Detecting faecal contamination sources to waterways using Proton Transfer Reaction - Mass Spectrometry

Abstract

Background: New Zealander’s take great pride in New Zealand’s well-known, clean, green image. To maintain this image a great deal of time and effort is put into monitoring and maintaining river water quality. The increase in dairying in New Zealand has increased the challenges associated with maintaining high water quality.

In New Zealand, dairy farming practices are different compared to the rest of the world. In New Zealand cows generally live the full year in paddocks grazing on pasture. This comes with unique challenges as the cow faecal material is deposited directly into the paddocks. From here the faecal material can travel via overland flow and subsurface pathways to rivers, streams and groundwater. The faecal material is of concern as it contains high levels of bacteria and a high solids content which can both affect water quality. Faecal material in the waterways can raise the bacterial levels to above the recognised safe limits for drinking and bathing. The high solids content of the faecal material results in a high biological oxygen demand (BOD) which depletes the waters of oxygen required by plants and animals. The high nutrient levels of the faecal material can also promote algal growth which can choke waterways. Proton Transfer Reaction Mass Spectrometry (PTR-MS) is a headspace analysis technique that detects volatile organic compounds (VOCs) in the air above a sample generating a VOC “fingerprint” specific to the sample. This technique was trialled to determine if it could be used as a technique to differentiate and quantify bacteria in waterways or used as a source tracking technique by differentiating faecal material from different animals (sheep, horse, deer and dairy cows).

Results: It was found that PTR-MS could differentiate between two species of bacteria (Escherichia coli and Pseudomonas aeruginosa). Mass ions specific to each bacterial species were identified with mass ion 49 (methanethiol) associated with the separation of P. aeruginosa, while mass ion 118 (indole) was associated with the separation of E. coli. Differentiation of cow, sheep, horse and deer faecal material was achieved with specific mass ions identified for each animal species. However, it was also determined that the VOC composition of sheep and cow faecal material changed over time as the fresh faecal material aged, this meant that the mass ions which differentiated the samples of fresh faecal material became less differentiating over time.

Conclusion: PTR-MS has potential to be used as a rapid, easy and relatively inexpensive means of faecal source tracking provided that the detection limit of VOCs from faecal slurries can be increased and that further studies can determine the impact of aging on the VOC profile of faecal material from different species.