Questioning the causes, effects, and management of dryland salinity in South Australia's Upper South East
This update published 27th April 2010
First published 24th January 2005
Some detrimental effects of deep drains are easily estimated (eg the volume of greenhouse gases emitted from previously wet or waterlogged soils high in organic matter, the financial cost of repairing drainage-induced sodic soils and sub-soils, and of replacing salt-tolerant pastures killed by drainage), and others are less easy to estimate but are nevertheless still tangible (eg visual pollution of unnatural engineering structures, damage caused to native flora and fauna by excessive dewatering of soils, and contamination of down-stream land and reserves with salts, acids, and heavy metals). The following provides a summary of some of the adverse effects of drains.
Although considered by some a tolerable disadvantage, a drain and its associated spoil heap is an ugly scar on the landscape, which is not helped by a program policy to allow disturbed soils to revegetate naturally. Sharod (2000) warned that drains might provide a route for invasion by weeds and plant disease, and evidence of weeds not previously seen has been noted in native vegetation through which Upper South East drains have been constructed (Wheeler et al, 2002).
Examples of what happens when there is no revegetation
..... weeds take over after 6 months!
The more aggressive weeds of the Upper South East, eg the poisonous European heliotrope, the legume-suppressing wire weed, and stock-damaging and legume-suppressing silver grass are thus likely to become a feature of Upper South East drains and adjacent land, as weeds grow uncontrolled and seeds blow into adjacent paddocks or are carried down-stream to the region's ephemeral and permanent wetlands, including the Coorong.
The Upper South East's Grand Canyon, hailed by some as a major engineering achievement, and by many others (including the Commonwealth Government) as major environmental vandalism, was constructed to allow the northern catchment drainage effluent flow into the Coorong at Salt Creek. Approximately 200 ha of native bushland and habitat were destroyed, although some estimates are as a high as 850ha (Elliot, 2000).
The natural outlet for Upper South East floodwaters lies 5km to the north, and which crossed the route of this drain! Specific measures originally required by the Commonwealth Government to re-open the natural outlet had included consideration of underground pipes, minimising impacts on endangered species, removing excavated soil, installing fauna crossings, and rehabilitating disturbed habitats!
The draft EIS (12NRC, 1993) required that "clearance of vegetation will need to be minimised and spoil banks revegetated with local native species". The above photograph was taken 5 years after this section of drain had been completed - very little has been done, and the drain sides are collapsing under the influence of rainfall and wind erosion.
Regulation 3(1)(c)(ii) of the Native Vegetation Act 1991 allows native vegetation to be cleared without approval when an environmental impact statement has been produced, although Native Vegetation Council opinion should be sought during the development of a management plan. An exemption to the regulation was sought from the SA State Parliament AFTER the northern catchment outlet was constructed, which, although encountering opposition in the Parliament's upper house, was still granted (Elliot, 2000).
Up to 2002, an estimated 245ha of native vegetation had been destroyed as a result of drain construction (Roger Ebsary, pers comm reported by Wheeler et al (2002)).
Sharod R et al (2000) warned that lowering of the watertable near a drain might cause some plants to die, or become less vigorous and less able to compete for space and nutrients, and to survive attacks by disease, pests or grazing animals. Subsequently, McEwan K et al (2002) report that in Tilley Swamp, a drain is causing the watercourse to dry, which is having a deleterious effect on the health of the rare wetland community (so described in the draft EIS (NRC, 1993)) of Melaleuca brevifolia (white-flowered paperbark/teatree), and Melaleuca halmaturorum (coastal paperbark/teatree) (Milne et al, 2001). Adverse effects extend to 850m, and 1200m from the drain for Melaleuca halmaturorum, but at these distances Melaleuca brevifolia health generally improved. At other locations, Melaleuca shrublands, Gahnia filum sedgeland (vulnerable, listed in SENRCC, 2003 ) and Selliera radicans +/- Wilsonia backhousei herbland (endangered, listed in SENRCC, 2003) are being displaced by samphire or salt scalds where salinity has increased.
At Deep Swamp, there has been a general decline in health of the entire community of both Melaleuca brevifolia and Melaleuca halmaturorum up to 850m from a drain. Extensive death of Gahnia filum sedgeland is being reported (McEwan K et al, 2002). At the Hanson-Tiver monitoring site, large-scale death of Melaleuca between 1998-2000 has been attributed to a drain (Milne T et al, 2001a).
McEwan K et al (2002) report that Melaleuca halmaturorum appears to respond adversely more rapidly to changes in hydrology. While annual groundwater fluctuations are found to assist in maintaining the health of Melaleuca in groundwater discharge areas, probably by leaching salts out of the root zone, death and other adverse health effects are possibly caused because their shallow roots are unable to respond rapidly enough to lowering watertables. Once the symptoms of health decline are visible, it may be too late to implement adjustments to the hydrological regime. Given that the majority of remnant vegetation occurs on the flats, large areas of native vegetation could be adversely impacted by drains, with adverse consequences also for biodiversity.
Melaleuca and Eucalyptus have deteriorated and died as a result of long-term waterlogging and high salinity. Salinity can also be caused by "mounding" of groundwater flows on the up-gradient side of wetlands. While salinity reduces the ability of flora to take up water, excessive or poorly controlled dewatering caused by poorly designed drains will have similar effects!
Puccinellia is dying in newly drained soils (McEwan K et al, 2002), despite it being a salt-tolerant grass recommended for pasture renovation (NRC, 1993), and proposed in the benefit:cost analysis for the program (Barber A, 1993). Major drains will reduce the growing season of perennial pastures within the zone of influence (1km) of a 2m drain, amounting to an estimated 10% annual production loss (Schrale G, 1987).
Poorly designed and positioned drains and stop banks (including weirs) have high potential to reduce flows to, drain, and/or contaminate wetlands. Studies in support of the Program (eg Armstrong D, 1992, and more recently Cox J, 2005) failed to model the impact of drains on wetlands, and remarkably, the draft EIS (NRC, 1993) failed to even mention aquatic species!
