Understanding groundwater quality – why it’s not easy

By Waiology 11/12/2013 1

By Chris Daughney and Magali Moreau

Un-muddying the Waters : Waiology : Oct-Dec 2013Groundwater resources are very important for New Zealand. Groundwater supplies about a third of our abstractive water needs and an even greater proportion of the water required by the agricultural sector.

There is understandably much concern about groundwater quality, particularly in terms of nitrate. High concentrations of nitrate in groundwater that is used for drinking can impair oxygen transport through the bloodstream, particularly in infants. High concentrations can also pose risks to aquatic ecosystems where nitrate-rich groundwater discharges to rivers, lakes and estuaries.

So what is the state of health of New Zealand’s aquifers, as far as nitrate goes? There are a couple of reasons why this question is not easy to answer.

The first challenge is to determine the concentration of nitrate that we should expect in the absence of human influence. It is hard to define this “baseline” because many of our aquifers are already impacted to some degree by human influence, meaning that nitrate concentrations are not representative of natural conditions. One elegant way to estimate the baseline condition is to compare the age of groundwater to the concentration of nitrate that it contains. Application of this technique shows that nitrate concentrations in New Zealand’s groundwater were typically below 1 mg/L (as NO3-N) before the export meat industry started. A similar conclusion was reached in a separate study that used a statistical technique (without water dating) to show that nitrate concentrations above 1.6 mg/l are probably indicative of human influence, whereas nitrate concentrations above 3.5 mg/L are almost certainly caused by human activity.

Idealised shape of the capture zone for a well in a homogeneous isotropic unconfined aquifer. The regional groundwater flow direction is from right to left (modified from Ministry of the Environment, British Columbia 2004).
Idealised shape of the capture zone for a well in a homogeneous isotropic unconfined aquifer. The regional groundwater flow direction is from right to left (modified from Ministry of the Environment, British Columbia 2004).

The second difficulty is that capture zones have been mapped for very few wells in New Zealand. A capture zone is the area of land through which rain or river water enters the aquifer and is ultimately extracted from the well of interest. The land use activities within the well’s capture zone can alter the groundwater quality. So if the capture zone has not been mapped, if we find nitrate concentrations above the baseline, the source of the nitrate may not be identifiable. This leaves us asking “where did this nitrate come from?”

A third difficulty is that groundwater flow is relatively slow. This results in a time lag between nitrate infiltration into the aquifer in a well’s capture zone and detection of that nitrate at the well some distance away. The groundwater dating techniques mentioned above can assist us to understand these time lags, but there are still many locations around New Zealand at which groundwater ages remain unknown. This leaves us with the question “when did this nitrate enter the aquifer?”

Returning to the question posed above, what is the state of health of New Zealand’s aquifers, as far as nitrate goes?

A recent survey evaluated the concentrations of nitrate in groundwater from over 900 wells across the country, using measurements made between 1995 and 2008. Roughly 1/3 of the tested wells had median nitrate concentrations above 3.5 mg/L, which as mentioned above is the upper threshold for natural conditions. The wells with above-baseline nitrate concentrations were found across the country, indicating pervasive degradation of groundwater quality. At roughly 5% of the wells tested, the nitrate concentration was in excess of the drinking water standard that is set for protection of human health.

Although relationships between groundwater nitrate concentrations and well depth were observed, no relationships to land use or land cover around the wells were detected. This lack of relationship between groundwater quality and land use around the wells is in fact a common result that has been observed in several studies in New Zealand and overseas. It can be explained by the factors presented above: it is hard to understand relationships between groundwater quality and land use unless the age and source (capture zone) of the groundwater are known.

Given the importance of groundwater to this country, and given the evidence that groundwater quality is pervasively degraded relative to natural conditions, there is urgency for better understanding of our aquifers. Determining groundwater age and mapping capture zones must become priority activities. Without the ability to unequivocally relate cause (land use) to effect (increasing nitrate concentrations), we will keep telling each other “it wasn’t me!”

Dr. Chris Daughney is the Director of the National Isotope Centre, and Magali Moreau is a groundwater geochemist, both GNS Science.

Ministry of Environment (British Columbia). (2004) Well Protection Toolkit Step 2 (electronic resource). 24 p.; last accessed 30/01/2013.
Morgenstern, U.: Daughney, C.J. 2012. Groundwater age for identification of baseline groundwater quality and impacts of landuse intensification – The National Groundwater Monitoring Programme of New Zealand. J. Hydrol., Vol. 456–457: 79–93.
Daughney, C.J.; Raiber, M.; Moreau-Fournier, M.; Morgenstern, U.; van der Raaij, R.W. 2012. Use of hierarchical cluster analysis to assess the representativeness of a baseline groundwater quality monitoring network : comparison of New Zealand’s national and regional groundwater monitoring programs. Hydrogeology journal, 20(1): 185-200
Daughney, C.; Randall, M. 2009. National Groundwater Quality Indicators Update: State and Trends 1995-2008, GNS Science Consultancy Report 2009/145. 60p. Prepared for Ministry for the Environment, Wellington, New Zealand.

One Response to “Understanding groundwater quality – why it’s not easy”

  • I assume it’s possible to get some indication of the groundwater age from atmospheric pollution ( bomb test isotopes, rained-out man-made halogenated molecules that don’t bind to soils ( CFCs, SF6?. )). Is that feasible and routinely performed?.

    Is it also possible to ascertain the nitrogen sources from ratios of concomitant precursor or degradation species, or are they degraded too quickly?. Are they bound to soils/biomass/carbon and don’t appear in the water. I suppose the question is, is it only oxidised nitrogen ( nitrate etc. ) measuably present in the water?.