APPLYING GE/SI RATIOS TO TRACE WEATHERING REACTIONS, HYDROLOGIC PATHWAYS AND COAL FLY ASH CONTAMINATION IN WATERSHEDS ACROSS THE UNITED STATES
Aguirre, Arnulfo Andres
The generation of solutes in the critical zone (CZ), the region from the top of the canopy to groundwater, is an essential process that sustains ecosystems. The chemical and physical mechanisms that weather rock are fundamental for nutrient cycling and the generation of reactive surfaces. These processes define the evolution and function of the CZ. In the following dissertation, topics of silicate weathering and solute transport are explored in multiple landscapes. Additionally, the application of Ge/Si as a geochemical and hydrologic tracer will be demonstrated in order to understand sources of solutes in streams, hydrologic pathways and as a tool to fingerprint coal ash contamination. Concentration-discharge (C-Q) patterns in streams are governed by solute generation mechanisms and transport processes. In the Kings River Experimental Watershed of California, we explored how C-Q relationships varied between two watersheds on a climate sequence. Using the Ge/Si, we observed that the mid-elevation catchment of Providence Creek was dominated by groundwater inputs even during periods of increased discharge resulting in chemostatic Si-Q relationships. In the higher elevation catchment of Bull Creek, significant leaching conditions due to snowmelt runoff resulted in dilution of Si-Q relationships. The sources of silicon to streams was further investigated in the Gordon Gulch catchment of Boulder Creek, CO. In this upper montane landscape with chemostatic Si-Q patterns, colloidal transport is a major contributor to the Si flux. By analyzing bulk chemistry and Ge/Si of a colloidal suspension, we identified periods of active colloidal mobilization during high stream discharge. Since average stream Ge/Si has a narrow range of 0.4-0.5 mol mol-1, the potential of Ge/Si as a tracer for coal ash was explored during the Dan River coal ash spill of 2014. By monitoring Ge/Si and Sb levels 10-months after the spill, hotspots of elevated ratios and Sb were identified as a result of post depositional processes of coal ash. Lastly, Ge/Si was applied to understand hydrologic pathways in the Bisley 1 catchment of the Luquillo Critical Zone Observatory. Our results describe a three end-member mixing system dominated by groundwater and followed by saprolite hydrologic pathways.
Colloids; Transport; Biogeochemistry; Germanium; Geochemistry; Weathering; Fly ash; Hydrologic sciences; silicon
Derry, Louis A.
Walter, Michael Todd; White, William M
Ph.D., Geological Sciences
Doctor of Philosophy
dissertation or thesis