Journal of Hydrology, 81 (1985) 93--109 93 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

[5]

G R O U N D W A T E R R E C H A R G E E S T I M A T I O N U S I N G C H L O R I D E , D E U T E R I U M A N D O X Y G E N - 1 8 P R O F I L E S IN T H E D E E P C O A S T A L

S A N D S O F W E S T E R N A U S T R A L I A

M.L. SHARMA and M.W. HUGHES

CSIRO Division of Groundwater research, Private Bag, P.O., Wembley, W.A. 6014 (Australia) CSIRO Division of Soils, Glen Osmond, S.A. 6056 (Australia)

(Received April 18, 1985; accepted for publication May 6, 1985)

ABSTRACT

Sharma, M.L. and Hughes, M.W., 1985. Groundwater recharge estimation using chloride, deuterium and oxygen-18 profiles in the deep coastal sands of Western Australia. J. Hydrol., 81: 93--109.

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 ground- water. 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 C1- concentration of groundwater was relatively uniform~ there was a considerable spatial variability in the depth distribution of C1- in the unsaturated zone. An approximate method of data analysis is developed. Application of this to C1- 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 distri- bution 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.

INTRODUCTION

E s t i m a t e s o f ra tes o f g r o u n d w a t e r recharge are basic t o m a n y issues such as d e v e l o p m e n t , m a i n t e n a n c e a n d u t i l i z a t i o n o f g r o u n d w a t e r r e sou rces , a n d a s s e s s m e n t a n d p r e v e n t i o n o f t he i r excess ive c o n t a m i n a t i o n . H o w e v e r , t ech- n i q u e s fo r e s t i m a t i n g loca l i sed or large-scale r echarge ra tes are n o t r ead i ly ava i lab le . A large g r o u n d w a t e r reservoi r , in the s a n d y aqu i f e r s o f t he S w a n Coas ta l P l a in o f W e s t e r n Aus t r a l i a , has s u p p l i e d in t he r e c e n t yea r s up to

94

42% of the water requirements for the capital, Perth (Pollett, 1981); and this reliance is likely to increase in the future. The total recharge, which occurs primarily from the natural precipitation, is estimated to be 350 × 106 m 3 yr -1 , amounting to about 11% of the annual rainfall (Allen, 1981}. However, there is a considerable variation in recharge rates due to changing land uses and topographical locations (Sharma et al., 1983). Because of spatial varia- bility of soil and vegetation parameters, the variation in recharge rates within an area of a plant community may also be large. Furthermore, the mechanisms of recharge are not adequately understood.

In an endeavour to develop and test the adequacy of techniques for estimating recharge soft samples were collected down the deep ( ~ 2 0 m) sandy profiles, and water extracts from these samples were analysed for three environmental tracers, Cl-, deuterium and oxygen-18. The objective was to interpret these profiles hydrologically, thereby estimating recharge rates to the groundwater.

SITE DESCRIPTION AND METHODS

Environmental details

The city of Perth (Western Australia) is located about midway on the Swan Coastal Plain which extends several hundred km in length but only a short distance (6--35 km) inland from the coast. The groundwater reservoir referred to earlier is located north of the Swan River (Fig. 1), and has an area of 2200 km 2 and stores some 20 × 109 m 3 of water (Allen, 1981) of acceptable quality ( < 5 0 0 m g l -~ , TDS). The aquifer is relatively shallow (~ 90 m) and primarily unconfined.

The area is underlain by coastal dunes originating from successive aeolian depositions. The sands are highly conductive, with a saturated hydraulic conductivity of the order of 30m per day. The approximate particle composition of these sands is 85--95% coarse sand, 5--10% fine sand with some fractions of silt and clay. This gives rise to their low water- holding capacity, which at 10 and 1500 kPa is of the order of 0.065 and 0.02 m 3 m -3 respectively. There is no large-scale surface runoff from the area. Occasionally localised runoff may occur due to water repellency of sands. The groundwater table forms a mound reaching a maximum elevation of 70 m above the sea level. The annual amplitude of change in the elevation of the water table varies from 0.2 to 4.5 m depending on soil, vegetation and topographical factors.

The region has a Mediterranean climate with distinct summer and winter seasons. The annual rainfall is of the order of 800 mm yr -1 , more than 80% of this falling during the winter period (May to October} and is the primary source of groundwater recharge. Potential evaporation on a yearly basis far exceeds the annual rainfall; Class-A pan evaporation being in order of 1800 mm yr -1 .

