REACTION PROGRESS AND GROUNDWATER RESIDENCE TIMES, THE CHEMICAL EVOLUTION OF GROUNDWATER IN A SILICATE BASED HEADWATER CATCHMENT FROM KINETICALLY LIMITED TOWARDS TRANSPORT LIMITED COMPOSITIONS
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Water-rock interactions drive silicate weathering in the Critical Zone, but therelationship between weathering reaction progress and groundwater residence time remains difficult to constrain because they are rarely observed directly. This thesis evaluates the extent of this relationship leveraging silicon stable isotopes (δ30Si) as a silicate weathering tracer. Springwaters from Sagehen Creek, a 27 km2 snowmelt-dominated catchment in theCentral Sierra Nevada were analyzed for dissolved chemistry, δ30Si, and CFC-12 apparent ages. Results show that δ30Si decreases systematically with increasing CFC- 12 piston-flow ages. Younger waters are isotopically heavier, consistent with early secondary mineral formation that preferentially removes 28Si from solution, whereas older waters are more chemically buffered and isotopically lighter. Findings suggest that δ30Si provides a time-sensitive tracer of silicate weathering reaction progress and helps capture the transition from kinetically limited toward more transport-limited weathering in groundwater systems. Findings suggest that silicate weathering reactions progress over decadal groundwater residence times.