To take one example, the Parrakie Wetlands, situated in the West Avenue Range Watercourse (7,500ha of remnant vegetation) in the central catchment, is home to the Southern Bell Frog, which is vulnerable throughout Australia, and which requires fresh water and vegetation in water for cover (Grear B, 2005). The Parrakie Wetlands are also home to the Yarra Pygmy-Perch, vulnerable throughout Australia and protected in South Australia (listed in SENRCC (2003)), and many other rare, vulnerable or endangered species, such as Mallee Fowl (vulnerable), Rosenbergs (or Heath) Goanna (rare), Beautiful Firetail (rare), Blue-Billed Duck (rare), and many other waterfowl. The Wetlands supports over 130 species of birds.
Southern Bell Frog
Yarra Pygmy Perch
Existing drainage works (Blackford and Fairview Drains) have already reduced flows and quality of water reaching the West Avenue Range Watercourse. Deep drains containing saline groundwater are now proposed for the watercourse, including a new drain adjacent to the Parrakie Wetlands. This poses a significant water contamination threat to the watercourse and the down-stream Henry Creek, as well as producing a barrier to the east-west fresh water flows that currently supply the Parrakie Wetlands.
Additional contamination concerns arise from the potentially high acidity of drain effluent and so-called black water. Many regions in the Upper South East have potential acid sulfate (high concentrations of iron and sulfate) soils, which could impact on the quality of drain effluent, by releasing toxic sulfate ions and heavy metals. However, carbonate concentrations are also generally high in Upper South East soils and groundwater, which should neutralise acidity, at least in the short-term. Increasing acidity might become a problem though when soluble salts are leached or diluted and carbonate levels lowered over time, eg during low flow periods or periods of fresh water flow. As soluble carbonates decline over time, acidity is expected to increase, releasing sulfates and heavy metals to down-stream areas.
Black water is caused by the decomposition of plant material not adapted to inundation, as well as chemical interactions between iron-rich soils and water. The black sludge (monosulfidic black ooze) that accumulates in the bottom of drains can also contribute to black water. Black water is generally non-acidic, has no dissolved oxygen, and kills all aquatic fauna that cannot escape. Artificial drainage has greatly increased the rate and overall amount of black water exported. (Smith, 2002)
Evidence for these reactions occurring in recently constructed Upper South East drains has been reported by Merry and Fitzpatrick (2005) and Fitzpatrick and Merry (2006), and there is anecdotal evidence of these reactions occurring in a number of other drains. Of particular concern is that the reports are on the analysis of 6-year old, 53km drain that forms a component of the proposed connector between the higher rainfall Lower South East and the Coorong.
Declining rainfall, a trend started over 50 years ago in the Upper South East and predicted to continue for at least another 50 years, is further reducing the frequency of significant fresh water flows necessary to export salt that is being concentrated in wetlands through evaporation. Increasing salinity has already adversely effected a number of wetlands in the Upper South East, and is likely to prove devastating to the fauna and flora of wetlands such as Parrakie, unless action is taken to at least maintain, or preferably increase, fresh water flows to Upper South East watercourses.
Conditions for Commonwealth Financial Assistance accompanying the 1993 proposals required that "measures be taken to minimise potential impacts ... on species listed under the Endangered Species Protection Act 1992 must be prepared to the satisfaction of the Australian Nature Conservation Agency". The Act was superseded by the Environment Protection and Biodiversity Conversation Act 200, which has even more stringent guidelines. The measures proposed by the Commonwealth appear to be viewed by the program as a hindrance to drain construction, rather than a guide!
Most of the salts present in groundwater and saline soils are either chlorides (Cl-), sulfates (SO42-), carbonates (CO32-) or bicarbonates (HCO3-) (all known as radicals) of calcium (Ca2+), magnesium (Mg2+), sodium (Na+), and potassium (K+). The superscripts +, 2+, -, and 2- indicate that each atom or molecule usually exists as an ion, caused by the loss or gain of electrons. This results in an electrostatic charge of sign and magnitude indicated by the superscript.
Ions and molecules of opposite electrostatic charge form so-called ionic bonds in stable molecules with zero net charge, called ionic molecules. The radicals are responsible for the characteristic properties of the molecule. Radicals existing as ions have a negative charge (known as an anion), and affect soil properties directly by increasing salinity. The forces that hold the individual atoms of a radical together are known as covalent bonds, which arise when electrons are shared. Covalent bonds are not as strong as ionic bonds.
Each of the salts has a unique solubility, which along with the composition of minerals through which rainfall leaches into the groundwater, dictates the concentration of salts present. When the salts are dissolved in water, they separate (also known as disassociate or dissociate) into cations (positively-charged molecules) and anions (negatively-charged molecules).
Dissolution occurs because the electrostatic charges on a water molecule (known as a polar molecule) are unequally distributed, resulting in the negatively charged oxygen atom and positively charged hydrogen atoms exerting attractive and repulsive forces on the cation and anion that form the salt molecule. If the salt molecule's ionic bond is weak (eg as with the sodium salts), the molecule separates easily into its constituent cation and anion - it dissolves. Water molecules then surround the ion, with the appropriate charge of the water molecule aligned with the ion. If the ionic bond is similar to or considerably stronger than the forces applied by the water molecules, then the salt is said to be either slightly soluble (eg gypsum) or insoluble (eg lime).
Dissolution of salts into their constituent ions also results in the electrical conductivity (shortened to EC) of the solution increasing, which is a direct measure of salinity. The ability of a solution to conduct electricity is related directly to the number of free ions it contains.
Dissolved salts cause plant cell dehydration by decreasing the osmotic potential of soil water. Water flows preferentially from high osmotic potential (low salt concentration) to low potential (high salts), so plant cells encounter an opposing force when extracting water from saline soil water. Osmotic potential is related directly to the concentration of solids in the water solution. If the solids are predominantly dissolved salts, then EC of the solution also provides a direct measure of osmotic potential.
Typical conversion factors given in the literature are:
|osmotic pressure (atmospheres)||=||ionic concentration (moles/litre) X 0.0821 X absolute temperature (K)|
|electrical conductivity (deciSiemens/metre)||=||ionic concentration (moles/litre) X (factor in range 0.008 - 0.019)|
The most common cations in arid and semi-arid areas are calcium, magnesium and sodium. Each of these cations is base-forming, meaning that they contribute to an increased OH- ion (hydroxide ion) concentration in the soil solution and a decrease in H+ ion (hydrogen ion) concentration. Where the concentration of OH- and H+ ions in solution are equal, the solution is said to have pH of 7. Where the concentration of H+ ions exceeds OH-, the solution is acidic and has pH less than 7. Where the concentration of OH- ions exceeds H+, the solution is alkaline and has a pH greater than 7.