Ca) 95

7C N

] C l e a r e d rural I~

D Native woodla

~ State forest pil

] U r b a n land

700mm R~

S( 0 5 I I

Cb) • 1 . 4

Banksia Woodland

o2 ° 5

, 3

50m L

Fig. 1. (a) Locality of the groundwater reservoir of the North Swan Coastal Plain, Western Australia, showing rainfall isohyets, current land use and the location of the experimental area. (b) The sampling plan for five profiles.

96

Because of the proximity of this area to the west coast, substantial chloride is deposited with the rain (Hingston and Gailitis, 1976). This chloride has been used as a tracer for water movement through the infertile sands which do not have any chloride of pedogenic origin (Allen, 1981; Sharma et al., 1983).

The native vegetation, consisting primarily of Banksia woodland, occupies about 30% of the land area overlying the water reservoir (Fig. 1). Over the years, the native vegetation has been progressively replaced by pine plan- tations and agriculture. This change in land use has affected the hydrological equilibrium, and the effect on the groundwater recharge is a concern to the Water Authori ty.

The study site

The study site is located in an extensive Banksia woodland communi ty (Fig. 1). The composition and density of various components of the vegetation communi ty are typical for the area. The sampling site is sur- rounded by similar vegetation for at least i km on all sides. As far as it could be ascertained, the native vegetation has been in its current state for over a century. The soil profiles consist of relatively uniform Bassendean sand, typical to the area (McArthur and Bettenay, 1960). Occasionally clay bands appear at depths below 10 m.

Sampling and analysis

Soil samples down the profile, from the surface to the groundwater table, were collected from five locations (Fig. lb) with one borehole per site. There was no marked change in soil and vegetation types except that the under- storey appears to be more dense at sites 1 and 4 than at sites 2, 5 and 3. The average topographical gradient was of the order of 5%. Cores of 4 8 m m diameter were taken mechanically. Profiles 1, 2 and 3 were drilled on October 4, 1982 after the winter rains and cores 4 and 5 were drilled on January 12, 1983 during mid-summer. Profiles 1--3 were 50 m apart, and 4 and 5 were drilled within 20 m of cores 1 and 2, respectively (Fig. lb) . The cores were sectioned into 250mm lengths and the sample was mixed thoroughly. A subsample was taken for water content and chloride determination and the remainder sealed in a screw-top glass jar for isotope analysis.

Gravimetric water contents were determined by drying the sample at 105°C. The dried soil was mixed with distilled water at a 2 :1 ratio and centrifuged for 90min. Chloride concentration was determined colori- metrically using mercuric thiocyanite and ferric ammonium sulphate (Dabin and Brion, 1966). Whenever necessary, organic colouring of the extract was removed by activated charcoal.

Water for isotope analysis was extracted from the bulk of the sample by

97

refluxing with toluene. Traces of toluene were removed from the water by warming with paraffin. Deuterium content was measured by evaporating 25 pl of the water sample over heated uranium metal in a circulating system and measuring deuter ium/hydrogen ratio in a mass spectrometer. Oxygen-18 was determined by the Epstein-Meyeda technique as described by Taylor (1973). Deuterium and oxygen-18 contents are expressed as 5-values, the deviations in parts per mille from the International Standard V-SMOW.

RESULTS AND DISCUSSION -- CHLORIDE PROFILES

Variation o f chloride and water content

Chloride concentrat ion.of soil water (C) and the corresponding volumetric soil water content (0) as a function of depth (z) for the five profiles are presented in Fig. 2. Approximate location and average {over 2 m) chloride concentration of the groundwater (C~) for each of the profiles are also shown. There is a considerable difference both in the shape as well as in the magnitude of C(z) for the five profiles, and therefore only general inter- pretations can be made. There is a rather small change in C with depth down to some 6- -10m, then C rises and approaches a rather large value which remains relatively high down to considerable depth (~ 18 m). With the depth approaching the groundwater table, C declines and approaches CG, which is usually much lower than the value of C in the bulk of the profile. An occasional sharp increase in C for short depth intervals in some profiles was associated with the presence of thin clay bands. Three of the profiles (1, 2, 3) were sampled in October (soon after the main recharge period), while the other two profiles (4, 5) were sampled in January (towards the middle of the summer period). This explains the higher C values in the surface layer (0--3 m) for the latter group of profiles. This seasonal effect is also apparent in the shape of soft-water profiles.

