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Drinking Water Quality in Scotland 2002

3. Overview of Water Quality in Scotland

Key Facts

Water supplies in Scotland

In Scotland 93% of water supplies are derived from surface water sources with the remainder being derived from groundwater sources. Water from all these sources is treated at 371 treatment works before distribution through 1,550 service reservoirs and over 45,900 km of mains. On average, about 1,560 Megalitres (Ml) of potable water is supplied each day to about 99% of the resident population.

In 2002 there were 482 water supply zones in Scotland. Figure 1 illustrates the distribution of the population by zone size and Figure 2 illustrates the distribution of the zones by population. Figure 3 shows the reduction in the number of zones since 1991 to the current number of 482. This reduction is a result of Scottish Water and its predecessors developing regional schemes to replace many of the smaller supplies.

Assessment of compliance (general points)

Schedule 2 to the Water Supply (Water Quality) (Scotland) Regulations 1990 sets out the number of samples that have to be taken for each parameter in each supply zone. In practice, each sample is tested for several parameters so the text and the tables relate to the number of determinations (tests) made for each individual parameter rather than to the number of samples taken. Also, if Scottish Ministers have authorised supply point sampling for certain parameters, the results for these parameters are recorded for each zone supplied from the point the sample was taken. A single result could therefore be recorded against 2 or more zones.

The number of samples specified in Schedule 2 is the minimum required to be taken per annum. The Drinking Water Quality Unit expects full compliance with the required sampling frequencies.

Sampling in 2002 did not meet the required frequencies. A shortfall of over 5,000 tests was identified for a range of parameters, none of which are crucial to the interpretation of results. The principal shortfall occurred for pesticide testing for which it is thought that samples may have been taken but not analysed within the time limit allowed for this parameter.

The Drinking Water Quality Unit has notified Scottish Water that it must pay close attention to the correct scheduling of regulatory samples. This is particularly important for tests for parameters such as pesticide, which are only of seasonal interest.

The term "total coliforms" refers to the parameter listed in Table C of Schedule 1 to the Regulations. It includes all coliform organisms whether faecal in origin or not. In the following sections, the term "coliforms" has been used for the total coliform parameter to simplify the text. The detection of coliforms in a sample is indicative of potential contamination that must be investigated. The presence of faecal coliforms (E.coli) in the same sample would indicate that the contamination was of faecal origin. It should be noted that while the Regulations require that faecal coliforms should be absent in all samples, they permit up to 5% of samples taken from each service reservoir, or from consumers' taps in any one zone, to contain coliforms. The Regulations only require summary information about microbiological results at service reservoirs and treatment works to be submitted. Where comment is made about compliance with the coliform standard at treatment works and service reservoirs, it should be borne in mind that the overall results given here may comprise a mixture of results for individual reservoirs, some of which pass and others which fail.

The 1990 Regulations prescribe concentrations or values for 57 parameters. In general, to be wholesome, water must not contain a parameter in excess of a Prescribed Concentration or Value (PCV); for total hardness and alkalinity (where water is softened or desalinated) the concentration must not be below a prescribed value and, in the case of the hydrogen ion parameter (pH) the value must lie in a range defined by a maximum and minimum prescribed value. For two parameters, total organic carbon and colony counts, the PCV is simply "no significant increase over that normally observed".

Seven of the 57 parameters with a prescribed concentration or value set out in the 1990 Regulations do not have a required sampling frequency assigned to them. The absence of a required sampling frequency means that it is not appropriate to include any determinations for those parameters when reporting the overall number of determinations carried out under the Regulations. When assessing compliance therefore, any determinations for these parameters are not included in the tabulated figures. Any failures to comply with the PCV's set for the seven parameters are reported in the text of the reports.

In particular cases, Scottish Ministers may authorise, under Regulation 4, the relaxation of a PCV. Relaxations may be authorised in emergencies, as a result of exceptional meteorological conditions or by reason of the nature and structure of the ground in the area from which the supply emanates. A relaxation may also be authorised if the supply is used solely for food production purposes and failure to meet the regulated standards does not affect the fitness for consumption of the final product. Regulation 5 places certain restrictions on the authorisation of relaxations. In particular, it requires that public health shall not be jeopardised and that any relaxation shall specify the extent to which the contravention of a PCV is authorised.

In assessing water quality in supply zones, the Drinking Water Quality Unit takes into account the existence of authorised relaxations. Thus, throughout the text and tables, reference to the contravention of a PCV implies that, where a relaxation is in place, a concentration or value greater than the authorised relaxation has been recorded. Concentrations or values up to the authorised relaxation are therefore not included in the number, or percentage, of PCV contraventions.

Although the text and tables make reference to contraventions of PCVs, there is no evidence that any of the contraventions were of such a magnitude or duration as to endanger the health of consumers. The PCV set for parameters that are of health significance are set with a wide margin of safety. For parameters that are of aesthetic significance, the standards set generally well below the level at which water would become unacceptable to consumers.
A contravention of a standard is not necessarily indicative that the water is unfit to drink.

