Pore-scale dual-domain flow and temporal variability in recharge, High Plains Aquifer, USA

Abstract

Between 2006 and 2010, heat dissipation probes monitored real-time matric potential at various depths in the vadose zone beneath two semiarid natural grassland sites within the High Plains aquifer. Monitoring results indicate 1) a downward potential for water movement, 2) at least 4 wetting fronts that rapidly propagated to previously unobserved depths (7.3 to 23.0 m below land surface), and 3) rapid water fluxes (0.9 to 6.8 mm/day) that likely resulted in recharge events during the period of observation. Measured water fluxes are orders of magnitude greater than previous estimates using convention tracer-based techniques that provide average water fluxes over decadal to millennial timescales. The rapid water fluxes are likely controlled by pore-scale dualdomain flow. Using observations from this study, I propose a new conceptual model of water flux in the vadose zone beneath natural grasslands in semiarid climates. The model conceptualizes monotonically decreasing total potential, and deep and rapid water movement that can result in episodic recharge events. The movement of water on daily to monthly timescales has important implications for the enhanced mobilization of some fraction of surface-derived contaminants in the vadose zone to groundwater. Most notably, interpretations of the observations and reported water fluxes from this study may account for the inconsistencies between observed groundwater chemistry and previously estimated chemical fluxes in the vadose zone of the High Plains aquifer.