From the soft-water profiles measured by a neutron probe in this area (M.L. Sharma, unpubl, data), it appears that water extraction by vegetation is limited to the top 10m. Variations of water content with depth in the lower part of the profile (below 10m) are related to variations in soil properties. Supporting data on root distribution confirmed that the major roots are indeed confined primarily to the top 5 m or so, therefore it would be reasonable to assume that water flux below 10 m is drainage below the root zone.

Computation o f recharge

For a vegetated land area where surface runoff is negligible, precipitation is the only source of water input and water movement is predominantly

98

0 0

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I t'. %1 I|

10 ; :

t.< 7" 2O

g

0 0.1 0 s",

t eL.

I

10 --~ "l

2O

Chloride concentration of sod water (mg/-1 )

500 0 500 0 500 0 500 0 500 I .I i" I I I I I I I I

2

|

. $'1o

"ll~ • • ~°"

- ° ~°"

l.i I I I

'i: 4 5

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• #;

_

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°° I °. • $ °

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1l Volumetric water content of soil (m 3 m 3 )

I

I, _c< , • ° " i $ s:" ,I

¢ . e

• ° _ _ • /.. IQ

0.2 0 0.1 0.2 0 0.1 0.2 0 0.1 0.2 0 0.1 0.2 I ." I I I I I

Ii Fig. 2. Depth distribution of chloride concentrat ion and volumetric water content for the five profiles beneath Banksia woodland in Western Australia.

one-dimensional, conservation of water can be expressed as:

P = E + R - t - A W (1)

where P is the precipitation, E is the evapotranspiration, AW is change in soil-water storage within the rooting zone and R is the drainage (recharge) below the maximum rooting depth z, all expressed as mm yr -1 . On a yearly basis AW can be assumed negligible• Considering mass balance of the environ- mental chloride for our sandy profiles, it can be justifiably assumed that there is no significant adsorption, exchange, t ransformation or expor t of chloride. Accepting pedogenic origin of chloride being negligible, it can be shown that:

PCp = Cz + A S (2)

99

where Cp and Cz are chloride concentrations in the precipitation and drainage water respectively, and AS is the change in chloride storage within the root zone. For deep sandy profiles, under relatively undisturbed soil-- vegetation system, steady state can be assumed and AS will be negligible. If long-term average values of Cp and P (i.e. Cp, P) are known, then the long- term average recharge rate (/~) can be estimated from:

[t = C pP Cz (3)

where Cz is the average chloride concentration of water draining below the root zone. It should be noted that here Cz is estimated as the depth-average chloride concentration in the unsaturated zone below the active root zone, and as such represents a long-term average concentration of the drainage water.

Since there is usually an uncertainty in defining the exact rooting depth, and also about the assumption as to whether a steady state has been reached, Sharma et al. (1983) suggested that Cz could be estimated by several different methods. Here we estimated Cz by three different methods, i.e.: (1) C1 (mean of C for the profile excluding the first 10m and the last 2 m); (2) C2 (mean C for the 2 m above the last 2 m); and (3) C~ (depth-average C for the groundwater).

Overall means of C1, C:, and CG based on the five profiles (representing areal values), and their corresponding recharge rates calculated by eqn. (3) are presented in Table 1. In these computations, an estimated long-term average value of precipitation (P) of 775 mm yr -1 was adopted, while a value of 12mg1-1 was used for Cp based on the work of Hingston and Gailitis (1976). This atmospheric C1- input amounts to about 9.3 g m - : yr -1 , and it is an estimate of dry fall plus wet fall collected by ralngauges. Thus it may be an under-estimate of the total C1- input, since it does not include possible addition of C1- through canopy impingement.

Although there is considerable variability between profiles (CV ~ 0.32) with respect to the various estimates of Cz in the unsaturated zone (C1, C: ), the areal recharge rates based on the two are not substantially different (Table 1). However, CG, which has very little variation from profile to profile (CV = 0.06), is substantially smaller than Cl or Cz • Measurements of CG as a function of time showed little change (~ 5%) during the year. The average recharge estimate based on C~ of 116 mm yr -1 is almost double that based on Ca or C:. Overall, C1 gives less recharge than C:. Accepting that the water extraction by roots is confined to the top 10 m, C1 should be the most appropriate measure of Cz and it should give an average value of recharge for the location. C: on the other hand, represents the concentration of water about to be added to the aquifer, thus a measure of the rate of most recent recharge.

