14C activity of DIC and DOC within a clayey-silt aquitard

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dc.contributor.authorTimms, Wen_AU
dc.contributor.authorHartland, Aen_AU
dc.contributor.authorJacobsen, GEen_AU
dc.contributor.authorCendón, DIen_AU
dc.contributor.authorCrane, Ren_AU
dc.contributor.authorMcGeeney, Den_AU
dc.date.accessioned2022-02-03T01:11:28Zen_AU
dc.date.available2022-02-03T01:11:28Zen_AU
dc.date.issued2015-07-10en_AU
dc.date.statistics2022-01-12en_AU
dc.description.abstractThe objective of this research was to use the 14C activity of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) to estimate the rate of vertical groundwater flow through thick clayey silt aquitards. These relatively low permeability strata may limit rates of recharge to underlying gravelly aquifers that are important water supplies. At a research site in the Mooki catchment of the Gunnedah Basin (31 031 '09.56"S, 1 50 028'07.70"E), groundwater was sampled from monitoring piezometers (n=6) that were installed in separate holes, with -3 m length intake screens at depths of 5 to 55 m below ground (BG). A 100 mm diameter core was drilled adjacent to the piezometers and porewater (n=5) was squeezed from -40 mm length clayey-silt core sections between the surface and -30 m depth. Both types of water samples were concentrated onto preloaded cartridges of conditioned ion-exchange resin for analysis. The 14C activity of DIC and DOC were then determined using standard AMS methods. Between 5 and 40 m depth, the 14C activity of DOC was 50 to 80 pMC, and DIC was 5 to 100 pMC, with very significant differences in activity for the entire depth sequence, except at shallow depths. Relatively minor differences between 14C activity of groundwater from piezometers versus squeezed 'point' samples from cores were also observed. At 40 mBG, the apparent age of waters is in the order of 2.2 Ka based on DOC results, and in the order of 27.6 Ka based on DIC results. While there is little divergence between 14C activity of DIC and DOC in waters at 5 mBG, the differences between inorganic and organic 14C activity generally increases with increasing depth. Further work is in progress to correct water residence times and evaluate these results in the context of water chemistry (eg. pH, alkalinity, DOC), and core geochemistry (eg. ubiquitous carbonate nodules through the core). At this site, the vertical hydraulic conductivity is -10-9 m/s (Timms et al., 2005) and evidence for buried palaeosols indicates a dual porosity medium (Crane et al., 2015) indicating that the aquitard could leak if a vertical hydraulic gradient was to develop. These preliminary findings suggest that in this geological setting the apparent age of groundwater determined from DIC could be a significant overestimate of fluid residence time, and that flow rates in a clayey-silt sequence are more dynamic than assumed by current conceptual models. en_AU
dc.description.sponsorshipAustralian Nuclear Science and Technology Organisation (ANSTO)en_AU
dc.identifier.citationTimms, W., Hartland, A., Jacobsen, G., Cendón D.I., Crane, R., & McGeeney, D. (2015). 14C activity of DIC and DOC within a clayey-silt aquitard. Presentation to the 13th Australasian Environment Isotope Conference (AEIC), Sydney, 8-10th July 2015. In 13th Australasian Environmental Isotope Conference, Sydney, Australia, 8-10th July 2015, Programme and Abstracts, (pp. 79).en_AU
dc.identifier.conferenceenddate10 July 2015en_AU
dc.identifier.conferencename13th Australasian Environment Isotope Conference (AEIC)en_AU
dc.identifier.conferenceplaceSydney, Australiaen_AU
dc.identifier.conferencestartdate8 July 2015en_AU
dc.identifier.pagination79en_AU
dc.identifier.urihttps://apo.ansto.gov.au/dspace/handle/10238/12773en_AU
dc.language.isoenen_AU
dc.publisherUniversity of New South Wales and Australian Nuclear Science and Technology Organisationen_AU
dc.subjectCarbon 14en_AU
dc.subjectClaysen_AU
dc.subjectSilten_AU
dc.subjectFluid flowen_AU
dc.subjectSedimentsen_AU
dc.subjectSoilsen_AU
dc.title14C activity of DIC and DOC within a clayey-silt aquitarden_AU
dc.typeConference Presentationen_AU
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