Hydrological Controls and Sources of Water for Shallow Groundwater of Underfit, Fine-Grained Floodplains

Abstract

An important mediating factor controlling the degree of connectivity between rivers and adjacent floodplains is the texture and structure of the floodplain alluvium. In fine-textured alluvium, especially shrink-swell clays, low hydraulic conductivity generally limits hydrologic flux, but the consequences of this for shallow groundwater have not been well investigated at the floodplain scale. We used shallow monitoring wells and stable isotopes to characterise the relative influence of river flooding and local precipitation on shallow groundwater across four underfit floodplains in the southeastern United States. Shallow groundwater elevation time series and isotopic data indicated precipitation was the most important control on shallow groundwater during the growing season overall. However, river level was important for controlling drainage and drying at all sites and controlled wet-up at low elevations sites, sites with assumed highly connected subsurface structures (sand and large silt), and after periods of low local precipitation. There was threshold behaviour in recession at topographically low sites, variability in recession timing that was unrelated to distance from the river and a large degree of spatial variability in isotopic composition of shallow groundwater across floodplains. All three of these observations, in combination with known patterns of water fluxes in Vertisols, suggest preferential flow through a discontinuous network of macropores made up of biogenic macropores, cracks and coarse soil layers, combined with substantial perching. We conclude that shallow groundwater in underfit, fine-grained floodplains moves in variably connected, variably saturated macropore networks that lead to locally idiosyncratic local water budgets and high spatiotemporal heterogeneity in water sources, flow paths, and residence times.