Karina T. Meredith

Groundwater residence time in a dissected and weathered sandstone plateau: Kulnura–Mangrove Mountain aquifer, NSW, Australia

Groundwater residence time in the Kulnura–Mangrove Mountain aquifer was assessed during a multi-year sampling programme using general hydrogeochemistry and isotopic tracers (H2O stable isotopes, δ13CDIC, 3H, 14C and 87Sr/86Sr). The study included whole-rock analysis from samples recovered during well construction at four sites to better characterise water–rock interactions. Based on hydrogeochemistry, isotopic tracers and mineral phase distribution from whole-rock XRD analysis, two main groundwater zones were differentiated (shallow and deep). The shallow zone contains oxidising Na–Cl-type waters, low pH, low SC and containing 3H and 14C activities consistent with modern groundwater and bomb pulse signatures (up to 116.9 pMC). In this shallow zone, the original Hawkesbury Sandstone has been deeply weathered, enhancing its storage capacity down to ∼50 m below ground surface in most areas and ∼90 m in the Peats Ridge area. The deeper groundwater zone was also relatively oxidised with a tendency towards Ca–HCO3-type waters, although with higher pH and SC, and no 3H and low 14C activities consistent with corrected residence times ranging from 11.8 to 0.9 ka BP. The original sandstone was found to be less weathered with depth, favouring the dissolution of dispersed carbonates and the transition from a semi-porous groundwater media flow in the shallow zone to fracture flow at depth, with both chemical and physical processes impacting on groundwater mean residence times. Detailed temporal and spatial sampling of groundwater revealed important inter-annual variations driven by groundwater extraction showing a progressive influx of modern groundwater found at >100 m in the Peats Ridge area. The progressive modernisation has exposed deeper parts of the aquifer to increased NO3− concentrations and evaporated irrigation waters. The change in chemistry of the groundwater, particularly the lowering of groundwater pH, has accelerated the dissolution of mineral phases that would generally be inactive within this sandstone aquifer triggering the mobilisation of elements such as aluminium in the aqueous phase.

Precipitation stable isotope variability and sub‐cloud evaporation processes in a semi‐arid region.

Cop Abstract: The stable isotopic (H/H and O/O) composition of precipitation has been used for a variety of hydrological and paleoclimate studies, a starting point for which is the behaviour of stable isotopes in modern precipitation. To this end, daily precipitation samples were collected over a 7-year period (2008–2014) at a semi-arid site located at the Macquarie Marshes, New South Wales (Australia). The samples were analysed for stable isotope composition, and factors affecting the isotopic variability were investigated. The best correlation between δO of precipitation was with local surface relative humidity. The reduced major axis precipitation weighted local meteoric water line was δH= 7.20 δO + 9.1. The lower slope and intercept (when compared with the Global Meteoric Water Line) are typical for a warm dry climate, where subcloud evaporation of raindrops is experienced. A previously published model to estimate the degree of subcloud evaporation and the subsequent isotopic modification of raindrops was enhanced to include the vertical temperature and humidity profile. The modelled results for raindrops of 1.0mm radius showed that on average, the measured D-excess (=δH 8 δO) was 19.8‰ lower than that at the base of the cloud, and 18% of the moisture was evaporated before ground level (smaller effects were modelled for larger raindrops). After estimating the isotopic signature at the base of the cloud, a number of data points still plotted below the global meteoric water line, suggesting that some of the moisture was sourced from previously evaporated water. Back trajectory analysis estimated that 38% of the moisture was sourced over land. Precipitation samples for which a larger proportion of the moisture was sourced over land were O and H-enriched in comparison to samples for which the majority of the moisture was sourced over the ocean. The most common weather systems resulting in precipitation were inland trough systems; however, only East Coast Lows contributed to a significant difference in the isotopic values. Copyright

