Quantification of Chemical Transfer From Soil to Runoff Under Simulated Rainfall and Runoff.

Although there is a conceptual understanding of processes governing chemical transport from soil to surface runoff, there are little experimental results actually quantifying these individual processes.  Experiments were conducted in a laboratory box to measure bromide (Br) loading from soil to runoff water where near-surface hydraulic gradient, rainfall and added runoff are carefully controlled.   By controlling the near-surface hydraulic gradient and the amount of rainfall and surface runoff, we quantified Br transport through each of the individual processes: i.e., 1) erosion; 2) convection under a vertical hydraulic gradient; 3) convection from surface flow or the Bernoulli Effect; and 4) diffusion.  Our data showed that rainfall alone can accelerate diffusion by as much as 11% at 30 mm/h to ~600% at 90 mm/h intensity. The significant rainfall intensity effect in chemical loading needs to be incorporated in current water quality models.