Evaluation of the filter paper technique for in situ sampling of solute transport in unsaturated soils and tuffs

Abstract

The performance of a filter paper sampling system for monitoring solute transport in unsaturated soils and tuffs was evaluated using numerical simulations. The sampling system consisted of a collection borehole and a sampling assembly designed specifically for the easy removal and replacement of a filter paper pad used to collect solution samples at locations throughout the borehole. Simulations were conducted to approximate an experimental setup at Busted Butte, Nevada, for two hypothetical soil formations: a Calico Hills tuff and a loamy sand. The purpose of the simulations was to evaluate any significant effects the sampling system might have on the prevailing flow and transport processes. The simulation results revealed that the overall disturbance of the system is a superposition of three major components caused by the installation and operation of the sampling device. Two of these components delayed the observed solute breakthroughs and hence increased the mean and variance of solute travel times, whereas the third disturbed the system by increasing the mean solute velocity. The analytical solution of the one‐dimensional convective‐dispersive equation in the fluid coordinate frame was introduced to approximate the system response at different levels of disturbance for a better understanding of the system dynamics. The general difficulties in interpreting flow and transport properties using field‐measured data are illustrated. To better interpret field data, it is important to understand the limitations a measuring device may impose on data analysis.