A contravention of a PCV is of significance. Because the water supplied at the time the sample was taken cannot be regarded as wholesome. This does not mean that the water was harmful to health or otherwise unfit for drinking, but it may mean, when considered along with other monitoring results, that the water quality needs improving in order to meet the high standards set in the Regulations. When considering microbiological results it is important to remember that samples that are taken at consumer's taps are susceptible to microbiological contamination due to the sanitary condition of the tap and associated plumbing.

The general rule that any contravention of a PCV constitutes a breach of the Regulations and thus causes the water to be regarded as unwholesome, does not apply to a number of standards specified in Regulation 3. These standards are:

a. the trihalomethanes (THM) standard, specified in regulation 3(3)(e), is only breached if the average concentration over the preceding 3 months exceeds the prescribed concentration of 100mg/l (THM being the aggregate of the concentrations of trichloromethane, dichlorobromomethane, dibromochloromethane and tribromomethane);

b. the sodium standard, specified in the 1990 Regulations Table A and regulation 3(5), is only breached if 20% or more of the determinations carried out over the preceding 36 months exceed the prescribed concentration of 150 mg/l;

c. the coliform standard in water supply zones, specified in the 1990 Regulations Table C of Schedule 1 and regulation 3(6), is only breached if 5% or more of determinations carried out over the preceding year contain coliforms (when less than 50 samples have been taken in the year, the criterion applies to the last 50 samples taken). A similar concession is permitted in service reservoirs; and

d. the standards for the parameters specified in the 1990 Regulations Table D of Schedule 1 and regulation 3(3)(d), are only breached if the average concentrations or values over the preceding 12 months exceed the prescribed concentrations or values.

In all the above cases, the number and percentage of individual determinations in excess of the relevant numerical PCV is shown in the tables on water quality in supply zones. However, only those zones (if any) which were non-compliant for those parameters on the basis described above are included in the part of the table listing non-compliant zones.

Microbiological quality of water leaving treatment works

Summary results from treatment works in 2002 are given below; figures for the previous four years are given for comparison.

Table 3.1 Summary results from treatment works with comparison for previous years

The results for 2002 indicate a rise in the number of failures for both coliforms and faecal coliforms. As in previous years, a detailed examination of the data ( Table 3.2) shows that the majority of these failures occurred at small water treatment works. Small works generally only have basic water treatment that is vulnerable to changes in raw water quality. Seasonal affects, especially high levels of precipitation, can overwhelm the works' ability to maintain microbiological integrity of the treated water. The summer of 2002 was notable for generally poor weather with at least two quite exceptional summer/autumn storms, which caused severe disruption to water treatment in many locations.

Table 3.2 Microbiological quality of water leaving treatment works in Scotland for 2002

Microbiological quality of water in service reservoirs

The sampling regime prescribed in the Regulations has over the years, revealed service reservoirs to be a particularly common source of contamination. Historically, many service reservoirs were built underground and grassed over. Consequently, they are prone to inward leakage from contaminated surface water. Scottish Water is addressing this problem by carrying out service reservoir refurbishment programmes. This usually involves excavation to expose and re-seal tank walls and roofs, and the replacement of covers and ventilators. Secondary disinfection is also being installed at some service reservoirs but this should only be used where the reservoirs are part of long distribution networks. In such cases there may be a need to boost disinfection levels to achieve a residual at the end of the network. However, secondary disinfection could disguise a more fundamental problem with a service reservoir and should only be used as a last resort.

Summary results for service reservoirs for 2002 compared to those for previous years are shown below:

Table 3.3 Summary results for service reservoirs in Scotland with comparison for previous years

The results for 2002 show that overall there was a significant rise in the number of failures for total coliforms during the year when compared with previous years. The Regulations require that there are no faecal coliforms present in any sample and for total coliforms the requirement for service reservoirs is that 95% of samples are free of contamination. Thus, these results suggest that Scottish Water still have work to undertake in order to comply with the Regulations. As in previous years, the smallest service reservoirs are failing at a higher rate than the larger service reservoirs as shown in Table 3.4.

Table 3.4 Microbiological quality in Service Reservoirs in Scotland for 2002

Microbiological quality of water in water supply zones

Summary results for the microbiological quality of water in supply zones in 2002 are given below; figures for previous years are given for comparison.

Table 3.5 Summary results for microbiological quality in supply zones with comparison for previous years

Results for 2002 show that overall there was a rise in the number of failures for both total coliforms and faecal coliforms during the year when compared with previous years. Table 3.6 provides a more detailed examination of the data and confirms that there was an overall increase in failures for total and faecal coliforms. The Drinking Water Quality Regulator will continue to monitor this situation and take appropriate action with Scottish Water.

Table 3.6 Microbiological quality of water at consumers taps

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