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Learning how groundwater is an important ingredient in the maintenance of Australian environments involves understanding biotic and abiotic features of groundwater systems; ground water dependent ecosystems ; and the impact of human-induced change on surface and groundwater ecosystems including thermal pollution, salinity; excessive evaporation and the effect of nutrients (such as phosphorous and nitrogen) on algal growth.
Biotic and abiotic features of a ground water system
Biotic features of any system are the living features of that system. Living features include plants and animals. In an environmental system, such as a local ground water system, biotic features are the plants and animals that depend directly, or indirectly, on ground water for their survival.
Current research indicates that there is significant microbiologically activity (in situ) in aquifers and aquitards. These organisms are therefore directly dependent on the ground water for their survival. Relevant application of this research is in ground water rehabilitation whereby particular microbial species can be encouraged to consume contaminant chemicals - this is called bioremediation (Bioremediation).
An 'oasis' (definition: fertile place in the desert) is an example of a ground water environment that supports life. Oases normally occur as areas of ground water discharge. Other examples of ground water environments that support life include 'ground water springs' where the same principles behind their formation apply. 'Ground water springs' are also sometimes noted for their particular colour or iridescence (strong blues or greens) - which is normally due to the hydrogeochemical properties along the ground water flow path.
The 'oasis' directly provides an environment for plants and animals. It is also noted that oasis's occur in some of the driest areas on Earth. As such these bodies of water are subjected to very high levels of evaporation. It is noted that although evaporation is very high, these are fresh water bodies and not saline (think Lawrence of Arabia in the desert comes across an oasis he is drinking fresh water not salt water). Implications of this are discussed below.
Other plants and animals are indirectly dependent of ground water systems, these ecosystems are now being referred to as ground water dependent ecosystems.
Ground Water Dependent Ecosystems
The inter-dependence of surface water and ground water systems is increasingly being recognised. Consider water flowing in a normal stream, creek or river. Water flowing in this creek, called stream flow, can be subdivided into flow contributed by surface runoff and flow contributed by ground water discharge through the bed of the creek. The contribution to stream flow of ground water discharge through the bed of the creek is referred to as base flow. These types of creeks are called gaining streams or effluent streams. An example of an effluent stream/creek is found where a river continues to flow through a desert where there is no obvious sign of surface runoff and that 'base flow' constitutes 100% of stream flow.
In a similar manner, a stream, creek or river can be an influent stream to the ground water system such that a portion of stream flow is lost as recharge to the ground water system. These types of creeks are also called losing streams. An example of an influent stream is the Murrumbidgee River. Here, the ground water system is 'recharged' by a portion of stream flow.
The zone of interaction between ground water and surface stream water is called the hyporheic zone. This zone lies beneath a streambed, and it fluctuates seasonally. It is an important zone, as upwelling of groundwater supplies stream organisms with nutrients, whilst down-welling stream water provides dissolved oxygen and organic matter to microbes and invertebrates in the hyporheic zone.
Effluent streams are by far the most common and 40 to 50% of total stream flow in Australia is derived from base flow (ground water discharge into the bed of the creek or river). As such, in many Australian environments, surface water flow is dependent on ground water flow. Therefore, surface water management requires ground water management. As you can see ground water plays a very important role in the stream flow of Australia's rivers and creeks.
Now, ground water dependent ecosystems (GDE's) can be considered are those which rely on contribution of 'base flow' to stream flow in a particular river or creek. A good example of a ground water dependent ecosystem are wetlands and marshes. Wetlands and marshes downstream of an irrigation area do not initially appear to be affected by ground water issues. However, as you have learned above, ground water discharge (base flow) can attribute a significant proportion of stream flow. It is also important to realise that the riparian requirement (conditions, water specific, required by wetland or marsh etc for survival) may not be exclusively quantitative but may also depend on frequency of flow. e.g. Snowy River, NSW. A continual trickle of water down a river system may be quantitatively similar to the conditions of the river before regulation but is not representative of the frequency distribution of that flow. i.e. it is necessary for the health of the river to be be sporadically in flood. These issues are now being recognised and included in management strategies with respect to surface water regulation. It is noted that regulation means damming and other flow control devices such as weirs.
