Direct stable isotope porewater equilibration and identification of groundwater processes in heterogeneous sedimentary rock

Katarina David; Wendy Timms; Andy Baker

doi:10.1016/j.scitotenv.2015.08.075

Direct stable isotope porewater equilibration and identification of groundwater processes in heterogeneous sedimentary rock

Sci Total Environ. 2015 Dec 15:538:1010-23. doi: 10.1016/j.scitotenv.2015.08.075. Epub 2015 Sep 28.

Authors

Katarina David¹, Wendy Timms², Andy Baker³

Affiliations

¹ School of Mining Engineering, UNSW Australia, NSW 2052, Australia; Connected Waters Initiative Research Centre, UNSW Australia, NSW 2052, Australia. Electronic address: k.david@student.unsw.edu.au.
² School of Mining Engineering, UNSW Australia, NSW 2052, Australia; Connected Waters Initiative Research Centre, UNSW Australia, NSW 2052, Australia.
³ Connected Waters Initiative Research Centre, UNSW Australia, NSW 2052, Australia.

PMID: 26383640
DOI: 10.1016/j.scitotenv.2015.08.075

Abstract

The off-axis integrated cavity output spectrometry (ICOS) method to analyse porewater isotopic composition has been successfully applied over the last decade in groundwater studies. This paper applies the off-axis ICOS method to analyse the porewater isotopic composition, attempts to use the isotopic shift in groundwater values along with simple geochemical mixing model to define the groundwater processes in the Sydney Basin, Australia. Complementary data included geophysical, hydrogeological, geochemical, and mineralogical investigations. Porewater from core samples were analysed for δ(18)O and δ(2)H from various sedimentary units in the Basin and compared to endpoint water members. Stable δ(18)O and δ(2)H values of porewaters in the Basin (-9.5 to 2.8‰ for δ(18)O and -41.9 to 7.9‰ for δ(2)H) covered a relatively narrow range in values. The variability in water isotopes reflects the variability of the input signal, which is the synoptic variability in isotopic composition of rainfall, and to a minor extent the subsequent evaporation. The porosity, bulk density and mineralogy data demonstrate the heterogeneity that adds the complexity to variations in the isotope profile with depth. The source of chloride in the sedimentary sequence was related to rock-water and cement/matrix-water interaction rather than to evaporation. The heterogeneous character of the sedimentary rock strata was supported by a change in pore pressures between units, density and variability in rock geochemical analyses obtained by using X-ray fluorescence (XRF) and X-ray power diffraction analyses. This research identified distinct hydrogeological zones in the Basin that were not previously defined by classic hydrogeological investigations. Isotopic signature of porewaters along the detailed vertical profile in combination with mineralogical, geochemical, geophysical and hydrogeological methods can provide useful information on groundwater movement in deep sedimentary environments. The findings of the study are valuable in management of sensitive ecosystems and potable resources above mining areas.

Keywords: Heterogeneity; Porewater; Sedimentary strata; Stable isotope.

Publication types

Research Support, Non-U.S. Gov't