Fate and Transport of Chemicals in the Capillary Fringe

Water in the capillary fringe (CF) moves horizontally with the groundwater, but the biogeochemical reactions in it may be more like those in the vadose zone above it than the groundwater below. A series of laboratory studies were conducted to assess the redox conditions and the fate of nitrate (NO₃), and bromide (Br) within the vadose zone-groundwater continuum. Two different sizes of columns (90- and 240-cm long), each with its front face constructed of clear polycarbonate sheet, were packed with different soils. Two 2.5-cm wide chambers on the sides of each column allowed solution application/drainage. In one experiment, four different soil materials were packed in small (90-cm long) columns and a water table (WT) was established at 38 cm below the soil surface. A series of redox probes were installed at 5 cm below and 5, 15 and 25 cm above the WT at 20, 42.5 and 65 from the inlet, and distilled water was passed through each column horizontally at different velocities. A redox gradient was formed within the CF, with reducing conditions developed near the WT while oxic conditions occurred in the upper part of CF. As lateral pore water velocity increased, a greater proportion of the CF became oxic. In a second experiment, large columns (240-cm long) were packed with sand containing little organic matter (OM) and field soil material containing 36 g kg^-1 OM. A solution containing NO₃ and Br was passed horizontally through each column at different rates under a constant WT. Although NO₃ was lost below WT, it persisted in a manner similar to Br while being transported through the CF. The degree of NO₃ loss was controlled by the redox potential along its flow path.