Sources of dissolved salts in the central Murray Basin, Australia

Chemical Geology, 111 (1994) 135-154 Elsevier Science B.V., Amsterdam

135

[SBI

Sources of dissolved salts in the central Murray Basin, Australia Blair F. Jones a, Jeffrey S. Hanor b and W.R. Evans c aU.S. Geological Survey, MS 432 National Center, Reston, VA 22092, USA bDepartment of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA CEnvironmental Geoscience & Groundwater Program, Australian Geological Survey Organisation, GPO Box 378, Canberra, A. C. T. 2601, Australia (Received September 21, 1992; revised and accepted May 19, 1993 )

ABSTRACT Large areas of the Australian continent contain scattered saline lakes underlain by shallow saline groundwaters of regional extent and debated origin. The normative salt composition of subsurface pore fluids extracted by squeezing cores collected during deep drilling at Piangil West 2 in the central Murray Basin in southeastern Australia, and of surface and shallow subsurface brines produced by subaerial evaporation in the nearby Lake Tyrrell systems, helps constrain interpretation of the origin of dissolved solutes in the groundwaters of this part of the continent. Although regional sedimentation in the Murray Basin has been dominantly continental except for a marine transgression in Oligocene-Pliocene time, most of the solutes in saline surface and subsurface waters in the central Murray Basin have a distinctly marine character. Some of the Tyrrell waters, to the southwest of Piangil West 2, show the increase in NaCI and decrease in sulfate salts expected with evaporative concentration and gypsum precipitation in an ephemeral saline lake or playa environment. The salt norms for most of the subsurface saline waters at Piangil West 2 are compatible with the dilution of variably fractionated marine bitterns slightly depleted in sodium salts, similar to the more evolved brines at Lake Tyrrell, which have recharged downward after evaporation at the surface and then dissolved a variable amount of gypsum at depth. Apparently over the last 0.5 Ma significant quantities of marine salt have been blown into the Murray Basin as aerosols which have subsequently been leached into shallow regional groundwater systems basin-wide, and have been transported laterally into areas of large evaporative loss in the central part of the basin. This origin for the solutes helps explain why the isotopic compositions of most of the subsurface saline waters at Piangil West 2 have a strong meteoric signature, whereas the dissolved salts in these waters appear similar to a marine assemblage.

I. Introduction

Large areas of the Australian continent contain scattered saline lakes and salinas and are underlain by shallow saline groundwaters of regional extent (Johnson, 1979; Lau et al., 1987; Evans, 1988 ). Some of these surface and near-surface brines are highly acidic and contain elevated concentrations of heavy metals (Macumber, 1983; Mann, 1983; Lyons et al., 1992). There has been considerable interest in determining whether the occurrence of these waters can shed light on the genesis of oreforming fluids in sedimentary basins (see, e.g., Lyons et al., 1988, 1990; Fegan et al., 1992). SSDI 0009-2541 (93) EO 169-T

In other areas of the continent, the progressive salinization of agricultural land and the upward discharge of saline groundwater into rivers and streams has lead to severe environmental problems (see, e.g., Brown et al., 1988; Ghassemi et al., 1989). Central to solving the general problem of the origin of shallow saline waters on the Australian continent is the documentation of the ultimate sources of their dissolved salts. As reviewed by Johnson (1979), Streich (1893) proposed nearly a century ago that the Australian salinas are the remnants of a marine transgression of continental extent. Jack (1921) was perhaps the first to propose in-

136

stead that the salts were derived from oceanic aerosols transported inland into internal drainage basins. Bonython (1956) suggested the fluids are merely evaporated river or lake waters. Some workers have favored the idea that the salts are connate in origin, i.e. that they are derived from seawater entrapped in sediments at the time of their deposition in a marine environment (Wopfner and Twidale, 1967; Johnson, 1979 ). The purpose of this paper is to present new information on the origin of solutes in subsurface brines in the central Murray Basin, southeastern Australia (Fig. 1 ). Nowhere on the Australian continent is the problem of determining the origin of saline waters of more practical importance than the Murray Basin, which contains some of the most important agricultural land on that continent. The clearing of land in the groundwater recharge zones, accompanied by irrigation development, has resuited in rising groundwater levels, evaporative concentration of salts, and the surface discharge of saline groundwaters (Brown et al., 1988 ). In order to determine how these salinity problems have developed and how they might be controlled, the then Australian Bureau of Mineral Resources, Geology and Geophysics (BMR), Canberra, in cooperation with various Australian state agencies, has been engaged in a study of the hydrogeology of the basin. In 1987, a borehole was drilled by the Victoria Department of Industry, Technology and Resources near the town of Piangil (Fig. 1 ) in northwestern Victoria through the entire thickness of Cenozoic basin fill to establish the hydrogeological role of the Geera Clay - - part of a regional low-permeability barrier to flow. As will be demonstrated, the isotopic and normative composition of pore fluids extracted from cores collected during drilling, and the normative composition of nearby shallow subsurface and saline lake brines (Teller et al., 1982; Macumber, 1983, 1984), constrain the interpretation of the ultimate origin of the dissolved salts in this portion of the basin.

B.F.JONESET AL.

; 0000 os -

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120o E

140 ° E

40 o S

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15U o E

145 ° E

Fig. 1. Maps showing the location of the Murray Basin and the Piangil and Lake Tyrrell study areas.

2. Study area

2.1. Regional setting The Murray Basin is a shallow, intracratonic sedimentary basin, ~ 300,000 km 2 in areal extent and 0.6 km in maximum thickness, which formed after the breakup and separation of the Australian and Antarctic continental lithospheric plates in Paleocene time (Brown, 1985 ). Regional sedimentation in the basin has been dominantly continental except for two

SOURCESOF DISSOLVEDSALTSIN THE CENTRALMURRAYBASIN,AUSTRALIA

137

400 WEST

EAST

200 -

Piano# Site

0-

-200

-400 Mesozoic - Paleozoic Basement

-600

0 t

1O0km I

-800

Fig. 2, Schematic west-east cross-section through the Murray Basin showing location of the Lake Tyrrell and Piangil sites relative to principal hydrogeologic units. Arrows show generalized groundwater flow directions. Modified from Evans (1988).

marine transgressions in Oligocene-Miocene and Pliocene times, when portions of the western half of the basin were flooded with seawater. There are three regional aquifer systems in the basin (Fig. 2): ( 1 ) the Renmark Group Aquifer, which forms the basal unit of the Cenozoic sequence of the Murray Basin; (2) the Murray Group limestone aquifer in the western part of the basin; and ( 3 ) the overlying Pliocene Sands regional aquifer system. The Geera Clay forms the principal part of a complex mid-Tertiary low-permeability barrier to regional groundwater flow - - both vertical and horizontal w i n the central part of the basin. Locally the Geera Clay vertically separates the Renmark Group and Pliocene Sands aquifers.

