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Browsing by Author "Webb, JA"

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Application of environmental isotopes to study aquifer interactions and their impact on groundwater salinisation in western Victoria
(Australian Institute of Nuclear Science and Engineering (AINSE), 2007-11) Raiber, M; Webb, JA; Jacobsen, GE; Chisari, R; Williams, AA; Neklapilova, B
Biomass uptake and fire as controls on groundwater solute evolution on a southeast Australian granite: Aboriginal land management hypothesis
(Copernicus Gesellschaft MBH, 2014-01-01) Dean, JF; Webb, JA; Jacobsen, GE; Chisari, R; Dresel, PE
The chemical composition of groundwater and surface water is often considered to be dominated by water-rock interactions, particularly weathering; however, it has been increasingly realised that plant uptake can deplete groundwater and surface water of nutrient elements. Here we show, using geochemical mass balance techniques, that water-rock interactions do not control the hydrochemistry at our study site within a granite terrain in southwest Victoria, Australia. Instead the chemical species provided by rainfall are depleted by plant biomass uptake and exported, predominantly through fire. Regular landscape burning by Aboriginal land users is hypothesized to have caused the depletion of chemical species in groundwater for at least the past 20 000 yr by accelerating the export of elements that would otherwise have been stored within the local biomass. These findings are likely to be applicable to silicate terrains throughout southeast Australia, as well as similar lithological and climatic regions elsewhere in the globe, and contrast with studies of groundwater and surface water chemistry in higher rainfall areas of the Northern Hemisphere, where water-rock interactions are the dominant hydrochemical control. © 2014, Copernicus Gesellschaft MBH.
Delineating groundwater flowpaths using 14C dating in the upper loddon catchment, central Victoria.
(Australian Institute of Nuclear Science and Engineering (AINSE), 2007-11) Hagerty, SK; Webb, JA; Jacobsen, GE
Radiocarbon dating of groundwater has been used to delineate flowpaths and calculate hydraulic conductivity as part of a detailed hydrogeological/hydrogeochemical study of the Tullaroop catchment in the Upper Loddon, central Victoria. The dating confirms that both the basalt and deep lead aquifers in the study area are recharged through scoria cones at volcanic eruption points. The radiocarbon dating also shows that flow paths in the basalt aquifer are complex since ages do not always increase downflow; any hydraulic conductivity calculations must take this into account. One anomalously old 14C date supports other hydrogeological evidence that suggests groundwater in the deep lead is trapped upflow of horsts created by ENE Cainozoic faults.
Environmental isotopes meet 3D geological modelling: Conceptualising recharge and structurally-controlled aquifer connectivity in the basalt plains of south-western Victoria, Australia
(Elsevier B. V., 2015-08) Raiber, M; Webb, JA; Cendón, DI; White, PA; Jacobsen, GE
To develop a conceptual model of recharge and inter-aquifer connectivity for a complex aquifer system in southwestern Victoria, Australia, a three-dimensional (3D) geological model was constructed and used to examine the influence of the regional geology on groundwater composition and age, as well as recharge mechanisms. The major aquifers are three basalts, differentiated on age and geomorphological features, and an underlying palaeovalley (deep lead) system. The upper fine-grained sediments of the deep lead form an aquitard that separates the basalt from the permeable sands and gravels beneath. Recharge calculations (chloride mass balance and water-table fluctuation methods), salinity, stable isotope, tritium and radiocarbon data show that volcanoes and the youngest basalts, although volumetrically minor, form preferential recharge areas for all the basalts (up to ∼30 mm/year), and also recharge the deep lead sediments through volcanic necks that penetrate the aquitard. Infiltration through the thick, clay-rich soils on the older basalts is small (typically ⩽ 1–2 mm/year) and saline, resulting in a progressive increase of groundwater salinity along the flow path in these basalts. The comparison of fault location with groundwater age in the deep lead aquifer suggests that in some areas, faults exert significant structural control on groundwater flow. This study demonstrates the value of integrating geological, hydrogeological and hydrochemical data to identify preferential recharge areas, inter-aquifer connectivity and the influence of faults on groundwater flow in a complex aquifer system. © 2015 Elsevier B.V.
