ENVIRONMENTAL ISOTOPE HYDROLOGY OF ... - Science Direct

Journal of Hydrology, 94 (1987) 89-107

89

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

E N V I R O N M E N T A L ISOTOPE HYDROLOGY OF SALINIZED E X P E R I M E N T A L CATCHMENTS

J.V. TURNER, A. ARAD* and C.D. JOHNSTON

Division of Groundwater Research, CSIRO, Private Bag, P.O. Wembley, W.A. 6014 (Australia) *Geological Survey of Israel, 30 Malkhe Yizrael Str., 95501, Jerusalem (Israel) (Accepted for publication February 25, 1987)

ABSTRACT Turner, J.V., Arad, A. and Johnston, C.D., 1987. Environmental isotope hydrology of salinized experimental catchments. In: A.J. Peck and D.R. Williamson (Editors), Hydrology and Salinity in the Collie River Basin, Western Australia. J. Hydrol., 94: 89-107. 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 65{~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 5180 and 52H values of groundwaters was used in association with chloride concentrations to provide information on mixing characteristics of groundwaters within the catchments.

INTRODUCTION

Previous research on water and solute storage and transport in the deeply weathered catchments of southwest Western Australia has documented many cases of significant solute accumulation in the unsaturated zone (Dimmock et al., 1974; Johnston and McArthur, 1981; Peck et al., 1981; Johnston, 1983). Clearing of native vegetation and subsequent mobilization of the accumulated solute has led to serious secondary salinization of agricultural land. While

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there is a general understanding of the major hydrologic processes leading to this type of salinization, detailed understanding of the mechanism of solute concentration and water and solute transport in these environments is lacking. A recent development has been the recognition that water table rise caused by enhanced recharge following clearing causes stored solute in the unsaturated zone to be mobilized by lateral groundwater flow rather than vertical movement (Johnston, 1986). Environmental isotope techniques have not been applied to study the mechanisms of solute accumulation or water and solute transport in the salinized agricultural regions of southwest Western Australia. This paper describes the environmental isotope hydrology of three salinized experimental catchments. The broad objective was to assess the value of these techniques in contributing to a detailed understanding of the hydrological processes occurring. More specifically the objectives were to determine the importance of evaporation as a mechanism of solute concentration and to compare the environmental isotope hydrology of two catchments, Ernies (E) and Salmon (S), that have not been cleared of native eucalypt forest with that of a cleared catchment at an advanced stage in the development of secondary salinization, Maringee Farm (M.F.). DESCRIPTION OF THE STUDY AREAS

The locations of the three catchments studied are shown in Fig. 1. The locations of bores and core samples taken within the catchments are shown in

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Fig. 1. Location map of Ernies (E), Salmon (S) and Maringee Farm (M.F.) catchments.

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Fig. 2. Location of bores and core samples within the catchments. Arrows indicate inferred direction of groundwater flow. Fig. 2. A detailed description of the geology, mineralogy, soils and vegetation of E and S catchments has been given by Bettenay et al. (1980) and a hydrogeological description of M.F. has been given by Martin (1984). The geology of the region consists of a Precambrian basement which at E and M.F. consists of even grained granites and dolerites and at S consists of interlayered gneisses and amphibolites. E and M.F. are typical of the eastern region in having a valley floor of low gradient and a deeply weathered but well-preserved regolith. By contrast, S is of moderate gradient and has been deeply incised by stream drainage. The basement rocks of the catchments have been deeply weathered resulting in lateritic profiles of up to 30 m thickness. These typically comprise vertical sequences of superficial deposits, duricrust, a ferruginous mottled zone and a pallid zone comprising predominantly kaolinite and quartz, which tends

92 to a partially weathered zone at depth. E and S have mean annual rainfalls of 820 and 1220 mm yr l, respectively. The permanent water table is at a depth of about 22m at E and at approximately 12 15m at S. Both catchments have ephemeral perched water tables which form during winter. The third catchment, M.F. is in a 600-700mmyr 1 rainfall region and was cleared of native vegetation in the mid 1960s. Before clearing, M.F. would be expected to have had a permanent water table at a depth of 1 ~ 2 0 m and significant solute accumulation in the unsaturated zone between 5 and 13m. In this respect, E may be regarded as representative of the situation at M.F. before clearing. Increased recharge since clearing at M.F. has resulted in mobilization of solute by its incorporation into horizontal groundwater flow as the water table has risen. This catchment is presently in an advanced state of secondary salinization and displays positive groundwater potentials, large areas of saline groundwater seepage and halite encrustation at the ground surface in groundwater discharge areas for 30 m on both sides of the stream. METHODS

Drilling and coring Core materials were obtained by hollow stem auger. Sections of core material from E and S were sealed in double polyethylene bags, placed in steel containers and stored at 4°C. Cores from M.F. were obtained with a split-core barrel lined with a t r a ns par e nt Mylar ® insert. This allowed recovery of 75 cm lengths of intact core material within a protective t ransparent sheath. These cores were frozen immediately after recovery from the field and stored at 10°C.