B~'AN~H~q'~V~-

- - -- )-- Pleistocene I

PARILLA

BOOKPURNONG

I J I

Pliocene

I --I

UPPER GEERA CLAY Miocene

E i-

LOWER GEERA CLAY

uJ a

_1

Oligocene

RENMARK GROUP

Eocene

2.2. Piangil West 2 site A bore (Piangil West 2) was drilled for BMR in 1987 through the entire Cenozoic sedimentary section to a depth of 392 m below ground surface near the town of Piangil as part of a program to evaluate the role of the Geera Clay on the regional hydrogeology of the basin. Intermittent core was collected to a depth of 100 m, and nearly continuous core collected from a depth interval of 100-230 m. Drill cuttings were collected from 230 m to total depth. The Cenozoic sedimentary section at the Piangil West 2 site (Fig. 3 ) is situated uncon-

GRANODIORITE

Devonian

Fig. 3. Stratigraphy at Piangil West 2 site. Exact ages of the units are not known.

formably above granodiorite of probable Devonian age. The Renmark Group, the basal aquifer in the drilled section, consists o f a 150m-thick sequence of fluvial and fluvial-lacustrine sediments of Eocene to Late Oligocene age (Brown, 1985; Brown and Stephenson, 1986 ). The Lower Renmark Group aquifer as defined

138

B.F. J O N E S

by Kellett (1990) is dominated by fine sand, silt and carbonaceous clay. The Renmark Group sediments grade upward into the 130m-thick sequence of Oligocene-Miocene muds, silts, and fine sands of the Geera Clay. The Lower Geera Clay, as defined here (Fig. 3 ), is comprised of marine silt and sand. The Upper Geera Clay was deposited in a fining-upward sequence of a marginal marine, tidal fiat setting (Radke, 1987) and is overlain by 20 m of muds tentatively assigned to the Bookpurnong Beds, a sequence of Upper Miocene clays deposited in a regressive, shallow marine environment (Fig. 4). The occurrence of probable gypsum/anhydrite, now replaced by dolomite, at depths of ~ 120 and ~ 130 m in the Upper Geera Clay and at 70 m toward the base of the overlying Parilla Sand (Fig. 4) attest to transient periods of hypersalinity during the deposition of these units (Radke, 1987 ). The Geera-Bookpurnong sequence is overlain by 80 m of PariUa Sand of Pliocene age. .5

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DIAGENESIS

This unit was deposited during a subsequent regional transgressive-regressive cycle, and is probably marine for most of its thickness at Piangil. The Parilla Sand forms part of the regional Pliocene Sands aquifer. The Blanchetown Clay, a Pleistocene freshwater to possibly brackish lacustrine unit, is the uppermost sedimentary unit in the general vicinity of Piangil. Blanchetown sediments were not encountered, however, at the Piangil West 2 drill site where the uppermost parts of the sequence are composed of gypsiferous sands of the Yamba Formation. These sediments contain chemical precipitates probably formed by evaporative concentration of regional Parilla waters during mid-Pleistocene/Holocene phases of continental aridity. The drill site is situated in a former lake that is currently becoming salinized. The direction of shallow groundwater flow in the Parilla aquifer at Piangil is to the northwest (Evans, 1988). Flow directions in the Lower Renmark Group aquifer, as inferred from data of Evans ( 1988 ), are subparallel to those in the Parilla.

2.3. Lake Tyrrell

~)

I

ETAL

I

POST-

COMPACT ON ~-

h g . 4. Oeposmonal environment and dlagenet~c mineralogy of the Bookpurnong Beds and Upper Geera Caly sediments at Piangil West 2 site. (Data from Radke, 1987.)

The Piangil West 2 drill site is situated 50 km northeast of Lake Tyrrell, a saline lake, developed within a similar stratigraphic setting as Piangil, which has received a great deal of study (Macumber, 1983, 1984). Although Piangil and Lake Tyrrell are not currently on the same regional flow paths, the groundwater flow regime at Lake Tyrrell, as documented by Macumber ( 1983 ), illustrates the processes of subaerial generation of brines currently operating in the central Murray basin. Three major hydrologic provinces having over an order of magnitude range in salinity have been recognized in the Parilla aquifer system at Lake Tyrrell (Fig. 5). These include: ( 1 ) the regional Parilla groundwater which has a salinity (of 37 g 1-1 ) close to that of seawater, and which discharges into the lake along its margins; (2) Lake Tyrrell reflux water,

139

SOURCES OF DISSOLVED SALTS IN THE CENTRAL MURRAY BASIN, AUSTRALIA

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Fig. 5. Hydrologicregime at Lake Tyrrell modified from Macumber (1984). Potentiometric surface contours in meters. Groundwater salinities in the cross-sectionare in percent. which has a salinity in excess of 200 g 1-1 produced by subaerial evaporative concentration in the open lake and which recharges the Parilla aquifer beneath the lake by density flow; and (3) T i m b o r a m - W a h p o o l reflux water of an intermediate salinity (typically 90-130 g 1-1 ) produced by subaerial evaporation and

downward density flow from the smaller Lakes T i m b o r a m and Wahpool immediately to the east of Lake Tyrrell. Despite their widely diffeting salinity, all three groundwater types have roughly similar proportions of major solutes. Considering that their hydrogeologic setting has been definitively described by Macumber

140

B.F.JONESETAL.

using high-pressure liquid chromatography (HPLC), for alkalinity using conventional titration techniques at BMR, and for 5D and 180 by CSIRO, Division of Water Resources, Canberra. Estimates of total dissolved salts (TDS) content were made by optical refractometry. Imprecision in the commercial HPLC techniques resulted in considerable scatter in chloride and sulfate analyses. The sodium values, as determined by ICP, appear to be the most accurate and precise of the analyses made in this study (based on comparison with refractometry results). Thus this constituent was used to evaluate charge balance relations and to calculate total salinity values. (The Na values are believed to have a relative precision of ~ + 5%.) Because the other major solute determinations were considered more reliable, chloride values were adjusted to achieve charge

(1983, 1984), it is useful to compare their chemical composition with the deeper waters at Piangil. 3. Pore-water chemistry at PiangU West 2

3.1. Sampling and analytical techniques Extraction of pore water from cored sediment samples from the Piangil West 2 borehole was accomplished using a hydraulic squeezer closely modeled after the apparatus used by the Deep Sea Drilling Program (Manheim, 1966). The squeezed-water samples (usually ,-. 5 ml) were filtered through 0.45-/1m membrane filters. Acidified aliquots were analyzed for cations by inductively coupled plasma spectrometry (ICP) at BMR. Unacidified aliquots were analyzed commercially for anions TABLE 1

Major solute chemical analyses of pore fluids (concentrations in ml 1-1 ) from Piangil West 2, Murray Basin, northwest Victoria, Australia (C1 balance corrected) Stratigraphic horizon

