evaluation of salt sources and loads in the upland areas of the murray–darling basin, australia

HYDROLOGICAL PROCESSESHydrol. Process. 23, 2485–2495 (2009)Published online 01 July 2009 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/hyp.7355

Evaluation of salt sources and loads in the upland areasof the Murray–Darling Basin, Australia

Ian White,1 Ben C. T. Macdonald,1* Peter D. Somerville1 and Robert Wasson2

1 Fenner School of Environment and Society, Australian National University, Canberra, ACT 0200, Australia2 Chancellery, Charles Darwin University, Darwin, NT 0800, Australia

Abstract:

Estimates of catchment salt balances throughout the Murray-Darling Basin (MDB) suggest mobilization of salt stores andgenerally increasing stream salinities. It is widely assumed these salts are rainfall-deposited cyclic salts of marine-aerosolorigin, concentrated by evapotranspiration, stored in groundwaters and mobilized by clearing of native vegetation. In this paper,the assumptions about the sources of salts, in upland catchments in the south-eastern region of the MDB are re-evaluated.The estimation of dissolved salts is re-examined using electrical conductivity (EC), the assessment of stream salt loads fromsporadic EC measurements and the sources of salts in upland catchments. It is concluded that, although evapotranspirationplays a significant role in the concentrations of salts in the lower MDB, mineral weathering is a major contributor to streamsalt loads in upland rivers of the MDB that supply over 80% of surface runoff to the Murray River. Copyright 2009 JohnWiley & Sons, Ltd.

Additional Supporting information may be found in the online version of this article.

KEY WORDS salinity; monitoring; evaluation; mineral weathering; water quality.

Received 20 August 2008; Accepted 17 April 2009

INTRODUCTION

Increases in stream salinity due to the altered hydrol-ogy of catchments resulting from land-use changes, cou-pled to frequent ENSO-related droughts, pose a majorchallenge to sustainable environmental management ofAustralia’s Murray–Darling Basin (MDB) (Figure 1).Increasing trends in stream salinity and in its surrogate,electrical conductivity (EC), in the lower South Aus-tralian reaches of the River Murray were recognized over30 years ago (Collett, 1978; Cunningham and Morton,1982; Morton and Cunningham, 1985). Estimates of theratio of chloride input in rainfall to the MDB to chlo-ride output in the lower River Murray have revealed thatsubsurface stores of salt are being mobilized in the MDB(Blackburn and McLeod, 1983; Simpson and Herczeg,1994). In the lower River Murray this is attributed tothe discharge of saline groundwater from MDB aquifersand of saline drainage from irrigation areas (Simpsonand Herczeg, 1991; Herczeg et al., 1993, 2001). In Aus-tralia these findings have generated widespread concernover stream salinization, dryland salinity, water alloca-tion and the sustainability of current land uses and ledto a $A1Ð4 billion National Action Plan for Salinity andWater Quality aimed at reversing salinity increases andmoving towards sustainable landscape management.

Jolly et al. (2001) analysed stream salinity trendsacross the whole MDB and this work provides the

* Correspondence to: Ben C. T. Macdonald, Fenner School of Envi-ronment and Society, Australian National University, Canberra, ACT0200, Australia. E-mail: [email protected]

first insight into the temporal and spatial changes inEC of the region’s major streams and was used toinfer the catchment salinity status within the MDB.The task of pulling together and analysing disparatesources of often sparse data in order to provide thefirst comprehensive analyses of trends in stream salinityand the salt balances of catchments throughout the largebasin such as the MDB by Jolly et al. (1997a, 2001)was clearly difficult and exacting. The use of a broad-brush approach employing engineering approximationsby Jolly et al. (1997a, 2001) was a rational approachto identify those saline ‘hot spots’ that needed moredetailed investigation and to allow the rapid developmentof policy and management options.

Jolly et al. (1997a, 2001) concluded that dry land salin-ity in catchments draining the western slopes of the GreatDividing Range was a significant contributor to increas-ing stream salinities in the Basin. They also comparedestimates of the influx of airborne oceanic salt aerosols(cyclic salts) deposited in rainfall to the calculated saltload exported by streams, using mostly sporadic measure-ments of stream EC to infer salt loads. A main assumptionwas that the only source of salt in the MDB is cyclicsalts of marine-aerosol origin (see Conyers et al., 2008),deposited by rainfall, concentrated by evapotranspirationand stored in groundwaters (Peck and Hurle, 1973). Inthis model, clearance of deep-rooted native vegetationand its replacement with shallow-rooted annual cropsincrease recharge, causing groundwater pressures to riseand increasing the discharge of stored saline ground-water into streams. Under these assumptions, estimates

Copyright 2009 John Wiley & Sons, Ltd.

