LABORATORY AND FIELD TESTING OF THE TURNER C6 MULTI-SENSOR SPECTROFLUOROMETER FOR GROUNDWATER TRACING

In the fall of 2008 the Geological Survey Program purchased a C6 Multi-Sensor Platform^TM submersible fluorometer from Turner Designs of Sunnyvale, California. The C6 submersible fluorometer is capable of integrating up to six Cyclops -7^TM fluorescence and turbidity sensors with the device’s standard temperature and pressure sensors. The C6 unit allows simultaneous detection of multiple fluorescent dyes and water quality parameters during extended remote deployments for continuous monitoring. Fluorescein, Rhodamine WT, optical brightener and turbidity probes were installed on the C6 unit.

Data from laboratory testing indicates that the unit is capable of detecting Rhodamine WT, fluorescein and Tinopal CBS-X (an optical brightener) in concentrations as low as 1 part per billion (PPB) in a low turbidity environment. All three dyes were detectible at concentrations of at least 10 PPB in turbid environments approximating 50 nephelometric turbidity units (NTUs). Fluorescence is measured in relative fluorescence units (RFU) and can be correlated to known concentrations. Background reflectance and fluorescence levels from the environment and other dyes were determined to be insignificant for the fluorescein and optical brightener probes. However, the Rhodamine WT probe gave false-positive readings when fluorescein dye is present due to spectral overlap from the fluorescein probe excitations source. The Rhodamine WT probe reports false readings of 58 ppb, 5.9 ppb, and 0.47 ppb at fluorescein concentrations of 1 part per million (PPM), 100 ppb, and 10 ppb, respectively, a range of 1 to 10 percent of the fluorescein concentration. Spectral overlap was reduced to less than 1 percent at all concentrations with the addition of a light shield to the Rhodamine WT probe. The addition of the light shield appeared to increase the accuracy of the Rhodamine WT probe by approximately 1 to 2 percent at the 10 PPB and 100 PPB concentration level, but reduced the accuracy by 7 percent at the 1 PPM concentration.