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Selwyn Johnston

INDEPENDENT COMMUNITY REPRESENTATIVES'

ADVOCATE

(Cairns... Far North Queensland)

 

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I appreciate your interest in the issues that effect not only Queenslanders, but all Australians. 

Please let me hear from you about your views on the issues that matter to your Family, your Community and your State.  

Sincerely,

Selwyn Johnston

 

One person, with the support of the community, can make a difference

 

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WATER - AUSTRALIA

AQUIFER RECHARGE

 

        INDEX

 

INTRODUCTION

AQUIFER RECHARGE

ATTRIBUTES OF GROUNDWATER

EQUUS BEDS GROUND-WATER RECHARGE PROJECT

GROUND WATER RECHARGE

     POTENTIAL SOLUTION

RECHARGED GROUND WATER BORES

WATER - AUSTRALIA'S FUTURE

 

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INTRODUCTION

Beneath the surface of the land lies a tremendous resource that many of us depend on for our very existence, yet often take for granted. This precious resource is ground water.

Ground water aquifers provide water for almost half of the state's population and about 90 percent of its rural residents. It is also an important source of water for community supplies, industrial needs and agricultural uses.

Far North Queensland (FNQ) has an abundant supply of ground water in a complex system of under-ground aquifers. Unlike some parts of the country, which receive very little precipitation, an abundance of annual monsoonal rainfall is constantly replenishing FNQs ground water. 

Although some areas of the state have experienced problems with quantity and quality of ground water, these problems have not yet proven severe. However, it is inevitable that future growth will continue to place increasing demands on this precious resource. It is critical to the future of the state that we strive to better understand the nature of our ground water resources, to help to ensure that our activities don't irreparably damage our supplies. 

What Is Ground Water? 

FNQ has a relatively abundant supply of both surface water and ground water. Fresh surface water includes the water in our rivers, creeks, streams and lakes. These sources make up the aboveground portion of our total fresh water supply. The part that lies below the earth's surface in saturated layers of sand, gravel or sedimentary rock, or in fractures in crystalline rock is called ground water. 

People tend to understand surface water much better than they do ground water. We can see surface water. We swim in it and fish in it. We can see that water levels decline during dry weather and rise when rainfall is plentiful. We can also see the effects of man-made pollution almost immediately. 

On the other hand, ground water is hidden. It is deep in the ground and is shrouded in many misconceptions and myths. 

For instance, some people believe that ground water originates in some mystical, pristine place far removed from man's influence. The fact is, almost all ground water found in FNQ originates within the state's boundaries, and many bores withdraw water which originates within a few hundred feet of the bore. 

Many people also believe that groundwater occurs in vast underground rivers or lakes. But with the exception of underground caverns and solution channels in some limestone aquifers, ground water almost always occurs in small pore spaces in layers of saturated sand, gravel or sedimentary rock or in cracks and fissures in crystalline rock. 

The Water Cycle groundwater makes up part of the earth's water cycle or hydrologic cycle, which is the continuous circulation of moisture and water on our planet. This cycle is in constant operation, moving water from the earth to the atmosphere by evaporation and back again to the earth's surface as precipitation, to produce stream flow and ground water flow. 

Of the water that falls to the earth's surface in the form of rainfall, some runs off the surface, some evaporates back to the atmosphere and some infiltrates into the ground. Part of the water that moves into the ground is taken up by plant roots and re-enters the atmosphere through transpiration. The rest percolates deeper into the earth and becomes ground water. This process is called recharge. 

Aquifers 

The word aquifer comes from the Latin words aqua, meaning water, and ferre, meaning to bear or carry. Thus an aquifer is a water-bearing geologic formation that can yield usable amounts of water. An aquifer may be a layer of gravel or sand, a layer of sandstone or limestone, or even a body of massive rock, such as granite, which has sizeable cracks and fissures. 

