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Water as a resource

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Water is the commonest substance on Earth; it exists on land, ... Meteoric water. fresh atmospheric water, which condenses in lakes and rivers and ground water. ... – PowerPoint PPT presentation

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Title: Water as a resource


1
Water
  • Water as a resource
  • Water is the most important resource studied in
    this course as it is the only one that is
    essential for human survival.
  • Water is the commonest substance on Earth it
    exists on land, in the atmosphere and in the
    oceans and lakes.
  • It occurs in different states solid (ice),
    liquid (water) and as gaseous water vapour.
  • Water may be pure, or it may contain dissolved
    substances, particularly salts oceans, lakes
    and ground water.
  • The term Water Resource usually applies to
    fresh water i.e. with low content of dissolved
    solids, suspended solids or biological
    substances.
  • Fresh water constitutes only a few of this
    total water. In some places it is very scarce.

2
Water
  • Renewable resource
  • Water differs from most other resources in that
    it is renewable driven in the Hydrological Cycle
    by the energy of the sun and by gravity.

3
Water
  • Renewable resource
  • Rainfall globally is irregular or seasonal so its
    renewability is not instantaneous. Rain on
    mountain tops in remote areas may not be
    available for human consumption for days or
    years.
  • Extremes causes drought and desertification or
    flooding.
  • Modern Water Usage
  • The typical amount of water used at subsistence
    levels in developing countries is 20-40 litres
    per person per day, while in industrialised
    nations, it is 500 l/p/d.
  • Agriculture - 69 of world water usage. Industry
    - 23.
  • Water is used to produce energy energy is
    needed to produce water. Given enough energy,
    fresh water resources can be created from
    seawater. 300 Mjoules of energy will produce 1
    cubic metre of fresh water.

4
Water
  • Using water in different ways
  • Water is an important resource for Transport on
    seas, rivers, lakes and canals. Of the 3,200km
    of canals in Britain, very few are now used
    commercially, e.g. coal to power stations.
  • Abstractive
  • Water is temporarily lost as a resource.
    Domestic, irrigation, industrial, cooling (1
    lost to evaporation).
  • Non-Abstractive
  • Water is used without changing its path or
    quality. Hydroelectric, transport, recreation.

5
Water
  • Science of Water
  • Water has some peculiar but essential properties.
  • Hydrogen bonding
  • Hydrogen bonding is similar to methane, but water
    freezes at 0oC and boils at 100oC while methane
    freezes at -184oC and boils at -162oC. The
    difference is in the bonding (polar), which is
    much stronger on water
  • Energy properties
  • The energy stored, as water changes phase from
    ice to water to steam, is a major factor in
    distributing the energy from the Sun
  • Water has a high heat capacity (ability to store
    heat) due to the large number of hydrogen bonds,
    so it heats and cools slower than many
    substances. Consequently, large water bodies
    affect local climate and weather.

6
Water
  • Science of Water
  • Density properties
  • A VITAL property of water is that ice floats!
  • If this property were not present, ice would sink
    to the bottom of lakes, rivers and seas, and
    eventually the whole body of water would freeze,
    removing oxygen and killing life. It also allows
    vertical mixing of oxygen and nutrients.
  • Solvent properties
  • Water is a very good solvent. Polar liquids
    dissolve polar solids like NaCl and mixes
    completely with polar liquids like alcohol. Its
    ideal for transporting nutrients through the
    organic tissues.
  • Conversely, because it is a good solvent it is
    easily polluted.
  • Water will NOT mix with non-polar liquids like
    oil or fat.

7
Water
  • The Water Cycle
  • Water moves on, over and through the earth in a
    continuous cycle driven by the sun and gravity.
  • This water cycle (Hydrological Cycle) involves
    water as a liquid, solid and gas, which move in
    different ways around the cycle. The total water
    in the cycle remains virtually constant.
  • Meteoric water
  • fresh atmospheric water, which condenses in lakes
    and rivers and ground water .
  • Saline water
  • seawater in oceans, seas and some lakes.
  • Small amounts of magmatic waters (volcanic) and
    formation waters (held in sediments) can be added
    - both are usually saline.