2486 I. WHITE ET AL.

Figure 1. The Australian Murray Darling Basin. Sampling locations on the Murray River, 1—Jingellic; 2—Heywoods; 3—d/s Yarrawonga Wier;4—d/s Torrumbarry Wier; 5—Barham; 6—d/s Swan Hill; 7—d/s Wakool Junction; 8—Euston d/s Wier; 9—Red Cliffs; 10—Lock 9; 11—d/s RufusRiver Junction; 12—Merbein; 13—Lock 5; 14—Lock3; 15—Waikerie; 16—Morgan, 17—Murray Bridge; 18—Tailem Bend; and 19—Milang atLake Alexandrina. Data for the Murray River and its tributary sourced from Mackay et al. (1988) and presented in Supporting Information (Table I)

of the ratio of cyclic salt load exported in streamsto that imported in rainfall can be used to infer bothcatchment salt load output/input ratios (SO/SI) and theimpacts of post-European settlement land use changeon stream salinization throughout the MDB. The saltload SO/SI ratios estimated by Jolly et al. (1997a, 2001)revealed some surprises. Several upland catchments withhigher rainfall which are located in relatively undisturbedareas have salt SO/SI ratios considerably greater thanthe equilibrium value of 1 (Table I). This, together withother unexpectedly high ratios in upland catchments withhigher rainfalls and lower stream EC, suggests additionalsalt sources in MDB catchments other than the cyclic saltdeposition. Collett (1978), Mackay et al. (1988), Simpson

and Herczeg (1994) and Herczeg et al. (1993, 2001) havediscussed the importance of weathering from upland areasas a source of salt.

Over 20 years ago, Gunn (Gunn and Richardson,1979; Gunn, 1985) argued that the main source of saltsin groundwater in the New South Wales’s southerntablelands was from previously deeply weathered rocks,not cyclic salt. Gunn’s study showed that groundwatersalinity correlated with the extent of weathering of rocksin the Palaeozoic Tasman Fold Belt. In addition, soilacidity, which is either an actual or potential problem inlarge areas of the south-eastern MDB, could contribute toincreased weathering (Murray–Darling Basin MinisterialCouncil, 1987). This forms an alternate hypothesis for the

Copyright 2009 John Wiley & Sons, Ltd. Hydrol. Process. 23, 2485–2495 (2009)DOI: 10.1002/hyp

SALT SOURCES AND LOADS IN THE UPLAND AREAS OF MURRAY–DARLING BASIN 2487

Table I. Estimated average annual salt output/input ratios (SO/SI) for selected catchments based on chloride inputs in rain and streamoutputs of chloride compared with those estimated by Jolly et al. (1997a) based on TDS inputs and outputs. The weight ratio of

[Cl]/TDS used in this estimation is also shown

River Location SO/SI Jolly et al. (1997a) Mean EC Output [Cl]/TDS(kg kg�1)

Input [Cl]/TDS(kg kg�1)

SO/SI Chloride

Murray Heywoods 1Ð88 56 0Ð11 0Ð23 0Ð88Murray Yarrawonga 1Ð26 61 0Ð14 0Ð23 0Ð77Mitta Mitta Tallandoon 1Ð28 56 0Ð07 0Ð23 0Ð37Kiewa Bandiana 2Ð38 50 0Ð11 0Ð23 1Ð16Ovens Peechelba East 1Ð91 77 0Ð18 0Ð23 1Ð54Murrumbidgee Angle Crossing 1Ð40 138 0Ð14 0Ð27 0Ð79Murrumbidgee Halls Crossing 1Ð91 205 0Ð14 0Ð27 0Ð84Goodradigbee Wee Jasper 1Ð93 66 0Ð11 0Ð23 0Ð95Lachlan Forbes 4Ð56 440 0Ð17 0Ð23 3Ð39Macquarie Dubbo 2Ð90 330 0Ð14 0Ð30 1Ð44Castlereagh Coonamble 0Ð61 485 0Ð23 0Ð32 0Ð50

origin of salts in the upland catchments of higher rainfall:a store of salt from the intense and deep weathering ofTertiary rocks and a continuous current supply of saltfrom mineral weathering resulting from weak soil acidreactions.

In the south-eastern upland MDB, conflicting hypothe-ses for the sources of soluble salts in upland areas havefar-reaching land management and policy implicationsfor individual catchments, which suggest that the salin-ity sources and salt load output/input ratios should bereassessed.

In this paper the process of estimating stream saltloads and constructing salt load output/input ratios forupland areas of the MDB as well as assumptions aboutthe sources of salts will be evaluated. NaCl salt in thelower MDB catchment is a major contributor to salt loadin the River Murray, but mineral weathering as a resultof water–rock interactions in upland catchments couldbe a major contributor to salt loads in the upland reachesof the MDB.

METHODS

Relation between stream EC and total dissolved solids(TDS)

Often salinity is thought to be only the dissolved massof NaCl in water, which, while a useful approximation, istoo simplistic. In catchment management it is more usefulto consider salinity as the mass of all dissolved salts inwater. Hem (1992) found that for a variety of naturalwaters in the United States a simple approximationrelating the TDS or the concentration of a single ionspecies [X] to EC was useful for particular ranges of saltconcentration and a fixed suite of dissolved salt species:

TDS or [X] D A ð EC25 �1�

In Equation (1) TDS or [X] is conventionally in mg/l,EC25 is the measured EC (�S/cm) corrected to 25 °Cand A is not constant over a wide concentration rangeor for different water chemistries. For natural waters,Hem (1992) found that A varied from 0Ð54 to 0Ð96,

with the predominance in the range 0Ð55 to 0Ð75. Hestrongly advocated that the ionic composition of streamsshould be determined at each measurement site and usedto calibrate EC values. An inherent bias is introducedwhen Equation (1) is fitted to data because it emphasizeshigher EC values. There are also inherent limitations inconverting stream EC values into TDS or individual ionconcentrations since EC is a physical rather than chemicalproperty of water samples.