A groundwater recharge perspective on locating tree plantations within low-rainfall catchments to limit water resource losses
(Copernicus Publications, 2015-02-26) Dean, JF; Webb, JA; Jacobsen, GE; Chisari, R; Dresel, PE
Despite the many studies that consider the impacts of plantation forestry on groundwater recharge, and others that explore the spatial heterogeneity of recharge in low-rainfall regions, there is little marriage of the two subjects in forestry management guidelines and legislation. Here we carry out an in-depth analysis of the impact of reforestation on groundwater recharge in a low-rainfall (< 700 mm annually), high-evapotranspiration paired catchment characterized by ephemeral streams. Water table fluctuation (WTF) estimates of modern recharge indicate that little groundwater recharge occurs along the topographic highs of the catchments (average 18 mm yr−1); instead the steeper slopes in these areas direct runoff downslope to the lowland areas, where most recharge occurs (average 78 mm yr−1). Recharge estimates using the chloride mass balance (CMB) method were corrected by replacing the rainfall input Cl− value with that for streamflow, because most recharge occurs from infiltration of runoff through the streambed and adjacent low gradient slopes. The calculated CMB recharge values (average 10 mm yr−1) are lower than the WTF recharge values (average 47 mm yr−1), because they are representative of groundwater that was mostly recharged prior to European land clearance (> BP 200 years). The tree plantation has caused a progressive drawdown in groundwater levels due to tree water use; the decline is less in the upland areas. The results of this study show that spatial variations in recharge are important considerations for locating tree plantations. To conserve water resources for downstream users in low-rainfall, high-evapotranspiration regions, tree planting should be avoided in the dominant zone of recharge, i.e. the topographically low areas and along the drainage lines, and should be concentrated on the upper slopes, although this may negatively impact the economic viability of the plantation. © 2015 Author(s)
The importance of topographic controls on groundwater recharge for plantation forestry in southeast Australia
(International Association of Hydrogeologists, 2013-09-16) Dean, JF; Webb, JA; Jacobsen, GE; Chisari, R; Dresel, PE
The trade off between water use and carbon sequestration by F plantation forestry is the subject of ongoing debate in Australia. Many tree plantations are planted without consideration for their impact on groundwater recharge. The spatial variation of recharge across a landscape can provide useful information on where to best situate a plantation while minimising its impact on groundwater recharge. Recharge is generally assumed to take place at topographic high points across a landscape, especially when rocky outcrops occur. This study analysed recharge rates at several points across a small paired catchment site to determine the most prevalent areas of recharge. Water-table fluctuations, Cl- mass balances, tritium and 14C ages were used to estimate recharge at 23 points across a 0.8 km2 catchment covered predominantly in a blue gum tree plantation, and an adjacent 0.5 km2 pasture catchment, in southwest Victoria, Australia. The CI- mass, balance method and water-table fluctuation methods were modified to better reflect the recharge processes taking place at the study site, including longer term hydrograph fluctuations and input from streams. Hydrographs from data loggers (since 2009) in bores situated close to the drainage line show considerably more recharge taking place compared to bores situated further upslope. Cl-mass balances show a similar trend, as do the hydrographs for the past 20-30 years, indicating that the majority of recharge is occurring along the drainage lines and towards the bottom of the slopes. Hydrograph fluctuations from these areas of the study site also often show significant response to runoff, measured at weirs on both catchments. Fracture heterogeneity contributes to some of the recharge variations, but the general trend is for recharge to occur more consistently in the topographically lower areas. ln order to minimise the interception of groundwater recharge by tree plantations in regions similar to the study area, the bulk of the plantation should be situated on the upper slopes, away from drainage lines and the immediately adjacent slopes. This would have less of an effect on the groundwater levels, as the plantation is not intercepting recharge in the zones where the majority of infiltration is occurring.
