Temporal, spatial and biotic variability in transpirate tritium near the LFBG and HIFAR: observations and working hypotheses
Date
2010-08-31
Authors
Twining, JR
Hankin, SI
Hughes, CE
Journal Title
Journal ISSN
Volume Title
Publisher
South Pacific Radioactivity Association
Abstract
Over the period from July 2007 to March 2009, transpired water samples were collected from trees and shrubs growing over the trenches at the Little Forest Burial Ground (LFBG), a near-surface, low-level, nuclear waste repository dating from the 1960's near ANSTO at Lucas Heights. Tritium was detected in these samples but its concentrations were variable over time as well as within and between plants. Also, the values determined were less than those measured in adjacent groundwater monitoring bores. Apart from background cosmogenic and residual fallout tritium, other potential sources available to the plants have been seepage from a nearby municipal landfill and tritium dispersed atmospherically from the HlFAR reactor, approximately 1.6 km to the south. To assess the potential contributions of these multiple sources and the extent to which seepage from the trenches is affecting plant transpirate tritium, transects away from the trenches and another from HIFAR through the LFBG and beyond were also sampled. These show some clear contributions from at least two sources. Variability between different branches of single plants at any one sampling time is hypothesised to be due to heterogeneous concentrations of tritium in the soil zones from which the plants are acquiring their moisture, together with poor mixing between xylem streams within the plants. Variability in transpirate tritium between sampling times for individual plants is hypothesised to be caused by rainfall dilution in near surface soils together with differences in water potentials across the active root zone. Soil water potential, an interaction between mechanical pressures (incl. gravity), osmotic (chemical) potential and matrix effects (e.g. surface tension), is influenced by rainfall and evaporation as well as by groundwater flow. The best correlation observed so far between transpirate tritium concentrations over time and the balance between rainfall and
evaporation is a 14 day leading average in the latter. However, this time-scale from a rain event is expected to be too short to have any influence on water in the root zone based on current models of piston flow recharge. Further considerations are required.
Description
Keywords
Tritium, HIFAR Reactor, ANSTO, New South Wales, Australia, Sampling, Evaporation, Soils, Ground water, Roots, Rain, Trees
Citation
Twining, J., Hankin, S., & Hughes, C. (2020). Temporal, spatial and biotic variability in transpirate tritium near the LFBG and HIFAR: observations and working hypotheses. Paper presented to the 11th South Pacific Environmental Radioactivity Association Conference "SPERA 2010", Crowne Plaza, Surfers Paradise, Gold Coast, Australia, 31 August - 3 September 2010.