MEASUREMENT OF SOLUTE TRANSPORT DURING CONSTANT INFILTRATION FROM A POINT-SOURCE

Theory describing transport from a point source is well developed, but not tested because of the lack of experimental data. There are few field studies because sampling requires either destructive soil coring or vacuum solution samplers. The objective of this study was to develop a method of measuring quickly and nondestructively the vertical flux of solute below a drip (point) source of water. The method uses vertically installed time domain reflectometry (TDR) probes making it appropriate for field studies. Water is dripped between the vertical probes at a constant rate until the wetting front has passed the ends of the probes and the measured soil water content is not changing. At this time, the water source is switched to one with a constant concentration of electrolyte tracer, dripping at the same rate. The relative change in TDR impedance is related to the advance of the solute front. Thus, measurements of average solute velocity and cumulative travel time as a function of depth are obtained. At a low drip rate into a sandy soil, the measured advance of the solute front vs. the cubed root of time was similar to that predicted from the theory of gravity-free infiltration. Identical measurements were obtained for the advance of solute-free water into a soil with a high background concentration of solute. Methods of estimating solute dispersivity and the relative influence of gravity vs. capillarity from the measurements were suggested but not tested. In particular a method of estimating alpha, the ratio of hydraulic conductivity and matric flux potential, is suggested using existing solutions of the linearized equation for flux from a point source.