The use of the value A D 0Ð64 in Equation (1) has beenadopted widely for waters in the MDB as an engineeringapproximation to estimate stream salt loads. This valuenow appears to have been adopted as the ‘standard’value for waters in Australia (Mackay et al., 1988;Simpson and Herczeg, 1994; Jolly et al., 1997a,b, 2001;Williamson et al., 1997; Walker et al., 1998) despite thefact that Pearce and Close (1983) suggest that A D 0Ð56is more appropriate for the River Murray. EC valuesof streams in the MDB vary over a wide range, froma minimum of 23 µS/cm to as high as 34 900 µS/cm(Mackay et al., 1988). In addition, the chemistry ofheadwaters in higher rainfall areas differs substantiallyfrom that of lowland streams influenced by groundwaterdischarge and irrigation returns (Mackay et al., 1988;Simpson and Herczeg, 1991, 1994; Herczeg et al., 1993).It seems therefore unlikely that the simple Equation (1)with A D 0Ð64 should hold over such a wide range of saltconcentrations and water chemistries in the MDB.

While not having the correct limiting behaviour at verylow concentrations (Robinson and Stokes, 1970), a powerlaw provides an excellent fit to conductivity data andTDS or single ion concentrations [X] of mixed electrolytesolutions over a wide range of concentrations:

TDS or [X] D a.ECb �2�

Here a and b are constants. The advantages ofEquation (2) over other semi-empirical forms are thatthere are only two empirical parameters; Equation (2)weights EC values over a wide range, not just the highest;and it gives TDS or [X] D 0 when EC D 0. Historical datafrom the MDB will be used to evaluate the proceduresfor converting EC to TDS.



Estimation of salt loads in streams with sparse EC data

The task of estimating long-term stream salt loadsfrom daily river flow and sporadic EC data is difficult.Often, no better than once-monthly EC readings werecollected until recently. To overcome this difficulty, Jollyet al. (1997a, 2001) first calculated the daily salt loads(SL) (tonnes/day) for each date on which EC had beenmeasured, and used the measured daily stream flow Q(Ml/day) and Equation (1) with A D 0Ð60:

SL D TDS ð Q/1000 D 0Ð60 ð EC ð Q/1000 �3�

The factor 1000 in Equation (3) is a consequence ofthe conversion of units. But Equation (1) with A D 0Ð60overestimates TDS in streams with low EC, so it mayhave a disproportionate effect on the calculated salt loadsexported by streams. In order to estimate daily salt loadsfor the lengthy periods with missing EC data, Jolly et al.(1997a, 2001) established regression relations betweenthe salt load and daily flow for selected gauging stations,when EC readings were separated by more than 7 days.

SL D G ð Q �4�

with G a constant. Most gauging sites had regressionswith coefficients of determination R2 greater than 0Ð9.Only a few sites analysed by Jolly et al. (1997a, 2001)required separate regressions for low flows and highflows, or for seasonal regimes, in order to obtain adequatecorrelations. These regression relations were then usedto estimate daily salt loads from the remaining largenumber of daily stream flows where there were noEC measurements. The above procedure is physicallysensible and is needed only when EC data is sparse. Withdaily EC data, daily salt loads can be calculated directly.

From Equations (3) and (4), the above procedureapplies a flow-weighted mean EC, either over the wholeflow regime or, in a few instances, for low or high flowsor seasonal regimes:

0Ð6 EC ð Q/1000 D G ð Q �5�

which simplifies to

EC D 1667 ð G �6�

where EC represents the flow-averaged EC.

Salt input/output ratios

In order to estimate cyclic salt output to input ratios(SO/SI) in catchments that export substantial mineralweathering products, it is necessary to use an assumedconservative tracer of cyclic salt, such as chloride. Theanalyses of Blackburn and McLeod (1983) provide esti-mates of [Cl] and TDS for rainfall inputs into the MDB.The median concentrations of sampled river waters,where possible (Johnson and Muir, 1977; Kelly, 1988;Mackay et al., 1988; Herczeg et al., 1993), have beenused to estimate [Cl] and TDS for salt output from theMDB rivers systems. A weighted ratio (SO/SI chloride)

has been used, where the [Cl]/TDS has been calculatedfor the rainfall and stream flow.

Ion and species ratios as indicators of catchmentweathering processes

The ratio bicarbonate to chloride [HCO3]/[Cl](meq l�1) provides a useful measure of the fraction ofsolutes added by mineral weathering relative to cyclicsalts sourced from rainfall (Mackay et al., 1988; Herczeget al., 2001). Other ratios such as ([Ca] C [Mg])/[Cl],([Na] C [K])/[Cl] and [H4SiO4]/[Na] on a meq l�1 basis,provide additional information about the input of ionsand species from mineral weathering.

RESULTS AND DISCUSSION

Relationship between stream EC and TDS

Despite the obvious nonlinearity, good linear correla-tions result when data is forced to fit Equation (1) overparticular concentration ranges; however, the value of Adepends on the EC range in question. Figure 2 illustratesthis using the measured equivalent conductances for solu-tions of NaCl at 25 °C (Robinson and Stokes, 1970). ForEC values close to that of seawater, A D 0Ð70 (Figure 2a)but over a range of EC closer to those found in manystreams in the MDB, A D 0Ð50 (Figure 2b). In this sim-ple example, adoption of A D 0Ð6 would overestimateTDS by over 22% in lower EC waters. The power-lawfit Equation (2) provides a better fit to the full range ofdata in Figure 2 with a D 0Ð382 and b D 1Ð04.

