Fluorescence as a measure of microbial contamination risk in groundwater

Research from Oxford and the British Geological Survey advances a new water quality monitoring approach.

Globally, a quarter of people lack access to water that is free from microbial contamination. In some countries that proportion is much higher. Consuming water that contains pathogens has serious health consequences, especially for children. Groundwater, which accounts for a third of global water supply, usually has better microbial quality compared to surface water, but it can be vulnerable to pollution and must be monitored to confirm when it is safe to drink.

Sampling for individual pathogens is costly and slow, so microbial water quality monitoring is usually designed around risk indicators, with Escherichia coli being the preferred choice. The presence of an indicator like E. coli tells us whether pathogens of faecal origin are likely to be present in the water. Although E. coli sampling is less onerous than measuring pathogen presence directly, it can still be prohibitively costly and time consuming – especially in low resource settings. Furthermore, pathogens (viruses in particular) may still be present in groundwater systems when E. coli is absent.

Motivated by the limitations of relying exclusively on E. coli, a research team from the University of Oxford (Saskia Nowicki, Katrina Charles, and Patrick Thomson) and the British Geological Survey (Dan Lapworth and Jade Ward) examined the potential for improving microbial risk assessments by using a complementary parameter, tryptophan-like fluorescence (TLF). Faecally contaminated water has high TLF intensity, which can be measured immediately in-situ without laboratory processing or expensive consumables.

The research was conducted in rural Kwale County, Kenya. In brief, the team found that TLF cannot be used as a proxy for E. coli on an individual sample basis, but it can add value to groundwater risk assessments by improving prioritisation of sampling and by increasing understanding of spatiotemporal variability. In addition to the low cost of sampling, precision is a key benefit of including TLF in monitoring: unlike bacteriological methods, in-situ fluorimetry has negligible method-induced variability. Factors like turbidity, changing temperature, and humic organic content can interfere with TLF measurement, but such interference is minimal in typical groundwaters. Nevertheless, potential interferences should be considered before including TLF in sampling programmes.

Further details:
Nowicki, S., Lapworth, D.J., Ward, J.S.T., Thomson, P., Charles, K., 2019. Tryptophan-like fluorescence as a measure of microbial contamination risk in groundwater. Sci. Total Environ. 646.

Funding
Lead author, Saskia Nowicki is a DPhil scholar supported by the UK Commonwealth Scholarship Commission and the University of Oxford. This research was also supported with funding from the UK Natural Environment Research Council, the UK Economic and Social Research Council and the UK Department for International Development for the UPGro programme on ‘Groundwater Risk Management for Growth and Development’.