M.W. Hughes

The use of natural tracers as indicators of soil-water movement in a temperate semi-arid region

Abstract In a semi-arid area of southern Australia a change in land use from Eucalyptus scrub to cropping with wheat is shown to have caused considerable change in the mechanism of the movement of soil water and the amount of deep drainage. Chloride concentrations of soil water have been used to show the mean annual amount of deep drainage increases from less than 0.1 to ∼ 3 mm yr. −1 following clearing of the native vegetation. The concentration of environmental tritium in soil water beneath the native vegetation is consistent with the hypothesis that some relatively recent water (post 1960) has penetrated to depths of at least 12m along channels occupied by living roots. Where the native vegetation has been cleared, no water which fell as rain since 1960 was found at depths greater than 2.5 m. 18 O and deuterium concentrations suggest that some water movement to the roots of the native vegetation is in the vapour phase.

Land clearance and river salinisation in the western Murray basin, Australia.

Abstract The clearing of native vegetation in a semi-arid region of southern Australia has led to increases in groundwater recharge of about two orders of magnitude. Although most of the clearing took place early this century, the generally deep water table along with the low rates of recharge means that there is a considerable delay in the response of the aquifer to the increased recharge. The rates of pre- and post-clearing recharge, and the time delay in aquifer response have been estimated using unsaturated zone chloride and matric suction profiles. Predictions of the time lag in aquifer response have been verified using bore hydrographs. The results of these analyses suggest that where the soils are light textured, and the water table is less than 40 m below the soil surface, it is now rising. Where the soils are heavier textured, it is estimated that the water table is rising only where it is less than 10 m below the soil surface. The effect of the increased recharge rates on the salinity of the River Murray, a major water resource, have been predicted using a groundwater model of the region. The predictions suggest that the salinity of the river will increase at about 1 μS cm −1 year −1 over the next 50 years and beyond.

The distribution of deuterium and 18O in dry soils 2. Experimental

Abstract This paper presents experimental data on the depth distribution of the stable isotopes 18 O and deuterium in soil water for columns of soil undergoing evaporation. Under steady-state and isothermal conditions in dry soil, isotope concentration rises from a low value at the soil surface to a maximum in the region where liquid transport of water dominates. The concentration then falls exponentially. We find agreement between observed isotope profiles and those calculated by theory developed in a previously published companion paper. Coefficients of determination for regressions of deuterium on 18 O delta values are high for all columns. Observed slopes ranged from 4.2 for saturated sand to 2.5 for sand having very low water content near the surface. This difference is explained in terms of increased path length for laminar diffusion of H 2 18 O and HDO in the vapour phase. This effect was verified by allowing water to evaporate through layers of dry porous material of varying thickness. In these experiments the slope varied between 3.9 and 2.7, showing reasonable agreement with that predicted by the model.

Recharge in karst and dune elements of a semi-arid landscape as indicated by natural isotopes and chloride

Abstract The rates and mechanisms of local recharge in a semi-arid environment have been investigated beneath two major landscape settings in the Murray Basin in South Australia; these were calcrete flats with sinkholes and sand dunes in adjacent landscape settings. Five minor settings were investigated and identified as undisturbed calcrete, primary (older) sinkholes, secondary (younger) sinkholes, sand dunes with native vegetation and sand dunes with introduced pasture. Data from a limited series of boreholes were interpreted to suggest that recharge varied from in excess of 100 mm yr.−1 for secondary sinkholes to less than 0.1 mm yr.−1 beneath sand dunes with native vegetation. The data suggest that recharge under sand dunes has increased by more than two orders of magnitude following clearing of the native vegetation ∼ 50 yr. ago. As ∼ 90% of an area of 10,000 km2 has been cleared, this clearing has ensured that inflow of saline groundwater to the adjacent River Murray will increase and, in view of the amount of salt stored in both the unsaturated and saturated zones, will continue for a very long time. At sites where the inferred recharge is lowest, profiles of the chloride concentration of soil water are consistent with the hypothesis that there is in excess of 50,000-yr. recharge stored in the thick unsaturated zone. Some evidence for changes in recharge rate over this time scale is seen.

