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Nitrates in groundwater: caught between a rock and a hard place?

Nitrate concentrations in shallow groundwater in the Lincoln area were, by 1983, already higher than desirable for drinking water.

This has stuck in my mind because neighbours warned us that the bore water on the property we had just bought was not safe for our toddler and young baby to drink! So, I took home bottles filled with water from one of Lincoln University's very deep bores. Thankfully it wasn’t long before Lincoln township’s public water supply was installed, this being supplied from two or three new deep bores.

This anecdote is consistent with Helen Rutter and her daughter Katherines analysis of Environment Canterbury’s groundwater quality data from the Selwyn Waihora zone, published in the Journal of Hydrology (NZ) in 2019. This shows that nitrate concentrations in Selwyn groundwater rose sharply from the 1960’s to the 1970’s —  see Figure 1. Most of the rise is understood to have occurred as a result of very high groundwater recharge (drainage through the soil) in 1979.

Figure 1: Change in nitrate concentration in Selwyn’s groundwater over time (decadal averages)

The timing of this abrupt increase is significant: it occurs before the substantial increase in irrigated area from about the mid-1980s, and before the rapid increase in the area of dairy farm land in Selwyn that began in the late 1990s. Removing dairy cows from the Plains won’t solve the nitrate problem.

If nitrate concentrations in groundwater are to be reduced to 1960s levels, which is consistent with what some are striving for, then one needs to know why they increased so rapidly in the 1970s. Factoring in the time lag between a change in land management and its effects showing up in groundwater quality suggests that changes made in the 1950s and 60s are the most likely cause.

While widespread adoption of a number of new practices and technologies occurred during this period, two stand out for me: significantly increased use of superphosphate in conjunction with large-scale pasture improvement using certified ryegrass and white clover seed. The resulting increase in soil fertility and pasture production enabled sheep numbers to increase significantly from 605,000 to 1.72 million over a thirty-year period — see Figure 2. The Department of Agriculture and the DSIR put considerable resource into promoting the use of superphosphate and high-quality pastures, from which the nation gained very considerable economic benefits.

Figure 2: Change in livestock numbers in Selwyn from 1950 to 1980

If increasing soil fertility turns out to be the root cause of the increase in nitrate concentrations in groundwater, it seems to me that we’re faced with a very difficult trade-off between soil quality and groundwater quality. High soil quality provides considerable socio-economic and environmental benefits, as does high groundwater quality. If it’s not possible to have high groundwater quality and, at the same time, high soil quality then as a community we have a difficult decision to make.

That is, “caught between a rock and a hard place”.

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