Post by Andy Baker, Professor researching groundwater, caves, past climate, organic carbon and more at the University of New South Wales, in Australia.
We often come across items that glow after being exposed to ultraviolet light. Fluorescent stickers can be bought for the ceilings of bedrooms; fluorescent hands on analogue clocks and watches; fluorescent markings on a car dashboard.
In all these examples, there are organic molecules that absorb energy in the form of ultra-violet light and can then re-emit that energy, in this case as visible light. We are talking fluorescence. Or when the emission of light is delayed, phosphorescence. It requires loosely-held electrons to be present in a molecule. The energy provided by ultraviolet light can excite these electrons to a higher energy level, and when the electrons return to a lower energy level, light is emitted. This emitted light has to be at a longer wavelength than the excitation energy, and if it occurs at wavelengths our eyes can detect, then we can see it. Hence the blue-green colours of watch hands and plastic ceiling stars.
What about groundwater? It’s the same process: if we shine ultraviolet light at groundwater samples, then they fluoresce due to the presence of organic molecules that are often present. Unfortunately, we can’t see any of this fluorescence with our eyes, as it is emitted in the middle- and long-range ultraviolet, so we must use detectors that can ‘see’ at these wavelengths. But that is relatively easy – charge coupled devices (CCDs), the same as you would find in a digital camera, detect in the ultraviolet. And we can add in improved light emitting diode (LED) technology, which can now produce higher-energy, shorter-wavelength ultraviolet light to excite fluorescent molecules. It’s the same technological improvement that means that you can now buy blue LEDs to decorate your house – have you noticed how they have become increasingly available – and potentially keeps you awake at night.
Why does groundwater glow in ultraviolet light? Firstly, it could be from natural organic matter. Organic matter is transported by rivers, which may be recharged to groundwater where rivers are ‘losing’. Or it might be leached from the overlying soil during rainfall recharge of groundwater. Or it might be desorbed from sedimentary material in the aquifer. Natural organic matter fluorescence tends to occur at longer ultraviolet wavelengths (360-400 nm) and provides a convenient way of detecting dissolved organic matter.
Secondly, groundwater samples might fluoresce due to the presence of microbial matter. In rivers and wastewater systems, the amount of fluorescence at shorter ultraviolet wavelengths (300-350 nm) has been observed to correlate with the amount of oxygen being consumed (the biochemical oxygen demand, or BOD). It is not possible to distinguish between individual or groups of microbial species, and researchers are still investigating exactly what is fluorescing. However, recent research has shown that there is significant potential in using hand-held fluorescent probes to determine microbial water quality in groundwater. Where there might be faecal contamination of groundwater used for drinking water supply, there is great advantage to this method as an immediate reading is possible in comparison with other methods which typically take 18-30 hours.
Thirdly, groundwater which is contaminated by organic matter may be detected if enough of the contaminant is fluorescent. For example, fluorescent whitening agents, also known as optical brighteners, may be added to detergents, shampoo and paper products to make items appear whiter. The molecules are designed to emit light at the blue-violet end of the visible light spectrum, and it counters any yellowing of aging fabric, paper or hair. Hence your clothes appear whiter, your hair blonder. Fluorescent whitening agents are removed during wastewater treatment and degrade in sunlight. But in the case of unlined landfill sites, fluorescent whitening agents can persist in contaminant plumes in the groundwater, making them a useful tracer.
So why does groundwater glow in ultraviolet light? It is all to do with any fluorescent organic matter that might be present in groundwater. And thanks to improvements in technology, we can now make measurements of this fluorescence using portable and handheld probes, in-situ, and rapidly. Increasingly adopted by surface water quality researchers and water engineers, is it time for the groundwater community to move on from groundwater flow to groundwater glow?