The Use of Tritiated Water to Study Water Circulation in a Sandy Soil in Senegal

The authors have carried out a study in Senegal on the behaviour, with respect to the soil solution, of a flow of water infiltrating a sandy soil under a constant head. The experimental set-up consisted of a cylindrical lysimeter 2 m high and 1 m in diameter. The initial water content of the soil was increased to near field capacity. A sheet of tritiated water 3 cm deep was then maintained on the soil surface. The drainage water was collected in 5-litre amounts and the radioactivity measured in the laboratory. The authors first compare the infiltration and drainage rates for dry and wet soil. From examination of the curve showing variations in the concentration of tritiated water in the drainage water as a function of the amount of drainage water it can be seen that the tritiated water) begins to appear in the drainage water before the water initially in the soil has been completely replaced. The concentration then increases rapidly and approaches the original value. From the shape of the curve the authors conclude that the soil solution is not immediately displaced in its entirety by the applied water, even in very sandy soil; instead, there is an intermediate phase during which the two solutions present are progressively mixed. This phase involves a total of 102 mm of water, of which 42 mm is tritiated water. The mixing occurs within an 80-cm thick soil layer. In conclusion, the authors discuss possible agronomic consequences of the processes studied. (author)