Ca

Mg

Na

K

SO4

C1

750 1,890 1,390

2,430 1,010 3,490

13,100 21,300 24,000

130 210 210

4,500 5,450 10,200

27,800 30,800 16,900

108.7 114.1 122.8 125.2 129.6 171.1 178.1 178.7 185.7

1,120 1,040 1,160 1,190 1,120 530 830 815 720

2,400 2,370 2,010 790 2,070 870 930 910 970

17,800 16,300 15,000 12,970 15,400 6,820 6,180 6,210 5,530

180 160 180 195 175 62 115 120 110

8,270 6,520 7,970 4,800 8,730 950 3,800 3,740 2,330

22,000 27,500 29,360 33,700 17,300 15,900 13,770 11,880 7,100

Lower Geera clay

192.2 198.4 219.8 220.1 227 227.3

290 240 420 420 350 470

390 385 535 490 420 600

3,780 3,610 3,290 3,470 3,200 3,980

67 39 55 67 67 70

840 880 915 960 880 1,260

7,360 7,000 10,750 7,790 5,010 10,280

Renmark Group clay

230.8 320

890 29

810 77

3,790 830

110 3.9

5,450 31

7,010 1,485

Parilla Sand Bookpurnong Beds

Upper Geeraclay

clay sand

sand

Depth (rn) 25.0 81.85 89.2

141

SOURCES OF DISSOLVED SALTS IN THE CENTRAL MURRAY BASIN, AUSTRALIA

balance prior to normative analysis. Further details of sample extraction, handling, and analysis are given by Hanor ( 1987 ).

3.2. Salinity and major dissolved ions The major solute compositions of waters at Piangil West 2 are given in Table 1. Most of the waters are dominated by Na and C1 with significant and varying proportions of Mg, SO4 and Ca. In general, the major-ion proportions are similar to those of seawater regardless of the TDS, as noted by Macumber (1984) for the waters of the Lake Tyrrell system. Details of water chemistry will be discussed in conjunction with a discussion of principal constituent variation and the results of the normative salt calculations. The variation in salinity (TDS) with depth in the pore fluids of Piangil West 2 is shown in Fig. 6. The vertical variation is systematic. The TDS of the shallowest available sample, from a nearby Parilla bore hole screened at a depth of 25 m, is 51 g 1-1. The salinity at the top of the Bookpurnong Formation is 88 g 1-1. Salinity then decreases in a nearly linear fashion

i

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-200

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20

,

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,

Isotopic composition of H20 in pore fluids from Piangil West 2, Murray Basin, northwest Victoria, Australia Depth

6180

613

(m)

(%o v s .

(%0 v s .

SMOW)

SMOW)

25.0

- 4.44

- 29.52

67.2

- 1.44

- 25.62

Parilla Lower Parilla

82.1

- 0.56

- 23.57

Bookpurnong

88.9

- 1.71

- 24.99

Bookpurnong

89.2

- 1.52

-21.56

Bookpurnong

Geera Geera U p p e r Geera U p p e r Geera U p p e r Geera U p p e r Geera U p p e r Geera U p p e r Geera U p p e r Geera U p p e r Geera U p p e r Geera Lower Geera Lower Geera Lower Geera Lower Geera Lower Geera Lower Geera Renmark

100.0

- 2.11

- 23.64

114.1

-

- 21.63

1.82

- 1.31

- 21.58 - 26.43

158.5

- 2.49

- 26.22

R

165.7

- 1.13

- 21.24

170.2

- 3.27

- 27.02

TDs,TDS' sumrefract°metry I

171.1

- 3.04

- 25.58

178.1

- 3.58

- 25.81

178.4

- 4.45

- 26.98

178.7

- 5.43

- 29.00

189.4

- 4.06

- 19.70

191.6

-2.51

- 17.63

215.0

- 2.00

- 15.94

219.8

- 2.81

- 22.25

227.0

- 2.58

- 22.19

230.8

- 2.50

- 24.60

310.0

- 6.24

- 36.69

i

60

,

i

80

100

Variation in pore-water salinity with depth at P i a n West 2 as determined by refractometry and sum of dissolved solids. P = P a r i l l a ; B=Bookpurnong Formation; UG = U p p e r Geera Clay; LG = Lower Geera; R = Renmark Group (see F i g . 3 ) . F i g . 6.

TABLE 2

- 2.61

TDS, ~L

gil

The 6D-6~80 compositions of pore waters from the Piangil West 2 site are given in Table 2; a plot of the data is shown in Fig. 7. The waters as a group are significantly lighter than either seawater or evaporated seawater. Also shown are weighted annual means for the

122.8

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3.3. Isotopic composition

150.1

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-400

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with depth to a value of 21 g 1- ~ at the Upper Geera-Lower Geera boundary. There is a pronounced change in slope in the salinity gradient at this boundary, and values of 13-16 g 1-~ appear to be typical of the Lower Geera Clay. The single available salinity measurement for the Renmark, which is of free-flowing water collected from the completed Piangil West 2 borehole, is 2.7 g 1-~. Details of the salinity structure for most of the Parilla and Renmark are not known.

Formation

Upper

Upper Upper

142

B.F. JONES ET AL.

(A) 0

10

Padlla

0

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-10 ,

m m

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Parilla-BookpurnongU Geera Trend

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.......... ~ 200 LowerGeera

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= .................................................. Ren

300'

Global MWL

. . . . . . . . . -6 -4 -2 0

Fig. 7. Isotopic c o m p o s i t i o n o f waters from Piangil West 2 site. Also s h o w n is the isotopic c o m p o s i t i o n o f rainwater from Melbourne, southeast of the M u r r a y Basin (Fig. 1 ). U = U p p e r .

period 1976-1983 of the isotopic composition of rainwater at Melbourne, Victoria, the closest regularly-reporting monitoring station, located 300 km southeast of the study site (IAEA, 1983, 1986). Fig. 8 shows plots of 51sO and 8D as a function of depth. It is convenient for the purposes of discussion to separate the waters at Piangil West 2 into three groups on the basis of their isotopic composition: (1) Parilla-Bookpurnong-Upper Geera waters; (2) Lower Geera waters; and (3) Renmark water. d D - ~ s O of the waters of the Parilla-Bookpurnong-Upper Geera group plot along a linear trend, which has as its lightest end member a water of apparently meteoric composition ( 8 1 ) = - 30°/oo, 8 ~ 8 0 = - 5%o) similar to modern-day Melbourne rainfall. The slope of this trend away from the Meteoric Water Line (MWL) toward heavier values is consistent with evaporation of water in an arid climate (Knauth and Beeunas, 1986 ). The variation in oxygen isotopic composition with depth (Fig. 8 ) shows some similarities to the variation in salinity with depth (Fig. 6). The waters become progressively heavier in both isotopes with depth from the top of the Parilla to the top of the Bookpurnong (Fig. 8). The 5~sovalues then show a reversal in trend and a pronounced return toward meteoric compositions

400 • , • , • , - , • , . , • -7 -6 -5 -4 -3 -2 -1 518 0 (B) 0 PadUa

100

.... ?o'p¢'~;~'o;~..... ,.Bo g . . . . . . . . . . . . . -. ~"-~.................... .................... J Upper Gemera

= a~ E.003002

. .Lo .~ ...................................

