GROUNDWATER RESIDENCE TIMES AND
PUKEKOHE AND BOMBAY BASALT AQUIFERS
CHEMISTRY
OF
THE
van der Raaij, R.W.
GNS Science
Aims
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 (van der
Raaij 2015). Concentrations often exceed the drinking water standards for New Zealand (Ministry
of Health, 2008) maximum acceptable value of 11.3 mg/L NO3-N and also the national bottom line
of 6.9 mg/L NO3-N (annual median) for nitrate toxicity in rivers (Ministry for the Environment,
2014); with implications for groundwater discharges to springs and streams.)
Method
Samples for age-tracers (tritium, CFCs, SF6) and for major ions, minor ions and metals were
collected at 10 sites in June 2014 using standard sampling protocols.
Figure 1.
Map of well locations sampled for groundwater MRT and chemistry. Map background is
simplified surface geology based on the QMap series (Edbrooke, 2001) with 20m contour
lines
Groundwater sampled from a well or other discharge point (e.g. spring) is a mixture of water with
variable residence times in the aquifer due to the convergence of different flow lines within the
aquifer at the discharge point. Groundwater age-dating therefore yields a MRT of water from all
converging flow lines. MRT have been calculated for all wells using the exponential-piston flow
model.
Results
MRT of groundwater from the sites tested in the Bombay and Pukekohe aquifer systems ranges
between 16 years and 99 years with fractions of exponential mixed flow ranging from 30% to
90%. Groundwater in Bombay aquifers shows an increase in MRT both in the direction of
groundwater flow and with depth. In comparison, groundwater from Pukekohe in the upper
aquifer generally has MRT younger than 50 years; meanwhile the oldest ages are seen in the
lower volcanic aquifer system.
Figure 2.
Transect showing groundwater MRT (in red) for the Bombay wells. The blue squares
represent wells which lie approximately along the transect 3-4 plotted on Figure 1. Vertical
lines indicate the length of open well from which water is drawn. The orange well is not
located along the transect but is included for comparison. The green line depicts the
approximate surface elevation. The arrows indicate the direction of groundwater flow as
implied by piezometric contours (Murphy, 1991). Note that the vertical scale is exaggerated for
figure clarity.
Groundwater in both volcanic aquifers is predominantly of type Mg-Na-Ca-HCO3-NO3-Cl or MgNa-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.
NO33-N (mg/L)
Statistically significant positive relationships with MRT are observed for pH, bicarbonate,
dissolved reactive phosphorus, potassium, and to a lesser extent, silica. Nitrate shows an inverse
relationship with groundwater MRT (Figure 3). This is common for analytes associated with landuse 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.
Changes in chemistry observed at SOE monitoring wells Rifle Range Shallow and BP Bombay
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.
30
25
20
15
10
5
0
Pukekohe
Bombay
R2 = 0.55
0
Figure 3.
20
40
60
MRT (years)
80
100
Plot showing the relationship between MRT and nitrate-nitrogen
References
Edbrooke, S.W. (compiler) 2001. Geology of the Auckland area. Institute of Geological & Nuclear Sciences
1:250,000 geological map 3. 1 sheet + 74p. Lower Hutt New Zealand. Institute of Geological & Nuclear
Sciences Ltd.
Ministry for the Environment. 2014. National Policy Statement for Freshwater Management 2014. Publication
No. ME1155, Ministry for the Environment, Wellington.
Ministry of Health. 2008. Drinking-water Standards for New Zealand 2005 (Revised 2008). Ministry of
Health, Wellington, New Zealand. 163 p. Available online at: http://www.moh.govt.nz.
Murphy G.K. 1991. Drury – Bombay groundwater investigation and interim management plan 1991.
Auckland Regional Council. Technical Publication 105. Aug 1991.
van der Raaij, R.W. 2015. Groundwater residence times and chemistry of the Pukekohe and Bombay basalt
aquifers. GNS Science Report 2015/11.