Contamination of public drinking water supply boreholes by bromate
(Contributed by Rob Sage, Veolia Water)
In 2001, Veolia Water Central discovered significant bromate pollution at two public water supply boreholes which abstract water from the Chalk aquifer north of London. Sampling of groundwater successfully identified the source of bromate to be a former chemical works at Sandridge, Hertfordshire. Further site investigation has established the extent of the contaminant plume and shown additional public water supply boreholes to be at risk. This article outlines the extent of the pollution and the steps taken to address it. Also highlighted are the difficulties in prosecuting those responsible so that full remediation of the source can be undertaken.
What is Bromate?
Bromate is a man-made chemical that was used in industrial and food manufacturing processes and does not occur naturally. Bromate can occur in water as a result of ozone-based water treatment processes where naturally occurring bromide is oxidised to form bromate. Bromate has been shown to be a carcinogen and in 2001 a water quality standard of 10 ug/l (micrograms per litre) was introduced.
Assessing the extent of bromate pollution
In May 2000, Veolia Water Central (VWC) (formerly Three Valleys Water) discovered bromate pollution at two of its groundwater sources abstracting from the chalk aquifer. The concentrations of bromate were 138ug/l at Hatfield and 7ug/l at Essendon. Hatfield, a 9 Ml/d (mega-litres per day) source, was immediately taken out of supply and an investigation initiated to determine the source of the contamination. Samples taken from private water supply wells and existing observation boreholes rapidly established the source to be that of a former chemical works at Sandridge, some 10 km up-gradient of Hatfield, which had been re-developed in the 1980s as residential properties. Some of the private water supply wells also had to be closed.
Limited site investigations at Sandridge have established high concentrations of both bromate and bromide in near surface boreholes. Further investigations down-gradient of Sandridge, including drilling of new observation boreholes, established the extent of the plume and indicated that it had travelled 20 km eastwards into the Lee Valley and was also impacting on Thames Water (TWUL) groundwater sources. The approximate extent of the plume in 2006 is shown below.
The shape of the plume contradicted the previous understanding of groundwater flow directions in this part of the Chalk aquifer and detailed studies have been undertaken to clarify this position, including tracer testing. This is the largest known contamination plume in the UK from a point source.
Ongoing monitoring by the Environment Agency (EA) showed that up-gradient of Hatfield, concentrations of both bromate and bromide were relatively stable, with a few locations increasing slightly and others decreasing slightly with time. However, down-gradient of Hatfield, concentrations showed an increasing trend. This was superimposed on an annual fluctuation that appeared to be influenced by recharge patterns as illustrated by the graph below for a borehole at Essendon.
Managing high bromate concentrations at the public water supply sources
Despite initial investment by VWC in a blending option for Essendon, by 2004 the increasing concentrations at both Essendon and some of the TWUL wells were causing significant concern to the water companies. High bromate concentrations would restrict the amount of water available for public supply, reduce operational flexibility and increase the cost of supplying customers with water.
Following extensive consultation and the installation of a system to pump the contaminated water to a waste sewer, VWC and TWUL undertook trial abstraction of groundwater from Hatfield in mid 2005. The objective was to see if re-introducing abstraction from this location would lower bromate levels in the down gradient sources. The contaminated groundwater was dosed with ferrous chloride to reduce the bromate to bromide and then discharged to a TWUL trunk sewer for dilution of the chloride and bromide. The waste stream was then subjected to normal treatment at existing TWUL sewage treatment works, before returning to the river Colne as effluent.
The initial trials were successful in achieving reductions in down-gradient concentrations of both bromate and bromide, but limitations on sewer capacity restricted the volumes that could be abstracted. Testing of the effluent from the sewerage works showed that the bromate had been reduced and that total bromide levels were not of concern. There was also no significant increase in chloride levels. Periods in which pumping from Hatfield was stopped resulted in rapid and significant increases in bromate and bromide concentrations at the down-gradient sources. Also, the concentration of bromate observed at Hatfield was heavily impacted by the rate of abstraction, with lower concentrations being seen at higher flow rates.
Modifications were made to the sewer and a second period of testing undertaken during 2006. This proved much more successful at allowing more continuous abstraction at higher rates, and in 2007 the Environment Agency issued VWC with an abstraction licence for the purposes of aquifer remediation. Abstraction has continued, and by the end of 2009, 2020 kg of bromate and 4667 kg of bromide had been removed from the aquifer system, treated and the water returned safely to the environment. A full plot of the bromate fluctuations at Essendon is shown above.
Long-term remediation of the source of bromate
In June 2002, the former chemical works site at Sandridge was designated contaminated land by St Albans District Council under Part IIA of the Environmental Protection Act 1990. As contamination of groundwater was occurring on site, responsibility for enforcement was passed to the EA, as the competent authority for groundwater protection. In 2005, the EA served Remediation Notices on two Appropriate Persons (Redlands Minerals Limited and Crest Nicholson Residential Plc) deemed responsible for the site. The Remediation Notices required the Appropriate Persons to undertake a number of assessment actions to study the pollution but, crucially, no requirement was made for them to remediate the aquifer. Both companies appealed against the findings that they were Appropriate Persons and a Public Inquiry was held in April 2007 to determine their appeals. Both VWC and TWUL gave evidence at this inquiry requesting that the Remediation Notices should be varied to require the Appropriate Persons to undertake interim remediation by meeting the costs of the water companies in carrying out their ‘pump and treat’ operation. The EA supported this position by the end of the Inquiry. The Inspector submitted his report to the Secretary of State in late 2007 recommending that the Appropriate Persons’ appeals be dismissed. Following two rounds of consultation, in July 2009 the Secretary of State dismissed the appeals and issued modified Remediation Notices which include the requirement to pay for the water companies’ costs of their pump and treat operation.
The Appropriate Persons sought permission of the High Court to challenge the Secretary of State’s decision by way of Judicial Review. The Court refused permission in early February 2010. The current remediation notice requires the Appropriate Persons to cover the costs of the pump and treat system at Hatfield and to takeover monitoring of the extent of the plume from the EA. It also requires them to undertake Assessment Actions to determine the amount of pollutant still existing at the source and the flux of contaminants leaving the source, including the capability of groundwater models to simulate this and identify suitable locations for additional pump and treat locations.
Ten years after being first discovered, however, there still has been no attempt at any clean up of contamination at the source and pollution of the aquifer continues. Only the action by the water companies to protect their own assets has had a beneficial impact on part of the plume. Significant expense has been incurred by the water companies and their customers, as well as the EA, and no one yet has contributed to reducing these costs.
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