LATERAL BROMIDE DISTRIBUTION IN A VERTIC CLAY SOIL

In order to control the transport of toxic elements, dissolved salts, and nutrients in agricultural areas, information on the spatial variab ility of field-scale transport properties is needed. To evaluate this for an unsaturated layered clayey soil, tracer tests were conducted at the Cherfech experimental field research station in Tunisia. Bromide- tagged water was infiltrated under pending conditions on a 21.7-m(2) h orizontal field plot equipped with 60 solute samplers, 15 neutron prob e access tubes, and 15 piezometers. Volumetric soil water content was measured by means of a neutron probe at five depths, and soil moisture samples were withdrawn through ceramic soil water samplers at four de pths, each with 15 suction samplers. The results showed typical eviden ce of preferential now, with a wide variety of travel times with depth . This was noted especially for deeper soil layers, which displayed a large horizontal variation. In two tracer experiments, the groundwater tracer concentration increased up to twice the concentration of the w ater in the unsaturated zone withdrawn from different depths. This sho ws that bypass directly to the groundwater, initially at 1.5 m depth, occurred under pending with chemigation.