Equation (3) is for a steady state condition, and it also inherently assumes that the profile is relatively uniform and movement of water is

100

TABLE 1

Various estimates of mean chloride concentration of leachate below the root zone (Cz) for the area under study (based on five profiles), their spatial variability (CV) and areal recharge rates (R) computed by eqn. (3). The profiles were sampled beneath a Banksia woodland in Western Australia

Measure of Cz Chloride concentration of Coefficient soil water (mg1-1 ) (Areal of variation mean based on five profiles) for C:

Areal recharge rates (R)

(mm yr- l ) /~ a s % of rainfall

C_1 195 0.32 51 6.6 146 0.24 67 8.6

C G 80 0.06 116 15.0

predominant ly vertical. For homogeneous profiles variation of C with respect to z should be confined primarily to the root zone, and as such variation below 10 m should be minimal. As can be seen (Fig. 2), there is some variability in C(z) even below 10 m which may be ascribed to vertical nonuni formi ty of the so;1 and seasonal variability of C1- input, rainfall and evapotranspiration. The overall higher value of C in the bulk o f the unsaturated zone below the root zone gives rise to substantially larger ( ~ 2 . 4 times) value of C1 compared with C~. This requires fur ther explanation and possible factors responsible for these differences are: (1) variable spatial scales; (2) variable time scales, including effect of seasonality; (3) variation in soil--vegetation characteristics with time; and (4) presence of preferred pathways for water movement through the profile.

It must be appreciated that the deep profiles of the site store several years o f recharge, thus C1 for each profile represents a localized value of Cz averaged over a number of years, while CG may represent Cz not for the location of the profile but rather an average over a substantially larger area. This is so because the highly conductive nature of the aquifer (Ks = 30 m per day, our unpubl, data) enables relatively large scale movement of ground- water to occur. However, due to the existence of relatively small hydraulic gradients (< 0.1%) over a period of several months (based on measurements in another experiment) , bulk water movement through the aquifer would be confined to a horizontal distance < l k m . Fur thermore , the direction of hydraulic gradients suggests that waterflow is likely to be away from the experimental site. Since the experimental site is surrounded on all sides to at least a distance of i km by similar vegetation, it is unlikely that C~ would be unduly influenced by groundwaters of substantially different chlorinity from the surrounding areas.

It must be realized that C1 and C~ represent mean chloride concentrat ions of leachate over different time scales. Although short-term seasonality effects are likely to be negligible below 10 m, longer-term (over several years) variation in rainfall and chloride input could contr ibute to differences

101 Time (yrs) required to accumulate Chloride

in the profile from precipitation (Average Chloride Fall = 9,3 g m 2yr l )

0 ( 5 10 15 20 25 , I I I I I I I I [

3

6

9

12

15

18

21

24 \ . \

27

30

Fig. 3. Depth distribution of cumulative chloride storage for the five profiles presented in Fig. 2.

between C1 and CG, since CG is likely to be much more damped against long-term variations of rainfall, chloride and weather. An examination of climatic records shows that there has not been any drastic changes in the climate for the last 50 yrs or so. The observed variation in CG over a period of 2 yrs was < 5%, and the vertical and horizontal variation in C~ was also found to be negligible. Under such conditions the overall mean value of C1, based on five profiles, should correspond closely to C~ provided water movement through the profile is uniform and there are no sources or sinks of solute in the profile as required for eqn. (3).

The possibility of changing soil--vegetation characteristics with time causing changes in evaportanspiration and hence in recharge rates is not very high either. As far as it could be ascertained, the soil and vegetation characteristics for the area have been stable over the last 100 yrs or so. Thus the observed large differences between C1 and CG can not be explained on the basis of seasonality or soil--vegetation modifications, particularly when the solute storage in the profiles can exceed 20 yrs of rainfall input (Fig. 3).

