Notes · Environmental ScienceUK · A-Levels

The hydrosphere

This chapter examines the water in and on the Earth and how it is used and managed. It covers the hydrological cycle and the distribution of the world's water, the human demand that leads to unsustainable exploitation of rivers and aquifers, the treatment of water for drinking and the treatment of sewage, and the new sources and management strategies that can make water supply more sustainable, throughout weighing environmental impact against cost and reliability.

4 sections~14 min reading time3 competenciesLevel Foundation 1 · Standard 2 · Advanced 1

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Exam profile

AO1 · Describe the hydrological cycle, water demand, treatment processes and supply optionsAO2 · Apply treatment and supply concepts to unfamiliar catchments and interpret abstraction and quality dataAO3 · Analyse the causes of unsustainable exploitation and evaluate the sustainability of water-management strategies

Operators:describeexplainapplyanalyseevaluatecalculate

basic level

AS-Level expects you to describe the hydrological cycle, the causes of water shortage, and the main stages of water and sewage treatment.

higher level

The full A-Level requires quantitative reasoning about abstraction and recharge and the evaluation of the sustainability and trade-offs of supply and management options.

Depth

Text

Contents · 4 sections

The hydrosphere

The hydrological cycle and the distribution of water#

FoundationAQA 7447 3.2.2

The hydrological cycle

Fig. 1Water cycles between the oceans, atmosphere and land, driven by solar energy and gravity.

The hydrological cycle is the continuous movement of water between the oceans, atmosphere and land, driven by energy from the Sun and by gravity. Water evaporates from the oceans and other surfaces and transpires from plants, forming water vapour; the vapour rises, cools and condenses into clouds and falls as precipitation; and the water then returns to the sea by surface runoff in rivers, or infiltrates the ground to become groundwater that moves slowly back to the sea. The cycle has no beginning or end, and the same water is used over and over, so fresh water is a renewable resource, though only if it is used no faster than the cycle replenishes it.

Although water is abundant, very little of it is accessible fresh water. The overwhelming majority is salt water in the oceans; most of the remaining fresh water is locked up in ice caps and glaciers or lies deep underground, leaving only a tiny fraction in the rivers, lakes and shallow groundwater that people can readily use. This uneven distribution, in both quantity and location, is the root of water scarcity: the problem is rarely a global shortage of water but a local shortage of accessible fresh water where and when it is needed.

The cycle also determines how quickly a source is renewed. Water in the atmosphere and in rivers turns over in days or weeks, so these are rapidly renewable; water in deep aquifers may have taken thousands of years to accumulate and is renewed extremely slowly, so heavy abstraction from such an aquifer is effectively mining a non-renewable store. Distinguishing rapidly renewed surface water from slowly renewed fossil groundwater is essential to judging whether a particular use is sustainable.

Climate and the hydrological cycle together explain why some regions have plentiful water and others little, and why water availability varies through the year. Regions under rising air in the circulation belts are wet; regions under sinking air are arid; and many places depend on a seasonal wet period or on meltwater. Because human demand does not match this natural pattern, water must often be stored, moved or treated, which is the subject of the rest of the chapter.

The distribution of the Earth's water

Fig. 2The distribution of the Earth's water (illustrative proportions): only a tiny fraction is accessible fresh water.

Worked example

Judging sustainable abstraction

An aquifer is recharged at about 20 million cubic metres per year, but a city abstracts 32 million cubic metres per year from it. Determine whether this use is sustainable and what will happen to the water table.

Compare abstraction and recharge
Abstraction (32 million) exceeds recharge (20 million).
Find the deficit
The store loses $32 - 20 = 12$ million cubic metres per year.
Conclude
Because abstraction exceeds recharge, the aquifer is being depleted and the water table will fall; the use is unsustainable.

Result: Abstraction exceeds recharge by 12 million cubic metres per year, so the aquifer is depleted and the use is unsustainable.

Typical mistakes

Treating all groundwater as rapidly renewable; deep fossil aquifers are renewed extremely slowly.
Saying the world is running out of water; the problem is the distribution and accessibility of fresh water, not the total amount.

Active revision

Explain why abstracting water from a deep, ancient aquifer may be unsustainable even though rivers fed by the same climate are renewable.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Water demand and unsustainable exploitation#

StandardAQA 7447 3.2.2

Causes and effects of over-abstraction

Fig. 3Over-abstraction propagates to falling water tables, dried wetlands, saltwater intrusion and subsidence.

Human demand for water comes from three broad sectors. Domestic use covers drinking, washing, sanitation and gardens; agricultural use, chiefly irrigation, is by far the largest globally; and industrial use includes cooling, processing and cleaning. Demand has risen with population growth, higher living standards and the spread of irrigated agriculture, and it is often greatest where and when natural supply is least, for example irrigating crops in a dry summer, so demand and supply are frequently out of step.