Climate Change and Groundwater

Human civilisations have for millennia depended on the stability of groundwater resources to survive dry or unreliable climates. While groundwater supplies are buffered against short-term effects of climate variability, they can be impacted over longer time frames through changes in rainfall, temperature, snowfall, melting of glaciers and permafrost and vegetation and land-use changes. Groundwater provides an archive of past climate variation by recording changes in recharge amount or the chemical and isotopic evolutionary history of a groundwater system. For example, in the Sahara desert of North Africa, radiocarbon dating of groundwater shows that a highly arid climate prevailed during the last ice age followed by more humid conditions up until approximately 4000 years ago. In northern America and Europe, massive meltwater recharge of aquifers that occurred as a result of the same ice age approximately 15,000–20,000 years ago has left distinctive stable isotope signatures that remain today. The groundwater response to future climate change will be exacerbated by the heavy reliance that present day societies continue to place on groundwater, and the extensive modifications we have made to natural hydrological regimes. Models of groundwater response to climate change predict both increases and decreases in groundwater recharge and groundwater quality. Outcomes will be dependent on geographic location, and hydrological, biological and behavioural feedback mechanisms as natural systems and human civilisations struggle to cope with both climate change and our increasing demand for water.

Soil degradation due to the destruction of crystalline kaolinite and the formation of X-ray amorphous clays accompanying ephemeral saline groundwater discharge

The discharge of saline groundwater results in the formation of sodic soil scalds in irrigated, dryland and urban environments of southeastern Australia. Sodic soils are dispersive, and this leads to soil erosion and a loss in agricultural productive capacity. These sodic soils commonly show polygonal cracking and pressure ridges indicating the presence of swelling clays. Infrared spectroscopy of scald surfaces and XRD (X-ray diffraction) analyses of the clay fractions of the sodic soils show the presence of amorphous clays, smectite, illite or mixed smectite/illite layer clays. Non-salinised soils adjacent to the salt scalds are commonly predominantly kaolinitic. SEM images and normative EDS mineral analyses of the clay fractions of these soils show that crystalline particles, predominantly of kaolinite, are progressively replaced by poorly crystalline smectite, illite and amorphous material. Normative mineral analyses determined from the bulk soil composition, based on a derived composition of submicron clay particles, show that increased soil salinity correlates with a higher percentage of X-ray amorphous clays of a smectitic composition. By comparison with studies from elsewhere, we propose a mechanism by which kaolinite is transformed into smectite and illite or mixed layer smectite/illite, which proceeds by dissolution and subsequent crystallisation, rather than by solid-state transformation. We propose that the crypto-crystalline or amorphous nature of the clays produced is largely a function of cycles of varying mineral stability produced by alternating periods of saline water discharge and freshwater flushing. The transformation of primary kaolinite to smectite/illite within saline discharge zones explains the spectral changes that allow such saline discharge areas to be mapped with hyperspectral imagery.

Identifying the source of atmospheric moisture over arid deserts using stable isotopes (2H and 18O) in precipitation

State Key Laboratory of Hydrology‐Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, China 2 Institute of Isotope Hydrology, College of Earth Sciences and Engineering, Hohai University, Nanjing 210098, China Australian Nuclear Science and Technology Organisation, Kirrawee DC, New South Wales 2232, Australia Correspondence Wenbo Rao and Bin Yong, State Key Laboratory of Hydrology‐Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, China. Email: raowenbo@163.com; yongbin@hhu. edu.cn Funding information Fundamental Research Funds for the Central Universities, Grant/Award Number: 2017B19614; National Natural Science Foundation of China, Grant/Award Numbers: 40973001, 41271041 and 41273015

Carbon dynamics in a Late Quaternary-age coastal limestone aquifer system undergoing saltwater intrusion