8
Water
  • The Water Cycle cont..
  • Storage in the Hydrosphere
  • The hydrosphere includes those parts of the earth
    that are mainly water, e.g. oceans. Ice caps,
    lakes, rivers are all temporary natural
    reservoirs in the water cycle.
  • Atmosphere contains 15 x 103km3 of water
    (reservoir)
  • The worlds lakes contain large volumes but ½ are
    saline 80 of the freshwater occurs in only 40
    large lakes (Great Lakes).

9
Water
  • The Water Cycle cont..
  • Precipitation
  • The atmosphere is one of the smallest reservoirs,
    but is the most important source of water.
    Rising air and water vapour expands (decrease in
    pressure) and cools _at_ 1oC per 100m in altitude.
  • As air cools it condenses to form droplets 0.001
    0.1mm Precipitation occurs when water droplets
    reach 1mm or when ice crystals form.
  • The net effect of the water cycle is to transfer
    40x103 km3 of fresh water from the oceans to the
    land each year by precipitation.

10
Water
  • The Water Cycle
  • Interception, evaporation transpiration
  • Interception precipitation stopped from
    reaching the ground by vegetation and direct
    evaporation to the atmosphere.
  • Dependent upon vegetation type density and
    season. 25-35, coniferous forest, 15-25, broad
    leaf, 14-19 from grassland.
  • Water reaching the ground becomes part of the
    overland flow which may soak into soil rock or
    evaporate. Rate of evaporation increases with
    temperature and it is linked with humidity.
    The greater the humidity, the less the
    evaporation.
  • Vegetation increases the amount of water returned
    to the air by transpiration.

11
Water
  • Ground water
  • Infiltration
  • Precipitation may run off or sink into the
    ground. The rate of infiltration depends on
    PERMEABILITY.
  • The water table is the boundary between the
    aeration zone and the saturated zone.

Underground water filtration process, aeration
zone, saturated zone and groundwater.
12
Water
  • Ground water
  • Groundwater movement
  • Water flows in response to pressure gradients
    from high pressure to low pressure.
  • The difference in pressure between 2 points at a
    horizontal distance l metres apart on a sloping
    water table difference in height h. The slope
    of the water table is the hydraulic gradient (HG)
    h/ l.
  • The speed of ground water movement v relates to
    the hydraulic gradient by Darcys Law
  • v K h/ l in metres/second or metres/day,
  • K Hydraulic conductivity

13
Water
  • Ground water
  • Groundwater movement cont..
  • Rocks fall into two categories based on their
    hydraulic conductivity (HC), permeable and
    impermeable.
  • Permeable rocks have HC of 1m/day or more.
  • The flow rate will be a combination of the
    hydraulic gradient and the hydraulic
    conductivity. e.g. Water in rocks with 1m/day HC
    may only move at 3mm/day if the hydraulic
    gradient is low.
  • Volume and speed are related by the equation
  • Q (quantity of water) Av
  • v speed through a section of rock with
    cross-sectional area A.
  • Darcys Law can be substituted into this to give
    the volume in terms of HC and HG. Q AK h/l

14
Water
  • Ground water
  • Porosity
  • Porosity is the amount of water a rock can store.
  • porosity pore volume x 100 total
    volume
  • It is controlled by the grain size, shape,
    sorting, cementation and fracturing.

1 cm
15
Water
  • Ground water
  • Permeability
  • Porosity is how much water can be stored
  • Permeability is a measure of properties which
    determine how fast water can flow.
  • For rocks to be high permeability they must have
    interconnected pores. Coarser sands and gravels
    are more permeable than silts (due to surface
    tension).

16
Water
  • Ground water
  • Aquifers
  • An aquifer is a rock which is sufficiently porous
    to hold water and permeable enough to allow water
    to flow.
  • The best aquifers are unconsolidated sands
    gravels.

