Nitrates in Groundwater and Impacts of Climate Change

Helen Rutter (Aqualinc Research Ltd)

Nitrate-N1 concentrations in groundwater (and groundwater-fed streams and rivers) are not constant over time, and even locations when concentrations are increasing or decreasing do not show a linear trend. The transport of nitrates from the land surface to a stream or other receiving water body is complex and not well understood. Changes in rainfall intensity or long-term patterns have the potential to impact nitrate leaching from the land and subsurface. High winter recharge has been found to be associated with elevated nitrate concentrations, and recent investigations in Canterbury have highlighted just how important rainfall is in terms of driving nitrate leaching. On a broad scale, the high winter recharge in the later 1970s is thought to be responsible for high nitrate-N concentrations in the 1970s in some Canterbury regions that still haven’t been exceeded (Rutter and Rutter, 2018).

The potential impacts of climate change on nitrate-N concentrations are broader than just the likely increase in leaching, as warmer or drier soils also affect the soil microbiology that influences the nitrogen cycle within the subsurface. Existing approaches to reduce nitrate-N leaching have focused on fertiliser or livestock management. However, these have resulted in sometimes only modest reductions in nitrate-N losses (Bowles et al., 2018). There is increasing evidence that climate change will influence agricultural management and plant– soil–microorganism interactions and subsequently the nitrogen cycle processes, limiting the benefits of common practices to reduce nitrate-N losses.

To understand the nature of climate change impacts on groundwater nitrate-N concentrations, three areas need to be addressed:
1. Source: What are the likely changes to agricultural practices and how may these affect nitrate-N leaching from the soil zone?
2. Pathway: What are the likely changes to groundwater recharge mechanisms and groundwater levels?
3. Receptor: What are the likely changes to nitrate-N concentrations in groundwater and the consequential impact on groundwater receptors?

This paper primarily focuses on the pathway, and the impacts that climate change may have on that. However, it is important to also consider the source and receptor components.

The available data show that the transport of nitrate-N through aquifer systems is complex and not well understood. The rapid response to rainfall, even at considerable depth, shows that contaminants can be transported very rapidly. With more extreme rainfall events under climate change, there could be more opportunity for greater losses of nitrate-N from the root zone and into the groundwater system.

Although their transport mechanisms are somewhat different, microbes can get transported with the rapidly recharging water and have very obvious and immediate health risks. This is supported by recent sampling post-July 2022 that has shown increased exceedance of microbes in groundwater.

This paper will present data from recent sampling and monitoring to assess the impacts of long term rainfall and extreme events on nitrate-N concentrations. The potential implications in terms of our understanding of nitrate sampling results and trends will be discussed, and further research needs will be identified that will add to our understanding of nutrient transport.