An aquifer may be anywhere from a few feet to several hundred feet thick. It may lie just below the earth's surface or hundreds or even thousands of feet down. 

Aquifer materials may be classified as consolidated or unconsolidated rock. Consolidated rock (often called bedrock) may consist of sandstone, limestone, granite or other rock. Unconsolidated rock consists of granular material such as sand, gravel and clay. 

The quantity of water a rock can contain depends on the rock's porosity, the total amount of spaces among the grains or in cracks that can fill with water. If water is to move through rock, the pores must be connected to one another. If the rock has a great many connected pore spaces big enough that water can move freely through them, it is permeable. 

Aquifers consisting of sand or gravel contain relatively large interconnected spaces between particles and will generally yield sizeable quantities of water. On the other hand, clay may contain a considerable amount of water and yet the pore spaces are so small that water cannot move freely between them. Therefore, clay layers tend to impede water movement and are not productive aquifers. Some of the most productive aquifers in FNQ consist of sedimentary rocks such as limestone, dolomite and sandstone. These typically contain many solution channels and interconnected pores, which hold water and allow it to move easily. 

Crystalline rock, such as granite, contains very little pore space and has very low permeability. However, nearly all consolidated rock formations of this type are broken by cracks, fractures or faults, which may enlarge over time. These cracks tend to hold water and, when intercepted by a bore, will often yield usable quantities of water. 

In many areas there may be multiple aquifers stacked on top of one another. These distinct layers of water-bearing material are often separated by impermeable layers of clay or rock, which prevent water from moving readily from one aquifer to another. These impermeable layers are called confining layers or confining beds.

An aquifer, which does not have a confining layer above it, is said to be unconfined. The upper surface of the saturated zone in such an aquifer is referred to as the water table. These aquifers occur in almost all areas of the state and are commonly called water table aquifers.

In water table aquifers, water may move readily from surface sources such as streams and rivers to ground water and vice-versa. The water level in these aquifers fluctuates readily with changes in weather patterns. An aquifer lying beneath a confining layer is commonly called a confined or artesian aquifer. As the water flows beneath the confining layer, the impermeable layer above it essentially traps it. 

Consequently, the water in the aquifer may be confined under pressure. When a bore is drilled into such an aquifer, this artesian pressure will cause the water level in the bore to rise above the point where the bore intercepted the aquifer. The level to which water will rise into tightly cased bores from artesian aquifers is called the potentiometric surface. 

If a bore is drilled in a low-lying area where the surface of the ground is lower than the potentiometric surface, water will flow from the bore under its own pressure. Such a bore is known as a flowing artesian bore. 

Since artesian aquifers are overlain by confining layers, recharge to the aquifer can only occur in places where the confining layer leaks, is absent, or where the aquifer is exposed at the ground surface. These areas are known as outcrop areas or recharge areas. 

Ground water is always moving by the force of gravity from recharge areas to discharge areas. Contrary to popular belief, ground water movement is generally very slow, typically only a few feet per year. However, in more permeable zones, such as solution channels in limestone or fractures in crystalline rock, it may move as fast as several feet per day. 

The force of gravity moves water toward areas of lower elevation. Ground water, particularly from the water table aquifers, typically discharges into streams, lakes and wetlands. Where the water table intercepts the ground surface, water can discharge, forming a spring. 

Because of differing geologic features and landforms in varying parts of FNQ, there are substantial differences in ground water conditions from one area to another. These features affect ground water quantity and quality. 

Water table aquifers are present in each of the physiographic areas. They are usually unconfined and are used for domestic and livestock supplies in most areas. Shallow bores tapping the water table aquifer are especially prevalent in rural areas where they are often used for domestic supply and livestock watering. 

Crystalline rock aquifers are used primarily for private water supplies and livestock watering. It is commonly believed that groundwater in this part of the state is not sufficient to supply such uses as community supplies and industry. Consequently, large water users in FNQ have relied primarily on surface water.

