Borehole Catchment Protection and Drinking Water Safety Plans

Case Study

Severn TrentESI, now part of Stantec, was commissioned by several water companies to review water quality information from their groundwater sources and to provide inputs into the Drinking Water Safety Plans for these sites.


Borehole Catchment Protection and Drinking Water Safety PlansThe primary objectives of a water safety plan in protecting human health and ensuring good water supply practice are the minimisation of contamination of source waters, the reduction or removal of contamination through appropriate treatment processes and the prevention of contamination in the distribution network and the domestic distribution system. This requires a thorough understanding of the water supply chain, ongoing operational monitoring and documentation of management arrangements. Understanding the risk factors around borehole construction and the immediate catchment of the borehole is a significant part of this process.

The Project

ESI has advised many of the UK’s major water companies on water quality issues at boreholes. For Severn Trent Water Ltd (STWL), ESI provided staff on secondment to support the development of Drinking Water Safety Plans for all its groundwater sources. Staff worked closely with other relevant specialists from STWL to review historical water quality data (particularly bacteria) based on site inspections. As a result of developing these plans, a number of actions were identified to minimise risks to water quality. ESI has subsequently provided support to help resolve these.


Borehole Catchment Protection and Drinking Water Safety Plans_2_Location RemovedDuring routine sampling in January 2010, STWL detected two pesticides, diuron and linuron, at concentrations above the 0.1 µg/l drinking water standard at one of its water treatment works. The contamination was traced to one of the boreholes feeding the works. In response to the contamination incident, STWL ceased abstraction and removed the 3 Ml/day source from supply. STWL was unable to re-commence abstraction until the pesticide risk had been reduced to an acceptable level which increased the risk of loss of supply should there be high customer demand (i.e. a dry summer) or failure of equipment at other sources. STWL commissioned ESI to investigate the source and extent of pesticide contamination in the aquifer, and to provide recommendations to aid the prompt return to supply of the source.


ESI was also contracted by DWI to review the raw water quality data collected by the water companies since 2009. Water companies in England and Wales have a legal requirement to monitor the quality of raw water abstracted for drinking water supplies. The purpose of such monitoring is to inform drinking water safety plan risk assessments for each water treatment works and associated supply system. This approach assesses and manages hazards and hazardous events that could potentially impact on the source of the water used for public supplied. This work has provided a first review of these data regarding:

  • The quality of the data provided by individual companies;
  • The variability of parameters provided by individual companies;
  • The variability of data for the same parameters provided by different companies


This DWI project concluded that:

  • Most water companies provided sufficient data to assess general water quality parameters such as turbidity, nitrogen species, hydrogen ion (pH), colour, metals, etc.
  • Most companies provided data on pesticides and other organic chemicals.
  • A large number of pesticide concentrations never rose above the level of detection.
  • This could indicate a risk to water supplies, if the water companies schedule analysis by pesticide group rather than by individual pesticides.
  • ESI recommended that the analytical methods used for some pesticides should be reviewed to ensure use of the correct LOD (limit of detection) and the correct units.