"Dirty dairying" on southwest Otago's tile-drained and mole-drained farms has for many years been in the headlines and now the Otago Regional Council has a scientific report proving intensive agriculture does affect water quality.
Results from the 12-month study of water quality in the Pomahaka catchment were presented to the council's environmental science committee on Thursday.
Water quality, habitat condition and ecological values were assessed for the study, with 15 waterways and 21 tile drains (12 on dairy farms, nine on sheep) sampled fortnightly to assess phosphorus, nitrogen, ammonia, E.coli and suspended solid levels.
Environmental resource scientist Justin Kitto said in a report to the committee the study showed tile-draining dairy farms were typically well above effects-based water quality guidelines for nutrients and had substantially higher concentration of contaminants than tile-draining sheep farms: "Often . . . [high contamination levels] are the result of inappropriate use of dairy shed effluent when the soil is saturated."
All catchments with more than 30% of it under dairy farming had poor water quality.
Results showed sedimentation was an issue all year around, at all flow levels, he said.
"Sedimentation control is critical as it can smother habitat, harbour bacteria and bind phosphorus."
Environmental Information and Science director John Threlfall said the study put the facts and figures to problems the council was already aware of and would be used as the basis for future discharge standards under the regional council-proposed rural water quality strategy.
"Hopefully, it is a great start to where we can pitch improvements."
Cr Stephen Woodhead said he was disappointed the results pointed to inappropriate rates of effluent application, given increasing compliance work by the council and the fact landowners knew samples were being taken.
Given much of the problem concerned land-use issues, it gave him hope progress could be made "reasonably simply", he said.
The report highlighted the importance of the sedimentation issue, if the council was going to seek the goal of swimmable water, Cr Woodhead said.
