Part 1
Overview of Water Quality in England and Wales
For 2000, a further improvement in overall compliance with 99.83% of 2,684,294 tests carried out complying with the Regulations.
Coliforms absent from all samples at 1,206 (86.89%) of the 1,388 water treatment works.
Coliforms absent from at least 95% of samples (the regulatory requirement) at 4,764 (99.60%) of the 4,782 service reservoirs.
Continued improvement in compliance in water supply zones with 99.81% of the 1,844,891 tests carried out in zones complying fully with the numerical standards.
Notable improvement in compliance with standards for iron, PAH, turbidity and lead.
Almost full compliance with the pesticides standards maintained.
Summary Information
Number of water companies*: 27 Number of consumers supplied: 52.1 million Amount of water supplied: 15,300 Ml/d Proportion from surface water sources: 68% Proportion from groundwater sources: 32% Number of treatment works: 1,388 Number of service reservoirs: 4,782 Number of water supply zones: 2,316 Total length of mains: 323,000 km
* Including inset appointments Introduction Link to published report
Overview Format
This part of the Report deals with the overall quality of drinking water supplied during 2000 to almost 52 million consumers in England and Wales, by the 27 water companies.
The Overview Chapter is split into five general sections as follows:
- Overall Water Quality in 2000 – gives summary statistics on water supply information in England and Wales, levels of overall compliance with regulatory standards, and a brief comparison with previous years.
- Microbiological Quality of Water Leaving Treatment Works – expands on the information in the previous section and provides further data and analysis on the compliance with standards for water leaving water treatment works in 2000, compared with previous years.
- Microbiological Quality of Water in Service Reservoirs – similarly expands on the information in the Overall Water Quality section and provides additional data and analysis on the compliance with standards that apply to water in service reservoirs.
- Water Quality in Water Supply Zones – whereas compliance at treatment works and in service reservoirs is based solely on microbiological parameters, compliance in water supply zones is based on all 55 parameters with numerical standards. This Section provides greater detail on how well these standards were met in 2000 and compares the results with those achieved over the last ten years.
- Comparisons of Drinking Water Quality between Companies – many factors, such as variations in the nature and character of particular water sources across the country and the respective sizes of different companies, make direct comparison of compliance data meaningless. Statistical techniques, allowing comparisons to be made, have been developed. The Inspectorate uses two indices: the Overall Quality Index and the Operational Performance Index to allow meaningful comparison.
Statistical Significance
Throughout this Report, and particularly in this Section, the term "statistically significant" is commonly used where comparisons are made between data from different years. Often, apparent differences in proportion of samples breaching the standards are affected by factors other than water quality, such as differences in the overall number of tests carried out, or changes to the total number of treatment works, service reservoirs or zones. Also, as the number of tests carried out is often very large, and the number of breaches is comparatively small, in many cases the differences are sufficiently small to be attributable to random variations.
Therefore, in each case ‘significance’ has been tested using accepted statistical techniques (normal approximation to the binomial distribution; 95% confidence interval) to determine those cases where statistically significant differences can be said to have occurred.
Where changes have been found to be ‘not statistically significantly different’ it indicates that, whilst the numbers may be different, no strong conclusions can be drawn concerning trends.
Presentation of Data
This year, for ease of reading, greater use has been made of tables and charts to present the water quality data.
For each of the sections dealing with treatment works, service reservoirs and water supply zones, data for 2000 is presented in table form. Comparative data from 1999 and 1998 has been included in each case.
Where information concerning overall compliance or numbers of non-compliant samples is shown, histograms have been used to represent the data for each year. In cases of individual parameters, line charts have been used since they were considered the most effective form to show the trends in compliance with standards over the last ten years. The contributions of individual parameters to the overall level of non-compliance, in water supply zones, in 2000 is shown in the form of a pie chart.
Overall Water Quality in 2000
At water treatment works, in service reservoirs and in water supply zones, the companies carried out a total of 2,684,294 tests in 2000. Of these, 99.83% demonstrated compliance with the relevant water quality standards. This is a relatively small percentage increase compared with 1999 (99.82%) but the number of breaches of the standards reduced from 5,148 in 1999 to 4,475 in 2000.
At 1,206 (86.89%) of the 1,388 sampling points at water treatment works, coliforms were absent from all samples.