Figure 3 highlights the magnitude of spatial variability in the chloride storage within a relatively small area. Taking chloride input to the system as 9.3 gm -2 yr -1 and assuming that the water movement occurs uniformly through the soil matrix, the chloride storage in the 10--20 m depth interval for each of the five profiles amounts to an equivalent of some 7.8, 13.8, 15.8, 8.0, and 13.0 yrs of the chloride input from rain. On the other hand, if

102

we assume that CG (= 8 0 m g l - ' ) represents the C1- concentration of areal leachate, the 10 - -20m depth interval should only have a chloride storage equivalent to some 4 .3yrs of rainfall chloride input. One possible explanation for these differences may be the presence of preferred pathways for water movement through the profile. If so, some portion of rain water may pass directly to the groundwater with minimal interaction with the soil matrix, as such providing a bimodal system for groundwater recharge. Accepting this hypothesis we propose a simple model for partitioning the total recharge into two components.

An analysis for bimodal system

It is assumed that water moves through the soil matrix as well as through preferred pathways, and the conservation of mass of water and solutes is preserved. It is also assumed that the instantaneous water flux through preferred pathways (qp) is much larger than that through the soil matrix (qm). Thus water associated with qp lasts only for short periods of time and is not available to be taken up by the plant roots. It is further assumed that water and solutes passing through preferred pathways interact negligibly with the soil matrix. As such, the average chloride concentration associated with qp can be approximated by Cp. Water flux through the matrix is subjected to plant uptake, and therefore its solute concentration increases in proport ion to evapotranspiration. Thus the solute concentration of leachate in the unsaturated zone below the rooting depth would be that of the draining water which passes through the soil matrix. Over a long period of time (> 1 yr) under undisturbed land-use conditions, a quasi-steady state can be justifiably assumed for the soil matrix in the profile below the root zone.

Using these concepts, and through conservation of mass, the total average recharge rate (/~) computed by eqn. (3) can be parti t ioned into two com- ponents as follows:

~- Rp -{- /1~ m (4)

while:

R C G = R p C p -~- R m Cz ( 5 )

Here /~, and Rm are average recharge rates throug_h the preferred path- ways and the soil matrix, respectively. Provided CG, Cp and Cz are known, /~p and/~m can be computed as follows:

= = (7)

103

TABLE 2

Summary of recharge rate (mmyr -l ) computed by eqn. (3) based on CG and its com- ponents computed by eqns. (6) and (7) with C1 as estimator of Cz for the five profiles sampled beneath Banksia woodland in Western Australia

Profiles

1 2 3 4 5 Mean CV

C1 Rm 53 37 28 57 60 47 0.29 Rp 60 92 86 57 52 69 0.26

CG R 113(14.6) a 129 (16.6) 114 (14.8) 114 (14.8) 112 (14.5) 116 (15.0) 0.06

aValues in parenthesis represent recharge as a percentage of average annual precipitation.

In these derivations it is assumed that the two components of water (i.e. those through preferred pathways and matrix), are thoroughly mixed follow- ing recharge of the aquifer. Recharge components Rm and Rp, computed by eqns. (6) and (7) for the five profiles, are summarized in Table 2.

Although our analysis is approximate, it nevertheless illustrates an important aspect, i.e., that water movement through the profile does not occur uniformly through the soil matrix. Considerably larger C1- concen- tration of leachate in the deep profiles below the root zone in comparison with CG suggests injection of some water of low chlorinity directly to or close to the phreatic surface. There is a large variability in the proport ion of Rp and Rm for the five profiles (Table 2). Nevertheless it appears that approximately 50% of the recharging water has to pass through preferred pathways to explain the observed chloride distributions in the profiles. Using these computat ions, it can be shown that on an average, the 10--20 m depth interval of these profiles stores some 11 yrs of recharge occurring through the soil matrix. These results may be somewhat surprising, particularly for relatively "uni form" looking sands which do not seem to have any obvious structures. The contr ibution of preferred pathways (macropores) to water flow in the field soils is being increasingly realized (Baven and German, 1982; German et al., 1984}. Recent experimental evidence (M.L. Sharma, 1984, unpubl, data) suggests that ~ 0.1% of the cross-sectional area of the soil surface needs to be occupied by effective water-conducting macropores, in order to increase the water flux density through the soil by more than an order of magnitude. From the analysis of lateritic duplex profiles using somewhat different models, Peck et al. (1981) and Johnston (1983) con- cluded that in those profiles water flux through the matrix ranged from being negligible (0.4 mm yr -1 ) to small (4.5 mm yr -1 ), while the estimated total recharge rates for the profiles were respectively 3.5 and 150 mm yr -~ . Thus the flux through preferred pathways in those profiles was several times more than through the matrix. Subsequently, the presence of such preferred

