Water and soil degradation in the Keynes Catchment, South Australia

Transcription

1 CSIRO Water and soil degradation in the Keynes Catchment, South Australia 2. Electrical conductivity, ph and chloride concentrations of the regolith Jim Cox, Phil Davies and Leonie Spouncer TECHNICAL REPORT CSIRO Land and Water Technical Report 4/99, October 1999 CSIRO LAND and WATER

2 Water and soil degradation in the Keynes Catchment, South Australia 2. Electrical conductivity, ph and chloride concentrations of the regolith by Jim Cox, Leonie Spouncer and Phil Davies

3 1 Abstract Salinisation of soils, streams and groundwaters are major problems in the Mt Lofty Ranges, South Australia. An understanding of both the source of salt and the catchment hydrology which mobilises the salt is needed to help understand land degradation processes and determine the best land management practices. As hydrological conditions change seasonally, groundwater monitoring in piezometers is an integral part of the study of salinisation. Eight piezometer nests were installed in the regolith above fractured sandstone and schist in the Keynes catchment in the Mt Lofty Ranges. The site is one of the South Australian National Dryland Salinity Focus Catchments. The regolith in the highest parts of the catchment was more acidic than lower in the landscape and had the lowest EC (less than 1 ds/m). At most sites, the increase in ph with depth was very high (up to 3 ph units) with most change within the top few metres. Surface soils at some sites (not the lowest parts of the landscape) were very acidic (ph 4.7). There was a clear rapid increase in stored salt (chloride) in the regolith with a decrease in elevation. Over 46 t/ha of salt are stored in the regolith within the valley. There is very little salt in the regolith at about 41 m above the Australian Height Datum (AHD) or higher. Introduction It is now well accepted that the hydrological balance of the southern Australian agricultural zone has been drastically altered by the large-scale removal of native vegetation over the last 12 years and its replacement with pastures and crops (e.g. Peck and Williamson 1987). Annual, shallow rooted pastures and crops have relatively little leaf area compared with the evergreen native vegetation and thus intercept and transpire

4 2 less water (Farrington et al. 1992, Scott and Sudmeyer 1993). This allows a larger amount of rainfall to percolate passed the root zone to the groundwater, causing them to rise (e.g. Nulsen and Baxter 1982, Richardson and Narayan 1995). Once the groundwater is within 1.5 m of the soil surface, the onset of dryland salinity is rapid (Nulsen and Henschke 1981). The relationship between rising groundwater and the rapid onset of dryland salinity in the texture-contrast soils (Chittleborough 1992) over granites and dolerites of Western Australia and basalts of Victoria are well documented (e.g. Engel et al. 1987, McFarlane and George 1992, George and Conacher 1993, Salama et al. 1993a). The development of saline seepage in deep sands is also well understood (e.g. George 1991). There is now a large volume of literature on the impact of land clearance on changes in groundwater salinity (e.g. Salama et al. 1993b) and on streams (Wood 1924, Salinity Committee 1984, Ruprecht and Schofield 1989, Schofield and Ruprecht 1989, Allison et al. 199). Large areas of texture-contrast soils in the Mt Lofty Ranges overlie fractured metasediments (Daily et al. 1976). These regions are often associated with the development of waterlogging, dryland salinity and acid-sulfate conditions (e.g. Cox et al. 1996, Fitzpatrick et al. 1996). To date, little is understood about how quickly the groundwater, above fractured rock systems, are rising, the seasonal response of groundwater to rainfall, nor the variability in its quality. This report presents the electrical conductivity, ph and chloride concentrations in the regolith in a 14 ha catchment in the Mt Lofty Ranges, South Australia. It is the first stage of a major study assessing soil and water degradation in this region.

5 3 Materials and methods Site The Keynes catchment is located north-east of Adelaide near the town of Keyneton in the Adelaide hills (Fig. 1). The catchment is 39 to 42 m above sea level, 2 ha in size and located in the western part of the Murray-Darling Basin. Climate, vegetation and soils in the catchment are typical of the region. The long-term (1 year) average rainfall at Keyneton is 54 mm. Rainfall only exceeds potential evaporation from May to September (Table 1). Land clearance commenced 12 years ago and was completed 4 years ago for grazing by sheep and cattle. The sown pastures are mostly a mix of annual grasses and clovers. The texture-contrast soils of the Keynes catchment (McMurray and Cox 1995a, 1995b) have formed over metasediments (Daily et al. 1976).