The OH- ion is found in all alkalis (bases that are soluble in water) and in alkaline solutions. A base is a substance which reacts with an acid to form a salt and water. The OH- ion is also found in all aqueous solutions (in which water is the solvent) because of the dissociation of water, where molecules continuously separate into H+ and OH- ions and then recombine again to form H2O. Because water has an equal number of H+ and OH- ions, it has a pH of 7.
Bases are usually metal oxides or hydroxides. So-called Group I hydroxides, eg sodium hydroxide (NaOH) and potassium hydroxide (KOH), are soluble in water, and Group II hydroxides are sparingly soluble, eg magnesium hydroxide (Mg(OH)2) and calcium hydroxide (Ca(OH)2). Group I and II metal oxides and hydroxides that are soluble in water are known as alkalis.
Typical solubilities for salts of interest, expressed in moles/litre at a temperature of about 21C, are (from Seelig, 2000, and others):
Sodium chloride (NaCl) - 6.15
Sodium sulfate (Na2SO4) - 3.45
Magnesium sulfate (MgSO4) - 2.18
Sodium bicarbonate (NaHCO3) - 1.22
Calcium sulfate (gypsum, CaSO4 . H2O) - 0.0154
Calcium carbonate (lime, CaCO3) - 0.00014
In practice, solubility is more complicated than implied by the list. Sodium sulfate solubility is highly sensitive to tempertaure and increase more than ten-fold from 0C (0.15 - 0.34 moles/litre, depending on whether molecule is of anhydrous, heptahydrous, or decahydrous form) to 32.4C (1.5 - 3.5 moles/litre). When several salts are in solution, their interaction influences solubility. Gypsum solubility increases up to three-fold in the presence of sodium chloride, but sodium sulfate solubility decreases markedly. High pH, for example associated with dissolved sodium bicarbonate, suppresses calcium carbonate but increases gypsum solubility.
These interactions could thus lead to increased concentrations of sodium sulfate in soils if sodium chloride is leached, for example by drainage. Furthermore, because sodium sulfate is more soluble at high ambient temepratures, a greater amount of the salt could rise to the soil surface through capillary action during hot, dry summer months, but less might leach back to the groundwater in cooler, wetter months. The sensitivity of gypsum to sodium chloride concentrations also suggests that gypsum applications to sodic soils will be more effectively transmitted through soil profiles when sodium chloride concentrations are high, such as before deep drainage.
Salts more soluble than gypsum are considered to cause salinity (Seelig, 2000). Solubility rules for ionic compounds can be found in Anon (2005a). The unit moles/litre is a measure of the number of molecules that can be dissolved in one litre of saturated water solution, usually at a temperature of around 20C. One mole comprises 6x1023 molecules of the salt of interest, where the number is known as Avagadro's constant or number.
The Group number refers to the number of electrons in the outermost electron shell, ie Group I metals (eg sodium and potassium) have a single electron, Group II metals (eg magnesium and calcium) have two electrons, and Group III (eg aluminium) have three electrons. Atoms within these three Groups are attracted to and held by charges on colloids in the soil, eg organic matter and clays. The charge sites on colloids is known as the exchange complex of soils. The Group I and II, metals because of their higher concentrations, usually dominate the exchange complex.
Extremely high pH (>8.5) occurs in sodic soils (defined as when sodium is attached to more than about 10% of the soil's, usually clay, exchange sites) when sodium carbonate (Na2CO3) or sodium bicarbonate (NaHCO3) is present. The sodium, carbonate, and bicarbonate ions react with water to produce hydroxide ions, having bonded the hydrogen ion to respectively the exchange site, to the carbonate ion to produce a bicarbonate ion (HCO3-), or to the bicarbonate ion to produce carbonic acid (H2CO3). Soluble carbonate salts are usually found in coarse structured soils. Micronutirents such as iron, manganese, zinc, copper, and cobalt are all much less available at pH greater than 7. (Seelig, 2000)
Other atoms of interest in soils include carbon (Group IV), nitrogen and phosphorous (Group V), oxygen and sulfur (Group VI), and chlorine (Group VII).
While sodium and potassium bicarbonates can exist as solid salts, calcium and magnesium bicarbonates are only found in solution. When soil moisture is reduced, either by evaporation, plant uptake, or drainage, calcium bicarbonate (Ca(HCO3)2), which is usually more common than magnesium bicarbonate, decomposes into the solid calcium carbonate (CaCO3), carbon dioxide, and water. This process results in calcium being removed from colloids, in particular clay particles, while sodium is left behind, creating a sodium-dominated (sodic) soils from a calcium-dominated soil. Carbonates when present are usually found at pH>8, which also causes calcium and magnesium to precipitate out of solution when the soil dries. (Warrence et al, 2004)
Water with high sulfate content is corrosive to concrete structures. The sulfate solution penetrates the concrete and reacts with calcium to form calcium sulfate, which precipitates out within the pores. This destroys the integrity of concrete by changing it to non-cementing material (calcium sulfate) that forms crystals in voids. Type V cement has high resistance to sulfate corrosion, unlike Type I (standard Portland cement) and Type II cement. (Seelig, 2000)
Large areas of the Upper South East have soils with high sulfate content, which have potential to become so-called acid sulfate soils. The soils were normally waterlogged, which prevented the sulfate layers oxidising and becoming acidic. The calcareous soils and sands of the Upper South East result in relatively high concentrations of soluble and solid bicarbonates and carbonates in the groundwater and soils, which generally neutralise any acidity formed on drainage, at least in the short term. Other chemical reactions also occur.
High soil pH in the Upper South East is usually associated with sodium bicarbonate and sodium carbonate, which are relatively very soluble salts that are easily leached from sandy soils. Calcium and magnesium salts will also be present, but at much lower concentrations(pers comm, Merry, 2004). High pH decomposes clays, resulting in hard pans forming in the lower top soil horizons and restrict root growth (Fitzpatrick et al, 2003). While decreasing the availability of critical nutrients for plant growth, eg P, N, Ca, Mg, Fe, Mn, Cu and Zn, high pH usually only occurs during the dry months when plant growth is negligible.
