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Browsing by Author "Cartwright, I"

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Constraining groundwater flow, residence ties, interaquifer mixing and aquifer properties using environmental isotopes in the southeast Murray Basin, Australia
(Elsevier B.V., 2012-09-01) Cartwright, I; Weaver, TR; Cendón, DI; Fifield, LK; Tweed, SO; Petrides, B; Swane, IC
Environmental isotopes (particularly δ18O, δ2H, and δ13C values, 87Sr/86Sr ratios, and a14C) constrain geochemical processes, recharge distribution and rates, and inter-aquifer mixing in the Riverine Province of the southern Murray Basin. Due to methanogenesis and the variable δ13C values of matrix calcite, δ13C values are highly variable and it is difficult to correct 14C ages using δ13C values alone. In catchments where δ13C values, 87Sr/86Sr ratios, and major ion geochemistry yield similar a14C corrections, ∼15% of the C is derived from the aquifer matrix in the silicate-dominated aquifers, and this value may be used to correct ages in other catchments. Most groundwater has a14C above background (∼2 pMC) implying that residence times are <30 ka. Catchments containing saline groundwater generally record older 14C ages compared to catchments that contain lower salinity groundwater, which is consistent with evapotranspiration being the major hydrogeochemical process. However, some low salinity groundwater in the west of the Riverine Province has residence times of >30 ka probably resulting from episodic recharge during infrequent high rainfall episodes. Mixing between shallower and deeper groundwater results in 14C ages being poorly correlated with distance from the basin margins in many catchments; however, groundwater flow in palaeovalleys where the deeper Calivil–Renmark Formation is coarser grained and has high hydraulic conductivities is considerably more simple with little inter-aquifer mixing. Despite the range of ages, δ18O and δ2H values of groundwater in the Riverine Province do not preserve a record of changing climate; this is probably due to the absence of extreme climatic variations, such as glaciations, and the fact that the area is not significantly impacted by monsoonal systems. © 2020 Elsevier B.V
Differences in groundwater and chloride residence times in saline groundwater: the Barwon River Catchment of Southeast Australia
(Elsevier B. V., 2017-02-20) Howcroft, W; Cartwright, I; Fifield, LK; Cendón, DI
The residence times of groundwater and chloride and the processes contributing to the development of saline (total dissolved solids (TDS) up to 45,379 mg/L) groundwater within the Barwon River Catchment of southeast Australia were investigated using major ion, stable isotope (δ18O, δ2H, and δ13C) and radioactive isotope (3H, 14C, 36Cl) geochemistry. The elevated groundwater salinity in the region is primarily due to evapotranspiration and recycling of solutes in saline lakes with minor contributions from weathering of halite, silicate and calcite minerals. Groundwater residence times estimated from 14C vary from modern to ~ 20 ka; for groundwater with lower 14C activities, the estimated residence times vary significantly depending on the assumed flow model and the 14C activity of recharge. Chloride residence times downgradient of Lake Murdeduke (a saline through-flow lake in the centre of the catchment) are greater than the corresponding groundwater residence times due to the recycling of Cl within the lake. Precise estimates of chloride residence time could not be determined using 36Cl due to R36Cl in precipitation being lower than that of groundwater. This is most likely due to R36Cl values in rainfall having been higher in the past than they are at present due to climate variability. δ18O, δ2H, and δ13C values also suggest that the region has experienced increasingly more evaporative conditions with time. The results of this study demonstrate that, while Cl is a useful tracer of hydrological processes, it must be applied carefully in arid and semi-arid regions of the world. In particular, recharge rates calculated using chloride mass balance may be underestimated where recycling of Cl has occurred. Crown Copyright © 2017 Published by Elsevier B.V.
Environmental isotopes as indicators of aquitard effectiveness, Murray Basin, Australia
(CRC Press, 2010) Cartwright, I; Weaver, TR; Cendón, DI; Swane, IC
The distribution of δ¹³C values,⁸⁷Sr/⁸⁶Sr ratios, and¹⁴C activities (a¹⁴C) show that considerable inter-aquifer flow occurs in the Wimmera Region of the Murray Basin, southern Australia. Many of the potential aquitards, (e.g., Bookpurnong Beds and Ettrick Marl) are ineffective barriers to groundwater flow; only the Geera Clay forms an effective aquitard and restricts inter-aquifer leakage between the Loxton-Parilla Sands and/or the Murray Group and the underlying Renmark Formation. This study illustrates the utility of environmental isotopes in determining inter-aquifer flow and aquitard effectiveness in regions where direct studies of aquitards is lacking.
Environmental isotopes as indicators of inter-aquifer mixing, Wimmera region, Murray Basin, Southeast Australia.
(Elsevier, 2010-10-20) Cartwright, I; Weaver, TR; Cendón, DI; Swane, I
Complex groundwater flow systems in confined aquifers that result from geological structures, stratigraphic changes, or the absence of efficient aquitards are difficult to constrain using physical parameters alone. Despite a relatively simple aquifer configuration, the distribution of groundwater total dissolved solids (TDS) concentrations, δ13C values, 87Sr/86Sr ratios, and 14C activities (a14C) in groundwater in the Wimmera region of the southern Murray Basin implies that considerable inter-aquifer flow has occurred. Given the presence of both silicate and carbonate aquifers, δ13C values and 87Sr/86Sr ratios are the key parameters that demonstrate inter-aquifer flow. Locally, between 40 and 95% of water from one aquifer has infiltrated the underlying aquifer homogenising many aspects of the groundwater geochemistry. Groundwater residence times estimated from a14C range from modern to > 30 ka and the distribution of 14C residence times confirm that inter-aquifer flow is regional scale and long term. Recharge of the deepest aquifers occurs across a broad region and not solely at the basin margins. Vertical leakage rates are ~ 6–10 × 10−3 m/year and long-term recharge rates 0.1–0.2 mm/year (< 1% of annual rainfall). Groundwater from this region is a locally valuable resource and failure to recognise that inter-aquifer flow occurs threatens the sustainability of this resource. © 2010, Elsevier Ltd.
Geochemical indicators of the origins and evolution of methane in groundwater: Gippsland Basin, Australia
(Springer, 2016-08-06) Currell, MJ; Banfield, D; Cartwright, I; Cendón, DI
Recent expansion of shale and coal seam gas production worldwide has increased the need for geochemical studies in aquifers near gas deposits, to determine processes impacting groundwater quality and better understand the origins and behavior of dissolved hydrocarbons. We determined dissolved methane concentrations (n = 36) and δ 13C and δ 2H values (n = 31) in methane and groundwater from the 46,000-km 2 Gippsland Basin in southeast Australia. The basin contains important water supply aquifers and is a potential target for future unconventional gas development. Dissolved methane concentrations ranged from 0.0035 to 30 mg/L (median = 8.3 mg/L) and were significantly higher in the deep Lower Tertiary Aquifer (median = 19 mg/L) than the shallower Upper Tertiary Aquifer (median = 3.45 mg/L). Groundwater δ 13C DIC values ranged from -26.4 to -0.4 ‰ and were generally higher in groundwater with high methane concentrations (mean δ 13C DIC = -9.5 ‰ for samples with >3 mg/L CH 4 vs. -16.2 ‰ in all others), which is consistent with bacterial methanogenesis. Methane had δ 13C CH4 values of -97.5 to -31.8 ‰ and δ 2H CH4 values of -391 to -204 ‰ that were also consistent with bacterial methane, excluding one site with δ 13C CH4 values of -31.8 to -37.9 ‰, where methane may have been thermogenic. Methane from different regions and aquifers had distinctive stable isotope values, indicating differences in the substrate and/or methanogenesis mechanism. Methane in the Upper Tertiary Aquifer in Central Gippsland had lower δ 13C CH4 (-83.7 to -97.5 ‰) and δ 2H CH4 (-236 to -391 ‰) values than in the deeper Lower Tertiary Aquifer (δ 13C CH4 = -45.8 to -66.2 ‰ and δ 2H CH4 = -204 to -311 ‰). The particularly low δ 13C CH4 values in the former group may indicate methanogenesis at least partly through carbonate reduction. In deeper groundwater, isotopic values were more consistent with acetate fermentation. Not all methane at a given depth and location is interpreted as being necessarily produced in situ. We propose that high dissolved sulphate concentrations in combination with high methane concentrations can indicate gas resulting from contamination and/or rapid migration as opposed to in situ bacterial production or long-term migration. Isotopes of methane and dissolved inorganic carbon (DIC) serve as further lines of evidence to distinguish methane sources. The study demonstrates the value of isotopic characterisation of groundwater including dissolved gases in basins containing hydrocarbons. Copyright © 2016, Springer Nature
Groundwater mean residence times of a subtropical barrier sand island
(European Geosciences Union, 2020-03-19) Hofmann, H; Newborn, D; Cartwright, I; Cendón, DI; Raiber, M
Fresh groundwater on barrier islands is affected by changing sea levels and precipitation variability due to climate change and is also vulnerable to anthropogenic processes, such as contamination and groundwater over-abstraction. Constraining groundwater mean residence times (MRTs) and flow paths is essential for understanding and managing these resources. This study uses tritium (3H) and carbon-14 (14C) to determine the MRTs of groundwater along a transect across subtropical North Stradbroke Island, south-east Queensland, Australia. Hydraulic properties, major ion geochemistry and stable isotopes are used to validate residence times and to identify the processes responsible for their variability. 3H activities range from less than 0.01 to 1 TU (tritium units), which are values lower than those of local average rainfall (1.6–2.0 TU). 14C concentrations range from 62.5 to 111 pMC (percent modern carbon). Estimated MRTs determined using lumped parameter models and 3H activities range from 37 to more than 50 years. Recharge occurs over the entire island, and groundwater MRTs generally increase vertically and laterally towards the coastal discharge areas, although no systematic pattern is observed. MRTs estimated from 14C concentrations display similar spatial relationships but have a much greater range (from modern to approximately 5000 years). Water diversion and retention by lower-permeability units in the unsaturated parts of the dune systems are the most likely course for relatively long MRTs to date. The results indicate that the internal structures within the dune systems increase MRTs in the groundwater system and potentially divert flow paths. The structures produce perched aquifer systems that are wide-spread and have a significant influence on regional recharge. The geochemical composition of groundwater remains relatively consistent throughout the island, with the only irregularities attributed to old groundwater stored within coastal peat. The outcomes of this study enhance the understanding of groundwater flow, recharge diversion and inhibition for large coastal sand masses in general, especially for older sand masses that have developed structures from pedogenesis and dune movement. With respect to south-east Queensland, it allows the existing regional groundwater flow model to be refined by incorporating independent MRTs to test models' validity. The location of this large fresh groundwater reservoir, in dry and populous south-east Queensland, means that its potential to be used as a water source is always high. Background information on aquifer distribution and groundwater MRTs is crucial to better validate impact assessment for water abstraction. © Author(s) 2020
Increasing Australian-Indonesian monsoon rainfall linked to early Holocene sea-level rise.
(Nature Publishing Group, 2009-09) Griffiths, ML; Drysdale, RN; Gagan, MK; Zhao, JX; Ayliffe, LK; Hellstrom, JC; Hantoro, WS; Frisia, S; Feng, YX; Cartwright, I; Pierre, ES; Fischer, MJ; Suwargadi, BW
The Australian-Indonesian summer monsoon affects rainfall variability and hence terrestrial productivity in the densely populated tropical Indo-Pacific region. It has been proposed that the main control of summer monsoon precipitation on millennial timescales is local insolation(1-3), but unravelling the mechanisms that have influenced monsoon variability and teleconnections has proven difficult, owing to the lack of high-resolution records of past monsoon behaviour. Here we present a precisely dated reconstruction of monsoon rainfall over the past 12,000 years, based on oxygen isotope measurements from two stalagmites collected in southeast Indonesia. We show that the summer monsoon precipitation increased during the Younger Dryas cooling event, when Atlantic meridional overturning circulation was relatively weak(4). Monsoon precipitation intensified even more rapidly from 11,000 to 7,000 years ago, when the Indonesian continental shelf was flooded by global sea-level rise(5-7). We suggest that the intensification during the Younger Dryas cooling was caused by enhanced winter monsoon outflow from Asia and a related southward migration of the intertropical convergence zone(8). However, the early Holocene intensification of monsoon precipitation was driven by sea-level rise, which increased the supply of moisture to the Indonesian archipelago. © 2009, Nature Publishing Group.
A multi-tracer approach to quantifying groundwater inflows to an upland river; assessing the influence of variable groundwater chemistry
(John Wiley & Sons, Inc., 2013-11-27) Atkinson, AP; Cartwright, I; Gilfedder, BS; Hofmann, H; Unland, NP; Cendón, DI; Chisari, R
Understanding the behaviour and variability of environmental tracers is important for their use in estimating groundwater discharge to rivers. This study utilizes a multi-tracer approach to quantify groundwater discharge into a 27 km upland reach of the Gellibrand River in southwest Victoria, Australia. Ten sampling campaigns were conducted between March 2011 and June 2012, and the distribution of 222Rn activities, Cl and 3H concentrations imply the river receives substantial groundwater inflows. Mass balances based on 222Rn, Cl and 3H yield estimates of groundwater inflows that agree to within ± 12%, with cumulative inflows in individual campaigns ranging from 24 346 to 88 467 m3/day along the studied river section. Groundwater discharge accounts for between 10 and 50% of river flow dependent on the time of year, with a high proportion (>40 %) of groundwater sustaining summer flows. Groundwater inflow is largely governed by regional groundwater flowpaths; between 50 and 90% of total groundwater inflows occur along a narrow 5–10 km section where the river intersects the Eastern View Formation, a major regional aquifer. Groundwater 222Rn activities over the 16 month period were spatially heterogeneous across the catchment, ranging between 2000 Bq/m3 and 16 175 Bq/m3. Although groundwater 222Rn activities display temporal variation, spatial variation in groundwater 222Rn is a key control on 222Rn mass balances in river catchments where groundwater and river 222Rn activities are within an order of magnitude of each other. Calculated groundwater discharges vary from 8.4 to 15 m3/m/day when groundwater 222Rn activities are varied by ± 1 σ. © 2013 John Wiley & Sons, Ltd.
Physical hydrogeology and environmental isotopes to constrain the age, origins, and stability of a low-salinity groundwater lens formed by periodic river recharge: Murray Basin, Australia.
(Elsevier, 2010-01-15) Cartwright, I; Weaver, TR; Simmons, CT; Fifield, LK; Lawrence, CR; Chisari, R; Varley, S
A low-salinity (total dissolved solids, TDS, <5000 mg/L) groundwater lens underlies the Murray River in the Colignan–Nyah region of northern Victoria, Australia. Hydraulic heads, surface water elevations, δ18O values, major ion geochemistry, 14C activities, and 3H concentrations show that the lens is recharged from the Murray River largely through the riverbank with limited recharge through the floodplain. Recharge of the lens occurs mainly at high river levels and the low-salinity groundwater forms baseflow to some river reaches during times of low river levels. Within the lens, flow through the shallow Channel Sands and deeper Parilla Sands aquifers is sub-horizontal. While the Blanchetown Clay locally separates the Channel Sands and the Parilla Sands, the occurrence of recently recharged low-salinity groundwater below the Blanchetown Clay suggests that there is considerable leakage through this unit, implying that it is not an efficient aquitard. The lateral margin of the lens with the regional groundwater (TDS >25,000 mg/L) is marked by a hectometer to kilometer scale transition in TDS concentrations that is not stratigraphically controlled. Rather this boundary represents a mixing zone with the regional groundwater, the position of which is controlled by the rate of recharge from the river. The lens is part of an active and dynamic hydrogeological system that responds over years to decades to changes in river levels. The lens has shrunk during the drought of the late 1990s to the mid 2000s, and it will continue to shrink unless regular high flows in the Murray River are re-established. Over longer timescales, the rise of the regional water table due to land clearing will increase the hydraulic gradient between the regional groundwater and the groundwater in the lens, which will also cause it to degrade. Replacement of low-salinity groundwater in the lens with saline groundwater will ultimately increase the salinity of the Murray River reducing its utility for water supply and impacting riverine ecosystems. © 2010, Elsevier Ltd.
Post-glacial coupling of the Australasian monsoon and teleconnections to the North Atlantic: new insights from Indonesian speleothems
(GNS Science, 2009-05-15) Griffiths, ML; Drysdale, RN; Gagan, MK; Zhao, JK; Ayliffe, LK; Hellstrom, JC; Hantoro, WS; Frisia, S; Feng, YX; Cartwright, I; St Pierre, E; Fisher, M; Suwargadi, B
The Australasian monsoon system orchestrates rainfall variability and terrestrial productivity in the densely populated region of the tropical Indo-Pacific. A clear understanding of the dominant mechanisms governing its variability has been difficult to resolve, partly because we currently lack high-resolution proxy records of past monsoon behaviour, particularly for the southern tropics. Here we provide a radiometrically dated reconstruction of Australian-Indonesian summer monsoon (AISM) rainfall based on oxygen isotopes and trace element data in stalagmites from southern Indonesia. The multi-proxy records are tied to age-depth models constructed from 62 TIMS and MC-ICP-MS U-series ages, covering the period 0 to 12.6 ka B.P. The record shows that the AISM was anti-phased with the East Asian summer monsoon (EASM) on orbital to millennial-centennial timescales over the past 12.6 ka. At the orbital-scale, local summer insolation was an important driver of opposing changes in AISM and EASM rainfall. However, a slight mismatch between the AISM and insolation from 9 to 11 ka B.P. is concurrent with the sharp rise in eustatic sealevel, which apparently increased the supply of northwesterly summer monsoon moisture to the Indonesian maritime continents. At millennial-centennial timescales, the oxygen isotope and trace element records show that periods of weakened North Atlantic meridional overturning circulation and cooling, including the Younger Dryas cold stage, are in phase with sharp increases in AISM rainfall. The connection between the AISM and a cooler North Atlantic is probably due to enhanced outflow from the Asian winter monsoon and associated southward migration of the intertropical convergence zone. These interhemispheric connections were dominant until ~6.5 ka, when the El Niño-Southern Oscillation became the governing influence on AISM variability.
Residence times and mixing of water in river banks: implications for recharge and groundwater–surface water exchange
(Copernicus Publications, 2014-12-12) Unland, NP; Cartwright, I; Cendón, DI; Chisari, R
Bank exchange processes within 50 m of the Tambo River, southeast Australia, have been investigated through the combined use of 3H and 14C. Groundwater residence times increase towards the Tambo River, which suggests the absence of significant bank storage. Major ion concentrations and δ2H and δ18O values of bank water also indicate that bank infiltration does not significantly impact groundwater chemistry under baseflow and post-flood conditions, suggesting that the gaining nature of the river may be driving the return of bank storage water back into the Tambo River within days of peak flood conditions. The covariance between 3H and 14C indicates the leakage and mixing between old (~17 200 years) groundwater from a semi-confined aquifer and younger groundwater (<100 years) near the river, where confining layers are less prevalent. It is likely that the upward infiltration of deeper groundwater from the semi-confined aquifer during flooding limits bank infiltration. Furthermore, the more saline deeper groundwater likely controls the geochemistry of water in the river bank, minimising the chemical impact that bank infiltration has in this setting. These processes, coupled with the strongly gaining nature of the Tambo River are likely to be the factors reducing the chemical impact of bank storage in this setting. This study illustrates the complex nature of river groundwater interactions and the potential downfall in assuming simple or idealised conditions when conducting hydrogeological studies.© Author(s) 2014. CC Attribution 3.0 License.
Residence times of bank storage and return flows and the influence on river water chemistry in the upper Barwon River, Australia
(Elsevier B. V., 2019-02) Howcroft, W; Cartwright, I; Cendón, DI
Documenting the sources and residence times of water that contributes to streamflow is important for understanding processes in river catchments. The residence times of bank storage and return flow and its influence on river water chemistry in the upper Barwon River of southeast Australia were investigated using stable (18O, 2H, and 13C) and radioactive (3H and 36Cl) isotopes, major ion geochemistry, river discharge data, and electrical conductivity (EC)-discharge hysteresis. Elevated 3H activities following high winter flows indicate that bank storage and return flow contributes to river discharge for at least several months. However, EC-discharge hysteresis patterns suggest that individual storm events make additional contributions to bank storage and return flow throughout the year over periods of a few weeks. 3H activities in the upper Barwon River are >1.75 TU throughout the year, suggesting that the contribution of older regional groundwater, which has 3H activities <0.04 TU, is relatively minor in comparison to bank return flows. However, downstream trends in total dissolved solids (TDS) concentrations, δ13C values and R36Cl values demonstrate that regional groundwater inflows deliver solutes to the river. Estimates of regional groundwater inflows are mainly in the range 8–33% of total stream flow. The R36Cl values of river water in the upper Barwon catchment are between 37 and 46, which are significantly higher than those of modern rainfall (∼20). The high R36Cl values may reflect retardation of bomb-pulse 36Cl due to plant uptake and recycling in the soil zone, which suggests Cl residence times of up to ∼60 years. The results of this study demonstrate that river water is comprised of both young and old water and that managing rivers and near-river environments should include careful consideration of both inputs. © 2018 Elsevier Ltd.
A review of radioactive isotopes and other residence time tracers in understanding groundwater recharge: Possibilities, challenges, and limitations
(Elsevier B. V., 2017-12) Cartwright, I; Cendón, DI; Currell, MJ; Meredith, KT
Documenting the location and magnitude of groundwater recharge is critical for understanding groundwater flow systems. Radioactive tracers, notably 14C, 3H, 36Cl, and the noble gases, together with other tracers whose concentrations vary over time, such as the chlorofluorocarbons or sulfur hexafluoride, are commonly used to estimate recharge rates. This review discusses some of the advantages and problems of using these tracers to estimate recharge rates. The suite of tracers allows recharge to be estimated over timescales ranging from a few years to several hundred thousand years, which allows both the long-term and modern behaviour of groundwater systems to be documented. All tracers record mean residence times and mean recharge rates rather than a specific age and date of recharge. The timescale over which recharge rates are averaged increases with the mean residence time. This is an advantage in providing representative recharge rates but presents a problem in comparing recharge rates derived from these tracers with those from other techniques, such as water table fluctuations or lysimeters. In addition to issues relating to the sampling and interpretation of specific tracers, macroscopic dispersion and mixing in groundwater flow systems limit how precisely groundwater residence times and recharge rates may be estimated. Additionally, many recharge studies have utilised existing infrastructure that may not be ideal for this purpose (e.g., wells with long screens that sample groundwater several kilometres from the recharge area). Ideal recharge studies would collect sufficient information to optimise the use of specific tracers and minimise the problems of mixing and dispersion. © 2017 Elsevier B.V
A review of the use of radiocarbon to estimate groundwater residence times in semi-arid and arid areas
(Elsevier, 2020-01) Cartwright, I; Currell, MJ; Cendón, DI; Meredith, KT
Groundwater is an important resource in arid and semi-arid regions and determining its residence times is critical for sustainable use. Radiocarbon (14C) is currently the primary geochemical tracer for determining residence times of regional groundwater systems. The analysis of 14C contents of dissolved inorganic carbon (DIC) became more straightforward following the development of accelerator mass spectrometry, which has led to an increase in the number of studies using 14C. However, the interpretation of 14C data is not always straightforward and many studies consider relatively few of the multiple processes that may affect the 14C contents of DIC in groundwater. Commonly, studies have focussed on correcting 14C contents for closed-system dissolution of 14C-free calcite, which is a near-ubiquitous process. However, especially in semi-arid and arid areas, uncertainties in the initial 14C contents and δ13C values of recharge due to the presence of low-14C soil CO2 in the deep unsaturated zone, recharge by rivers, or open-system calcite dissolution pose problems for mass balance calculations. Additionally, processes such as methanogenesis and mineralisation of organic carbon may be locally important. Most studies also assume a constant atmospheric 14C content and non-dispersive piston flow in aquifers, which results in residence times being underestimated and makes it difficult to compare the groundwater archive to other palaeoclimate or palaeoenvironment records. Additionally, mixing of water within aquifers, diffusion of 14C between low and high permeability layers, and sampling from multiple units in long-screen wells may limit whether a meaningful residence time can be determined. Overall, while it is relatively straightforward to estimate broad ranges of residence times or determine general patterns of groundwater flow, the quest to quantify residence times, flow rates, and recharge remains a challenge. The use of multiple radioactive tracers, better characterisation of δ13C values and 14C contents of the potential sources of DIC, and more critical assessment of flow systems will improve the utilisation of this important tracer. © 2019 Elsevier B.V
Sources and transit times of water in headwater temperate rainforest streams
(Copernicus Publications, 2019-04-07) Cartwright, I; Atkinson, AP; Gilfedder, BS; Hofmann, H; Cendón, DI; Morgenstern, U
Headwater catchments are important sources of water for many river systems. Unlike lower reaches of rivers that are frequently connected to alluvial aquifers, headwater catchments are commonly developed on indurated rocks that lack extensive groundwater systems. The observation, however, that many headwater streams are perennial implies that streamflow is sustained by water contained in fractures, soils, and/or the regolith. Understanding the sources and transit times of water that generates streamflow in headwater streams is important for understanding catchment functioning and predicting the response of catchments to changing climate or land use. This study determines water sources and transit times in first-order streams from a temperate rainforest in the Otway Ranges, southeast Australia. Comparison of the major ion geochemistry of soil water, water flowing through soil pipes (macropores), and groundwater from the riparian zone adjacent to the stream indicates that water from soil pipes is the major contributor to streamflow. By contrast, riparian zone groundwater and water from elsewhere within the soils contributes little to streamflow. The streams are gaining and the lack of riparian zone groundwater inputs may be due to the presence of low hydraulic-conductivity organic-rich streambed sediments or compartmentalisation of shallow groundwater by clays in the weathered rocks. Similarly, pockets of isolated water within the soils that are not connected to the soil pipes also exist. The stream water has tritium (3H) activities of 1.80 to 2.06 TU, with slightly higher activities recorded during the higher winter flows. The water from the soil pipes has 3H activities of 1.80 to 2.25 TU, the riparian zone groundwater has 3H activities of 1.35 to 2.39 TU, and one sample of soil water has a 3H activity of 2.22 TU. The 3H activities of all these catchment waters are significantly lower than those of modern rainfall (2.6 to 3.0 TU), and mean transit times calculated using a range of lumped parameter models are between 3 and 57 years. These mean transit times are consistent with the waters being resident in the catchment for sufficient time for weathering reactions and evapotranspiration to occur. While the discharge from the soil pipes increases following periods of high rainfall, this water is stored for several years within the catchment before discharge (probably within the weathered regolith). Thus, the increase in discharge is not the simple transmission of recent rainfall through the macropores but mobilisation of younger stores of water as the catchment wets up. The long mean transit times of the stream water imply that it is derived from a relatively large store (>108 m3) and is buffered against year-on-year variations in rainfall. However, longer-term variations in rainfall or land use will likely impact streamflow. © Author(s) 2018. CC Attribution 4.0 license.
Stable isotopes as indicators of dissolved methane sources and cycling in the Gippsland Basin, Victoria
(National Centre for Groundwater Research And Training, 2017-07-12) Currell, MJ; Cendón, DI; Cartwright, I
A suite of environmental tracers were analysed from the Gippsland Basin, Victoria, to determine the origins of methane in groundwater and characterise the biogeochemical and physical processes controlling its occurrence and cycling. Water samples were collected from a range of depths and lithology, and were analysed for stable isotopes of methane plus a suite of other tracers - radiocarbon, noble gases (He-4, Ne, Ar), δ18O, δ2H and δ13CDIC. The data were analysed within the hydrogeological framework to characterise sources of methane in groundwater and identify possible transport processes. Methane isotopic compositions ranged widely throughout the system. Two predominant groups of methane were found, both of which were of bacterial origin. One group contained isotopes with typical acetate-fermentation signatures (δ13CCH4 = −45.8 to −66.2‰ and δ2HCH4 = −204 to −311‰), and largely occurred in deep groundwater, near the coast. The other group exhibited unusually depleted δ13CCH4 values by typical global standards (−83.7 to −97.5 ‰) and δ2HCH4 values between −236 and −391‰. This group is associated with relatively shallow groundwater, near areas of extensive lignite. Radiocarbon and He-4 data indicate that groundwater age increases with depth, however inter-aquifer mixing complicates age interpretations. Stable isotopes of water in the deepest parts of the system show relatively depleted δ18O and δ2H, suggesting isotopic modification during methanogenesis and/or depleted signatures associated with palaeo-recharge conditions. The study provides the first data on dual-isotopic compositions of methane in the Gippsland Basin, and new insights into sources and cycling of methane in a multi-layered sedimentary basin. The basin is one in which extensive groundwater extraction and mining activity occurs, which may be having ongoing effects on inter-aquifer connectivity for both water and gases.
Tracing the age, origins and hydrodynamics of groundwater and surface water exchange in river banks
(International Association of Hydrogeologists, 2013-09-17) Unland, NP; Cartwright, I; Cendón, DI; Chisari, R
It is common for groundwater-surface water assessments to be conducted within streams at discrete time intervals in order to characterise the gaining or losing nature of a stream and the volumetric flux of water between the two reservoirs. While these studies offer sound scientific information for one point in time, they often overlook the dynamics of groundwater and surface water interaction under changing hydrologic conditions. This study couples discrete sampling for hydrochemical parameters with the continuous monitoring of physical parameters at multiple locations. in taking this approach the interaction between river water and groundwater stored in river banks can be assessed over space and time, allowing for both the qualitative and quantitative impacts of water exchange to be assessed. Continuous analysis of groundwater head levels and electrical conductivity indicates the presence of a semi conned aquifer of increased salinity underlying an unconfined aquifer of lower salinity in the region. Carbon-14 and tritium results indicate that groundwater in the underlying aquifer is significantly older than that of the unconfined aquifer, with variable mixing between the two resulting a range of intermediate ages. While discrete sampling and temperature profiling of river water indicates a predominantly gaining system, reversal of hydraulic gradients during periods of increased rainfall and river discharge indicates a change to losing conditions. Although this indicates the occurrence of bank infiltration, an initial increase in groundwater electrical conductivity during increased river discharge suggests that increased leakage from the semi-confined aquifer dominates groundwater chemistry at these times. The degree to which this occurs varies between locations and the scale of discharge events. This study illustrates the complexity and variability by which groundwater-surface water interactions can occur within river banks.
