"The results of this paper are timely with Environment Canterbury recently declaring a nitrate emergency. Our study overwhelmingly confirms that Canterbury is the region with the largest issues with nitrate contamination in New Zealand. Using a dual nitrate isotope approach (“chemical fingerprint”), animal effluent and urine was identified as a primary cause of high nitrate levels found in these groundwaters. The isotope results are supported by routinely collected land use information that demonstrates a drastic change across the region over the past 30 years, which we have recently outlined here.

"For Canterbury alone, we estimated that of the 81,000 people that rely on their own private bore for drinking water, 6.8 % could have nitrate concentrations over the drinking water standard and 43.1% were over half the standard. These results suggest there is an elevated health risk for people relying on private drinking water in areas of concern identified in the study (Canterbury, Waikato and Southland), particularly for bottle-fed infants.

"Multiple regulatory agencies have started evidence reviews in response to large studies linking nitrate with multiple adverse health outcomes at levels far lower than the current drinking water standard (outlined here). While the reviews are still ongoing, their initiation and preliminary conclusions suggest a precautionary approach to controlling nitrate contamination in drinking water supplies is prudent."

"The paper describes a national survey that focussed on domestic self-suppliers of groundwater. Domestic supplies tend to rely on shallow groundwater sources so might be more likely to be vulnerable to contamination with nitrate from the land surface. The authors found that nitrate concentrations in Canterbury, Waikato and Southland stood out as being elevated which is not a surprise and tends to agree with the results of other studies. The annual survey carried out in 2024 by ECan showed 36% of wells in Canterbury had nitrate at or above ½ MAV - so the domestic well results presented are not really that different to the regional survey results.

"This is not to say that Canterbury doesn't have an issue with nitrate concentrations in groundwater: it does, and this is an issue that ECan have been addressing.

"The focus on sources of nitrate is on dairy, and it would be good to see a wider consideration of possible contributing land use. Many factors can affect nitrate concentrations in groundwater, including any agriculture (dairy, arable, horticulture) and things like septic tanks. One of the issues with domestic self-suppliers will be that they are likely to also have onsite wastewater treatment systems - the potential effects from this aren't mentioned. Trends and historic patterns are obviously not considered in this type of study, but they are important to consider - for example, Canterbury experienced a spike in nitrate concentrations in the late 1970s (pre-dairy intensification) which was attributed to a dry period in the early 1970s followed by a series of wetter than usual winters.

"The novel aspect of the paper is that it is a survey of domestic self-suppliers as opposed to a state of the environment-type survey. Based on Canterbury, it doesn't present any big surprises and is reassuring that domestic self-suppliers are not showing alarmingly different results relative to what is known in Canterbury. It's a good reminder for domestic self-suppliers to be aware of drinking water quality."

"This is an important study as it is the first to provide national data on levels of nitrate contamination of domestic drinking water used in rural areas across the country. It found that such contamination was common, with a high proportion (31%) of samples having markedly elevated levels of nitrate and 5.1% of samples above the maximum acceptable value (MAV). These water sources are not managed in the same way as registered drinking-water supplies covering urban areas which are sampled and tested regularly for chemical contaminants.

"These findings are a major public health concern. There is growing evidence that elevated nitrates in drinking water are associated with a range of health harms, including colorectal cancer and pre-term birth. The current MAV is likely to be above the level at which these effects occur, so should probably be revised downwards. High nitrate levels in fresh water also cause serious ecological harms.

"This study comes out just as Environment Canterbury has declared a regional “nitrate emergency,” citing rising nitrate levels in most monitored groundwater sites in that region. This formal recognition highlights worsening groundwater quality and the urgent need for stronger action to protect drinking water and public health.

"Now is the time for Central Government to act to minimise human exposure to nitrates. Unfortunately, Government is currently taking action to reduce regulatory controls on freshwater quality. That policy needs to be reversed, or at the very least put on hold while this new evidence is assessed.

"Nitrate contamination of fresh water in rural areas is primarily from the rapid expansion of dairy farming. Isotope testing in this current study showed that dairy effluent was the main source of nitrates. While water treatment to remove nitrates is possible, it is costly and unsustainable, making source water protection essential. Central government must consider limits on intensive dairying and invest in further research to reduce risks and restore drinking water quality and safety."

"The study of Rogers et al. provides useful further evidence of the nitrate content of groundwater in New Zealand, with a specific focus on self-supply sources. The following comments should be noted:

"While the study provides some information on the precision of the analytical methods used, no information is provided on the accuracy of these methods. Many journals require papers to include information on method accuracy so that the reader can judge how much weight to give analytical findings.

"Section 2.4 is titled “Nitrate exposure estimate…”, however, no estimate of nitrate exposure is provided – this would require information on, not only, the nitrate content of drinking-water but also the amount of drinking-water ingested and the exposure to nitrate from non-drinking-water sources, such as food and medication. However, this appears to be an oversight, as exposure is not further discussed in the paper and the paper more rightly focuses on compliance of sources with the maximum acceptable value (MAV) from the New Zealand Drinking-water standards.

"The abstract states that “…dairy effluent is identified as a primary cause of high nitrate….”. I believe this overstates the discrimination of the method and the text of the paper more accurately states that these high values are “likely to be primarily sourced from animal effluent such as dairy cow wastes”."

"Excessive nitrate contamination of freshwater harms both the health of our people and the environment. More attention is being given to nitrate contamination of drinking water sources, particularly as town supplies like Waimate and Gore breach our national drinking water standards for nitrate. Alongside this attention, it is essential that the public is aware that the Government has proposed a suite of changes to freshwater policy that will weaken the protections for drinking water sources, worsening an already serious, costly and intergenerational problem.

"What is important about this paper in particular is that it highlights the impacts on the drinking water of households on private groundwater supplies, situations where an individual home or a very small cluster of homes source their water from the ground themselves. These supplies are too frequently overlooked, especially by regional councils who are responsible for protecting the quality of groundwater.

"Where regional councils have failed to protect groundwater from nitrate contamination, as in Canterbury, they regularly and unfairly emphasise the responsibility of those people on private supplies to manage the quality of the water in their taps. However, as this paper points out, households cannot manage the land and water around them to protect against nitrate contamination. Individual households should not have to bear increasing financial and health burdens because the council is failing to do its duty to the community to protect their groundwater. Importantly, once groundwater is breaching the drinking water standard, the environmental impacts are likely to have started long ago as much lower concentrations of nitrate impact the health of waterways.

"What is useful, but troubling, is the number of samples in this study that breach half the maximum drinking water standard (5.65mg/L). Of course, those that breach the standard are very worrying but it is essential too that we see how many are heading towards breaching the standard. In Canterbury, the study found that 6.8 % of rural samples breached the drinking water standards (11.3mg/L) while 43.1% were above half the standard (5.65mg/L). The latest groundwater data from Environment Canterbury (ECan) indicates that more than 60% of monitored groundwater sites demonstrate a worsening trend. Putting this study and ECan’s information side by side, this means that more people in Canterbury are likely to face having drinking water that breaches human health standards in future.

"Two weeks ago, ECan declared a ‘nitrate emergency'. While this was good in the sense that the council was formally acknowledging the severity of the problem, ECan (and all other regional councils) need a far more robust investigation of their existing rules and plans to identify if they are working to protect their communities drinking water sources. If the rules and plans are not working (as this study and many others indicate), why not and how do they need to change. There is more detail on this this in an article published we last week through the Public Health Communication Centre. This urgently-needed work can be done regardless of the changes central government makes to policy and can be used by councils to make informed decisions about where to next."