The original Wilts and Berks went west from Abingdon and there were a few traces around Wantage - still shown on Ordnance Survey maps. Don't know where it went after that. Google can be quiteout of date; one place i walk has a sizeable lake that is just workings in one corner onGoogle earth.
I thought a massive reservoir was planned just to the west of Abingdon? maybe this is a feed to that?