Gary Hancock

Using Ra isotopes to examine transport processes controlling benthic fluxes into a shallow estuarine lagoon

Measurements of the benthic flux of four naturally occurring radium isotopes in a shallow lagoon in the Bega River estuary has provided information on the types and rates of transport processes operating in the lagoon sediments. The measurement techniques included Ra mass budgets of the lagoon, Ra fluxes into benthic chambers, and modelling of the pore water and solid phase Ra profiles in a sediment core. The sediment profile of 210Pb, and the solid phase and pore-water profiles of the longer-lived Ra isotopes, 228Ra (half-life 5.7 years) and 226Ra (half-life 1600 years), indicate bioturbation to a depth of 10 cm. A diffusion-bioturbation model has been used to assess the relative importance of molecular diffusion and bioturbation as transport processes controlling the benthic flux of Ra. The flux of the shortest-lived isotope, 224Ra (half-life 3.7 days), is not significantly enhanced by bioturbation, and its flux is consistent with diffusion-controlled release. However bioturbation enhances the 228Ra flux by a factor of more than two over the flux due to molecular diffusion alone. Modelled pore-water profiles and flux calculations are consistent with a bioturbation time scale between 0.5 and 2 years. The measured benthic flux of 226Ra is much greater than can be accounted for by the modelled profile, and may be due to slow 226Ra desorption from the sediment, variable sediment accumulation rates, or groundwater flow. Based on 226Ra pore-water and flux measurements at the time of this study, groundwater flow has an upper limit of 0.3 cm d−1.

Horizontal mixing of Great Barrier Reef waters: offshore diffusivity determined from radium isotope distribution

The Great Barrier Reef (GBR), northern Australia, is the largest coral reef system in the world and provides habitat for highly diverse tropical marine ecosystems. Mixing in the coastal waters of the GBR is an important parameter influencing the health of these ecosystems. We have used the distribution of the four naturally occurring radium isotopes to determine the rate of mixing of nearshore waters of the central part of the GBR lagoon with water from the Coral Sea. The observed radium distribution is modeled using a one-dimensional diffusion model. The model improves on previous radium offshore mixing models by incorporating the benthic flux of radium diffusing across the sediment-water interface and offshore changes in water column depth. We find that the inner lagoon diffusivity (<20 km offshore) is best estimated using the short-lived isotopes 224Ra and 223Ra. The concordance of K x estimated using the two different isotopes and the apparent consistency between measured riverine inflows to the lagoon and inflows inferred from the modeled salinity distribution provide confidence in the results. The mean value of K x for the inner lagoon region of the southern central zone between latitudes 15.8°S and 19.0°S (265 ± 36 m2 s−1) is more than twice that in the northern central zone (14.3°S to 15.8°S). This difference likely reflects the different reef matrix density in the two zones. The distribution of the longer-lived isotope 228Ra indicates more rapid mixing in the middle and outer lagoon. These results indicate that central GBR water within 20 km of coast is flushed with outer lagoon water on a timescale of 18–45 days, with the flushing time increasing northward.

Tareena Billabong – a palaeolimnological history of an ever-changing wetland, Chowilla Floodplain, lower Murray–Darling Basin, Australia

A 427-cm sediment core was extracted from Tareena Billabong, a Murray River floodplain wetland in the extreme south-west of New South Wales, Australia. Analysis of fossil diatoms and pollen, sediment 210Pb and 137Cs profiles and radiocarbon and luminescence dating reveal that Tareena Billabong has undergone substantial environmental change in its ~5000-year history. Shortly after its formation, the billabong was a freshwater lagoon with a diatom flora dominated by Synedra ulna and Planothidium lanceolatum. An increase in Aulacoseira granulata, a river plankton dominant today, reflects two phases of increased connectivity with the Murray River in the mid to late Holocene. A shift to lagoonal taxa after ~3000 years BP is attributed to water balance and river-flow changes, possibly associated with regional climate change. Importantly, it appears to have undergone an extended phase of increasing turbidity, and possibly wetland salinity, commencing ~3000 years BP. Sedimentation increased at least 15-fold in the European phase. Billabong salinity increased markedly soon after European settlement, reaching a peak in the late 1800s AD. While regulation then increased the degree of connection between the billabong with the River in the 1920s AD, salinity levels remained high. Increased salinity is revealed by increases in the diatom taxa Amphora spp., Cyclotella meneghiniana, Gyrosigma acuminatum, Planothidium delicatulum and Tryblionella hungarica and by declines in Casuarinaceae, Eucalyptus, Myriophyllum and Cyperaceae pollen. Tareena Billabong was subjected to considerable environmental pressures from the early stages of European settlement in terms of sediment load, hydrological change and salinity.

