The importance of unsaturated zone biogeochemical processes in determining groundwater composition, southeastern Australia Matthew Edwards & John Webb Abstract Analysis of soil, soil water and groundwater in the Mount William Creek catchment, southeastern Australia, shows that Mg2+ and Ca2+ within infiltrating rainfall are rapidly depleted by plant uptake and adsorption on clay minerals. Na+ and K+ may exhibit minor enrichment at shallow depths but are quickly readsorbed, so that cation/Cl– ratios typical of groundwater are observed in soil water within the upper 200cm of the soil profile for all species. The concentrations of K+ and Ca2+ in soil and groundwater are more depleted than Na+ and Mg2+ due to preferential uptake by vegetation. Removal of organic matter results in a continuing, long-term export of all major cations from the soil profiles. The processes of biogeochemical fractionation within the unsaturated zone rapidly modify the cation/Cl– ratios of infiltrating rainfall to values characteristic of seawater. These mechanisms may have reached steady state, because groundwaters with seawater ion/Cl– ratios are thousands of years old; the exchange sites on the soil clays are probably saturated, so cations supplied in rainfall are exported in organic matter and incorporated into recharge infiltrating into the groundwater. Much of the chemical evolution of groundwater traditionally attributed to processes within the aquifer is complete by the time recharge occurs; this evolutionary model may have broad application. Keywords Unsaturated zone . Cation exchange . Plant uptake . Hydrogeochemistry . Australia
Received: 5 April 2008 / Accepted: 23 February 2009 Published online: 27 March 2009 © Springer-Verlag 2009 M. Edwards : J. Webb Department of Environmental Geoscience, La Trobe University, Kingsbury Drive, Melbourne, Victoria 3086, Australia M. Edwards ()) ENSR Australia, 6/417 St Kilda Road, Melbourne, Victoria 3004, Australia e-mail:
[email protected] Tel.: +61-3-86992199 Fax: +61-3-86992122 Hydrogeology Journal (2009) 17: 1359–1374
Introduction The role of soil zone reactions in controlling groundwater chemistry is often regarded in the hydrogeochemical literature as subordinate to reactions occurring in the saturated zone of aquifer systems. Where aquifers are composed of reactive minerals such as carbonates, halides or unstable silicates, interactions between groundwater and the host lithologies dominate the evolution of groundwater composition (e.g. Garrels and Mackenzie 1967; Heathcote 1985; Cardenal et al. 1994; Kimblin 1995; Rosen and Jones 1998; Stuyfzand 1999; Toth 1999; Rademacher et al. 2001; Dogramaci and Herczeg 2002; Benedetti et al. 2003). However, in the semi-confined and unconfined alluvial aquifers of the Murray-Darling Basin, southeastern Australia, it is generally agreed that groundwater solutes are derived from rainfall and that the dominant evolutionary mechanism affecting groundwater chemistry is concentration by evapotranspiration (e.g. Dyson 1983; Arad and Evans 1987; Macumber 1991; Simpson and Herczeg 1994; Herczeg et al. 2001; Cartwright et al. 2004). Despite this, high-resolution rainfall chemistry, datasets from this area (e.g. Hutton and Leslie 1958; Blackburn and McLeod 1983; Simpson and Herczeg 1994; Bormann 2004) demonstrate that the ratios of most ions to Cl– within rainfall differ substantially from the groundwater ratios, indicating that processes apart from evapotranspiration are affecting the groundwater chemistry. Studies of many groundwater systems throughout Australia (e.g. Lawrence 1975; Arad and Evans 1987; Macumber 1991; Salama et al. 1993; Acworth and Jankowski 2001; Herczeg et al. 2001; Cartwright et al. 2004) and in other countries (e.g. Kimblin 1995; Elliot et al. 1999; Guler and Thyne 2004) show that groundwater major cation/Cl– ratios typically decline rapidly as the groundwater salinity increases, from values approximating those in local rainfall towards values more characteristic of seawater in many cases. To explain these trends, previous workers have cited mechanisms including mineral weathering (e.g. Jankowski and Acworth 1993; Cartwright et al. 2004), cation exchange (e.g. Kimblin 1995; Toth 1999; Acworth and Jankowski 2001; Bennetts et al. 2007), dissolution/precipitation reactions (e.g. Stuyfzand 1999; Herczeg et al. 2001; Guler and Thyne 2004), the formation of secondary clay minerals (e.g. DOI 10.1007/s10040-009-0449-8
