Solute transport in seasonal perched water tables in loess-derived soilscapes

Hydraulically restrictive fragipans and argillic horizons are a common feature of loessial soils in the Palouse region of northern Idaho, resulting in perched water development during the winter and early spring. The high relief of the region may result in lateral flow of perched water and solutes. This study was initiated to quantify the rates of solute movement through perched water at three sites receiving 610, 700, and 830 mm of annual precipitation. Three transects, perpendicular to slope contours, consisting of seven, nine, and seven sampling wells, were installed at each site. Five kilograms of KBr tracer were applied in a trench upslope from the transects. Water samples were drawn biweekly to test for Br-. Rate of Br- movement and saturated hydraulic conductivity (Ksat) of soil horizons were used to quantify flow of perched water. Results indicate that maximum observed Br- transport decreased with annual precipitation and was 86, 50, and 35 cm d-1 at the 830, 700, and 610-mm sites, respectively. Depth profiles for Ksat are similar among sites; values range from 63 to 129 cm d-1 in the Ap horizons and decrease with depth to 0.10 to 0.21 cm d-1 in the restrictive horizons. The E horizons immediately above the argillic-fragipan horizons have Ksat values from 1.2 to 5.2 cm d-1 suggesting that much of the rapid solute transport via perched water tables occurs in the more permeable horizons overlying the E horizons. Our results demonstrate that perched water flow through these landscapes may enhance agrichemical transport, thereby impacting both nutrient-use efficiency and local water quality