Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 16, 1996
Publication Date: N/A
Interpretive Summary: Surface water quality is a serious concern due to the increasing use of pesticides and fertilizers in modern agricultural systems. Many chemicals in lakes and streams have exceeded the toxic levels. To reduce the chemical contents in the surface water bodies, the ways and means of chemical transfer from soil to water runoff must be understood. Results of this study showed there exists a thin layer (probably less than 3-4 mm) below the soil surface, in which runoff, soil water, and infiltrating water are well mixed. The chemicals in this layer are available for losses in the surface water runoff, and the amounts of losses are proportional to water runoff volumes. The results also showed that chemical concentrations in the water runoff decreased rather rapidly with time, and this decreasing trend as well as the total chemical losses can be predicted with the thin layer concept. The findings of this study can be used to develop a useful tool for environmentalists to assess potential chemical losses in the water runoff under various conditions and to develop best management plans for controlling surface water contamination.
Technical Abstract: Soils were exposed to three consecutive simulated rains under dry, wet, and water table conditions with gypsum as a tracer placed at 5-mm depth to evaluate the extent and nature of chemical transfer from soil solution to runoff and to determine the effective depth of the mixing zone. No electrolyte release was detected in runoff during the dry run for all three soils. Initial electrical conductivity (EC) was 60-120 µS/cm, which decreased exponentially to <6 µS/cm after 5 min. of rain during the wet run (2 d after dry run). Initial EC was 250-400 µS/cm for the water table run, and reduced rapidly to 50 µS/cm. Results indicated that the complete mixing concept is valid, and the effective mixing depth appears to be <3-4 mm. Fast- and slow-rate processes can be identified. The fast-rate processes, driven by rain drop impact and confined to the mixing zone, cause an exponential depletion of chemicals in the mixing zone. The slow-rate processes, dominated by molecular diffusion and mechanical dispersion, describe chemical transfer to the mixing zone from below.