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Blake 1989; Salama et al. 1993) and mixing with connate seawater (e.g. Lawrence 1975; Elliot et al. 1999), with the stated or implied assumption that these processes occur below the water table. Until recently, only a few studies described how reactions within the unsaturated zone could affect groundwater major element composition (e.g. Spears and Reeves 1975; Drever and Smith 1978; Moss and Edmunds 1992). However, it is recognised that many soils are a hot spot of geochemical, particularly organic, activity due to the high microbial, fungal and root biomass present (Chorover et al. 2007). Microbial metabolic activity concentrated in the uppermost few millimeters of the soil strongly affects chemical gradients of nutrient species (Garcia-Pichel et al. 2003), and detailed studies on soil water composition (White et al. 2002, 2006) have demonstrated the influence of biological processes on the concentrations of some species. This suggests that soil zone processes can influence soil water and therefore groundwater composition to a degree that has often been neglected. This study assesses the evolution of groundwater chemistry in a small, partially cleared catchment in Victoria, southeastern Australia, through a detailed study of soil, soil water and groundwater chemistry. It presents an alternative explanation wherein most change in groundwater composition is attributed to soil zone processes, including plant nutrient cycling, previously neglected in the study of Australian groundwater chemistry.
colluvium, sub-divided into a basal Tertiary sand and gravel unit overlain by Quaternary sandy clay. Therefore three aquifers are present: Palaeozoic basement, semiconfined Tertiary sediments and Quaternary alluvium. Soil profiles analysed throughout the catchment (Fig. 1) fall between two distinct geological and physiographic end-members, represented by sites 27 and 5, both of which were selected for detailed sampling. Site 27 (SP27) is located adjacent to the Grampians-Gariwerd National Park (Fig. 1), in Quaternary transported silty-clays beneath natural Eucalypt forest. The site lies in a local valley floor at an elevation of ~190 m relative to Australian Height Datum (AHD) (equivalent to mean sea level), and slopes gently to the north and east. The soil composition is dominated mineralogically by kaolinite and quartz with minor anorthite, and consists of medium-grey clay grading downwards into light brown silty-sand at 2.0 m depth. The water table in the vicinity of SP27 is approximately 10 m below the ground surface, such that the surficial Quaternary alluvium is saturated at this locality. Site 5 (SP5) lies in cleared farmland on deeply weathered Silurian schist (Fig. 1). It is located on the crest of a broad ridge at ~240 m AHD with the surrounding terrain falling gently to the north and south. The water table lies at a depth of 8.5 m. The soil is a pale yellow silty clay with minor quartz gravel and is dominated mineralogically by kaolinite and quartz with minor illite/muscovite and clinochlore. The farmland at site 5 is used for sheep grazing; no fertilisers are applied, nor is there any evidence of historical fertiliser usage.
Site description The Mount William Creek catchment is located in the upper reaches of the Wimmera River catchment in western Victoria, southeastern Australia (Fig. 1), and lies on the southern margin of the Murray-Darling Basin, which occupies a large part of inland Australia. The study area has a temperate climate with mean annual rainfall of 595.5 mm at Moyston, in the south, and 533.5 mm at Stawell, in the northeast of the catchment. Pan evaporation exceeds rainfall in all but the winter months, with a mean annual total of 1501.1 mm at Stawell (Bureau of Meteorology 2003). The topography is dominated by the Mount William and Mount Difficult Ranges of the Grampians-Gariwerd National Park, which form high mountain escarpments in the west of the study area, and the relatively subdued, lower elevation Ararat Hills and Black Range in the east. Much of the catchment has been cleared for dryland agriculture (largely sheep grazing), excluding some significant tracts of native forest vegetation, particularly in the Grampians-Gariwerd National Park, which extends along the entire western boundary. The area is geologically complex, with a Palaeozoic basement consisting of Cambrian greenstones and turbiditic metasediments, Silurian schists and sandstones and a number of Devonian-age granitic intrusives (Cayley and Taylor 2001). The basement is overlain in the Mount William Creek valley by Cainozoic fluvial sediments and Hydrogeology Journal (2009) 17: 1359–1374
Site hydrogeology The Palaeozoic basement aquifer incorporates all the Palaeozoic rock units within the catchment, and is largely composed of crystalline rocks characterised by fracture porosity. The aquifer outcrops along the western catchment boundary as the densely vegetated Grampians Ranges, which consist of metamorphosed sandstones, through the centre of the catchment as a discontinuous ridge of metavolcanic greenstone, and extensively in the east of the catchment as rolling hills of granite and deeply weathered metamorphic rocks (Fig. 2). Throughout the central lowlands and tributary valleys the aquifer is concealed and confined to semi-confined beneath the Cainozoic sedimentary cover, and has a low hydraulic conductivity of the order of 0.8 m/day (Edwards 2006). The Tertiary Calivil Formation comprises unconsolidated sands and gravels, consisting predominantly of quartz, that lie in palaeodrainage lines beneath the present Mount William Creek floodplain, where deposits can reach 40 m in thickness and up to 6,000 m in width (Fig. 2). It is the dominant aquifer in the catchment because of the large volume of relatively porous sediment and is also known as the Tertiary sediments aquifer; it is responsible for much of the down-basin groundwater flow within the catchment (Edwards 2006). The aquifer does not outcrop and therefore receives recharge from other DOI 10.1007/s10040-009-0449-8
