December 18th, 2024
Environment Canterbury has released two new reports on groundwater quality in the region, it said today.
Groundwater Science Manager Carl Hanson said the report on our Annual Groundwater Quality Survey summarises the current state of groundwater quality throughout the region and provides analysis of trends over the past decade.
While the Risk Maps of Nitrate in Canterbury Groundwater 2022 report, produced every second year, is for people with private bores drawing water from shallow groundwater aquifers and used by Community and Public Health to help ensure safe drinking water, he said.
“These reports give a general picture of groundwater quality. Bore owners are responsible for their individual supplies. With the heavy rainfall the region has experienced this winter, we strongly recommend that people have their well water tested and make sure it is safe to drink.”
The groundwater quality survey takes place each spring, when groundwater levels are usually highest. Samples are collected from wells to help understand the state of the resource and assess changes in water quality over time.
“The results in the latest groundwater report are what we expected,” Hanson said. “They are similar to most years, noting that these are ‘snapshot’ surveys.”
Groundwater is vulnerable to contamination, including E. coli and nitrate. For the 2021 survey, 327 wells were sampled region wide. E. coli was detected in samples from 29 (9%) of these wells, and samples from 34 (10%) of the wells had nitrate concentrations above the Maximum Acceptable Value (MAV), the national drinking water standard set by Taumata Arowai.
The MAV is 50 milligrams per litre for nitrate, equivalent to 11.3 milligrams per litre of nitrate-nitrogen.
“The 10-year trend analysis shows that nitrate concentrations continue to increase in groundwater across the region,” Hanson said.
“All wells with nitrate and E. coli above the MAV are privately owned. The owners of these wells have been notified. Most of the wells are not used for drinking water. Some owners have installed filters if they are drinking the water. Others have replaced the wells, but we still monitor old wells for our long-term trend analyses.”
Read the Annual Groundwater Quality Survey - 2021 report (PDF File, 3.4MB).
The Risk Maps of Nitrate in Canterbury Groundwater 2022 shows the risk of shallow groundwater exceeding the MAV.
“The latest maps contain only minor changes from the previous version from two years ago,” Hanson said.
Read the full Risk Maps of Nitrate in Canterbury Groundwater - 2022 report (PDF File, 2.85MB).
Nitrate concentrations in Canterbury groundwater have been increasing for many years, due largely to nutrient losses from agricultural land.
“Environment Canterbury takes nitrate very seriously,” Hanson said. “We have some of the strictest land-use rules in New Zealand to protect the region’s water quality. Over time, these steps will help ensure that less nitrate enters Canterbury’s water.”
While Environment Canterbury is responsible for the health of all groundwater and surface water in the region, people with private drinking wells are responsible for ensuring they have a safe supply.
Hanson offers this advice: “Make sure your well is secure and test your water periodically. In most cases, samples can be taken from a kitchen tap and sent to a lab for testing at your own expense.”
Environment Canterbury sampled groundwater from 327 wells across the Canterbury region in our 2021 annual groundwater quality survey.
The samples from 34 wells (10%) had nitrate-nitrogen concentrations above the health-based Maximum Acceptable Value (MAV). This was higher than the previous year’s survey (20 wells or 6% of sampled wells), but closer to the 30 wells (9%) measured above the MAV in 2019.
Environment Canterbury found increasing trends in nitrate-nitrogen concentrations in 47% (30% very likely, 17% likely) of the wells with enough data to analyse trends over the past ten years. The concentrations in 27% of the wells showed no trend, and 27% of the wells showed decreasing trends (9% very likely, 18% likely).
E. coli were detected in the samples from 29 wells (9%), which was lower than the previous year’s survey (37 wells or 11% of sampled wells had E. coli detected in 2020).
Just over half (58%) of the groundwater samples in areas where there is likely high connectivity with surface water (70 out of a total of 121 samples) had nitrate-nitrogen concentrations greater than 2.4 mg/L. Baseflow from such groundwater could contribute to some lowland rivers failing to meet the National Bottom Line concentration (of 2.4 mg/L annual median nitrate-nitrogen).
Samples from 19 (16%) of the wells sampled in areas where there is likely high connectivity with surface water had dissolved reactive phosphorus (DRP) concentrations above 0.018 mg/L. Baseflow from this groundwater could contribute to some lowland rivers being given a D band classification for DRP (>0.018 mg/L annual median concentration).
The samples from some wells did not meet the aesthetic Guideline Value (GV) for hardness, iron, manganese, pH, and ammonia. These results were very similar to previous surveys.