In recent years, however, systematic bore-siting techniques have produced high-yielding bores (greater than 500 litres/min.) on a regular basis. Because surface water sources have been pushed to their limits in some areas, several studies are now under way to evaluate whether the use of groundwater can be increased in this region, particularly for community supplies. 

Because of the increased use of ground water over the past few decades, there is increasing concern about declining ground water levels and whether water is being removed faster than it is being recharged. 

Several factors cause ground water levels to fluctuate. These levels naturally rise and fall because of seasonal patterns of ground water recharge and storage. 

In FNQ, ground water levels tend to be highest in the winter and lowest in summer. In late spring, summer and early autumn, evaporation and transpiration by plants use up most of the water that would otherwise recharge the aquifer. At the same time, the aquifer is discharging water into streams, springs and bores. 

A seasonal decline in ground water levels results. In the late summer, winter and early spring, most plants are dormant and evaporation rates are low. Consequently, rains during this time of year tend to saturate the soil, stream levels rise, and ground water recharge occurs, resulting in water level increases. 

Longer-term changes in ground water levels may occur because of climate and pumping changes. Less ground water recharge will occur during dry years than in wet years. Several years of below normal rainfall will typically result in a gradual decline in water levels. 

Ground water levels can also be affected by pumping from bores. When water is pumped from a bore, the water level in the bore is drawn down, forming a cone-shaped depression on the water surface. This cone of depression is maintained as long as the bore is pumping but is usually localized and does not affect other bores in the area. 

However, when several high-capacity bores are pumping in the same vicinity, the cones of depression may overlap and cause a general lowering of the water level in an area. When this happens during a time of dry weather, the water level may drop to the point that shallower bores in the area go dry and the water level drops below the pump inlet in others. When this happens, even though the situation is usually temporary, it creates a great deal of concern about the use and allocation of our ground water resources. 

FNQ's ground water is of good quality in most areas and is suitable for most uses. Concentrations of impurities in ground water generally do not exceed the EPA's maximum contaminant levels for drinking water. There is no evidence of any significant deterioration of public drinking water supplies. Where human-related contamination has been detected, the effect has generally been local and has not caused widespread contamination of any of the aquifer systems. 

All ground waters in FNQ contain naturally occurring minerals in varying concentrations. It is not unusual for ground water to contain some minerals in high enough concentrations to cause problems with staining of plumbing fixtures and laundry, scale formation or objectionable tastes and odours. 

Other water quality problems have been detected by various state agencies, but these have been relatively isolated and limited to small areas. 

Ground Water Protection 

Protecting ground water from the effects of man's activities should be a major priority in order to preserve this valuable resource for future generations. Ground water, as a rule, moves very slowly. Once contaminated, an aquifer is very difficult (if not impossible) to clean up. It may take decades or even generations for nature to cleanse a contaminated aquifer. 

Some potential sources of ground water contamination include: 

      Septic tanks 

      Solid waste landfills 

      Leaking underground storage tanks 

      Municipal and industrial wastes 

      Animal wastes 

      Agricultural fertilizers and pesticides 

Any of these contamination sources can pollute ground water if not managed properly, but all are of special concern in those areas identified as major ground water recharge areas. In the future, these ground water recharge areas may warrant special protection in order to preserve the quality of the FNQ ground water. 

Besides man's ability to create pollutants, his activities may also create situations, which make contamination of ground water more likely. For instance, over pumping from bores in coastal areas may cause salt-water encroachment. Over pumping may also cause sinkholes to form in some areas. These sinkholes may breach the confining layer above an aquifer and allow contaminants from the surface to enter the aquifer. 

Bores, if not properly constructed, may allow water from the surface to carry contaminants into the aquifer, or they may allow water from a shallow, contaminated aquifer to mix with water in a deeper aquifer. Old, abandoned bores and agricultural drainage bores, if not filled, may also serve as conduits to allow surface contaminants to enter the aquifer. A particular risk is incurred when these old bores are used as disposal sites for household garbage, pesticide containers or other waste products. 

