The Extent and Nature of Rainfall‐soil Interaction in the Release of Soluble Chemicals to Runoff

Abstract Experiments were conducted with a simulated rainfall on boxes of three different soils (Ruston fine sandy loam, Ruston loam, and Parsons clay), prewetted with a bromide (4,000 ppm Br) solution. The bottom of the soil boxes (100 cm long by 15 cm wide by 10 cm deep) was either an impervious plate that allowed no downward infiltration through the soil during the rainfall, a pervious plate that allowed free infiltration, or a pervious plate covered with a thin layer of soil‐slurry, which allowed infiltration at a reduced level. In all the soils, Br concentrations in runoff from impervious‐base boxes were one to two orders of magnitude higher than in runoff from pervious‐base boxes. The concentrations in slurry‐base boxes were intermediate. This result and the shape of the concentration‐time curves showed that the concept of an effective complete mixing depth of rainwater and soil solution used in modeling the release of chemicals‐to‐runoff is not strictly valid. However, the concept may be applied as an approximation to the data from pervious boxes. Bromide concentrations of the soil in the impervious‐base boxes at the end of 1‐h rainfall events (6.8 cm of rain) showed that the Br was lost to runoff from as great as the 2.0‐cm depth. The amount released, however, decreased with depth. It is suggested that the transfer of chemical from below a thin soil surface layer may be due to pumping action of, or turbulence generated by, the raindrop impacts, and may be considered as an accelerated‐diffusion process. As a simplified approach, the mixing zone concept was modified and tested by incorporating an exponential decrease in the degree of mixing with depth, and a piston displacement of soil solution by infiltration. This approach seemed adequate as a first‐order approximation. The results suggest that clay‐pan soils, wet areas in a watershed, and tillage practices that increase permeability of the soil‐surface layer relative to that of the layer below will greatly increase the loss of soil chemicals to runoff.