Using multiple geochemical proxies to trace origin of gypsum (Gulf of Carpentaria, Australia, -70ka).

This paper discusses the geochemical signature of the Carpentaria evaporitic levels using minor-trace and rare-earth elements (REE) and Sr, O, C and S isotopes. The chemistry of these evaporites reveals important criteria for distinguishing between ancient marine and non-marine evaporites. © 2007, Sociedad Geologica de Espan

Palaeohydrology of the Mulhouse Basin: are fluid inclusions in halite tracers of past seawater composition?

Brine reactions processes were the most important factors controlling the major-ion evolution in the Oligocene, Mulhouse Basin (France) evaporite basin. The combined analysis of fluid inclusions in primary textures in halite by Cryo-SEM-EDS with sulfate-δ34S, δ18O and 87Sr/86Sr isotope ratios reveals hydrothermal inputs and recycling of Permian evaporites, particularly during advanced stages of evaporation in the Salt IV member which ended with sylvite formation. The lower part of the Salt IV evolved from an originally marine input. Sulfate-δ34S shows Oligocene marine-like signatures at the base of the member (Fig.1). However, enriched sulfate-δ18O reveals the importance of re-oxidation processes. As evaporation progressed other non-marine or marine-modified inputs from neighbouring basins became more important. This is demonstrated by an increase in K concentrations in brine inclusions, Br in halite and variations in sulfate isotopes trends and 87Sr/86Sr ratios. The recycling of previously precipitated evaporites was increasingly important with evaporation. Therefore, regardless of the apparent marine sequence (gypsum, halite, potassic salts), the existence of diverse inputs and the consequent chemical changes to the brine preclude the use of trapped brine inclusions in direct reconstruction of Oligocene seawater chemistry.European Association for Geochemistry; Geochemical Societ

Using 14C and 3H to understand groundwater flow and recharge in an aquifer window

Knowledge of groundwater residence times and recharge locations is vital to the sustainable management of groundwater resources. Here we investigate groundwater residence times and patterns of recharge in the Gellibrand Valley, southeast Australia, where outcropping aquifer sediments of the Eastern View Formation form an "aquifer window" that may receive diffuse recharge from rainfall and recharge from the Gellibrand River. To determine recharge patterns and groundwater flow paths, environmental isotopes (3H, 14C, δ13C, δ18O, δ2H) are used in conjunction with groundwater geochemistry and continuous monitoring of groundwater elevation and electrical conductivity. The water table fluctuates by 0.9 to 3.7 m annually, implying recharge rates of 90 and 372 mm yr−1. However, residence times of shallow (11 to 29 m) groundwater determined by 14C are between 100 and 10 000 years, 3H activities are negligible in most of the groundwater, and groundwater electrical conductivity remains constant over the period of study. Deeper groundwater with older 14C ages has lower δ18O values than younger, shallower groundwater, which is consistent with it being derived from greater altitudes. The combined geochemistry data indicate that local recharge from precipitation within the valley occurs through the aquifer window, however much of the groundwater in the Gellibrand Valley predominantly originates from the regional recharge zone, the Barongarook High. The Gellibrand Valley is a regional discharge zone with upward head gradients that limits local recharge to the upper 10 m of the aquifer. Additionally, the groundwater head gradients adjacent to the Gellibrand River are generally upwards, implying that it does not recharge the surrounding groundwater and has limited bank storage. 14C ages and Cl concentrations are well correlated and Cl concentrations may be used to provide a first-order estimate of groundwater residence times. Progressively lower chloride concentrations from 10 000 years BP to the present day are interpreted to indicate an increase in recharge rates on the Barongarook High. © Author(s) 2014

Groundwater residence time in the Kulnura-Mangrove Mountain Plateau (Gosford, NSW, Australia)