At 4,764 (99.60%) of the 4,782 sampling points at service reservoirs, total coliforms were absent from at least 95% of samples (the regulatory standard).
The overall number of tests failing the standards has decreased from 44,973 in 1991 to 4,475 in 2000. This represents a reduction of 90% over the last ten years.
Figure 1 below shows the overall compliance with the standards, specified in the Water Quality (Water Supply) Regulations 1989, over the last ten years. The improvement is best seen in Figure 2, which shows the decreasing number of breaches of the standards.
The improvement over the period is mainly due to significant reductions in the number of breaches of the total coliform and faecal coliform standards in service reservoirs, and of the iron, lead, turbidity, PAH and pesticides standards in zones. There have also been lesser, but notable, contributions from reductions in breaches of the standards for a number of other parameters such as nitrate, nitrite, manganese and pH.
Figure 1
Figure 2
A number of smaller treatment works have been closed as part of the implementation of the Cryptosporidium Regulations. A number of service reservoirs have also been abandoned because of operational changes in supply arrangements. This has resulted in a decrease in the number of test results reported for treatment works and service reservoirs.
There has been a similar steady reduction in the number of test results reported for water supply zones. This is partly because of changes in zone boundaries resulting in fewer zones. There has also been a steady decrease in the number of parameters sampled at the increased sampling frequency, and more instances where the reduced sampling frequency applies. This occurs as improvements in drinking water quality are achieved. The reduction in the number of tests means that there is a smaller impact on overall percentage compliance figures.
Microbiological Quality of Water Leaving Treatment Works
Figure 3 below shows the companies' performance in 2000, with data for 1999 and 1998 for comparison.
Figure 3 Microbiological Quality of Water Leaving Treatment Works
2000 1999 1998 Number of water treatment works 1,388 1,447 1,456
COLIFORMS
Total number of tests 173,544 183,773 186,640 - number containing coliforms 247 215 290 - % containing coliforms 0.14 0.12 0.16 Treatment works with coliforms detected 182 165 220 - % of all works 13.11 11.40 15.11
FAECAL COLIFORMS
Total number of tests 173,553 183,804 186,645 - number containing faecal coliforms 43 36 54 - % containing faecal coliforms 0.02 0.02 0.03 Treatment works with faecal coliforms detected 41 32 48 - % of all works 2.95 2.21 3.30
Figures 4 and 5 below show the companies’ performance in meeting the regulatory requirements at water treatment works over the last ten years.
In Figure 4, overall compliance with the regulatory requirements since 1991 is shown. Figure 5 shows the trend in the proportion of treatment works at which coliforms or faecal coliforms have been detected over the last ten years.
Figure 4
Figure 5
There has been a slight increase, since 1999, in the number of treatment works, and samples from treatment works, where both total and faecal coliforms were detected.
A total of 247 samples taken from 182 different treatment works (13.11% of all works) were found to contain total coliforms, compared with 215 samples from 165 treatment works (11.40%) in 1999. Faecal coliforms were detected in 43 samples taken from 41 treatment works (2.95% of all water) compared to 36 samples from 32 treatment works (2.21%) in 1999.
This increase is contrary to the overall ten-year trend, which has seen continued improvement in water quality leaving treatment works. At 99.92% the overall compliance is still very high, and is higher than in any year except for 1999 when it reached 99.93%.
Statistically, even after allowing for the reduction in the number of samples taken and the number of treatment works in operation, the number of samples containing total coliforms in 2000 is considered significantly higher than in 1999.
Examination of all 27 companies’ individual performance at treatment works compared with last year showed that, for 23 of the companies, there was no statistically significant difference from 1999. Two companies had shown a statistically significant improvement, but the performance of two companies had deteriorated significantly.
Many factors can influence compliance with microbiological standards, some of which the companies cannot control. However, companies showing significant deterioration in performance will be subject to enhanced audit and inspection by DWI during 2001.
It is important to point out that the detection of total coliforms and faecal coliforms in the treated water does not mean that the water is harmful to health. They are indicator organisms, not harmful themselves, which give a warning that more dangerous organisms may be present. They are easy to detect and their presence in water leaving a treatment works can indicate that the effectiveness of the treatment or disinfection processes has been reduced. Thus their presence is taken very seriously and action is taken to remove them.
