Estimating diffusion in heterogeneous groundwater systems using short-llived radio-isotopes and stable isotopes or bromine

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dc.contributor.authorPeterson, MAen_AU
dc.contributor.authorCendón, DIen_AU
dc.contributor.authorAndersen, MSen_AU
dc.contributor.authorMokhber-Shahin, Len_AU
dc.contributor.authorWong, HKYen_AU
dc.contributor.authorRowling, Ben_AU
dc.date.accessioned2020-06-09T02:18:19Zen_AU
dc.date.available2020-06-09T02:18:19Zen_AU
dc.date.issued2014-07-07en_AU
dc.date.statistics2020-05-15en_AU
dc.description.abstractDiffusion is an important and ubiquitous phenomenon in nature, but too often neglected or unmeasured in water resource hydrogeology or solute transport. Diffusion may, in fact, be the dominant process that dictates hydrogeochemistry and affects tracers. Conservative and age tracers are commonly used for water resource or contaminant plume transport estimations, but will give misleading results, if diffusion is ignored. Diffusion of tracers into aquitards, matrix pores of fractured rocks, blind fractures or other low conductivity zones lead to retardation and possible adsorption, exchange, precipitation or decay. This becomes increasingly important as heterogeneity of flow domains increase, for example, in fractured rock aquifers, interlayered sediments or aquifers associated with aquitards. Traditional methods of measuring diffusion coefficients in small slices of heterogeneous rock are unreliable for upscaling, so this study presents an alternative method based on lab-scale drill-core tests and suggests field-scale borehole tracer tests. Fick’s first law shows that diffusion rates are driven by concentration gradients. Short-lived radiotracers soon reach a steady-state concentration gradient with enhanced flux where diffusion rate equals decay, while stable tracers trend towards saturation and ever-decreasing fluxes. We compare diffusion of short-lived radiotracers 131I half-life 8 days) or 82Br (half-life 1.5 days) to their stable equivalent (I or Br) into 45–50 cm lengths of ~60 mm diameter drill core. Five cores were selected from three fractured rock environments: sandstone, limestone and metavolcanics. By regularly sampling and refilling the annulus with tracers around the enclosed core, we are able to discern differential in-diffusion between stable- and radio-tracers. For example, the annulus was sampled and refilled weekly with an (equivalent decayed) 131I activity of 22 Bq/g and within three weeks (2.6 half-lives) each core had reached a characteristic steady state flux. The net fluxes were 7 around 1.0 Bq/cm2/week in the sandstone cores, 0.2 Bq/cm2/week in the metavolcanics, and 0.05 Bq/cm2/week in the limestone. This was compared to stable iodine weekly refills at 2.6 mg/L, which gave ever-diminishing diffusion results. The net fluxes of stable iodine diminished steadily, e.g. over three weeks from 98 to 26 ng/cm2/week for the most porous (medium sandstone ~15%), and from 11 to 7 ng/cm2/week for the least porous (limestone ~2%). Experiments were also performed using 82Br and stable bromine, with sampling and refills performed on a daily (0.68 half-lives) basis. Similar trends were apparent, though the data was noisier due to more frequent refills and less time for diffusion to generate significant changes in the annulus reservoir solutions. This method enables analysis of drill cores for comparative effective diffusion coefficients of different systems. Quantitative interpretation is currently being refined. In principle, the method should be transferrable to single boreholes or tracer tests between multiple boreholes to gain larger scale representation of effective diffusion within a groundwater system. The normally confounding factors, such as dilution, advection, exchange, adsorption and precipitation, are negated by comparing the stable with radio-tracer results, as all isotopes of these elements are identically affected by such processes and losses. © Geological Society of Australia Incen_AU
dc.identifier.citationPeterson, M. A., Cendón, D., Andersen, M. S., Mokhber-Shahin, L. Wong, H., Rowling, B. (2014). Estimating diffusion in heterogeneous groundwater systems using short-llived radio-isotopes and stable isotopes or bromine. Presentation to the Australian Earth Sciences Convention 2014 (AESC 2014), 22nd Geological Convention, Newcastle NSW, 7-10 July 2014, (pp. 6-7). Retrieved from: http://aesc2014.gsa.org.au/assets/Various-reg-partner-opp-workshop-summ-/AESC-Abstract-Proceedings.pdfen_AU
dc.identifier.conferenceenddate10 July 2014en_AU
dc.identifier.conferencenameAustralian Earth Sciences Convention 2014, Newcastle NSW, AESC 2014. 22nd Geological Conventionen_AU
dc.identifier.conferenceplaceNewcastle, New South Walesen_AU
dc.identifier.conferencestartdate7 July 2014en_AU
dc.identifier.govdoc9549en_AU
dc.identifier.issn0729 011 Xen_AU
dc.identifier.pagination6-7en_AU
dc.identifier.urihttp://aesc2014.gsa.org.au/assets/Various-reg-partner-opp-workshop-summ-/AESC-Abstract-Proceedings.pdfen_AU
dc.identifier.urihttp://apo.ansto.gov.au/dspace/handle/10238/9518en_AU
dc.language.isoenen_AU
dc.publisherGeological Society of Australiaen_AU
dc.subjectDiffusionen_AU
dc.subjectGround wateren_AU
dc.subjectRadioisotopesen_AU
dc.subjectStable isotopesen_AU
dc.subjectIodineen_AU
dc.subjectBromineen_AU
dc.subjectWateren_AU
dc.subjectHydrologyen_AU
dc.subjectPlumesen_AU
dc.subjectTracer techniquesen_AU
dc.subjectPrecipitationen_AU
dc.subjectAquifersen_AU
dc.subjectSedimentsen_AU
dc.titleEstimating diffusion in heterogeneous groundwater systems using short-llived radio-isotopes and stable isotopes or bromineen_AU
dc.typeConference Abstracten_AU
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