The Kulnura-Mangrove Mountain plateau consists of the catchments of Mangrove, Narara, Mooney Mooney, and Ourimbah Creeks, and Wyong River. Groundwater plays a key role in sustaining stream flow within these catchments. Estimates indicate up to 50% of annual stream flow is derived from baseflow. The local community water supply relies on the groundwater within the elevated Hawkesbury- Narrabeen sandstone plateau. Furthermore, the Gosford-Wyong Councils’ Water Authority (WSA) is the third largest in NSW and utilises many of the streams flowing from the sandstone plateau for municipal water supply. It is anticipated that the WSA will provide municipal water for 319 000 persons by the year 2010. The increasing volumes of groundwater being extracted and changing land use have the potential to cause damage to the fresh water aquifer through contamination and aquifer depletion. A hydrogeochemical survey (2006-2009) has been conducted in NSW Dept of Water and Energy (DWE) monitoring wells across the plateau in order to determine groundwater residence times. Groundwater was analysed for major ions, minor and trace elements, H2O 18O and 2H, 13CDIC, 87Sr/86Sr, 14CDIC, and 3H, and complemented with mineralogical and isotopic information obtained from soil and drill chips collected during well construction. Water stable isotopes confirm the meteoric origin of the groundwater with most values plotting on the local meteoric water line. Localised evaporative trends suggest recharge with evaporated groundwater stored in ponds. Shallow groundwaters have 3H and 14C activities consistent with modern recharge (Fig 1). Carbon “bomb pulse” signatures of up to 116.8 pmC are found in the central areas of the plateau. The thin soils, lack of carbonates in the intensely weathered near-surface Hawkesbury sandstone, and the shallow depth of the water samples is consistent with the 3H results measured, suggesting minimal dilution of the original 14C. Input of this data into a southern hemisphere bomb pulse model [1] suggest potential recharge during the 1990´s, coinciding with sustained wet conditions and above average rainfalls experienced during this period. Fig. 1. 14C vs 3H plot of groundwater samples in the Kulnura- Mangrove Mountain Plateau Deeper groundwaters have lower 14C and 3H activities in some cases close to background level (Fig. 1). The quantifiable 3H suggests residence times of <70 a. However, non-corrected 14C residence times are submodern (>500 a). This apparent discrepancy can be explained by either mixing with older waters or dissolution of carbonates. The good correlation of total dissolved inorganic carbon (TDIC) and Ca (R2=0.8), 13CTDIC in groundwater and mineralogy results from drill chips suggest that dissolution of dispersed carbonates is taking place. The deepest groundwaters show the most difference in residence time across the study area. The eastern and western plateaus yield old groundwater with 14C corrected residence times of around 9 ka and 4 ka respectively. However, the groundwater at equivalent depths in the central plateau was found to be considerably younger with residence times of <70 a

Radiocarbon and geochemical constraints on shallow groundwater recharge in a large arid zone river, Cooper Creek, SW Queensland, Australia.

In the arid and semi-arid internally drained Lake Eyre Basin of central Australia, large mud dominated anabranching river systems transport monsoon derived floodwaters into the centre of the continent during the summer months, and subsequently spend much of the year under low to no flow conditions. Cooper Creek has the largest catchment in this basin, and in south west Queensland has a wide (20-60km) floodplain and multiple channel system. Enlarged channel segments, known as waterholes or billabongs, can retain water throughout much of the dry season, and their mud base can often be scoured during floods into the underlying sandy alluvium where the shallow groundwater table exists 3-5m below the base of the waterholes. Little is known of the groundwater recharge mechanisms in this ecologically important and hydrologically unregulated river system, thus a number of piezometer transects were construct across the floodplain between two waterholes to investigate groundwater recharge processes in further detail. Samples recovered from all piezometers were analysed for major-trace element, water stable isotopes (δ2H and δ180), 3H and 14C. Water stable isotopes reveal shallow groundwater is recharged by high magnitude, low frequency monsoonal flood events, with minor evaporative enrichment probably linked to recent smaller flooding events. 14C dating of dissolved inorganic carbon reveals recharge is most effective beneath the deepest channel segments of the waterholes, and that residence time of the shallow groundwater increases with distance from major waterholes, with the post 1950’s 14C bomb pulse signature present only in close proximity to the channels. 3H allows further refinement of the shallow groundwater residence times, with no 3H detected in groundwater over ~500m from the waterholes, indicating groundwater recharge is slow and restricted to major flooding events. The increase in groundwater residence time with distance from waterholes, is also accompanied by an abrupt increase in salinity, and suggests recent recharge has formed local freshwater lenses above the regional, more saline groundwater. This increase in salinity with increasing distance from the waterholes is not accompanied by an increase on the evaporative signal of water stable isotopes, suggesting evapotranspiration is the dominant mechanism of salinisation within the shallow groundwater beneath the floodplains and minor channels. This study demonstrates that detailed chemical analysis of groundwaters from arid and semi arid areas can provide a useful estimate of recharge where the remote location makes traditional detailed borehole monitoring difficult or impossible to achieve

Exploring the hydrochemical evolution of brines leading to sylvite precipitation in ancient evaporite basins.