For every breach, the companies concerned carried out investigations to determine the nature of the problem. In the vast majority of occasions, appropriate action was taken, and the Inspectorate considered the breaches of the standard to be either trivial, or unlikely to recur as a result of remedial actions taken by the companies. However, the Inspectorate has considered, or has taken enforcement action against, eight companies in respect of 14 treatment works to require the companies concerned to take the necessary measures to prevent further breaches.
Microbiological Quality of Water in Service Reservoirs
Figure 6 below shows the companies' performance in 2000, with data for 1999 and 1998 for comparison.
Figure 6 Microbiological Quality of Water in Service Reservoirs
2000 1999 1998 Number of service reservoirs 4,782 4,830 4,894
COLIFORMS
Total number of tests 246,133 248,615 252,375 - number containing coliforms 507 527 602 - % containing coliforms 0.21 0.21 0.24 Service reservoirs with coliforms detected 427 449 509 Service reservoirs with coliforms detected in more than 5% of samples 18 22 23 - % of all reservoirs 0.40 0.46 0.47
FAECAL COLIFORMS
Total number of tests 246,174 248,646 252,392 - number containing faecal coliforms 81 95 97 - % containing faecal coliforms 0.03 0.04 0.04 Service reservoirs with faecal coliforms detected 72 90 91 - % of all reservoirs 1.51 1.86 1.86
Figures 7 and 8 below show the companies’ performance in meeting the regulatory requirements at service reservoirs over the last ten years.
In Figure 7, overall compliance with the numerical standards for coliforms and faecal coliforms since 1991 is shown. Figure 8 shows the trend in the proportion of service reservoirs breaching the standards for coliforms or faecal coliforms over the last ten years.
It should be noted that, whilst Figure 8 shows the proportion of service reservoirs where coliforms were absent from at least 95% of samples taken over the year (the regulatory requirement), all incidences of detections of coliforms have been used in generating Figure 7.
Figure 7
Figure 8
In 2000, the number of service reservoirs, and samples from service reservoirs, containing total coliforms and faecal coliforms fell again. In all cases the numbers are lower now than at any time in the last ten years.
Only 18 (0.40%) of the 4,782 service reservoirs in England and Wales failed to comply fully with the standards for total coliforms, and only 72 (1.51%) failed to comply fully with the standard for faecal coliforms. This represents a reduction of 88% in the number of service reservoirs failing to comply with the standard for total coliforms, and a 78% reduction in the number failing for faecal coliforms, since 1991.
During 2000 a total of 492,307 microbiological tests were carried out on samples taken from service reservoirs. Total coliforms or faecal coliforms were only detected on 588 occasions, giving an overall compliance figure of 99.88%. This is the highest figure since the regulations came into force, and represents a reduction of 74% since 1991, when more than 2,200 tests failed to comply with the numerical standards.
On each occasion that coliforms or faecal coliforms were detected the companies carried out an investigation into the cause and, where necessary, carried out remedial actions to reduce the chances of a recurrence. The Inspectorate considered each of the breaches during the compliance assessment audit process, to determine whether further action was required. In most cases the breaches were considered trivial, or unlikely to recur because of the actions taken. However, enforcement action has been taken, or is under consideration, for breaches of the total coliform or faecal coliform standards at seven service reservoirs in five company areas.
Water Quality in Water Supply Zones
Figure 9 below shows the companies' performance in 2000, with data for 1999 and 1998 for comparison.
Figure 9 Water Quality in Water Supply Zones
Parameter Tests in 2000 Zones in 2000*: 2,316 Tests Taken Contravening PCV Non-compliant Number % 2000 1999 1998 Coliforms 149,683 843 0.56 8 4 15 Faecal coliforms 149,692 62 0.04 60 61 63 Colour 42,510 9 0.02 8 3 10 Turbidity 64,555 37 0.06 33 75 92 Odour 15,572 6 0.04 6 11 11 Taste 15,420 5 0.03 5 6 11 Hydrogen ion 65,259 37 0.06 27 25 42 Nitrate 32,755 27 0.08 10 10 11 Nitrite 34,672 1,031 2.97 184 203 194 Aluminium 37,600 38 0.10 35 41 41 Iron 50,854 656 1.29 375 440 500 Manganese 39,378 111 0.28 97 113 144 Lead 20,408 190 0.93 135 212 250 PAH 10,731 322 3.00 174 264 289 Trihalomethanes 11,294 129 1.14 45 27 70 Total pesticides 29,811 1 <0.01 1 4 2 Individual pesticides 606,979 45 0.01 44 81 48 All others 467,718 48 0.01 0 0 0 TOTAL 1,844,891 3,597 0.20 - - -
* 2,324 in 1999; 2,337 in 1998
Figure 10 below shows the overall compliance with water quality standards in companies’ water supply zones over the last ten years.
