USING SOIL MOISTURE PROBES TO MONITOR SOLUTE TRANSPORT IN ONE-DIMENSIONAL SOIL COLUMNS

One-dimensional soil columns are often used in the hydrogeology classroom to demonstrate principles of groundwater flow such as Darcy's law. Another topic that is often discussed in class, but not as easily demonstrated, is the transport of solutes in groundwater. Specifically, concepts such as breakthrough curves, solute dispersion, retardation, and average velocity may be difficult to demonstrate with a typical Darcy column. Our goal was to produce an experimental soil column design that would facilitate the demonstration of solute transport concepts in the classroom. The design uses commercially available, inexpensive (~$100/probe), soil moisture probes for in-situ measurement of changes in soil water electrical conductivity (EC) as ionic solutes (“tracers”) are transported through the column by flowing groundwater. We instrumented a column with probes every 5.5 cm. Tracer tests were run at various flow rates and tracer concentrations. The advance of the tracer through the column was clearly seen in the probe data. Velocity estimates based on the travel time of the breakthrough curve peak concentration between probes matched estimates from Darcy's Law reasonably well. No retardation was apparent for our soil; however, other soil/solute combinations could be used to demonstrate retardation. Though we found that the probe data could be used to assess velocity and to qualitatively demonstrate the concept of dispersion, it is not yet clear whether the probes can be used to quantitatively assess dispersion. In addition to facilitating the demonstration of solute transport, the column that we used had numerous manometers installed along the length of the column and could thus be used for the traditional Darcy experiment exploring the relationship between discharge, hydraulic conductivity, gradient, and area. For teachers or researchers interested in using the soil probes to demonstrate concepts related to flow and transport we recommend using at least two soil probes to demonstrate the use of a tracer test for the in-situ determination of groundwater velocity. A soil column designed and instrumented to address concepts of both flow and transport could be assembled for approximately $500, including two soil probes, soil probe datalogger, and column materials (e.g., PVC pipe, endcaps, and tubing for manometers).