Hydrologic mixing model based on weathering chemistry identifies saprolite as the source of inorganic colloids in a granitoid catchment

The production and export of inorganic colloids in environmental systems affects soil formation, biogeochemistry, and geomorphology. Inorganic colloids also carry otherwise immobile contaminants through the critical zone. While theoretical and experimental work has described how colloids move through porous media, no framework has been developed to predict inorganic colloid sources based on critical zone properties, such as soil type. We develop a method of rapidly investigating colloid sources in a granitoid headwater catchment and employ it to predict where colloids are produced in its weathering profile. Our field site, Gordon Gulch, is in the montane zone of the Colorado Rockies and is one of three catchments studied by the Boulder Creek Critical Zone Observatory. We develop a two-component mixing model of stream water chemistry using end-members based on the mineral stability of two groundwater wells, one screened partially in saprolite and one screened entirely in bedrock. We find that colloids are more abundant in stream water when saprolite water dominates, during spring meltwater flow. The relationship between colloid abundance and saprolite water is strongly non-linear at the outlet of the catchment. We describe a conceptual model of inorganic colloid production and export in the catchment based on my findings and prior work in the catchment. We predict that colloids are produced in the mobile regolith and upper saprolite, and stored in and exported to stream water from the mid to upper saprolite.