Report No DWI0248
ASSESSMENT OF MONITORING REQUIREMENTS FOR DRINKING WATER
Final Report to the Department of the Environment
The Water Supply (Water Quality) Regulations 1989 implement the EC Directive 80/778/EEC relating to the quality of water intended for human consumption. Monitoring and analysis requirements are covered by Parts IV and V.
The Department of the Environment let a contract to WRc to examine the present Regulations in England and Wales, to advise on how well they implement the Directive with respect to monitoring and whether the discretionary parts give adequate consumer protection.
A detailed comparison was made between parts IV and V of the Regulations and the requirements of the Directive, especially in relation to the point of sampling and sampling frequencies.
Results from statutory monitoring were obtained from nine selected water companies and were subjected to statistical analysis. In addition further information and views on monitoring were obtained from the companies by means of a questionnaire.
Comparison of the Regulations with the Directive
Detailed comparison of the Regulations with the Directive indicates that they meet the mandatory requirements of the Directive and in many respects are more comprehensive. The Directive states imprecisely that the water should be monitored 'at the point where it is made available to the user'. The Regulations require this to be at a tap unless a supply point has been authorised for specific parameters.
The Regulations introduced the concept of a maximum size of supply zone, whereas this is not limited in the Directive.
Statistical Evaluation of the Parameters
A concept of 'predictable' and 'unpredictable parameters has been developed based on historical data. Of the 41 parameters for which adequate data exist. approximately half can be defined at present as 'predictable'.Sampling Frequencies
The report examines the probability of detecting non-compliance at the frequencies used in the Regulations. The report shows, for example, that, assuming a true rate of compliance of 98%, the standard frequencies in the Regulations give a probability of detecting failure of 2% for the parameters in 'Table 4', and 90% for the much increased frequencies for microbiological parameters in 'Table 6', at the largest zone size. The probability increases as sampling frequency increases and compliance decreases.
The standard frequency for "Table 4" parameters is judged to be too low and it is recommended that the minimum sampling frequency should be increased to four per year.
The report presents a statistical approach for judging when reduced frequencies can be used for 'predictable' parameters, based on past results.
Relating the frequency of sampling to zone size is shown to be a valid approach for parameters which can change in distribution but that fixed number sampling is more appropriate for other parameters. Additionally it is shown that there is no statistical reason why the zone size limit should not be increased.
The present rule for increasing sampling frequency where failures occur is questioned in terms of cost-effective monitoring. More emphasis should be placed on other follow-up actions to be taken when exceedences occur.
In principle, fixed point sampling should be specified except for parameters which tend to give exceedences at particular locations in distribution. The scope for fixed point sampling should be explored further. However, there is some evidence that the number of parameters specified for random sampling in the Regulations could be increased.
There is considerable advantage in monitoring at authorised supply points for parameters whose concentrations do not change in distribution and there is a case for extending this approach to zones where supplies are blended.
Data supplied by water companies did not allow an examination of parameter behaviour out of normal working hours. However, seasonality should be covered by advice in the Guidance Document.
Overall the monitoring requirements in the Regulations implement the content and spirit of the Directive and compare well with regimes known to exist in other Member States. There are some areas, however, which could benefit from re-appraisal.
Given adequate historical data, parameters can be classed as 'predictable' or 'unpredictable'. This distinction, together with a knowledge of whether concentrations change in distribution, could assist in the development of monitoring strategy.
Copies of this report may be available as an Acrobat pdf download under the 'Pre 2000 Reports' heading on the DWI website.