Quantification and heterogeneity of infiltration and transport in alluvial floodplains

In order to protect drinking water systems and aquatic ecosystems, all critical nutrient source areas and transport mechanisms need to be characterized. It is hypothesized that hydrologic heterogeneities (e.g., macropores and gravel outcrops) in the subsurface of floodplains play an integral role in impacting flow and contaminant transport between the soil surface and shallow alluvial aquifers which are intricately connected to streams. Infiltration is often assumed to be uniform at the field scale, but this neglects the high spatial variability common in anisotropic, heterogeneous alluvial floodplain soils. In the Ozark ecoregion, for example, the erosion of carbonate bedrock (primarily limestone) by slightly acidic water has left a large residuum of chert gravel in Ozark soils, with floodplains generally consisting of coarse chert gravel overlain by a mantle (1 to 300 cm) of gravelly loam or silt loam. The process of alluvial sediment deposition is highly variable, and can cause gravel layers to outcrop on the soil surface at various locations within a floodplain. The objective of this research was to quantify heterogeneity in infiltration rates at three floodplain sites in the Ozark ecoregion of Oklahoma and Arkansas. Innovative field studies, including plot scale (1 by 1 m and 3 by 3 m) solute injection experiments along with geophysical imaging, were performed on both gravel outcrops and non-gravel outcrops. Plots maintained a constant head of 3 to 10 cm for up to 48 hours. Infiltration rates varied from 0.8 to 70 cm/h, and varied considerably even within a single floodplain. Electrical resistivity imaging was used to identify zones of preferential flow as well as characterize subsurface soil layering. Fluid samples from observation wells outside the plot (0.5 m from the boundary) indicated nonuniform subsurface flow and transport. Phosphorus was detected in the groundwater for 6 of the 12 plots and was positively correlated to the presence of gravel outcrops. Results indicated that flow paths are sub-meter scale for detecting infiltrating solutions. Tension infiltrometers showed that macropore flow accounted for approximately 85% to 99% of the total infiltration.