Field-scale transport from a buried line source in variably saturated soil

Abstract

Lateral subsurface flow through the upper soil layers (stormflow zone) during storm events has been shown to be a dominant mechanism of contaminant transport in forested watersheds. Data bases for multi-region flow and transport modeling for hydrogeologic conditions where stormflow predominates are lacking. Direct measurement of the tracer flux under field-scale conditions are non-existent. The objective of this paper was to evaluate the significance of three hydrologic pore regions to stormflow. Two tracer releases were made from a buried line source during storm events and the spatial and temporal variability in solute concentration and the tracer fluxes were measured. During one of the injections, macropore flow was extremely rapid with solute transport to a downslope trench 65 m from the line source taking just 3.2h. Mesopore flow appeared to be significant for short distances in that tracer movement to solution samplers just 3 m downslope of the line source occurred within 3 h of the release. Soil sampling 6 months after the second release revealed that the tracer plume was refracted in the direction of the fractured bedding plane, and therefore did not coincide with the array of samplers for distances greater than 13 m downslope of the source. Soil sampling data suggested that micropores served as a sink/source for Br − with 47% of the non-reactive tracer remaining immobilized by micropores at the termination of the study. Interaction between the upper 2 m of the stormflow zone and the groundwater system was believed minimal; however, lateral flow below 2 m was concluded to be significant.