By Carl Hanson
Nitrate concentrations in Canterbury groundwater have been prominent in the media recently. Headlines have included phrases like “ticking time bomb”, “scaremongering” and “freaking out much of Canterbury”.
What I want to do in this article is to present the state of nitrate concentrations in Canterbury groundwater, and the trends we see in those concentrations, as objectively as I can, avoiding any emotive language.
First, the concentrations. Based on the data from our regional long-term monitoring programme, which includes approximately 300 wells distributed across the region, nitrate concentrations in Canterbury groundwater fall into two groups:
- About 30% of the samples we collect have concentrations less than 1 mg/L (recorded as nitrate nitrogen).
- The rest of the samples have concentrations distributed over a broader range, with an average of about 5 to 6 mg/L and the highest concentrations exceeding 20 mg/L.
Groundwater with low nitrate concentrations is found in the areas coloured green on the map to the right. This groundwater is derived mainly from alpine rivers, which have low nitrate concentrations themselves. In addition, the groundwater in some areas is anoxic as a result of organic material in the aquifer sediments. We find this particularly along the northern coast of the Canterbury Plains, where the land has historically been covered by swamps. Nitrate is generally not compatible with anoxic environments.
Groundwater with higher concentrations is found in the areas coloured yellow and red on the map in Figure 1. This covers most parts of the region where groundwater is used. The red colouring shows areas where we frequently measure concentrations greater than the New Zealand drinking-water standard (the Maximum Acceptable Value, or MAV, equal to 11.3 mg/L when reported as nitrate nitrogen).
In the yellow areas, concentrations are variable. They change from one location to another, they change with depth, and they change over time. Nitrate concentrations often display a seasonal pattern, with the highest concentrations occurring in the springtime after winter rainfall. Within these yellow-coloured areas, concentrations are generally below the MAV, but they may exceed the MAV in some years and/or at some locations.
There have been some questions in the media as to the source of the nitrate contamination. In my opinion, farming is the main source. Point sources like septic tanks and wastewater discharges contribute discrete, localised plumes of contamination, and in some cases the resulting concentrations are quite high. But we see elevated nitrate concentrations across the entire region. Nitrate contamination is ubiquitous. If you drill a well almost anywhere within the yellow and red areas on the map above, you will find groundwater with nitrate concentrations that are higher than they would be naturally. Contamination of this extent can only be explained by diffuse leaching from the farmland that covers the region. There are no other plausible sources.
Regarding trends, we can say comfortably that nitrate concentrations are increasing. Over the past ten years (2003-2012), we’ve noted increasing trends in about 30% of the wells in our regional-scale long-term monitoring programme. In contrast, we find decreasing trends in only 3% of the wells. Concentrations in the rest of the programme wells rise and fall over time but show no clear long-term trends, either upward or downward.
The results have been similar since our monitoring programme began in the 1980s. For any time period that we look at, we find that while most of the wells in the survey show no clear trend, the trends that do show up are mostly increasing. The increases are distributed across the region.
Concentrations have probably been increasing since farming began in Canterbury. As more land has been cultivated and farmed, as more fertiliser has been added to increase crop production, and as more stock have been grazed on the paddocks, the nitrogen balance in the soil has increased, and more nitrate has been available to leach into the groundwater.
Finding ways to minimise nitrate leaching from farming is a key part of Environment Canterbury’s work under the Canterbury Water Management Strategy.
Carl Hanson leads the groundwater quality team at Environment Canterbury.