Ground water dependent ecosystems include wetlands, hanging swamps, mangroves, billabongs and springs. These environments are discussed in the Nature Conservation Council of NSW - Groundwater Dependent Ecosystems. Previous policy decisions regarding ground water allocation have neglected the importance of ground water dependant ecosystems. Any subsequent reduction in the water supply to such ecosystems can significantly reduce fauna and flora abundance.
This highlights an important issue. Ground water systems have been traditionally considered infinite resources chiefly because of an 'out of sight, out of mind' approach to resource management. Ground water systems are in, what is called, dynamic equilibrium. Input to the ground water system as recharge (rainfall, infiltration etc) is balanced by output of the ground water system as discharge (base flow to streams etc). As well, due to the residence time of ground water, paleorecharge issues are also important (as discussed in Section 8.4.1). Thus, the system can be balanced but it is not static.
When you alter the system equilibrium by abstracting ground water then, although the system may not adjust instanteously, there will, logically, be less ground water available for discharge. i.e. less discharge as base flow to streams.
Groundwater abstraction scheme will, inevitably, reduce discharge from the ground water system. If there are ecosystems dependent on this 'base flow' or ground water discharge then they will obviously be affected.
Groundwater abstraction occurs primarily for irrigation purposes - either for stock or intensive agriculture - also for town domestic supply. As such, ground water that is abstracted is typically re-applied to the ground surface as irrigation water. Apart from the enhanced losses due to evaporation and evapotranspiration of crops, the system would appear to be somewhat re-balanced by an additional input component deemed infiltration. Although there is an additional degree of infiltration due to leakage from the ground surface into the ground water system, the combination of evapotranspiration and export of produce, on the whole, is a net loss of water to the ground water - surface water system.
Increasingly this concept is being recognised in the determination of safe yield. Safe yield is loosely defined as 70% (relative) or so of the defined recharge (rainfall or effluent stream etc) to the ground water system. Due to the difficult nature of determining sustainable aquifer yields in the past there are now currently many instances of a need to reduce ground water allocations which will prove very controversial. Again a concept such as 'safe yield' is/was only an approximate of the riparian requirements of GDEs. The next step, recently, has been to quantify this 'riparian requirement' more exactly and then reduce the available recharge accordingly, prior to allocation of water available for abstraction. The results of this type of modelling increasingly dispels the 'infinite resource' assumption in that it becomes obvious that the system was initially already in equilibrium.
Solubility of oxygen and carbon dioxide in water and temperature dependence
The amount of dissolved oxygen, O2 in water is an important indicator water quality. Water fully saturated with air at 1 atm (standard pressure 101.3 kPa) and 20oC contains about 9 ppm (~9 mg/L) of O2. Oxygen is necessary for fish and other aquatic life for survival. i.e. cold-water fish require 5 ppm (~5 mg/L) for survival. Note that ppm (mg/kg) = mg/L if density is 1 kg/L.
Aerobic bacteria consume dissolved oxygen in order to oxidise organic materials and so meet their energy requirements. This organic material that bacteria are able to oxidise is said to be biodegradable. Overall reaction is as follows:
where CH2O, a carbohydrate, is used as a simplification for organic material. Decay of organic matter is an oxidation reaction which may occur in soil, water and also within aquifers where fossil organic matter can be present as peat, lignite etc. The process uses oxygen, or other electron acceptors, and produces carbonic acid (CO2(aq)).
As such, the solubility of O2 and CO2 is very important in surface water systems.
In contrast to solid solutes, the solubility of gases in water decreases with increasing temperature. e.g. carbonated beverages go 'flat' as they are allowed to warm; as the temperature of solution increases, the solubility of CO2 decreases, and CO2 (g) escapes from solution. The decreased solubility of O2 in water as temperature increases is one of the effects of thermal pollution of surface water bodies. The effect is particularly serious in deep lakes because warm water is less dense than cold water. Thus, cold water tends to remain on top, near the surface. This situation impedes the dissolving of oxygen into the deeper layers, thus stifling respiration of aquatic life requiring oxygen.