6 4 Fig. 1. Location of the study site (Keynes catchment). WA NT SA QLD NSW Angaston Keyneton Somme Catchment site Sedan VIC Ck Marne STUDY SITE TAS Springton Saunders Ck River Mt Pleasant ADELAIDE Lobethal Murray River Woodside SCALE 5 1 Verdun kilometres Table 1. Average long term (3 yr) rainfall (mm) and potential evaporation (mm) at Keyneton. Month J F M A M J J A S O N D Annual Rainfall Evaporation Source: Bureau of Meteorology Piezometer installation Twenty-three piezometers, comprising seven nests of three piezometers and one nest of two piezometers were installed throughout the catchment (Fig. 2). Holes were drilled with 12 mm diameter solid flight augers. The piezometers were completed by lining the drill holes with 5 mm Class 9 UPVC pipe. A specially slotted Class 12 UPVC module encased in a dros-pak nylon sleeve with an outer packing of washed quartz sand was installed over the bottom 1 m (or.5 m in the piezometers < 4 m deep; Thompson

7 5 et al., 1992). The holes were sealed with bentonite and backfilled. Where soils were unstable, piezometer installation depth was less than the depth drilled. Morphological characteristics, texture and colour were recorded every 1 m, as drilling progressed, using the methods of Northcote (1979) and McDonald et al. (199). The ph of the drill cuttings was measured in the laboratory within 12 hours. There are two requirements for dryland salinity, a source of salt and also water to transport the salt to the soil surface. The amount of deep stored salt was determined by collecting drill cuttings every 1 m and analysing them, using standard techniques, for EC and Cl (Rayment and Higginson 1992). Further details are in Thompson et al. (1994) and Cox and Reynolds (1995). Fig. 2. Location of piezometers within the catchment.

8 6 Results Piezometer details The maximum depth that could be drilled into the regolith (before reaching bedrock) was about 2 m in the valley, in the centre of the catchment. Depths of all piezometers are presented in Table 2. On the hill slopes, the regolith was thinner and the deepest piezometers were drilled to about 1 m. Table 2. Piezometer depths (m). Site Piezometer Depth K1 P1, P2, P3 3.3, 1.9, 15.1 K2 P1, P2, P3 3.1, 1.7, 16.2 K3 P1, P2, P3 3.4, 9.9, 14.4 K4 P1, P2, P3 3.4, 9.7, 12.2 K5 P1, P2, P3 3.3, 1.7, 18.6 K6 P1, P2, P3 3., 9.4, 1.1 K7 P1, P2, P3 3.2, 11.4, 15.7 K8 P1,P2 3.4, 1.8 Lithology, ph and EC of the regolith The regolith below site K1 was a yellowish brown (1YR5/6) sandy clay loam to light grey (5YR7/2) silty clay loam. The regolith below site K2 was a dark greyish brown (2.5YR4/4) to olive brown (2.5Y4/2) silty loam to silty clay loam. The regolith below site K3 was an olive brown (2.5Y4/4) to olive (5Y5/3) silty loam. The regolith below site K4 was olive (5Y4/3) silty loam to fine sandy loam. The regolith below site K6 was olive (5Y4/2) to greyish brown (2.5Y 5/2) sandy clay loam. The regolith below site K7 was an olive grey (5Y 4/2) to dark grey (5Y 4/1) silty loam. The regolith below site K8 was a yellowish brown (1YR5/6) to olive yellow (2.5Y6/6) silty clay loam.

9 7 The EC of the regolith below site K1 averaged.57 ds/m (range , Appendix 1, Fig. 3). The EC at K2 averaged.73 ds/m which was higher than at K1 but the range ( ds/m) was similar. The EC at K3 was highest in the catchment (average.95 ds/m; range ds/m) due mainly to a very high level at the soil surface. This site was wet for much of the year, drying out only in summer (data not shown). The EC at K4 averaged.61 ds/m (range ds/m). The EC at K5 averaged.72 ds/m (range ds/m). The EC at K6 averaged.21 ds/m (range ds/m). The EC at K7 was the lowest in the catchment and averaged.7 ds/m (range.5-.1 ds/m). The EC at K8 was low and similar to K7 with an average of.7 ds/m (range.6-.1 ds/m). Sites K1, K2 and K5 had neutral surfaces and were alkaline with depth. Sites K3 and K4 were alkaline throughout. Site K6 was very acidic throughout. Site K7 was very acidic at the surface and neutral at depth. Site K8 was moderately acidic at the surface and neutral at depth. The most acidic soils generally occurred at the highest points in the catchment (K6, K7 and K8) and had the lowest ECs. At most sites, the increase in ph with depth was very high (up to 3 ph units) with most change within the top few metres. Surface soils at some sites (K6 and K7) were very acidic (4.7). These sites were located relatively high in the landscape.

