A multiplexed fiber optic miniprobe system for measuring solute transport in soil

A fiber optic miniprobe (FOMP) system has been recently developed, capable of measuring solute breakthrough curves (BTCs) in situ and in real time at a single point within the soil matrix. We have multiplexed this system to a llow continuous operation of up to 20 of these FOMPs simultaneously. The sy stem was used to measure replicated multipoint BTCs of a fluorescent tracer in a large silica sand column, and allowed verification of the multiplexed FOMP system's ability to measure transport at a point scale. We performed seven miscible displacement studies (runs) generating 140 BTCs to examine s ystem performance, transport variability measured at a small scale, and the sensitivity of that variability to changes in initial pulse concentration (C-0). The convective-dispersive equation (CDE) was adequate to describe tr ansport in the silica sand (r(2) > 0.96). However, tails observed in all th e BTCs were attributed to an immobile phase by fitting several BTCs to a re sident concentration form of the mobile-immobile model (MIM). The mass reco very of the tracer ranged from 73 to 127% with an average of 98%. Changes i n column averaged transport parameters for the seven miscible displacement studies were minimal; however, there appears to be no consistency in transp ort at the measurement stale of the probes. Small changes in C-0 were not f ound to significantly alter or change the variability in transport paramete rs. The multiplexed fiber optic system was found to be an accurate tool for small-scale characterization of solute transport in soil.