Investigating the origin of salinity and aquifier interaction in a sesonally pumped confined aquifier system in southeast Australia (Western Port Basin)
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Date
2014-07-07
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Geological Society of Australia
Abstract
This ongoing study aims to identify the different sources of fresh/saline water, pathways, mechanisms of groundwater salinisation, and factors controlling salt-water intrusion in the Western Port Basin aquifer (Victoria, Australia). The multi-layered aquifer is characterised by variable distributions of fresh and saline groundwater at the coastline, which are speculated to result due to geological heterogeneity, and the influence of past sea levels. The spatial salinity distribution observed indicates that the typical situation of a dense saltwater wedge underlying fresh groundwater does not apply. Further to natural drivers, an understanding of the potential short and long term effects of seasonal pumping for irrigation in the basin is desired, particularly whether it induces migration of water from other areas in the basin or causes leakage from low-permeability horizons.
The impact of seasonal pumping on the variation and distribution of salinity was examined at a nested bore site 500 m from the coast. Analyses of chloride concentration and δ18O in groundwater indicate various degrees of mixing between seawater and fresh meteoric groundwater. Multi-depth sampling in the upper aquifer, identified high salinity water with ~19% seawater mixture above slightly less saline water – with ~17% seawater; while samples from the lower aquifer yielded significantly fresher groundwater (~4 to 7% seawater). This is likely to be due to the relatively good connection between the upper aquifer and the ocean, and low-permeability material separating the shallow and deeper aquifers. Seasonal salinity changes were observed as a response to pumping, with salinity in the
upper aquifer decreased by ~15.5% at the onset of pumping, and salinisation gradually increased once pumping ceased by ~22%.
Recent 14C dating of groundwater in the proximity of the pumping area revealed low radiocarbon activities (<25 pMC) and a lack of tritium, which rules out the possibility of modern recharge reaching the pumping area. A thick Holocene clay cap (ranging from 10 to 30 m) covering significant areas and extending beneath the bay, limits rainfall recharge to the basin margins where the aquifers outcrop. Beneath the shallow bay a paleoriver channel incised deep into the bay floor sediments expose the aquifer to potential vertical and horizontal input from modern seawater. However, the lack of long-term salinisation despite decades of pumping, and the relatively old groundwater ages, imply buffering against saline intrusion by stores of relict fresh water. Incorporation of these results into a conceptual model will contribute to an improved understanding of the groundwater flow system and
the long-term implications of pumping on future groundwater sustainability in the Western Port Basin.
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Keywords
Victoria, Australia, Salinity, Saline aquifers, Aquifers, Fresh water, Coastal regions, Sea level, Irrigation, Weather, Chlorine
Citation
Lee, S., Currell, M. & Cendón, D. (2014). Investigating the origin of salinity and aquifier interaction in a sesonally pumped confined aquifier system in southeast Australia (Western Port Basin). Presentation to the Australian Earth Sciences Convention 2014 (AESC 2014), 22nd Geological Convention, Newcastle NSW, 7-10 July 2014, (pp. 63-64). Retrieved from: http://aesc2014.gsa.org.au/assets/Various-reg-partner-opp-workshop-summ-/AESC-Abstract-Proceedings.pdf