Laboratory and field testing of a remotely deployed, multi-sensor fluorometer for groundwater tracing in an Ozark environment

Abstract

In fall of 2008, the Missouri Geological Survey Program, Missouri Department of Natural Resources began testing a remotely deployed, multi-sensor submersible fluorometer. The submersible data logging fluorometer is capable of integrating up to six sensors, including fluorescence and turbidity, with the device’s standard temperature and pressure sensors. Fluorescein, Rhodamine WT™ (Acid Red 388, manufactured by Chemcentral), optical brightener, and turbidity probes were installed on the unit. The instrument measures fluorescence in relative fluorescence units (RFU) that can be correlated to known concentrations. Data from laboratory testing indicated that the unit was capable of detecting tested water tracers in concentrations as low as one part per billion (ppb) in a zero turbidity environment and at concentrations of 10 ppb, even in turbid water. Background reflectance and fluorescence levels from organics in the environment and other tracers were determined to be insignificant for the fluorescein and optical brightener sensors. However, the Rhodamine WT probe gave false-positive readings when fluorescein dye was present due to spectral overlap from the fluorescein sensors excitations source. Spectral overlap was reduced to <1 % at all concentrations with the addition of a light-reducing shield to the Rhodamine WT sensor. The unit allowed simultaneous detection of multiple fluorescent water tracers and measured water quality parameters during extended remote deployments for continuous field monitoring. Field testing indicated the fluorometer was reliable detecting the three tracers tested and more precisely delineating the time of travel for tracers than conventional methods. Field testing revealed the unit provides more confidence in the detection of optical brightener than using carbon packet sampling methods and was less time consuming that collecting and analyzing numerous water samples.