Groundwater survey highlights ongoing nitrate challenge despite fewer E. coli detections

Our latest groundwater survey has found little change in the proportion of wells exceeding drinking water standards for nitrate, while significantly fewer wells showed evidence of E. coli.

The 2025 Annual Groundwater Quality Survey sampled groundwater from 358 wells across Canterbury, providing a regional snapshot of groundwater quality and insights into long-term trends. 

Nitrate still trending up in more than half of wells tested

Results showed 38 wells (10.6 per cent) had nitrate-nitrogen concentrations above the drinking water Maximum Acceptable Value (MAV) of 11.3mg/L, similar to the 10 per cent recorded in 2024. However, the number of wells where E. coli was detected dropped from 49 in 2024 to 23 in 2025.

Water and Land Science Manager Dr Elaine Moriarty said the results reinforce the importance of continued monitoring and testing of private water supplies, as well as sustained efforts to improve groundwater quality.

"Groundwater is hugely important to Canterbury. It's the source of drinking water for many communities and it also feeds a lot of our lowland springs and streams."

"This survey shows some encouraging signs, particularly the reduction in E. coli detections, but it also confirms that nitrate remains a long-term challenge."

Higher nitrate concentrations were usually found in areas with more intensive land use. Of the wells assessed for long-term trends, 59 per cent showed increasing nitrate concentrations over the past decade, while 24 per cent showed decreasing trends, with the remaining 17 per cent showing no clear trend.

Groundwater quality can also impact lowland ecosystems

The survey also highlighted the connection between groundwater quality and the health of Canterbury's waterways. More than half of the shallow groundwater samples assessed in areas connected to rivers and streams had nitrate concentrations above the surface water bottom line of 2.4 mg/L, meaning they could contribute to ecological impacts in surface water.

Dr Moriarty said actions to improve groundwater quality can take years to become visible.

"In parts of Canterbury with deep groundwater the water we sample today may have entered the ground many years ago. This means changes in how land is managed may take a long time to show up in monitoring results."

"That's why annual surveys like this are so important. They help us track long-term trends, understand where risks remain, and provide communities with reliable information about the state of their groundwater."

The survey also included a one-off assessment of lead concentrations. Lead was detected in some samples at very low levels, but no wells exceeded the drinking water standard.

Download the full report: Annual Groundwater Quality Survey 2025 (PDF file, 3.4MB)