When water is taken faster than the hydrological cycle replaces it, exploitation becomes unsustainable. Over-abstraction from rivers reduces their flow, harming aquatic life, concentrating pollutants and sometimes drying reaches completely; over-abstraction from aquifers lowers the water table, so wells must be deepened, springs and wetlands dry up, and the land may subside. These effects show that water resources are finite in practice and that the cost of over-use falls on ecosystems and on future users.

In coastal aquifers, excessive abstraction causes a particular problem: as fresh groundwater is removed, salt water is drawn in from the sea to replace it, a process called saltwater intrusion, which contaminates the supply and can ruin an aquifer for drinking and irrigation for a very long time. Similar damage follows from the salinisation of irrigated land, where evaporation concentrates salts in the soil, an issue revisited in the chapters on soils and agriculture. These are examples of how solving one water problem can create another.

The consequences of unsustainable exploitation are environmental, economic and social. Ecosystems dependent on rivers and wetlands are degraded; the cost of obtaining water rises as sources deepen or must be treated or transported; and competition for scarce water can cause conflict between users, regions and even countries sharing a river. Recognising that water is a shared and limited resource, and that its over-use has these knock-on effects, sets up the case for the treatment, new sources and demand management considered next.

Worked example

Explaining saltwater intrusion

A coastal town increases its abstraction of groundwater and the water from its wells gradually becomes salty. Explain why.

Effect on the fresh groundwater
Increased abstraction removes fresh groundwater faster than it is recharged, lowering the fresh-water level in the aquifer.
Movement of salt water
The pressure of the fresh water no longer holds back the seawater, so salt water is drawn inland into the aquifer to replace the fresh water removed.
Consequence
The wells now draw brackish water; because it takes a very long time for the salt to be flushed out, the aquifer is effectively spoilt for drinking.

Result: Over-abstraction let salt water intrude into the aquifer, contaminating the supply for a long time.

Typical mistakes

Assuming agriculture is a minor user; irrigation is usually the largest single use of water globally.
Overlooking saltwater intrusion as a specific and long-lasting consequence of coastal over-abstraction.

Active revision

Explain why over-abstraction of groundwater near the coast can make an aquifer unusable for drinking water.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Treating water: potable supply and sewage treatment#

StandardAQA 7447 3.2.2

A sewage-treatment works

Fig. 4Sewage passes through screening, primary settlement, biological (activated-sludge) treatment and final settlement.

Water for drinking must be made potable, meaning safe and pleasant to drink. Treatment for a public supply typically screens out large debris, then allows fine particles to settle, often after adding a coagulant that clumps them together, before passing the water through filters (sand or membrane) to remove the remaining particles and many microbes, and finally disinfecting it, usually with chlorine, to kill the remaining pathogens. The order reflects a logic of removing progressively finer material and then sterilising, and each stage targets a specific kind of contaminant.

Sewage (wastewater) must be treated before it is returned to the environment, because its high content of organic matter and nutrients would otherwise cause the oxygen depletion and eutrophication met in the previous chapter. Treatment proceeds in stages. Preliminary treatment screens out large solids and grit; primary treatment lets suspended solids settle out as sludge in settlement tanks; secondary (biological) treatment uses microorganisms to break down the dissolved and fine organic matter, either in aeration tanks where air is bubbled through an activated sludge of microbes, or over trickling filter beds; and a final settlement removes the microbial floc.

The heart of sewage treatment is the biological stage, where aerobic microorganisms respire the organic pollutants, greatly reducing the biochemical oxygen demand of the effluent so that it can be discharged without stripping oxygen from the receiving river. Where necessary, tertiary treatment removes remaining nutrients (nitrate and phosphate) and pathogens, for example by further biological or chemical treatment, to protect sensitive waters from eutrophication. The sludge removed at each settlement stage is itself treated, often by anaerobic digestion, which produces methane that can be burnt for energy and a residue that can be used on land.

Treatment is effective but has costs in energy, chemicals and infrastructure, and it is only as good as its weakest stage: a failure of disinfection risks disease, and inadequate secondary treatment risks oxygen depletion downstream. Being able to describe the purpose of each stage, and to reason about what would happen if a stage were missed or overloaded, is exactly the applied understanding the examination looks for, and it links water management back to pollution and the nitrogen cycle.

\text{percentage reduction} = \frac{\text{initial} - \text{final}}{\text{initial}} \times 100

Percentage reduction

Used to express, for example, how much the biochemical oxygen demand of sewage is reduced by treatment; a higher percentage means more effective treatment.

Worked example

Calculating a reduction in BOD

Raw sewage has a biochemical oxygen demand of 300 mg per litre; after full treatment the effluent has a BOD of 15 mg per litre. Calculate the percentage reduction in BOD.

