Nitrates warning remains

Nitrate levels have fallen slightly in a rural Waimate District water supply but ongoing warnings about the dangers of drinking it remain in place as Environment Canterbury suggests the problem could continue for weeks or months.

The Waimate District Council’s warning to users not to drink the water from the Lower Waihao Rural (including Waikakahi East) Water Scheme was issued on August 6 as the nitrate level surged past the Drinking Water Standards Maximum Acceptable Value (MAV) of 50mg/L (milligrams per litre) to 51.3mg/L.

The council also advised boiling the water does not remove nitrates and warned the concentrations are especially relevant to infants and pregnant women.

The council’s asset group manager Dan Mitchell said yesterday that while the ‘‘current levels are just below’’ the MAV of 50mg/L’’ the warning for the more than 600 consumers will remain in place until further notice, adding it was ‘‘too early to change tact’’.

Carl Hanson, a groundwater science manager with ECan, said the supply’s intake is ‘‘a shallow well located in an area of intensive farming’’.

‘‘Our monitoring has shown nitrate concentrations in shallow groundwater in the area to be increasing over the past 20 to 30 years.

‘‘This was a driver for the setting of limits for nitrate losses that are now included in the Canterbury Land and Water Regional Plan.’’

Hanson said the only immediate solution to the problem would be for the Waimate District Council to find an alternative water source with lower nitrate concentrations.

‘‘The nitrate concentrations in the current supply may decrease again over time, but this could take weeks or months.’’

Mitchell said their experience is that the move from flood irrigation (border dykes) to pivot irrigation has had a profound effect on groundwater quality.

‘‘Flushing of the near surface is now only occurring during significant rain events and is likely to result in the elevated nitrate issue that we are observing now.

‘‘It should be noted that the primary source of nitrates is likely to be chemical fertilisers and/or animal discharges.’’

Hanson said ‘‘regular compliance monitoring inspections are carried out in relation to all land use consents and, to date, we are unaware of any specific noncompliances to land use activity in the immediate catchment’’.

‘‘We will continue to look at all potential nitrate contributions in the wider catchment and work closely with the Waimate District Council on any additional findings.’’

Mitchell said the district council has been aware of ‘‘fluctuating’’ nitrate levels since March 2021 which was when online monitoring was installed as a precautionary measure.

‘‘Council has investigated numerous alternative sources over the last 12 years or so. These investigations have been unsuccessful. We are currently constructing a new treatment plant and this design has provision for nitrate removal.

‘‘The difficulty is consenting this process (ECan) as there is a requirement to discharge byproduct back in to the environment.’’

In the the meantime, the district council has supplied safe drinking water in temporary tanks at halls in Glenavy and Morven and also advised of a potable water tap source at Victoria Park, Hillary St, in Waimate.

Mitchell said the council was not ‘‘at this stage’’ intending to increase the number of places safe drinking water can be sourced from.

‘‘Council is utilising the Civil Defence welfare team and knowledge to assist those unable to make it to the temporary supply points.’’

Mitchell said all of other public Waimate District supplies and nitrate levels are well within acceptable levels.