Renmark

400 . . . . . . . . . . . . . . -40 -30 -20 Fig. 8. Variation in

-10

t~lSo a n d b~D vs. d e p t h at Piangil site.

at the sand bed which marks the Upper GeeraLower Geera boundary. Below this level both 6180 - and 8D-values increase in the Lower Geera and then decrease markedly in the Renmark. The waters of the Lower Geera show a distinctly different dD vs. 5180 signature (Fig. 7 ). These waters as a group plot in a compositional field above the Parilla-BookpurnongUpper Geera trend and somewhat closer to Standard Mean Ocean Water (SMOW). The heaviest water in the Lower Geera occurs at a depth of 220 m from where the isotopic compositions become lighter upward toward the Upper Geera-Lower Geera contact and downward toward the top of the Renmark. The details of the isotopic composition of waters in the Renmark are not known. The single available value, which is for the regional Renmark Group aquifer, is meteoric in char-

SOURCESOF DISSOLVEDSALTSIN THE CENTRALMURRAYBASIN,AUSTRALIA

acter. The Renmark water is significantly lighter than either the inferred end-member for the Parilla-Bookpurnong-Upper Geera trend or contemporary Melbourne rainwater. It is clear that the isotopic composition of the waters at Piangil West 2 have been modified since the deposition of their host sediments. The original marine water molecules of the Bookpurnong Beds and Upper Geera Clay pore waters have apparently been quantitatively displaced by a mixture of meteoric and evaporated meteoric H20, probably by mechanisms such as outlined by Ferguson et al. (1992). It is likely that the latter have been introduced vertically downward into the system from the overlying PariUa aquifer and that the former have been introduced from waters migrating laterally through the sand beds, particularly at the Upper Geera-Lower Geera contact. Although both &D and &~80 increase markedly with depth in the Parilla Sand and decrease notably at the Upper Geera-Lower Geera boundary and in the Renmark Group, the two isotopes appear to vary inconsistently with depth from the Bookpurnong Beds to the middle of the Lower Geera Clay. A few of the Lower Geera points on the 6D vs. &180 plot (Fig. 7 ) appear to define a separate evaporation trend compared to the larger number of values from higher in the section. The two trends readily extrapolate to the extremes for Melbourne precipitation, with the Lower Geera trend intersecting the meteoric line at a position suggesting warmer temperatures for recharge waters. It is perhaps more likely that the somewhat heavier isotopic values are vestiges of mixing with residual marine pore fluids in the Geera Clay muds.

143

the probable first-rank changes in pore-water salinity which have occurred in the individual sedimentary units at the site since deposition. A comparison of present-day salinities with estimated connate pore-water salinities, as inferred from the probable environments of deposition of the individual sedimentary units at the site (Brown, 1985; Radke, 1987), is shown as a function of depth in Fig. 9. The term connate salinity is used here to refer to the salinity of fluid entrapped in a particular layer of sediment at the time of the deposition of that layer. Note the great increase in TDS which has occurred in the Parilla and Upper Geera Clay in the upper part of the section and the loss which has occurred in the Lower Geera Clay. Hanor (1987) has estimated there has been a net addition of 3.103 kg m -2 of dissolved salt to the groundwater system at Piangil West 2 over its entire vertical extent of 380 m. Scatter plots (Fig. 10) were used to examine the covariance between major solutes in pore fluids from Piangil West 2 and to compare them with similar constituent relations in analyses from the Lake Tyrrell system (data from Macumber, 1983). The first four diagrams present the major cations (Na, K, Mg, Ca) plotted against chloride. In evaluating the geochemical evolution of waters in evapora0

E.

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LG

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3.4. Salt balance

0

Connate Salinity Present Salinity

. . . . . . . . . . . . . . . . . . . 20 40 60 80

100

TOS, g/L

Although some details of the past depositional history and present salinity structure at Piangil West 2 are not fully understood, sufficient information is available to speculate on

Fig. 9. Comparison of estimated connate and present porewater salinities at Piangil West 2. Note the great increase in TDS which has occurred in the PadUa and Upper Geera, and the decrease in the Lower Geera. For explanation ot letters, see caption to Fig. 6.

144

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seawater

ratio

/

15000

1500'

•

• i I

.J

seawater~atio

10000

/ /

E

1 -S •

1000

/

t,

(.)

•

I

•// /

•

•

•

•

5000

0~ 0

50000

100000

150000

200000

• 50000

looooo

CI, mg/L

lsoooo

200000

CI, mg/L I

30000

I

i

A Parilla Bookpunong _1

"~ 2oooo. E

•

g

seawater

if)

ratio

Upper Geera Lower Geera Renmark

• Tyrrell

100OO •

500

•

.1~ i

1000

1500

Sea Water

2000

Ca, mg/L Fig. 10. Scatter diagrams of major cations vs. chloride, and calcium vs. sulfate for pore waters from the Piangil West 2 bore and the Lake Tyrrell system.

tive basins the assumption is made that C1 is conserved in solution until halite saturation is reached (Jones et al., 1977; Eugster and Jones, 1979 ). Processes affecting the gain or loss of specific solutes are then recognized by deviation from a linear relation with chloride. Sodium and chloride are clearly the dominant constituents in both the Piangil and Lake Tyrrell waters. The plotted values for these sol-

utes are referenced to the ratio found in seawater and halite crusts, the most likely sources of these constituents. Among the pore fluids from the Piangil West 2 site, the only sodium value clearly exceeding a 1:1 molar NaC1 ratio is for a sample from the base of the Bookpurnong Beds, and is most likely the result of weathering of sodic feldspar. Other Piangil pore fluids are slightly enriched in chloride, proba-

