4.6 Fluorescence Spectroscopy

Fluorescence spectroscopy (also known as fluorimetry or spectrofluorometry) is a type of electromagnetic spectroscopy that analyzes fluorescence from a DOC sample. It involves using a beam of light, usually ultraviolet light, that excites the electrons in molecules of organic compounds and causes them to emit light at a different wavelength; typically, but not necessarily, visible light. At low DOC concentrations typical of groundwater, the fluorescence intensity will generally be proportional to the concentration of the fluorophore, that is, the part of the DOC that fluoresces. Unlike in UV/visible spectroscopy, ‘standard’ device-independent spectra are not easily attained. Several factors influence and distort the spectra and corrections are necessary to attain ‘true’, i.e., machine-independent, spectra. This complicates the use of fluorescence spectroscopy for application to groundwater.

An example of how fluorescence spectroscopy has been applied to two hydrologically different aquifers in the United Kingdom was given by Lapworth and others (2008). A VarianTM Cary Eclipse fluorescence spectrometer was used for the fluorescence analysis. Excitation (Ex) wavelengths were set between 200 and 400 nm with a 5 nm bandwidth and emission (Em) wavelengths were set between 250 and 500 nm with a 2 nm bandwidth. The results were entered into what is known as an excitation-emission matrix (EEM) with the excitation wavelength (nm) entered on the x-axis, the emission wavelength (nm) on the y-axis, and fluorescence intensity (au) on the z-axis. Total fluorescence is determined by summing the intensity across the whole EEM after masking interfering peaks such as those for water (Lapworth et al., 2008).

Two study sites tapping Permo-Triassic Sandstones were chosen for this study, one being the Penrith Sandstone of Cumbria and one in the Sherwood Sandstone of South Yorkshire. Both aquifers are regionally important sources of public water supply but have contrasting hydrogeological settings. The Penrith Sandstone is unconfined and is locally recharged directly by atmospheric precipitation. In contrast, the Sherwood Sandstone consists of multiple confined and unconfined aquifers that are more removed from direct atmospheric recharge. Both study sites have oxidizing conditions based on dissolved oxygen concentrations, and therefore changes in fluorescence are not expected to be due to changing redox gradients. Clear differences were observed in the fluorescence profiles of the two aquifers (Figure 17).

Graph showing changes in total fluorescence with depth

Figure 17  Changes in total fluorescence with depth in the Penrith Sandstone (site A) and the Sherwood Sandstone (site B). Depth is expressed as meters below ground level (mbgl).

First, the total intensity of Penrith DOC was much lower than Sherwood DOC. Secondly, the total intensity of Sherwood DOC decreased initially with well depth and then remained relatively constant. In contrast, the total intensity of Sherwood DOC decreased more gradually with well depth. What could explain these observed differences? One possibility is land use. The Penrith Sandstone is located in a rural setting whereas the Sherwood sandstone is located in a suburban setting. Another possibility is that there are differences in DOC concentrations, based on the strong correlation that was observed between intensity and DOC concentrations (r2 of 0.58, p = 0.05). It is also possible that differences in hydrologic setting (confined versus unconfined) may affect the fluorescent properties of DOC. Finally, differences in the lithology between the different sandstones might explain the differences. So, while fluorescence spectroscopy can reveal spatial and temporal differences in the fluorescent properties of groundwater DOC, the significance and/or causes of those differences are difficult to determine.


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