Markall, B., Anglian Water Services, UK(free)
Background In December 2006, AW were made aware of the use of fire‐fighting foam which contained perfluoroctane sulphonate (PFOS) and related compounds on an airbase in our region. As a precaution AW sampled the adjacent WTW. The raw water was found to contain 3.7 µg/l PFOS, along with lower concentrations of five associated compounds. The final water was found to be clear of PFOS. Water Treatment Works We abstract groundwater for public supply from a shallow chalk aquifer via a single borehole at the site which is hydraulically downstream of this airbase. The chalk aquifer is vulnerable to contamination due to the limited thickness of protective cover. In addition, the hydraulic properties of the aquifer are such that contaminants can migrate under natural and artificial hydraulic gradients at velocities up to 50 m/day. The chemical structure of PFOS makes it extremely resistant to chemical, physical or biological breakdown. The existing treatment is by forced draft aeration and 5 GAC adsorbers, orthophosphoric acid dosing, disinfection and then blended with water from other groundwater sources in the area in the contact tank on site. The GAC used at the site is Chemviron F400 grade, and the site is designed to give an empty bed contact time (EBCT) of 30 minutes at maximum plant flow. All water then passes to a reservoir where it is further blended with water from other groundwater sources and delivered to a Public Water Supply Zone serving a population of 57,382. The site has been subjected to a number of groundwater investigations to define contaminant plumes associated with aviation fuel, solvents and unleaded fuel.
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