More testing is needed in the Pomahaka River catchment to fully understand the effects of land use on the area's water quality, the Otago Regional Council says.
Water quality in the lower Pomahaka catchment has been deteriorating for years, while land use intensified, the council's environmental science committee was told last week.
Last year, the council started a 12-month water quality sampling programme to get a better understanding of the effects of land use on water quality in the area.
The area's poor draining soils had resulted in tile and mole drainage being installed to improve grazing.
Environmental scientist Eric Brown said in a report those drains were one of the main factors leading to the declining water quality.
Results after eight months of sampling showed water quality was poor in both dairy and sheep tile drain areas and supported the idea that tile drains themselves were not necessarily the primary cause of pollution.
Instead, it was the way tile drained lands were used, he said.
There was some evidence other sources of surface water contamination might include the lack of stock exclusion from streams in areas of low-intensity sheep, beef and deer grazing.
Environmental information and science director John Threlfall said the report showed there were a lot of unknowns in the catchment which would not be identified without winter testing.
Cr Stephen Woodhead was pleased to see indications tile drains were not the primary cause of the pollution.
The exclusion of stock idea was interesting given only dairy farms were required to fence their waterways, he said.
Chief executive Graeme Martin said the information would be used to formulate ways to deal with any water quality problems and any policy changes.
The committee recommended sampling continue until November.