The ‘compliance figures’ in this chart have been derived by taking the total number of tests taken in zones and the number of occasions on which one or more sample has breached the relevant numerical standard (‘point breaches’). This does not take into account parameters such as total coliforms where a zone is not considered non-compliant unless more than 5% of samples taken contain coliforms, or certain other parameters where compliance with the standard is based on an average of results. The data are, however, considered a useful indicator of the trend in water quality in supply zones over the last ten years
Figure 10
Since 1991 there has been a 91% reduction in the number of test results from zones which failed to comply with the relevant standards. In 2000, only 3,597(0.19%) of the 1,844,873 tests carried out in zones failed to comply fully with the standards, compared with 41,724 samples that failed in 1991.
Trends over the last ten years in compliance with the standards for the 55 individual parameters which have a numerical standard, and which companies must test for in all of their water quality zones, are represented in the following six charts (Figures 11 to 16). In this case the percentage figures are calculated from data for zonal compliance and therefore do take account of cases where compliance is based on a percentage, or on average results.
For clarity, the parameters have been arranged in groups of broadly similar character, and the charts below generated according to these groups, but it should be stressed that these groupings do not have any scientific or regulatory significance. The groups are categorised as follows.
- Microbiological Parameters: namely total and faecal coliforms;
- ‘Physical’ Parameters: parameters which, in general terms, affect the appearance, taste or odour of the water;
- ‘Metals’ Parameters: trace metals, such as iron or lead, which may be detected in drinking water;
- ‘Organic’ Parameters: parameters such as pesticides, and disinfection by-products which companies regularly monitor for;
- ‘Ionic’ Parameters: other ionic parameters (pH, nitrate and nitrite);
- All Other Parameters: a combined figure for the remaining 38 parameters, regularly tested for, but rarely found at non-compliant levels.
For further information on a specific parameter, please refer to the Quality Standards Table in the introduction to Part 2 of this Report.
Figure 11
Figure 12
Figure 13
Figure 14
Figure 15
Figure 16
Non-Compliant Zones
The meaning of the term 'non-compliant' needs to be borne in mind when considering the tables and charts in this section of the Report. In many cases, a zone may have become non-compliant because a single result has breached a standard. Such breaches have been regarded as trivial in the compliance assessment and do not indicate a serious problem with water quality.
In other cases the breaches, although not trivial, are considered unlikely to recur as a result of remedial actions taken by the companies, or are being remedied through undertakings given by the companies concerned to carry out appropriate improvement programmes to overcome the problem and achieve compliance.
Enforcement action has been considered in a small number of cases, where the contravention is likely to recur and where no remedial action is in place. Such action has been considered, or is under consideration, for breaches of standards for 13 parameters in 41 (1.78%) water supply zones. Full details of this enforcement action are given in Part 3, Section E, of this Report and in individual company sections.
Figure 17 below shows those parameters for which breaches of standards have occurred in 2000. Each is shown as a percentage of the 3,597 individual breaches in water supply zones. A commentary on significant parameters is given below.
Figure 17
Microbiological Parameters
The number of zones contravening the total coliform standard has shown a slight increase since last year. The increase is not statistically significant and is still lower than in any year except 1999. The number of zones contravening the faecal coliform standard has shown a further small reduction since 1999 although this reduction is not considered statistically significant.
Breaches of the standards in samples taken from consumers' taps can sometimes be caused by the condition of the plumbing and therefore do not necessarily reflect fully the microbiological quality of the water supplied. However, companies are expected to take appropriate action in the event of any failure of the microbiological standards. The Inspectorate requires evidence, including repeat sampling and sampling from related points, before accepting that the condition of a consumer's tap may be responsible for the breach.
Some of the contravening zones are already covered by undertakings. For the remainder, the Inspectorate is ensuring that appropriate steps are taken by the companies to restore compliance with the standard.
