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Papers by Glenn Harrington

Water Resources Research

Carbon-14 (14C) has been widely used to estimate groundwater recharge rates in arid regions, and ... more Carbon-14 (14C) has been widely used to estimate groundwater recharge rates in arid regions, and is increasingly being used as a tool to assist numerical model calibration. However, lack of knowledge on 14C inputs to groundwater potentially limits its reliability for constraining spatial variability in recharge. In this study, we use direct measurements of 14C in the unsaturated zone to develop a 14C input map for a regional scale unconfined aquifer in the Ti Tree Basin in central Australia. The map is used as a boundary condition for a 3-D groundwater flow and solute transport model for the basin. The model is calibrated to both groundwater 14C activity and groundwater level, and calibration is achieved by varying recharge rates in 18 hydrogeological zones. We test the sensitivity of the calibration to both the 14C boundary condition, and the number or recharge zones used. The calibrated recharge rates help resolve the conceptual model for the basin, and demonstrate that spatially distributed discharge (through evapotranspiration) is an important part of the water balance. This approach demonstrates the importance of boundary conditions for 14C transport modeling (14C input activity), for improving estimates of spatial variability in recharge and discharge.

Bayesian Network models Traditional knowledge Water resource planning Knowledge integration Groun... more Bayesian Network models Traditional knowledge Water resource planning Knowledge integration Groundwater-surface water interaction Aboriginal s u m m a r y

Ground Water Monit Remediat, 2000

The installation of gas-filled diffusion samplers into small-diameter boreholes results in a sign... more The installation of gas-filled diffusion samplers into small-diameter boreholes results in a significant reduction of the dissolved gas concentration around the sampler. In aquifers where the diffusive flux of solutes outpaces advective transport, the process that governs the equilibration time of a sampler is the resupply of solutes by diffusion from the aquifer. The authors have derived a solution that

Groundwater recharge is one of the more difficult components of the hydrological cycle to estimat... more Groundwater recharge is one of the more difficult components of the hydrological cycle to estimate but one that is becoming increasingly important as Australia turns to groundwater resources for future economic development. Also of concern is groundwater discharge. The extraction of groundwater by pumping inevitably reduces groundwater discharge to rivers where the two are connected. Knowledge of both groundwater recharge and discharge is required for effective management of groundwater resources.

Hydrological Processes, 2015

IAH Selected Paper Series, volume 9, 2007

Water Resources Research, 2011

Elevated helium in river water indicates discharge of regional groundwaterRegional discharge can ... more Elevated helium in river water indicates discharge of regional groundwaterRegional discharge can be separated from locally derived groundwater dischargeTotal, local and regional groundwater discharge volumes can be calculated

Journal of Hydrology, 2015

ABSTRACT Groundwater age tracers are often measured to help constrain estimates of groundwater re... more ABSTRACT Groundwater age tracers are often measured to help constrain estimates of groundwater recharge, especially in arid environments where other methods are unsuitable. However multiple processes can influence the shape of vertical tracer profiles in an aquifer including (1) variation in tracer input concentrations from the unsaturated zone, (2) the role of diffusion in transporting tracer into the aquifer when fluxes are low and (3) spatial variability in recharge. This study demonstrates the influence of spatially variable recharge and spatially variable carbon-14 (14C) activities in the unsaturated zone on vertical 14C profiles in groundwater. Through groundwater flow and solute transport modelling, we demonstrate that recharge estimated from single point measurements of 14C may be wrong more than an order of magnitude when unsaturated zone 14C activities and recharge vary spatially. We then present a case study from the Ti Tree Basin in arid central Australia, where detailed profiles of 14C activity in unsaturated zone gas and groundwater have been measured, and spatial variability in unsaturated zone 14C is observed (ranging from 54 to 106 pMC above the watertable). Through modelling our data, we show that when unsaturated zone 14C activities are known, measurement of the 14C profile can help constrain estimates of recharge and its spatial variability. This approach improves our understanding of groundwater flow in the Ti Tree Basin, by showing mountain front recharge to be an important mechanism.

ABSTRACT The isotopic compositions of soil water and groundwaters in arid and semi-arid zones are... more ABSTRACT The isotopic compositions of soil water and groundwaters in arid and semi-arid zones are always different from the mean composition of rainfall. Although evaporative processes always remove the lighter isotopes (1H and 16O) to the vapour phase, arid zone groundwaters are invariably depleted in the heavy isotopes (2H and 18O) relative to mean present day rainfall. We compare two sites, one in semi-arid South Australia and the other in arid Central Australia that have a similar mean annual rainfall (250 to 300 mm/a), very high potential evapotranspiration (2500 and 3500 mm/a respectively) but very different rainfall patterns (winter dominated versus summer monsoonal). We aim to evaluate whether inferences from groundwater \delta2H and \delta18O reveal information about palaeorecharge, or recharge mechanisms or a combination of both. Recharge to the unconfined limestone aquifer in the Mallee area of South Australia occurs annually via widespread (diffuse) infiltration of winter dominant rainfall. This process is reflected in soil and groundwater isotopic compositions that plot relatively close to both the Local Meteoric Water Line and the volume-weighted mean composition of winter rainfall, and have a deuterium excess (\delta2H-8.\delta18O) of between +2 and +8 for the freshest samples. Groundwater recharge to the arid Ti-Tree Basin occurs predominantly by inputs of partially-evaporated surface water from ephemeral rivers and flood-plains following rare, high-intensity storms that are derived from monsoonal activity to the north of Australia. These extreme events result in groundwater and soil water stable isotope compositions being significantly depleted in the heavy isotopes relative to the mean composition of rainfall and a deuterium excess of between minus 8 and +3 in the freshest groundwaters.