104

TABLE 3

Average monthly rainfall (P, mm) for Perth, Western Australia, and their weighted mean concentration of deuterium (6D), oxygen-18 (Sls) and displacement ( d ) a n d their respective standard deviations (a) (from IAEA, 1981)

Month P 5 D (Y n ~ is o n d (~ n

Jan 7 - 10.1 25.3 7 1.45 3.75 8 - 1.09 12.80 7 Feb 12 -- 12.4 21.7 6 -- 0.94 3.39 8 0.92 8.35 6 Mar 16 -- 7.9 6.7 7 -- 1.23 1.09 8 0.97 5.69 7 Apr 47 -- 16.3 10.9 8 -- 3.56 1.31 10 12.39 4.57 8 May 113 -- 12.2 6.9 9 -- 3.60 1.20 10 15.29 4.76 9 Jun 172 -- 15.2 9.9 9 -- 4.14 1.11 9 17.93 4.74 9 Jul 163 -- 15.6 5.0 9 - 4.18 0.51 9 17.92 3.84 9 Aug 119 -- 12.8 7.2 9 -- 3.58 0.75 9 15.84 3.76 9 Sep 80 -- 10.1 4.6 9 -- 3.17 0.71 9 15.33 2.35 9 Oct 56 -- 14.7 5.4 9 -- 3.67 1.20 9 14.66 5.26 9 Nov 20 -- 7.9 18.2 9 -- 2.39 2.71 9 11.20 5.20 9 Dec 9 -- 6.4 12.5 8 -- 2.32 2.01 8 12.11 5.63 8

p a t h w a y s f o r a l a t e r i t i c so i l has b e e n d e m o n s t r a t e d e x p e r i m e n t a l l y ( J o h n s t o n

e t a l . , 1 9 8 3 ) .

RESULTS AND DISCUSSION -- STABLE ISOTOPE PROFILES

V a l u e s o f d e u t e r i u m (6D) a n d o x y g e n - 1 8 ( 5 1 s ) in p r e c i p i t a t i o n v a r y w i t h t i m e o v e r a l a rge r a n g e d u e t o c h a n g i n g m e t e o r o l o g i c a l c o n d i t i o n s . P re - c i p i t a t i o n s a m p l e s have n o t b e e n t a k e n f r o m t h e s i t e i n v e s t i g a t e d b u t s a m p l e s o f r a in have b e e n t a k e n a t P e r t h , a p p r o x i m a t e l y 50 k m s o u t h o f t h e s a m p l i n g s i t e s , o v e r a p e r i o d o f 14 y r s a n d a n a l y s e d b y t h e I n t e r n a t i o n a l A t o m i c E n e r g y A g e n c y . T h e s e o b s e r v a t i o n s have b e e n s u m m a r i z e d ( I A E A , 1 9 8 1 ) a n d T a b l e 3 is r e p r o d u c e d f r o m t h e i r d a t a . T h e v e r y h igh s t a n d a r d d e v i a t i o n va lue s f o r e a c h m o n t h , p a r t i c u l a r l y f o r t h e s u m m e r m o n t h s N o v e m b e r t o F e b r u a r y , s h o w t h e d i f f i c u l t y in e s t i m a t i n g a r e l i a b l e i n p u t f u n c t i o n f o r 5D a n d 5 is f r o m such a l i m i t e d d a t a se t . P r e c i p i t a t i o n a m o u n t w e i g h t e d m e a n s f o r t h e s e d a t a are ~D ~---13.4 a n d 51s - - - - 3 . 4 . T h e m e a n va lue s f o r t h e w i n t e r m o n t h s M a y t o A u g u s t a r e - - 1 4 . 2 a n d - - 3 . 8 , r e s p e c t i v e l y a n d t h e s e a r e t h e m o r e r ea l i s t i c v a l u e s f o r t h i s s t u d y as i t is o n l y r a i n f a l l f a l l ing in t h e s e m o n t h s w h i c h c o n t r i b u t e s t o r e c h a r g e .

A s wil l b e s h o w n l a t e r , b o t h s o i l - w a t e r a n d g r o u n d w a t e r s a m p l e s a n a l y s e d in t h i s s t u d y a re i s o t o p i c a l l y l i g h t e r t h a n t h e m e a n va lue s f o r P e r t h r a i n f a l l , d e s p i t e s o m e e n r i c h m e n t d u e t o e v a p o r a t i o n . T h e r e a r e t w o p o s s i b l e r e a s o n s f o r th i s .