Iron
There has been a decrease in the number of water supply zones failing to comply with the iron standard during 2000. This is the fifth successive year in which the number of zones not complying with the iron standard has reduced. The 15% decrease, from 440 zones not complying in 1999 to 375 in 2000, is statistically significant. The steady downward trend since 1995, which has seen the number of non-complaint zones drop by 39%, is also statistically significant.
There are no health risks associated with these breaches of the iron standard. However, iron can affect the taste and appearance of the water, and give cause for concern. Many of the breaches are the result of localised problems within the distribution system, especially where there are unlined cast iron mains. In a few cases the breaches are due to inadequate removal of iron at a treatment works.
Nearly all the breaches occurred in zones that are included in undertakings. These have been given by the companies to carry out improvements in the distribution system or at treatment works. The steady underlying trend of improving compliance with the iron standard can be attributed to the work being carried out under these undertakings. These improvements should accelerate as the programmes of work are progressed.
Lead
There has been a steady decrease every year since 1991 in the number of zones not complying with the lead standard. A total of 135 (5.8%) zones failed to comply fully with the standard for lead in 2000. This is a significant improvement compared with 1991, when 661 (25.6%) zones did not comply.
This continued improvement largely reflects the additional treatment that companies have installed, or are installing, under the terms of their undertakings in respect of lead. Such treatment makes the water less able to dissolve lead from pipework, i.e. it makes it less plumbosolvent.
The main sources of lead in drinking water are lead service pipes, i.e. the connection from the main to the property, or other internal lead plumbing, which may be present in older buildings. When considering breaches of the lead standard, it is particularly important to bear in mind that the nature and condition of the pipework at the sampling location may greatly influence the outcome. The detection of lead in samples taken from a particular zone does not necessarily mean that the water is plumbosolvent. As long as lead continues to be present in household plumbing it is likely that occasional breaches of the lead standard will occur in water at consumers' taps.
The current standard for lead is 50 µg/l. The new Drinking Water Directive sets a more stringent interim standard for lead of 25 µg/l, to be achieved by the end of 2003, and a final standard of 10 µg/l, to be met by the end of 2013. The Inspectorate has required companies to submit strategies and legally binding programmes of work for achieving compliance with these tighter standards.
Nitrite
The number of zones contravening the standard for nitrite has shown a slight decrease since last year. The decrease is not statistically significant but has resulted in the lowest level of non-compliance to date. However, with 1,031 individual tests found to contain nitrite above the standard, it was the most commonly failing parameter.
Detection of nitrite above the current standard of 0.1 mg/l is usually associated with the use of chloramines as a disinfectant. Chloramines are formed by the reaction of chlorine and ammonia, which is introduced into the water supply as an alternative disinfectant to chlorine. This method of disinfection has the benefit of reducing consumer complaints of chlorine taste or odour. Over time, and under certain conditions, chloramines can break down and one of the by-products formed is nitrite.
Where chloramination is used, companies are working to optimise the process to minimise the risk of nitrite production. Since 1991, the number of zones not complying with the standard has decreased from 317 (12.3%) to 184 (7.9%). Like all parameters, there is a wide margin of safety between the standard and the concentration which is considered potentially harmful.
The new EC Drinking Water Directive contains a revised combined standard for nitrite and nitrate, which is based on a formula and recognises the relationship between the two parameters. The Directive also introduces a separate individual nitrite standard which applies at the treatment works, rather than in water supply zones, to take account of the use of chloramines. These changes have been taken into account when making the new Water Supply (Water Quality) Regulations 2000, which implement the Directive.
Polycyclic Aromatic Hydrocarbons (PAH)
A steady rise in the number of zones not complying with the standard for PAH was apparent from 1991 to 1996. This trend was reversed in 1997 and the downward trend has continued into 2000; non-compliance is now at its lowest recorded level (4.9% of all zones) in the last ten years. The decrease of 34% of zones not complying since 1999 is statistically significant, and demonstrates the benefits of the programmes of work being carried out under the companies' distribution system undertakings.
PAH is derived from the distribution system and is often a localised problem. Breaches of the standard in a zone does not mean that the entire water supply in that zone is non-compliant. The short term remedial measures taken by water companies to alleviate acute water quality problems, such as discolouration, may give rise to breaches of the PAH standard. The distribution undertakings require companies to take account of non-trivial breaches of the PAH standard when assessing priorities for mains renovation.
