The Impact Of Domestic Wastewater Effluent On Private Wells: An Evaluation Of Contamination Fingerprinting Techniques

Households in many rural areas worldwide rely on septic tanks, simple pit latrines or other means of wastewater disposal. Many such households are not served by a piped mains water supply, but rather obtain their water supplies from local wells. Sampling studies of private wells in Ireland and elsewhere have shown many wells to be contaminated, with sources of microbial contamination known to include domestic wastewater treatment systems, as well as practices associated with intensive agricultural production. While the microbial quality of private well water is commonly assessed using faecal indicator bacteria (FIB), such as E. coli, FIB are not source-specific, and provide no information as to the origin of the contamination. A range of chemical and microbiological fingerprinting techniques has been investigated in an attempt to identify a robust method for apportioning private well contamination to a specific source. Fingerprinting methods evaluated include ionic ratios, fluorescent whitening compounds, faecal sterol profiles, artificial sweeteners, caffeine, pharmaceuticals and human specific Bacteroidales 16S rRNA genetic markers. A total of 212 Irish households that depend on private wells and domestic wastewater treatment systems, commonly septic tanks, were evaluated by site assessment surveys. A once-off sampling and analysis campaign of these wells found that 15% were contaminated with E. coli. Subsequent monitoring of 24 selected wells found 45% to be contaminated with E. coli on at least one occasion. The application of fingerprinting techniques to these monitored wells found that ionic ratio analysis, specifically the use of chloride/bromide and potassium/sodium ratios, is a useful low-cost fingerprinting technique capable of identifying impacts from human wastewater and organic agricultural contamination, respectively. The artificial sweetener acesulfame was detected on several occasions in a number of monitored wells, indicating its conservative nature and potential use as a fingerprinting technique for human wastewater. However, fluorescent whitening compounds, faecal sterols and caffeine were not detected in any wells, suggesting low suitability. Whilst human specific Bacteroidales genetic markers were detected, further work is required to identify how the culture-independent nature of the method relates to faecal contamination.