Iron oxides accumulate at the top of clay sub-soils, and in drain walls, where they clog soil pores, and restrict salt leaching and effluent flow into the drain (Fitzpatrick et al, 2003 and 2004).
Recent animal health problems in the Upper South East led to the analysis of animal tissue and pasture samples, with the aim of identifying mobilised soil nutritional and toxic elements. While many nutritional elements were found at levels in an acceptable range (including boron, calcium, chromium, cobalt, iodine, iron, lithium, magnesium, manganese, molybdenum, phosphorous, potassium, selenium, silica, sodium, sulfur, vanadium and zinc), a range of toxic elements at levels that will cause chronic health problems were routinely measured in pasture samples. The toxic elements included aluminium, antimony, arsenic, barium, cadmium, lead, mercury, nickel, silver, strontium, tungsten, and uranium.
The animal health problems were concluded to have arisen from soil drying arising from increasing droughts and/or drainage. Soil drying allows solubilising of the toxic elements by oxidation. Furthermore, drainage has potential to not only oxidise and mobilise these toxic elements, but also leach them laterally to drains where they become concentrated. (pers comm Parsons, 2005)
A stinking iron monosulfidic black ooze (MBO) forms in the bottom of newly constructed drains and adjacent soils. When mixed with water, the iron monosulfide in MBO can react within minutes to completely consume dissolved oxygen (Bush, 2004). The deoxygenation risk increases at higher salinities (Merry and Fitzpatrick, 2005). The reducing conditions in MBO (substances undergo reduction if they lose oxygen, gain hydrogen, or gain electrons) produce methane (CH4), nitrous oxide (N2O), and hydrogen sulfide (H2S) (Fitzpatrick, 2004). H2S is believed to be formed by sulfate reducing bacteria acting on sulfates in the groundwater (O'Sullivan et al, 2005)(Sawlowicz, c. 2001). The MBO is also a source of other toxic ions (principally carbonates (CO32-) and bicarbonates (HCO3-)), a scavenger of heavy metals, and, if exposed to atmospheric oxygen, a source of sulfuric acid (Fitzpatrick, 2004).
Removal of MBO during desilting will result in the rapid formation of sulfuric acid. When stirred up by rough water on windy days, in particular when the water is shallow during drier months, the H2S gas produced is toxic and noxious, and can be smelled over long distances. MBO particles are easily mobilised and washed down-stream when stirred up by wind (see photograph below right), or at times of increased flows (Merry and Fitzpatrick, 2005).
Iron sulfides are commonly found in the anoxic (low oxygen) sediments of drains, where bacterial sulfate reduction is very active and the amount of available metals, mainly iron (Fe), is sufficient. The process begins with the reaction of bisulfide and ferrous ions to form soluble Fe(HS)2 and FeHS-, which can be a very fast process. Transformation to other forms then occurs by the addition of sulfur or loss of iron. Recent theories postulate that earliest life forms arose from similar iron/sulfur chemical transitions occurring near deep sea hydrothermal vents. (Sawlowicz, c. 2001)
Iron monosulfide (FeS, which in different crystalline or framboid forms is known as mackinawite and troilite) sediments are commonly found in drains in acid sulfate soil areas, and can contribute more than 30% of the unconsolidated sludge in a drain base. The sulfides play an important role in improving water quality by absorbing contaminants, including acidity and metals. It is believed that freshly deposited iron monosulfide is largely amorphous (ie non-crystalline), but over time undergoes a transformation to crystalline forms, such as mackinawite, troilite, and greigite (Fe3S4), and then the more stable pyrite (FeS2) (Bush, 2000). The sediments, in particular, the amorphous and hence more highly reactive particles, are prone to oxidation during drain cleaning or drought, producing acidity, and releasing sulfate ions and metals (eg iron and aluminium) into the drainage system (Hicks et al, 2002)(Smith, 2004).
The orange surface that forms on the MBO comprises iron oxyhydroxysulfates, which are oxidised iron sulfides. They also scavenge other elements, such as arsenic and cadmium (Fitzpatrick et al, 2003a).
Acid release can be reduced by not clearing drains. Holding water back using drop board culverts has also been shown to prevent acid groundwater seeping into drains, and hence reducing acid flows. Filling in deep drains and replacing them with shallow v-drains is considered to be a more reliable and effective long-term solution. (Smith, 2002)
Upon oxidation with water present, pyrite undergoes a chemical transition into ferrous sulfate (FeSO4) and sulfuric acid (H2SO4). Subsequently, bacterial oxidation (by Thiobacillus ferrooxidans) converts ferrous sulfate and sulfuric acid into ferric sulfate (Fe2(SO4)3) and water. A chemical reaction then converts ferric sulfate and pyrite into ferrous sulfate and elemental sulfur. The elemental sulfur may then be oxidised in water in another bacterial process (by Thiobacillus thioxidans) to form sulfuric acid, which then favours the continuation of the earlier described transitions (Reclamation and Management, Abrol, 1988). Hicks (2002) and Fitzpatrick (2003a) describe another oxidation process for pyrite in water, which produces Fe(OH)3 and sulfuric acid.
The concentration of carbonate anions is always less than for chloride in the Upper South East, which is the most dominant anion found in groundwater (Fig 27, O'Driscoll, 1960). Sulfate anions are also generally found in subordinate and variable percentages. Presumably, over time, carbonate concentrations in drain effluent will decline, giving rise to a lowering of effluent pH, and thus an increase in sulfates and heavy metals in solution. The risk of this happening is likely to be high if the proposed Lower South East to Coorong connector is approved by the South Australian State Government. The absence of carbonate anions in groundwater leads to highly acidic groundwaters and drainage effluent, which are found in several areas in Australia (eg Fitzpatrick, 2003; Rogers, 2005).