SOURCES OF DISSOLVED SALTS IN THE CENTRAL MURRAY BASIN, AUSTRALIA

bly the result of fractionation (e.g., sulfate precipitation) prior to recharge, but possibly related to a small, and normally unfavored, exchange ofNa ÷ for alkaline-earth cations. The waters from the Lake Tyrrell system appear to fit the seawater ratio well except for the most concentrated brines, which probably contain the most recycled halite. The relationship of magnesium to chloride in the Piangil West 2 pore waters follows the seawater ratio closely, except again for Bookpurnong Beds fluid. The more concentrated waters from the Lake Tyrrell area generally approach the seawater ratio, but there is considerable scatter, and a few points show substantial depletion. In contrast, the strong deviation of all analyses from the seawater ratio for potassium to chloride seems to increase with TDS regardless of the scatter, suggesting loss of K from solution throughout evaporative concentration, initially on clays and subsequently by precipitation of alunite and jarosite (Long et al., 1992a, b). The most dramatic difference in composition between Piangil and Lake Tyrrell waters is in the plot of calcium vs. chloride. All but the most dilute Piangil West 2 fluids exceed the seawater ratio of Ca to C1, whereas the Tyrrell brines are strongly depleted in Ca. To a lesser extent, the same relationship is seen with regard to sulfate and chloride. The situation is clarified by the relation of calcium to sulfate. The controlling influence of gypsum sources is indicated by the limiting nature of the 1:1 ratio of Ca to SO4. The more concentrated Lake Tyrrell samples reflect the depletion of calcium expected for precipitation of gypsum from fluids where sulfate exceeds calcium. It is interesting to note that of the Piangil fluids only the Bookpurnong waters are close to saturation with gypsum, except for the sample from the Folly Point spring at Lake Tyrrell, which has the highest alkali sulfate proportion of any reported for the area (Teller et al., 1982). Comparison of the major solute relationships for the Piangil and Tyrrell waters (Long

145

et al., 1992a, b) with similar plots given by McArthur et al. (1989) for saline waters from the closed basins of the Yilgarn Block in the southeast of Western Australia suggests overall similarities in surficial processes at Lake Tyrrell but significant differences from processes affecting the Piangil West 2 groundwaters. The Yilgarn data suggest no excess weathering or recycled inputs of sodium and magnesium, and no indication of long-term re-solution of calcium sulfate. The strontium and sulfur isotopic evidence of McArthur et al. (1989) and Chivas et al. ( 1991 ) for the dominance of marine aerosol inputs to the Yilgarn basins is quite definitive. Considering the similarity in solute matrix between the basins, and the estimated addition of salt beyond what would have come from connate sources alone, it is apparent that aerosols have contributed significantly to Murray Basin as well as Yilgarn Block solutes. 4. Normative salt calculations

4.1. Techniques The computer program SNORM (Bodine and Jones, 1986) calculates a salt norm from the chemical composition of a natural water. The salt norm is the quantitative ideal equilibrium mineral assemblage that would crystallize if the water evaporated to dryness at 25 °C and 1 bar pressure under atmospheric partial pressure of CO2. SNORM proportions solute concentrations to achieve charge balance. It quantitatively distributes solutes into normative salts allowing only stable associations based on the Gibbs phase rule, available free energy values, and observed low-temperature mineral associations. Although most natural water compositions represent multiple solute origins, results of normative calculations of waters world-wide suggest three major hydrochemical categories: meteoric or weathering waters that are characterized by normative alkali-bearing sulfate and/or carbonate salts; marine-like waters that

146

are chloride-rich with a halite-Mg-salt association similar to the seawater norm; and diagenetic waters that are frequently of marine origin but yield normative Ca-bearing chlorides, indicating solute alteration by secondary mineral reactions. The solute source or reaction process within each of the above categories is commonly indicated by the presence or absence of diagnostic salts and their relative abundance in the normative salt assemblage. Normative analysis has been used to elucidate solute origin and distribution in complex saline groundwater systems such as the Permian Rustler Formation of southeastern New Mexico, U.S.A. (Bodine and Jones, 1990) and the Miocene Madrid Basin aquifer of Spain (Jones and Llamas, 1989). A simplified representation of the salt norm which avoids consideration of complex evaporite mineralogy is made possible by definition of the "simple-salt assemblage" (Bodine and Jones, 1986). This assemblage is constructed by the computer program SNORM from the true salt norm through separation of each major-ion normative salt into its simple-salt components (omitting minor solutes). The simple-salt assemblage includes the carbonates, sulfates and chlorides of calcium, magnesium, sodium and potassium. 4.2. Piangil norms

The results of SNORM calculations obtained from the pore fluids of Piangil West 2 are given in Table 3 along with the normative composition of seawater for comparison. These waters are all dominated by NaC1, with K-salts constituting < 1% of the norm. The major normative simple salts are presented individually in anhydrous weight percent, but Mg- and SO4-salts are also given as group totals for more detailed comparison. At the Piangil West 2 site, the interstitial fluid of the Parilla Sand is 1.5 times more concentrated than the near-seawater salinity reported by Macumber ( 1983, 1984) for the re-

B.F. JONES ET AL.

gional Parilla aquifer (Table 3), and this increase is reflected primarily in the MgC12 component of the norm (8.3% vs. 14.8%). In contrast, the pore water of the upper Bookpurnong Beds is depleted in both total normative Mg- and SO4-salts relative to the regional Parilia aquifer. Below the upper Bookpurnong Beds the norms of Piangil West 2 fluids contrast with the seemingly simple monotonic change with depth seen in the decrease of salinity values (Fig. 6 ). The deeper Bookpurnong pore-water sample is normatively very similar to that from the top of the Geera Clay, though more saline. Generally, overall normative characteristics of interstitial fluids in the Upper and Lower Geera appear related to the stratigraphic horizon (Fig. 11 ). The pore fluids of the Upper Geera Clay are lower in total Mg-salts, but generally higher in total sulfate than the Parilla Sand at Piangil. Total normative Mg- and SO4-salts in the pore fluids of the Upper Geera Clay are both usually higher than those of the underlying Lower Geera, except for the NaCl-depleted fluid at 23 l-m depth. Conversely, normative CaSO4 in pore water from below the upper Bookpurnong Beds generally increases with the decreasing salinity at greater depth in the Geera Clay. The Lower Geera pore fluids provide an interesting contrast to those of the Upper Geera Clay. The uppermost six norms for the Lower Geera Clay are similar to seawater in total Mgand SO4-salts, despite the waters having a salinity one-third that of normal seawater. This is a result of a higher proportion of CaSO4 and MgC12 than in the Upper Geera norms. In the norm for the two samples at 220 m the proportional increase in CaSO4 to total SO4-salts is largely balanced by a decrease in NaC1 rather than an increase of MgSO4, which leads to the appearance of small amounts of normative CaCI:. The only available norm for the Renmark Group aquifer contains the most CaC12 of all of the waters at Piangil West 2, very little SO4, and low total Mg-salts. The most anoma-

147

SOURCES OF DISSOLVED SALTSIN THE CENTRAL MURRAYBASIN,AUSTRALIA TABLE 3 Salt norms of pore fluids (in wt%) extracted from core of Piangil West 2, Murray Basin (C1 balance corrected) Stratigraphic horizon

Depth (m )

TDS (g 1- t )

