Investigations

This section contains summaries of reports on drinking water
quality incidents and investigations.

Report on drinking water quality incident at:
Lintrathen Zone Water Treatment Works in August 2007

Scottish Water Executive Summary

Lintrathen water treatment works (WTW) supplies direct to the Lintrathen water supply zone (WSZ) within which is a distribution main to the north of Brechin which supplies a small number of rural properties.

The 15" Spun Iron (SI) Lintrathen Trunk Main (TM) connects to a 2" Asbestos Cement (AC) distribution main at Lundie Castle some 8.5 km north of Brechin. This main extends southward for approximately 5.5 km and terminates at a WSZ boundary control valve at Mill of Brathinch Farm.

On 2nd August 2007 a customer notification of water rising in a field at Chapleton Farm generated a site visit and a repair of the 2" AC main commencing on the morning of 3rd August. The site of the burst was in a grass field occupied by cattle and throughout the excavation activity to the main, positive water pressure was maintained through the careful operation of a line valve upstream of the burst. Downstream of the burst location the main supplies 7 properties at 3 locations, Cairndrum Farm, Lummingtom Farm and Bankhead Farm, a population of about 20.

The repair of the main involved the removal of a 10 foot section of AC pipe and replacement with a section of pvc pipework connected by maxifit couplings on to the ends of the existing AC mains. The flow of water from the open upstream pipe continued during the course of the repair and the level of water in the excavation kept below the invert level of the open downstream pipe by the use of a pump.

DOMS and HCoP procedures were followed throughout the course of the repair activity. Following the completion of the repair the line valve was opened and the main charged. A hydrant close to the WSZ boundary control valve was partially opened and allowed to run overnight before a post repair sample was taken at Lummington Farm from an outside tap at the farm cottage on 4th August. The main was flushed for a further two days from the hydrant after the sample was taken as is normally done on this main following a burst repair in order to clear any discoloured water and to flush the main downstream of the last connection a length of some 750 metres.

The post repair sample failed and resamples taken from inside taps at Lummington Farm and at Bankhead Farm on 8th August both passed without any further action being taken. Chlorine residuals of the initial sample were extremely low and of the resamples were acceptable. Low chlorine is possibly the reason for failure during repair of water main. No contact was made with customers following the initial sample failure result and it was agreed with the CPHM to review results when resamples were available.

The distance of private supply pipework from the main to the outside tap at Lummington Farm is approximately 200 metres.

Chlorine residuals in the 2"AC main are being monitored for a period of at least six weeks. The delay between the dates of the original sample and the resamples has been investigated and action taken.

DWQR Assessment on the incident

The DWQR's assessment of this bacteriological failure in the distribution system is that it may have been caused by the repair of a burst on a short length of a two inch diameter pipe. The repair was carried out while the system was pressurised so the risk of contamination during the repair would have been less than if the repair had been carried out with the water turned off. Scottish Water (SW) claim that all their relevant procedures to minimise the risk of contamination were followed. However, SW is unable to provide any hard evidence of compliance with procedures other than a statement to the effect that an interview took place with the relevant staff which confirmed that procedures had been followed. SW state that the lack of a chlorine residual in the sample that failed suggests that there may be an issue with the condition of the main. However, two resamples taken 4 days later from (a) the same place as this first sample and (b) a different location further downstream on the same length of main, show reasonable levels of chlorine residuals. DWQR therefore considers that there is a strong possibility that the failed sample is likely to have been caused by the lack of strict adherence to the relevant procedures, that the main may have been contaminated as a result and that it was a failure to follow procedures that used up the chlorine residual and caused the associated sample to fail.

The two day delay in taking the re-samples is not acceptable. It is noted that the network operator could not contact the scientific adviser on Monday 6 August about taking a re-sample and that on Tuesday 7 August when he did make contact it was too late due to other commitments that day. Re-samples were therefore not taken until Wednesday 8 August.

DWQR notes the actions being taken as a result of this incident. Namely, to:

1. Check chlorine residuals on the main and to flush it if these fall below 0.10 mg/l until the residual recovers and undertake further action if the residuals continue to fall.
2. Remind the operator that instructions for taking re-samples must be carried out in the timescale specified.

DWQR has also asked SW to review its procedures in respect of ensuring hygienic conditions during any work on the distribution system which should include provision for ensuring that there is a satisfactory chlorine residual in the system around the working area before work commences and to ensure that this remains so during the work. If the residual is not adequate for the protection of public health, provision should be made to increase the residual locally whilst the work is carried out.