Design of a flow-through electrical conductivity probe

Solute transport through soil is a complex process that requires accurate data for successful understanding and prediction of chemical movement. Non-binding conservative tracers, such as CaCl₂, move with soil water, increase the soil water solution electrical conductivity (EC), and thus can be used to determine important soil water and chemical transport parameters. Although post-processing and analysis can be used to determine EC values, and thus tracer concentration, "real time" tracer concentration data would allow for more efficient and direct EC determinations. Therefore, an easy to fabricate, flow-through EC probe has been designed. The funnel like device, which is made of Teflon to reduce chemical adsorption, uses surface tension to hold approximately one drop of column effluent. Probe calibration performed using KCl standards yielded a second order polynomial regression with an R² value of 0.99. The flow-through EC probe was validated against EC values in column effluent samples collected with a fraction collector and then analyzed with a bench-top EC meter. The comparison between the flow-through probe and the bench-top EC meter yielded a low root mean square error (0.12 dS m^-1 compared to tap water EC ≈ 0.7 dS m^-1), with a linear regression slope of 0.95 (n = 84). In addition, there were no significant differences in transport parameters determined by CXTFIT, a model that estimates solute transport in porous media, based on EC results from the flow-through probe and the bench-top meter.