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Drinking Water Quality in Scotland 2002

The overall water quality results for 2002

The majority of determinations made for water quality during 2002 complied with the standards set in the Regulations. Over 148,000 determinations were carried out on tap samples for individual parameters and 1,076 of these determinations failed to meet the relevant standards representing 0.72% of all samples.

Table 3.7 Key drinking water quality parameters for Scotland 2002

Notes *means PCV or relaxed PCV **means all other parameters reported on for which a numerical standard exists

Table 3.7 gives a summary of water quality in 2002 in supply zones across Scotland for 10 key parameters.

In addition, all other parameters that have numerical standards are reported in the "All Others" count in the last item of Table 3.7.

The physico-chemical parameters listed in Table 3.7 are discussed below. Summary results for each parameter for the period 1998-2002 are also included.

Colour: Colour is an aesthetic parameter and treatment is given to remove or reduce it to produce water that will have an acceptable appearance to customers. The colour of raw water may vary considerably according to seasonal and weather conditions. Variations in the raw water mean that many supplies, especially the smaller ones, exceed the standard for colour.

As noted elsewhere 2002 was a year of poor weather, in particular there were some exceptional storm events. Parameters such as colour are indicative of such events where the treatment process available in many rural areas are primitive with little capability to remove colour. The results for 2002 did not reflect this general observation because it is unlikely that tests scheduled at the start of the year will coincide with storm events.. The replacement of old water works with modern units capable of removing colour will solve this problem.

Table 3.8 Colour results for 2002 with comparisons for previous years

Turbidity: Turbidity is used as an assessment of the fine particles suspended in water. This parameter is often, but not always, associated with colour and has many of the same unacceptable characteristics.

Table 3.9 Turbidity with comparisons for previous years

Hydrogen: ion (pH) is a measure of the acidity of a water and the Regulations set limits for pH outside which the water is deemed unwholesome. The Regulations prescribe that pH should lie between 5.5 and 9.5. There are a number of reasons for the natural pH of many Scottish waters being outwith the regulatory limit. These include:

• upland waters used for water supply generally contain acidic organic material derived from peat.

• hard rock areas in the south west and north of Scotland suffer as a result of acid rain. Excessive acidity occurs because the runoff has very low levels of dissolved solids and consequently poor buffering capacity.

• deposition of marine salt in water catchments following severe storms at sea can cause rapid fluctuation in raw water pH.

The natural pH of most Scottish raw waters is altered during treatment by the addition of chemicals to aid coagulation and filtration and is finally corrected to control the corrosion of water mains and the uptake of lead, copper and zinc in household plumbing. Failures of pH in treated water are therefore most often related to a failure of the treatment plant and can be high or low pH failures depending on the fault. Another cause of failure arises from water mains with cementitous lining materials. Some water of high purity is prone to a rise in pH when in contact with such pipes.

Table 3.10 Hydrogen ion with comparisons for previous years

Aluminium: Aluminium is a natural constituent of many water sources, particularly upland surface waters. Aluminium compounds also play an important part in water treatment as coagulants to remove suspended matter and impurities, including pathogenic organisms. The Regulations set a standard for aluminium of 200 micrograms (mg) per litre. This is based on aesthetic considerations since high concentrations may cause unacceptable discoloration of the water.

Many water supplies contain concentrations of aluminium that exceed the standard from time to time. This can occur either because of natural changes in the raw water quality or because of poor control of the treatment process.

In areas where there is a relaxation of the regulatory standard, consumers' intake of aluminium from water will still be lower than the average from food and other sources and should be well within acceptable levels of intake established by the World Health Organisation.

Table 3.11 Aluminium with comparisons for previous years

Iron and Manganese: Upland surface waters which are naturally acidic because of low concentrations of bicarbonates tend to dissolve, or form compounds of, iron and manganese from the soils and minerals in the catchment. Significant seasonal variations in concentrations of iron and manganese can occur but there can also be variations in manganese concentrations due to disturbance of accumulated deposits on the beds of reservoirs when the water is drawn down or when it circulates.

The standards for iron and manganese are set at levels that ensure they will not be responsible for unpleasant tastes, dirty water or staining. Iron and manganese are not health-related parameters. For that reason, and because these are substances that occur naturally in the raw water, relaxations have been authorised in supply zones where the standards are exceeded from time to time.