Sylvite is a very common mineral in ancient evaporite deposits. Due to the absence of current deposits, the natural geochemical mechanism/s for synsedimentary sylvite precipitation and accumulation are not well understood. Numerous sylvite deposits or portions of them have been described as a result of diagenesis (i.e. Sergipe subbasin, Brasil). However, a number of deposits have been described as synsdimentary or being formed during primary evaporite deposition. It is the last group of deposits that can be studied to better understand the hydrochemical processes taking place in the brine at the onset of sylvite precipitation. The Salt IV sylvite beds from the Mulhouse potash basin, Alsace (France) have been described as synsedimentary in origin (LOWENSTEIN and SPENCER, 1990; CENDON et al., 2008). While sylvite in itself does not contain fluid inclusions viable for micro analysis, primary textures in neighboring halite are used as a proxy to understand brine evolution. Two halite-sylvite cycles from the B1 and B2 layers of the potash lower seam were selected. These exhibited clear primary halite crystal textures with sylvite adapting to an irregular halite sedimentary surface and finishing with a flat surface. The nine halite samples, selected at centimeter scale, provided close to 100 single fluid inclusion analyses, representing both the transition towards sylvite precipitation and the post sylvite precipitation. The fluid inclusion analyses revealed strong fluctuations in K concentration, well over the analytical error (<10%). These variations, in the same halite crystal, seem aligned in growth bands, with fluid inclusions within a certain growth band showing practically identical K concentrations, while neighboring bands exhibit a different concentration. Overall, the closer we are from a sylvite layer the higher K concentrations are. However, strong fluctuations continue when growth bands are compared. This pattern shows cycles of increasing K concentration along parallel growth bands with sharp falls followed by the initiation of a new increasing trend. The small “growth band” scale of the K concentration variations, suggests very sensitive processes within the brine with potential environmental changes (i.e. seasonal variations, day-night temperature fluctuations cycles) leading towards the final mass precipitation of a sylvite layer

Installation of a pilot experimental trench at the Little Forest legacy site

During 2017, a pilot experimental trench was constructed at the Little Forest Legacy Site (LFLS). The objective of installing this trench was to facilitate experimental field-work aimed at further characterising the site, in particular the hydrology of the excavated trenches and of the near-surface layers in which the trenches are located. The test trench is of similar depth to the waste disposal trenches at the legacy site (3 metres) and extends 6 m in length. However, unlike the disposal trenches, the experimental trench contains no waste materials of any kind. Instead, the trench contains a number of sampling points and other instrumentation, and is back filled with river gravel to provide a uniform composition and maintain structural stability. It is intended that the pilot trench will be followed by other trenches with specific experimental objectives. The purposes of this report are to discuss the background, rationale for, and implementation of the facility; to provide a detailed description of the pilot trench; and to compile information and photographs documenting the excavation process. Although some preliminary hydrological data and comparisons with the legacy trenches are presented, the scientific data will be fully discussed and interpreted in future scientific reports

Analysis of environmental isotopes in groundwater to understand the response of vulnerable coastal aquifer to pumping: Western Port Basin, south-eastern Australia

The response of a multi-layered coastal aquifer in southeast Australia to decades of groundwater pumping, and the groundwater age, flow paths and salinization processes were examined using isotopic tracers. Groundwater radiocarbon and tritium contents decline with distance and depth away from basin margins; however, in the main zone of pumping, radiocarbon activities are generally homogeneous within a given depth horizon. A lack of tritium and low radiocarbon activities (< 25 pMC) in groundwater in and around the pumping areas indicate that seasonal recovery of water levels is related to capture of old water with low radioisotope activities, rather than arrival of recently recharged water. Mechanisms facilitating seasonal recovery include release of water from low-permeability layers and horizontal transfer of water from undeveloped parts of the basin. Overall stability in seasonally recovered water levels and salinities for the past three decades indicate that the system has reached a dynamic equilibrium with respect to water balance and salinity, following a major change in flow paths and solute distributions after initial development. Groundwater delta O-18, delta H-2 and chloride contents indicate mixing between fresh meteoric-derived groundwater and marine water at the coast, with the most saline groundwater approximating an 80:20 mixture of fresh to oceanic water. © 2013, Springer