Natural waters, of course, consist of more than asingle salt. The relation between median TDS (mg/L)and median EC25 for samples from the River Murrayand its main tributaries (Mackay et al., 1988; Figure 3a)can be fitted by the power law or the linear regres-sion methods. However, the linear regression methodunderpredicts the TDS values at lower salinities rela-tive to the power law. Figure 3b gives a logarithmic plotof the relation between median chloride ion concentra-tion [Cl] and median EC25 for samples from the RiverMurray and its main tributaries (Mackay et al., 1988).The solid line (Figure 3) shows the fit of the data toEquation (1) where A D 0Ð27. Despite the high correla-tion coefficient (r2 D 0Ð98), the logarithmic plot showsthat Equation (1) is weighted towards the largest EC mea-surement. Equation (1) systematically overestimates [Cl]at low concentrations by up to an order of magnitude.When [Cl] data are forced into a linear fit, a physi-cally implausible negative intercept is often needed toobtain an adequate fit of the actual non-linear dependence(Simpson and Herczeg, 1991, 1994). The power law fitusing Equation (2) [Cl] D 0Ð0006 ð EC1Ð344 fits the dataequally well and more plausibly gives [Cl] D 0 whenEC D 0.

The range of water chemistries and solute concen-trations across the MDB require individual calibrationsunder a range of flow conditions at each major salinity



TDS= 0.50 x ECP =9.3e-9

0

200

400

600

800

1000

1200

1400

0 0.5 1 1.5 2 2.5

EC25(mS/cm)

TD

S (m

g/L

)

TDS= 0.70 x ECP =5.1e-15

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

0 50 100 150

EC25(mS/cm)

TD

S (m

g/L

)A B

Figure 2. Relationship between TDS and measured EC at 25 °C, for solutions of NaCl at concentrations (Robinson and Stokes, 1970) up to amaximum EC of (a) 90000 µS/cm, where A D 0.70 and (b) 2320 µS/cm, where A D 0.50

y = 0.95x0.92

R2 = 0.99

y = 0.5188x

R2 = 0.9910

100

1000

10000

10 100 1000 10000

EC25(µS/cm)

TD

S (m

g/L

)

River Murray

Tributaries

y = 0.0006x1.3445

R2 = 0.97

y = 0.0076x

R2 = 0.98

0.01

0.1

1

10

100

10 100 1000 10000

EC25(µS/cm)

Cl (

mm

ol(-

)/L)

River Murray

Tributaries

Figure 3. Logarithmic plot of the relation between (a) median TDS (mg l�1) and median EC25 (�S/cm) and (b) the median chloride ion concentration(mg l�1) and the median EC25 (�S/cm) for water samples along the River Murray and its main tributaries. The solid line is the fit of the data to

Equation (1) with A D 0.267. Data are from Mackay et al. (1988)

gauging station in the MDB. As an interim measure, thepower law models (Figure 3)

TDS D 0Ð95EC0Ð92 or [Cl] D 0Ð0006EC1Ð3445 �7�

provide a useful estimate of median TDS or [Cl] valuesfor the entire River Murray. The conversion of EC toTDS will also be influenced by turbidity in a preliminarystudy of river chemistry in Australia. Bormans et al.(2001) associated elevated relative levels of Ca and Mgwith increased concentrations of suspended clays at highflows.

Dependence of EC on stream flow

Low EC occurs during high flows, when most of thesoluble salt is exported (Figure 4). In the lower RiverMurray, EC or chloride concentrations [Cl] are weaklyinversely related to the stream flow rate Q (Figure 4):

EC or [Cl] / Q�g �8�

The exponent g in Equation (8) appears to range from0Ð41 to 0Ð45 for [Cl], while g ³ 0Ð31 for EC for monthly

Figure 4. Dependence of the mean annual river electrical conductivity atgauging site kilometre point (KP) 87 with median annual flow at KP 71,both on the lower Murray River. Data are from Simpson and Herczeg

(1991, 1994)

or annual data in the lower South Australian reaches ofthe MDB (Cunningham and Morton, 1983; Morton andCunningham, 1985; Simpson and Herczeg, 1994).



The weak inverse dependence of EC on flow inEquation (8) does not hold over the entire MDB. Insome upland catchments, stream EC appears not to becorrelated with flow (Mackay et al., 1988). The universalapplication of Equation (1) with A D 0Ð60 across allstreams in the MDB could lead to overestimates of saltload exports and consequently of salt output to inputratios, particularly in upland locations with lower ECvalues or at higher flow rates. This could contribute to theapparently anomalous salt output/input ratios that havebeen found in some upland catchments in higher rainfallregions (Jolly et al., 1997a, 2001).

Simpson and Herczeg (1994) noted that the monthlymean chloride concentrations in the lower River Murray(and EC as in Figure 4) varied by less than an orderof magnitude despite the over two orders of magnitudevariation in monthly flows (implying g � 0Ð5 in Equation(7)). This has been attributed to the enhanced salinegroundwater discharge in the MDB during the monthsof high surface discharge, which implies that the timescales of response of groundwater flows to rainfall aresimilar to that of surface runoff over monthly time scales.Collett (1978) and Mackay et al. (1988), on the otherhand, pointed out that the relation between EC and flowdepends on the origin of the dissolved salt in the streamand argued that the major source of dissolved salts inthe upper reaches of rivers is from mineral weathering ofsoils and rocks.

