A new report on water quality trends on rivers in Waikato show that while some aspects have improved, nitrogen levels from pastoral farming remain a concern.
The Waikato Regional Council report covers trends from 1993 to 2012.
Water scientist Bill Vant said nitrogen concentrations in the Waikato and other rivers have continued to increase and because of soil structure it could take some time for that to change.
He said there are also water clarity and turbidity issues in many places.
On the other hand levels of ammonia, a toxic form of nitrogen have fallen in many rivers in Waikato.