Monitoring of Suspended Sediment Mineralogy in Puerto-Rican Rivers: Effects of Flowrate and Lithology

Abstract

Climate change induced changes in river flow dynamics have the potential to change the composition of suspended sediments in crucial tropical river ecosystems, possibly affecting their resiliency. This study investigates how changes in river discharge and bedrock lithology affected the physiochemical nature of river suspended sediments over a typical year in three Puerto-Rican rivers. Suspended sediment samples were collected on filter membranes in 2006 from three watersheds of differing lithology (quartz-diorite, volcaniclastic, and mixed lithology) in the Luquillo Mountains, Puerto-Rico. By monitoring changes in suspended sediment mineralogical composition (determined by XRD and SEM) as a function of discharge, we determined how sediment loads responded to changes in hydrological input in a typical year. Results showed that bedrock lithology influenced river suspended sediment mineralogy, with the fraction of crystalline versus amorphous material strongly influenced by the dominant lithology of the watershed. Crystalline phases were associated with granodiorite bedrock compared to amorphous material dominating the volcaniclastic watersheds. Thus, the mineralogy of suspended sediments in the river systems was controlled by secondary minerals. Mineralogical results showed that, bearing quantitative changes upon hydrological events, suspended sediments in all three watersheds returned to baseline composition post storm events, suggesting that the three watersheds are resilient to the events recorded that year. While the long-term mineralogical analysis of the evolution of suspended material in the studied rivers provided insights into river response to hydrologic events, it also proved technically challenging as materials in suspension in such pristine rivers are sparse and poorly crystalline.