Market gardens ‘higher risk’ for nitrates
Groundwater monitoring shows that nitrate levels continue to exceed drinking water standards under some sections of the Waimea Plains.
A Tasman District Council report covering the findings of that monitoring and the results of a soil survey of the eastern plains says the indications are that market gardening is a ‘‘higher-risk land use for nitrate leaching’’ compared with the other main land uses in the study area – pasture, pipfruit and viticulture.
In topsoil samples, market garden sites had higher nitrate and other nutrient levels. In subsoil samples, mean nitrate levels were three to four times higher under market gardens than the other land uses.
‘‘Market gardening stood out above the rest,’’ council land resource scientist Bernard Simmonds on Thursday told councillors.
However, contemporary land practices – primarily the use of fertilisers – are believed to be just one source of the nitrates, for which elevated concentrations have been recorded in the aquifers since the 1970s.
The other main contributor is thought to be a former piggery that operated near the Richmond foothills and closed in the 1980s. Its activities are believed to have resulted in a ‘‘plume of nitrates’’ in the groundwater that is moving north towards the Waimea Inlet.
Simmonds said further testing of the former piggery site was planned, to investigate if it still presented a risk to the groundwater.
Eighty sites were sampled for the soil survey on the eastern plains during the winters of 2017 and 2018, at topsoil depths of 7.5 to 17.5 centimetres, and subsoil depths of 30 to 40cm.
Topsoil nitrate levels were not of concern, Simmonds said, because ‘‘that’s where you want it, and the plants can take it up readily’’.
However, at subsoil depths there was a higher risk of leaching, he said.
That subsoil level was also close to the rooting depths of many vegetables grown on the plains, ‘‘and therefore represents a potential depth limit for vegetable nitrate uptake’’.
The plains’ soils were some of the best in the region ‘‘and work really well with irrigation’’, Simmonds said, but they had a high potential for nutrient leaching because they were well drained and had a rapid permeability.
Monitoring indicated that nitrate levels above the drinking water standards in some bores appeared to be linked with high rainfall.
A sharp increase in nitrate concentrations after high rainfall in August and September 2019 suggested that ‘‘leaching from the soil and overlying land use into the aquifer was occurring’’, the council report says.
The work will be used to ‘‘inform discussions with landowners and stakeholders on mitigating the impacts of land use on nitrate levels in groundwater, and will be considered in the upcoming plan change process on nutrient management plans in the Waimea Plains, as well as review of the Tasman Resource Management Plan’’.
‘‘We do not believe that a more urgent intervention is required on the basis of these results,’’ the report says.
However, some councillors, including Dean McNamara, wanted more urgency in the issue.
‘‘You told us that . . . market gardening is not being managed in a way that secures nitrates once it gets below a certain level,’’ McNamara said. ‘‘Is there something we can do with farm practices to reduce this risk?’’
Simmonds said visits to leading market gardeners had revealed that ‘‘their practices for nutrient and irrigation management are excellent’’.
Councillor Dana Wensley said she had always been told that the former piggery was the problem, and this was the first clear data in her mind that ‘‘there’s a problem with land use on the plains’’.
‘‘I feel uneasy with just sitting on this information.’’
Councillor Chris Hill said she hoped there was ‘‘a real sense of urgency’’ for the next steps to fill the knowledge gap.
‘‘It seems highly alarming to me.’’