Natural-series radionuclides in traditional North Australian aboriginal foods

Activity concentrations of the radionuclides 226Ra, 210Pb, 210Po, 238U, 234U, 230Th, 232Th and 227Ac were measured in edible flesh of traditional Aboriginal food items from the Magela and Cooper Creek systems in the tropical Northern Territory of Australia. Fish, buffalo, pig, magpie goose, filesnake, goanna, turtle, freshwater shrimp and freshwater crocodile were studied. Activity concentrations in water were also measured to enable the calculation of concentration ratios (CRs). For most edible flesh samples, activity concentrations followed the approximate order: 210Po≫226Ra>210Pb>[234U∼238U]>[230Th∼232Th]. The 210Po/210Pb activity ratio was particularly high (greater than 100) for pig flesh. CRs for fish species fall into two groups. Group 1 (bony bream and sleepy cod) had CRs about five times higher than for group 2 (eight other species). CRs for turtle flesh were similar to those for fish in group 1, while those for turtle liver were about a factor of 10 higher. CRs for magpie goose, filesnake, freshwater shrimp, goanna and crocodile flesh were also of the same order as for fish in groups 1 or 2. Calculations of dose resulting from release of wastewaters from uranium mining operations in the region show that the dominant pathway would be uptake of radionuclides, especially 226Ra, by freshwater mussels, followed by radionuclide uptake by fish.

Onset and persistence of cyanobacterial blooms in a large impounded tropical river, Australia

The dynamic interplay between physical, chemical and biological factors in the development and persistence of cyanobacterial blooms in impounded rivers is an important topic. Over a 3-year study period, variable climatic conditions were recorded in the Fitzroy River, Queensland, Australia, which is a typical, impounded lowland tropical river. Post-flood turbidity reduced the available light in the well-mixed water column to levels insufficient for cyanobacterial growth. Only when the water column stratified and the slowly sinking particles dropped from the surface layer did the ratio of surface mixed layer depth to euphotic depth approach 1, allowing cyanobacterial growth. By the time the light climate became favorable, most of the dissolved nutrients had been scavenged from the water column by settling particles or sequestered by fringing macrophytes and other biogeochemical processes. Cyanobacterial blooms dominated by Cylindrospermopsis raciborskii persisted for several months until the next flood flushed the system. The cyanobacterial species dominating that environment were very small and had high specific phosphorus uptake rates. Their nutrient requirement was met by transfer across the oxycline driven by regular high wind mixing events, entraining nutrient-rich bottom waters. Nutrient fluxes from the sediments into the anoxic bottom layer were sufficient to replace the bottom nutrients lost to the surface layer.

Phosphorus dynamics in Australian lowland rivers

In freshwater systems, phosphorus is adsorbed predominantly to clay within the sediments. Assuming a linear adsorption/desorption isotherm, rapid equilibrium adsorption, and transport by molecular diffusion, estimates are derived for (a) the rates of exchange between the adsorbed phosphorus pool in the sediments and the dissolved pool in the water column and (b) time scales to re-establish equilibrium after a step change in the water column phosphorus concentration. For oxic sediments, the time scale is of the order of tens of days. Anoxic release is much faster;the time scale is tens of minutes. The release of treated sewage at Narrabri abruptly raises the phosphorus concentration in the Namoi River. The concentration only returns to its original level 10–20 km downstream. A sediment adsorptive-uptake model underestimates the downstream phosphorus uptake rates. An alternative model, based on biotic uptake by Cladophora , describes reality better. It treats phosphorus transfer as controlled by physical transport processes and by the phosphorus uptake capacity of the biota. We show also that carp resuspension is faster than diffusion (6 v. 28 days) in restoring phosphorus concentrations in the water column after perturbation by rapid algal drawdown.