Fortunately, at present there have not been any cases of widespread man-made contamination of any of the major aquifers in FNQ. Where contamination has been detected in bores it has typically been attributed to sources near the bore site, often immediately adjacent to the bore. 

FNQ's ground water is one of our most precious resources and every effort should be made to preserve the integrity of this important commodity for now as well as for future generation.

 

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AQUIFER RECHARGE

 

Precipitation, Runoff, Infiltration, Evaporation, Transpiration, Aquifer Recharge

What is an aquifer? 

An aquifer is a body of geologic material that can supply useful quantities of ground water to natural springs and water wells. 

What is aquifer recharge? 

Aquifer recharge is the process by which rainwater seeps down through the soil into an underlying aquifer. There are many natural processes that determine how much rainwater actually reaches and replenishes an aquifer instead of being evaporated, consumed by plants and animals, or simply running off the ground surface into streams, rivers, lakes, and oceans. 

Why is important to map aquifer recharge? 

The protection of our water resources is an important for everyone in the State concerned with the quality and availability of clean drinking water. Previous water conservation and supply programs have proven to be insufficient for adequately protecting our drinking-water supplies. About half of the water used by humans for daily living is extracted from the ground. Therefore it is important to identify those parts of the State where our ground water is most likely to be replenished so that we can attempt to protect these vital resources from pollution and any land-use practices that will decrease the quality and availability of clean water. 

How to map aquifer recharge areas? 

The method uses rainfall data from climate-monitoring stations, maps showing how the land surface is currently used (residential, agricultural, commercial, wooded, pavement, etc.), what kind of soils occur at the earth's surface, and the extent of wetlands (creeks, streams, rivers, lakes, marshes, and bogs). These data are combined using scientific methods to determine how much ground water is available in any particular area for recharge to the local aquifer. How much of this water will actually make it into the aquifer is also predicted based on how much water can usually be pumped from water wells drilled into the aquifer.

 

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ATTRIBUTES OF GROUNDWATER

There is more ground water than surface water

Is less expensive and economic resource

Is sustainable and reliable source of water supply

Is relatively less vulnerable to pollution

Is usually of high bacteriological purity

Is free of pathogenic organisms

Needs little treatment before use

Has no turbidity and colour

Has distinct health advantage as an alternative for lower sanitary quality surface water

Is usually universally available

Resource can be instantly developed and used

Has low vulnerability to drought

Is key to life in arid and semi-arid regions

Ground water is source of dry weather flow in rivers and streams

Artificial ground water recharge has long been recognized as a means of introducing water into the ground water system to store water, reduce pumping lifts, salvage storm-water runoff, or enhance ground water quality.

In effect, ground water aquifers (saturated rock or sediment that yield water in economic quantities to bores or springs) are used as water-storage facilities instead of constructing surface-water reservoirs.

Artificial ground water recharge can be accomplished by surface spreading or ponding of water in areas where surficial deposits are highly permeable, or by injection of surface water into an aquifer using bores. Interest in artificial ground water recharge has increased in recent years due to declining water levels in many aquifer systems around the world.

Aquifer storage and recovery projects involve the storage of water in an aquifer via artificial ground water recharge when water is available (usually during spring runoff), and recovery of the stored water from the aquifer when water is needed (usually late summer).

Although losses of water stored via artificial ground water recharge do occur, primarily by water moving vertically or laterally out of the target aquifer before recovery, the sometimes-significant losses of water through evaporation from surface-water storage facilities are largely avoided.

 

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A POTENTIAL SOLUTION

Aquifer storage and recovery within the Mulgrave Aquifer, either via land-surface infiltration or injection bores, offers a potential solution to the problems associated with the water-level decline in the City of Cairns area.

 

 

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Written and Authorised by Selwyn Johnston, Cairns FNQ 4870