ANSTO :: Browsing by Author "Peterson, MA"

Browsing by Author "Peterson, MA"

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Application of stable noble gases, 85Kr and 39Ar to investigate the freshwater lens on Rottnest Island, Western Australia
(Goldschmidt, 2017-08-13) Kersting, A; Aeschbach, W; Deslandes, A; Meredith, KT; Peterson, MA; Purtschert, R; Suckow, A
We report on a multi-tracer study of a freshwater lens on Rottnest Island west of Perth (Western Australia). The potable water supply of this carbonate island is entirely based on a shallow freshwater lens ‘floating’ on more saline water. Former studies (Bryan 2016) identified rain as the sole source of the fresh groundwater and that this very vulnerable system is threatened by anthropogenic usage and reduced rainfall due to climate change, causing salt water intrusion.Since the freshwater lens only has a thickness of approx. 20m, even the short screens of the observation wells (1-2m length) cause significant mixing of water of different ages. The combination of tritium (3H) and radiocarbon (14C) used earlier cannot resolve details of the age distribution because of the low values for tritium in precipitation on the southern hemisphere and because of mixing corrosion of the carbonate aquifer together with seawater intrusion changing the initial radiocarbon values. Therefore,this study focussed on the applicationof 85Kr(half-life 10,8yr) and 39Ar (half-life 269yr) as well as the stable noble gases. Here 85Kr complements 3H to resolve the component of very young waterin the age distribution,whereas39Ar fillsthe dating gap between 3H and 14C. The heavy noblegases (Ar, Kr, Xe) can give additional information on the infiltration of freshwater or seawater and 4He can identify admixtures of old water.
Assessment of the aquifers in the West Canning Basin-Pardoo - application of isotopic and hydrogeochemical techniques
(Australian Nuclear Science and Technology Organisation, 2014-01) Meredith, KT; Cendón, DI; Hankin, SI; Peterson, MA; Hollins, SE
The benefits of a multidisciplinary team model for groundwater-surface water investigations, Thirlmere Lakes, NSW.
(National Centre for Groundwater Research And Training, & Australian Chapter International Association Of Hydrogeologists, 2019-11-25) Cowley, KL; Cohen, TJ; Forbes, MS; Barber, E; Allenby, J; Andersen, MS; Anibas, C; Glamore, W; Chen, SY; Johnson, F; Timms, W; David, K; McMillan, T; Cendón, DI; Peterson, MA; Hughes, CE; Krogh, M
The Thirlmere Lakes Research Program (TLRP) is a four-year collaborative multidisciplinary program designed to gain a whole-of-system understanding of the hydro-dynamics of a complex lake environment. The program was established from concerns that proximal aquifer interference activities were factors in recent lake level declines. Five research teams were established to investigate five adjacent lakes set within an entrenched meander bend located south-west of Sydney. The project involved lithological, geochemical and geochronological analysis from lake beds and surrounding slopes to understand lake evolution and determine potential past lake-drying events. Further geological understanding of the lake area was obtained from resistivity imaging (RI), ground penetrating radar (GPR), and analysis of rock cores that were drilled from two deep bores adjacent the lakes. Development of water balance budgets involved fine-scale on-site meteorological measurements including on-site evapotranspiration monitoring, combined with high-resolution bathymetry from RTK GPS, LiDAR surveying and drone photogrammetry. Groundwater-surface water interactions were measured using lake-bed multilevel temperature and pressure arrays, hydraulic head measurements and fine-scale isotope, major ion and environmental tracer time-series analysis. Preliminary findings indicate that the five lakes have been separated for over ~100,000 years and that the lakes themselves contain sediment that is possibly up to 250,000 years old. Assessing the modern dynamics we show that current lake level declines during a period of low rainfall are largely evaporation dominated. One lake however appears to have greater water storage in adjacent sediments providing compensatory inflows. In a second lake, there are indications of localised connectivity with shallow (≤18m) groundwater, but no evidence of connectivity with deeper aquifers. Geological surveys indicate a clay layer 6-8 m below the lakes and spatial variations in both sediment and rock geology. The influence of these geological features, including structures projecting towards the lakes, on groundwater storage and flow is the focus of ongoing research as is temporal variability and lake interactions at different lake levels. The benefits of the multidisciplinary team model include refining the research targeting areas of uncertainty and to enhance and calibrate each team’s results. This approach will provide a comprehensive whole-of-system model of the evolution and hydro-dynamics of a complex lake system. © The Authors
The canary or the coalmine? Isotopic evidence of drying climate versus groundwater outflow as the cause for recent losses from Thirlmere Lakes, NSW
(National Centre for Groundwater Research And Training, & Australian Chapter International Association Of Hydrogeologists, 2019-11-24) Peterson, MA; Cendón, DI; Hughes, CE; Crawford, J; Hankin, SI; Krogh, M; Cowley, KL; Cohen, TJ; Andersen, MS; Anibas, C; Glamore, W; Chen, SY; Timms, W; McMillan, T
