Jeffrey V. Turner

The mechanisms of catchment flow processes using natural variations in deuterium and oxygen-18

Abstract Stable isotope compositions and chloride concentrations in rainfall, deep and shallow groundwaters and streamflow have been used to investigate the flow processes through a small catchment in southwest Western Australia. In addition to the stable isotope and chloride data, daily rainfall, streamflow rate, streamflow chloride concentrations and shallow and deep groundwater hydrographs were used in an integrated approach to determine the mechanisms of streamflow generation. The range in δ18O and δ2H of rainfall events incident to the catchment during 1985 was 9.0 and 60‰, respectively, and was large enough to be useful in tracing flows generated from them through the catchment. During the season, four separate rainfall events were analysed using stream hydrograph separation methods based on δ18O, δ2H and chloride. Results showed that between 60 and 95% of the streamflow generated from the respective events had originated from pre-event shallow groundwater within the catchment. The seasonal pattern in the streamflow isotopic composition during 1985 was also analysed. Most significant rainfall events during the season generated an almost immediate response in the streamflow isotopic composition. The response to individual rainfall events was strongest early in the season but diminished as the season progressed. This attenuation with time indicated isotopic mixing and dilution of individual rainfall events with the increasing storage of shallow groundwater as the season progressed. Kalman filtering methods were used to investigate the function linking rainfall volume and streamflow and also the average time lag between rain and flow, using δ values in rainfall as time markers. The analysis showed a range of “ages” for the streamflow from about 20 to about 50 days lag after rainfall, tending to decrease after rainfall events and then increase again. The estimated value of the lag was always significantly greater than zero. Deep and shallow groundwaters in the 1985 season were readily distinguished from each other in δ18O-δ2H space. Streamflow isotopic composition matched that of the shallow groundwater system for almost the entire season. Thus the isotopic evidence was in accord with chemical evidence that streamflow originated primarily from preceding rainfall after a comparatively short residence time in the shallow groundwater system.

Environmental isotope hydrology of salinized experimental catchments

Deuterium, oxygen-18, tritium and chloride concentrations were used in three salinized experimental catchments to gain insight into the mechanism of solute concentration and flow processes in the saturated and unsaturated zones. The three experimental catchments were studied because of their location in different rainfall regions, their status with respect to clearing of native vegetation and with respect to secondary salinization. Two uncleared catchments have average annual rainfalls of approximately 820 and 1220 mm, respectively. The third cleared catchment has an annual rainfall of 650–750 mm. This catchment was in an advanced state of secondary salinization and displayed large areas of saline groundwater discharge with halite encrustation at the ground surface. The stable isotope compositions of the solution phase in solute bulge profiles in the unsaturated zone showed a close agreement with the amount-weighted mean isotopic composition of rainfall and only surficial evidence of isotopic enrichment due to evaporation. Evaporation from the soil surface plays a minor role as a mechanism of solute concentration in the unsaturated zone. The dominant process of solute concentration in the unsaturated zone was ion exclusion during uptake of water by tree roots which was evidently a solute but not isotope fractionating process. Tritium analyses of unsaturated zone water and grondwater indicated movement of recent recharge to 7–10 m depth at the low rainfall site but over the full depth of the 15 m unsaturated zone at the higher rainfall site. The variability in δ18O and δ2H values of groundwaters was used in association with chloride concentrations to provide information on mixing characteristics of groundwaters within the catchments.