Monosulfidic Black Ooze
Black Ooze Stirred Up On A Windy Day
One of the most contentious issues has been whether or not the Coorong requires, and would benefit, from discharge of drainage effluent. The key concerns are volumes and water quality, especially in relation to impacts on salinity levels, and nutrient and heavy metal concentrations. The State Government and NRC position is that while drainage, stop banks and weirs constructed since 1864 altered flows to the Coorong, its current hypersaline (40,000-45,000ppm) conditions closely approximate those that existed before 1864, and that drainage flows up to a 10-year average of 40GL/year could be accommodated.
Anecdotal evidence from Aboriginals, and other evidence, that much larger flows occurred prior to 1864 were dismissed (10.1, NRC, 1994). However, the Coffey MPW report (NRC, 1994a) noted that massive freshwater flows into the Coorong appear to increase the attractiveness of the habitat for some birds. Other reports cited by Coffey MPW, and anecdotal evidence (eg PWC (1999) and Gary Hera-Singh of the Southern Fisherman's Association) also suggest that the State Government and NRC position is wrong.
Excess nutrients in the Coorong may lead to algal blooms, and excess heavy metals in the drains (now habitat for fish and birds), wetlands and Coorong may accumulate to toxic levels lethal to key food species. Metals of significance are copper and zinc (194, NRC, 1993), both of which have lowered toxicity potential in alkaline drainage waters. It is also not certain that heavy metals will remain in sediments in a shallow wind-affected lagoon. Further investigations were recommended (10.3.9, NRC, 1994). Dispersing sodic clay particles suspended in drainage flows will also carry attached nutrients to down-stream wetlands and to the Coorong.
An increase in the concentration of certain gases in the atmosphere has been attributed to the so-called greenhouse effect, which is causing global temperatures to rise and the climate to change.
The effect arises from high energy solar radiation (mainly at visible wavelengths, in the form of light) being absorbed at the earth's surface and re-radiated in the form of radiant heat (at lower energy infrared wavelengths). Atmospheric absorption of light is less than at infrared wavelengths, which results in some of the incident solar energy being trapped in the form of heat energy. The temperature of the atmosphere then rises until an equilibrium point occurs when the energy of the incoming solar radiation is balanced by losses to space of heat and reflected solar energy.
Some of the incident solar energy is reflected directly back to space. White regions (snow, ice, and clouds) will reflect almost all incident solar radiation back to space. Darker regions, such as deserts, will absorb some of the incident solar radiation, but also reflect a large proportion back to to space. Dark regions, such as those covered with dark soils and vegetation will absorb almost all incident solar radiation. However, vegetation also uses some of the incident solar radiation to produce chemical energy, in the photosynthesis process, which converts carbon dioxide (CO2) from the atmosphere into sugars stored in plants. The sugars then produce heat energy when they are broken down during biological processes (eg digestion, decaying) and burning, as well as greenhouse gases such as CO2 and methane (CH4).
The greenhouse effect is a naturally occurring phenomenon dominated by water vapour in the atmosphere absorbing and re-radiating energy at infrared wavelengths. Without the greenhouse effect, mean temperatures at the earth's surface would be considerably lower than they are today. However, the current atmospheric equilibrium is being adversely affected by an increase in the concentrations of other infrared absorbing gases, principally CO2, which has risen from about 280 parts per million (ppm) by volume to about 380 ppm since 1750. Half of this rise has occurred since about 1975.
Infrared emissions from the earth's surface span a range of wavelengths, across which water vapour does not absorb uniformly. One of the wavelength regions is where, coincidentally, infrared absorption by water vapour is low but emissions from the earth are a maximum. This is also a wavelength region where infrared absorption by CO2 also peaks, which is a reason why CO2 emissions are a major concern. Other greenhouse gases absorb infrared radiation at different infrared wavelengths, many of which coincide with a region of high absorption by water vapour. Their net effect on global warming is therefore likely to be less, which climate change skeptics claim is a flaw in the global warming argument. The likely effect of increasing CO2 in the atmosphere though should definitely not be ignored.
While a precise understanding of the complex greenhouse gas interactions occurring in the atmosphere is not yet known, the overwhelming body of opinion predicts that increasing concentrations of man-made greenhouse gases are leading to increasing atmospheric temperatures, which in turn is increasing the capacity of the atmosphere to hold more water vapour. This process thus reinforces the adverse effects of growing concentrations of man-made greenhouse gases.
Other processes reinforce the greenhouse effect at the earth's surface. These include the loss of large areas of ice and snow resulting in less solar radiation being reflected directly back to space, and more being absorbed and re-radiated as infrared energy. Soil processes, such as soil drying, result in the oxidisation of organic matter and the production CO2.
The International Panel on Climate Change (IPCC) has recently published best estimates of temperature equilibrium conditions under different greenhouse scenarios. This work, and others, including details of the different greenhouse gases, their sources, effects, and persistence in the atmosphere, can be found by searching for "climate change" on the internet. A brief summary of the greenhouse effect, with several links to other relevant information can be found on Wikipedia, in particular go to its page on greenhouse gases.
CO2 is produced when the more soluble calcium bicarbonate, Ca(HCO3)2, decomposes on drying into the relatively insoluble calcium carbonate (CaCO3) (Warrence et al, 2004).
The reducing conditions that produce MBO in the drain base also produce methane (CH4), nitrous oxide (N2O), and hydrogen sulfide (H2S) (Fitzpatrick, 2004). CH4 and N2O are greenhouse gases, which have approximately 22X and 300X more global warming potential than CO2 (Blasing et al, 2005)(Pew, 2005).
Hicks (2002) estimated the long term average release of carbon from drained acid sulfate soils in North Queensland to be the equivalent of 121 tonnes of CO2 per year per hectare, arising from the decomposition of organic carbon and reaction with soil carbonates. Previously undrained soils will have elevated levels of organic matter, which on draining will oxidise to increase CO2 emissions (11, Halley et al, 1992). Upper South East top-soils appear to lose about 0.5% of organic carbon after drainage, which equates to CO2 emissions of about 25 tonnes/ha.
