Release of a Soluble Chemical from Soil to Runoff

Abstract

ABSTRACT THE assumption that there is a certain effective depth of surface soil within which the soil water mixes com-pletely with rain water in the release of soil chemicals to infiltration and runoff was tested, using Br as a tracer. Experiments were conducted with simulated rainfall on boxes of soil that were prewetted with a Br solution. The following variables were also examined: soil slope, soil cover, rainfall intensity, and bottom of the soil box either permeable or impermeable. In permeable boxes, most of the Br was lost from the mixing zone within 5 min after the start of rainfall, and there was no Br in runoff after 10 min. During this early period, the effective depth of mixing was small and changed appreciably with time. At simulated rainfall intensity of 6.5 cm/h and slope of 4 percent, time-averaged effective depth of interaction for Br was a little over 1 mm. The transient nature of the ef-fective depth of interaction and its average value agreed well with the small-time values found earlier by using 32P as a tracer for similar soil, slope, and rainfall conditions. Doubling the rainfall intensity resulted in a faster development of the interacting zone which averaged 1.77 mm in effective depth. Changing soil slope to 16 percent increased the time-averaged effective depth by about 18 percent, and covering the soil surface with a fine nylon screen decreased it by 30 percent. The effects of successive levels of slope (4, 8, 12, and 16 percent) or soil covers (none, coarse, medium, and fine screen) on Br concentrations in runoff with time were not clearly separated, due to small changes and relatively large ex-perimental variability. The Br concentrations in runoff from saturated soil boxes with impermeable bottoms were several times higher and remained at a high level during four con-secutive 30-min rainfall applications, as compared with those from permeable boxes. The concept of one effective soil depth of uniform mixing did not seem to hold for these data. The increased movement of Br to runoff from impermeable boxes was only partly attributable to sub-surface interflow. Pumping action of raindrops impact-ing a saturated soil system, in which downward percola-tion is prevented, is an important mechanism of bringing a mobile chemical from below the normal mixing zone to he surface. Further work is required on this phe-nomenon, because of its potential for adding a tremen-dous amount of chemicals to runoff from shallow soils or soils with B horizons (subsoils) of very low permeability.