NaCI

KC1

MgC12

Parilla Sand

25.0

46

72.0

0.5

14.8

5.5

Bookpurnong Beds

81.85 89.2

65 82

83.5 74.8

0.5 0.5

5.8 8.4

Upper Geera Clay

108.7 114.1 122.8 125.2 129.6

60 57 52 41 53

75,1 75,1 73.8 80.5 73.9

0.6 0.5 0.7 0.9 0.6

171.1 178.1 185.7

23 23 21

76.8 69.2 68.1

192.2 198.4 219.8 220.1 227 227.3 230.8

12 12 14 12 11 14 17

clay

Lower Geera Clay

clay

Renmark Group

320

Seawater

surface

2.5

35

Total Mg-salts

Total SO4-salts

7.2

22.0

12.7

8.5 5.8

1.7 10.5

7.5 18.9

10.2 16.3

6.4 9.3 5.3 2.9 4.0

6.3 6.2 7.6 9.8 7.2

11.6 8.8 12.6 5.9 14.3

18.0 18.1 17.9 8.8 18.3

17.9 15.0 20.2 15.7 21.5

0.5 1.0 1.0

15.1 7.9 15.1

6.0 12.1 11.8

9.8 3.6

17.6 17.7 18.7

6.0 21.9 15.4

78.2 78.8 73.6 71.9 73.3 71.1 57.0

1.0 0.6 0.8 1.0 1.2 0.9 0.2

11.3 10.4 15.2 15.4 14.2 15.7 0.2

8.0 7.0 9.5 11.1 10.7 11.2 17.9

1.4 3.2 0.3 1.0 23.5

K2SO4:1.2

12.7 13.6 15.2 15.4 14.5 16.7 23.7

9.4 10.2 9.5 11.1 11.0 12.2 42.6

84.2

0.3

12.0

1.7

-

CaCI2:1.8

12.0

1.7

78.2

2.2

9.2

4.0

6.1

-

15.3

10.1

lous norm of all was obtained from the pore water in the clay layer at the base of the Lower Geera Clay, assuming the water analysis to be accurate. The increased total SO4-salts in this norm, accompanied by an equivalent decrease in NaC1, are approximately double the highest value seen in the Upper Geera Clay. In all the pore fluids of Piangil West 2 the NaC1 content never varies greater than 15% from the seawater value except for one point in the Lower Geera, despite a more than 7-fold range in salinity. The salt norms appear to reflect not only some overall differences between formations, but lithologic variations between particular horizons within these formations as

CaSO4

MgSO4

Other

CaC12:1.6

CaC12:0.4 0.1

well. Thus a maximum in MgSO4 corresponds to an evaporitic mud layer at ~ 130-m depth in the Upper Geera Clay, and the highest porefluid SO4 proportion for any strata was in the clay at the bottom of the Lower Geera (Figs. 3 and 11 ). Similarly, the highest percentages of MgC12 in the Upper Geera correlate with thin sand horizons. The proportion of total Mg-salts is similar in most Upper Geera norms and the total SO4 percentages are 1.5-2 times higher than seawater. Thus pore fluids from sandy horizons appear anomalously low in SO4, especially at the 171-m horizon. However, the pore water at the base of the Upper Geera Clay seems more depleted in NaC1 relative to sea-

148

B.F, JONES

Regional Parilla

Regional Padlla

.- . . . . . . . . . . . . . . . . . . . . .

ET AL.

"-.....

o p

~_--~-

100'

100

'

...........

]g .......... UG

=-

r .........

•...... L_G_.

..200'

....

E

UG

E a

'-i

t-

20o

a 300 ¸

300

....

4O0

, ....

50

,

60

.

.

70

.

}

,

.

.

.

.

80

400

. . . . . . . . . . . . . . . . . . . . .

0.0

90

0.5

NaCI Norm, wt %

1.0

1..

2.0

2.5

KCI Norm wt %

Regional Parilla

i---2~

1.5

Regional Parilla

................

100 ¸

E

,oo.

-200

°

[] 300

40(3

•400

5

10

15

~

20

10

MgCI2 Norm, wt % Regional . Parilla

-Z._-. . . .

30

Regional Parilla

"...................

~-

100

100" UG

E ~.

20

Total Mg Salts, wt%

E

20o

~.

171

200.

~.... iL =....................... _ U~LG?_.--~'"

a 300

R

.300'

4OO 10

20

30

40

50

Total SO4 Salts, wt %

i

400 0

10

20

CaSO4 Norm, wt %

Fig. 11. Plots of the variation with depth in the concentration of normative salts in Piangil West 2 waters. The vertical line marked SW in each diagram represents the normative concentration of that salt in seawater. For explanation of other letters, see caption to Fig, 6.

water. Also, an alternation of high with lower SO4 levels relative to seawater or the Parilla Sand marks each pair of Upper Geera intervals sampled. The proportion of K in the pore fluids of Piangil West 2 is generally less than half that of seawater and, with the exception of the 231-m

layer at the base of the Lower Geera, is associated with chloride. The high of the 231-m layer balances over 80% of the K. Otherwise, the K-salt distribution is only vaguely associated with the stratigraphic horizon; the pore fluids of the section above the clay, at 130 m in the Upper Geera, contain an average one5

0

4

SOURCES OF DISSOLVED SALTS IN THE CENTRAL MURRAY BASIN, AUSTRALIA

third less KC1 than the samples from the section below (except for the sample at 171 m). 4.3. Lake Tyrrell norms Detailed explanation of Piangil West 2 interstitial fluid norms can be attempted by reference to the Lake Tyrrell system. As noted in Section 2.3, Macumber ( 1983, 1984) has subdivided the groundwater regime into three hydrochemical provinces differentiated by salinity. Salt norms for analyses presented by Teller et al. (1982) and Macumber (1983), as representative of each of these three provinces, are given in Table 3. Significant normative differences exist within these groups and can be related to the details of their geochemical evolution. The similarity of seawater norms to Macumber's (1983) average Parilla Sand water and the Tyrrell 1 bore (Table 4) of Teller et al. (1982) indicates a marine origin for the major solute matrix in the Tyrrell system. The other analyses grouped with the Parilla Sand by these authors illustrate an increase in normative NaC1 and decrease in normative SO4-salts expected with the observed evaporative concentration and gypsum precipitation in the highly ephemeral saline lake or playa environment. Norms for associated saline springs suggest this trend continues irregularly through dilution, dissolution and recycling of the halite component, which is precipitated only under more highly saline conditions accompanying surface drying. Some of the total SO4-salt depletion can be attributed to the precipitation of additional non-marine CaSO4 by Ca introduced into solution through acid-sulfate (Macumber, 1984) hydrolysis of carbonate or Ca-bearing silicate (e.g., plagioclase) in near-surface sediments (Long et al., 1992a, b). Further evolution in water composition is reflected in the first norm of Table 3 classified by Teller et al. (1982) as Lake Tyrrell reflux, which represents capillary or surficial playa concentrate. The alkali sulfate component in the Folly Pt. spring (Table