Although all PAH breaches are investigated, detailed examination of the results shows that, in almost all cases, the contravention of the standard has been caused by only one of the six indicator substances determined in the definition of PAH, namely fluoranthene.
The new EC Drinking Water Directive no longer includes fluoranthene as an indicator substance in the PAH standard and this has been taken into account during the making of the new Regulations to implement the Directive. Water distribution undertakings have been adjusted accordingly to give priority to breaches of other standards.
Pesticides (Individual and Total)
During 2000, seven individual pesticides were detected above 0.1µg/l in 45 samples. In every instance the concentrations found corresponded to exposures far smaller than those known to be harmful or likely to affect health.
Comparing 2000 with 1999, there has been a significant (36%) increase in the number of zones complying with the individual pesticides standards. There was also an increase in zones complying with the total pesticide standard, but this was not considered statistically significant. Over the last ten years, and particularly since 1995, there has been a very significant reduction in the number of zones breaching the standards for both total and individual pesticides. These reductions reflect the completion and commissioning of most of the improvement programmes in which companies are engaged to deal with pesticides, although changes in pesticide usage may also be reflected in some of these reductions.
The difference between 1991, when 23,159 (2.8%) of individual pesticide samples failed the standard, and 2000, when only 45 (<0.01%) samples failed the standard, demonstrates the effectiveness of the enforcement process and the use of undertakings. Pesticides have now been virtually eliminated from drinking water.
Turbidity
There has been a statistically significant reduction in the number of zones failing to meet the turbidity standard compared with last year. In 2000, 33 (1.4%) zones breached the standard compared with 75 (3.2%) in 1999. This improvement is also likely to be associated with the programmes of work on distribution systems.
Manganese
There has been a further reduction in the number of zones failing to meet the manganese standard compared with last year, but the difference is not considered statistically significant. In 2000, 97 (4.2%) zones breached the standard compared with 113 (4.9%) in 1999.
The majority of the breaches occurred in zones that are included in undertakings. The steady underlying trend of improving compliance with the manganese standard since 1995 can be attributed to the work carried out under these undertakings.
Other parameters
In 2000, all zones complied with the Regulations in respect of 31 of the 55 parameters with numerical standards. The standards for a further four parameters (copper, zinc, surfactants and temperature) were breached on only one occasion in a single zone.
Only 11 individual parameters failed the standards in more than 1% of the total number of zones (see Figure 17). Each of the individual breaches are shown in the tables in the individual company sections in Part 2. The Inspectorate has assessed every contravention and enforcement action is under consideration, or has been taken where appropriate.
Improvements in Zones Since 1991
Since the publication of the 1993 Chief Inspector’s Report, the Inspectorate has used a series of “target” diagrams in the overview chapter to illustrate levels of zonal compliance for key parameters. Over the last few years, as compliance has approached 100%, improvements between consecutive years have become increasingly difficult to discern.
In this year’s overview section two target diagrams have been included. One represents the position in 1991 (the earliest year that all the relevant data was available), the other shows the most recent position in 2000. The significant improvements over 10 years are immediately apparent when comparing these diagrams.
Summary Information
Figure 18 below provides a summary of the key information for each of the 27 water companies, listed alphabetically.
It is not designed to make any direct comparisons between companies. The issue of comparison is covered in the next section.
