P.G. Cook

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.

Recent advances in dating young groundwater: chlorofluorocarbons, 3H3He and 85Kr

Abstract Chlorofluorocarbons, 3 H 3 He and 85Kr are tracers of atmospheric origin than can be used to date groundwater over periods from 0 to 40 years. In suitable aquifers, measured groundwater ages can be used to estimate groundwater flow paths, and vertical and horizontal flow velocities. In simple, piston flow systems they can be used to estimate groundwater recharge rates with an accuracy of 20% or less, better than can be achieved with traditional hydraulic-based methods. Groundwater dating methods have also been used to reconstruct past releases of contaminants to aquifers.

Spatial variability of groundwater recharge in a semiarid region

Abstract Chloride profiles and electromagnetic techniques are used to estimate rates of groundwater recharge. The use of electromagnetic techniques allows large numbers of estimates to be made in a relatively short period of time, thus permitting statistical analysis of the spatial variability. Recharge rates appear to be approximately log-normally distributed, in accordance with published results for infiltration rate and hydraulic conductivity. The spatial structure is approximately defined by a spherical semivariogram. The results derived are applied to a study of groundwater salinisation. Predictions for future recharge and salt loads to the aquifer are made based on the estimated recharge distribution.

A new chloride leaching approach to the estimation of diffuse recharge following a change in land use

Abstract A new approach has been developed to estimate the increase in groundwater recharge following land-use modification. The approach uses the degree of leaching of chloride to quantify soilwater drainage below the root zone which ultimately leads to groundwater recharge. It is more general than similar previously reported techniques for analysing transient chloride profiles to infer recharge rates, and hence has wider application. We have applied the technique to a field situation in southern Australia where clearing of native vegetation for agricultural production leads to large increases in groundwater recharge. The examples serve to demonstrate the technique and some of the practical difficulties in the application of solutes techniques to recharge estimation.

The application of electromagnetic techniques to groundwater recharge investigations

Abstract The use of electrical methods for estimating spatial patterns of groundwater recharge was evaluated at a field site in southeastern Australia. Here, recharge increased from less than 0.2 mm year −1 under native Eucalyptus vegetation, to between 1 and 14 mm year −1 under dryland agriculture. This increase in recharge results in progressive leaching of salts in the soil profile. Differences in recharge can be estimated from differences in depth of leaching. The estimated recharge rates are correlated with soil texture, with higher recharge rates generally occurring through sandier soils. The relationships of recharge to salt content and soil texture both contribute to lower apparent electrical conductivities for higher recharge rates. The effect of recharge rate on measured apparent electrical conductivities was modelled for various geophysical devices (including frequency-domain (FEM) and time-domain (TEM) electromagnetic instruments and direct current resistivity). The soil-texture effect was shown to have a greater effect than the solute leaching effect in determining the correlation between recharge and apparent electrical conductivity. Analysis of sensitivity to geological noise showed that variations in soil type below 2 m could disguise any correlation. Correlations between recharge rate, measured at core sites from chloride tracer techniques, and apparent electrical conductivity, measured with FEM electromagnetic devices, supported the conclusions of the model. For DC resistivity and TEM methods, correlations between recharge and apparent electrical conductivity were not significant, although for resistivity this may be due partly to the small number of measurements made. The FEM device most sensitive to variations in recharge had an operating frequency of 9.8 kHz. At lower frequencies the sensitivity is reduced, as the instruments are sensing too deeply. The poor correlations for TEM, as compared with FEM, are due probably to the relatively deeper penetration of the TEM instrument used in the study, rather than any inherent differences between the techniques. Because the major reason for the correlation between recharge and apparent electrical conductivity is soil texture, in this area the geophysical devices are mostly mapping soil type.

A helicopter-borne electromagnetic survey to delineate groundwater recharge rates

Groundwater recharge is one of the most difficult components of the water balance to measure. For this reason, electromagnetic methods have been used to infer its variability from measurements of apparent electrical conductivity. In this study, groundwater recharge was estimated at 20 sites using unsaturated zone chloride methods. Interpolation between drill sites was accomplished with the aid of a helicopter-borne electromagnetic survey (DIGHEMIV). Correlations between recharge and apparent electrical conductivity were only significant (R2 = 65%) at the highest frequency (56,000 Hz). Using these single-frequency data, variations in recharge were mapped over an area of 32 km2. Recharge, as inferred from the electromagnetic data, appears to be lognormally distributed, and varies from less than 1 to more than 50 mm yr−1. Within the study region, spatially averaged recharge can be estimated from the electromagnetic data, with an accuracy of approximately −60%, +140% (90% probability). This is comparable to the estimation accuracy when surface electromagnetic methods are used. Aerial electromagnetic methods appear very useful for identifying areas of high and low recharge over large regions.

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 importance of considering diffusion when using carbon-14 to estimate groundwater recharge to an unconfined aquifer

A number of studies have used 14C to estimate ages of unconfined ground water and to infer recharge rates. Some of these studies have been in arid or semi-arid regions, and the inferred recharge rates have been very low. The implicit assumption has been that the vertical movement of 14C within the unconfined ground water is entirely by convective processes. In this paper we extend the ground water 14C model of previous workers to include diffusion within the aquifer, and show that for recharge rates less than approximately 1 mm year−1, diffusion can be important in defining the 14C distribution. For this reason it is not possible to use 14C profiles to estimate recharge in these cases. We also show how neglecting diffusion can lead to serious underestimates of groundwater ages in unconfined aquifers where recharge rates are similarly low.