Water and solute transport in the shallow subsurface of a riverine wetland natural levee

In riverine wetlands, the rate and magnitude of water exchanged between river channels and adjacent wetlands have a major influence on hydrologic function and associated aquatic habitat and water quality. River channels are generally separated from backswamps by natural levees; however, preferential flow may allow potentially substantial water and solute exchange between them. Determining the influence of preferential flow on water and solute transport in natural levees requires an innovative approach that allows measurement of transport rates at a scale large enough to capture field-scale variability. To test the hypothesis that preferential flow is important to subsurface water movement through natural levees, we measured hydraulic gradients and solute tracers in a 10 x 9 m grid of 19 shallow (2m) monitoring wells within a large volume (300 m3) of natural levee in the Atchafalaya Basin, Louisiana. In addition to measuring transient responses to natural events, we constructed a small reservoir on the backswamp side of the levee to create a hydraulic gradient from the swamp to the adjacent river channel. We used a simple linear system approach to model residence time distributions of water and solute within the natural levee. Overall, hydraulic response to forcing events, whether from rain or reservoir filling, was rapid in all wells (mean pressure wave velocity 2.5 E-2 m/s) and relatively uniform; in comparison, tracer transport was much slower (mean 3.2 E-4 m/s) and more variable. Spatially, mean subsurface tracer velocities varied over several orders of magnitude: between 1.6 E-7 and 6.8 E-5 m/s under a 12-cm hydraulic gradient and between 2.1 E-7 and 2.2 E-3 m/s under a 70-cm hydraulic gradient. Comparatively greater variability of chemical verses hydraulic behavior indicates multiple functioning transport mechanisms in natural levee sediments and suggests preferential flow; thus, methods that estimate transport rates based on standard point measurements likely greatly underestimate exchange at the field scale. Results suggest that preferential, subsurface flow through natural levees may provide hydrologic connectivity that is meaningful to basin-scale biogeochemical processes