A new Certified Reference Material for radionuclides in Irish sea sediment (IAEA-385)

A new Certified Reference Material (CRM) for radionuclides in sediment (IAEA-385) is described and the results of the certification process are presented. Eleven radionuclides ((40)K, (137)Cs, (226)Ra, (228)Ra, (230)Th, (232)Th, (234)U, (238)U, (238)Pu, (239+240)Pu and (241)Am) have been certified and information mass activities with 95% confidence intervals are given for seven other radionuclides ((90)Sr, (210)Pb((210)Po), (235)U, (239)Pu, (240)Pu and (241)Pu). Results for less frequently reported radionuclides ((60)Co, (99)Tc, (134)Cs, (155)Eu, (224)Ra and (239)Np) and information on some activity and mass ratios are also reported. The CRM can be used for quality assurance/quality control of the analysis of radionuclides in sediment samples, for the development and validation of analytical methods and for training purposes.

Uptake of uranium and thorium series radionuclides by the waterlily, Nymphaea violacea

Abstract The waterlily Nymphaea violacea is a major aquatic macrophyte in the waters of the Alligator Rivers Region, Northern Teritory, Australia. It is also a traditional Aboriginal diet item, and is considered to be potentially one of the main contributors to the effective dose equivalent arising from consumption of so called ‘bush’ food in the region. Because of the proximity of the Ranger Uranium Mine (RUM), the activity concentrations of the U and Th series radionuclides have been studied in water, sediment and waterlily during different seasons at five sites downstream of the mine site. The objectives of the study are: 1. 1. To identify the major source of radionuclide uptake by the plant; i.e. water or sediment. 2. 2. To assess the concentration factors/ratios needed for predicting the radiation exposure of the critical group resulting from any discharge of water to the aquatic environment from the Ranger uranium mine. 3. 3. To estimate the natural radiation exposure of the public arising from consumption of waterlilies. Results show that sediment is the major source of U and Th series radionuclides to waterlily roots, rhizomes and foliage. Waterlily to sediment concentration ratios, based on wet weights, are of the order of 0·01–0·03 for roots and rhizomes and 0·005–0·015 for foliage. The naturally occurring committed effective dose equivalent (CEDE) from an estimated annual intake of 3 kg waterlily rhizomes from the forage areas are of the order of 0·02 mSv.

The release and persistence of radioactive anthropogenic nuclides

Abstract Atmospheric testing of nuclear weapons during the period 1945–1980 ushered in the ‘atomic age’ and released large quantities of anthropogenic radiogenic nuclides into the atmosphere. These radionuclides were subsequently deposited as fallout to the entire surface of the planet. While many have decayed to negligible levels, long-lived radionuclides persist and will do so for thousands of years. Isotopes of plutonium, 239Pu (half-life 24 100 years) and 240Pu (half-life 6563 years), provide the best chronological markers for the onset of this anthropogenic event both now and into the future due to their long half-lives, particle-reactivity, and the fact that they were present in negligible quantities prior to anthropogenic production and release. Chronostratigraphic markers established by distinct Pu concentration profiles and Pu isotope changes in sediment sequences and ice and coral cores can provide high-resolution dating over the last 60 years. However, even though fallout has ceased, it is found that the Pu inventory currently held in surface soil layers and the oceans will continue to supply Pu to sediment deposition zones for millennia and centuries, respectively. The delivery of this Pu will depend on soil erosion and bioturbation rates, and the rate of removal of dissolved Pu from the ocean.

Against the tide: the freshening of naturally saline coastal lakes, southeastern South Australia

Diatom analyses of sediment cores extracted from three lakes in coastal southeastern South Australia reveal that, for most of the mid-late Holocene, they were shallow, brackish to saline systems with limited flow of water from continental sources. The construction of a substantial network of drains in the early years of settlement, to maximise transportation and agricultural production through wet winters, lead to abrupt freshening of the lakes. Interestingly, despite substantial nutrient loads to Lake Bonney SE (there are two Lakes Bonney and Frome in South Australia, which is why the lakes in the southeast of the state are differentiated with ‘SE’) associated with the commissioning and expansion of pulp and paper mills, a wastewater treatment plant discharge and agricultural runoff, there is only moderate evidence of nutrient enrichment in the lake, possibly because the post-impact assemblages are dominated by taxa with broad ecological preferences. Despite being preserved within a conservation park, eutrophication associated with agriculture is evident in the diatom assemblages of Lake Frome SE, which has a catchment more than twice that of Lake Bonney SE. Mullins Swamp, on the other hand, supports few indicators of eutrophic conditions. The freshening of these lakes is against the tide of salinisation from rising saline groundwaters in most wetlands across southeastern Australia.