The iron and manganese results for 2002 continue to show an improvement over previous years. However, there were some significant dirty water events during the year, not picked up by regulatory sampling.

Scottish Water now has a network management system in place which is designed to identify the operational shortcomings of water mains. Quality failures such as iron and manganese are taken into account along with many other problems. The improvement programme arising from this management system will deliver quality benefits as well as fewer bursts and reduced leakage.

Table 3.12 Iron results for 2002 with comparisons for previous years

Table 3.13 Manganese results for 2002 with comparisons for previous years

Lead: Water supplied from the mains does not contain any significant amount of lead. The lead recorded in determinations, carried out on samples of water taken from consumers' taps, is derived from the action of the water on lead piping between the main and the tap. The soft acidic water typical of many Scottish upland sources is particularly likely to have this effect. To overcome this effect the water is treated, where practical, to reduce the tendency for lead to be dissolved from plumbing. The 1990 Regulations set a standard for lead of 50 micrograms per litre at the time of supply. Where there is a risk of the lead standard being exceeded after the time of supply, the Regulations require that the water be treated if a significant reduction in lead concentrations can be achieved and treatment is reasonably practicable. Due to the property-specific nature of lead failures, the percentage of determinations failing year on year can vary considerably.

The World Health Organisation (WHO) published guidelines for drinking water quality in 1993. These included a guideline value for lead of 10 micrograms (mg) per litre. In December 1998 the European Commission published a revised Drinking Water Directive which reflects the WHO guideline for lead. The Directive states that the standard for lead at customers' taps should be reduced to 10 mg/l by 2013, with an interim standard of 25 mg/l to be achieved by the end of 2003.

During years 2000 to 2002 a major lead sampling survey was carried out to determine what work Scottish Water must do to implement the Water Supply (Water Quality) (Scotland) Regulations 2001. In particular the installation of, or optimisation of, orthophosphate at Water Treatment Works to further reduce the uptake of lead into drinking water. The data, when analysed fully, should provide a statistically robust estimate of the number of houses with some element of lead in their drinking water supply pipes.

Table 3.14 Lead results for 2002 with comparisons for previous years

Total Trihalomethanes: Upland surface waters, from which most Scottish supplies are drawn, are likely to contain significant quantities of natural organic substances. When the water is disinfected using chlorine these can react with the chlorine to form by-products. The group of substances known as trihalomethanes (THM) is used to monitor this process. The revised Drinking Water Directive sets an interim standard of 150 mg/l for THM to be achieved by 2003 and a final standard of 100 mg/l to be achieved by 2008. There is no standard for THM in the original Drinking Water Directive but the current Regulations set a UK standard for THM of 100 mg/l as an average over 3 months. Although there is no strong evidence of any health risk from chlorination by-products in water supplies, medical advisers have recommend that, as a matter of prudence, action should be taken to reduce THM concentrations where they persistently exceed the standard. However, action should only be taken where it can be carried out without compromising disinfection.

Improvements in the microbiological quality of water supplies have, at the same time, resulted in an increase in the failure rate for THM. The authorities recognise that as well as taking measures to improve the disinfection of water supplies they must also take steps to bring about compliance with the THM standard. The large number of undischarged undertakings reflects the increasing commitment to controlling THM formation along with improving microbiological quality. Many of the zones where failures occur are small rural supplies, having at best only simple treatment prior to chlorination.

The steady improving trend in THM compliance since 1998 was stopped in 2002. The failure rate in 2002 was 17.1%, this compares with 15.5% in 2001. The increase in failure is due mainly to poor raw water quality in some zones. The number of zones failing to meet the required standard continued to improve reflecting the investment programme to date. The investment programme being implemented by Scottish Water over the next 5 years should see improvements at all water works in Scotland, which currently are unable to treat water adequately to prevent THM formation.

Table 3.15 Total THM's results for 2002 with comparisons for previous years

All Other Parameters: The final item on Table 3.7 shows that in 2002 over 76,000 determinations were made on the wide range of other parameters in the Regulations. Out of this number there were 111 failures, but these were practically all trivial in nature. As noted elsewhere in this chapter there was a shortfall of over 5,000 determinations, mainly pesticide and PAH.

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