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Title: A nine-year record of groundwater environmental tracer variations in a weathered sandstone plateau aquifer
Authors: Cendón, DI
Hankin, SI
Hughes, CE
Meredith, KT
Peterson, MA
Scheiber, L
Shimizu, Y
Keywords: Ground water
Southern Oscillation
Indian Ocean
New South Wales
Issue Date: 15-Dec-2016
Publisher: American Geophysical Union
Citation: Cendón, D. I., Hankin, S. I., Hughes, C. E., Meredith, K., Peterson, M., Scheiber, L., & Shimizu, Y. (2016). A nine-year record of groundwater environmental tracer variations in a weathered sandstone plateau aquifer. Paper presented to the AGU Fall Meeting, San Francisco, 12-16 December 2016. Retrieved from
Abstract: Most groundwater isotopic studies are limited to one snapshot in time due to high costs associated with sampling and analytical procedures. The timing of sampling within long-term seasonal climatic cycles may affect interpretations, particularly in unconfined or semi-confined aquifer systems. To test the potential influence of decadal climatic trends, particularly on groundwater residence time, we have combined results from a multi-year sampling programme. Hydrogeochemistry and isotopic tracer analysis including H2O stable isotopes, δ13CDIC, 3H, 14CDIC for all samples and 87Sr/86Sr and NO3-δ15N, have been applied to groundwater recovered from the Kulnura – Mangrove Mountain aquifer hosted by a weathered sandstone plateau within the Sydney Basin (Australia). In general, the study area is characterised by alternating dry and wet periods that can be prolonged as they are linked to wider climatic events such as El Niño, La Niña and modulated by the Indian Ocean Dipole. The region experienced above average rainfall from 1985-1990 followed by generally drier conditions (1991-2007) and slightly wetter conditions to 2015. Groundwater results from the first years (2006-2010), under generally dry conditions resulted in lower groundwater levels, revealed important inter-annual variations. These are interpreted to be locally driven by groundwater extraction, resulting in a progressive influx of modern groundwater. The progressive input of modern water has exposed deeper parts of the aquifer to increased NO3- concentrations of anthropogenic origin. The change in chemistry of the groundwater, particularly the lowering of groundwater pH, has accelerated the dissolution of carbonate mineral phases that in turn affects 14C residence time assessments. Subsequent sampling results (2012-2015), under higher rainfall conditions, suggest modern recharge in areas previously without measurable tritium activities. The complex interplay between recharge, anthropogenic influences and climate may be further complicated by the local irregularities in the sandstone weathering profile and the transition to preferential groundwater fracture-flow with depth. © AGU 2016
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