(1 ) A n " a m o u n t " e f f e c t w i t h h e a v i e r r a i n f a l l e v e n t s w i t h l i g h t e r i s o t o p e c o m p o s i t i o n s (Cra ig , 1 9 6 1 ) c o n t r i b u t i n g m o r e t o r e c h a r g e . Th i s e f f e c t w o u l d n o t b e o b v i o u s in t h e r a i n f a l l s a m p l e s a n a l y s e d as t h e s e a r e b u l k s a m p l e s f o r

105

each month. Nevertheless, some winter values as low as 5 D --- - - 3 3 and 518 = - -6 have been observed.

(2) The "rain-out" effect of Dansgaard (1964) with the site being further from the ocean than the Perth rainfall station. For Australian rainfall stations, Hughes and Allison (1984) report a weak negative correlation between stable isotope composition and distance from the coast in the direction of the prevailing winds. The correlation is not good enough to be used predictively but does suggest that isotope composition in rainfall would be lighter at the investigation site than at Perth.

Whilst individual values of 5 D and 5 is in rainfall vary over a large range, they are linearly related such that 5D = a518 + d (Craig, 1961). Perth rain- fall data, using monthly amount weighted means, gives an equation of 5D = 5.3 51S + 4.3 with an r 2 of only 0.83 (Fig. 4). The low slope and poor correlation are due to direct evaporation of much of the summer rain. When only mean winter-month data are plotted (Fig. 4) they fall close to a line of slope 8 but with a value of 16 for d. This high value for d is to be expected because Perth rainfall generally comes from water vapour originat- ing from the Indian Ocean evaporating into an atmosphere of low humidity (Gat, 1983). Gat also states that d will not change appreciably as this air mass looses water by precipitation. It can then be expected that the winter rainfall at the sampling site will have a relationship close to 5 D m 8 518 + 16.

Allison et al. (1984a) discuss the way in which isotopic composition of rain water may be modified in the unsaturated zone of the soil. They con- clude that transpiration has little or no effect, but that evaporation at or near the soil surface can result in strong enrichment of the heavy isotopes along a line of slope less than the meteoric-water line. They postulate that this water when displaced downwards and mixed with subsequent rainfall, will have a stable isotope composition which will plot on a line parallel to, but displaced from, the meteoric line. They further postulate that this displacement will be proportional to the inverse of the square root of recharge.

Values of 5D and 51s for the five profiles are plotted in Fig. 5. Profiles 1--3 were sampled in early spring and show little enrichment near the surface while profiles 4 and 5 which were sampled during summer show considerable enrichment in the top metre. The 5D VS. 51S plots for the surface to 2 m samples for the latter profiles (Fig. 6a) lie on a line with slope 2.6. These data are consistent with profiles reported for other parts of southern Australia (Allison et al., 1984b). The enriched water in this part of the profile will be subsequently mixed with incoming rainfall and moved down the profile.

As discussed earlier, the profiles show a general decrease in water content and an increase in chloride concentration with depth -- corresponding to the maximum depth of water extraction by plant roots as suggested earlier, and the isotope data above this depth represents water added by rain during winter. Mean 5 D and 51s values were calculated for each profile for the

106

+20 -

/ ~ O ~)D= 5'3 ~)18 +4.3 Q. _10

~'~ r e c i p i t d amount --20 weighted monthly means --30 / O Summer months (Oct. to April)

• Winter months (May to Sept.)

J I J J I I I I I --6 --5 --4 --3 --2 --1 0 +1 +2

~18 in Precipitation

Fig. 4. Plot o f m o n t h l y mean values o f 5 D and 5 is o f Perth rainfall.

"~D / ~)18 In Soil W a t e r

20-10 4 2 20 10 -4 2 20-10 4 ~ -20 10 4 2 -20 10 04 " ' , -1 ~ ~ ° ~'~ ,'

8

a 16 ~" ."

