Interchange of Infiltrating and Resident Water in Partially Saturated Media
The interplay between resident water (“old water”) and infiltrating water (“new water”) in porous media is examined through experiments in an idealized 2D glass micromodel. The analysis is motivated by the recognition that two complementary processes occur as water migrates through the vadose zone: infiltration, when water advances downward through this zone, and drainage, which follows infiltration events (precipitation and irrigation) when air displaces water as the water migrates deeper into the subsurface. Residual water saturation after a cycle of drainage is controlled by pores and pore clusters that retain “old water”. During an infiltration cycle, “new water” flowing through the porous medium will intermittently connect to these clusters and then will leave a new pattern of disconnected clusters containing, presumably, both old and new water. In the micromodel experiments, image analysis is used to quantify the miscible interplay between old and new water over a cycle of imbibition by new water and drainage by air in a domain that is partially saturated with old water. We demonstrate that some old water remains in the system at long times within stable water pockets; these pockets may remain stable even after a second cycle of infiltration. An implication of this finding is that water scarcity under dry climate conditions may increase the influence of mixing of infiltrating and resident water on water quality, particularly in the uppermost aquifer layers.
KeywordsPore Space Vadose Zone Water Injection Volumetric Fraction Pore Scale
Maxime Gouet-Kaplan is thanked for performing the experiments.
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