Mesoscale connectivity through a natural levee

Natural levees are potentially locally important zones of lateral seepage between stream channels and flood-plain backswamps, because their relatively coarser soils provide pathways of high hydraulic conductivity in an otherwise low conductivity system. Therefore, understanding the rates and mechanisms of subsurface exchange of water and solutes through natural levees may be necessary for understanding biogeochemical cycling in floodplains. We measured imposed hydraulic gradients and solute tracers in 19 shallow monitoring wells within a 580m(3) volume of natural levee in the Atchafalaya Basin, Louisiana. We modeled residence time distributions of pressure and tracers using a simple linear system to quantify spatially variable transport velocities and infer dominant flow mechanisms at a mesoscale. The spatial mean velocity of pressure transport was faster than the mean velocity of tracer transport by two orders of magnitude (1.7x10(-2) and 4.6x10(-4) ms(-1), respectively), and the variance of pressure velocities was less than the variance of tracer velocities by seven orders of magnitude (1.4x10(4) min(2) and 7.9x10(11) min(2), respectively). Higher spatial variability of tracer velocities compared to pressure velocities indicates different functioning mechanisms of mass versus energy transport and suggests preferential flow. Effective hydraulic conductivities, which ranged in magnitude from 10(-1) to 10(3) md(-1), were higher than would be predicted by soil texture. We conclude that, in this fine-grained system, preferential flow paths control water and solute exchange through natural levees. These findings are important for future studies of water and solute cycling in riverine wetlands, and rates of exchange may be particularly useful for modeling water and nutrient budgets in similar systems.