Figure 18
Company Name Consumers Supplied Water Supplied (Ml/d) ProportionSurface Water Proportion Ground Water Number of Treatment Works Number of Service Reservoirs Number of Zones Length of Mains (Km) Samples taken at Treatment Works Failures for Total Coliforms Failures for Faecal Coliforms Samples taken at Service Reservoirs Failures for Total Coliforms Failures for Faecal Coliforms Total Samples Taken in Zones Failures in Zones Total Tests Taken Total Number of Failures % Compliance Albion 300 0.083 100% 0% 0 0 1 0 0 0 0 0 0 0 391 1 391 1 99.74% Anglian 4,000,000 1,200 50% 50% 139 380 189 35,233 29,536 7 0 40,928 31 1 138,166 737 208,630 776 99.63% Bournemouth & West Hants. 431,000 157 80% 20% 8 23 15 2,800 4,200 0 0 2,376 1 0 9,370 13 15,946 14 99.91% Bristol 1,086,000 290 75% 25% 15 181 52 6,500 6,072 5 0 18,234 8 2 18,981 27 43,287 42 99.90% Cambridge 291,700 70 0% 100% 23 33 14 2,194 3,294 7 0 3,432 5 0 4,480 9 11,206 21 99.81% Cholderton 2,100 0.6 0% 100% 1 1 1 44 104 0 0 104 0 0 313 0 521 0 100.00% Dee Valley 257,100 73 96% 4% 8 31 18 1,953 2,752 1 0 3,160 2 2 6,916 8 12,828 13 99.90% Dwr Cymru 2,800,000 1,000 94% 6% 109 564 183 24,900 25,946 7 2 58,822 66 12 75,145 310 159,913 397 99.75% Essex & Suffolk 1,700,000 477 95% 5% 26 110 66 8,370 8,384 6 1 10,804 43 3 73,931 76 93,119 129 99.86% Folkestone & Dover 160,000 49 15% 85% 15 13 10 1,068 2,468 1 1 1,330 0 0 3,504 1 7,302 3 99.96% Hartlepool 91,000 35 0% 100% 3 6 4 513 662 0 624 0 0 1,182 1 2,468 1 99.96% Mid Kent 563,300 159 12% 88% 31 68 21 4,167 5,956 3 0 6,928 4 1 14,628 45 27,512 53 99.81% North Surrey 480,000 139 85% 15% 4 22 14 2,500 2,234 3 1 2,212 2 0 8,872 7 13,318 13 99.90% North West 6,800,000 1,940 92% 8% 161 387 305 42,200 47,562 90 18 40,704 53 8 252,359 752 340,625 921 99.73% Northumbrian 2,600,000 758 95% 5% 41 249 111 16,515 14,054 6 2 25,292 29 15 153,717 178 193,063 230 99.88% Portsmouth 647,000 176.4 13% 87% 20 43 23 3,200 6,782 12 1 4,316 9 2 14,898 14 25,996 38 99.85% Severn Trent 7,356,000 2,000 65% 35% 164 614 286 41,000 31,142 15 0 62,482 44 4 137,487 180 231,111 243 99.90% South East 1,400,000 370 28% 72% 63 172 90 9,078 11,128 12 7 17,578 37 7 44,119 109 72,825 172 99.76% South Staffordshire 1,200,000 330 50% 50% 21 37 39 5,738 6,728 1 0 3,724 9 0 26,565 21 37,017 31 99.92% South West 1,500,000 446 90% 10% 36 351 81 15,000 16,171 10 1 37,005 35 5 65,775 128 118,951 179 99.85% Southern 2,220,000 585 30% 70% 98 211 116 13,302 17,237 5 1 21,915 10 0 38,750 69 77,902 85 99.89% Sutton & East Surrey 634,000 152 15% 85% 9 33 22 3,374 4,551 0 0 3,536 0 0 17,387 99 25,474 99 99.61% Tendring Hundred 145,000 30 20% 80% 3 8 4 930 1,262 0 0 898 0 0 2,551 5 4,711 5 99.89% Thames 7,500,000 2,500 75% 25% 99 371 236 31,000 37,632 32 1 37,460 57 3 377,602 405 452,694 498 99.89% Three Valleys 2,400,000 699 33% 67% 77 117 98 11,000 15,714 10 2 11,856 10 1 54,457 77 82,027 100 99.88% Wessex 1,200,000 385 20% 80% 93 325 103 10,800 21,478 8 2 34,263 25 5 96,408 65 152,149 105 99.93% Yorkshire 4,710,000 1,257 78% 22% 121 432 214 30,000 24,048 6 3 42,324 27 10 206,937 260 273,309 306 99.89% Totals 52,174,500 15,278 1,388 4,782 2,316 323,379 347,097 247 43 492,307 507 81 1,844,891 3,597 2,684,295 4,475 99.83% Comparisons of Drinking Water Quality between Companies
Consumers and organisations frequently ask the Inspectorate how drinking water quality varies from water company to water company.
The Regulations cover 55 parameters with numerical standards, some of which are more important than others in respect of public health and aesthetic considerations. Also, companies vary considerably in size supplying anywhere between 300 and 7.5 million consumers.
