Nitrates: Known Knowns and Known Unknowns

Helen Rutter – Aqualinc Research Ltd

In spite of changes to date, water quality is continuing to decline, amid increasing concerns
around freshwater and drinking water quality. To address this, we need effective and equitable
limit-setting processes that improve water quality, enhance/restore Te Mana o te Wai, and
provide drinking-water safety. However, we do not currently have a good enough
understanding the amount of nitrate stored in groundwater, and the time-lags involved in its
transport, which is a pre-requisite for doing this.
A recent analysis of trends in nitrate-N concentrations across Canterbury indicates that the
increasing concentrations that we are now observing, are not only being driven by recent
dairying, but also by much earlier land use activities. However, the exact nature of the link
between land use and water quality is not clear. Across Canterbury, the impacts of historic
and current land use activities on water quality in different areas are quite marked, with wells
in the Ashburton zone being close to the drinking water standards of 11.3 mg/l, whereas wells
under much of Christchurch have less than 1 mg/l. Nitrates started to increase in all areas
prior to the boom of dairying in the late 1990s, and it may be that the full effects of dairy have
yet to be seen.
We can explain some of the reasons for the variability. There are “known knowns” such as
historic local land use, depth to groundwater, and influence of river recharge on groundwater
quality. However, there is a lot that remains to be explained: the “known unknowns”, including
time lags for nitrates to move through the groundwater system, and the ability of the system
to remove nitrates (the attenuation capacity). In particular, whether the natural heterogeneity
of our aquifer systems is resulting in variable transport rates, with some nitrate moving rapidly,
and some being a slow-moving, “load to come”, the impacts of which are not yet being seen.
This paper explores existing data to show what we already understand about nitrate trends in
Canterbury, and assesses what further work needs to be done to start to provide councils with
the answers they are going to need to address freshwater and drinking water reforms.
If we want to achieve an improvement in drinking water and freshwater quality, we need to
first understand what drives nitrate concentrations and what causes their variability. This
should be of increasing concern to councils, given the likely requirements of current freshwater
reforms and drinking water management reform.