Profile 1 Profile 2 Profile :

4 -2 0

Prof i le 4 Profile

Fig. 5. Depth dis tr ibut ion o f ~D and ~ 18 for the same five prof i les used in Fig. 2.

depth intervals 1--10 m and from 10 to 2 m above the water table and these are plotted in Fig. 6b. Whilst the mean values for each profile have a high standard error (approximately 38 and 0.55 for deuterium and oxygen-18, respectively), there appears to be a consistent difference between the two depth invervals plotted. Both sets of data appear to lie on lines approxi- mately parallel to the Perth winter precipitation line but displaced from it. The small displacement for the shallower depths is probably caused by the small evaporation during the winter months as the soil profile is charged. This water would be further enriched as more water is lost during summer. The data from below 10 m represent water which remained in the profile at the end of summer and has since been displaced downwards. It is this displacement which may indicate recharge rate. The mean of this data for all five holes is 5D = - - 2 3 . 4 and 518 = - - 4 . 1 and the mean value of the displacement is 6.6 + 0.6.

107

~D

+20

+10

0

--10

- - 20

- 3 0

+10

Ram ~)D = 8 ~18 + 16 O --10

Q O ~o

- - 20

0.95)

I I 1 I I 1 I I I I - - 30 --4 0 +4 +8 +12

a) 318

O O 2 m--10rn: ~D=8~18 + 14 L ", 10 m -- (W.T. -- 2 m): ~D=8~18 +9 [] •Groundwater: ~D- 8(~18 + 12

R a i n ~D =8 ~18 +

i ~ 1 I I

--6 --5 --4 --3 --2 --1

b) ~)18

Fig. 6. (a) Plots o f 6 D vs, 518 for rainfall and for soi l water in the surface 2 ra depth interval for prof i les 4 and 5. (b) Plots o f 6 D vs. 618 for soil water and groundwater for the five profi les .

Data for the groundwater samples from the saturated zone beneath each of the five profiles are also plotted in Fig. 6b. They have a mean value of - - 2 1 . 9 + 1.7 for 5D and - -4 .3 + 0.1 for 51s and give a mean displacement from the meteoric line of 4.3 + 1.3. The smaller displacement for ground- water than for soil water in the unsaturated zone supports the bimodal system for recharge as proposed previously based on chloride data.

The relative amounts of matrix and preferred pathway recharge can not be estimated reliably from the stable isotope data because of the large uncertainty of the rainfall input data. However, the mean values of matrix and preferred recharge rates, Rm and Rp based on C1 in Table 2 can be used to estimate the 5-values of rainfall required to change the observed isotope composit ion of soil water to that of groundwater. This calculation, based on eqn. (5), gives 5D = --20.9" and 5~s = - -4 .4 . These values fall close to the meteoric-water line and thus are consistent with the proposed model incorporating preferred pathways for recharge.

CONCLUDING DISCUSSION

Water will be lost from the near-surface portion of the soil profile both by direct evaporation from the soil and by transpiration. Both these

108

mechanisms will result in an increase in chloride concentration of soil water, but only direct evaporation will result in a shift from the meteoric line in the relationship between 5D and 51s. Allison et al. (1984a)present data from a sand dune near Adelaide (South Australia) which suggest that a very similar relationship exists between chloride concentration and isotope enrichment under both pasture and bare soft. However, the experimental site of this s tudy is vegetated with perennial Banksia woodland. The data discussed here suggest that whilst the soil water is being removed from near the surface by both evaporation and transpiration, considerable amounts of water are being lost by transpiration from depths greater than i m. This transpiration loss may vary considerably from site to site as inferred from the spatial variation of chloride profiles. The overall small enrichment of isotope in the unsaturated zone suggests that water loss by evaporation is a relatively small component of the total water loss and, as such, reliable estimates or recharge cannot be made from the stable isotope data.

The analyses of data both on the depth distribution of our C1- as well as isotope suggest that water movement does not occur uniformly through the profile. Application of an approximate model, using the C1- data, indicates that some 50% of the recharging water may be bypassing the main soil matrix, while reaching the phreatic surface. We do not have direct obser- vations on the presence of such preferred pathways. It is recommended that further research be conducted to quantify spatial distribution and the role of such preferred pathways to groundwater recharge. Our results show con- siderable spatial variability in the recharge. Such variations should be taken into account in assessing the potential of the water resource, and in the development of predictive models for long-term maintenance of the resource.

ACKNOWLEDGEM ENTS

We acknowledge the capable technical assistance provided by Mark Fernie. This research was supported by a grant from the Metropolitan Water Authori ty , Western Australia.

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