Estimation of daily salt loads in streams with sparse ECdata

Figure 5 shows a typical regression between salt loadand daily flow for a gauging station on the MurrumbidgeeRiver below Hay Weir (site 410136), over the periodDecember 1984 to April 2000. The Murrumbidgee is amajor tributary of the Murray (Figure 1). This site is oneof the few where there is a reasonably long record of bothEC and water chemistry. It is influenced by upstreamdiversions, as well by inter-basin transfers, and liesdownstream of a major irrigation area. Equation (4) givesG D 0Ð1038, which produces a flow weight mean ECof EC D 173 µS/cm (Equation (5)) at the MurrumbidgeeRiver below the Hay Weir. This mean EC is onlyslightly higher than the arithmetic mean EC, 164 µS/cm(� D 62 µS/cm), over the period. The fact that the flow-weighted mean is greater than the arithmetic meansuggests that stream salinity may increase slightly withflow at this site perhaps due to drainage from upstreamirrigation areas.

Sporadic sampling of stream EC does impose a filteron EC–stream flow relationships, and spot measurementsof EC are seldom made at the extremes of river flows.In the example shown in Figure 5, the maximum flowat which EC was measured was 34 020 Ml day�1,while the peak flow over the period December 1984to April 2000 was 53 340 Ml day�1. The minimumflow was 37 Ml day�1 but the lowest daily flow atwhich laboratory EC measurements were made was518 Ml day�1.

Figure 5. The correlation between daily salt load estimated from mea-sured stream EC using SL D 0Ð6 ð EC ð Q and daily river flow for theMurrumbidgee River below the Hay Weir (site 410136) for the period

December 1984 to April 2000

Collett (1978) and Mackay et al. (1988) argued that,at locations where EC is insensitive to flow, mineralweathering may be an important source of dissolved saltin the stream. Gunn (Gunn and Richardson, 1979; Gunn,1985) went further and proposed that deeply weatheredrocks were the source of groundwater salinity in thesouthern tablelands of NSW. It appears that Equation (5)applies to all flows at many of the sites examined byJolly et al. (1997a, 2001), apart from some of the moresaline Victorian rivers and some sites in the Namoi RiverCatchment. These studies suggest that not all salt loadsin streams in the MDB are sourced from stored cyclicsalts.

Sources of salts in the lower MDB

The catchment salt budgets constructed for the MDBare often based on the assumption that all soluble ionscontributing to stream EC are sourced from cyclic salts ofmarine origin. Chemical and isotopic evidence suggeststhat this is predominantly so in the lower MDB, wherecyclic salts concentrated by evapotranspiration over geo-logical time scales are discharged from brackish andsaline aquifers and from irrigation drainage (Herczeget al., 1993, 2001; Simpson and Herczeg, 1991). How-ever, this assumption is less secure in upland catchmentswhere mineral weathering may be an important sourceof dissolved salts. In this section, the main weatheringreactions and sources of salts identified by researchers inthe lower MDB are summarized.

The cations that are mobilized depend on the com-position of the bedrock minerals and clays, which inturn correlate with bedrock lithology. The MDB includesgranitoid and Paleozoic sediment terrains, which mayresult in more available Al and Mg, Fe and Ca ionsrespectively. Acid-induced weathering of carbonates inthe regolith is believed to be the major mineral weath-ering process for groundwater evolution in the MDB(Herczeg et al., 2001). The major source of weak acid



is believed to be dissolved CO2, principally from soilrespiration (Herczeg et al., 1991).

Soil acidification is a major concern in south-easternand central catchments in the MDB, and approximately25% of the area contains problem acid and moderatelyacid soils or soils that have the potential to be acid.Slattery et al. (1998) have shown disturbing rates ofweathering of fine clay fractions in continuously croppedacidic soils over a 12-year period in the south-easternregion of the MDB. The enhanced export of base cationsfrom acidifying catchments has the potential to driveincreased stream salinization.

Importance of mineral weathering in upland catchmentsto stream chemistry

Figure 6 shows transects of [HCO3]/[Cl] and ([Ca] C[Mg])/[Cl] from median ion concentrations along theRiver Murray and Darling River (Mackay et al., 1988;Simpson and Herczeg, 1993). Also shown are the medianratios for these species in ‘uncontaminated’ rainfall across

0

1

2

3

4

5

6

0 500 1000 1500 2000 2500

Distance upstream (km)

Ion

rati

o (m

eq/L

)

Murray Basin [HCO3]/[Cl]

Murray Basin ([Ca]+[Mg]/[Cl])

Darling Basin [HCO3]/[Cl]

Darling Basin ([Ca]+[Mg]/[Cl])

Rain

Murrumbidgee

River

Lower

Castlereagh

River

Campaspe &

Goulburn River

Barr Creek

Figure 6. Ion ratios (meq l�1) for median concentrations of weatheringproducts in the Murray River upstream of the Murray mouth (data fromMackay et al., 1988; Herczeg et al., 1993) and the Darling River upstreamof the Murray–Darling junction. The median ratio for both sets of species

in rain across the MDB is also shown (dashed line)

the Basin, both close to 0Ð53 (meq l�1/meq l�1) (Black-burn and McLeod, 1983; Simpson and Herczeg, 1994).Groundwaters in the lower MDB have [HCO3]/[Cl] thatrange between 0Ð02 for saline groundwater to 0Ð6 for freshgroundwater (Herczeg et al., 2001).