Furthermore, drinking water quality in all water companies is very high with the overall percentage of tests complying with the standards approaching, and even equalling, 100%. Meaningful comparisons have to be able to detect small differences in compliance between companies that are statistically significant and have not occurred by chance.
To allow a meaningful comparison to be made, DWI publishes, for each company, an overall quality index, based on 15 key parameters, and an operational performance index based on six of those key parameters: iron, manganese, aluminium, turbidity, faecal coliforms and trihalomethanes.
Last year’s results were reported in detail in DWI Information Letter 9/99 ‘Drinking Water Quality - Comparative Measures’. It included a copy of the WRc report ‘Annual Compliance with Drinking Water Quality Parameters - an Improved Statistical Approach’ which gives a full explanation of the statistical basis for the calculation of these indexes. Full details of this year’s results have been provided in a similar Information Letter recently sent to all water companies. These reports are available on the DWI web site.
It is important to note that the simple percentage of tests complying is a different measure from the overall quality and operational performance indexes. These different measures must not be confused with one another, as they are not directly comparable.
Overall Quality Index
Figure 19 lists the water companies in decreasing order of overall quality index for the calendar year 2000 in three columns; statistically significantly below average, not statistically significantly different from average; and statistically significantly above average. It should be noted that for the smaller water companies with relatively low numbers of samples it is not possible to show whether their index is statistically significantly different from average.
Figure 19 Overall Quality Index: 2000
Company Index Statistically significantly below average Not statistically significantly different from average Statistically significantly above average 100.00
100.00
99.98
99.96
99.95
99.94
99.94
99.94
99.93
99.93
99.93
99.92
99.92
99.91
99.91
99.90
99.90
99.88
99.86
99.79
99.79
99.78
99.77
99.77
99.75
99.74
99.71
99.30
Northumbrian
North West
Albion
Cholderton
Hartlepool
Portsmouth
Bristol
Bournemouth & W. Hants
Folkestone & Dover
North Surrey
Dee Valley
Cambridge
Sutton & East Surrey
Dwr Cymru
England and Wales
South East Water
Yorkshire
Tendring Hundred
Mid Kent
South West
Anglian
South Staffordshire
Severn Trent
Wessex
Essex and Suffolk
Southern
Three Valleys
Thames
Trends in the overall quality index over the calendar years 1996 to 2000 inclusive have been assessed to determine whether there is a significantly improving trend, no statistically significant trend or a significantly deteriorating trend. Figure 20 shows that there were no companies with a significantly deteriorating trend and for two companies, Dwr Cymru and Severn Trent, there is a significantly improving trend. All other companies showed no significant trend.
Figure 20 Trend in Overall Quality Index for 1996 to 2000
Statistically significant deteriorating trend Statistically significant improving trend - Dwr Cymru - Severn Trent Operational Performance Index
Figure 21 lists the water companies in decreasing order of operational performance index for the calendar year 2000 in three columns in a similar manner to the overall quality index.
Figure 21 Operational Performance Index: 2000
Company Index Statistically significantly below average Not statistically significantly different from average Statistically significantly above average 100
100
100
100
99.99
99.95
99.95
99.95
99.94
99.92
99.92
99.91
99.90
99.90
99.90
99.89
99.88
99.81
99.76
99.68
99.66
99.61
99.57
99.52
99.49
99.47
99.45
98.73
Yorkshire
Northumbrian
South West
North West
Albion
Cholderton
Folkestone & Dover
Hartlepool
Portsmouth
Cambridge
Bristol
North Surrey
Bournemouth & W.Hants
Sutton & East Surrey
Dee Valley
South East Water
England and Wales
Dwr Cymru
Mid Kent
Tendring Hundred
Thames
Three Valleys
South Staffordshire
Anglian
Severn Trent
Southern
Essex and Suffolk
Wessex
Figure 22 shows the trend in the operational performance index over the calendar years 1996 to 2000 inclusive. No companies show a significantly deteriorating trend and there is a significantly improving trend for five companies, shown below. Again, all other companies showed no significant trend.
Figure 22 Trend in Operational Performance Index for 1996 to 2000
Statistically significant deteriorating trend Statistically significant improving trend -
-
Anglian
Dwr Cymru
-
-
-
Essex & Suffolk
South Staffordshire
South West
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Published 11 July 2001
Updated 27 September 2001
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