Method for in situ field calibration of fiber optic miniprobes

A fiber optic. miniprobe (FOMP) system, based on remote fluorometry, has be en recently developed for in situ real-time measurement of solute transport processes in soil. In order for the FOMP output light intensity measuremen ts to be converted to fluorescent tracer concentration, an in situ calibrat ion is necessary for each probe. The conventional calibration method consis ts of leaching the entire soil unit of interest with several pore volumes o f tracer solution for each calibration step. The steady signal measured by the probes at each step is then related to tracer concentration. This proce dure works well for typical laboratory soil columns but is impractical for fiber optic calibration in longer soil columns or field conditions because of the Large amount of time and tracer required. As a result, a "point" cal ibration method has been developed that consists of injecting standard solu tions directly into the small soil volume surrounding the measurement tip o f the probe. Comparisons of the conventional leaching method with the injec tion calibration techniques confirmed the new procedures work well in both silica sand and clay loam soil columns. As a result, the new calibration te chnique was tested in a heterogeneous field soil and provided a simple and accurate approach to calibrating FOMPs for in situ real-time measurement of solute transport processes in the field.