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Fig. 1 Location of the Mount William Creek catchment study area, showing sample sites
aquifers. It is heterogenous and anisotropic, due to a moderately well defined fining upwards sequence and strong lateral variability; hydraulic conductivities are estimated to range from 1.0 m/day up to 100 m/day. The Quaternary alluvial aquifer consists mostly of extensive alluvium, which blankets much of the lower relief area along Mount William Creek and tributary valleys to depths of up to 25 m (Edwards 2006; Fig. 2). The sediments are clay-rich and unconsolidated, with minor sand stringers evident during drilling, both in this study and previous investigations (e.g. Harrison 1993). This aquifer is characterised by a low hydraulic conductivity of the order of 1.0 m/day. Hydrogeology Journal (2009) 17: 1359–1374
Potentiometric and groundwater geochemical evidence presented by Edwards (2006) generally indicates a low degree of cross-formational interaction throughout the majority of the study area where groundwater movement is dominated by lateral flow; however, vertical flow is significant in areas of relatively high recharge. Recharge occurs dominantly from direct rainfall accessions and is highly variable throughout the catchment, ranging from 50–100 mm/year in the Grampians Range and associated colluvial slopes to 0.2 mm/year in areas of well-developed regolith beneath remnant native vegetation (Edwards and Webb 2006). Transient and steady-state soil water Cl– mass balance modelling (Edwards 2006) show DOI 10.1007/s10040-009-0449-8
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Fig. 2 Areal extents of the major aquifer systems, Mount William Creek catchment
that recharge at SP5 and SP27 is of the order of 8 and 0.2 mm/year, respectively, consistent with the physiography of the sites.
Sampling and analytical methods Groundwater A total of 79 groundwater bores were sampled during this study (Fig. 1), ranging in depth from 5 to 142 m, with a typical depth of ~20 m. Before collection of groundwater samples, all bores were purged of at least three bore volumes of water using a Grundfos SQ1–80N electric submersible pump for bores with ≥100 mm diameter casings, and a PVC bailer for bores with smaller diameter casings. Groundwater samples were collected in 120-ml polyethylene vials following the stabilisation and measurement of the physical parameters EC, pH, Eh and temperature; samples were immediately refrigerated for transport back to the laboratory. All groundwater samples were filtered through 0.45 µm cellulose nitrate filters prior to analysis. Ca2+ , Mg2+ and K+ concentrations were determined on acidified samples containing 10% LiCl, using a GBC 933 Plus Atomic Absorption Spectrophotometer (AAS) with an Hydrogeology Journal (2009) 17: 1359–1374
air-acetylene flame. Samples were diluted with deionised water to fit the working ranges for Ca2+ , Mg2+ and K+ of 0.1– 4.0, 0.5–2.0 and 0.5–4.0 mg/L, respectively. Na+ was determined on acidified samples using a Sherwood Model 410 flame photometer, after dilution to fit the working range of 0–30 mg/L. Cl– and other anion concentrations were determined on unacidified samples using a Phenomenex A300 anion peak ion chromatograph with a 1.7 mM NaHCO3/1.8 mM Na2CO3 eluent, 25 mM H2SO4 regenerate solution, and flow rate of 1.5 ml/min. The samples were diluted to fit the working range for Cl– of 0–250 mg/L. Alkalinity was determined by standard titration to pH 4.5 with HCl following the method of Rayment and Higginson (1992). Selected groundwater samples were collected and analysed for 14C concentration. Samples were collected in acid-washed (HNO3) 1-L glass bottles, rinsed three times with sample water and filled so that headspace was negligible, in order to minimise atmospheric contamination. Analysis was performed using the ANTARES tandem accelerator at ANSTO, Sydney (Fink et al. 2004). The average error associated with the percent modern carbon (pMC) values here is 0.3%, and the age limit is 50,000± 200 years. Age determinations made on the basis of pMC values are uncorrected for the addition of radiogenically dead DOI 10.1007/s10040-009-0449-8
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carbon, due to the paucity of carbonate minerals or preserved organic material within the aquifers (Edwards 2006).
Soils A total of 36 soil profiles (Fig. 1) were sampled for a minimum analytical suite of soil moisture, EC1:5, pH1:5 and Cl–. Soil samples were collected using a Gemco solid flight auger-drilling rig in the winter of 2005. Sub-samples were taken by hand at the top, middle and bottom of every 50cm flight length during drilling, and combined to form a single sample that represents the entire 50-cm interval. This technique ensured that representative samples were collected and that cross contamination was kept to a minimum. Samples were immediately stored in zip-lock plastic bags and refrigerated for transport back to the laboratory. Soil moisture contents were determined gravimetrically by weighing sub-samples before and after oven drying at 105°C. The EC1:5 and pH1:5 were determined by adding 25 ml of deionised water to 5 g of air-dried and sieved soil (8 m).
K+ The concentration of K+ in soil water is only detectable in the upper 50 cm of the profiles at both sites (Table 3; Fig. 4). The soluble K+/Cl– ratio at the top of the profile approximates that of rainfall in SP27 and exceeds that of rainfall in SP5, indicating an additional source of K+ in SP5. This may represent the input of K+ from the breakdown of organic material and/or desorption of K+ from clays by cation exchange with Ca2+ and/or Mg2+. Because soluble Ca2+ is strongly depleted in the shallow subsurface (2,000 mg/L TDS) in the study area to have ion/Cl– ratios typical of seawater; groundwaters sampled have 14C
ages ranging from ~2,500 up to ~16,500 years (Table 2). This implies that the cation-removal processes in the unsaturated zone have been operating at a more or less constant rate for thousands of years. Despite 14C age determinations being derived from groundwater in the semiconfined Calivil Formation, these waters have the same relative ion concentrations as those waters in the overlying alluvium, due in large part to the fact that recharge occurs via the overlying unit. The exchangeable cation content of the soils in the study area is less than might be expected from this time frame. Assuming a bulk soil density of 1.6 tons/m3 (typical of soils in this part of Australia; Dyson and Jenkin 1981), 1 m2 of the upper 50 cm of the soil profiles at both sites contains ~800 kg of soil with 35–50 moles of exchangeable cations (K+ , Ca2+ , Na+ and Mg2+; from Table 3); assuming all exchangeable cations were derived from infiltrating pore waters, this represents 500– 900 years’ supply of cations in rainfall (from Table 1). The exchangeable cation content of the entire 200 cm thickness of both profiles is 160–190 moles/m2, representing 2,000–3,500 years’ supply of cations in rainfall. Since the unsaturated zone processes responsible for the seawater-type ion/Cl– ratios in the higher salinity groundwaters appear to have been operating for many thousands of years, this indicates that the concentration of exchangeable base cations in the soil profiles may have reached a steady state, implying that continuing net removal of base cations by cation exchange is no longer occurring. The total exchangeable cation content of the kaolinite in the upper 50 cm of soil profile SP5 (0.08 mol kg–1; from Tables 3 and 4) lies within the range of experimental cation exchange capacities for kaolinites in southeastern Aus-
Table 5 Depletion of cation species in groundwater relative to a theoretical composition derived by the concentration of local rainfall, using Cl– as the reference ion Average rainfall (mg/L)a Highest salinity groundwater (mg/L)b Normalised rainfall (mg/L)c Depletion (%)
Na+
Ca2+
Mg2+
K+
Cl–
1.3 4,700 4,562 –3d
0.9 46 3,271 99
0.3 662 1,146 42
0.4 16 1,391 99
2.3 8,094 8,094 0
Shows strong depletion of Ca2+ and K+ in Mount William Creek catchment groundwaters relative to other cations a Volume weighted average composition from Table 1 b Cation and Cl– analysis of highest salinity groundwater (Bore 5312) c Average rainfall concentrations normalised to Cl– of highest salinity groundwater d Negative value indicates enrichment of Na+ , due to small errors Hydrogeology Journal (2009) 17: 1359–1374
DOI 10.1007/s10040-009-0449-8
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tralia (0.028–0.244 mol kg ; Ma and Eggleton 1999), so the clays in the upper parts of the soil profiles in the study area may be saturated with respect to base cation storage. However, continuing, long-term removal of ions by plant uptake is occurring. Ions taken up by vegetation from both the soil solution and exchange sites are returned to the soil through the continual breakdown of organic matter, and this may be responsible for the occasional groundwater cation/Cl– ratios exceeding those in local rainfall (Fig. 3), as shown elsewhere (Jobbagy and Jackson 2004). However, this return of ions to the soil is disrupted by the net erosional loss of particulate organic material through overland transport (sheetflow and rivers, e.g. Hopmans et al. 1987; White et al. 2002); White et al. (2006) reported high Ca2+ concentrations in surface waters draining grassland terraces in coastal California, which they attributed to seasonal biomass cycling. In addition, organic material is lost as smoke during bushfires (e.g. Stewart and Flinn 1985), and, in farmed areas, through removal of agricultural products (animals and crops). Significant amounts of both Ca2+ and K+ can also be lost by accumulation on plant leaves during photosynthesis and dispersion by wind (Artaxo and Orsini 1987; de Mello 2001). Therefore the removal of organic matter will result in a continuing, long-term export of ions, particularly Ca2+ and K+, from soil profiles, and the ions most strongly depleted in Mount William Creek groundwaters are Ca2+ and K+ (Table 5). It is therefore possible that a steady state scenario has become established at the study site; the soil clays are probably saturated with respect to base cation storage, and the cations supplied in rainfall are partly exported in organic matter and partly incorporated into recharge infiltrating downwards into the groundwater. These processes are apparently sufficient to maintain the patterns observed.
Relationship between unsaturated zone processes and groundwater salinity and ion/Cl– ratios across the catchment The results of this study demonstrate that, in the study area, much of the chemical evolution of the groundwaters normally attributed to groundwater-aquifer interactions in fact occurs prior to recharge. How do these processes cause the variability in the major ion/Cl– ratios across the catchment and why, once the salinity exceeds ~2,000 mg/ L, is there a further downgradient increase in salinity within the aquifers in the study area without any change in ion/Cl– ratios (Fig. 3)? The elevated parts of the study area have relatively thin soil cover and are characterised by rapid recharge (as shown by groundwater Cl– mass balance and hydrograph data; Edwards 2006; Edwards and Webb 2006). Because of the limited time that this recharge spends in the unsaturated zone, it undergoes relatively little evapotranspiration and interaction with plants and clays, so that the base cation/Cl– ratios are often only slightly or moderately depleted compared to rainfall. This is clearly shown by the Hydrogeology Journal (2009) 17: 1359–1374
2+
–
+
–
Ca /Cl and K /Cl plots (Fig. 3). In contrast, over the low relief remainder of the catchment, where soil and regolith profiles are thicker and better developed (e.g. SP5 and SP27), recharge is slow and diffuse (Edwards 2006; Edwards and Webb 2006), allowing sufficient time for evapotranspiration to increase the salinity and for unsaturated zone processes to modify the ion/Cl– ratios so that they resemble those of dilute seawater (Figs. 3 and 4). There is a concomitant downflow increase in groundwater salinity and decrease in ion/Cl– ratios across Mount William Creek catchment up to salinites of ~2,000 mg/L TDS (the value at which seawater ion/Cl– ratios become established; Fig. 3). This is caused by mixing of more saline diffuse recharge that has undergone evapotranspiration and ion depletion in the unsaturated zone with fresher, less depleted groundwater recharged in the higher elevation parts of the catchment. Mass balance mixing calculations using Na+ and Cl– demonstrate the amount of mixing. Adding dilute groundwater with relatively high ion/Cl ratios (
