I was going to reply to this with a statement something to the effect that there are a couple of hazardous landfill sites dotted around Church and as such there may well be some seepage into the river. I did a quick google on Wm. Blythes, because I know they process a few heavy metals. Look what's turned up:
Fine William Blythes Church - Google Search
I don't recall reading 'owt about this in either the Observer or the LT, but I suppose they missed it because Blyths HQ is down in Essex. Just a shame Blythes can't move their entire chemical plant down there.
Things could have been pretty nasty.