If the Upper South East drains lower watertables up to 200m either side (Program staff claim up to 5km!), and half of the area drained in the Upper South East is underlain with potential acid sulfate soils containing soil carbonates, then over a million tonnes per year of CO2 will be produced. If the predominant source of CO2 emissions is from oxidisation of soil organic carbon, and the previously wet top soil is dried to a distance 1km either side of a drain, then over three million tonnes of CO2 could be released as a result of drainage. Because a planning principle of the South East Natural Resource Management Plan (17, SENRCC, 2003) requires that "future development should be either greenhouse neutral or result in a net decrease in greenhouse gas emissions", the Upper South East drainage program appears to be a major breach of this principle, unless it has been compensated for by other means!
A tree plantation of about 14,000ha would be needed to absorb a million tonnes a year of CO2, but mature trees should be removed, and not allowed to rot or burn, which releases CO2 back into the environment. Trees are about 50% by weight carbon, and absorb CO2 through their leaves from the atmosphere by photosynthesis, which with water converts CO2 into glucose and oxygen. Mature trees grow less rapidly and thus have a lower intake of CO2.
Methane (CH4) is exhaled by ruminants in significant quantities. Because an objective of the drainage program is to improve agricultural productivity, it is reasonable to expect that cattle and sheep numbers will increase in the region.
Assuming stocking rates improve by an average 1dse/ha across the region (680,000ha), this would mean that the equivalent of an additional 68,000 cows could be stocked. According to UCal (2003), each cow produces approximately 600 l/day of CH4, which is about 150 kg/year (0.68kg/m3 at 1bar and 15C (Air Liquide, 2005)), or a little over 10,000 tonnes/year for the additional cattle. Australian cattle are reported to be much more prolific producers of methane, producing of the order of 1.8 tonnes/head/year (MLA, 2005a), which is assumed to be an error!
Grazing is not a greenhouse gas neutral enterprise, because most of the carbon in CO2 absorbed by crops and pastures is subsequently ingested by stock and then exhaled in the form of the 22X more damaging CH4.
In order to compensate for the additional CH4 production, the equivalent of about 223,000 tonnes of CO2 (ie 22 x 10,127) would need to be removed from the atmosphere to make the project greenhouse neutral, or 3,500ha of additional trees would need to be planted.
In order to compensate for the increased CO2 and CH4 emissions arising from the drainage program, an additional 45,000ha (just under 7% of the region) should be planted to trees.
Intensive agricultural practices enabled by drainage also increase emissions of N2O. Reporting for the Australian Greenhouse Office, Dalal et al (2003) noted that N2O emissions from agricultural soils come from nitrogen fertilisers (32%), soil disturbance (38%) and animal waste (30%). Since 1990, N2O emissions from soil disturbance and intensive livestock production have steadily increased. There has also been a fast increase in nitrogen fertiliser use for cereal production, accounting for 70% of the total fertiliser use in 2000 compared with just over 50% in 1990.
The Upper South East was traditionally grazing country prior to drainage, but there are increasing numbers of farmers now turning to cereal grain production following drainage.
Estimates of N2O emissions from agricultural sources vary markedly, depending upon the enterprise. They range from about 10 kg N2O/ha (claimed to be 5 tonnes/ha of CO2 equivalent) from fertilised dairy paddocks, 5 kg N2O/ha (2.5 tonnes/ha of CO2 equivalent) from N fertiliser applied to cereal crops, down to 0.2kg N2O/ha (about 100 kg/ha of CO2 equivalent) grazed pasture and rangelands.
Other sources of greenhouse gas emissions, eg emitted by machinery involved in drain construction, or by other machinery associated with more intensive agricultural enterprises arising in the region after drainage, would need to be included in the greenhouse gas calculations.
Sodicity problems only manifest after saline soil is drained, and in some cases might not be apparent for years or decades. Its adverse effects have been observed for thousands of years. The processes that cause and prevent sodicity problems arising in drained saline soils have been understood for decades in countries such as India, Peru, Egypt, US, and UK (Quirk, 1999)(Warrence et al, 2004)(Ag Bureau of SA, 2005). A 1994 description of sodicity induced by draining saline land in South Australia (Fritsch et al, 1994) was believed to be the first describing such a process in Australia (Fitzpatrick et al, 2001), although the cause and treatment of sodicity had been understood in Australia since at least the mid 1950s (Quirk et al, 1955).
Sodicity is caused by excessive amounts of sodium from salts becoming adsorbed onto (attached to) the surfaces of microscopic particles of clay and organic matter. Weight-for-weight, organic matter has the capacity to adsorb up to 30X more sodium and other beneficial elements than clay (NSW DPI, 2004). Each sodium ion (existing as a cation) is attracted to negatively-charged sites on the surface of the soil colloid particles, known as adsorption or exchange sites. In addition to sodium, the most common soil cations are calcium, magnesium, potassium, ammonium, and hydrogen (Mengel, 2005).
Clay particles are plate-like in appearance with broken irregular edges. Negatively-charged sites generally exist on either side of each plate. Positive charges also exist, generally around the plate edges. Other negatively-charged sites exist on soil particles, caused by the unbalanced replacement of Group III ions with Group II within the clay structure, or by the dissociation of organic and hydrous oxide molecules (13, Halley et al, 1992). Under saline conditions, and under normal conditions when soils are not sodic, individual soil particles are attracted to each other by the forces existing between negative and positive charges and form jumbled aggregates in a process called flocculation. The existence of the attractive forces is the reason for the cohesiveness and structural stability of these soils.
The higher porosity of these soils arises from the large spaces between flocculated particles (known as micro-pores), and between larger aggregates (known as macro-pores). Soil porosity is important because it provides space to hold soil water and atmosphere, allows unobstructed penetration of roots, and assists in drainage, including leaching of salts from saline soils if present. Coarser structured soils also have a shorter capillary rise distance, so salts stay lower in the soil profile.
For plants to access water and nutrients, their roots must be able to adequately explore the soil volume. Since water becomes limited towards the end of the growing season in the Upper South East, and soils generally dry from the top down, optimum productivity is typically found where plants can get their roots into the deeper sub-soil layers. However, sodicity makes soil less able to hold soil water and more difficult for roots to penetrate and thus limits the soil volume explored - productivity is adversely affected (Baldock, 2003). Well structured soils comprise about 50% pores and 50% mineral and organic matter (3, Halley et al, 1992).
