Is climate change impacting ground water nitrates?
Climate change is having an impact in New Zealand and there are well-recognised issues including warming temperature and sea level rise, as well as the wider consequences, such as more extreme rainfall events and more prolonged droughts. There has been much focus on issues such as increased water requirements for irrigation or impacts on groundwater levels and hence on water availability, but the impacts on groundwater quality have had limited investigation.
Whilst we can’t easily predict how the changing climate will affect how much nitrate ends up in our groundwater and surface water systems, we can observe how existing weather events affect the measurements we can make now. We do know that high winter recharge is associated with elevated nitrate concentrations, as shown by the response to the extreme weather events in May 2021 and July 2022 where monitoring showed elevated nitrate concentrations for considerable periods of time following the rainfall. Recent investigations in Canterbury have highlighted just how important rainfall, and particularly the onset of winter recharge, can be in terms of driving nitrate leaching. One sensor in northern Canterbury shows an immediate response at the start of winter recharge. With concentrations rising from less than 1 mg/l nitrate-N to up to around 25 mg/l. Interestingly, the nitrate concentrations and response to rainfall are not directly correlated with rainfall amounts: it appears that some relatively small rainfall events can result in marked increases in nitrate concentration.
Based on available research, it appears that nitrogen accumulates in the soil profile during periods when there is no recharge: this is partly due to the lack of water moving through the soils and taking soluble nitrate with it. However, it is also partly because the conversion of insoluble organic nitrogen to soluble nitrate is controlled by microbial activity. During periods of low soil moisture, the microbes cannot thrive, and this conversion is limited, meaning insoluble organic nitrogen accumulates in the soil. On rewetting, the microbes thrive and convert nitrogen into soluble nitrate. The combination of large amounts of soluble nitrate becoming available and lack of uptake by plants, at the same time as recharge occurring, can result in what have been described as “hot moments” with very high concentrations of nitrate being transported to groundwater. More extreme weather events, in terms of prolonged drought combined with more extreme rainfall events are likely to result in increasing “hot moments”. Avoiding dry soil conditions through irrigation, when nitrogen can accumulate due to the lack of microbial activity as well as lack of uptake by plants, has the potential to reduce transport of nitrate when recharge is re-started. Balanced against this, is the need to avoid excess leaching due to rainfall when soils are maintained close to field capacity through irrigation.
The effects of climate change on nitrate concentrations in groundwater are hugely complex, not only due to uncertainties around the impacts of climate change itself, but the unknowns and uncertainties around how climate change will affect both soil microbiological processes and recharge.
The available work suggests that climate change is likely to result in worsening nitrate concentrations. The effects on Canterbury groundwater are, as yet, unproved, but maintaining good soil moisture conditions, whilst avoiding over-irrigation, may prove to be the best approach. Currently there are as many questions as answers but we can’t ignore the fact that recharge is a significant driver of nitrate concentrations, and as it changes in response to more extreme dry or wet periods, we can expect to see a concurrent change in nitrates concentrations.