The Thirlmere Lakes Research Program (TLRP) is a collaboration investigating water loss mechanisms in recent drying of five adjacent lakes, located 75 km south-west of Sydney. Some stakeholders and previous studies have perceived a correlation with local longwall coal mining history and suspect deep fracture outflow. Others suggest the lakes are simply responding to a drier climate, serving as the canary in the broader climate-change ‘coal mine’. ANSTO has applied recurrent isotopic and chemical monitoring of the lakes and adjacent groundwater over two years to unravel some of the mystery of their recent water losses. Each lake behaved uniquely, but they shared some common trends. Steady enrichment of stable water isotopes, 2H and 18O, indicates the dominance of evaporation, with minimal losses to groundwater or through transpiration. Lake Cl/Br ratios were very low and clustered in three groups, two trending away from initial ratios indicative of groundwater input. 3H and 14C show recent rainfall and/or runoff as the main contributors to lake waters, with apparent ages in the adjacent shallow groundwater up to several decades. High levels of 222Rn from shallow bores suggest a close association between the peats enclosing the lakes and 238 U from ancient erosion, or proximity of an underlying shale lens. The only deep piezometer (72-84 m) near the lakes showed negligible contributions from the lakes or recent surface water. The trends in isotopic and chemical parameters infer that evaporation is sufficient to explain recent water losses from most of these perched lakes. Trends in some lakes hint that these had previous inputs from groundwater. While the historical variability of groundwater input to the lakes remains unknown, there is no current evidence of major losses to groundwater. Thirlmere Lakes will exist only intermittently under dry climate conditions. © The Authors
Characterisation of groundwater dissolved organic matter using LC-OCD: implications for water treatment
(Elsevier, 2021-01-01) Rutlidge, H; McDonough, LK; Oudone, PP; Andersen, MS; Meredith, KT; Chinu, K; Peterson, MA; Baker, AA
The polarity and molecular weight of dissolved organic matter (DOM) is an important factor determining the treatability of water for domestic supply. DOM in surface water and groundwater is comprised of a mixture of carbon with varying molecular weight ranges, with its composition driven by DOM sources and processing. Here, we present the largest dataset of chromatographic DOM in surface and groundwater samples (n = 246) using liquid chromatography organic carbon detection (LCsingle bondOCD). Our data represents four categories (surface water, hyporheic zone water, local groundwater, and regional groundwater) from five different sites across Australia. In all environments, high molecular weight hydrophilic DOM such as biopolymers (BP) and humic substances (HS) are present in surface waters and are processed out of groundwater as it moves from surface water and hyporheic zones into shallow local groundwater and deeper regional groundwaters. This results in a higher percentage of low molecular weight neutrals (LMWN) and hydrophobic organic carbon (HOC) in deeper regional groundwaters. Our findings indicate that the presence of sedimentary organic matter strongly influence the character of surface and groundwater DOM, resulting in groundwater with higher HS aromaticity and molecular weight, and reduced percentage of LMWNs. We also observe highly variable hydrophilic / HOC ratios in groundwater at all sites, with 9.60% and 25.64% of samples at sites containing sedimentary peat layers and non-sedimentary peat sites respectively containing only hydrophilic dissolved organic carbon (DOC). We identify average hydrophilic / HOC ratios of 4.35 ± 3.76 and 7.53 ± 5.32 at sites containing sedimentary peat layers and non-sedimentary peat sites respectively where both hydrophilic DOC and HOC are present. Overall our results suggest that fractured rock and alluvial aquifers in sedimentary organic carbon poor environments may contain DOC which is better suited to ozonation, biologically activated carbon filtration powdered activated carbon, suspended ion exchange treatment or magnetic ion exchange resin since DOC is more hydrophilic and of lower molecular weight and lower aromaticity. Aquifers located near sedimentary organic matter layers may benefit from pre-treatment by coagulation/flocculation, sedimentation and sand filtration which have high removal efficiency for high molecular weight and polar compounds. © 2021 Elsevier Ltd.
Comparing interglacials in eastern Australia: a multi-proxy investigation of a new sedimentary record
(Elsevier, 2021-01-01) Forbes, MS; Cohen, TJ; Jacobs, Z; Marx, SK; Barber, E; Dodson, JR; Zamora, A; Cadd, H; Franke, A; Constantine, M; Mooney, SD; Short, J; Tibby, J; Parker, A; Cendón, DI; Peterson, MA; Tyler, JJ; Swallow, E; Haines, HA; Gadd, PS; Woodward, CA
The widespread formation of organic rich sediments in south-east Australia during the Holocene (Marine Isotope Stage [MIS] 1) reflects the return of wetter and warmer climates following the Last Glacial Maximum (LGM). Yet, little is known about whether a similar event occurred in the region during the previous interglacial (MIS 5e). A 6.8 m sediment core (#LC2) from the now ephemeral Lake Couridjah, Greater Blue Mountains World Heritage Area, Australia, provides insight into this question. Organic rich sediments associated with both MIS 1 and 5e are identified using 14C and optically stimulated luminescence (OSL) dating techniques. Also apparent are less organic sedimentary units representing MIS 6, 5d and 2 and a large depositional hiatus. Sediment δ13C values (−34 to −26‰) suggests that C3 vegetation dominates the organic matter source through the entire sequence. The pollen record highlights the prevalence of sclerophyll trees and shrubs, with local hydrological changes driving variations in the abundance of aquatic and lake-margin species. The upper Holocene sediment (0–1.7 m) is rich in organic matter, including high concentrations of total organic carbon (TOC; 20–40%), fine charcoal and macrophyte remains. These sediments are also characterised by a large proportion of epiphytic diatoms and a substantial biogenic component (chironomids and midges). These attributes, combined with low δ13C and δ15N values, and C:N ratios of approximately 20, indicate a stable peat system in a swamp like setting, under the modern/Holocene climate. In comparison, the lower organic rich unit (MIS 5e-d) has less TOC (5–10%), is relatively higher in δ13C and δ15N, and is devoid of macrophyte remains and biogenic material. Characterisation of the organic matter pool using 13C-NMR spectroscopy identified a strong decomposition signal in the MIS 5e organic sediments relative to MIS 1. Thus the observed shifts in δ13C, δ15N and C:N data between the two periods reflects changes in the organic matter pool, driven by decompositional processes, rather than environmental conditions. Despite this, high proportions of aquatic pollen taxa and planktonic diatoms in the MIS 5e–d deposits, and their absence in the Holocene indicates that last interglacial Lake Couridjah was deeper and, or, had more permanent water, than the current one. ©2020 Elsevier Ltd.
Differentiating between the d13C signature from environmental conditions and SOM cycling in eastern Australian peat sediments
(Australasian Quaternary Association (AQUA), 2021-07-08) Forbes, MS; Cohen, TJ; Marx, SK; Sherborne-Higgins, B; Cadd, H; Francke, A; Cendón, DI; Peterson, MA; Mooney, SD; Constantine, M; Boesl, F; Kobayashi, Y; Mazumder, D
The analysis of stable carbon isotopes is commonly used in Quaternary science to reconstruct the environmental conditions and vegetation contributions to sedimentary sequences. However, the measured d13C signature of the total organic matter (OM) pool can also reflect other complexities within depositional environments. The peats of the Thirlmere Lakes system in the southern section of the Blue Mountains World Heritage Area provides an excellent opportunity to closely scrutinise such d13C dynamics. These deposits are rich in TOC (20-40%) meaning analytical techniques such as 13C-NMR, used to characterise the OM pool, can be applied effectively. Furthermore, the identification of several peat units deposited over the last ~130 ka allows for temporal comparisons. d13C values determined for a 7 m sediment sequence from Lake Couridjah representing both the MIS 1 and MIS 5e interglacial periods vary by up to 4 to 6‰. These trends were subsequently identified in two other sediment sequences (Lake Baraba and Lake Werri Berri) proximal to Lake Couridjah. Initially we interpreted our results as reflecting a C3 dominated vegetation environment with MIS 1 wetter than MIS 5e, following the established relationship between water stress and d13C enrichment. However, spectral analysis of the OM pool indicates that d13C is driven by changing OM dynamics rather than large changes in environmental conditions. In these environments, the greater presence of carbohydrates (i.e. cellulose) in MIS 1 result in more depleted d13C values. In contrast, the MIS 5e peat is dominated by relative inert OM C fractions including charcoal and lipids (such as leaf waxes), which influences environmental proxies such as C/N. Thus, it is likely that the older MIS 5e peat is a more decomposed version of the active MIS 1 peat, and thus differentiating environmental conditions between the two using d13C alone is not particularly illuminating. To overcome this, we describe the d13C values for a coarse charcoal and high temperature hydrogen pyrolysis fractions, modern vegetation, catchment POC and DOC, and n-alkanes composition and generate catchment carbon models for both MIS 1 and MIS5e. Finally comparing the size of the OM pools of both interglacial deposits can provide useful information in estimating the carbon storage capacity of peat deposits in eastern Australia over these time scales. © The Authors.
Differentiating between the d13C signature from environmental conditions and SOM cycling in eastern Australian peat sediments
(Australasian Environmental Isotope Conference, 2022-11-14) Forbes, MS; Cohen, TJ; Marx, SK; Sherborne-Higgins, B; Cadd, H; Francke, A; Cendón, DI; Peterson, MA; Mooney, SD; Constantine, M; Boesl, F; Kobayashi, Y; Mazumder, D
The analysis of stable carbon isotopes is commonly used in Quaternary science to reconstruct the environmental conditions and vegetation contributions to sedimentary sequences. However, the measured d13C signature of the total organic matter (OM) pool can also reflect other complexities within depositional environments. The peats of the Thirlmere Lakes system in the southern section of the Blue Mountains World Heritage Area provides an excellent opportunity to closely scrutinise such d13C dynamics. These deposits are rich in TOC (20-40%) meaning analytical techniques such as 13C-NMR, used to characterise the OM pool, can be applied effectively. Furthermore, the identification of several peat units deposited over the last ~130 ka allows for temporal comparisons. d13C values determined for a 7 m sediment sequence from Lake Couridjah representing both the MIS 1 and MIS 5e interglacial periods vary by up to 4 to 6‰. These trends were subsequently identified in two other sediment sequences (Lake Baraba and Lake Werri Berri) proximal to Lake Couridjah. Initially we interpreted our results as reflecting a C3 dominated vegetation environment with MIS 1 wetter than MIS 5e, following the established relationship between water stress and d13C enrichment. However, spectral analysis of the OM pool indicates that d13C is driven by changing OM dynamics rather than large changes in environmental conditions. In these environments, the greater presence of carbohydrates (i.e. cellulose) in MIS 1 result in more depleted d13C values. In contrast, the MIS 5e peat is dominated by relative inert OM C fractions including charcoal and lipids (such as leaf waxes), which influences environmental proxies such as C/N. Thus, it is likely that the older MIS 5e peat is a more decomposed version of the active MIS 1 peat, and thus differentiating environmental conditions between the two using d13C alone is not particularly illuminating. To overcome this, we describe the d13C values for a coarse charcoal and high temperature hydrogen pyrolysis fractions, modern vegetation, catchment POC and DOC, and n-alkanes composition and generate catchment carbon models for both MIS 1 and MIS5e. Finally comparing the size of the OM pools of both interglacial deposits can provide useful information in estimating the carbon storage capacity of peat deposits in eastern Australia over these time scales.
Discrete interval groundwater samples from uncased boreholes in the Hawkesbury Sandstone, NSW, Australia, reveal mixed results.
(International Association of Hydrogeologists, 2013-09-19) Peterson, MA; Cendón, DI; Hankin, SI; Chisari, R
The effects of subsidence and related fracturing from longwall coal mining have raised community concern around the Southern Coalfields of NSW. Shallow fracturing of bedrock streambeds has led to draining of perennial pools and localised diversion within some streams. Consequently, the risk of diverting surface water and shallow groundwaters towards underground mining operations has also been debated in the community. A Zone-of-Interest Groundwater Sampler (ZoIGS) with straddle packers and multiple pressure sensors was used to collect samples from discrete 5 m intervals within uncased 100 mm boreholes. Up to 6 intervals were successfully sampled in each borehole, to a maximum depth of 137 m below ground level. Hydrogeochemical information analysed comprised major ions, water stable isotopes, 3H and 14CDIC . The results were compared to geophysical data and pressure recovery after purges for each of the intervals. Each borehole displayed a unique vertical arrangement of apparent ages, hydraulic conductivities, mixing and pressure gradients. Hydrogeochemical results showed that some of the more evolved groundwater at depth, with corrected 14C ages up to a few thousand years, was also displaying a 3H signature of mixing with much younger waters. In some cases age profiles were overturned with depth, with younger water occurring below older water. Geophysical and pressure recovery data showed that each borehole intersected a number of aquifers, separated by shale aquitards up to five metres thick. The shale beds are sometimes closely associated with high horizontal hydraulic conductivities and outflow zones. 3H indicative of younger water did not always coincide with the higher hydraulic conductivity zones. The results describe a complex heterogeneous system within the Hawkesbury Sandstone, with multiple flow paths contributing to mixing of old and young groundwater at depth. The ability to sample and investigate numerous discrete intervals within uncased boreholes gave an opportunity for better understanding the system, with minimal infrastructure.
Discrete interval sampling investigations into the relationship between upland swamps and siltstone aquitards within the Hawkesbury Sandstone, Sydney Basin
(Australasian Environmental Isotope Conference, 2009-12-03) Peterson, MA; Mitry, WH; Waring, CL
The Southern Coalfields of the Sydney Basin underlie large tracts of Sydney’s water catchment area. This area comprises a dissected plateau of near-horizontal strata, typically capped by the quartzose Hawkesbury Sandstone, with dams constructed across some of the deeper valleys. Many of the perennial tributaries arise within small upland swamps of several hectares size, which exist near the watershed divides. The swamps seem to be reliant on lateral groundwater flowpaths and iron-rich basal rock bars, both of which appear to be controlled by coherent siltstone aquitards within the Hawkesbury Sandstone. Here we present some results from a baseline study of one upland swamp, comparing the swamp with samples, geology and measurements from an open borehole on an adjacent ridge. Swamps and hillslopes were sampled from surface water and piezometers; including field measurements, stable and radioisotopes and general chemistry. Discrete intervals within the nearby borehole were selected based on drill core and other logging results that inferred naturally fractured layers of higher hydraulic conductivity, separated by the siltstones and other coherent sandstones. Five selected zones were isolated, sampled and measured using ANSTO’s zone-of- interest groundwater sampler (ZOIGS). Isolation of 5 m intervals is achieved using inflatable straddle packers, with vibrating wire piezometers measuring pressures above, within and below the isolated zone. Effective isolation by the packers and the siltstone aquitards was confirmed by head differences and independent recovery curves of adjacent zones during purging and sampling. Recovery curves were also used to infer effective hydraulic conductivities for each isolated zone. Tritium, deuterium, oxygen-18 and other results show that the lower portion of the swamp, just above the outcropping siltstone layer, had a higher contribution of groundwater. Exposure of the groundwater to the atmosphere at this point causes dissolved iron to oxidise and precipitate, which hardens the basal rock bar of the swamp against nick-point erosion. The basal rock bars therefore maintain a low hydraulic gradient and perched water table within the swamps. The two significant siltstone layers in the borehole were replicated at similar elevations within the swamp and in two previous boreholes in the area, implying that they may be quite extensive. Examination of the elevations of basal and mid-swamp rockbars of over 20 swamps in the local area indicates that many coincide with the level of these two siltstone layers. The implication is that the aquitard properties of the siltstone may be responsible for both the formation and survival of the swamps. Longwall coal mining often leads to subsidence and fracturing of the strata, sometimes resulting in changes to the surface-groundwater and aquifer connectivities. Fracturing of the aquitards and/or rock bars could dewater upland swamps if mining is not managed carefully. BHP Billiton Illawarra Coal Pty Ltd supported this and other baseline studies to better understand the upland swamps before undermining proceeds.
Estimating diffusion in heterogeneous groundwater systems using short-llived radio-isotopes and stable isotopes or bromine
(Geological Society of Australia, 2014-07-07) Peterson, MA; Cendón, DI; Andersen, MS; Mokhber-Shahin, L; Wong, HKY; Rowling, B
Diffusion is an important and ubiquitous phenomenon in nature, but too often neglected or unmeasured in water resource hydrogeology or solute transport. Diffusion may, in fact, be the dominant process that dictates hydrogeochemistry and affects tracers. Conservative and age tracers are commonly used for water resource or contaminant plume transport estimations, but will give misleading results, if diffusion is ignored. Diffusion of tracers into aquitards, matrix pores of fractured rocks, blind fractures or other low conductivity zones lead to retardation and possible adsorption, exchange, precipitation or decay. This becomes increasingly important as heterogeneity of flow domains increase, for example, in fractured rock aquifers, interlayered sediments or aquifers associated with aquitards. Traditional methods of measuring diffusion coefficients in small slices of heterogeneous rock are unreliable for upscaling, so this study presents an alternative method based on lab-scale drill-core tests and suggests field-scale borehole tracer tests. Fick’s first law shows that diffusion rates are driven by concentration gradients. Short-lived radiotracers soon reach a steady-state concentration gradient with enhanced flux where diffusion rate equals decay, while stable tracers trend towards saturation and ever-decreasing fluxes. We compare diffusion of short-lived radiotracers 131I half-life 8 days) or 82Br (half-life 1.5 days) to their stable equivalent (I or Br) into 45–50 cm lengths of ~60 mm diameter drill core. Five cores were selected from three fractured rock environments: sandstone, limestone and metavolcanics. By regularly sampling and refilling the annulus with tracers around the enclosed core, we are able to discern differential in-diffusion between stable- and radio-tracers. For example, the annulus was sampled and refilled weekly with an (equivalent decayed) 131I activity of 22 Bq/g and within three weeks (2.6 half-lives) each core had reached a characteristic steady state flux. The net fluxes were 7 around 1.0 Bq/cm2/week in the sandstone cores, 0.2 Bq/cm2/week in the metavolcanics, and 0.05 Bq/cm2/week in the limestone. This was compared to stable iodine weekly refills at 2.6 mg/L, which gave ever-diminishing diffusion results. The net fluxes of stable iodine diminished steadily, e.g. over three weeks from 98 to 26 ng/cm2/week for the most porous (medium sandstone ~15%), and from 11 to 7 ng/cm2/week for the least porous (limestone ~2%). Experiments were also performed using 82Br and stable bromine, with sampling and refills performed on a daily (0.68 half-lives) basis. Similar trends were apparent, though the data was noisier due to more frequent refills and less time for diffusion to generate significant changes in the annulus reservoir solutions. This method enables analysis of drill cores for comparative effective diffusion coefficients of different systems. Quantitative interpretation is currently being refined. In principle, the method should be transferrable to single boreholes or tracer tests between multiple boreholes to gain larger scale representation of effective diffusion within a groundwater system. The normally confounding factors, such as dilution, advection, exchange, adsorption and precipitation, are negated by comparing the stable with radio-tracer results, as all isotopes of these elements are identically affected by such processes and losses. © Geological Society of Australia Inc
Evolution of dissolved inorganic carbon in groundwater recharged by cyclones and groundwater age estimations using the 14C statistical approach
(Elsevier B. V., 2018-01-01) Meredith, KT; Han, LF; Cendón, DI; Crawford, J; Hankin, SI; Peterson, MA; Hollins, SE
The Canning Basin is the largest sedimentary basin in Western Australia and is located in one of the most cyclone prone regions of Australia. Despite its importance as a future resource, limited groundwater data is available for the Basin. The main aims of this paper are to provide a detailed understanding of the source of groundwater recharge, the chemical evolution of dissolved inorganic carbon (DIC) and provide groundwater age estimations using radiocarbon (14CDIC). To do this we combine hydrochemical and isotopic techniques to investigate the type of precipitation that recharge the aquifer and identify the carbon processes influencing 14CDIC, δ13CDIC, and [DIC]. This enables us to select an appropriate model for calculating radiocarbon ages in groundwater. The aquifer was found to be recharged by precipitation originating from tropical cyclones imparting lower average δ2H and δ18O values in groundwater (−56.9‰ and −7.87‰, respectively). Water recharges the soil zone rapidly after these events and the groundwater undergoes silicate mineral weathering and clay mineral transformation processes. It was also found that partial carbonate dissolution processes occur within the saturated zone under closed system conditions. Additionally, the processes could be lumped into a pseudo-first-order process and the age could be estimated using the 14C statistical approach. In the single-sample-based 14C models, 14C0 is the initial 14CDIC value used in the decay equation that considers only 14C decay rate. A major advantage of using the statistical approach is that both 14C decay and geochemical processes that cause the decrease in 14CDIC are accounted for in the calculation. The 14CDIC values of groundwater were found to increase from 89 pmc in the south east to around 16 pmc along the groundwater flow path towards the coast indicating ages ranging from modern to 5.3 ka. A test of the sensitivity of this method showed that a ∼15% error could be found for the oldest water. This error was low when compared to single-sample-based models. This study not only provides the first groundwater age estimations for the Canning Basin but is the first groundwater dating study to test the sensitivity of the statistical approach and provide meaningful error calculations for groundwater dating. Crown Copyright © 2017 Published by Elsevier Ltd.
Fertilizers rule REYs: agricultural catchments of Eastern Australia
(Goldschmidt, 2019-08-18) Cendón, DI; Harris, SJ; Kelly, BFJ; Peterson, MA; Hankin, SI; Rowling, B; Watson, J; Xiao, S
Lanthanides, generally named Rare Earth Elements (REE), are part of the internal transition metals forming a group of 15 elements with very similar chemical characteristics and physical properties. REEs and Yttrium (named REY) are widely used to understand geochemical processes. The increasing use of REYs in technology as well as their presence as a by-product in some fertilizers has opened new pathways for these metals to enter the water cycle, thus making REYs tracers of anthropogenic activity. In this study we investigate the concentration and distribution of REYs in two predominantly agricultural catchments of Eastern Australia: the Namoi River with a 43,000 km2 catchment, which forms part of the headwaters of the Murray- Darling Basin; and the Nogoa River with a 27,600 km2 catchment, which forms part of the Fitzroy River catchment, the largest in eastern Australia flowing into the Great Barrier Reef. Bi-monthly sampling during 18 months was conducted at seven selected sites along both rivers. The [REY] in water samples were analyzed by automated chelation pre-concentration (SeaFast, ESI), followed by ICP-MS. Samples were automatically loaded onto a loop and injected to an iminodiacetate column that chelates REY allowing matrix Na+, Cl-, Ca2+, Mg2+ and, more importantly, Ba2+ ions to be rinsed out. The pre-concentration process allowed a ~20-fold increase in concentration. Results are compared to those obtained from commonly used fertilizers in the region. REY trends suggest a link to the fertilizers used in both catchments. No regional variations were apparent, possibly due to the prevailing dry conditions during the sampling period. Stream flow was controlled by dam releases in the upper ridges for both catchments.
Groundwater residence time in a dissected and weathered sandstone plateau: Kulnura–Mangrove Mountain aquifer, NSW, Australia
(Taylor Francis Online, 2014-04-14) Cendón, DI; Hankin, SI; Williams, JP; Van der Ley, M; Peterson, MA; Hughes, CE; Meredith, KT; Graham, IT; Hollins, SE; Levchenko, VA; Chisari, R
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. © 2020 Informa UK Limited
Groundwater tracing with nucleogenic 36Cl in West Canning Basin, Western Australia
(European Geosciences Union, 2017-04-26) Wilcken, KM; Cendón, DI; Meredith, KT; Simon, KJ; Stopic, A; Peterson, MA; Hankin, SI
Chlorine-36 has been used over the past 20-30 years as a groundwater tracer in many hydrological studies and is a well-established dating technique. Given the half-life of 301 kyr it is well suited for dating of ‘old’ groundwater between 50 kyr - 1 Myr. A challenge associated with utilising 36Cl as a tracer is that it can be produced via three different pathways that will influence the result based on the unique hydrogeological setting of a study area. Typically the dominant source of 36Cl in groundwater is atmospheric 36Cl that is produced at troposphere and stratosphere via interaction of cosmic-ray protons and secondary neutrons with Ar. However, the secondary cosmic-ray neutrons can similarly produce 36Cl in surface rocks particularly at high elevations. Also nucleogenic production of 36Cl at subsurface environments can become significant, especially if U and/or Th concentrations are high. Delineating and quantifying these processes is essential when using 36Cl as a groundwater dating tool. In contrast to a conservative situation where atmospheric 36Cl dominates, we present a study in the West Canning Basin located in the Pilbara region of Western Australia, where the 36Cl/Cl ratio increases from 30 10-15 near the recharge zone to 100 10-15 over a 60 km of flow path within a confined aquifer. Additional isotopic evidence (14C and 87Sr/86Sr) in groundwater, mineralogy (X-Ray diffraction) and elemental analysis (Neutron Activation) of whole-rock powder samples from the aquifer and overlying geological units, is used to establish an interpretation that nucleogenic 36Cl production is effectively the only potential process to explain the data. Nucleogenic production can influence the groundwater 36Cl content in two different ways: (1) as an additional input of Cl with a 36Cl/Cl ratio that reflects the neutron flux within the particular mineralogy; or (2) via “in-situ” production of 36Cl directly in the groundwater from the dissolved 35Cl where the rate is dictated by the neutron flux in the aquifer and the residence time. For this study, the whole-rock elemental composition is used to calculate the nucleogenic production within the aquifer and confining units of the groundwater system. This effectively ruled out both the Wallal sandstone aquifer and the overlying confining unit of the Jarlemai clay aquitard as the sources of 36Cl. Here we explore the idea that the underlying granite is the most likely source of 36Cl. © Author(s) 2017. CC Attribution 3.0 License.
Isotopic evidence for nitrate sources and controls on denitrification in groundwater beneath an irrigated agricultural district
(Elsevier, 2022-04-15) Harris, SJ; Cendón, DI; Hankin, SI; Peterson, MA; Xiao, S; Kelly, BFJ
The application of N fertilisers to enhance crop yield is common throughout the world. Many crops have historically been, or are still, fertilised with N in excess of the crop requirements. A portion of the excess N is transported into underlying aquifers in the form of NO3−, which is potentially discharged to surface waters. Denitrification can reduce the severity of NO3− export from groundwater. We sought to understand the occurrence and hydrogeochemical controls on denitrification in NO3−-rich aquifers beneath the Emerald Irrigation Area (EIA), Queensland, Australia, a region of extensive cotton and cereal production. Multiple stable isotope (in H2O, NO3−, DIC, DOC and SO42−) and radioactive isotope (3H and 36Cl) tracers were used to develop a conceptual N process model. Fertiliser-derived N is likely incorporated and retained in the soil organic N pool prior to its mineralisation, nitrification, and migration into aquifers. This process, alongside the near absence of other anthropogenic N sources, results in a homogenised groundwater NO3− isotopic signature that allows for denitrification trends to be distinguished. Regional-scale denitrification manifests as groundwater becomes increasingly anaerobic during flow from an upgradient basalt aquifer to a downgradient alluvial aquifer. Dilution and denitrification occurs in localised electron donor-rich suboxic hyporheic zones beneath leaking irrigation channels. Using approximated isotope enrichment factors, estimates of regional-scale NO3− removal ranges from 22 to 93% (average: 63%), and from 57 to 91% (average: 79%) beneath leaking irrigation channels. In the predominantly oxic upgradient basalt aquifer, raised groundwater tables create pathways for NO3− to be transported to adjacent surface waters. In the alluvial aquifer, the transfer of NO3− is limited both physically (through groundwater-surface water disconnection) and chemically (through denitrification). These observations underscore the need to understand regional- and local-scale hydrogeological processes when assessing the impacts of groundwater NO3− on adjacent and end of system ecosystems. © The Authors 2022, Published by Elsevier B.V. CC BY-NC-ND license
Mine subsidence induced hydraulic connection tested by geochemical and geophysical tracing techniques.
(Mine Subsidence Technical Society, 2007-11-27) Waring, CL; Peterson, MA
Changes in surface and groundwater hydrology induced by mine subsidence near surface fracturing can also subtly alter the geochemistry. More rapid drainage of surface and groundwater along fracture pathways may lower the standing water level allowing air to penetrate further, accelerating rock weathering and leaching reactions. However, these sulphide oxidation and carbonate dissolution reaction products are normal constituents of groundwater and their presence does not necessarily indicate an impact of mine subsidence. Changes to the groundwater recharge – discharge regime due to fracturing will also affect the age of the groundwater. Whilst the groundwater age is not a significant water quality parameter it may unambiguously distinguish changes to the hydrology, where conventional chemistry may be difficult to interpret. Mine subsidence fracturing may also cause hydraulic connection between previously isolated aquifers or ultimately between the surface and the mine. Isotopic geochemical dating (35S, 3H, 14C) and tracing (δ13C, δ2H, δ18O, δ34S) techniques are used to distinguish the origin of the groundwater and potential isolation or fracture connections. Isolated groundwater may show a dissolved gas signature similar to petroleum gas within the Bulgo Sandstone. To identify the geochemical impact of mine subsidence fracturing, pre and post mining profiles are being assembled for comparison. Groundwater age profiles also help to constrain and verify groundwater flow models. However, to produce useful 14C dates from carbon dissolved in groundwater differences between carbon species (DIC dissolved inorganic carbon, DOC dissolved organic carbon and dissolved methane / ethane) caused by interaction with siderite in the aquifer are recognised and addressed. A nuclear geophysical logging technique, Prompt Gamma Neutron Activation Analysis (PGNAA) is also used to trace the flow of an injected salt solution into fractures and into the porous and permeable sandstone surrounding the borehole. The variable distance the salt tracer moves into the porous rock under a known pressure increase (above standing water level), over a known time and tracer volume allows calculation of hydraulic conductivity at 20cm increments along the length of the borehole. If there is significant flow of the tracer into fractures and beyond the PGNAA measurement range a relative tracer movement distance is provided by the PGNAA log, rather than hydraulic conductivity. Other relevant lithological and hydraulic parameters such as porosity may be derived from measured Si, H, Cl, ±Fe, ±Al elemental abundance provided by PGNAA borehole logging. Borehole sampling of aquifer water at narrow discrete intervals for geochemical profiling requires isolating the aquifer segment or individual fracture flow from the rest of the borehole. A dual packer apparatus is used to take narrow (2, 5, 10m) discrete interval samples or measure individual fracture flows.
A new diffusion method for heterogeneous drill cores using radioactive bromide or iodide
(National Centre for Groundwater Research And Training, 2017-07-12) Peterson, MA; Cendón, DI; Andersen, MS
Diffusion is sometimes a dominant process affecting radiotracers commonly used to quantify water resources, such as 3H, 14C and 36Cl. However, measuring diffusion in heterogeneous environments including fractured rock currently requires many samples and assumptions. Typical diffusion coefficients derived from small rock samples are insufficient to determine diffusive losses in such settings. Larger representative lengths of drill cores are often available, but current diffusion methods cannot utilise large samples. Isolating the diffusive fluxes from other tracer losses, such as adsorption or precipitation, within a representative sample over a reasonable period is necessary to gauge and correct for diffusive tracer losses. A paired radioactive-stable tracer radial diffusion method using lengths of drill core in a close-fitting acrylic tube diffusion cell has been devised and tested in the laboratory. With frequent injections, the radioactive decay of the short-lived radiotracer (82Br or 131I) maintains a steeper activity gradient and therefore diffusive flux into the core than the accumulating stable isotope of the same ion (Br or I). Therefore, the normalised radiotracer lost from the diffusion cell reservoir (1-At/A0) remains greater than the diminishing stable tracer losses (1-Ct/C0). The range of possible diffusive loss becomes [(0.5 + Ct/C0 – At/A0) ± (0.5 - Ct/2C0)], which converges logarithmically with repeated injections. Within two weeks, five drill cores yielded daily diffusive reservoir losses for bromide tracers: coarse sandstone 10±2%, medium sandstone 11±1%, massive limestone 2±1%, basalt 2±1% and basaltic breccia 3±1%. The iodide results were very similar once corrected for relative diffusion coefficients and duration, though higher in one basalt core. Larger, representative heterogeneous cores can now be assessed. The derived rate of tracer loss can be applied to correct for diffusive losses of commonly used environmental radiotracers. This provides a previously unavailable correction for determining flow paths, rates and ultimately water resources in heterogeneous environments.
A nine-year record of groundwater environmental tracer variations in a weathered sandstone plateau aquifer
(American Geophysical Union, 2016-12-15) Cendón, DI; Hankin, SI; Hughes, CE; Meredith, KT; Peterson, MA; Scheiber, L; Shimizu, Y
Most groundwater isotopic studies are limited to one snapshot in time due to high costs associated with sampling and analytical procedures. The timing of sampling within long-term seasonal climatic cycles may affect interpretations, particularly in unconfined or semi-confined aquifer systems. To test the potential influence of decadal climatic trends, particularly on groundwater residence time, we have combined results from a multi-year sampling programme. Hydrogeochemistry and isotopic tracer analysis including H2O stable isotopes, δ13CDIC, 3H, 14CDIC for all samples and 87Sr/86Sr and NO3-δ15N, have been applied to groundwater recovered from the Kulnura – Mangrove Mountain aquifer hosted by a weathered sandstone plateau within the Sydney Basin (Australia). In general, the study area is characterised by alternating dry and wet periods that can be prolonged as they are linked to wider climatic events such as El Niño, La Niña and modulated by the Indian Ocean Dipole. The region experienced above average rainfall from 1985-1990 followed by generally drier conditions (1991-2007) and slightly wetter conditions to 2015. Groundwater results from the first years (2006-2010), under generally dry conditions resulted in lower groundwater levels, revealed important inter-annual variations. These are interpreted to be locally driven by groundwater extraction, resulting in a progressive influx of modern groundwater. The progressive input of modern water has exposed deeper parts of the aquifer to increased NO3- concentrations of anthropogenic origin. The change in chemistry of the groundwater, particularly the lowering of groundwater pH, has accelerated the dissolution of carbonate mineral phases that in turn affects 14C residence time assessments. Subsequent sampling results (2012-2015), under higher rainfall conditions, suggest modern recharge in areas previously without measurable tritium activities. The complex interplay between recharge, anthropogenic influences and climate may be further complicated by the local irregularities in the sandstone weathering profile and the transition to preferential groundwater fracture-flow with depth. © AGU 2016
Nitrogen cycling dynamics in a humid subtropical climate: insights from the Nogoa River sub-catchment, central Queensland, Australia
(Copernicus Publications, 2019-04-07) Harris, SJ; Cendón, DI; Peterson, MA; Hankin, SI; Watson, J; Xiao, S; Kelly, BFJ
The Nogoa River sub-catchment, Queensland, Australia, supports a multimillion-dollar agricultural sector. For the last decade, efforts have been made to monitor river nitrate loads (Fitzroy partnership for River Health, 2017), which may affect sensitive ecosystems downstream, such as the World Heritage-listed Great Barrier Reef (Brodie et al. 2012). Research into nitrous oxide, which arises from both the oxidation of ammonium fertilisers and/or reduction of subsequent nitrate, is also very important due to its increasing impact on the atmosphere. An integrated approach that considers the interactions between atmosphere, river water and groundwater nitrogen compounds is thus integral to closing the nitrogen cycle in the region. Nitrogen fertiliser contributions to greenhouse gas emissions, riverine environments and aquifers remain uncertain for several reasons: (1) ad-hoc river water sampling frequency and infrequent shallow groundwater sampling; (2) a lack of isotopic evidence for attributing sources and highlighting attenuation processes; (3) poor understanding of groundwater recharge pathways, residence times, and contributions to the Nogoa River; and (4) a lack of quantification of river water and groundwater nitrous oxide concentrations and emissions. In this poster, we present hydro-geochemical data (major ions, N2O-N, δ2H-H2O and δ18O-H2O, δ15N-NO-3 and δ18O-NO- 3, and natural radioactive tracers) from seven sites along the Nogoa River that were repeatedly sampled over a 1-year period, and from 24 shallow groundwater bores sampled in October 2018. A comparison with historical major ion groundwater data reveals that nitrate concentrations have increased due to continued fertiliser input over the past ∽ 20 years, reaching up to 25 mg L-1 NO- 3 -N. Dual nitrate isotopes (δ15N and δ18O) reveal that denitrification occurs in both the shallow groundwater and Nogoa River samples, and suggest a predominant fertiliser source of nitrate. The data will be placed in the wider context of recharge pathways, residence times and contributions to the Nogoa River, and will be used to understand the interplay between the river and alluvial aquifer nitrate and nitrous oxide emissions. © Author(s) 2019. CC Attribution 4.0 license.

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