Understanding groundwater dynamics on barrier islands using geochronological data: an example from North Stradbroke Island, South-east Queensland
(National Centre for Groundwater Research And Training, 2015-11-03) Hofmann, H; Newborn, D; Cartwright, I; Cendón, DI; Raiber, M
Freshwater lenses underneath barrier islands are dynamic systems affected by changing sea levels and groundwater use. They are vulnerable to contamination and over-abstraction. Residence times of fresh groundwater in barrier islands are poorly understood and have mostly been assessed by modelling approaches and estimates without fundamental validation with chronological estimations. Assessing residence time and recharge rates will improve significantly our understanding of hydrological processes of coastal environments that will in turn allow us to make informed decisions on groundwater use and environmental protection. This project focused on groundwater recharge rates and residence times of the fresh water aquifer system of North Stradbroke Island, south-east Queensland, Australia. Groundwater bores, wetlands and submarine groundwater discharge points in the tidal areas (wonky holes) were sampled along a transect across the island and were analysed for major ion chemistry and stable isotopes (δ2H, δ18O, δ13C) in combination with 3H, 14C analysis and 222Rn. Calculated 3H using a 90% exponential-piston flow model and 14C ages range from 12 to >100 years and modern to 3770 years, respectively, indicating a highly heterogeneous aquifer system with mixing from low and high conductive areas. The major ion chemistry in combination with stable and radiogenic isotopes suggests that a significant groundwater component derives from the fractured rock basement and older sedimentary formations underlying the sand dunes of the island. The results help refining the conceptual and numerical groundwater flow model for North Stradbroke Island in this particular case but also demonstrate the possible complexity of barrier island hydrogeology.
Understanding the sources and transit times of water sustaining streamflow in upland catchments
(National Centre for Groundwater Research And Training, & Australian Chapter International Association Of Hydrogeologists, 2019-11-24) Cartwright, I; Atkinson, AP; Gilfedder, BS; Hofmann, H; Cendón, DI; Morgenstern, U
Headwater catchments are important sources of water in many rivers. While headwater catchments are commonly developed on indurated rocks without extensive groundwater systems, the observation that many headwater streams are perennial implies that they are sustained by water in fractures, soils, or the regolith. Understanding the sources and transit times of water in headwater streams is important for understanding catchment functioning and predicting the impacts of changing climate or land use. This study uses major ion geochemistry and tritium (3H) to determine water sources and transit times in first-order streams in the Otway Ranges, southeast Australia. Comparison of the geochemistry of soil water, water from soil pipes (macropores), and riparian groundwater indicates that macropore flow is the major contributor to streamflow. The streams are gaining and the lack of riparian zone groundwater inputs may be due to the presence of low hydraulic-conductivity organic-rich streambed sediments or compartmentalisation of shallow groundwater by clays in the weathered rocks. Similarly, much of the soil water exists in isolated pockets of isolated water that are not connected to the soil pipes. The stream water has tritium (3H) activities of 1.80 to 2.06 TU. These are significantly lower than the 3H activities of modern rainfall (2.6 to 3.0 TU), even during the higher winter flows. The water from the soil pipes has 3H activities of 1.80 to 2.25 TU, the riparian zone groundwater has 3H activities of 1.35 to 2.39 TU, and one sample of soil water has a 3H activity of 2.22 TU. Mean transit times calculated using a range of lumped parameter models are between 3 and 57 years. Relatively long mean transit times are consistent with the major ion geochemistry that implies that waters are resident for sufficient time for weathering reactions and evapotranspiration to have occurred. While the discharge from the soil pipes increases following periods of high rainfall, the long mean transit times implies that this water is stored for several years within the regolith before discharge, with storage volumes estimated as >108 m3. Thus the increase in streamflow is not the simple transmission of recent rainfall through the macropores but mobilisation of existing catchment stores. The streams will be buffered against year-on-year variations in rainfall but are vulnerable to longer-term variations in rainfall or land use. Management of these catchments needs to consider the impacts on the macropores and the delayed responses caused by the large storage volumes. © The Authors
Using 14C and 3H to understand groundwater flow and recharge in an aquifer window
(Copernicus Publications, 2014-12-09) Atkinson, AP; Cartwright, I; Gilfedder, BS; Cendón, DI; Unland, NP; Hofmann, H
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.

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