This study investigates the inorganic and organic aspects of the carbon cycle in groundwaters throughout the freshwater lens and transition zone of a carbonate island aquifer and identifies the transformation of carbon throughout the system. We determined 14C and 13C carbon isotope values for both DIC and DOC in groundwaters, and investigated the composition of DOC throughout the aquifer. In combination with hydrochemical and 3H measurements, the chemical evolution of groundwaters was then traced from the unsaturated zone to the deeper saline zone. The data revealed three distinct water types: Fresh (F), Transition zone 1 (T1) and Transition zone 2 (T2) groundwaters. The 3H values in F and T1 samples indicate that these groundwaters are mostly modern. 14CDOC values are higher than 14CDIC values and are well correlated with 3H values. F and T1 groundwater geochemistry is dominated by carbonate mineral recrystallisation reactions that add dead carbon to the groundwater. T2 groundwaters are deeper, saline and characterised by an absence of 3H, lower 14CDOC values and a different DOC composition, namely a higher proportion of Humic Substances relative to total DOC. The T2 groundwaters are suggested to result from either the slow circulation of water within the seawater wedge, or from old remnant seawater caused by past sea level highstands. While further investigations are required to identify the origin of the T2 groundwaters, this study has identified their occurrence and shown that they did not evolve along the same pathway as fresh groundwaters. This study has also shown that a combined approach using 14C and 13C carbon isotope values for both DIC and DOC and the composition of DOC, as well as hydrochemical and 3H measurements, can provide invaluable information regarding the transformation of carbon in a groundwater system and the evolution of fresh groundwater recharge.

Rainfall isotope variations over the Australian continent – Implications for hydrology and isoscape applications

This paper presents a continental scale interpretation of δ2H and δ18O in Australian precipitation, incorporating historical GNIP data at seven sites (1962-2002) and 8-12 years of new monthly data from 15 sites from 2003 to 2014. The more than doubling of stations and the significant time series duration allow for an improved analysis of Australian precipitation isotopes. Local meteoric water lines were developed for each site, and for the Australian continent. When the annual precipitation weighted values were used, the Australian meteoric water line was δ2H = 8.3 δ18O + 14.1‰. Precipitation amount was found to be a stronger driver of precipitation isotopes than temperature at most sites, particularly those affected by tropical cyclones and the monsoon. Latitude, elevation and distance from the coast were found to be stronger drivers of spatial variability than temperature or rainfall amount. Annual isoscapes of δ2H, δ18O and deuterium excess were developed, providing an improved tool to estimate precipitation isotope inputs to hydrological systems. Because of the complex climate, weather and oceanic moisture sources affecting Australia, regional groupings were used instead of the climate zone approach and additional data was included to improve the coverage in data poor regions. Regression equations for the isoscape were derived using latitude, altitude and distance from the coast as predictor variables. We demonstrate how this isoscape can be used as a tool for interpreting groundwater recharge processes using examples from across Queensland and New South Wales, including the Murray Darling Basin. Groundwater isotopes at sites where direct local recharge occurs are similar to rainfall, but for inland sites, which are often arid or semi-arid, a disconnect between shallow groundwater and local rainfall is observed; the departure in deuterium excess for these sites increases with aridity and distance from the headwaters where flooding originates.

Characterization of the subsurface architecture and identification of potential groundwater paths in a clay-rich floodplain using multi-electrode resistivity imaging

ABSTRACT The interaction between surface water and groundwater in clay-rich fluvial environments can be complex and is generally poorly understood. Airborne electromagnetic surveys are often used for characterizing regional groundwater systems, but they are constrained by the resolution of the method. A resistivity imaging survey has been carried out in the Macquarie Marshes (New South Wales, Australia) in combination with water chemical sampling. The results have enabled the identification of buried palaeochannels and the location of potential recharge points. The data have been compared with previously published airborne electromagnetic data in the same area. Deeper less conductive features suggest that there is a potential connection between the Great Artesian Basin and groundwater contained within the shallow sand aquifer. Even though the chemistry of the groundwater samples does not indicate interaction with the Great Artesian Basin, the observed discontinuity in the saprolite implies potential for this to happen in other locations.