Field Evaluation of Travel Times and Flow Mechanisms in the Mississippi Delta Vadose Zone Using Tracers

In the Bogue Phalia basin in the Delta region of Mississippi, as in many farmed areas, intensive application of agricultural chemicals has led to their detection in surface and ground water; however, contributing unsaturated zone processes are not well understood. The fine textured soils often exhibit surface ponding and runoff after irrigation and rainfall as well as extensive surface cracking during extended dry periods. Fields are typically land-formed to promote surface flow into irrigation ditches and streams that feed into larger river ecosystems. Downward flow of water below the root zone is considered minimal; regional ground- water models predict only 5 percent or less of precipitation recharges the heavily-used alluvial aquifer. In this study we assessed transport within and below the root zone of a fallow soybean field with a 2-m ring infiltration test including tracers and subsurface instrumentation for sampling and for measuring water content and matric potential. Seven months after tracer application, we collected 47 continuous cores for tracer extraction to define the extent of water movement. Water movement was rapid below the pond, traveling up to 0.21 cm/s, indicating the importance of vertical preferential flow paths. Lateral flow of water at shallow depths was extensive and spatially non uniform, reaching 10 m from the pond within 3 months. Within 2 months, the wetting front reached a textural boundary between the silty soil and sandy alluvium at 5 m. The aquifer was historically confined by the silty material prior to extensive irrigation pumping that has lowered the water table from about 2 m to 12 m. Preliminary results indicate that after 7 months any water breaking through the soil-alluvium boundary, which now acts as a capillary barrier within the vadose zone, likely does so as unstable finger flow (fine-over-coarse layering has been observed in lab studies to initiate fingering) which is difficult to detect with point measurements. Preferential flow in the vadose zone may contribute to contaminant transport in both surface water, through shallow lateral flow to irrigation ditches, and ground water, through vertical finger flow.