Continuous modeling of hyporheic exchange explains chemostasis in glacial meltwater streams, Antarctica

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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 nearly equal to rates of water flux through a catchment. However, the physical controls on solute mobilization and production, which drive 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 9 years of hydrologic record, we observe Si chemostasis, based on historical Si-Q regressions. We propose that hyporheic exchange maintains chemostasis of weathering-derived solutes, given that there is no lateral hillslope flow and no deep groundwater contribution to MDV streams. We test this hypothesis by developing novel hyporheic end-member mixing models (HEMMs) to estimate hyporheic exchange flux during the 6-12 week flow seasons on four streams, over 9 seasons of flow record.  The model simulations reveal that 5-53% of total annual streamflow is turned over through the hyporheic zone. A greater portion of annual streamflow is turned over through hyporheic zones on longer streams, compared to shorter streams. Si-Q regressions were re-computed using HEMM simulated hyporheic flux rates (Si-Q_HZ) and confirm that hyporheic fluxes exert a control on Si concentrations in all streams.  Results from this study confirm the hydrochemical significance of hyporheic zones in the MDVs, specifically the ability of hyporheic zones to be dynamic Si sources.