| dc.description.abstract | The absence of a public water supply network in many rural areas of Ireland means that approximately 162,000 households rely on private wells for their water provision. In many cases such rural areas are also not connected to a centralised sewerage network and therefore depend on onsite domestic wastewater treatment systems (DWWTS). The necessity for the disposal of domestic wastewater and the attainment of safe drinking water within a spatially confined household site requires an in-depth understanding of the contaminant transport and attenuation processes for the DWWTS effluent - in parallel with suitable water well design and construction - in order to ensure groundwater resources (and hence human health) are adequately protected.
Previous sampling studies of private wells in Ireland have, however, shown a proportion to be contaminated, with sources of microbial contamination known to include DWWTS, as well as practices associated with intensive agricultural production. As exposure to faecal contaminated water is associated with acute gastrointestinal illness, which in some cases can prove fatal, it is important that sources and pathways of well contamination are identified and remedied. However, 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, making remedial actions and the refinement of best practices in onsite wastewater treatment and private well protection challenging. The overall aim of this research was to assess the impact of DWWTS on the well water quality of private wells in Ireland, through the application of a range of chemical and microbiological contamination fingerprinting techniques, including an evaluation of their ability to apportion contamination to a specific source.
A total of 212 households dependent on private wells and DWWTS were evaluated by individual site assessments that recorded variables relating to the hydrogeological setting, the individual construction features of each well and its siting relative to potential sources of contamination. The findings demonstrated that a significant proportion of wells were often inadequately sealed and protected against contamination via direct ingress. Subsequent sampling and analysis of each well for a range of chemical and microbial water quality indicators found that 15% were indeed contaminated with E. coli.
Through the development of a multiple logistic regression statistical model, the site assessment and water quality data were analysed to identify the specific factors associated with private well contamination. While geological setting and siting with respect to a DWWTS were not found to be significantly associated with the occurrence of contamination, poor wellhead condition and proximity to a potential point source of agricultural contamination were statistically significantly associated with the presence of E. coli.
A representative subset of 24 wells was monitored over a 14-month period to assess temporal variation in well water quality. This monitoring period identified significant temporal variation in quality, with 58 % of the wells testing positive for E. coli on at least one occasion.
These wells were then used to evaluate a range of chemical and microbiological fingerprinting techniques. Fingerprinting methods evaluated include ionic ratios (K/Na and Cl/Br), fluorescent whitening compounds, faecal sterol profiles, artificial sweeteners, caffeine, pharmaceuticals (carbamazepine and sulfamethoxazole) and human specific Bacteroidales 16S rRNA genetic markers.
The performance of each fingerprinting technique as well as the resources (and hence costs) required for their application varied considerably. Three methods/compounds (K/Na, Cl/Br and acesulfame) showed promise in identifying sources of well contamination. Elevated K/Na and Cl/Br ratios were found to be an indicator of potential impacts from decaying organic matter (such as farmyard waste) and DWWTS, respectively. The artificial sweetener acesulfame was detected on several occasions in a number of faecally-contaminated wells. Its conservative nature and anthropogenic origin suggest its potential use as a fingerprinting technique for human wastewater. In contrast, carbamazepine, sulfamethoxazole, caffeine, FWC and faecal sterol profile analysis appear to show reduced potential due to limited or non-detection. Whilst human and bovine specific Bacteroidales genetic markers were detected, further work is required to identify how the culture-independent nature of the method relates to faecal contamination.
The findings from this research were used to develop a stepped approach that could be applied to any private well in Ireland (and potentially internationally) in order to identify well water quality and the source(s) of any contamination that is identified. This tiered approach has the potential to identify the well water quality, its susceptibility to contamination, the source of contamination if any present and, hence, any remedial works that are required. As this research has indicated that a significant number of private wells are at least intermittently contaminated, the application of the tiered approach across Ireland is recommended to ensure groundwater and human health are protected. | en |
