Groundwater modernisation and associated chemical changes in a Hawkesbury sandstone acquifer (Kulnura–Mangrove Mountain, NSW, Australia)
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Date
2014-07-07
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Geological Society of Australia
Abstract
Land and groundwater usage has the potential to influence the groundwater chemistry of an aquifer. Progressive modernisation of groundwater, variation in pH and associated water/rock reactions have been identified in areas of the Kulnura–Mangrove Mountain aquifer (KMMA).
Detailed temporal and spatial sampling of groundwater (general hydrogeochemistry, H2O stable isotopes, δ13CDIC 3H, 14C and 87Sr/86Sr) revealed important inter-annual variations driven by groundwater extraction showing a progressive influx of modern groundwater at >100 m depth in some areas. In the Peats Ridge plateau, shallow groundwater samples show high 14C bomb pulse signatures, indicating modern recharged groundwater, while deeper groundwater shows a yearly increase in modern 14C inputs, instead of lower a14C values, as observed in other wells and generally expected. Values evolved from 36.1 pMC (5.2 ka BP) in 2007, to modern values of 103 pMC in 2010 with the latest sample in 2012 failing to graphitise, probably due to the high CO2 generally linked in the study area
with modern groundwater. The 3H activities have also evolved from values below the quantification limit in 2007 and 2008 to values of ~1.1 TU in 2012.
The minimal buffering capacity of the quartzose sandstone aquifer, at least in its upper zone where dispersed carbonates have long been dissolved, means that shallow groundwater generally has a low pH. Limited historical data (1998) shows higher pH for all samples compared to the same wells analysed for this work. However, it is in the central area where pH changes are most evident. During 2007, groundwater pH was similar to that expected for samples at similar depths with consistent groundwater residence times; however, successive samples show a shift to lower pH similar to those found in much shallower samples, as well as modern groundwater ages. Groundwater extraction is therefore causing an inflow of modern waters at depth with associated acidification.
An important consequence of acidification is the capacity to mobilise trace metals. Of particular interest is aluminium as it has been linked to enhanced risks of cognitive decline for subjects with a high daily intake from drinking water (≥ 3.7 μM·day−1). Shallow samples in the Mangrove Mountain area and some of the deeper samples with Al concentrations of ~3.45 μM are a risk for average drinking water intakes. The movement of low pH shallow
groundwater is causing an increase in Al concentrations, particularly in the central area of the KMMA, and this may be affecting groundwater for local consumption or that recovered in bottling plants. © Geological Society of Australia Inc
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Keywords
Ground water, Fluid withdrawal, Water wells, Drinking water, Aluminium, Australia
Citation
Cendón, D., Hankin, S., Williams, J. P., & Graham, P. (2014). Groundwater modernisation and associated chemical changes in a Hawkesbury sandstone acquifer (Kulnura–Mangrove Mountain, NSW, Australia). Presentation to the Australian Earth Sciences Convention 2014, Newcastle NSW, AESC 2014, 22nd Geological Convention, 7-10 July 2014, (pp. 9). Retrieved from: http://aesc2014.gsa.org.au/assets/Various-reg-partner-opp-workshop-summ-/AESC-Abstract-Proceedings.pdf