Earlier studies of the Pomahaka, Kuriwao and Clydevale-Wairuna aquifers determined they were made of gravel alluvium but a review found they were weathered, hard rock, council groundwater resource scientist Rebecca Morris said in a report.
''The new information has implications for water management, as the Regional Plan: Water for Otago is based on those earlier studies.''
That meant restrictions on takes for the Pomahaka and Kuriwao aquifers were based on the connection alluvial aquifers had to surface water.
As a result, the basins were included in the regional plan C-series maps in the groundwater protection zones.
However, the majority of the basins were hard rock and not connected to surface water, making the current groundwater management for the basins ''overly restrictive''.
''Consideration should therefore be given to removing the Pomahaka and Kuriwao aquifers from the C-series maps.''
A rock aquifer relied on cracks and fractures in the rock to transmit water, so the availability of long-yielding water was localised, which meant it was difficult to set allocation limits.
Instead, the review found potential alluvial ribbons on two reaches of the Pomahaka and one reach of the Waipahi Stream which could be considered if the water plan was to be amended, she said.
The ribbon aquifers might be vulnerable to leachate contamination because of their connection to surface water.
''Consideration should, therefore, be given to defining the ribbon aquifers as nitrogen-sensitive zones.''
Additional work was required to determine what leaching rate of nitrogen should be applied, she said.
The report was received by the council's technical committee and referred to the policy committee for further consideration.