The solubility of oxygen at 1 atm pressure and 20oC (standard temperature and pressure) is ~1.4 mmol/L (mM).
range of salinities that can be tolerated by common aquatic and terrestrial organisms
The quality of water (both surface water and ground water) can be very important, especially if water composition is high in salts. There are many different types of salts including chlorides (e.g. NaCl - halite) and sulphates (e.g. FeS2 - pyrite , CaSO2.2H2O - gypsum).
The quality of water can significantly affect the available uses of that water due to the limitation in tolerance to salt by common aquatic and terrestrial organisms. If water is to be used for purposes such as drinking or irrigation, it is necessary to know the concentration of salts (and often the types of salts) which are dissolved.
The salt content of a source of water may be indicated in several ways:
1. Electrical conductivity
Field measurement using an EC meter, gives an approximation of total salt content. It is usually expressed is either uS/cm (EC units) or mS/cm.
where uS/cm is microSiemens per centimetre (EC units).
There is a reasonably well defined relationship between total dissolved salt (TDS, mg/L) and electrical conductivity as recorded by an electrical conductivity meter (EC, uS/cm):
where A is correlation factor ranging between 0.55 and 0.8 for ground waters (somewhat site specific); higher values can be associated with water high in sulphate concentration and lower values can be associated with water high in bicarbonate concentration.
2. Total Dissolved Solids (TDS)
commonly calculated from major ion concentrations (major ions in water chemistry are Na+, K+, Ca2+, Mg2+, HCO3-, SO42-, Cl- and sometimes NO4-) which is indicative of total salinity.
TDS is expressed in units of mg/L. TDS may also be determined gravimetrically by weighing the mass of salts left by evaporating a known volume of water to dryness. Total dissolved solids may comprise sodium, potassium, calcium, magnesium, chloride, sulfate, bicarbonate, carbonate, silica, organic matter, fluoride, iron, manganese, nitrate and phosphate.
Specific salt measures that reflect the hazard of specific ions to soil and plant health are sodium adsorption ratio (SAR); exchangeable sodium ration (ESR) and magnesium hazard (MH)
By far the most common measure is TDS (mg/L) where the advised (aesthetic) limit for total dissolved salts is 500 mg/L. The NHMRC/ARMCANZ Australian Drinking Water Guidelines (ADWG) provides the Australian community and the water supply industry with guidance on what constitutes good quality drinking water (Australian and New Zealand Drinking Water Guidelines). These guidelines are specifically tailored to Australia and New Zealand and are, mostly, in line with similar World Health Organisation (WHO) guidelines for water quality.
Potential impact of excessive water evaporation from terrestrial and inland aquatic habitats
Salinisation is often described as being primary or secondary. The term primary salinisation is restricted to naturally occurring salinity and is used to refer to saline lakes, playas and the effects of dryland salinity. The term secondary salinity has been proposed to refer to salinity caused by human interference in the hydrological cycle.
It is difficult to agree with the classification of 'secondary salinity' in that in many cases, human activity has only accelerated or re-activated a naturally occurring process. A more useful classification would be simply natural and irrigation salinity.
Natural salinity occurs associated with saline lakes and playas and in the erosion of saline soils. Saline lakes and plays occur wherever the long term input of salt exceeds the long term output of salt (internally draining basin). e.g. Lake Eyre, The Dead Sea. The development of salinity in a closed basin may be interrupted or partially reversed by changes to the hydrological budget. i.e. Lake George. Lake George periodically contains fresh water overlying saline deposits at depth.
Playas are shallow saline lakes where salt is precipitated. They can occur in internally draining basins (closed basins) or along coastal area where ground water discharges. Playa conditions exist widely throughout western NSW and northern Victoria where ground water discharges to the surface and is concentrated by evaporation. The ground water at depth is not saline but the continuous slow discharge of water in an arid environment eventually builds up enough salt that direct precipitation of salt occurs.
Irrigation salinity occurs where irrigation water is applied without the possibility of drainage. It is exacerbated in areas where there are already salt concentrations in soil, possibly from previous arid phases, but seems to occur almost unavoidably if drainage systems are not in place.
