Groundwater residence times and chemistry of the Pukekohe and Bombay basalt aquifers

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van der Raaij, R.W. 2015 Groundwater residence times and chemistry of the Pukekohe and Bombay basalt aquifers. Lower Hutt, N.Z.: GNS Science. GNS Science report 2015/11 50 p.

Abstract: A study of groundwater Mean Residence Time (MRT) and the relationship to groundwater chemistry has been carried out in the Bombay and Pukekohe volcanic aquifer systems in response to high nitrate concentrations observed in groundwater from these systems. Nitrate concentrations often exceed the drinking water standards for New Zealand (Ministry of Health, 2008) maximum acceptable value of 11.3mg/L NO3-N and also the national bottom line of 6.9mg/L NO3-N (annual median) for nitrate toxicity in rivers (Ministry for the Environment, 2014). This has implications for groundwater discharges to springs and streams. MRT of groundwater from the sites tested in the Bombay and Pukekohe aquifer systems ranges between 16 years and 99 years. Groundwater in Bombay aquifers shows an increase in MRT both in the direction of groundwater flow and with depth. Groundwater from Pukekohe in the upper aquifer generally has MRT younger than 50 years; and the oldest ages are seen in the lower volcanic aquifer system. Groundwater in both volcanic aquifers is predominantly of type Mg-Na-Ca-HCO3-NO3-Cl or Mg-Na-Ca-NO3-HCO3-Cl. The dominance of magnesium reflects the basalt mineralogy i.e., the presence of magnesium-rich olivines and pyroxenes. Nitrate is the dominant anion in younger groundwater, but is not present in older groundwater where bicarbonate concentrations are much higher. Statistically significant positive relationships with MRT are observed for pH, bicarbonate, dissolved reactive phosphorus, potassium, and to a lesser extent, silica. These analytes have the potential to be used as proxies for groundwater age. To develop these as proxies, additional age-tracer data and chemistry should be collected to refine the groundwater residence time models. Iron, manganese, and ammonia also show relationships with groundwater age, which are primarily controlled by the groundwater redox state. Nitrate concentrations show an inverse relationship with groundwater MRT. This is common for analytes associated with land-use changes and intensification. The chemistry of younger waters reflects the impacts of recent land-use, while older water retains the chemical signature of less-impacted recharge sources. However, it is difficult to use such a relationship as a groundwater age proxy as the variability in nitrate concentrations for young groundwaters is primarily a function of land-use, not MRT. Although old groundwater at some wells is anoxic, there is no evidence of denitrification. Changes in chemistry observed at two Auckland Council “State of the Environment” monitoring wells indicate that pumping-induced changes to the aquifer flow regimes may be occurring. This may have consequences on the groundwater age structure of the aquifer systems. (auth)