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https://consult.environment-agency.gov.uk/hnl/land-contamination-at-st-leonards-court-sandridge/
Lots of detail in the documents at the bottom.
Part of the problem seems to be a shallow layer of the land was contaminated. The developers cleared the site, removed the hard standing and left the plot empty for a while which allowed the contamination to be washed into the aquifer where it is slowly washing out over many generations.
It was a housing developer that wrecked the Hertfordshire chalk aquifer. Bit of bromate never harmed anyone?