Sources of Salt
Salinity is measured as concentration of salt. All waters contain some salt. Rainfall is no exception, in that it naturally carries small concentrations of salt derived primarily from oceanic sources (sea spray etc.).
Rain water is a considered a primary source of salt with concentrations typically between 10 mg/L to 30 mg/L. For an area where rainfall is 500 mm per year this equates to 150 kg of salt per hectare.
There is no problem however if there is sufficient excess rainfall over evaporation that this salt is carried to the ocean by run-off or percolation to ground water and discharged as base flow. If this salt is not flushed from the soil by leaching but is accumulated in some manner then there can be a problem.
Areas adjacent to the coast, given predominantly on-shore winds, receive substantial quantities of salt in rainfall. This is considered the primary cause of much of the salinity found in south-west Western Australia.
Salt from Rock Weathering
The weathering of rock will release cations associated with the silicate struction of the particular mineral. This can provide a steady source of salt to a ground water system, however can not be the source of large quantities of salt.
The geochemical process involved in weathering relies on an efficient mechanism for removal of weathering products away from the reaction site. If significant weathering is occurring then this must be associated with a continual flux of clean water from recharge. If a flux of water is maintained then the salt can not be concentrated.
An exception to this general rule will be if rocks contain connate salt. Some shales, which are deposited under marine conditions, still contain significant quantities of salt. Ground water in shale country is often of poor quality. Connate salt can also be described as relic salt.
Aeolian Deposits of Dust and Salt
There has always been significant quantities of dust (loess) transported around the world. e.g. dust storm in Melbourne but in the 80's. These processes continue to the present day.
Where winds blow over dry salt lake beds, considerable salt will become entrained in the dust. In Australia, the salt an dust has been blown east from the Murray-Darling Basin and re-deposited on the Tablelands. Subsequent erosion of these deposits can therefore provide significant salt loads.
Irrigation water adds to the salt load in the soil in proportion to the soluble salt content of the particular water and also the quantity of water applied. Even in the case of excellent quality irrigation water containing 250 mg/L of soluble slat (TDS), the amount of added per hectare per year can be substantial. Given this salt concentration and a crop that requires between 6000 m3 and 10000 m3 of water per year, one hectare will receive between 1.5 and 2.5 tonnes of salt.
From this simple calculation we can derive some important findings. If there is insufficient or inadequate drainage from this irrigation system, then this quantity of salt will accumulate within the soil profile each year. If there is no leakage from the system then a finiteness has been introduced. Over time productivity (in terms of crop yield) will continue to fall. Therefore ought there be as much attention given to the development of drainage for these systems as there is for irrigation?
Role of nutrients (phosphorous and nitrogen) in algal growth
Other ground water quality parameters include contribution of nutrients (chiefly Phosphorous, PO43- and Nitrogen N-NO3) to ground water quality as well as dissolved oxygen content (DO, mg/L).
Dissolved oxygen content and nutrient concentration within ground water aquifers (whether by naturally occurring processes or contamination) plays an important role in hydrogeochemistry of ground water and therefore significantly affects ground water quality. However, it is far more important to understand that ground water-surface water interaction and it is through this interaction that problems can develop. Contamination of ground water through the excessive use of fertilizers (especially phosphorous- and nitrogen-based fertilizers) as well as from, now augmented, non-biodegradable domestic washing powders and liquids contributes to contamination of surface water sources through ground water-surface water interaction.
In most cases, however, this runoff from agricultural and urbanised land contribute to Nitrogen and Phosphorous concentrations in downstream creeks and waterways. In low flow conditions these higher concentrations of nutrients can contribute to triggering of excessive algal growth (termed algal bloom). Some of these 'algal blooms' can be a serious problem with respect to water supply (e.g. water reservoir supplies for major towns and cities) resulting in significantly reduced dissolved oxygen concentrations (changing surface water from aerobic to anaerobic with resultant change in water chemistry).
Eutrophication is the naturally evolution of stagnant water bodies into marshes and wetlands which, in turn, evolve into plains (peats etc.). It is recommended further reading is undertaken with respect to these surface water issues.