In Situ Water Quality Monitoring Using an Optical Multiparameter Sensor Probe

Abstract

Optical methods such as ultraviolet/visible (UV/Vis) and fluorescence spectroscopy are well-established analytical techniques for in situ water quality monitoring. A broad range of bio-logical and chemical contaminants in different concentration ranges can be detected using these methods. The availability of results in real time allows a quick response to water quality changes. The measuring devices are configured as portable multi-parameter probes. However, their specification and data processing typically cannot be changed by users, or only with difficulties. Therefore, we developed a submersible sensor probe, which combines UV/Vis and fluorescence spectroscopy together with a flexible data processing platform. Due to its modular design in the hardware and software, the sensing system can be modified to the specific application. The dimension of the waterproof enclosure with a diameter of 100 mm permits also its application in groundwater monitoring wells. As a light source for fluorescence spectroscopy, we constructed an LED array that can be equipped with four different LEDs. A miniaturized deuterium-tungsten light source (200-1100 nm) was used for UV/Vis spectroscopy. A miniaturized spectrometer with a spectral range between 225 and 1000 nm permits the detection of complete spectra for both methods.

Keywords: UV/Vis spectroscopy; fluorescence spectroscopy; in situ water monitoring; model–view–controller architecture; submersible sensor probe.

Figure 1

Sensor concept and final UV/Vis–fluorescence submersible sensor probe.

Figure 2

Measuring cell with two 90° collimators (a) and LED array (b).

Figure 3

Schematic of the LED driver.

Figure 4

Flow diagram of Model–View–Controller architecture.

Figure 5

Schematic structure of sensor configuration for field applications.

Figure 6

Spectra of (a) nitrate and (b) humic acids and results of quantitative measurements of (c) nitrate (peak intensities at 241 nm, concentrations as NO₃-N) and (d) humic acids (peak intensities at 254 nm) using UV/Vis spectroscopy.

Figure 7

Fluorescence emission spectra of quinine sulfate (a) as proxy for FDOM after excitation at 340 nm and calibration (b) using peak intensities at λ_em = 385 nm.

Figure 8

Results of turbidity measurements using UV/Vis spectroscopy (a) and using scattered light measurement (b).

Figure 9

Dashboard for water monitoring campaign on the Elbe River. Water temperature, water depth, turbidity and FDOM are displayed together with the geo-position.

Figure 10

Spatial data plot of the turbidity measurements of the monitoring campaign at the river Elbe (315 single measurements).