Chemical and isotopic analysis Unsaturated zone solutions were extracted from core materials for chemical analysis either by immiscible displacement with freon (Whelan and Barrow, 1980) or with deionised water in a 1:5 soil-water ratio extract. The chloride concentration in the original unsaturated zone solution was determined by back calculation. Chloride determinations from both these methods of extraction are shown in Fig. 4. Extraction of water from unsaturated-zone core materials for 5180, ~ H and tritium analysis from E and S was carried out by azeotropic distillation with toluene. The 52H analyses on water from the M.F. core materials shown in Fig. 10, were carried out using a single-step microdistillation method (Turner and Gailitis, in prep.). Deuterium analyses were carried out by reduction of the water to hydrogen over zinc at 450°C following standard methods (Coleman et al., 1982). Oxygen-18 analyses were carried out by either the standard EpsteinMayeda carbon dioxide equilibration method (Roether, 1970) or, for very small

93 w a t e r samples, by r e a c t i o n with g u a n i d i n e h y d r o c h l o r i d e , using a m e t h o d modified after D u g a n et al. (1985). Stable isotope ratios were d e t e r m i n e d using a VG SIRA 9 mass spectrometer. Results are r e p o r t e d in s t a n d a r d 5 n o t a t i o n r e l a t i v e to V-SMOW w h e r e 5sample.SMOW ~--- (R~,mple/RsMow - 1) × 1000%o and R is the r a t i o lsO/160 or 2H/1H. T r i t i u m c o n c e n t r a t i o n s were d e t e r m i n e d on u n e n r i c h e d w a t e r samples by liquid scintillation c o u n t i n g with v e r y low background. Results are r e p o r t e d in t r i t i u m units (TU). RESULTS AND DISCUSSION

Stable isotopic composition of rainfall Rainfall samples were collected at the sites of coring (water b a l a n c e site 2 (WBS 2) at E and w a t e r b a l a n c e site 3 (WBS 3) at S) over a period of 19 months. F i g u r e 3 shows the 3180 and 52H of rainfall from both catchments. The a m o u n t weighted 5 values for E rainfall were 5180 = - 4.72%0 and 52H = - 19.3%0 and for S rainfall 5180 = - 4.72%0 and 52H = - 19.5%0. Thus t h e r e was no signific a n t difference in the a m o u n t - w e i g h t e d m e a n isotopic composition of the rainfall from e i t h e r c a t c h m e n t . S e p a r a t e rainfall collections for isotopic analysis were not made at M.F. and the rainfall values from S and E t a k e n as r e p r e s e n t a t i v e of rainfall at M.F. The 51sO-52H regression for rainfall has a slope of 6.3 and an i n t e r c e p t of 9.8 which establishes the local m e t e o r i c w a t e r line (MWL) for the region. The local M W L is an i m p o r t a n t r e f e r e n c e for s u b s e q u e n t m e a s u r e m e n t s on u n s a t u r a t e d and saturated-zone waters. As can be seen in Fig. 3, the rainfall compositions were variable and some c o n s e c u t i v e rainfall events at S differed by up to 6%0 in 5180 and 38%0 in 52H. T r i t i u m c o n c e n t r a t i o n s in rainfall h a v e declined to t h e i r prebomb level of approximately 8 TU. 10 ~

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95

Chloride, ~lso, 52H and tritium in E catchment A total of four cores were obtained at E (WBS 2). Core E2 was taken in December 1982 and cores 8253, 8254 and 8255 in December 1983. The location of these cores is shown in the inset to Fig. 2. The chloride concentrations of unsaturated zone solutions are shown in Fig. 4. Analysis of these types of chloride profile in terms of water and solute transport have been given by Johnston (1983). The stable isotope and tritium analyses of unsaturated zone water were made to determine whether the mechanism of solute concentration was evaporation from the soil surface and whether this caused subsequent vertical upward movement. Alternatively, the uptake of water by tree roots and subsequent transpiration of unsaturated-zone water could be responsible for solute concentration. The former process would be expected to impart a significant isotopic evaporation signature to the residual water in the unsaturated zone, and could be identified by that water being isotopically enriched and falling to the right-hand side of the local MWL in 5'sO-5~H space. Uptake of water by tree roots and its subsequent transpiration, however was not expected to result in any change to the isotopic composition of the residual unsaturated-zone water. (White et al., 1985; Zimmerman et al., 1967). The profiles of 51~O and 52H from E (WBS 2) are shown in Fig. 5. They are remarkably uniform from approximately 2 m down, and have an isotopic composition within the range of the amount-weighted mean in precipitation. A common feature of the isotope profiles is the marked isotopic enrichment at the ground surface caused by evaporation. Such surface enrichment due to evaporation has been observed and predicted by Barnes and Allison (1984). A second common and distinctive feature of the isotope profiles is the depletion at depths between 0.5 and 2 m. This is observed in each profile independently of its location or the time of sampling. Such surface minima in stable isotopic composition are a common characteristic of profiles taken in quite different field environments (Bath et al., 1983; Allison et al., 1983). Barnes and Allison (1984) have predicted that a slight depletion in isotopic composition would occur during nonisothermal evaporation from soil surfaces which fit the evaporative conditions at this site. However, the magnitude of the predicted effect is small and may not be large enough to explain the observed extent of depletion. An alternative explanation is that water stored in the ephemeral perched aquifer at this site is depleted because of selective infiltration of the more intense rainfall events which tend to be isotopically depleted. Significantly, there was no variation in the isotopic composition through the peak in chloride concentration over the depth range 3-16 m. This showed that evaporation from the ground surface played only a minor role as a mechanism of solute concentration and further, that it was restricted to being a surficial process. It must be concluded then that the mechanism of solute concentration in the unsaturated zone was ion exclusion during uptake of water by tree roots. A plot of the unsaturated zone waters in 51sO-~2H space is shown in Fig. l l a .

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