HYPORHEIC EXCHANGE EXPLAINS CHEMOSTASIS IN GLACIAL MELTWATER STREAMS, ANTARCTICA

Chemostasis (comparatively minor change of solute concentrations over a wide range of discharge) of weathering-derived solutes (e.g., silica) is commonly observed in temperate streams, indicating that general rates of solute mobilization and production in the catchment are proportional to rates of water flux through a catchment. However, the physical controls on solute mobilization and production driving chemostasis, are not well understood. In the streams of the McMurdo Dry Valleys (MDVs) of Antarctica, glacial meltwater is the dominant (>98%) source of streamflow. Over 23 years of hydrologic record, we observe Si chemostasis, based on historical Si-Q regressions. We propose that long residence time “old water” contributions from the extended hyporheic zone maintain chemostasis, given that there is no lateral hillslope flow and no deep groundwater contribution to MDV streams. We test this hypothesis by developing a hyporheic end-member mixing model (HEMM) to estimate old water contributions from the hyporheic zone on four streams across a range of discharge conditions. Model simulations reveal that lateral inflow rates of old water are proportional to in-channel discharge rates, according to power-scaling relationship. A greater portion of total discharge is composed of old water on longer streams, compared to shorter streams. End-member sensitivity analysis shows that estimated old water contributions are most sensitive to changes in the old water end-member concentration. However, during high flows on short streams, estimates of old water contributions are more sensitive to changes in glacier water end-member concentration.