Under saline conditions, sodicity problems are suppressed. However, when saline-sodic soils are drained, fresh water moving through the soil causes the loosely-bound sodium atoms to move away from the negatively-charged sites and form a positively-charged "cloud" around each soil particle. This suppresses the effects of the negatively-charged sites, and the attractive forces between each particle are weakened. When the concentration of sodium atoms exceeds a threshold of about 5% - 10% (Lamond, 1992)(Quirk, 1999b)(DWLBC, 2004), attractive forces are suppressed, and repulsive forces start to dominate. The percentage of sodium atoms relative to all atoms attached to soil particles is known as the exchangeable sodium percentage (ESP).
Above the sodicity threshold, aggregates of clay and organic matter disintegrate. Dispersing soil particles become suspended in water, which becomes cloudy (turbid). At the soil's surface, the impact of rain drops and leaching of salts causes saline-sodic soil aggregates to break down, producing dense, sealed and crusted surfaces; waterlogging is commonly associated with such soils.
When soil moisture becomes reduced, either by evaporation, plant uptake, or drainage, soluble calcium bicarbonate (Ca(HCO3)2) decomposes into the solid calcium carbonate (CaCO3), carbon dioxide, and water. This process results in calcium being removed from colloids, leaving sodium behind (Warrence et al, 2004), which increases the effective ESP of the soils.
Impermeable surfaces and sub-soil hard pans are formed with seasonal wetting and drying, as sodic soil particles disperse and move into and clog the soil's pores. Water and atmosphere essential for healthy plant growth is displaced. This leads to an increase in waterlogging, restricts growth of plant roots, reduces the ability of plants to take up water and nutrients, and slows down leaching of salts. Extensive waterlogging experienced in areas where the watertable is known to be well below the surface is more likely to be caused by poor soil structure and sealed top- and sub-soils. Sodic soils are sticky when wet, and when dry become hard, cloddy and crusty (Lamond, 1992). Sealed sodic surfaces if disturbed become dusty and highly susceptible to water and wind erosion.
Adverse effects of sodicity have been observed in Upper South East soils 2 years after drainage, at distances up to 400m from a drain, and are likely to impact on the future ability of the soils to grow plants and continue to leach salts (Fitzpatrick et al, 2003). In some cases, soil changes have been rapid and agronomic impacts severe (Fraser, 2006). Sandy top-soils become readily leached and seasonally dry, but gypsum is commonly required to prevent the development of hard capping in sub-soils, which prevent root penetration and further salt leaching (McEwan et al, 2002).
After several years, dispersed sodic particles drift down through a soil's profile, which becomes more dense. A manifestation of this effect are "black bogs" that have been noted close to recently constructed drains in central Upper South East, where soils have collapsed into compressed, sunken, infertile patches (DWLBC, 2004). A Federal Government grant worth $110,000 has been awarded to the Keilira Farm Management Group, which (in conjunction with the University of Adelaide) is endeavouring to identify farm management techniques that will enable these soils to be reclaimed (Howell, 2005).
Soil sodicity also makes its presence felt on drain banks, which slump, tunnel, and collapse, as clay disintegrates and particles disperse. They are then carried away by rainfall, by discharge from adjacent land through drain walls, and by the flow of drain effluent. CSIRO recommend revegetating drain banks to prevent these effects from happening (Fitzpatrick et al, 2003).
Over time, erosion tunnels, formed by seepage from adjacent land, will expand in diameter when fresh water flows through them during periods of high rainfall, and flushes away dispersed clay from the tunnel wall. The tunnel will eventually collapse to form an erosion gully.
Dispersed clay particles suspended in the drain effluent (also containing dissolved nutrients, in particular highly soluble nitrates and phosphates, and pesticides washed off paddocks) will be carried down-stream where they can feed algal blooms in wetlands (Quirk, 1999), and potentially the Coorong. Lake George in the region's south has already been severely affected in this way by drain effluent.
Dispersed clay washed from drain banks
and suspended in drain effluent after rain.
Erosion caused by dispersion of sodic clays.
Erosion tunnel forming in sodic drain bank ...
... which will grow back into the paddock
and collapse to form an erosion gully.
(photograph by CRC Soil & Land Management)
Infertile patches ("black bogs") formed after drainage
The other major element of interest in discussions on sodicity is calcium, which is more tightly bound to clay and organic matter than sodium, and does not result in repulsive forces between particles when saline soils are drained. An empirical relationship (the Gapon equation - see (Quirk, 1999b) and (Seelig, 2005)) describes the physical equilibrium condition that exists between adsorbed and dissolved ions. The Gapon equation predicts that an increase in the concentration of calcium or magnesium ions in the soil water will result in adsorbed sodium ions being replaced until a new equilibrium condition is reached, ie sodicity is reduced.
The Gapon relationship is the basis of statements (US (Lamond, 1992)(Buchanan, 1993), UK (Halley et al, 1992) and CSIRO (Fitzpatrick et al, 2003)) that saline-sodic soils should always be pre-treated before drainage, to ensure that sodium is replaced by calcium before the salt concentration is allowed to fall and the soils loses its permeability. State Government reporting (18, Barnett, 2000) warned that if ground water is saline, much greater care must be taken with drainage. These warnings were provided to the Upper South East program but ignored, and farmers were not warned of the consequences of failing to follow them. In the event that pre-treatment is not possible, weirs should be installed to prevent de-watering until treatment has been applied. A number of horror stories from the irrigation industry graphically illustrate the results of ignoring this advice, eg (Quirk, 1999a)(Quirk, 1999b). When fresher water follows the use of even slightly saline irrigation water, sodicity problems occur, resulting in a major breakdown of soil structure.
Reclamation of sodic soils, and in particular saline-sodic soils, is considerably more difficult than saline soil reclamation, because dispersed soils are impervious to water and resist leaching of salts from the soil profile (Buchanan, 1993).
Lamond (1992) and Quirk (1999b) provide guidance on the quantity of gypsum required to correct sodicity. The recommended rates to remediate soils to a depth of 30cm are a little over 4tonnes per hectare of gypsum for each milliequivalent of sodium present per 100g of soil. Typically, remediation requires of the order of 5 tonnes/ha of gypsum applied to top-soils and slotted in sub-soils, which is a significant and unexpected additional cost. In sodic alkaline soils, doubling of rates is recommended because of the reduced solubility of gypsum at high pH (Rengasamy et al, 2005).
