Groundwater residence time in the Condamine River Alluvial Aquifer (SE-QLD)

Many gigalitres of groundwater have been extracted from the Condamine River Alluvial Aquifer (CRAA) since the 1960s. These groundwater withdrawals have stressed the system and locally altered the groundwater flow paths. Isotopes can provide powerful insights into recharge pathways, flow direction, and the sustainability of groundwater withdrawals from alluvial aquifers. To address some of the regional groundwater concerns we must characterise alluvial groundwater residence time.A total of 31 groundwater samples were collected from privately owned irrigation bores and Qld-DNRM government monitoring bores in the region between Condamine Plains and Dalby. Parameters analysed included: 3H, 14CDIC, 222Rn, 87Sr/86Sr, δ13CDIC, water δ2H and δ18O, sulfate δ34S and δ18O as well major, trace and REE elements.Distance from primary recharge areas (rivers) provides the main control on groundwater residence time in the CRAA. This is supported by the following observations:1) Groundwater between the Condamine River and its northern branch has low TDS (~400 mg/L), is Na-HCO3-type and has detectable 3H, indicating a proportion of modern recharge (<70 years);2) Groundwater east of the northern branch has higher TDS (~700 mg/L) and is Na-HCO3- -type with increasing eastern inputs. No 3H is detected and 14C shows sub-modern groundwater (~500 years);3) Groundwater along the eastern and western boundaries of the alluvium or samples retrieved from the Walloon Coal Measures (WCM) have high TDS (1,250-19,770 mg/L) and are Na-Cl-type. Residence times in the upper WCM increase along the flow path to the west from modern to 32,000 years on the western side.Groundwater residence time distributions provide a visualisation of recharge processes and delineate areas where groundwater withdrawals are less sustainable within the CRAA

Accumulative evidence highlighting that the Narrabri and Gunnedah formations are mythical

The Narrabri and Gunnedah Formations, used to describe the valley-filling sedimentary sequences in portions of the Murray-Darling Basin, have never been formally defined. The hydrogeological evidence for naming these formations is reviewed in the context of modern sedimentary models. Are we using the right architectural model?Hundreds of lithological logs from the Murrumbidgee, Namoi, and Gwydir catchments are used to examine the evolution of each alluvial aquifer. For each depth interval, the catchment-wide proportions of coarse (gravel, sand) and fine (silt, clay) sediments is determined. Sediment size distributions are then examined in the context of past climates and the conceptual inland fluvial model for distributive fluvial systems. Vertical hydraulic connectivity is examined using new hydrogeochemical data and nested groundwater hydrograph sets.All systems show the core features of aggradational distributive fluvial systems. The valley-filling sequences for all catchments examined transitioned from high energy wet environments at depth, dominated by sand and gravel deposits, through to the modern-day low-energy silt and clay dominated depositional environments. Gravel and sand deposits dominate in the proximal portion of the catchment, and low energy silt and clay deposits dominate in the distal portions. The apparent existence of the Narrabri and Gunnedah Formations is due to changing sediment grain size proportion and channel fill sand connectivity. Both the facies and hydrograph analyses show that semi-confining layers are only local. Extensive hydrogeochemical data from the Namoi catchment show continuity of mixing between basement and surface inflows.All catchments have many sedimentary architectural features consistent with the distributive fluvial system model, and reflect changing climate throughout the Neogene and Quaternary. Use of the Narrabri and Gunnedah Formation nomenclature, which has been incorporated into the National Aquifer Framework, is not supported by either the sedimentological, hydrograph or hydrogeochemical record

Biogeochemical constraints on the origin of methane in an alluvial aquifer: Evidence for the upward migration of methane from underlying coal measures

Geochemical and microbiological indicators of methane (CH4) production, oxidation and migration processes in groundwater are important to understand when attributing sources of gas. The processes controlling the natural occurrence of CH4in groundwater must be understood, especially when considering the potential impacts of the global expansion of coal seam gas (CSG) production on groundwater quality and quantity. We use geochemical and microbiological data, along with measurements of CH4isotopic composition (δ13C-CH4), to determine the processes acting upon CH4in a freshwater alluvial aquifer that directly overlies coal measures targeted for CSG production in Australia. Measurements of CH4indicate that there is biogenic CH4in the aquifer; however, microbial data indicate that there are no methanogenic archaea in the groundwater. In addition, geochemical data, particularly the isotopes of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC), as well as the concentration of SO42-, indicate limited potential for methanogenesis in situ. Microbial community analysis also shows that aerobic oxidation of CH4occurs in the alluvial aquifer. The combination of microbiological and geochemical indicators suggests that the most likely source of CH4, where it was present in the freshwater aquifer, is the upward migration of CH4from the underlying coal measures

Fugitive methane emissions from natural, urban, agricultural, and energy-production landscapes of eastern Australia

