If the pollution was only at the Tarell river mouth - the BBC's 'Tarrell' is incorrect - then the finger of suspicion would point at Brecon Livestock Market, if only due to previous
form.
http://www.newswales.co.uk/index.php...nt&F=1&id=4108
"
Council cautioned over river pollution
1/6/2001
Three separate pollution incidents of the Afon Tarell at Brecon have resulted in Powys County Council accepting a formal caution from Environment Agency Wales.
The incidents involved discharges of effluent from the Brecon Livestock Market site on 5th April, 9th May and 14th June 2000"