Biotic, temporal and spatial variability of tritium concentrations in transpirate samples collected in the vicinity of a near-surface low-level nuclear waste disposal site and nearby research reactor

Abstract

The results of a 21 month sampling program measuring tritium in tree transpirate with respect to local sources are reported. The aim was to assess the potential of tree transpirate to indicate the presence of sub-surface seepage plumes. Transpirate gathered from trees near low-level nuclear waste disposal trenches contained activity concentrations of (3)H that were significantly higher (up to ∼700 Bq L(-1)) than local background levels (0-10 Bq L(-1)). The effects of the waste source declined rapidly with distance to be at background levels within 10s of metres. A research reactor 1.6 km south of the site contributed significant (p < 0.01) local fallout (3)H but its influence did not reach as far as the disposal trenches. The elevated (3)H levels in transpirate were, however, substantially lower than groundwater concentrations measured across the site (ranging from 0 to 91% with a median of 2%). Temporal patterns of tree transpirate (3)H, together with local meteorological observations, indicate that soil water within the active root zones comprised a mixture of seepage and rainfall infiltration. The degree of mixing was variable given that the soil water activity concentrations were heterogeneous at a scale equivalent to the effective rooting volume of the trees. In addition, water taken up by roots was not well mixed within the trees. Based on correlation modelling, net rainfall less evaporation (a surrogate for infiltration) over a period of from 2 to 3 weeks prior to sampling seems to be the optimum predictor of transpirate (3)H variability for any sampled tree at this site. The results demonstrate successful use of (3)H in transpirate from trees to indicate the presence and general extent of sub-surface contamination at a low-level nuclear waste site.

Trench ‘Bathtubbing’ and Surface Plutonium Contamination at a Legacy Radioactive Waste Site

“…An unacidified 100 mL sample was also collected for tritium measurement by liquid scintillation using purification and counting procedures described elsewhere. 20 …”

“…For groundwater samples, 10 L of water was collected for radiochemical analysis, while for trench water samples, 500 mL was sufficient. An unacidified 100 mL sample was also collected for tritium measurement by liquid scintillation using purification and counting procedures described elsewhere …”

“…An unacidified 100 mL sample was also collected for tritium measurement by liquid scintillation using purification and counting procedures described elsewhere. 20 Surface soils were generally collected with a push-tube sampler, which obtains a sample of approximately 500 g from the top 10 cm of soil. The soil sampling pattern consisted of points in circles at a radius of 1 and 3 m from the trenchsampler and then samples taken at 6 and 10 m distance in the downhill direction from the trench sampler.…”

Assessing doses to terrestrial wildlife at a radioactive waste disposal site: Inter-comparison of modelling approaches

“…The trenches are covered by~1 m of clay-rich soil layer, which in turn is overlain by topsoil and a well-developed and maintained grass-dominated vegetative cover. Due to its relatively dry and clay-rich conditions, only limited transport of radionuclides away from the trenches has been documented by the ongoing site monitoring system (Hughes et al, 2010;Twining et al, 2011).…”

How mobile is tritiated water through unsaturated cement-based materials? New insights from two complementary approaches

“…Secondly, tritiated water can occur under both liquid and vapor forms in unsaturated cement-based materials used in several types of nuclear facilities, e.g. near-surface disposal facilities, fusion reactor or tritium handling facility [13][14][15][16][17]. Under these conditions, further investigations are thus necessary to determine if HTO diffusion rates are as high as gas diffusive rates [6] or as low as the ionic solutes ones [4] when dehydrating.…”