149

3) most likely results from siliciclastic weathering (e.g., sodic feldspar) or alkaline-earth exchange for sodium on resulting clays. Compared to the compositional trends of the surface waters, evaporatively concentrated, but sodium chloride and non-Ca sulfate undersaturated, saline lake brines, recharged into the subsurface by density differences, retain their more soluble Na-Mg-sulfate and -chloride components. The extent of saturation and precipitative loss of mixed Ca- and Na-sulfates determines the degree of normative shift toward a more magnesian assemblage. Thus in the subsurface waters of the Lake Tyrrell area below the zone of capillarity, total Mg-salts are conserved or actually increased relative to seawater, except where recycling of surficial halite crusts by dissolution causes an increase in the proportion of NaC1. Total SO4-salts in dilute surface waters can be increased in the subsurface by re-solution of CaSO4 from evaporitic strata as noted in the Upper Geera, whereas the downward transport (diffusive or advective) of residual hypersaline solute matrix would have been depleted in proportion of SO4 by surficial CaSO 4 precipitation. All these effects can be seen in different examples grouped by Teller et al. ( 1982, table 1 ) as Lake Tyrrell reflux (Table 3; Folly Pt. 1, the 83C bore, and Bimbourie 1001 ). In contrast, the representative Lake Tyrrell reflux presented by Macumber ( 1983, 1984) is essentially the normative equivalent of surface brine ponds at saturation with halite, which are depleted in both Mg and SO4 relative to seawater. The norm for the 83C bore water (Table 4) appears the most evolved by evaporative concentration without sulfate precipitation. Its composition is somewhat lower in NaC1 than seawater, which is readily attributable to precipitation of halite, and it is enriched in both MgSO4 and MgC1. At the same time the less saline Timboram-Wahpool reflux and lower Tyrrell Creek analyses given by Teller et al. (1982) are most notably depleted with respect to total SO4, despite slightly highe! proportions of CaSO4. Thus the normative

B.F. JONES ET AL.

150 TABLE 4

Representative salt norms (in wt%) from Lake Tyrrell system, Murray Basin Depth (m) Seawater

TDS (g 1-1 )

NaCI

KC1

MgCI2

CaSO4

MgSO4

MgCO3 Total Mg-salts

Total SO4-salts

surface

35

78.2

2.2

9.2

4.0

6.1

-

l 5.3

10.1

< 80 6-16

37 39

78.6 78.7

0.4 0.6

8.3 9.3

3.7 3.2

9.0 8.3

-

17.3 17.6

12.7 11.5

surface 48 surface 0.5

315 89.1 173 283

89.6 1.0 87.0 90.6

0.4 6.3 1.O -

6.6 0.6 6.8 -

0.5 3.0 0.6 1.2

2.8

9.3 alk. SOa: 7.6

9.4 3.6 11.4 0.7

3.7

? 45-55 ~ 40

271 257 232

77.4 84.2 85.1

0.5 0.6 0.5

8.8 6.4 6.0

0.5 0.6 0.8

12.8 8.1 7.6

0.03

21.6 14.5 13.6

13.3 8.7 8.4

20 ~ 30

121 114

78.3 79.3

0.8 0.6

14.2 11.0

1.7 2.4

4.7 6.6

.07

18.9 17.6

6.2 9.0

surface surface surface

83 117 312

82.3 79.8 83.0

0.3 0.6 0.6

0.9 7.6 8.0

4.7 3.6 0.5

11.8 8.3 8.0

.19 .13 -

12.7 15.9 16.0

16.5 11.9 8.5

(A) Parilla Sand and associated: [ 1 ] Nangilor-Colignan [2] Tyrrell 1 bore

(B) Lake Tvrrell: [3 ] [2 ] [ 2] [2]

Lake Tyrrell, northern end Spring A, N W end surface Spring B, 100 m from A Folly Point 1

4.6 0.7

5.2 9.5

(C) Lake Tyrrell reflux: [2] 83C bore [2] Bimbourie 1001 [ 1 ] Lake Tyrrell reflux, ave. ( n = 12)

(D) Timboran- WahPool reflux: [2] Tyrrell 84B bore [ 1 ] Timboram-Wahpool reflux, ave. ( n = 8 )

(E) Tyrrell Springs: [1] N N E Tyrrell 2 [ 1 ] SE Tyrrell [1] T - P 2 N W T y r r e l l

References: [ I ] = M a c u m b e r ( 1984); [ 2 ] = Teller et al. ( 1982, table 1 ); [ 3 ] = Teller et al. ( 1982, table 3).

analyses support the salinity-based Lake Tyrrell area hydrochemical provinces of Macumber ( 1983, 1984), but clearly illustrate the extensive compositional variance which occurs within each province. The compositional variability of the Lake Tyrrell system is undoubtedly a complex function of rates of evaporation, rainfall, runoff and recharge. This interrelationship determines at what stage in the chemical evolution of a surficial brine it is recharged into the subsurface. Of course, the drive for downwelling is primarily the density of the surficial brine itself, but downward recharge can be enhanced by increased lake levels as well. The increase in hy-

draulic head resulting from lake level rise may not induce much mixing if the density contrast between inflow and residual brine is sufficient to maintain stratification. The hydrologic cross-sections of Macumber ( 1983, 1984) indicate that groundwaters underlying the shallower Lakes Timboram and Wahpool in the Lake Tyrrell system typically are less saline than Lake TyrreU recharge waters. This might be a consequence of less total salt accumulation in these smaller basins prior to recharge, and thus a greater dilution by the Parilla groundwaters. However, because of a closer approach to complete drying in the smaller lakes, their waters are more geo-

SOURCES OF DISSOLVEDSALTS IN THE CENTRAL MURRAY BASIN,AUSTRALIA

151

titatively assessed by Wood and Sanford (1990) and Sanford and Wood ( 1991 ).

chemically evolved by evaporative concentration and fractional salt precipitation than the Tyrrell waters. Indeed, this is reflected in the higher MgC12 and lower total SO4 components of Timboram-Wahpool reflux norms. With the addition of a sodium component to the Tyrrell area saline lake or playa brines by surficial weathering and soil water (as seen at Folly Pt., Table 4) the next salt to reach saturation after gypsum should be Na2SO4, not halite, though mirabilite or thenardite have not been reported. With any fractional precipitate loss of sodium sulfate, Timboram-Wahpool-type reflux can appear similar in composition to a post-halite bittern, even with more frequent, dilute, a n d / o r lower volume recharge events. The profound effect of the leakage ratio, (inp u t ) / ( o u t p u t ) , for such basins on brine evolution and evaporite formation has been quan-

5.4. Comparison of results Except for the regional Parilla Sand waters, all of the brines from the Lake Tyrrell system are more concentrated than the pore fluids of Piangil West 2. However, the normative resuits suggest some distinct similarities in solute distribution and the controlling processes (Fig. 12). The Piangil Parilla Sand, lower Bookpurnong and Upper Geera Clay pore-fluid norms are somewhat similar to those for subsurface Lake Tyrrell reflux, with a generally high MgSO4 component, as well as high total Mg- and SO4-salts. In contrast, norms of pore waters from the Lower Geera, with the exception of the clay layer at 231 m, are more akin to Timboram-Wahpool reflux, e.g. high in

EVAPORATIVE CONCENTRATION

~_

- Lake Tyrrell SEA

~Tyrrell

REGIONAL I PARILLA ~ ,

WATER AOO,FERI \ '

/

-

~ /~...__~_

/

gV0su

\ [

~.

