SO 4 2−, H 2S, and 3H concentrations along the flow system from wells varying 20 to 80 m in depth suggest that the aquifer–aquitard alluvial fan system is interconnected between different depths. Enriched groundwater 18O and 2H signatures in the distal end of the flow system in Area A are most likely the result of the introduction of evaporated waters from the surface, further evidence that the system is interconnected.