Tracing groundwater flow in granite landscapes using δ18O, δ2H, 3H and 14C in the Upper Wimmera catchment, western Victoria
(Australasian Environmental Isotope Conference, 2009-12-03) Hagerty, SK; Webb, JA; Dyson, P; Hocking, M; Jacobsen, GE; Chisari, R; Poulson, S
Previous work [1,2] has suggested that low relief, highly weathered granites in the Upper Wimmera in western Victoria are a significant source of salts exported to the catchment via groundwater, whereas high relief, less weathered granites are a source of fresh, good quality groundwater. In order to understand the processes of salt accumulation and movement in the catchment, groundwater recharge and flow has been investigated using the isotope ratios δ18O and δ2H, and the isotopes 3H and 14C. The Upper Wimmera forms part of the Murray Darling Basin [Figure 1], near the township of Ararat in western Victoria. The main geological units in the area are: the highly weathered and fractured basement of Ordovician aged metasediments, the Devonian aged granites that intruded the metasediments (with surrounding Quaternary-aged colluvial aprons), and the overlying Quaternary alluvial sediments of the Shepparton Formation. These three geological units also form the main aquifers of interest in the study area. Stable isotope ratios (δ18O and δ2H) suggest that the groundwater forms two groups; one that plots along the local meteoric water line and is relatively depleted in the heavy isotopes, and another that is more enriched in heavy isotopes, and plots on an evaporation trendline with a slope of 4.1. This latter group of groundwater is also much more saline on average (9.4 mS/cm compared to 2.6 mS/cm). However this is not due to open-water evaporation; the fractionation of isotopes seen in the evaporated water can only account for a loss of <10% of the original water body. However the high salinities observed require a loss of at least 99.5% of the original water body. Hence, the high salinity is likely due to transpiration by vegetation, as this process does not affect stable isotope ratios. Twenty three of these groundwater samples were analysed for 3H and 10 samples contained detectable tritium, suggesting that these samples are ‘modern’ (less than ~50 years old). All of these 10 samples are in the group of groundwater that has undergone evaporation, suggesting that this group is also younger on average. The radiocarbon data agrees with the tritium data; 38 samples were analysed for 14C and the samples in the group of evaporated groundwater are younger on average (78 pMC compared to 60 pMC). The radiocarbon data also shows that vertical flow is an important component of the flow system in weathered granite colluvium, and to a lesser extent in the Ordovician basement. However vertical flow does not appear to be a significant component in the Shepparton Formation.
Using environmental tracers and geological modelling to understand the effect of aquifer composition on groundwater quality in the basalt plains of Western Victoria (Australia)
(Australasian Environmental Isotope Conference, 2009-12-03) Raiber, M; Webb, JA; White, PA; Cendón, DI; Jacobsen, GE
In the basalt plains of south-western Victoria, Australia, good-quality groundwaters are found only in small areas whereas poor quality groundwater with salinities sometimes in excess of seawater concentrations typically dominates most aquifers. As a result, the limited groundwater resources that are useful for domestic and agricultural purposes are under considerable pressure, and the prevailing drought which has affected Australia and especially its southeast since 1997 has further reduced the availability of surface waters and increased the reliance on groundwaters as the principal source for water supplies. In the present study, groundwater ages from more than 70 radiocarbon and tritium measurements of groundwater from a shallow basalt aquifer and an underlying Tertiary Deep Lead System (Streatham Deep Lead System) were used in combination with the analysis of the groundwater major ion composition, stable isotopes (deuterium and δ18O) and 87Sr/86Sr ratios to understand groundwater processes in the basalt plains of western Victoria. A three-dimensional geological model was developed in order to interpret and visualise the geological relationship between the major aquifers; this 3D geological model was also used to model the distribution of groundwater properties such as groundwater age and salinity within the major aquifers using the minimum tension interpolation technique of the EarthVision software. Recharge rates derived from independent chemical and physical estimation techniques provided further evidence on the link between the aquifer composition, aquifer interactions and the quality of groundwater. The study shows that groundwater is typically young (i.e. less than 100 years BP) and fresh near the major volcanoes, not only in the shallow basalt aquifer but also in the underlying deep lead aquifer. With increasing distance along the flow path away from these volcanic centres, both the groundwater age and salinity progressively increase within the shallow and deeper aquifers, yielding maximum radiocarbon groundwater ages of up to approximately 35000 yrs. BP. The comparison of groundwater age, salinity, recharge rates and soil thickness indicates that the soil structure and thickness are the major controls on groundwater quality. The absence of any significant soil development on the volcanoes facilitates rapid recharge through fractures with recharge rates of up to ~30 mm/yr, whereas the thick, heavy soils developed on basalts away from the volcanoes allow no substantial recharge to the aquifers, resulting in estimated recharge rates typically less than 1-2 mm/yr and primarily old and saline groundwaters here. The analysis and visualisation of groundwater age and groundwater quality parameters in 3D using a three-dimensional geological model is a useful tool to achieve a better understanding of the link between geology and groundwater salinity in the basalt plains of western Victoria. The knowledge gained from this study will assist water managers for example with the selection of suitable locations for the drilling of drought relieve bores and it will also help to highlight areas particularly vulnerable to aquifer over-exploitation and facilitate the optimisation of management strategies.

Browsing by Author "Webb, JA"

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