Mineral weathering is the predominant source of dis-solved species in the MDB for locations more than about1500 km upstream of the river mouth (Figure 6). In theseupper reaches, weathering adds over four times (meq l�1)the solutes than does cyclic salt as illustrated in Figure 6.When the ion ratios for median water chemistries in theMurray Basin are plotted against the [Cl] (Figure 7) asimple mixing model appears to describe the [HCO3]/[Cl]chemistry. This is not the case for ([Ca] C [Mg])/[Cl],where the inputs of Barr Creek and the Loddon Rivercause the ([Ca] C [Mg])/[Cl] ratio to rise systematicallyabove that for [HCO3]/[Cl] (Figures 6 and 7).

Locations with [HCO3]/[Cl] and ([Ca] C [Mg])/[Cl]>1 show where mineral weathering contributes as muchsolutes to stream salt loads as those from cyclic salt.Ratios greater than 1 occur in upstream catchments thatdrain the south-western slopes of the Great DividingRange and in the Darling River. The large contributionof mineral weathering in the Darling River (Table I) is ofno surprise (Mackay et al., 1988; Simpson and Herczeg,1994). There are extensive deposits of highly calcareoussoils in its lower catchment and a large area of naturallyacid, skeletal soils over the Cobar peneplain in mid-catchment (Murray–Darling Basin Ministerial Council,1987).

The upper reaches of the Murray, the Mitta Mitta,Kiewa, Bogan (midstream), Darling, Murrumbidgee,Lachlan, Macquarie, Ovens, Broken Creek, and theCastlereagh (above Coonamble), all transport substantialquantities of mineral weathering products (Table I). Largeareas of these eastern and southern catchments have beenidentified as having acid soil problems or potential prob-lems (Murray–Darling Basin Ministerial Council, 1987).Mineral weathering clearly contributes significantly tostream salt loads in a substantial number of the rivers

0.01

0.1

1

10

100

0.01 0.1 1 10 100

Chloride (meq/L)

Bic

arbo

nate

(m

eq/L

)

River Murray

TributariesDarling River

MurrumbidgeeRiver

River Murray D/SSwan Hill

Wakool &Gouburn Rivers

Campaspe River

Lake Victoria

Barr Creek

0.01

0.1

1

10

100

0.01 0.1 1 10 100

Chloride (meq/L)

Cac

lciu

m+M

ages

ium

(m

eq/L

)

River Murray

Tributaries

Darling River

MurrumbidgeeRiver

River Murray D/SSwan Hill

Barr Creek

Campaspe River

Lake Victoria

Wakool &Gouburn Rivers

Figure 7. Ion ratios (meq l�1) indicative of mineral weathering as a function of the median chloride ion concentration for median water chemistriesin the River Murray (data from Mackay et al. (1988)). Dashed line is the 1 : 1 ratio



that drain the western slopes of the Great Dividing Range,together with the Darling River (Figure 5). Those tribu-taries with [HCO3]/[Cl] >1 contribute over 80% of therunoff (including diversions from the Snowy River) tothe lower MDB (Simpson and Herczeg, 1991), and hencemaintenance of water quality within these catchments isimportant for Basin saltinity management.

The headwaters of the Castlereagh River, near Coon-abarabran (Figure 1), have ratios greater than 6 forsome species (Figure 8), similar to the headwaters ofthe River Murray (Figure 6). Mineral weathering shoulddepend on the pH. The most dominant mineral in theWarrumbungles Mountains near Coonabarabran is albite(Johnson and Muir, 1977), and when it is weatheredto kaolinite, the mole ratio [H4SiO4]/[Na] should equal2. In the Castlereagh River, which has its headwatersin the Warrumbungles, the mole ratio of weatheringproducts [H4SiO4]/[Na] increases as pH decreases inthe higher rainfall headwaters; however, the maximummole ratio reached at lower pH is [H4SiO4]/[Na] D 0Ð5(Figure 9). However, this pH dependence may be fortu-itous since diatoms remove the dissolved silica in streams(Mackay et al., 1988). The pH increases downstream asthe Castlereagh River flows through the calcareous soilsof the Darling catchment.

Only the Castlereagh M/S and D/S, the Goulburn,Wakool, Campaspe, Barr Creek and Lakes Victoria andAlexandrina (Milang) (Table I) have weathering ratiosless than 1. Cyclic salt inputs from the Victorian riversand the Wakool are well documented (Collett, 1978;Morton and Cunningham, 1985; Mackay et al., 1988;Simpson et al., 1993). The site in Lake Alexandrinais subject to seawater intrusion during dry periods.The chemistry of the Castlereagh River changes as theriver crosses the Triassic Napperby–Merrygoen beds(Figure 8). Both [Cl] and [Na] increase when the rivertransects these carbonaceous shale beds (Johnson andMuir, 1977). Despite this, ion ratios indicative of mineralweathering are all greater than 1 along a substantial

Figure 8. Ion ratios (meq l�1) indicative of mineral weathering for theCastlereagh River in a transect downstream from the head of the river(May 1973). Also shown is the geology intersected by the river (P DPilliga sanstone; G D Garrawilla lavas, N-M D Napperby-Merrygoenshale beds; QA D Quaternary alluvium)(data from Johnson and Muir

(1977))

Figure 9. Mole ratios for the mineral weathering products of albite[H4SiO4]/[Na] as a function of river pH in a transect downstream fromthe head of the Castlereagh River (May 1973). Lower pH occurs in theheadwaters of the Warrumbungle Mountains [data from Johnson and Muir

(1977)]

length of the river (Figure 8). Similar variations instream geochemistry are found in the Cudgegong River,a tributary of the Macquarie River, and it is believedthat the source of chloride in the headwaters is connateand hosted in sedimentary geological formations from thePermian (Muir and Johnson, 1979). The lower reachesof the Cudgegong River and the Macquarie River (site1800 m upstream Figure 6) have [HCO3]/[Cl] >1.

