Browsing by Author "Unland, NP"

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    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.
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    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.
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    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.
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    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.