Nitrate levels linked to growers
Agricultural and livestock land uses are the primary sources of nitrate contamination in the waters on and under the Waimea Plains, a new report says.
A summary of existing science from catchment management consultant Andrew Fenemor for the Tasman District Council said monthly groundwater data suggested that historic contamination from a piggery that closed in the 1980s has ‘‘likely passed and that the nitrate signature in these wells is caused by local and upstream intensive land uses, particularly market gardening’’.
Financial analyses suggest that favoured land uses for expansion once the Waimea dam is complete would be hops, apples and vegetables.
Monitoring and modelling shows that land uses with the highest nitrogen losses are, from highest to lowest, dairy, outdoor vegetables, hops, grapes and apples, the report says.
Among other conclusions in the report is a finding that the most sensitive soils for nitrate leaching are the stony Ranzau soils with lower waterholding capacity. Ranzau soils cover a large swath of the eastern plains on which there are market gardens.
Another conclusion is that Pearl Creek, Neimann Creek and Borck Creek, all spring-fed streams, are receiving waters for high-nitrogen groundwaters.
‘‘[T]here is already sufficient science information to adequately inform development of a policy response for managing nitrates on the Waimea Plains,’’ the report says.
‘‘Development of the council’s nutrient management approach should be the priority now.’’
Fenemor’s report was discussed at a meeting of growers that included Waimea Irrigators and Water Users Inc consultant Brian Halstead.
Halstead said to mitigate nitrogen losses, market gardeners could consider shifting operations to land west of the Waimea River, which had ‘‘heavier soils’’ that were less sensitive to nitrate leaching.
Growers could also consider reducing crop rotations or reverting to grazing or crops ‘‘that don’t need copious amounts of nitrogen’’.
‘‘There will be some economic pain if growers are forced to reduce fertiliser and growing techniques for field crops,’’ Halstead said.
Councillor Kit Maling, who was also at that meeting, said he understood that the nitrate ‘‘hot spots’’ in some areas related to farming practices carried out ‘‘some years ago’’. Less nitrogen was used now.
‘‘Growers are going to have to come up with farm plans and fertiliser plans,’’ Maling said.
The level of nitrates in the water has long been a concern. A council survey in 2016 found the nitrate concentrations in some areas did not meet the drinking water standards.
Tests of some bores in 2019, arranged by Waimea Plains landowner Cathy Hughson and fellow Tasman District resident Lew Solomon, also showed some nitrate levels exceeded drinking water standards.
A 33-year Danish study of 2.7 million people found nitrates in drinking water may be linked to colorectal cancer. It found statistically significant increased risks at levels above about 1mg/L nitrate-nitrogen.