In the mid-catchment of the River Murray, the ionratios of Goulburn and Campaspe Rivers are less than1 (Figure 6). The chemistry of the lower reaches ofthe Campaspe River is due to salt remobilization withinthe catchment principally from the ‘Shepparton Aquifer’which has TDS values up to 13 000 mg/l and ionratios <1 close to the River Murray (see Arad andEvans, 1987). Similarly, the shallow Shepparton forma-tion ground waters in lower Goulburn River catchmentare saline and have ion ratios <1 (see Cartwright andWeaver, 2005) and the surface waters are characterizedby groundwater discharge. These sites are typical for thelower Murray catchment where cyclic salts are dischargedinto the surface waters from groundwater stores.

Mineral weathering products and stream salt loads

To demonstrate the contribution of mineral weatheringproducts to stream salt loads in the MDB, we havecalculated the cumulative salt load due to Ca, Mg andHCO3 ions for the Murrumbidgee River downstream ofHay Weir at site 410136 (Figure 10) and compared it tothe total salt load estimated by Jolly et al. (1997a) forthis site.

Water chemistry analyses on the Murrumbidgee Riverdownstream of the Hay Weir (site 410136) for the period1995 to 2000 provide a relation between the dissolvedsolids (DSCaCMgCHCO3 (mg/l)) due to Ca, Mg and HCO3

and EC: DSCaCMgCHCO3 D 2Ð298 ð EC0Ð671 (R2 D 0Ð95).This was then used to determine the relation betweenthe salt load due to these species SLCaCMgCHCO3 (tonnesday�1), and daily stream flow SLCaCMgCHCO3 D 0Ð0727 ðQ (r2 D 0Ð99). The procedure of Jolly et al. (1997a,



Figure 10. Cumulative salt load due to the weathering products Ca, Mgand HCO3 in the Murrumbidgee downstream of Hay Weir (site 410136)compared with the total cumulative ‘cyclic’ salt load estimated by Jolly

et al. (1997a)

Figure 11. Relation between the ion ratios (meq l�1/meql�1) indicativeof weathering and the reciprocal of the chloride concentration for the

Murrumbidgee River downstream of Hay Weir (site 410136)

2001) was then followed to estimate the cumulativesalt load due to Ca, Mg and HCO3. The cumulativesalt load due to Ca, Mg and HCO3 at this site onthe lower Murrumbidgee (Figure 10) over the decade1985–1994 is 73% of the total salt load estimated byJolly et al. (1997a). While not all this material is sourceddirectly from weathering products, it is clear that mineralweathering is a major contributor to stream salt load atthis site.

The [HCO3]/[Cl] and [Ca C Mg]/[Cl] ratios for thissite on the Murrumbidgee (Figure 11) lie on almostidentical mixing lines. Together with [Na C K]/[Cl], allratios are greater than 1 and all increase as [Cl] decreases(1/[Cl] increases). The extrapolated values of these ratiosat high chloride concentrations are identical to thosefound in the lower River Murray (Figure 7). The mixingrelation for [HCO3]/[Cl] is also very similar to that forthe Murray (Figure 7).

A simple interpretation of the lower Murrumbidgee siteis that waters there are the result of mixing between two

end members. One is a groundwater-like component withhigher chloride concentrations and relative composition(ion ratios) similar to those of MDB groundwaters (Her-czeg et al., 2001). The other, a low-chloride component,is predominantly composed of mineral weathering prod-ucts, which may be due to waters released from upstreamimpoundments as well as from surface runoff. The lat-ter component contributes mineral weathering productsto the river’s total salt load. The assumption that all saltexported in the river at this site is cyclic salt is inconsis-tent with the river chemistry and leads to a considerableoverestimate of the marine-aerosol-sourced cyclic saltload and the salt load output/input ratio.

Salt load output/input ratios

Blackburn and McLeod (1983) recognized that theTDS of the rainfall samples collected across the MDBincluded significant wind-blown terrestrial dust. Simp-son and Herczeg (1994) attempted to separate dust-contaminated sites by removing stations with calcium tosodium ratios [Ca]/[Na] (meq l�1/meq l�1) greater than1. Jolly et al. (1997a, 2001) also adopted that approach.It was assumed that dust contamination was due to windre-suspension of local, mostly carbonate, evaporate min-erals. A more detailed study of both rainfall and dryfallinputs of salts across the MDB would be valuable, partic-ularly in the drier western and the wetter eastern regions.

The ‘uncontaminated’ rainfall sites identified by Simp-son and Herczeg (1994) still show evidence of terrestrialinputs. The sites have Ca/Mg ratios (meq l�1/meq l�1)between 3Ð7 to 16Ð5 times greater than for seawater, andbecause of this Blackburn and McLeod (1983) used rain-fall data for the very wet 1974/75 period to estimatethe MDB salt budget. In addition, Simpson and Herczeg(1994) used dissolved Cl alone as a conservative tracerfor marine cyclic salt in MDB salt budgets. In contrast,Jolly et al. (1997a, 2001) used the TDS in rainfall at‘uncontaminated’ sites and stream TDS inferred from ECmeasurements to construct catchment salt budgets.

