The Ashburton water zone continues to stand out for the wrong reasons in Environment Canterbury’s (ECan) annual groundwater quality survey.
ECan has released the survey results for 2021, which tests for levels of contaminants in bores and wells across the region in spring, and it shows nitrate levels continue to rise.
Across the 10 Canterbury Water Management Strategy Zones, the Ashburton water zone accounted for 53 (16 per cent) of the 327 wells in the survey.
Of the 327 wells, 34 exceeded the maximum acceptable value (MAV), the national drinking water standard set by Taumata Arowai, for nitrate-nitrogen (over 11.3mg/L) of which 19 (55 per cent) were in Ashburton.
A further 22 sites in the district registered between 5.65 to 11.3mg/l while only three were below 1mg/L.
The proportion of total samples exceeding the MAV was 10 per cent which is higher than the 6 per cent found in 2020, a year when the region experienced below-average recharge conditions.
“At a regional scale, we see the greatest increases in groundwater nitrate concentrations after wet winters when there’s enough rainfall to flush nitrate out of the soil and down into the underlying groundwater,” ECan’s groundwater science manager Carl Hanson said.
The 10-year trend analysis shows that nitrate concentrations continue to increase in groundwater across the region, Hanson said.
“Nitrate concentrations in groundwater beneath the Canterbury Plains in the Ashburton zone continue to be high relative to many other parts of Canterbury,” he said.
“North of the Ashburton/Hakatere River, and around Tinwald, this has been the case for many years.
“In other parts of the Hinds/Hekeao area, concentrations have increased considerably over the past 20 years as a result of changes in farming and irrigation practices.”
The Managed Aquifer Recharge trials in Hinds have shown some promising results, he said, “but more work is needed before we can judge their overall success”.
The Ashburton zone also accounted for 10 of the 29 wells that exceeded the MAV for E. coli levels (one bacterium per 100 millilitres of water).
All wells with nitrate and E. coli above the MAV are privately owned and the owners have been notified.

• By Jonathan Leask