Preferential Flow Path Effects on Subsurface Contaminant Transport in Alluvial Floodplains

Abstract

For strongly sorbing contaminants, transport from upland areas to surface water systems is typically considered to be due to surface runoff with subsurface transport assumed negligible. However, certain local conditions can lead to an environment where subsurface transport to streams may be significant, a source of contamination not alleviated by current best management practices (e.g. riparian buffers). The Ozark region, including parts of Oklahoma, Arkansas, and Missouri, is characterized by cherty, gravelly soils and gravel bed streams. Previous research identified a preferential flow path (PFP) at a field site along the Barren Fork Creek in northeastern Oklahoma. With the subsoils having hydraulic conductivities on the order of 100 to 500 m/d, the previous research demonstrated that even a sorbing contaminant such as phosphorus can be transported in significant quantities through the subsurface. The objective of the current project was to determine the connectivity of the PFP to the stream and to further evaluate the hypothesis that the alluvial groundwater acts as a transient storage zone, providing a contaminant sink during high flow and a contaminant source during baseflow. A trench was installed above the PFP with the bottom of the trench at the topsoil/alluvial gravel interface. Piezometers were installed along the PFP and throughout the riparian floodplain, which was mapped with electrical resistivity equipment. Water was pumped into the trench to maintain a constant head, and a conservative tracer (Rhodamine WT) was injected into the trench. Water table elevations were recorded real-time using water level loggers and water samples were collected throughout the experiment. Results of the experiment demonstrated preferential movement of Rhodamine WT along the perched preferential flow pathway, infiltration of Rhodamine WT into the alluvial groundwater system, and then transport in the alluvial system as influenced by the stream/aquifer dynamics. This research demonstrated the importance of physical heterogeneity in affecting contaminant transport even in coarse gravel, alluvial subsoils.