1. Introduction
There are an estimated 140 000 Private Water Supplies ( PWS) in the UK (Reid et al., 2003). Although larger supplies, and those used for commercial purposes, are sampled frequently and the quality of the water tested, many of these supplies, particularly those supplying single households, are subject to either intermittent or no testing. In Scotland alone, there are an estimated 60 000 people who are served by supplies subject to low frequency of sampling. Shepherd (2000) suggested that the microbiological quality in three private water supplies, examined at weekly intervals over a two year period, was characterised by satisfactory water quality for long periods but with rapid and heavy pollution evident following rainfall events. It is this highly episodic pattern of water quality deterioration that makes the design of appropriate sampling protocols for risk and regulatory assessment difficult. Work in North East Scotland (Lilly et al., 2003) demonstrated that few PWS remain free of contamination for long periods and that heavy or prolonged rainfall was the main factor in reducing microbial water quality.
One approach to overcome any shortfalls in low frequency sampling is to develop and utilise a risk assessment procedure where the hazards likely to lead to a reduction in water quality are identified. By making improvements to the design of collection chambers or restricting animal access to the source, the water quality may be improved. Any risk assessment can be either quantitative where individual risk factors are scored and summed to give an overall risk or it can be a simple check-box approach that gives the owners and managers of such supplies a toolkit by which to effect changes to the collection, storage and distribution of potable water. One difficultly with a quantitative risk assessment is that some of the individual factors assessed have greater or lesser effects on water quality and need to have weightings applied to reflect this.
The Macaulay Institute and the Centre for Research into Environment and Health were asked by the Scottish Executive to combine their respective water quality datasets in order to attempt to identify weightings for various factors likely to influence water quality in PWS that could be subsequently used in quantified risk assessments. This report describes the two datasets (from North east Scotland and West Wales) and the relationship between rainfall and water quality in these datasets. The datasets were then used to validate a quantified and a semi-quantified risk assessment. The datasets were combined (as requested by the Scottish Executive) and used to examine whether logistical analyses could help identify other factors likely to affect water quality and, if so, what would be an appropriate weighting for use within quantitative risk assessments.