10 8 Fig. 3. Electrical conductivity of drill cuttings. Electrical Conductivity (ds/m) KH1 EC KH2 EC KH3 EC KH4 EC Depth (m) 5 KH5 EC KH6 EC KH7 EC KH8 EC

11 9 Fig. 4. ph of the drill cuttings. ph (in CaCl 2 ) KH1 ph KH2 ph KH3 ph KH4 ph Depth (m) 2 5 KH5 ph KH6 ph KH7 ph KH8 ph 25

12 1 Salt storage Figure 5 (a and b) shows the pattern of salt stored below the surface at two piezometer sites (K3 and K7; see Fig. 2 for their locations). In general, there was little salt stored in the hilltops (e.g. K7) as the salt has been leached (Table 3). A salt bulge (maximum chloride) occurred at 13 m depth. However, there are high levels of salt stored in the valleys (e.g. K3) as salt accumulates in the clays. Most surface salt was in the lower elevated parts of the catchment, near piezometers K2 and K3. Fig. 5. Salt stored below a hilltop and a valley site. a. Salt storage, K3 (valley site) Chloride (mg/kg) Depth (m) b. Salt storage, K7 (hilltop site)

13 11 Chloride (mg/kg) Depth (m) KH7 Table 3. Salt storage within the regolith (t/ha). Piezometer nest Salt storage K1 143 K2 462 K3 343 K4 185 K5 32 K6 355 K7 8 K8 5 There was a strong relationship between salt storage and elevation in the catchment (Fig. 6). Salt storage (t/ha) = -.38 x Elevation 2 (m AHD) x Elevation r 2 =.79.

14 12 Fig. 6. Relationship between salt storage and elevation. 6 5 y = -.387x x R 2 =.79 Salt storage (t/ha) Elevation (m AHD) Chloride content of the drill cuttings was clearly related to EC (Fig. 7). Chloride (mg/kg) = x EC (ds/m) 7.4, r 2 =.98. Fig. 7. Correlation between the chloride content and electrical conductivity of the drill cuttings. Cl concentration (mg/kg) Chloride = EC r 2 =.98 K1 K2 K3 K4 K5 K6 K7 K Electrical Conductivity (ds/m)

15 13 Discussion and conclusions The most acidic soils (as low as 4.7) generally occurred at the surface at the highest points in the catchment. At most sites, the increase in ph with depth was rapid over the first few metres then remained fairly constant. The soils in the upper part of the catchment also had the lowest ECs. There was a clear relationship between salt storage and elevation in the landscape. Whether this relationship holds throughout the Mt Lofty Ranges or only applies to this catchment is not known. On average, about 2, kg/ha of salt are stored within the regolith. About 2-3 kg/ha of this salt is discharged through the erosion gully in the catchment (see the third report in this series) and thus (ignoring cyclic salt inputs) it would take over 1 years to flush the stored salt from the catchment under current hydrological conditions. Acknowledgments Thanks to Graham and Melanie Keynes on whose property the research is being carried out. Jim Thompson and Michael Reynolds helped with the drilling of piezometers and soil collection. Adrian Beech did the chemical analyses. Funding was provided by the Australian Centre for International Agricultural Research. Greg Rinder and Bob Schuster drafted some of the figures. References Allison, G.B., Cook, P.G., Barnett, S.R., Walker, G.R., Jolly, I.D., and Hughes, M.W Land clearance and river salinisation in the western Murray Basin, Australia. Journal of Hydrology 119, 1-2.

16 14 Brown, B Transportation of the salts required in the formation of acid sulphate soils through a saline rocky microcatchment. Unpublished Honours Thesis Dept of Soil Science, University of Adelaide. Chittleborough, D. J Formation and pedology of duplex soils. Australian Journal of Experimental Agriculture 32, Cox, J.W. and Reynolds, M.B Hydrogeological Results from the Keynes Catchment, South Australia (Stage 2) Technical Report 5/1995 CSIRO Division of Soils. Cox, J.W., Fritsch, E. and Fitzpatrick, R.W Interpretation of soil features produced by modern and ancient processes in degraded landscapes: VII. Water duration. Australian Journal of Soil Research 34, Cox, J.W., Davies, P., and Spouncer, L Water and soil degradation in the Keynes catchment, South Australia 7. Seasonal changes in groundwater quality. CSIRO Land and Water Technical Report. Daily, B., Firman, J.B., Forbes, B.G., and Lindsay, J.M Geology In: Natural History of the Adelaide Region (ed Twidale, C.R., Tyler, M.J. and Webb, B.P) Royal Society of South Australia Inc. Engel, R., McFarlane, D.J., and Street, G The influence of dolerite dykes on saline seeps in south-western Australia. Australian Journal of Soil Research 25, Farrington, P., Salama, R.B., Watson, G.D., and Bartle, G.A Water use of agricultural and native plants in a western Australian wheatbelt catchment. Agricultural Water Management 22, Fitzpatrick, R. W., Fritsch, E., and Self, P Interpretation of soil features produced by ancient and modern processes in degraded landscapes: V. Development of saline sulfidic features in non-tidal seepage areas. Geoderma 69, George, R.J Management of sandplain seeps in the wheatbelt, Western Australia. Agricultural Water Management 19, George, R. J., and Conacher, A. J Interactions between perched and saprolite aquifers on a small, salt-affected and deeply weathered hillslope. Earth Surface Processes and Landforms 18,