The water balance of a small lake using stable isotopes and tritium

Abstract A small maar lake, known as the Blue Lake, set in a karstic region of southeastern Australia, provides the municipal water supply for a population of ∼ 20,000. The lake has a volume of 3.6·10 7 m 3 , of which 10–15% is pumped from it each year. The lake is recharged almost entirely from groundwater and the main objective of this study was to estimate the total groundwater inflow and outflow rates. Estimation of groundwater throughflow in a lake is difficult to assess using classical hydrological techniques and an alternative method involving measurement of the concentrations of the environmental isotopes 3 H, 18 O, 2 H and 14 C in the lake water and in the recharging groundwater was used to establish the lake-water balance. The water-balance calculations indicated a total groundwater inflow to Blue Lake of between 5.0·10 6 and 6.5·10 6 m 3 yr. −1 , corresponding to a residence time of water in the lake of ∼ 6 yr. It was not possible to derive the relative proportions of inflow to the lake from the two possible source aquifers, using these isotopes, because their concentrations did not show a sufficiently large contrast to distinguish the two water sources.

Assessment methodologies for submarine groundwater discharge

Submarine groundwater discharge (SGD) in the coastal zone is recognized as a potentially significant material pathway from the land to the ocean. This chapter provides an overview on several methodologies used to estimate SGDs. Measurements of SGD using “manual seepage meters” show that consistent and reliable results can be obtained if one is aware of and careful to prevent known artifacts. New “automated seepage meters” help understand the hydrological and coastal oceanographic processes with longer-term and higher-resolution measurements. Direct measurements of SGD by seepage meters and piezometers in local areas may be scaled up to a regional basis by use of natural geochemical and geophysical tracers. Water balance estimates, although useful for rough estimates, are usually not very precise because the uncertainties in the various terms used to construct the balance are often on the same order as the groundwater discharge being evaluated. Estimates of SGD via analytical and numerical methods depend mainly on the evaluations of the thickness of the aquifers and representative hydraulic conductivities, of which well-constrained values are usually difficult to obtain.

Observation of water and solute movement in a saline, bare soil, groundwater seepage area, Western Australia. Part 1: Movement of water in near-surface soils in summer

In order to elucidate the relationship between evaporation, salinisation, and annual water and salt balances in semi-arid and arid regions, hydrological and meteorological observations were undertaken over 3 years in a small, salinised, bare-soil, groundwater seepage area in Western Australia. This paper focuses on water behaviour near a bare saline soil surface during the dry summer. Analysis of observed data on soil vapour density using a vapour diffusion transfer model can account for the daily upward vapour flux from the soil surface that occurs in midsummer. The dry soil undergoes cycles of drying during the day, accompanied by salt crust formation and wetting during the night. In late summer, the same zones show a wetting trend owing to a marked atmospheric vapour invasion and condensation at night regardless of evaporation during daytime. The daily average vapour flux at the ground surface in mid- and late-summer, respectively, estimated through the vapour transfer model in the dry soil layer was ~0.35 and 0.03 mm/day. Comparison of vapour fluxes at the ground surface measured with a portable surface evaporimeter with modelled estimates of vapour transport in soil showed agreement of the proposed model to field results at low wind speed, but not at the higher wind speeds. This identifies the active role of turbulent surface wind speed on vapour transfer in the dry soil layer below the ground surface.

Observation of water and solute movement in a saline, bare soil, groundwater seepage area, Western Australia. Part 2. Annual water and solute balances

Intensive hydrological and meteorological observations were used over 2 years at a small, salinised, bare soil, groundwater seepage area in Western Australia to determine annual water and solute balances and the processes mobilising and discharging saline groundwater. The study area of 40 m by 40 m comprised a two-layered (duplex) soil, with an upper permeable layer 0.3–0.4 m thick and a less permeable underlying layer. During the wet winter season, groundwater discharge from the study area corresponded to ~45% of the rainfall amount. The annual salt discharge from the area via groundwater flow was calculated to be ~2.1 kg/m2 as NaCl, which was almost the same as the salt mass stored in the upper layer in summer. A salt-discharge model was developed and showed that even though the bare soil, groundwater seepage area occupies only a few per cent of the total catchment area, the salt discharge from it was a large component of that from the total area. During the dry summer season, lateral groundwater flow did not occur. Rainfall that recharged and became stored water in the previous winter became the source of evaporation in the following summer, with 60–70% of the evaporated water in summer derived from stored water of the previous winter.