I wanp I refit

~"~ Oxidize sulfide

~ plus

Saltponds [:t" brine springs ]

peripheral springs ~ - ~ / ~

dissolve sdmate El. Ca carbonate

~-~

ppt. Mg, Na2SO 4

precipitate halite

~'- ~

ycle halite

~dissolve

Lower Bookpurnong waters Upper Geere

Clay weters ~ Lower Geere Clay waters Renrnark Group waters

Fig. 12. Schematicstratigraphic, hydrochemicaland hydrodynamicrelationshipssuggestedby calculatedsalt norms for the Lake Tyrrell system and pore waters at PiangilWest 2.

152

MgC12 but usually closer to seawater in total Mg- and SO4-salts. The higher CaSO4 content of all Piangil porefluid norms reflects their lower salinity and less control of CaSO4 by gypsum saturation. With a single exception at 125.2-m depth, the Piangil pore fluids from below the upper Bookpurnong contain normative levels of NaC1 equal to or less than seawater. The Piangil pore-fluid norm for the upper Bookpurnong Beds resembles that for the springs at the NW end of Lake Tyrrell, though higher in CaSO4 and lower in NaC1. This is most likely related to depth variation in gypsum and halite being recycled. However, this fluid also has the lowest proportion of Mg-salts of any sample, except for surface brines of the Lake Tyrrell system, and raises the possibility of magnesium loss through clay interaction (Jones, 1986) or factional precipitative loss of an alkali-magnesium double salt (e.g., bloedite). Most of the Piangil West 2 results are compatible with the dilution of variably fractionated bitterns slightly depleted in Na-salts, which have been recharged downward from the surface and then have dissolved a variable amount of CaSO4. Both the Piangil and Lake Tyrrell brines are depleted in proportion of potassium relative to seawater, usually by > 50%, suggesting uptake by clay (or perhaps alumino-sulfates, especially at Tyrrell; Lyons et al., 1992). The association of high MgSO4 with clay seams in both the Upper and Lower Geera at Piangil West 2 is illustrated in the pore-fluid norms from 130- and 230-m depth. This can result from the dissolution of dolomite plus gypsum and the precipitation of calcite or "dedolomitization", leaving MgSO4 in solution. Additional variation in the norms may be related to differential diffusion or diagenetic reaction appearing in solution data as K or Mg for Ca exchange, and producing CaC12 in norms for pore fluids below clay layers.

B.F. JONES ET AL.

6. Conclusions The chemical, isotopic and normative composition of subsurface waters in Piangil West 2 and the normative evaluation of the evolution of shallow saline waters in the Lake Tyrrell system permit some conclusions to be made regarding the origin of solutes in saline waters of the central Murray Basin. The solutes in nearly all of the waters we have studied have a pronounced marine signature with modifications reflecting subaerial evaporative loss of salts and local dissolution and exchange with the surrounding sediments at depth. The composition and salinities of these waters cannot be explained solely by either diagenetic processes or by the simple evaporation of waters associated with continental weathering, as suggested by Bonython (1956). Mass-balance considerations indicate that the source of these marinetype solutes cannot simply have been connate seawater buried in the Geera and Bookpurnong sediments at the time of their deposition, as there has been significant import of solutes downward from the sands of the Parilla into the underlying marine silts and clays of the Geera Clay. It is possible, as has been proposed by previous authors (e.g., Lawrence, 1975), and demonstrated for the Yilgarn (McArthur et al., 1989), that significant quantities of additional marine salt, blown into the Murray Basin as aerosols, have subsequently been dissolved in shallow regional groundwater systems basin-wide, and have been transported by groundwater into areas of large evaporative loss in the central part of the basin. This origin for the solutes would help explain why the isotopic compositions of most of the subsurface saline waters at Piangil West 2 borehole have a strong meteoric signature, whereas the dissolved salts appear so similar to a marine assemblage. In addition, the data indicate that the Piangil West 2 pore fluids from

SOURCES OF DISSOLVED SALTS IN THE CENTRAL MURRAY BASIN, AUSTRALIA

the Parilla Sand-Geera Clay strata are dominated by fluctuations in downward recharge of variably evaporated surface waters, such as in the Lake Tyrrell system, rather than just mixing with the less-saline Renmark Group aquifer fluids below. The normative analyses have aided the unraveling of the various hydrologic and mineralogic processes involved.

Acknowledgements J.S.H. would like to thank Peter J. Cook for the opportunity to spend part of a sabbatical in 1987 with the Hydrogeology Group at the Bureau of Mineral Resources, Canberra, participating in the Murray Basin Project; more recent work on Murray basin material has been supported in part by NSF Grant EAR9019342. W.R.E. publishes with the permission of the Executive Director, AGSO. We also thank J.R. Kellett and C.M. Brown for useful discussions on the Murray Basin. J. Spring and G.F. Sparksman assisted in sample preparation and analysis, and isotopic analyses were by J.M. Olley. We are very grateful for review of the manuscript by Briant Kimball, D.T. Long and J.K. Bohlke, and for aid in preparation of the typescript by Marge Shapira and of Fig. 12 by Leslie Robinson. Publication approved by the Director, U.S. Geological Survey.

References Bodine, Jr., M.W. and Jones, B.F., 1986. The SALT NORM: A quantitative chemical-mineralogical characterization of natural waters. U.S. Geol. Surv., Res. Invest. Rep. 86-4086, 130 pp. Bodine, Jr., M.W. and Jones, B.F., 1990. Normative analysis of groundwaters from the Rustler Formation associated with the Waste Isolation Pilot Plant (WlPP), southeastern New Mexico. Fluid-Mineral Interactions: A Tribute to H.P. Eugster. Geochem. Soc., Washington, D.C., Spec. Publ. No. 2, pp. 213-269. Bonython, C.W., 1956. The salt of Lake Eyre - - Its occurrence in Madigan Gulf and its possible origin. R. Soc. S. Aust. Trans. 79: 66-90. Brown, C.M., 1985. Murray basin, southeastern Aus-

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