CHARACTERIZATION OF IONIC TRACER TRANSPORT IN GROUNDWATER USING INEXPENSIVE SOIL MOISTURE PROBES

Determination of in-situ solute (tracer) transport rates using sampling piezometers to collect samples, which are then analyzed in the laboratory, can be time consuming. Geophysical techniques such as time domain reflectometery can measure conductivity changes induced by ionic tracers in-situ but the equipment is expensive. The goal of this research was to determine whether inexpensive soil moisture probes could be used to effectively and accurately measure ionic tracer breakthrough curves in a saturated sand. The relationship between probe output (voltage) and solution conductivity was first determined in water over a range conductivities for three models of probes. The experiments showed a generally predictable and reproducible relationship between voltage and conductivity so a second series of calibration experiments was conducted in saturated sand. Finally, the probes were used to measure tracer breakthrough curves in a laboratory flow cell experiment and a field-scale push pull experiment. Potential difficulties in applying this technique include voltage variations due to deployment method, the need for using high concentrations of tracer in settings with high background conductivities, possible density effects when high concentrations are used, and the fact that the technique is not ion-specific. Despite these difficulties, the probes appear to be promising for use in tracer experiments in shallow, saturated soil because they are sensitive to changes in solution conductivity, are inexpensive (~$100/probe), and can be read using an automated datalogger (~$200).