17 15 McDonald, R.C., Isbell, R.F., Speight, J.G., Walker, J., and Hopkins, M.S Australian Soil and Land Survey Field Handbook, 2nd edn. Inkata Press, Melbourne. pp McFarlane, D.J., and George, R.J Factors affecting dryland salinity in two wheatbelt catchments in Western Australia. Australian Journal of Soil Research 3, McMuray, L.S., and Cox, J.W. 1995a. Soil morphological features down a convex toposequence: Keyneton, South Australia. Technical Report 2/95, Cooperative Research Centre for Soil and Land Management, Adelaide, South Australia. McMuray, L.S., and Cox, J.W. 1995b. Soil morphological features down a concave toposequence: Keyneton, South Australia. Technical Report 3/95, Cooperative Research Centre for Soil and Land Management, Adelaide, South Australia. Northcote, K.H A Factual Key for the Recognition of Australian Soils. 4th edn. Rellim Tech., Glenside, South Australia. Nulsen, R.A., and Henschke, C.J Groundwater systems associated with secondary salinity in Western Australia. Agricultural Water Management 4, Nulsen, R.A. and Baxter, I.N The potential of agronomic manipulation for controlling salinity in Western Australia. Journal of the Australian Institute of Agricultural Science 48, Peck, A.J., and Williamson, D.R Effects of forest clearing on groundwater. Journal of Hydrology 94, Rayment, G.E. and Higginson, F.R Australian Laboratory Handbook of Soil and Water Chemical Methods, Inkata Press, Sydney, Australia. Richardson, S.B. and Narayan, K.A The effectiveness of management options for dryland salinity control at Wanilla, South Australia. Agricultural Water Management 29, Ruprecht, J.K., and Schofield, N.J Analysis of streamflow generation following deforestation in southwest Western Australia. Journal of Hydrology 15, Salama, R.B., Farrington, P., Bartle, G.A., and Watson, G.D. 1993a. Salinity trends in the wheatbelt of Western Australia: results of water and salt balance studies from Cuballing catchment. Journal of Hydrology 145,

18 16 Salama, R.B., Farrington, P., Bartle, G.A., and Watson, G.D. 1993b. The chemical evolution of groundwater in a first order catchment and the process of salt accumulation in the soil profile. Journal of Hydrology 143, Salinity Committee Salt of the Earth: Final Report on the Causes, Effects and Controls of Land and River Salinity in Victoria. Parliament of Victoria, Australia. Schofield, N.J., and Ruprecht, J.K Regional analysis of stream salinization in southwest Western Australia. Journal of Hydrology 112, Scott, P.R. and Sudmeyer, R.A Evapotranspiration from agricultural plant communities in the high rainfall zone of the southwest of Western Australia. Journal of Hydrology 146, Thompson, J. Mc., Cox, J.W. and Reynolds, M.B Physical characterisation of soils associated with piezometer installations in land degradation studies in the Keynes Catchment, Keyneton, South Australia (Stage 1) Technical Report 42/1994 CSIRO Division of Soils. Thompson, J.Mc., Williamson, D.R., Fitzpatrick, R.W. and Davies, P.J Piezometer design, installation and monitoring techniques to study groundwater and perched watertables. CSIRO, Division of Soils. Technical Report 28/1992. Wood, W.E Increase of salt in soil and stream following the destruction of the native vegetation. Journal of the Royal Society of Western Australia 1,

19 17 Appendix 1: Soil chemical properties of the drill cuttings DEPTH EC ph ph Cl (m) (ds/m) in H 2 in.1m CaCl 2 mg/kg K K K

20 K K K

21 K K