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Mixed human, animal and wildfowl sources possible

When the site survey has been completed and the likely source of contamination identified is unclear or likely to be a mixture of human and/or animals and/or birds, then the initial step is to sample and identify if there are elevated levels of E. coli present in the sample. Elevated levels of E. coli are dependent on the water type e.g. when dealing with recreational waterways we have found that >200 CFU/ml of E. coli is likely to illicit useful information from the toolbox of assays. However, we have also been able to identify faecal sources when 50 CFU/ml was the level of E. coli in the water sample. So based on the need for an answer it may be worthwhile for water managers to trial a low E. coli level with the realisation that levels of contaminants may be too low for detection.

Initial investigation needs to begin with all of the PCR markers that are likely to be relevant to the location. If human sources are suspected then FWA analysis should also be part of the toolbox. Faecal sterol analysis can be very helpful in estimating the contribution of faecal pollution from a combination of herbivore sources and human sources. Levels of coprostanol (the main human sterol) and 24-ethylcoprostanol (the main herbivore sterol) are used to estimate the percentage faecal contribution from humans and herbivores. Faecal sterol analysis would be recommended where a sample has numbers of E. coli >500CFU/ml and the detection of at least one human and one herbivore PCR marker is backed up by FWA analysis.

A combination of all three of the assays: FWA, PCR markers and Faecal Sterol analysis is required to give a high certainty of the correct identification of a faecal source host. It is also important to understand the intermittent nature of faecal pollution which maybe dependent on weather conditions and other environmental factors. Therefore greater emphasis can be placed on faecal source tracking when there are results from multiple samples collected at separate time intervals from the same water site.