Solute transport in the capillary fringe and shallow groundwater: Field evaluation

Lateral transport of waterborne contaminants is generally assumed to occur below the water table (WT), but recent laboratory studies have suggested that subsurface lateral solute transport could occur above the WT through the capillary fringe (CF). The objective of this study was to evaluate the horizontal transport of solutes in the CF and shallow groundwater (SGW). Two consecutive field experiments were conducted in a Leon sand (Aeric Alaquod) at a site with a shallow WT. In both experiments, a bromide (Br-) solution was applied to the bottom of an auger hole dug to within 10 cm above the projected CF. Movement of Br- in the subsurface was monitored by collecting CF and SGW samples using nests of tension samplers installed at radial distances of 60, 120, 220, and 320 cm from the application spot. Each nest of samplers contained a tension sampler at 45, 60, 75, 90, and 105 cm depth. Bromide transport was monitored for 58 d in the first experiment while the more detailed second experiment was conducted for 84 d. Peak Br- concentration generally occurred in the upper 60 cm of the soil where the CF was located for most of the experiment. The Br- plume that entered the CF moved horizontally in it until Br- was partially moved into the SGW by the fluctuating WT. In the second experiment, approximately 48% of the Br- detected in the CF at a distance of 320 cm from the application spot was still in the CF after 15 cm of rain through 59 d.