Modern cavity ringdown spectroscopy systems (CRDS) enable the continuous measurement of methane concentration. This allows for improved quantification of greenhouse gas emissions associated with various natural and human landscapes. We present a subset of over 4000 km of continuous methane surveying along the east coast of Australia, made using a Picarro G2301 CRDS, deployed in a utility vehicle with an air inlet above the roof at 2.2 mAGL. Measurements were made every 5 seconds to a precision of <0.5 ppb for CH4. These surveys were undertaken during dry daytime hours and all measurements were moisture corrected. We compare the concentration of methane in the near surface atmosphere adjacent to open-cut coal mines, unconventional gas developments (coal seam gas; CSG), and leaks detected in cities and country towns. In areas of dryland crops the median methane concentration was 1.78 ppm, while in the irrigation districts located on vertisol soils the concentration was as low as 1.76 ppm, which may indicate that these soils are a sink for methane. In the Hunter Valley, New South Wales, open-cut coal mining district we mapped a continuous 50 km interval where the concentration of methane exceeded 1.80 ppm. The median concentration in this interval was 2.02 ppm. Peak readings were beyond the range of the reliable measurement (in excess of 3.00 ppm). This extended plume is an amalgamation of plumes from 17 major pits 1 to 10 km in length. Adjacent to CSG developments in the Surat Basin, southeast Queensland, only small anomalies were detected near the well-heads. Throughout the vast majority of the gas fields the concentration of methane was below 1.80 ppm. The largest source of fugitive methane associated with CSG was off-gassing methane from the co-produced water holding ponds. At one location the down wind plume had a cross section of approximately 1 km where the concentration of methane was above 1.80 ppm. The median concentration within this section was 1.82 ppm, with a peak reading of 2.11 ppm. The ambient air methane concentration was always higher in urban environments compared to the surrounding countryside. Along one major road in Sydney we mapped an interval that extended for 6 km where the concentration was greater than 1.80 ppm. The median concentration in this interval was 1.90 ppm, with a peak reading of 1.97 ppm. This high reading in an urban setting is most likely due to leaks from the domestic gas distribution system. Methane leaks were detected in all country towns. Our measurements show that at the point of resource extraction the methane emission footprint of CSG is smaller than that of open-cut coal mining. However, leaking gas from urban centers must be added to the fugitive emissions of CSG to calculate the total fugitive emission footprint of CSG, which may therefore not be as low as claimed in the national greenhouse gas accounts. Our results highlight the need for additional continuous monitoring of methane emissions from all sectors, and for the full life-cycle of energy resources to be considered

A multi-tracer approach to constraining artesian groundwater discharge into an alluvial aquifer

© Author(s) 2017. Understanding pathways of recharge to alluvial aquifers is important for maintaining sustainable access to groundwater resources. Water balance modelling is often used to proportion recharge components and guide sustainable groundwater allocations. However, it is not common practice to use hydrochemical evidence to inform and constrain these models. Here we compare geochemical versus water balance model estimates of artesian discharge into an alluvial aquifer, and demonstrate why multi-tracer geochemical analyses should be used as a critical component of water budget assessments. We selected a site in Australia where the Great Artesian Basin (GAB), the largest artesian basin in the world, discharges into the Lower Namoi Alluvium (LNA), an extensively modelled aquifer, to convey the utility of our approach. Water stable isotopes and the concentrations of Na+ and HCO3g suggest a continuum of mixing in the alluvial aquifer between the GAB (artesian component) and surface recharge, whilst isotopic tracers (3H, 14C, and 36Cl) indicate that the alluvial groundwater is a mixture of groundwaters with residence times of 70 years and groundwater that is potentially hundreds of thousands of years old, which is consistent with that of the GAB. In addition, Clg concentrations provide a means to calculate a percentage estimate of the artesian contribution to the alluvial groundwater. In some locations, an artesian contribution of up to 70g is evident from the geochemical analyses, a finding that contrasts with previous regional-scale water balance modelling estimates that attributed 22g of all inflow for the corresponding zone within the LNA to GAB discharge. Our results show that hydrochemical investigations need to be undertaken as part of developing the conceptual framework of a catchment water balance model, as they can improve our understanding of recharge pathways and better constrain artesian discharge to an alluvial aquifer

The case for refining bottom-up methane emission inventories using top-down measurements

Bottom-up global methane emission estimates are important for guiding policy development and mitigation strategies. Such inventories enable rapid and consistent proportioning of emissions by industrial sectors and land use at various scales from city to country to global. There has been limited use of top-down measurements to guide refining emission inventories. Here we compare the EDGAR gridmap data version 4.2 with over 5000 km of daytime ground level mobile atmospheric methane surveys in eastern Australia. The landscapes and industries surveyed include: urban environments, dryland farming, intensive livestock farming (both beef and lamb), irrigation agriculture, open cut and underground coal mining, and coal seam gas production.Daytime mobile methane surveys over a 2-year period show that at the landscape scale there is a high level of repeatability for the mole fraction of methane measured in the ground level atmosphere. Such consistency in the mole fraction of methane indicates that these data can be used as a proxy for flux.A scatter plot of the EDGAR emission gridmap Log[ton substance / 0.1 degree x 0.1 degree / year] versus the median mole fraction of methane / 0.1 degree x 0.1 degree in the ground level atmosphere highlights that the extent of elevated methane emissions associated with coal mining in the Hunter coalfields, which covers an area of 56 km by 24 km, has been under-represented in the EDGAR input data.Our results also show that methane emissions from country towns (population < 100,000) are underesti- mated in the EDGAR inventory. This is possibly due to poor information on the extent of urban gas leaks.Given the uncertainties associated with the base land use and industry data for each country, we generalise the Australian observations to the global inventory with caution. The extensive comparison of top-down measurements versus the EDGAR version 4.2 methane gridmaps highlights the need for adjustments to the base resource data and/or the emission factors applied for coal mining, especially emissions from underground-mine venting. Also, more detail is required on the areal extent and rate of leakage from the gas distribution systems. This is likely to be the case for many other countries. Our results highlight the value of mobile methane surveys for guiding the refinement of bottom-up emission estimates, and they also suggest the expansion of all forms of top-down emission estimates would result in reduced uncertainty in the global methane budget