Analytical Modeling of Soil Solution Monitoring by Diffusion in Porous Cups

Abstract

There is increasing interest toward in situ solution monitoring of soil chemicals for agricultural, industrial, and ecological purposes. Rather than extracting soil solution, a series of laboratory experiments was conducted to evaluate a diffusion equilibration technique, providing for real time in situ soil solution nitrate concentration via UV absorption spectroscopy. Experiments allowed for diffusion of nitrate from an outside reservoir into a porous cup. The experimental data were compared with model predictions using three different analytical solutions: (i) an exact solution (EX model) assuming radial symmetry that includes a porous cup with a porous wall and a finite size outside reservoir, (ii) an approximate solution (RC model) that assumes an infinitely large supply reservoir with a porous cup wall that does not influence ion diffusion, and (iii) and a model based on an electric circuit analog (ECA model). The results indicate that the ECA solution provides the best match overall to the experimental data.