Transient hydrological conditions implied by chloride mass balance in southeast Australian rivers

Transient hydrological conditions implied by chloride mass balance in southeast Australian rivers

Chemical Geology 357 (2013) 29–40 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo Tran...

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Chemical Geology 357 (2013) 29–40

Contents lists available at ScienceDirect

Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo

Transient hydrological conditions implied by chloride mass balance in southeast Australian rivers Ian Cartwright a,b,⁎, Benjamin Gilfedder a,b,c, Harald Hofmann a,b a b c

School of Geosciences, Monash University, Clayton, Vic. 3800, Australia National Centre for Groundwater Research and Training, GPO box 2100, Flinders University, Adelaide, SA 5001, Australia Limnologische Forschungsstation, Universität Bayreuth, Universitätsstr. 30, 95447 Bayreuth, Germany

a r t i c l e

i n f o

Article history: Received 26 April 2013 Received in revised form 13 August 2013 Accepted 14 August 2013 Available online 20 August 2013 Editor: David R. Hilton Keywords: Cl Groundwater–surface water interactions Landscape change Salinity

a b s t r a c t A robust correlation between electrical conductivity (EC) values and Cl concentrations in river water from southeast Australia allows detailed Cl fluxes to be calculated from continuous EC and river discharge records. Many Victorian rivers export significantly more Cl than is delivered to their catchments by rainfall. Cl* is defined as the mass of Cl exported in the rivers relative to that input by rainfall over a multi-year period (Cl* = 100% indicates that the river exports the same mass of Cl as is input by rainfall). There is a systematic relationship between catchment type and Cl*. Rivers draining cleared plains have Cl* values between 50 and 750%, rivers draining volcanic plains have Cl* values of 770–1600%, whereas rivers with large forested upland catchments have Cl* values of 50–110%. These values are minima as they do not account for Cl exported by groundwater from the catchments. The calculations are based on long-term (up to 22 year) records that span drought and high rainfall periods. The magnitude of Cl* is far higher than can be explained by errors in the calculations or variability in rainfall and runoff, and Cl/Br ratios preclude halite dissolution as a source of Cl. The excess Cl reflects hydrological changes in the catchments. Land clearing on the cleared plains has caused the rise of regional water tables which results in the export of Cl from saline groundwater via increased baseflow to the river systems. Drainage systems on the volcanic plains are re-establishing following impoundment by recent (b4.5 Ma) lava flows; Cl which accumulated in shallow groundwater around saline lakes and marshes developed on these volcanic plains is now being exported via the rivers. The upland catchments have undergone less landscape change and may be in chemical balance. The methodology outlined here provides a straightforward assessment of whether catchments are in chemical balance that may in turn indicate whether they are undergoing hydrological changes. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Assessing if groundwater and surface water systems are in chemical mass balance, that is where the flux of solutes out of a catchment balances solute inputs, is important for understanding whether catchments are changing hydrologically. Solutes in surface water and groundwater are derived from precipitation (rainfall, snowfall, and dry deposition) and the dissolution of minerals. Solutes are exported from catchments via rivers and groundwater, and may also be consumed by mineral precipitation (Fig. 1). Assuming that the boundaries of the surface water and groundwater catchments coincide, the solute balance is: P  XP þ

X

RX ¼ SWo  XSWo þ GWo  XGWo þ ΔST  XST

ð1Þ

(Cartwright et al., 2013), where P is precipitation, SWo is surface water outflow, and GWo is groundwater outflow (all in m3/year). ΔST accounts for changes to the volume of water stored in the catchment ⁎ Corresponding author at: School of Geosciences, Monash University, Clayton, Vic. 3800, Australia. E-mail address: [email protected] (I. Cartwright). 0009-2541/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chemgeo.2013.08.028

due to changes in soil moisture, surface storage, or fluctuations in the water table; ΔST N 0 m3/year implies an increase in water held in storage due to, for example, a rise in the water table. X is the concentration (kg/m3) of the solute of interest in the various components, and the product of the terms gives solute fluxes in kg/year within the catchment. ΣRX is the net rate (kg/year) in the catchment for mineral reactions that produce or consume solute X; ΣRX = 0 kg/year for solutes that have no mineral sources or sinks. Other terms could be included in the mass balance in specific catchments, for example to take into account groundwater or surface water extraction, diversion of surface water from adjacent catchments, or groundwater inflows from adjacent catchments. Detailed long-term rainfall records are available for many parts of the world via national meteorological bureaux. River discharges are well constrained in many catchments; for example, the United States Geological Survey Water Data for the Nation programme in the USA (www.waterdata.usgs.gov) and the National River Flow Archive in the UK (www.ceh.ac.uk/data/nrfa) contain river discharge records that span several decades or longer. Groundwater levels and river and groundwater chemistry data are also locally available (e.g., United States Geological Survey: www.waterdata.usgs.gov and British Geological

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Fig. 1. Schematic depiction of solute and water fluxes in a catchment (modified from Cartwright et al., 2013); terms are defined in the text.

Survey: www.bgs.ac.uk/research/groundwaterdatabases). Combined with estimates of rainfall chemistry (e.g., Commonwealth Scientific and Industrial Research Organisation, 2012), these data allow many of the solute fluxes in Fig. 1 to be calculated. Groundwater fluxes, however, are less well known (although they may be broadly estimated from hydraulic conductivities and hydraulic heads) and there are very few constrains on catchment-wide rates of mineral dissolution or precipitation. While it is difficult to perform accurate solute balances on catchments without these data, it is possible to identify catchments that export significantly more solutes than are input via rainfall. Although the fluxes are not well known, groundwater generally exports solutes from catchments (i.e., GWo N 0 m3/year) and even in closed basins GWo = 0 m3/year. Thus, if the flux of a tracer for which ΣRX = 0 kg/year out of a catchment via the river system exceeds the input from rainfall (i.e., if SW ∗ XSW N P ∗ XP) the catchment is a net exporter of solutes. There are several reasons that catchments may be net exporters of solutes, including: 1) rising water tables increasing baseflow to rivers that results in the increased export of solutes stored in groundwater via the river systems (Peck and Hurle, 1973; Allison et al., 1990; Ghassemi et al., 1995); 2) draining of saline marshes which exports solutes that have accumulated over long periods in the soil zone and shallow groundwater (Bennetts et al., 2006; Cartwright et al., 2009, 2013); 3) release of organically-bound solutes during degradation of soils (Bastviken et al., 2006; Svensson et al., 2012) and/or; 4) rivers in coastal areas exporting solutes that accumulated in groundwater from ocean water during previous higher sea levels (Arakel and Ridley, 1986). All these scenarios result from landscapes undergoing hydrological changes where the rivers are exporting solutes that have accumulated in the catchments over decades to millions of years. 1.1. Cl mass balance in catchments The role of biogeochemical processes in soils in retaining or releasing solutes, particularly Cl, is well known (e.g., Bastviken et al., 2006, 2007;

