Development and Use of Photometric Sensors for Contaminent Transport Studies

Amanda Treadaway, Cambridge University
Geotechnical Engineering Group


A new technique has been developed for monitoring contaminant transport in-situ under controlled conditions, which produces an approximately real time continuous output.The fibre-optic photometric sensor has been found to be a powerful tool for the investigation of contaminant transport phenomena and a great advance on the less elegant technique of obtaining samples for laboratory analysis.This valuable new device, which complements existing in-situ measurement techniques, uses an easy to measure parameter, light absorbance, which is directly related to concentration and thus produces linear calibrations.The system is highly versatile with the potential to detect any aqueous ionic or organic contaminant that absorbs light, provided a suitable wavelength is used

The original photometric detection system produced promising results in one-dimensional unaccelerated sand dispersion column tests conducted using short pulses of dye and CuSO4 as contaminants.However, a preliminary beam centrifuge test revealed that the early photometric detection system was detrimentally effected by increased gravity; improvements were made to solve this and other problems, including excessive baseline noise and drift.The final photometric detection system was then used in a number of accelerated contaminant transport experiments.This relatively inexpensive equipment was robust enough to function satisfactorily when mounted on a geotechnical centrifuge at 50-g and when used in high-pressure accelerated tests, provided measures were taken to keep air and fine particles out of the sensor bodies.

The photometric sensors have been successfully used to monitor concentrations of copper and food dyes, including Brilliant Blue FCF dye, in saturated soil beds of sand or sand incorporating a thin E-grade kaolin layer.They have produced sensible, reproducible results that have been readily explained using theory and supply information: on advection rates, longitudinal dispersion coefficients and sorption.Longitudinal dispersion coefficients and retardation factors were directly determined from the experimental results (an equation was derived to ascertain longitudinal dispersion coefficients from pollution plumes) and compared with values for these parameters obtained by fitting the CXTFIT model to the experimental results.Peclet numbers were calculated and the dominant dispersive mechanisms were assessed.Retardation factors were obtained from the results of batch equilibrium tests using equilibrium sorption models.Comparison between the retardation factors determined from the batch equilibrium tests and the contaminant transport experiments indicated that sorption was non-equilibrium.

Keywords aqueous contaminants, batch equilibrium tests, Brilliant Blue FCF dye, clay liner, contaminant transport, copper sulphate, CXTFIT, food dye, E-grade kaolin, fibre-optic, geotechnical centrifuge, high-pressure, longitudinal dispersion coefficient, one-dimensional flow, Peclet number, photometric sensor, retardation factor, sand, saturated soils, sorption.