Groundwater recharge estimation using chloride, deuterium and oxygen-18 profiles in the deep coastal sands of Western Australia

The depth distributions of environmental chloride, deuterium and oxygen-18 in the deep sands (> 20 m) were monitored with a view to estimate average recharge to groundwater. The investigated area is located on the southwestern coast of Western Australia and supports a perennial native woodland vegetation. By using a steady state model based on the conservation of chloride, the average areal recharge was estimated to be approximately 15% of the average precipitation (775 mm yr−1). Whilst the Cl− concentration of groundwater was relatively uniform, there was a considerable spatial variability in the depth distribution of Cl− in the unsaturated zone. An approximate method of data analysis is developed. Application of this to Cl− data shows that possibly some 50% of the total annual recharge occurs via movement of water through preferred pathways, thus bypassing the soil matrix. Stable isotope (deuterium and oxygen-18) data show that there has been some isotopic enrichment due to evaporation of the rainfall, but the extent of enrichment appears to be too small to estimate recharge rates reliably. It is argued that relatively small isotopic enrichment occurs in the system studied since the direct evaporation from soil is a rather small component of the total evapotranspiration. The observations on the depth distribution of isotopes qualitatively support the presence of a bimodal system for water movement through the profile, as was suggested by the analysis of chloride data.

The movement of isotopes of water during evaporation from a bare soil surface

Stable isotopes of water (2H, 18O) at natural levels have been used to measure steady-state evaporation from soil surfaces in situations where other techniques would fail. In an endeavour to generalise the technique, we have investigated the movement of isotopes during nonsteady evaporation, especially during second-stage evaporation. Equations describing the isotope profiles are developed and it is found that the isotope concentration is a function of the Boltzmann variable (depth time−12). The equations are then solved under the assumption that the evaporation front is narrow. Seven packed sand columns were allowed to evaporate under controlled conditions and then sectioned for analysis of water content and isotopic composition. The results agree generally with the theory but there are difficulties with using isotopes to estimate cumulative evaporation.

Comparison of recharge to groundwater under pasture and forest using environmental tritium

Abstract Recharge to groundwater under pasture and forest at a site on the Gambier Plain, southern Australia, has been investigated by making use of environmental tritium. Water from the top 20 cm of the shallow unconfined aquifer was sampled at locations both in pasture and forest during October 1970 and February 1971. For several sites along a groundwater streamline, the mean tritium concentration of groundwater beneath the forest was 1.8 TU, while that under pasture was 12 TU. The mean depths to water were approximately the same for both land covers, being 6.0 m and 5.6 m respectively. These results lead to the conclusion that there is virtually no recharge to groundwater beneath the forest.

The calibration of frequency-domain electromagnetic induction meters and their possible use in recharge studies

Abstract Published models of soil conductivity are used to develop a calibration for frequency-domain electromagnetic induction meters. The calibration is portable, requiring only a knowledge of certain soil properties. One critical soil property is the impedance factor, a function of the soil water content. We select an impedance factor from the literature which, when incorporated in the calibration, provides the best fit to measured apparent electrical conductivity. The calibration derived is used to test the ability of frequency-domain electromagnetic induction meters to estimate groundwater recharge rates in a semi-arid area of South Australia. Two analytical models relating total (or cumulative) recharge to soil solute profiles are used to derive theoretical relationships between electrical conductivity and total recharge. These modelled relationships provide a good fit to field data, suggesting that electromagnetic techniques have value in recharge studies.

The use of environmental tritium to estimate recharge to a South-Australian aquifer

Abstract The tritium concentration of groundwater samples has been used to estimate the amount of water moving laterally into an aquifer as well as the quantity of local recharge. A multicompartment model used to calculate the variation of tritium concentration within the aquifer, predicts that the lateral input of water is 24 · 10 6 m 3 year −1 , while local recharge is 27 mm year −1 . These results are compared with conventional hydrogeological estimates of 18 · 10 6 m 3 year −1 and 70 mm year −1 , respectively.

Tritium fallout in southern Australia and its hydrologic implications

The tritium concentration in rainfall collected at 11 sampling stations in southern Australia has been determined, for the years 1966–1970. The concentration, which generally is of magnitude 30 to 50 atoms of tritium per 1018 atoms of all hydrogen species, shows well-defined peaks in spring and summer rainfall, and increases significantly at sampling locations towards the interior of the continent. The level of tritium is still considerably higher than the natural abundance produced by cosmic ray bombardment of the stratosphere, but after reaching a peak, it is now declining and presumably will approach the pre-bomb concentration of about 6 T.U. The purpose of the analyses is to obtain tritium input functions for water entering the underground storages, as a necessary first step in using tritium concentration of groundwater to determine residence times and the inventory of water resources.