Water Resources Research

Carbon-14 (14C) has been widely used to estimate groundwater recharge rates in arid regions, and ... more Carbon-14 (14C) has been widely used to estimate groundwater recharge rates in arid regions, and is increasingly being used as a tool to assist numerical model calibration. However, lack of knowledge on 14C inputs to groundwater potentially limits its reliability for constraining spatial variability in recharge. In this study, we use direct measurements of 14C in the unsaturated zone to develop a 14C input map for a regional scale unconfined aquifer in the Ti Tree Basin in central Australia. The map is used as a boundary condition for a 3-D groundwater flow and solute transport model for the basin. The model is calibrated to both groundwater 14C activity and groundwater level, and calibration is achieved by varying recharge rates in 18 hydrogeological zones. We test the sensitivity of the calibration to both the 14C boundary condition, and the number or recharge zones used. The calibrated recharge rates help resolve the conceptual model for the basin, and demonstrate that spatially distributed discharge (through evapotranspiration) is an important part of the water balance. This approach demonstrates the importance of boundary conditions for 14C transport modeling (14C input activity), for improving estimates of spatial variability in recharge and discharge.

Bayesian Network models Traditional knowledge Water resource planning Knowledge integration Groun... more Bayesian Network models Traditional knowledge Water resource planning Knowledge integration Groundwater-surface water interaction Aboriginal s u m m a r y

Ground Water Monit Remediat, 2000

The installation of gas-filled diffusion samplers into small-diameter boreholes results in a sign... more The installation of gas-filled diffusion samplers into small-diameter boreholes results in a significant reduction of the dissolved gas concentration around the sampler. In aquifers where the diffusive flux of solutes outpaces advective transport, the process that governs the equilibration time of a sampler is the resupply of solutes by diffusion from the aquifer. The authors have derived a solution that

Groundwater recharge is one of the more difficult components of the hydrological cycle to estimat... more Groundwater recharge is one of the more difficult components of the hydrological cycle to estimate but one that is becoming increasingly important as Australia turns to groundwater resources for future economic development. Also of concern is groundwater discharge. The extraction of groundwater by pumping inevitably reduces groundwater discharge to rivers where the two are connected. Knowledge of both groundwater recharge and discharge is required for effective management of groundwater resources.

Hydrological Processes, 2015

IAH Selected Paper Series, volume 9, 2007

Water Resources Research, 2011

Elevated helium in river water indicates discharge of regional groundwaterRegional discharge can ... more Elevated helium in river water indicates discharge of regional groundwaterRegional discharge can be separated from locally derived groundwater dischargeTotal, local and regional groundwater discharge volumes can be calculated

Journal of Hydrology, 2015

ABSTRACT Groundwater age tracers are often measured to help constrain estimates of groundwater re... more ABSTRACT Groundwater age tracers are often measured to help constrain estimates of groundwater recharge, especially in arid environments where other methods are unsuitable. However multiple processes can influence the shape of vertical tracer profiles in an aquifer including (1) variation in tracer input concentrations from the unsaturated zone, (2) the role of diffusion in transporting tracer into the aquifer when fluxes are low and (3) spatial variability in recharge. This study demonstrates the influence of spatially variable recharge and spatially variable carbon-14 (14C) activities in the unsaturated zone on vertical 14C profiles in groundwater. Through groundwater flow and solute transport modelling, we demonstrate that recharge estimated from single point measurements of 14C may be wrong more than an order of magnitude when unsaturated zone 14C activities and recharge vary spatially. We then present a case study from the Ti Tree Basin in arid central Australia, where detailed profiles of 14C activity in unsaturated zone gas and groundwater have been measured, and spatial variability in unsaturated zone 14C is observed (ranging from 54 to 106 pMC above the watertable). Through modelling our data, we show that when unsaturated zone 14C activities are known, measurement of the 14C profile can help constrain estimates of recharge and its spatial variability. This approach improves our understanding of groundwater flow in the Ti Tree Basin, by showing mountain front recharge to be an important mechanism.

ABSTRACT The isotopic compositions of soil water and groundwaters in arid and semi-arid zones are... more ABSTRACT The isotopic compositions of soil water and groundwaters in arid and semi-arid zones are always different from the mean composition of rainfall. Although evaporative processes always remove the lighter isotopes (1H and 16O) to the vapour phase, arid zone groundwaters are invariably depleted in the heavy isotopes (2H and 18O) relative to mean present day rainfall. We compare two sites, one in semi-arid South Australia and the other in arid Central Australia that have a similar mean annual rainfall (250 to 300 mm/a), very high potential evapotranspiration (2500 and 3500 mm/a respectively) but very different rainfall patterns (winter dominated versus summer monsoonal). We aim to evaluate whether inferences from groundwater \delta2H and \delta18O reveal information about palaeorecharge, or recharge mechanisms or a combination of both. Recharge to the unconfined limestone aquifer in the Mallee area of South Australia occurs annually via widespread (diffuse) infiltration of winter dominant rainfall. This process is reflected in soil and groundwater isotopic compositions that plot relatively close to both the Local Meteoric Water Line and the volume-weighted mean composition of winter rainfall, and have a deuterium excess (\delta2H-8.\delta18O) of between +2 and +8 for the freshest samples. Groundwater recharge to the arid Ti-Tree Basin occurs predominantly by inputs of partially-evaporated surface water from ephemeral rivers and flood-plains following rare, high-intensity storms that are derived from monsoonal activity to the north of Australia. These extreme events result in groundwater and soil water stable isotope compositions being significantly depleted in the heavy isotopes relative to the mean composition of rainfall and a deuterium excess of between minus 8 and +3 in the freshest groundwaters.