Our Ref: DWI/2/10 Event 30/10 Date: 9 June ... - Derry City Council

3. Water treatment works performance
3.1 The following is a list of nine water treatment works where contraventions were
reported and events notified through normal reporting procedures during the period of
this incident. Further assessment of these events will be undertaken by the
Inspectorate through normal assessment and reporting procedures.
3.2 During the incident final water quality at nine works was affected in the short term and
where the standards were breached these were notified to the Inspectorate in
accordance with regulatory reporting requirements. These are listed below:
Cabragh / Gortlenaghan WTWs (iron and manganese);
Carmoney WTWs (turbidity);
Dorisland WTWs (aluminium);
Caugh Hill WTWs (turbidity);
Lough Macrory WTWs (aluminium);
Ballinrees WTWs (aluminium);
Dunore Point WTWs (aluminium);
Lenamore Springs WTWs (turbidity); and
Killylane WTWs (turbidity and aluminium).
A range of problems at the WTWs including: instrument faults; response to alarms;
freeze/thaw damage: effects of low temperature on the coagulation process; and
increased production, were all factors in one or more of these events.
4. Distribution networks / systems recovery
4.1 Throughout this incident there was significant disruption within the distribution network
caused as water mains burst requiring repair, as operational changes were made as
water was redistributed throughout the system or when water was re-introduced to
networks following restoration of supplies. Such operations would have caused
disturbance within the mains which may have led to water quality problems being
encountered in some areas. As part of good operational practice, it is the
Inspectorates opinion that NI Water should have robust procedures in place to
mitigate, as best it can, against any risks of contamination being introduced into the
distribution systems when undertaking these activities.
4.2 It is the Inspectorates opinion that, when a significant loss of supplies has been
experienced, a programme of water quality checks through an operational sampling
programme is necessary, and that such a programme should be related to the
assessment of the potential risks to public health. The monitoring programme is
necessary to verify that there has not been a breach of the integrity of the distribution
network such as may be caused by: multiple repairs of burst pipework and fittings;
valving operations; sediment disturbance when service reservoirs are being refilled
following significant depletion; and reintroducing water into the distribution network.
4.3 It is not evident from the information provided that such a programme was in place
during this incident. The Inspectorate is concerned that there appears to have been a
reliance on compliance monitoring, with no clear information available on any specific
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