Point measurement of solute transport processes in soil using fiber optic sensors

Many of the mechanistic studies of solute transport in soil require nondestructive measurement of wafer now and solute transport parameters at temporal and spatial resolutions far beyond the capabilities of our existing methodologies. Using remote fiber optic fluorometry (RFF) techniques, a new methodology has been developed which allows in situ measurement of solute transport processes in soil in red time and on a continuous basis. The system consists of transmitting a constant beam of light through the input leg of a bifurcated fiber optic miniprobe to a location of interest within the soil matrix. At the probes tip, the incoming light interacts with the soil matrix where it is partially absorbed and partially reflected back into the probe. The reflected signal is transmitted through the output leg to a photodetector and quantified. The intensity of the output signal, which is constant under steady conditions, changes when a plume of fluorescent water tracer passes through the soil matrix in front of the probe, allowing for in situ measurement of a solute breakthrough curve (BTC) at the "point" of observation. Using this system, a series of miscible displacement studies was performed in 20-cm-long, 5-cm-diameter soil columns in which a fiber optic miniprohe (3.00 mm in diameter) was horizontally inserted at a column's midpoint. Details of the fiber optic system, including its components, setup, and operation, and the results of more than 15 BTCs in different soils will be discussed, along with an overview of the advantages and disadvantages of this new methodology.