Svensson et al., 2012). Organic matter in soils retains Cl either via ionexchange or chlorination (Bastviken et al., 2006). If rates of organic matter formation and mineralisation are similar, there may be no net change to soil Cl concentrations. However, where soils are degrading or if soil water residence times or O2 concentrations change, some of this soil Cl may be released (Oberg and Sanden, 2005; Bastviken et al., 2006; Svensson et al., 2012). Calculation of Cl mass balances due to changes in Cl retention in soils has been mainly carried out in small upland forested catchments. Assessments of Cl mass balances in larger catchments where groundwater represents a significant store of solutes that are exported through the river systems are less common. Peck and Hurle (1973) showed that cleared catchments in Western Australia export up to an order of magnitude more Cl than is input from rainfall, whereas adjacent forested catchments were close to being in chemical mass balance. Simpson and Herczeg (1994) determined that the River Murray in Australia exported approximately twice the annual input of Cl from the Murray–Darling Basin, which they attributed to rising water tables and increased export of groundwater-derived solutes via the river system. Cartwright et al. (2013) determined that the Upper Barwon catchment (Victoria, Australia) exported up to 2230% of the Cl input by rainfall, which was attributed to the re-establishment of river systems on a young basalt landscape exporting Cl that had accumulated in shallow groundwater around saline marshes and lakes and/or the rise in the regional water table following land clearing. In this study we examine multiyear Cl mass balance in 17 catchments with different landuse and geology in southeast Australia. Specifically, we assess whether the catchments are exporting more Cl via the river systems than is delivered annually and are consequently out of chemical and hydraulic equilibrium. Finally, we evaluate the landscape changes that may produce hydraulic disequilibrium, identify regions that contribute most to the surface water salinity, and assess the timescales over which changes to the hydrologic systems occur. Most of the studies that have addressed solute balances have focussed on anthropogenic changes; here we also examine the impacts of long-term

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natural changes to hydrologic systems. For the following reasons, we have limited our analysis to Victorian catchments. Firstly water databases in Australia are state based and it is advantageous to use data from a single database where the methods of collection and reporting are uniform. Secondly, the assessment of inter-annual variations in Cl fluxes is simpler if the sites display similar climate variations. The timing and duration of high-rainfall and drought periods are similar throughout southeast Australia but are different from that elsewhere on the continent (Bureau of Meteorology, 2012). 2. Geological, physiographic, and hydrological setting Average annual rainfall in Victoria varies from ~300 mm in the northwest to ~1500 mm in the Victorian Alps (Fig. 2a) (Bureau of Meteorology, 2012). Inter-annual rainfall in southeast Australia is highly variable; the period covered by this study (1989–2011) encompasses periods of average to above average rainfall (1989–1993, 1995–1996, and 2009–2010) and a major drought (1997–2008) (Bureau of Meteorology, 2012). The average annual rainfall between 1989 and 2011 in Victoria was 630 mm, which is ~25 mm below the long-term average. Cl concentrations in rainfall and dry deposition away from the coast in southeast Australia vary between 0.9 and 3.8 mg/L with a median Cl concentration of ~1.5 mg/L (Blackburn and McLeod, 1983; Commonwealth Scientific and Industrial Research Organisation, 2012). The geology of Victoria may be divided into three broad zones (Department of Primary Industries, 2012). A basement of variably metamorphosed and indurated Palaeozoic sediments, volcanic rocks, and igneous intrusions crops out in the east and centre of the state (Fig. 2b). The Palaeozoic basement forms ranges of hills and mountains with steeply incised river valleys, notably in the Victorian Alps and along the Great Dividing Range. The major sedimentary basins (Murray, Gippsland, and Otway Basins) comprise Mesozoic to Recent sediments that overlie the Palaeozoic rocks. The topography of these areas is more subdued, especially that of the Murray Basin in the northwest which is characterised by extensive low-relief plains (Lawrence, 1988). The Newer Volcanics Province comprises extensive Quaternary basaltic flows and pyroclastic deposits in southeast Victoria. The eruption of the basalts buried the pre-existing landscape, blocking drainage courses and forming lakes and marshes in depressions in the lava surface (Joyce, 1988; Dahlhaus et al., 2008). Many of these lakes are endorheic or have very limited outflows (Cartwright et al., 2009; Tweed et al., 2011). The lava flows are younger than 4.5 Ma (Price et al., 2003) and following their eruption the draining systems have partially re-established draining the lakes and marshes (Edwards et al., 1996; Dahlhaus et al., 2008). Groundwater flow within the Palaeozoic basement is largely fracture-hosted and, while transmissivities are locally high in areas of intense fracturing, overall the basement has low hydraulic conductivities (Department of Primary Industries, 2012; Department of Sustainability and Environment, 2012). The shallow groundwater in much of the basement areas has total dissolved solids (TDS) concentrations of b1000 mg/L (Fig. 2c). Shallow groundwater in the sedimentary basins has variable TDS concentrations (Fig. 2c). Shallow groundwater in the Murray Basin has TDS concentrations that range from b1000 mg/L to N13,000 mg/L, with TDS concentrations as high as 100,000 mg/L locally recorded (Lawrence, 1988). Shallow groundwater in the Gippsland and Otway Basins generally has TDS concentrations between 500 and 5000 mg/L (Petrides and Cartwright, 2006; Hofmann and Cartwright, 2013). Shallow groundwater in the volcanic plains generally has TDS concentrations of 3500–13,000 mg/L; although locally TDS concentrations of up to 80,000 mg/L are recorded (Bennetts et al., 2006; Dahlhaus et al., 2008; Cartwright et al., 2009). Despite the wide range of TDS concentrations and the variety of aquifer lithologies, the geochemical processes controlling groundwater geochemistry are similar throughout southeast Australia (e.g., Arad and Evans, 1987; Herczeg et al., 2001; Cartwright et al., 2004, 2006; Bennetts et al.,

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2006; Petrides and Cartwright, 2006; Cartwright et al., 2008; Hofmann and Cartwright, 2013). Except for groundwater within ~5 km of the coast, δ18O and δ2H values are significantly lower than those of ocean water and cluster around the values of local rainfall implying that there is no connate water in the aquifers. Molar Cl/Br ratios in the groundwater are generally b1000, implying that halite dissolution is negligible; this conclusion is consistent with the lack of significant halite deposits within the aquifers. Groundwater geochemistry is largely controlled by evapotranspiration of rainfall during recharge with minor silicate weathering and minor precipitation and/or dissolution of carbonate and gypsum. Cation exchange (especially the sorption of Na onto clays and the release of Ca and Mg) modifies the composition of the most saline groundwater. Landuse in Victoria also corresponds to the geology. Much of the upland basement areas are vegetated with remnant native eucalypt forests or plantation forestry (both eucalypt and pine). Catchments in these areas have commonly been cleared for agriculture only along the floodplains close to the rivers. The sedimentary basins and the basalt plains are more extensively cleared for agriculture that comprises dryland grazing and cropping with zones of irrigation, especially close to the major rivers (Department of Sustainability and Environment, 2012). Land clearing over the last 200 years has caused significant changes to the hydrogeology. The replacement of deep-rooted native vegetation with shallow-rooted crops significantly decreased transpiration rates and increased recharge rates. This landscape change caused water tables in many catchments to rise which has increased baseflow to rivers and converted ephemeral rivers and lakes to permanent water bodies (Allison et al., 1990; Ghassemi et al., 1995). 3. Data sources and methodology The river catchments in this study (Tables 1, 2, Fig. 2a) are designated by their dominant land use and geology as: cleared plains (rivers primarily draining large sedimentary basins where much of the land in the catchment is cleared for agriculture); NVP (rivers primarily draining the basalts of the Newer Volcanics Province in catchments that contain lakes and swamps in dammed drainage courses, again much of the land has been cleared for agriculture); and upland (rivers primarily draining the mountainous areas in the east where much of the catchment sides and headwaters are forested). The studied catchments have multiyear continuous electrical conductivity (EC) and discharge records and well-defined catchment areas draining through the individual gauges (Water Resources Data Warehouse, 2012). The Wimmera and Corangamite catchments are internally drained; all other catchments drain to the Murray River or the ocean. The lowermost gauges with EC and discharge records are located towards the end of the catchment, meaning that the behaviour of the catchment as a whole is captured. The Wannon River is a tributary of the Glenelg River; however, since the confluence is below the lowermost gauge on the Glenelg for which there is EC data, they have be considered as separate catchments. Otherwise tributaries are included in the catchment of the main river. EC and discharge are recorded on a sub-daily basis (typically 30–180 min). There are also ~ 3650 individual measurements of Cl (generally made using ion chromatography) and EC (from calibrated handheld meters) in many of the catchments over a wide range of salinities (Fig. 3). The overall Cl vs. EC correlation has a R2 of 0.994, which allows Cl concentrations to be robustly calculated from EC values. The longest EC records extend from 1989 to the present day (Table 1); discharge records are typically longer, in some cases covering several decades. For the time periods considered in this study (Table 1), the records have b10% missing EC data and b5% missing discharge data with data gaps distributed throughout (i.e., there is no bias for extreme events not being recorded). While EC and Cl are well correlated (Fig. 3), EC (or Cl) vs. discharge relationships in some catchments are irregular, for example Cl vs. discharge in the Ovens catchment has a

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I. Cartwright et al. / Chemical Geology 357 (2013) 29–40

R2 of ~0.5 (Fig. 5). Thus portions of records that have discharge data but which are missing EC data have been omitted rather that estimates of EC values being made. Similarly, no attempts have been made to infill missing discharge data. Many rivers in Victoria have very little or no flow during summer periods, especially during the drought years of 2006–2008. River discharge values of zero were regarded as genuine (rather than artefacts of gauge malfunctions) where they occurred during the summer months and where multiple gauges in the catchment and/or adjacent catchments also recorded zero flow. Daily Cl fluxes were calculated from the sub-daily discharge data and Cl estimates. Cl and water fluxes are presented as both the total flux during the time that the records exist and yearly averages (Table 1), which allows comparison between catchments with different lengths of records. Annual river discharges in southeast Australia vary significantly and one or two years of high discharge may dominate the total discharge on decadal timescales (Fig. 4). Because of this year-on-year variability, median Cl fluxes or fluxes from individually years are less informative than the average fluxes in determining Cl mass balances. Rainfall totals are from the Bureau of Meteorology (2012). Average annual rainfall for each catchment was calculated by distance weighted interpolation using long-term average rainfall of all available rainfall stations in that catchment; for many rainfall stations there is over 100 years of data. For the initial calculations, Cl concentrations in rainfall were assumed to be 1.5 mg/L (Blackburn and McLeod, 1983; Commonwealth Scientific and Industrial Research Organisation, 2012). The average Cl input for each catchment or subcatchment was calculated from the average catchment rainfall and the area of catchment upstream of the gauge (Table 1). We define the parameter Cl* as the mass of Cl exported from the catchment over the entire monitoring period relative to the Cl input by rainfall over that time; Cl* = 100% indicates a catchment where the river exports the same mass of Cl as is delivered by rainfall. 4. Results 4.1. Flow characteristics of Victorian rivers For the time periods considered in this study the total discharge of the rivers varies from 1.4 to 24% of total rainfall (Table 2). The upland rivers have higher relative discharges (8–24%) than those from the cleared plains (1.4–14%) or the NVP (2.1–7.1%), reflecting the higher rainfall and steeper slopes in the upland areas that result in higher runoff. Most of the rivers have discharge records that extend for several decades (Water Resources Data Warehouse, 2012). The net discharge during the time periods considered in this study (Table 2) are similar to those from the longer records, indicating that the flow conditions captured in this study are typical of longer-term conditions. The variation in the annual discharge of Victorian Rivers reflects the annual variation in rainfall (Fig. 4); in particular, discharge in all the rivers was low in the drought years 2002 and 2007. 4.2. Cl concentrations and fluxes Calculated Cl concentrations at the lowermost gauges of the rivers from this study range from 1 to 8260 mg/L (Table 2, Fig. 5). While rivers in all of the catchments record low Cl concentrations during periods of high discharge, there is a significant difference in the median and maximum Cl concentrations between rivers from different regions (Table 1). Median and maximum Cl concentrations at the lowermost gauges in the upland rivers are 4–22 mg/L and 12–64 mg/L, respectively. Median and

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maximum Cl concentrations at the lowermost gauges in the NVP rivers are 574–1875 mg/L and 1144–4068 mg/L, respectively. Median and maximum Cl concentrations at the lowermost gauges in the cleared plains rivers are 20–1589 mg/L and 108–6978 mg/L, respectively; this group of rivers has the widest variability in Cl concentrations. Within the subcatchments there is commonly a wide range of median and maximum Cl concentrations (Table 1). For example, median Cl concentrations at individual gauges in the Barwon and Loddon catchments range from 231 to 3490 mg/L and 27 to 1900 mg/L, respectively. Likewise, the Goulburn River includes the Sunday Creek tributary that has a median Cl concentration of 209 mg/L, significantly higher than the 10–20 mg/L median Cl concentrations at the gauges on the main river. There is an inverse correlation between Cl concentrations and discharge in all catchments (Fig. 5). Because the variation in discharge is much greater than the variation in Cl concentrations, however, there is a positive correlation between Cl fluxes and discharge (i.e., the export of Cl by the rivers is highest at high flows even though the concentration of Cl in the rivers at such times is low: Fig. 5). The relationships between discharge and Cl concentrations or fluxes are closely similar for both calculated and measured Cl concentrations as illustrated for the three catchments in Fig. 5. Average Cl fluxes per hectare of catchment for the lowermost gauges range from 3.8 to 69 kg/ha/year for the cleared plains, 76 to 123 kg/ha/year for the NVP catchments, and 0.59 to 15 kg/ha/year for the upland catchments (Table 2; Fig. 6). Some rivers, however, such as the Wimmera, have relatively modest average annual Cl fluxes (13 kg/ha/year) compared with other cleared plains rivers despite the Wimmera River recording the highest Cl concentrations of the cleared plains rivers. This reflects the fact that Wimmera River has long periods of low or no flow during summer, especially in drought years. 4.3. Assessment of Cl mass balance Cl* values at the individual gauges range from 45 to (Fig. 7; Tables 1 and 2). For the lowermost gauges, Cl* is 50–750% for the cleared plains rivers, 770–1600% for the NVP rivers, and 50–110% for the upland rivers. The largest values of Cl* are commonly recorded from relatively small areas within a catchment; for example, Birregurra Creek (area = 102 km2, Cl* = 5130%) within the Barwon catchment (total area = 2713 km2, overall Cl* = 1600%) or the Dundas River (area =211 km2, Cl* = 34,650%) in the Glenelg catchment (area 3841 km2, overall Cl* = 770%). In most catchments, the lowermost gauges have lower Cl* values than many of the other gauges. This is the result of the location of the monitoring infrastructure, which was partially instituted for assessing salinity as an environmental problem, being focussed on saline tributaries or reaches. The Barwon catchment has a very wide range of Cl* values that are both considerably higher and lower than the Cl* value at the lowermost gauge; this is due to the mixing saline outflows from the upper catchment (through the Inverleigh gauge) with fresher inflows largely from the Leigh and Moorabool tributaries. In most catchments the annual percentage of Cl exported via the rivers relative to the input from rainfall varies considerably (Fig. 8); this reflects the large variations in annual discharge (e.g. Fig. 4) that translate into large variations in Cl fluxes (Fig. 5). 5. Discussion Many of the Victorian catchments export significantly greater amounts of Cl than are input by rainfall. Before discussing hydrological

Fig. 2. a. Location of the study catchments in Victoria (shaded in inset map), grouped according to catchment type. Cleared Plains: AV = Avoca; BK = Broken; CP = Campaspe; GB = Goulburn; LD = Loddon; LT = La Trobe; WM = Wimmera. Newer Volcanic Province: BW = Barwon; CG = Corangamite; GE = Glenelg; HK = Hopkins; WN = Wannon. Uplands: KW = Kiewa; MM = Mitta Mitta; OV = Ovens; TS = Thomson. Gauges are the lowermost in each catchment. Contours show mean annual rainfall. b. Summary geological map of Victoria; NVP = Newer Volcanics Province. c. Distribution of Total Dissolved Solids (TDS) in groundwater from the shallowest unconfined aquifers. Data from Bureau of Meteorology (2012), Department of Primary Industries (2012), Victorian Water Resources Data Warehouse (2012).

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Table 1 Cl inputs, fluxes and Cl* for Victorian catchments. Gaugea

Median Cl

Maximum Cl

Area

Cl inputb

Net Cl inputc

Net Cl fluxc

Cl*d

Average Cl fluxd

Cl fluxe

mg/L

mg/L

km2

106 kg/year

106 kg

106 kg

%

106 kg/year

kg/ha/year

1217 1589

2165 6978

78 2668

0.063 2.15

0.88 43.0

9.32 323

1060 750

0.667 17.5

86 66

27 28 18 26

89 108 106 115

845 270 4750 8230

0.615 0.196 3.46 5.99

11.1 3.53 62.3 108

5.16 2.22 32.5 56.9

46 60 50 50

0.309 0.118 1.9 3.14

Campaspe: cleared plains, rainfall 450 mm Millewa Ck* 1990–2011 173 Axe Ck* 1991–2011 280 Mullers Ck* 1990–2011 178 Mt Pleasant Ck* 1995–2011 187 Coliban 2007–2011 59 Redesdale 1990–2011 189 Echuca 1993–2011 362

1770 822 1770 2132 96 442 2756

32 34 198 248 306 629 2820

0.022 0.023 0.134 0.167 0.207 0.425 1.9

0.46 0.46 2.81 2.67 0.83 8.93 34.2

1.07 5.94 5.42 11.1 3.63 84.4 72.6

230 1290 195 415 440 945 210

0.052 0.277 0.251 0.512 0.801 4.15 4.61

Goulburn: cleared plains, rainfall 480 mm Sunday Ck* 1989–2011 209 Trawool 1991–2011 10 Murchison 1991–2011 20

1295 93 108

340 7335 10770

0.243 5.28 7.76

5.35 106 155

60.4 414 272

1130 390 175

2.69 19.3 13.2

79 26 12

Latrobe: cleared plains, rainfall 740 mm Scarnies Bridge 1997–2011 66 Kilmarny 1997–2011 75

159 157

3850 4464

4.27 4.69

59.8 65.7

394 431

660 660

26.7 30.7

69 69

Loddon: cleared plains, rainfall 400 mm Middle Ck* 1995–2011 Calivil Ck* 1989–2011 Nine Mile Ck* 1988–2011 Bet Bet Ck* 1991–2011 Barr Ck* 1991–2011 Serpentine 1990–2011 Kerang 1997–2011

770 775 347 1232 1900 338 27

3129 13017 9753 4199 35655 1230 5340

270 650 1800 1850 2850 8558 14100

0.162 0.39 1.08 1.11 1.17 5.13 8.46

2.59 8.58 24.8 22.2 23.4 108 118

9.1 78.4 42.1 155 693 446 366

350 915 170 700 2960 415 310

0.557 3.7 1.84 7.59 48.5 21.1 15.6

21 57 10 41 170 25 11

Wimmera: cleared plains, rainfall 400 mm Mt Cole Ck* 2003–2011 915 Concogella Ck* 1993–2011 1687 Mt William Ck* 1993–2011 915 Richardson River* 1989–2011 6945 Glynwyllin 1992–2011 816 Horsham 1993–2011 301 591 Locheil 1993–2011 Dimboola 1990–2011 556

2211 4204 2211 56201 2550 1151 1438 73156

144 239 652 1831 1357 4066 6540 11040

0.086 0.179 0.469 1.10 0.814 2.44 3.92 6.62

0.69 3.22 8.44 24.2 15.5 43.9 70.6 139

8.01 36.0 51.9 84.3 172 91.7 142 291

1165 1120 615 350 1110 210 200 210

0.881 1.96 2.88 3.99 9.25 4.87 7.72 14.3

61 82 44 22 68 12 12 13

Barwon: basalt, rainfall 530 mm Warrambine Ck* 1995–2011 Gerangamete Ck* 1994–2011 Birregurra Ck* 1994–2011 Moorabool River* 2003–2011 Leigh River* 1994–2011 Ricketts Marsh 1993–2011 Kildean Lane 1993–2011 Winchelsea 2000–2011 Inverleigh 1989–2011 Pollocksford 1994–2011

1716 712 3490 231 421 252 323 376 562 574

2631 4997 14469 948 842 1058 1585 1400 3368 1442

57 60 102 150 1125 590 865 1050 1270 2713

0.043 0.048 0.081 0.119 0.894 0.471 0.687 0.836 1.01 2.16

0.69 0.82 1.38 0.95 15.2 8.48 12.4 9.20 22.2 36.7

2.36 1.26 70.6 8.62 162 143 254 144 442 588

350 155 5130 905 1065 1685 2055 1565 1990 1600

0.135 0.071 1.97 1.11 9.29 7.86 14 12.6 21.4 33.5

24 12 193 74 83 133 162 120 169 123

Corangamite: basalt, rainfall 725 mm Deans Creek* 1993–2011 Browns 1993–2011 Woady Yallock 1993–2011

578 2978 1875

1390 8263 4068

49 226 1158

0.0533 0.246 1.26

0.96 4.43 22.7

17.2 26.3 181

1790 595 800

0.955 1.59 9.67

195 70 84

Glenelg: basalt, rainfall 660 mm Dundas* 1999–2011 Grange Burn* 2003–2011 Fulham 1996–2011 Harrow 2002–2011 Burkes 2001–2011 Dergholm 2004–2011

2337 1139 726 1159 1171 1472

5768 1575 2931 3491 2656 3212

211 997 2010 2437 3375 3841

0.209 0.987 1.99 2.41 3.34 3.80

2.51 7.90 29.9 21.7 33.4 26.6

869 119 228 149 253 205

34650 1510 760 690 760 770

44.4 14.5 14.6 23.3 24.2 29.1

2104 145 73 96 72 76

Hopkins: basalt, rainfall 680 mm Wycliffe 1995–2011 Hopkins Falls 1996–2011

6455 1569

22647 6807

1350 8355

1.37 8.52

21.9 136

341 1420

1555 1040

21.6 92.3

160 110

Data range

Avoca: cleared plains, rainfall 540 mm Amphitheatre 1997–2011 Coonooer 1991–2011 Broken: cleared plains, rainfall 485 mm Bosey Ck* 1993–2011 Kattamite 1993–2011 Rices Weir 1993–2011 Gowangardie 1993–2011

3.7 4.4 4.0 3.8

16 81 13 21 26 66 16

I. Cartwright et al. / Chemical Geology 357 (2013) 29–40

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Table 1 (continued) Gaugea

Median Cl

Maximum Cl

Area

Cl inputb

Net Cl inputc

Net Cl fluxc

Cl*d

Average Cl fluxd

Cl fluxe

mg/L

mg/L

km2

106 kg/year

106 kg

106 kg

%

106 kg/year

kg/ha/year

Wannon: basalt, rainfall 675 mm Henty 1996–2011

1324

3923

4159

4.21

63.2

767

1210

49.3

119

Kiewa: uplands, rainfall 1250 mm Kiewa 2002–2011 Bandiana 2002–2011

5 4

26 12

1145 1655

1.72 2.48

15.5 22.3

17.6 24.5

115 110

1.81 2.5

16 15

Mitta Mitta: uplands, rainfall 1200 mm Hinnomunjie 2004–2011 6 Colemans 2003–2011 4 Tallandoon 2004–2011 7

18 27 17

1533 3634 4720

2.76 6.54 8.49

19.3 52.3 59.4

9.43 24.6 30.6

50 45 50

1.24 3.02 4.13

8.1 8.3 8.8

Ovens: uplands, rainfall 1250 mm Peechelba 1999–2011

7

61

6239

9.36

112

83.6

75

6.75

11

Thomson: uplands, rainfall 1000 mm Wandocka 2005–2011 Bundalhuha 1997–2011

11 16

29 40

1417 3528

2.13 5.31

12.8 74.3

9.69 51.7

75 70

1.53 3.51

11 10

a b c d e

Data range

Catchments shown on Fig. 1, * indicates tributary. Cl input calculated from annual rainfall in catchment and Cl concentration of 1.5 mg/L. Calculated over entire monitoring period. Mass of Cl exported by the river relative to that input by rainfall over entire monitoring period. Annualised fluxes.

models that may account for this, the origins of Cl in these catchments must be established and the uncertainties in the calculations assessed. 5.1. Origins of Cl in Victorian rivers Cl in rivers may be derived from rainfall via surface runoff, halite dissolution, and/or groundwater inflows while in-river evaporation may also increase Cl concentrations. There are minor occurrences of nearsurface halite deposits in some of the catchments, mainly in salt lakes (Long et al., 1992; Chambers et al., 1996; Dahlhaus et al., 2008; Bowen and Benison, 2009; Long et al., 2009; Tweed et al., 2011). However the total area of halite deposits is small (≪1% of the landscape) and many of the saline lakes are endorheic and do not drain into the rivers. As for the groundwater, molar Cl/Br ratios in the rivers are generally b1000 (Cartwright et al., 2004, 2009, 2013; Bennetts et al., 2006). The absence of Cl/Br ratios close to those in halite (~104: McCaffrey et al., 1987) and the lack of halite in the landscape precludes halite dissolution

as a significant source of Cl (c.f., Herczeg and Edmunds, 2000). River water in southeast Australia commonly has δ18O and δ2H values that define trends beneath the meteoric water line (Simpson and Herczeg, 1991; Cartwright et al., 2009, 2011, 2013; Meredith et al., 2009), implying that it has undergone evaporation. However, the very high Cl concentrations of the rivers (up to 8260 mg/L) are unlikely to be caused solely by evaporation. The displacement of δ18O values of the river water along local evaporation lines reported in those studies is generally b5‰, implying b25% evaporation (Gonfiantini, 1986). Thus evaporation can only account for a small increase in Cl concentrations over those in rainfall. Many of the rivers in southeast Australia that flow for most of the year receive groundwater inflows (Allison et al., 1990; Simpson and Herczeg, 1994; Dahlhaus et al., 2000; Cartwright et al., 2013). The high Cl concentrations in the rivers most probably reflect the inflows of saline shallow groundwater and/or water from the unsaturated zone that provides water to rivers via increased interflow following high rainfall events (c.f. Winter, 1999).

Table 2 Water fluxes and ranges of Cl fluxes and excesses for the lowermost gauges in the Victorian catchments. Catchment

Avoca Broken Campaspe Goulburn Latrobe Loddon Wimmera Barwon Corangamite Glenelg Hopkins Wannon Kiewa Mitta Mitta Ovens Thompson a b c

Classification

Cleared plains Cleared plains Cleared plains Cleared plains Cleared plains Cleared plains Cleared plains Basalt Basalt Basalt Basalt Basalt Uplands Upland Upland Upland

Gauge

Coonooer Gowangardie Echuca Murchison Kilmarny Kerang Dimboola Pollocksford Woady Yallock Derholm Hopkins Falls Henty Bandiana Tallandoon Peechelba Bundalguha

Data range

1990–2011 1993–2011 1993–2011 1991–2011 1997–2011 1997–2011 1990–2011 1994–2011 1993–2011 2004–2011 1996–2011 1996–2011 2002–2011 2004–2011 1999–2011 1997–2011

Area

Net water flux

Net Cl*a

Range Cl fluxb

Annual Cl exportsc

km2

%

%

106 kg/year

%

2668 8230 2820 10770 4464 14100 11040 2713 1158 3841 8355 4159 1655 4720 6239 3538

4.9 3.7 1.8 14 14 3.3 1.4 7.1 2.4 2.1 2.3 3.3 24 18 12 8.1

750 50 210 175 660 310 210 1600 800 770 1040 1210 110 50 75 70

0.42–55.3 (12.5) 0.352–18.3 (1.85) 0.463–17.4 (4.28) 1.96–50.7 (4.73) 17.1–71.0 (590) 0.326–94.1 (22.0) 0–64.3 (6.00) 8.81–79.7 (29.8) 2.46–19.6 (8.78) 5.95–78.8 (22.6) 28.3–193 (84.2) 12.3–105 (46.1) 0.766–5.25 (2.22) 1.28–8.6 (2.83) 1.24–14.7 (7.40) 1.43–10.4 (2.45)

20–2670 6–305 24–910 25–650 370–1510 4–1120 0–970 410–3690 200–1560 160–2070 330–2270 290–2490 30–210 15–100 15–160 30–200

Calculated using entire record. Range calculated using years with lowest and highest discharges, median value in brackets. Range of annual Cl exports as a percentage of annual input.

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I. Cartwright et al. / Chemical Geology 357 (2013) 29–40

5.2. Errors and uncertainties There are numerous uncertainties in calculation of Cl* that need to be assessed to determine whether they can account for the calculated excess mass of Cl being exported by some of the rivers.

Fig. 3. Cl vs. EC for Victorian rivers included in this study. Data from Victorian Water Resources Data Warehouse (2012).

Fig. 4. a. Variation in annual rainfall for Victoria between 1990 and 2011; data from Bureau of Meteorology (2012). b. Variation in annual discharge for the Corangamite (Newer Volcanics Province); Avoca (Cleared Plains) and Ovens (Upland) rivers between 1990 and 2011. Note the very low discharge in 2002 and 2006 and the large range in annual discharges; other rivers show similar trends. Data from Victorian Water Resources Data Warehouse (2012).

1) River discharge. The variation in Cl fluxes is largely a function of river discharge (Fig. 5), which in turn varies with annual precipitation (Fig. 4). Thus, if the periods assessed in this study included mainly high precipitation years, long-term Cl exports would be overestimated. The records used in this study encompass the significant drought period of 1997–2008 as well as higher rainfall periods such as 1989–1993 and 2009–2010. The average annual river discharges of many rivers over the monitoring periods are similar to or lower than their long-term averages (Water Resources Data Warehouse, 2012) and for most catchments similar Cl* values are estimated if the median rather than the average annual Cl flux is used in the calculations (Fig. 7). In addition, for several catchments, especially the NVP rivers, the lowest Cl flux recorded in any one year is higher than the annual Cl input (Fig. 8). While variations in river discharge do impact the calculated Cl fluxes, they cannot account for the observed mismatches between Cl input and output. 2) Rainfall amounts. Variation in rainfall amounts and consequently variations in the delivery of Cl must be taken into account. The calculations used rainfall totals that are long-term averages for the catchments as a whole. This introduces some errors for the uppermost gauges in catchments with large rainfall gradients; however, the errors will be smaller for the lowermost gauges that capture water derived from throughout the catchment. There is minimal difference to the calculations if the long-term average of rainfall or the rainfall averages from 1989 to 2011 (which coincide with the duration of the EC records) are used. Rainfall in southeast Australia between 1989 and 2011 was ~5% lower than the long-term averages (Bureau of Meteorology, 2012) and using the lower values would decrease the estimate of Cl input and consequently increase the Cl* values. 3) Cl concentration of rainfall. The Cl concentration of rainfall probably represents the largest uncertainty. The adopted value of 1.5 mg/L represents the median value for rainfall in southeast Australia, except for within a few km of the coast (Blackburn and McLeod, 1983; Commonwealth Scientific and Industrial Research Organisation, 2012) and is within the range of inland rainfall globally (Drever, 1997). However, Blackburn and McLeod (1983) report Cl concentrations in inland Victoria are locally as high as 3.9 mg/L (possibly due to Cl from dry deposition) and as low as 0.9 mg/L. Increasing the assumed Cl concentration of rainfall will decrease the estimated Cl* values; however, even varying the Cl concentrations between 1 and 2.5 mg/L, which represents the 10th and 90th deciles of the Blackburn and McLeod (1983) dataset, makes little difference to the overall conclusions (Fig. 7). 4) Groundwater inflows. The calculations assume that groundwater only exports Cl from the catchments. Significant inflows of groundwater with high Cl concentrations from outside the boundaries of the surface water catchments represent a possible additional source of Cl. The boundaries of many of the surface water and groundwater catchments coincide; however, locally groundwater may infiltrate from adjacent catchments. For example the upper Barwon catchment derives some groundwater from the adjacent Corangamite catchment that ultimately discharges to the Barwon River (Cartwright et al., 2013). At steady state, the additional Cl provided by these groundwater inflows is that which is delivered by rainfall to the recharge area from which that groundwater is derived. In the case of the upper Barwon catchment this amounts to an extra 20% of Cl input upstream of the Inverleigh gauge and 9% additional Cl for the catchment as a whole. This again introduces minor uncertainties in specific catchments but does not explain the overall Cl imbalances.

I. Cartwright et al. / Chemical Geology 357 (2013) 29–40

37

Fig. 5. Variation in Cl concentrations and Cl fluxes from the Corangamite (a, b), Avoca (c, d), and Ovens (e, f) rivers; small diamonds were calculated from the EC data as discussed in the text, larger circles are from measured Cl concentrations. Note the inverse correlation between Cl concentrations and discharge but the positive correlation between Cl flux and discharge. Data from Victorian Water Resources Data Warehouse (2012).

Fig. 6. Annualised Cl export from the catchments as measured at the lowermost gauge (data from Table 1). Cleared Plains: AV = Avoca; BK = Broken; CP = Campaspe; GB = Goulburn; LD = Loddon; LT = La Trobe; WM = Wimmera. Newer Volcanic Province (NVP): BW = Barwon; CG = Corangamite; GE = Glenelg; HK = Hopkins; WN = Wannon. Uplands: KW = Kiewa; MM = Mitta Mitta; OV = Ovens; TS = Thomson.

5) Other uncertainties. Other uncertainties include the measured surface area of the catchment upstream of the various gauges. Remeasurement of areas of a selection of catchments using digital topographic maps produced area estimates that were within a few percent of those in Table 1. There will also be minor uncertainties in the EC measurements (most commercial EC loggers are precise to within 0.5%) and in the conversion of EC values to Cl concentrations; however, these are small relative to the magnitude of the Cl imbalance. The observation that the calculated and measured Cl concentrations define similar relationships to discharge (Fig. 5) indicates that estimating Cl concentrations from EC values is robust. There are also possible errors in the rating curves from the gauges used to estimate discharge. A recent assessment of the rating curves suggest that errors may be as high as 30% at low flow but are b5% at medium or high flow conditions (Scanlon et al., 2008). Since the majority of Cl is exported at high flow (Fig. 4), this has a small impact on the calculated values. 6) River regulation. Many of the rivers in this study have reservoirs that modify their natural flows. The reservoirs generally provide additional water to the rivers during low rainfall years and are allowed to fill thus reducing river discharge during high-rainfall periods. River regulation via the reservoirs will thus impact the Cl fluxes in any given year but not the multi-year averages.

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I. Cartwright et al. / Chemical Geology 357 (2013) 29–40

It is difficult to assign precise uncertainties for these calculations. The error bars in Fig. 7 assume that the uncertainties are uncorrelated with a 10% error in rainfall totals, a 50% error in the Cl concentration of rainfall, a 10% error in discharge arising from gauging uncertainties, and a 10% error in the Cl concentrations derived from the EC values. These values, which probably overestimate some of the uncertainties, translate into an overall uncertainty of ~60%. While this is significant, it does not alter the conclusions that most of these catchments are exporting significantly more Cl than is input via rainfall.

5.3. Sources of excess Cl

Fig. 7. a. The relative mass of Cl exported from catchments relative to that input by rainfall (Cl*) for all gauges in the catchments calculated from the entire records for each gauge; uncertainties are calculated for the lowermost gauge. b. Comparison between Cl* calculated for the entire monitoring period and Cl* calculated using the median annual Cl export. Data from Tables 1 and 2. Cleared Plains: AV = Avoca, BK = Broken; CP = Campaspe; GB = Goulburn; LD = Loddon; LT = La Trobe; WM = Wimmera. Newer Volcanic Province (NVP): BW = Barwon; CG = Corangamite; GE = Glenelg; HK = Hopkins; WN = Wannon. Uplands: KW = Kiewa; MM = Mitta Mitta; OV = Ovens; TS = Thomson.

We propose that the excess Cl exported by many of the river catchments in southeast Australia is due to changes in the hydrological balance and suggest two scenarios. Firstly, following the eruption of the Newer Volcanic Plains Basalts, lake and wetland systems developed due to lava flows blocking prior drainage systems. Subsequent erosion of the basalts has gradually re-established the drainage systems and caused the progressive draining of the lakes and wetlands. This type of landscape change has been documented in the Barwon and Corangamite catchments (Fig. 1). Basaltic flows erupted between 100 ka and 4.5 Ma (Price et al., 2003) dammed the upper Barwon River forming a large lake system that extended into the adjacent Corangamite catchment (Joyce, 1988). Erosion of the basalts gradually reduced lake levels creating a series of disconnected saline lakes and wetlands in the Barwon and Corangamite catchments (e.g., present day Lake Corangamite and Lake Murdeduke). Dating of lunettes in the Corangamite catchment indicates that this occurred between 30 and 10 ka (Edwards et al., 1996; Dahlhaus et al., 2008). Other Newer Volcanics Province catchments, such as the Hopkins, contain lakes that have developed on previous drainage lines (Cartwright et al., 2009). Lakes and wetlands across the Newer Volcanic Province have restricted inflows and outflows and are generally brackish to hypersaline (Dahlhaus et al., 2008). Lakes that lie above the local water table and throughflow lakes recharge the shallow groundwater. Discharge lakes that lie in topographic lows also recharge the groundwater when brines become sufficiently dense that hydraulic gradients are reversed or when rainfall and runoff raises the lake level. Groundwater throughout the Newer Volcanics Province has locally very highly TDS concentrations due to it being recharged from these lakes and wetlands (Edwards et al., 1996; Bennetts et al., 2006; Dahlhaus et al., 2008; Cartwright et al., 2009; Tweed et al., 2011). Re-establishment of the river systems changes the closed-nature of the catchments and groundwater will be exported via the rivers. This results in the Cl that accumulated in the shallow groundwater as well as Cl from the lakes and wetlands to be exported by the rivers. A second scenario, especially for the cleared plains catchments, is that land clearing which replaced native vegetation by grasslands and

Fig. 8. Range of annual Cl exports as a percentage of Cl inputs from rainfall at the lowermost gauges. The large variation in annual Cl exports is due to the variability in annual discharge (Fig. 4). Data from Tables 1 and 2. Cleared Plains: AV = Avoca; BK = Broken; CP = Campaspe; GB = Goulburn; LD = Loddon; LT = La Trobe; WM = Wimmera. Newer Volcanic Province (NVP): BW = Barwon; CG = Corangamite; GE = Glenelg; HK = Hopkins; WN = Wannon. Uplands: KW = Kiewa; MM = Mitta Mitta; OV = Ovens; TS = Thomson.

I. Cartwright et al. / Chemical Geology 357 (2013) 29–40

crops has caused increased recharge that has resulted in a rise of the regional water table (Calf et al., 1986; Allison et al., 1990; Ghassemi et al., 1995). In turn this increases the baseflow component to the rivers resulting in the export of Cl with relatively long residence times from the groundwater. The magnitude of the excess Cl exported by rivers in the cleared plains catchments in Victoria is similar to the Western Australian catchments (300–2000%: Peck and Hurle, 1973), which was also attributed to a similar mechanism of land clearing and rising water tables. Both the removal of lakes and wetlands and land clearing results in a decrease in net evapotranspiration. Because evapotranspiration is the major process that controls groundwater salinity, groundwater Cl concentrations will steadily decline. As Cl is provided to the rivers via baseflow, this will also result in an eventual decrease in the salinity of the rivers. The timeframes over which these changes will occur are difficult to estimate. As regional flow systems take hundreds to thousands of years to adjust to major hydrological changes (e.g., Ghassemi et al., 1995), the disequilibrium conditions may persist over similar timeframes. The NVP rivers, which have the largest Cl* values, are still responding to changes in landscape and hydrology that occurred several thousand years ago. Breakdown of soils where Cl is bound to organic matter may also release Cl to surface water (Oberg and Sanden, 2005; Bastviken et al., 2006, 2007; Svensson et al., 2012). However, the magnitude of excess Cl in the Victorian Rivers is generally higher than where this has been documented; for example, Cl exports in 32 temperate northern hemisphere catchments discussed by Svensson et al. (2012) are up to 130% of inputs. Additionally, Australian soils have low concentrations of organic matter relative to northern hemisphere soils (Oades, 1988) and their Cl retention capacity is likely to be lower. Given that the rivers in southeast Australia have inflows of locally saline groundwater, it is more likely that changes to the groundwater systems described above are the main causes of the excess Cl; although locally the breakdown of soil organic matter may provide additional Cl. The low salinity rivers, notable those draining the uplands, may be exporting similar amounts of Cl to that received in rainfall. Unlike the rivers on the cleared plains, the upland rivers retain significant proportions of native forest in their upper reaches and the area of cleared land is relatively small. This has led to fewer changes to water table levels in these catchments and the amount of organic matter in the soils may also not be changing. Similarly, unlike the rivers draining the basalts, there is little evidence of recent natural changes to drainage systems. However, without knowledge of the Cl outflows in groundwater from those catchments it is not possible to definitively assess whether these catchments are exporting more Cl than is currently being delivered. The northern Victorian rivers are tributaries of the Murray River. Salinities in the Murray River have increased over the last few decades and declining surface water quality is of environmental concern (Simpson and Herczeg, 1994; Cartwright et al., 2011; Murray Darling Basin Association, 2013). Although comprising b 20% of the area of the Murray–Darling Basin, the Victorian rivers account for ~ 50% of the discharge of the Murray River. Cleared plains catchments such as the Avoca, Goulburn, and Loddon that have median Cl fluxes of 4.7 − 22 × 106 kg/year (Table 2) are contributing significant quantities of Cl to the River Murray. Within those catchments, individual tributaries such as Sunday Creek (Goulburn) or Bet Bet and Barr Creeks (Loddon) contribute disproportionally large Cl fluxes (Table 1). Land management, specifically replanting trees to lower water tables or water interception schemes, should be focussed on these saline subcatchments. 6. Conclusions Many rivers in southeast Australia export significantly more Cl than is input to their catchments by rainfall, indicating that they are undergoing hydrological changes. There are two landscape changes that cause

39

the majority of these changes. Firstly, as has been documented elsewhere (Peck and Hurle, 1973; Allison et al., 1990: Ghassemi et al, 1995) land clearing has resulted in increased recharge and a rise in the water table which in turn causes the export of Cl from groundwater into the river systems. The second mechanism is one of natural landscape change that results from re-establishment of drainage systems following the eruption of basaltic lava flows. To our knowledge, linking large-scale hydrological changes to progressive landscape changes that follow recent volcanism has not been attempted. Much of the focus in Australia and elsewhere has been on how anthropogenic landscape changes may disturb the hydraulic balance (e.g. Ghassemi et al., 1995); however, this study illustrates that long-term natural changes also play a role. The magnitude of Cl* in the NVP rivers is greater than those from the cleared plains, illustrating that these natural landscape changes locally have a more profound impact on the hydrogeology than anthropogenic impacts. This information will help inform the debate as to the extent to which natural and anthropogenic processes produce the observed high salinities in river systems (e.g., Dahlhaus et al., 2000, 2008), which in turn directs management options. The observation that the NVP river systems are still responding to landscape changes that occurred several thousand years ago illustrates that the timescales for the geochemistry of these catchments to re-establish steady state conditions are long. The methodology utilised here is most readily applied to solutes that are not produced or consumed by mineral dissolution and to catchments where the groundwater and surface water basins coincide. Application of this technique is also simplified where long-term continuous EC records are available; however, first order estimates may be made from discharge data and sporadic Cl measurements if Cl flux vs. discharge relationships can be established (Fig. 5). Assessment of the geochemical mass balance in low salinity catchments where the net export of Cl via the river system is less than the rainfall input requires the fluxes via the groundwater system also be understood. This study also illustrates that where there is considerable inter-annual variation in river discharge and Cl fluxes, records that span several years are ideally required to calculate mass balances as annual estimates may be significantly different from the long-term average (Fig. 8). Additionally, given that there may be concentrations of monitoring infrastructure on minor tributaries that have high salinities, care must be taken in upscaling observations from individual gauges if monitoring data for the whole catchment does not exist.

Acknowledgements We would like to thank the Department of Sustainability and Environment for their ongoing support of the Victorian Water Resources Data Warehouse without which studies such as this would not be possible. This work was supported by the P3 programme of the ARC-NWI funded National Centre for Groundwater Research and Training. Darren Bennetts and an anonymous reviewer provided helpful comments on this work.

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