Effects of Rainfall and Surface Flow On Chemical Diffusion From Soil to Runoff Water.

Although basic processes of diffusion and convection have been used to quantify chemical transport from soil to surface runoff, there are little research results actually showed how these processes were affected by rainfall and surface flow. We developed a laboratory flow cell and a sequence of experiments that measured bromide transport, by convection and diffusion, from soil to runoff water under three different soil hydraulic conditions: i.e., free drainage, saturation and artesian seepage.   Experiments were conducted under three rainfall intensities and added surface flows ranged from 1 to 100 times the equivalent rainfall rate.  Our results showed that rainfall impact increased the chemical transport from soil to surface runoff, with the most pronounced effect, i.e., up to 19 times greater than the static diffusion, under artesian seepage condition.   An increased surface flow and water depth diminished the raindrop impact effect on chemical diffusion and chemical loading can be quantified from the static diffusion process with the consideration of flow induced dilution.  We believe this is the first quantitative effort in measuring components of chemical transport process from soil to runoff water.