 Many of our waterways are already saturated with nitrogen (and its evil twin, phosphorus), such that regional councils are spending tens of millions of ratepayers' - and taxpayers' - dollars trying to clean them up. But we haven't even begun to see the worst of it. Beneath every dairy district, there is a subterranean tidal wave of nitrogen seeping inexorably downhill, into groundwater, into creeks, then rivers, then estuaries. A