Find the reduction
Reduction = $300 - 15 = 285$ mg per litre.
Express as a percentage of the initial
$\dfrac{285}{300} \times 100$ .
$\frac{285}{300} \times 100 = 95\%$
Interpret
A 95% reduction shows the treatment, especially the biological stage, has removed most of the oxygen-demanding organic matter, so the effluent is much safer to discharge.

Result: The treatment reduces the BOD by 95%, making the effluent much safer for the river.

Typical mistakes

Confusing potable-water treatment (making water safe to drink) with sewage treatment (cleaning wastewater before discharge).
Forgetting that the secondary stage is biological and reduces the biochemical oxygen demand, which is the key to protecting the river.

Active revision

Explain why the secondary (biological) stage of sewage treatment is essential if the effluent is to be discharged into a small river.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

New sources and sustainable water management#

AdvancedAQA 7447 3.2.2

Desalination by reverse osmosis

Fig. 5Reverse osmosis forces water through a membrane that holds back the salt, giving fresh water and a brine waste.

Where existing sources cannot meet demand sustainably, new sources can be developed. Reservoirs behind dams store water from wet periods for use in dry ones and can generate hydroelectricity, but they flood land and habitats, trap sediment, and change the flow and ecology of the river downstream. Water can be transferred by pipeline or canal from a wetter region to a drier one, but transfer schemes are expensive and can damage the donor catchment. Rainwater harvesting captures water locally for reuse. Each option has a different balance of yield, cost and environmental impact.

Desalination removes salt from seawater or brackish water to produce fresh water, most commonly by reverse osmosis, in which water is forced under high pressure through a semi-permeable membrane that holds back the salt, leaving fresh water on one side and concentrated brine on the other. Desalination provides a reliable supply independent of rainfall, which is invaluable in arid coastal regions, but it uses a great deal of energy (and so, unless powered by low-carbon energy, adds to carbon emissions) and produces hot, salty brine that must be disposed of without harming coastal ecosystems. It is a technical solution whose sustainability depends on how it is powered and how the brine is managed.

Just as important as new supply is demand management: reducing the water that is needed in the first place. This includes reducing losses from leaking pipes, using water-efficient appliances and irrigation methods such as drip irrigation that lose less to evaporation, recycling water and reusing greywater (lightly used water from washing) for purposes that do not need drinking-quality water, and using pricing and metering to encourage careful use. Demand management is often the cheapest and least damaging way to close a supply gap, because saving water avoids the cost and impact of developing and treating new sources.

Sustainable water management combines these approaches according to local conditions, and its evaluation weighs reliability, cost, energy use and environmental impact against one another. A good answer recognises that there is rarely a single best solution: a desalination plant may secure supply but at high energy cost, a reservoir may store water but flood a valley, and demand management may be cheap but insufficient alone. The most sustainable strategy usually reduces demand first and then meets the remaining need with the least damaging combination of sources, an argument that anticipates the sustainability chapter.

Worked example

Evaluating a supply option

A dry region can either build a desalination plant or a large reservoir. Compare the two options and recommend an approach.

Desalination
Provides a reliable supply independent of rainfall but uses much energy and produces brine, so its carbon and coastal impacts depend on its power source and brine disposal.
Reservoir
Stores rainfall cheaply and can generate hydroelectricity but floods land and habitats and alters the river downstream, and its yield depends on rainfall.
Recommend
Neither is impact-free; the region should first reduce demand through efficiency and reuse, then meet the residual need with whichever option has the lower local impact, ideally a low-carbon-powered desalination plant if rainfall is unreliable.

Result: Both options have significant impacts; the sustainable approach reduces demand first, then chooses the least damaging source for the remaining need.

Typical mistakes

Presenting desalination as impact-free; it is energy-intensive and produces brine that must be disposed of carefully.
Ignoring demand management and assuming shortages can only be solved by building new supply.

Active revision

An arid coastal city is considering a desalination plant. Evaluate this option, considering reliability, energy use and environmental impact, and suggest one demand-management measure it should also adopt.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

References & sources

Sources

AQA

AQA AS and A-level Environmental Science (7447) specification

Department for Education

GCE AS and A level subject content

The hydrosphere

4 sections~14 min reading time3 competenciesLevel Foundation 1 · Standard 2 · Advanced 1

The hydrosphere

Key points

Judging sustainable abstraction

Key points

Explaining saltwater intrusion

Key points

Calculating a reduction in BOD

Key points

Evaluating a supply option

Sources

The hydrosphere

Key points

Judging sustainable abstraction

Key points

Explaining saltwater intrusion

Key points

Calculating a reduction in BOD

Key points

Evaluating a supply option

Sources