When determining gypsum requirements, the measurement accuracy of exchangeable ions will need to be accurately determined confirmed. Despite quoting results to 4 significant figures, figures for exchangeable sodium can be over-stated by 100% for samples taken from saline soils, if soluble sodium is not removed (Bing So et al, 2004)!
PIRSA implies that sodicity is not a negative aspect of drainage (Cook, 2005), which is contrary to national and international opinion. While PIRSA also notes that displacement of loosely bound potassium from exchange sites by calcium is a negative aspect of gypsum application, the cost of not treating sodicity is even greater. Wherever rainfall exceeds evapotranspiration, beneficial cations, including potassium and calcium, will always be displaced from the soil's exchange complex by the hydrogen ions from water. These beneficial ions will then be leached in drainage water, in a process that results in soils becoming more acid (26, Halley et al, 1992). In turn, displacement of exchangeable ions by calcium, magnesium or potassium, eg during lime, gypsum or potash application, will result in exchangeable hydrogen and sodium ions being displaced into the soil solution, in a process that will also affect soil pH.
Calcium chloride, a major waste bi-product of the sodium carbonate industry (Chemlink, 1997) is reported to be more effective than gypsum in treating sodicity, and significant stockpiles exist pending disposal at sea (Chemlink, 1997a). Transport, spreading, and incorporation costs usually make treatment of sodicity with calcium chloride uneconomical. However, a drainage program on the scale of that conducted in the Upper South East might change this balance. Calcium nitrate is reported to be up to 100X more effective than gypsum at treating sodicity, and recommended rates are 0.5T/ha (pers comm Hignet, Dec 2004).
Other methods of remediating sodicity include liberating calcium in the soil if it exists in significant quantities, eg in calcareous soils. The process can occur naturally, as a result of water absorbing CO2 expired by soil organisms to produce carbonic acid (H2CO3), which in turn reacts with the less soluble calcium carbonate (CaCO3) to produce the more soluble calcium bicarbonate (Ca(HCO3)). Acidification can also occur using fertilisers such as urea, MAP, DAP, and ammonium sulfate (the most acidifying nitrogen-based fertiliser). Sulfur applied directly to soil, which reacts to produce sulfuric acid, has been used successfully to release calcium. Sulfuric acid has been applied directly to soils, but it can have a major adverse effect on soil biota.
A less aggressive method of liberating calcium involves exploiting plant root processes, in particular of legumes, which produce organic acids that promote the release of calcium. This technique has been used successfully to address sodicity in drained saline land in India and Egypt , where soils are rich in calcium carbonate (Oosterbahn, 2003). Australian researchers are also investigating this process (Rengasamy, 2001). Acidification has the added benefit of lowering high, damaging pHs, which often occur after draining saline soils.
If soil conditions are favourable, a multi-faceted attack on sodicity employing a combination of gypsum, acidifying fertilisers, and legumes (principally deep-rooted lucerne) will probably be optimum and most cost-effective. PIRSA recommend against lucerne because of a continuing possibility of waterlogging on drained soils. However, gypsum, acidifying fertiliser and lucerne should all work together to reduce this probability. Lucerne has a higher tolerance to ESP (at 40-60) than clovers and tall fescues (20-40), but lower tolerance than tall wheatgrass (>60) (Abrol et al, 1988).
A so-called fresh water lens (fresh water layer ) is formed on the top of saline groundwater during periods when rainfall exceeds evaporation and evapotranspiration. The process also leaches salts that might have accumulated in the top-soils and at the surface back down to the underlying saline groundwater. Where top-soils are coarse structured, capillary rise distances can be as little as 10 - 15cm, or where there is good surface cover, which results in low evaporation, groundwater is unlikely to have high salinity at the surface. However, perennial vegetation could still result in salinity concentrating in the root zone as a result of evapotranspiration concentration.
Where soil conditions and vegetation are suitable, the fresh water lens does not dissipate during the summer months, and so provides an ongoing source of stored water for plant growth, irrigation and stock watering. Any drainage that removes both surface water and lowers watertables will result in the lens thinning or being removed completely. In general, the coarser structured soils that are likely to have a significant fresh water lens will be the most effectively drained!
Drainage that removes surface water and lowers watertables could therefore have a devastating effect on agricultural productivity in these areas, and on the availability of fresh water flows to wetlands.
Wind erosion of unprotected drain banks will be an ongoing maintenance problem, which could have been reduced by revegetation. The Eyre Peninsula NRM Group in conjunction with PIRSA Rural Solutions recommend establishing grass on drain banks comprising fine sand as soon as possible (Dowie, 2005). Previously stable land around gateways and used for lanes is also subject to drying and significant wind erosion, which will need expensive rubbling to stabilise.
The first photograph below shows a seriously wind eroded drain bank 6 months after construction. The dashed line is the original position of a 4m wide, 150m long ledge used by excavators to construct the drain. The arrows point to what remains of the ledge after being exposed to wind for just 6 months. Tonnes of sand have been blown into the adjacent farmer's paddocks, and into the drain where it almost blocked the flow of upstream effluent.
Serious wind erosion of a berm (land between spoil and drain) occurred less than a month after the drain was constructed, but was halted by stabilising blowing sands with cereal rye (a drought tolerant cereal used in the Upper South East for sand stabilisation), sown privately at a cost of about $500/km (less than 1% of the cost of constructing 1km of drain)!
Serious Wind Erosion After 6 Months.
Wind Blowing Sand Into Adjacent Paddock.
Areas of major erosion and blockages within the Northern Catchment drain network were identified using Google Earth. Other areas probably exist but are not so clearly visble. Locations are given in latitude and longitude (both degrees) separated by a space. To zoom into an area, either click on a two number group or copy and paste into a Google Earth Fly To window, and press enter.
Extended areas between the following points:
|-36.1629 139.7538 to -36.1432 139.7889|
|-36.1364 139.7944 to -36.1216 139.8138|
|-36.1176 139.8386 to -36.1117 139.8514|
|-36.0935 139.8570 to -36.0798 139.8642|
Mt Charles Drain:
Bunbury Drain, from junction with
Taunta Hut Drain at: