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After the explosion of the Chernobyl Nuclear Power Plant in April 1986, contaminated material was buried in shallow trenches within the exclusion zone. A (90)Sr plume was evidenced downgradient of one of these trenches, trench T22. Due to its conservative properties, (36)Cl is investigated here as a potential tracer to determine the maximal extent of the contamination plume from the trench in groundwater. (36)Cl/Cl ratios measured in groundwater, trench soil water and leaf leachates are 1-5 orders of magnitude higher than the theoretical natural (36)Cl/Cl ratio. This contamination occurred after the Chernobyl explosion and currently persists. Trench T22 acts as an obvious modern point source of (36)Cl, however other sources have to be involved to explain such contamination. (36)Cl contamination of groundwater can be explained by dilution of trench soil water by uncontaminated water (rainwater or deep groundwater). With a plume extending further than that of (90)Sr, radionuclide which is impacted by retention and decay processes, (36)Cl can be considered as a suitable tracer of contamination from the trench in groundwater provided that modern release processes of (36)Cl from trench soil are better characterized., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Abstract: This study uses 3H concentrations, 14C activities (a14C), 87Sr/86Sr ratios, and δ13C values to constrain calcite dissolution in groundwater from the Ovens catchment SE Australia. Taken in isolation, the δ13C values of dissolved organic C (DIC) and 87Sr/86Sr ratios in the Ovens groundwater imply that there has been significant calcite dissolution. However, the covariance of 3H and 14C and the calculated initial 14C activities (a0 14C) imply that most groundwater cannot have dissolved more than 20% of 14C-free calcite under closed-system conditions. Rather, calcite dissolution must have been partially an open-system process allowing 13C and 14C to re-equilibrate with CO2 in the unsaturated zone. Recognising that open-system calcite dissolution has occurred is important for dating deeper groundwater that is removed from its recharge area in this and other basins. The study is one of the first to use 14C and 3H to constrain the degree of calcite dissolution and illustrates that it is a valuable tool for assessing geochemical processes in recharge areas. [Copyright &y& Elsevier]

The Palm Valley Oasis (Finke Gorge National Park) in arid central Australia is characterised by large stands of red cabbage palm trees (Livistona mariae). How these unique plants, over 1000 km away from nearest relatives in the tropical parts of northern Australia persist, has long fascinated visitors. The hydrogeology of this area helps explain this phenomenon. Stable isotope (δ2H, δ8O) analyses shows groundwater to have a uniform composition that plots on or near a local meteoric water line. Carbon-14 results are observed to vary throughout this aquifer from effectively dead (<4%) to 87% modern carbon. Ratios of chlorine-36 to chloride range from 130 to 290×10-15 36Cl/Cl. In this region atmospheric 36Cl/Cl ratio is around 300×10-15. Thus an age range of around 300 ka is indicated if, as is apparent radioactive decay is the only significant cause of 36Cl/Cl variation within the aquifer. The classic homogenous aquifer with varying surface topography flow model is the simplest conceptual model that need be invoked to explain these data. Complexities, associated with local topography flow cells superimposed on the regional gradient, may mean groundwater with markedly different flow path lengths has been sampled. This potential flow path complexity, which is also evidenced by slight variation in groundwater cation ratios, can account for the distribution of isotope age data throughout the aquifer. Given the likely very slow travel times indicated by this aquifer''s hydraulic properties, age differences of the magnitude indicated from chlorine-36 data are feasible. The likely slow travel times (>100 ka) along some flow paths indicate groundwater discharge would endure through arid phases associated with Quaternary climate oscillations. Such a flow system can explain the persistence of this population of Palms and also highlight the possibility that Palm Valley has acted as a flora refuge since at least the mid Pleistocene. [Copyright &y& Elsevier]

The residence times of groundwater and chloride and the processes contributing to the development of saline (total dissolved solids (TDS) up to 45,379 mg/L) groundwater within the Barwon River Catchment of southeast Australia were investigated using major ion, stable isotope (δ 18 O, δ 2 H, and δ 13 C) and radioactive isotope ( 3 H, 14 C, 36 Cl) geochemistry. The elevated groundwater salinity in the region is primarily due to evapotranspiration and recycling of solutes in saline lakes with minor contributions from weathering of halite, silicate and calcite minerals. Groundwater residence times estimated from 14 C vary from modern to ~ 20 ka; for groundwater with lower 14 C activities, the estimated residence times vary significantly depending on the assumed flow model and the 14 C activity of recharge. Chloride residence times downgradient of Lake Murdeduke (a saline through-flow lake in the centre of the catchment) are greater than the corresponding groundwater residence times due to the recycling of Cl within the lake. Precise estimates of chloride residence time could not be determined using 36 Cl due to R 36 Cl in precipitation being lower than that of groundwater. This is most likely due to R 36 Cl values in rainfall having been higher in the past than they are at present due to climate variability. δ 18 O, δ 2 H, and δ 13 C values also suggest that the region has experienced increasingly more evaporative conditions with time. The results of this study demonstrate that, while Cl is a useful tracer of hydrological processes, it must be applied carefully in arid and semi-arid regions of the world. In particular, recharge rates calculated using chloride mass balance may be underestimated where recycling of Cl has occurred. [ABSTRACT FROM AUTHOR]

In nature, primordial 236 U has long since decayed to concentrations below detection. However, measurement of 236 U produced in-situ by neutron capture on 235 U in high-grade uranium deposits is made possible by recent advances in accelerator mass spectrometry (AMS). The detection of appreciable quantities of 236 U in groundwaters may reflect local uranium mineralisation, and thus prove useful in uranium exploration and potential age and ore grade estimations. Nine mineralised sediments from the South Australian Beverley North sandstone-hosted uranium deposits have 236 U/ 238 U ratios ranging from (1.57 ± 0.43) × 10 − 12 to (9.09 ± 0.55) × 10 − 12 , and U concentrations that vary by almost three orders of magnitude, ranging from 78.9 to 24,200 μg/g. Overall, the samples with the highest [U] have higher 236 U/ 238 U ratios, consistent with the generation of higher neutron fluxes with one notable exception with anomalously high [U] and a relatively low 236 U/ 238 U ratio. The observed variability in the 236 U/ 238 U ratio both within the deposits themselves, and between deposits may reflect heterogeneous mineralogy, elemental composition and water contents, which can affect the neutron flux generated within the samples. A single groundwater sampled within mineralisation from the Pepegoona West deposit yielded a 236 U/ 238 U ratio of (6.57 ± 2.97) × 10 − 12 . This is the first published data detecting natural, non-anthropogenic 236 U in groundwater in contact with a uranium deposit. The 236 U/ 238 U isotopic composition of the single groundwater sample is indistinguishable from that of the mineralised sediments from the same deposit. This is interpreted to reflect isotopic equilibration between the mineralisation and groundwater, rather than the in-situ production of 236 U by neutron capture on dissolved 235 U in the waters due to the low [U] typical of these highly reducing groundwaters. 236 U appears to have limited mobility in the Pepegoona West groundwater system, as evidenced by the lack of signature in groundwaters sampled from nearby wells in low-grade and un-mineralised portions of the deposit. This suggests that the detection of 236 U in the highly reducing groundwaters prevalent in this area may not be applicable as a proxy for uranium mineralisation. However, use of this technique as a potential exploration tool may have greater success in other areas with different hydrogeological conditions, specifically where the groundwaters are oxidising and uranium has a greater solubility as U(VI) complexes. [ABSTRACT FROM AUTHOR]

Abstract: Systematic variations in Cl/Br ratios together with R36Cl values, 14C activities, and δ 18O values reflect differences in groundwater recharge and define groundwater flow paths in the Riverine Province of the southeast Murray Basin. In groundwater from the shallowest Shepparton Formation, homogenisation of Cl/Br and δ 18O values and a decline in 14C activities with depth imply that vertical flow dominates. The 14C activities define variations in pre-land clearing recharge. In the palaeovalleys of present day rivers (“deep leads”), which contain sandier sediments, higher recharge (0.5 to 1.4 mm/year) produced relatively fresh groundwater (TDS<3000 mg/L). By contrast, away from the deep leads, lower recharge (0.1 to 0.4 mm/year) resulted in higher degrees of evapotranspiration producing more saline groundwater (TDS up to 60,000 mg/L). Cl/Br ratios correlate with these differences in recharge and groundwater salinity. Groundwater from the deep leads has average molar Cl/Br ratios of 530 to 660, while that from adjacent areas has average molar Cl/Br ratios of 980 to 1090. The variation in Cl/Br ratios is interpreted as due to small differences in the volume of windblown halite that the groundwater has dissolved during recharge, as confirmed by an inverse correlation between Cl/Br ratios and R36Cl values in groundwater from the Goulburn subcatchment. Zones of groundwater with variable Cl/Br ratios in the deeper Calivil–Renmark aquifer also allow the detection of rapid leakage from the surface that has the potential to compromise groundwater resources. [Copyright &y& Elsevier]

Documenting the origins, residence times, and movement of groundwater and the solutes that it contains is critical to understanding hydrogeological systems. This study uses Cl mass balance to determine the Cl accession time (i.e. the time required for Cl to accumulate) and 36 Cl to estimate the residence times of Cl in the Victorian portion of the Murray Basin, southeast Australia. Much of the Murray Basin contains saline groundwater with total dissolved solids (TDS) concentrations commonly > 14,000 mg/L and locally up to 300,000 mg/L. The total mass of Cl stored in the Victorian portion of the basin is estimated as between 12,400 and 47,100 MT. Using present day rainfall totals and Cl concentrations in rainfall, the Cl accession time is 170 to 650 ka. Aquifer thicknesses and groundwater salinity both increase westwards in this part of the Murray Basin. Consequently, the Cl accession times increase westward from 0.1–0.6 ka to 286–1080 ka. By contrast, 14 C activities of the majority of the groundwater are > 2 pMC, and commonly much higher. Notwithstanding the difficulty in correcting 14 C residence times, the widespread occurrence of groundwater with above background 14 C activities implies that groundwater residence times are generally < 30 ka, which is substantially shorter than the Cl accession times. R 36 Cl (the ratio of 36 Cl to total Cl × 10 15 ) values of the groundwater are between 20 and 230, and are uncorrelated with Cl concentrations. While it is difficult to determine precise Cl residence times, the observation that the R 36 Cl values are significantly higher than those that represent secular equilibrium with the aquifer matrix (R 36 Cl of 5 to 10) indicates that they are up to a few hundred thousand years and similar to the Cl accession times. The R 36 Cl values together with the geology of the aquifers, stable isotope ratios, and major ion geochemistry precludes halite dissolution, incorporation of connate water, or the long-term diffusion of Cl from clays as mechanisms of producing the elevated Cl concentrations. Rather, it is most likely that the high Cl concentrations result from recycling of solutes in saline lakes and playas or repeated cycles of evapotranspiration in the unsaturated zone. [ABSTRACT FROM AUTHOR]

Summary: A low-salinity (total dissolved solids, TDS, <5000mg/L) groundwater lens underlies the Murray River in the Colignan–Nyah region of northern Victoria, Australia. Hydraulic heads, surface water elevations, δ18O values, major ion geochemistry, 14C activities, and 3H concentrations show that the lens is recharged from the Murray River largely through the riverbank with limited recharge through the floodplain. Recharge of the lens occurs mainly at high river levels and the low-salinity groundwater forms baseflow to some river reaches during times of low river levels. Within the lens, flow through the shallow Channel Sands and deeper Parilla Sands aquifers is sub-horizontal. While the Blanchetown Clay locally separates the Channel Sands and the Parilla Sands, the occurrence of recently recharged low-salinity groundwater below the Blanchetown Clay suggests that there is considerable leakage through this unit, implying that it is not an efficient aquitard. The lateral margin of the lens with the regional groundwater (TDS >25,000mg/L) is marked by a hectometer to kilometer scale transition in TDS concentrations that is not stratigraphically controlled. Rather this boundary represents a mixing zone with the regional groundwater, the position of which is controlled by the rate of recharge from the river. The lens is part of an active and dynamic hydrogeological system that responds over years to decades to changes in river levels. The lens has shrunk during the drought of the late 1990s to the mid 2000s, and it will continue to shrink unless regular high flows in the Murray River are re-established. Over longer timescales, the rise of the regional water table due to land clearing will increase the hydraulic gradient between the regional groundwater and the groundwater in the lens, which will also cause it to degrade. Replacement of low-salinity groundwater in the lens with saline groundwater will ultimately increase the salinity of the Murray River reducing its utility for water supply and impacting riverine ecosystems. [Copyright &y& Elsevier]