When Cl, which is a conservative ion, is used as atracer for cyclic salts, the salt load output/input ratios forupland catchments are, in most cases, halved (Table II).Catchments can be seen to either be in equilibrium, or insome cases storing cyclic salt. The one exception is theLachlan River at Forbes, which remains an exporter ofstored cycle salt. The assumption that all TDS in streamsis due to marine-origin cyclic salts is not justified formany of the upland catchments of the MDB that havebeen examined in this paper. Mineral weathering, mainlydue to the presence of weak acids in soil, contributes asmuch salt load to upland streams as does the input ofcyclic salt (Table II).

CONCLUSION

The use of a ‘universal’ simple correlation betweentotal dissolved salts and stream electrical conductivity(TDS D 0Ð6ð EC25) for all rivers in the MDB may cause



Table II. Ion ratios from median concentrations of species indicative of mineral weathering for selected locations in the MDB (Datafrom Johnson and Muir, 1977; Kelly, 1988; Mackay et al., 1988; Simpson and Herczeg, 1993, 1994). Values of [HCO3]/Cl and

[CaCMg]/[Cl] >1 indicate significant mineral weathering

Source Locationa [Cl] (µeq l�1) [HCO3]/[Cl](eq eq�1)

[Ca C Mg]/[Cl](eq eq.�1)

[Na C K]/[Cl](eq eq�1)

Murray U/S 2322 km 54 6Ð12 4Ð51 2Ð60Mitta Mitta Confl. MR 2204 km 76 4Ð95 4Ð93 2Ð45Kiewa Confl. MR 2179 km 107 3Ð21 1Ð93 1Ð76Bogan M/S Site 421039 310 2Ð91 2Ð19 2Ð18Darling Confl. MR 825 km 1269 2Ð47 2Ð15 1Ð59Murrumbidgee D/S Confl. MR 1244 km 451 2Ð07 2Ð49 1Ð67Macquarie U/S Dubbo, Site 421001 846 2Ð03 2Ð46 1Ð19Lachlan Site 412036 1156 2Ð01 1Ð73 0Ð98Macquarie D/S Site 421012 1354 2Ð00 2Ð47 1Ð32Murrumbidgee M/S Site 410136 409 1Ð95 2Ð05 1Ð32Macquarie M/S Site 421006 1044 1Ð95 2Ð20 1Ð13Ovens Confl. MR 1995 km 254 1Ð68 1Ð13 1Ð39Broken Creek Confl. MR 1729 km 536 1Ð56 1Ð45 1Ð73Murray U/S 1505 km 395 1Ð33 1Ð18 1Ð33Castlereagh M/S Coonamble Site 420005 2510 1Ð12 1Ð15 1Ð24Castlereagh D/S Site CR18 2708 0Ð81 0Ð97 0Ð89Lake Victoria Confl. MR 700 km 2144 0Ð67 0Ð83 1Ð10Goulburn Confl. MR 1709 km 1156 0Ð64 0Ð70 1Ð24Wakool Confl. MR 1266 km 1410 0Ð52 0Ð77 1Ð04Milang Confl. MR 45km 4175 0Ð47 0Ð64 1Ð11Murray M/S 1391 km 1410 0Ð40 0Ð65 0Ð97Murray D/S 87 km 4823 0Ð32 0Ð51 0Ð94Campaspe Confl MR 1689 km 5077 0Ð30 0Ð59 0Ð75Barr Creek Confl. MR 1444 km 42 310 0Ð06 0Ð38 0Ð59Rainwater Basin-wide 37 0Ð54 0Ð51 1Ð41

a Distances represent distances upstream of the mouth of the Murray. The confluences of major tributaries with the Murray River (Confl. MR) arealso given as distances upstream of the Murray mouth. Site numbers are gauging station numbers.

an overestimate of salt loads in streams with lower EC orat high flows. In the MDB, because of the wide range ofstream salinities, water chemistries and the non-linearityof the relation between EC and TDS, it is unlikely thatthis ‘universal’ proportionality is appropriate. Instead,individual calibrations using chemical analyses at majorgauging stations are required under a range of flow andseasonal conditions.

It has been widely assumed that the principal source ofsalinity in streams throughout the MDB is the dischargeof cyclic salts concentrated in groundwaters. While cyclicsalts dominate in the lower MDB, mineral weathering isan equally important source of these dissolved solutesin upland catchments. Increasing EC trends in somestreams in the MDB could therefore be partly the resultof increased mineral weathering, predominantly in soils.Soil acidity is a major concern over large areas of theMDB. The extent to which soil acidity contributes tostream salinity remains to be determined.

ACKNOWLEDGEMENTS

We thank Myriam Bormans and Phillip Ford of CSIROLand and Water for provision of access to variousdatasets and for helpful and stimulating discussions.The raw water quality dataset for the MurrumbidgeeRiver was provided by the former NSW Departmentof Land and Water Conservation. The assistance of

Stuart Pengelly, DLWC, in expediting access is gratefullyacknowledged. We are grateful to Ross Cunningham ofthe Fenner School of Environment and Society, ANU,for his insights into the power-law behaviour of naturalsystems and John Passioura of CSIRO Plant Industry forexpanding our horizons.

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evaluation of salt sources and loads in the upland areas of the murray–darling basin, australia

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