17
Water
  • Ground water
  • Aquicludes are saturated beds, formations or
    group of formations which can neither release
    water from storage nor transmit water through it.
  • Acquitards are saturated beds, formations or
    group of formations which yield in insignificant
    quantities of water when compared to aquifers,
    but allow appreciable amount of water leakage.

18
Water
  • Ground water
  • Aquifers
  • Unconfined Aquifer
  • Confined Aquifer

19
Water
  • Ground water
  • Exploration for groundwater
  • Use surface geology maps to determine rock types
  • Determine porosity permeability
  • Identify possible aquifers
  • Remote sensing can be used to establish land
    surface characteristics (e.g. vegetation changes)
    which can indicate sub-surface variations.
  • The geological structure is important. The
    general dip and any folding will influence the
    direction of flow.
  • Fault zones can act as major aquifers.
  • Resistivity surveys and investigation boreholes
    are often used in exploration and assessment.

20
Water
  • Surface water
  • Springs
  • Many streams and rivers have springs as their
    source or are fed by them along their courses,
  • Most springs are water table springs and can be
    of several different kinds, depending on the
    geology.
  • Valley springs
  • Ground surface intersects the water table
  • Stratum springs
  • Downward percolation is interrupted by
    impermeable rock which usually results in a
    spring line.
  • Solution channel springs
  • Water travelling through limestone re-emerges at
    surface.

21
Water
  • Surface water
  • River flow
  • Throughflow
  • Rivers can receive water by throughflow, i.e.
    water which flows in the aeration zone without
    getting below the water table.
  • Baseflow
  • Baseflow is the groundwater available and depends
    on the topography, geology, vegetation and
    season. It will be low in a catchment area of
    impermeable rocks.

22
Water
  • Surface water
  • River flow
  • Discharge
  • This is the volume of water passing through a
    point in a given time.
  • Q Av
  • Where, Q quantity A cross- sectional area
    v flow rate

23
Water
  • Surface water
  • Reservoirs
  • Reservoirs have been used to store water for
    thousands of years. Most are built to increase
    water supplies, while some are used to generate
    hydroelectric power and others used to regulate
    water flow to prevent flooding.
  • Most of Britains reservoirs are DIRECT SUPPLY.
    They store water for steady release by pipeline.
  • River regulation reservoirs are built to store
    water for the release to rivers, the rivers being
    used as the mode of transport of water, so pipes
    are not needed.

24
Water
  • Surface water
  • Sites for reservoirs
  • Dam a river valley
  • Factors to be considered
  • Adequate supply of high quality water
  • Minimal environmental effects
  • Elevation providing natural flow using gravity
  • Watertight base sides
  • No geological hazards due to rock
    instability/earthquakes…
  • Suitability of site (general)
  • Superficial deposits
  • Peat should be removed
  • Clays can be useful

25
Water
  • Surface water
  • Dams Gravity Dams
  • Two types are commonly used which are
    fundamentally different ways of withstanding the
    PRESSURE of water.
  • This system depends on its own weight to
    withstand deformation and movement.
  • Usually made up of selected unconsolidated
    material such as clay or broken rock, these are
    called earth dams. They usually have an
    impermeable clay cone, to reduce seepage through
    the dam. The sides are usually covered by broken
    blocks rock or concrete to reduce erosion by rain
    or waves.
  • Gravity dams can also be built entirely piled
    masonry or concrete. The largest in Britain, the
    Kielder Dam is an earth dam- e.g. Aswan High Dam.

26
Water
  • Surface water
  • Dams Gravity Dams
  • Aswan High Dam, shown below, is a rock filled
    gravity dam, 1.2km long. It incorporates an
    upstream cofferdam (A) and a downstream cofferdam
    (B).

27
Water
  • Surface water
  • Dams Wall Dams
  • Wall dams are usually only built in narrow
    valleys when a relatively high dam is needed.
    They must be strong and impermeable and are
    usually constructed of masonry or concrete. Their
    strength is often increased by making them convex
    (toward the reservoir) or with buttresses on the
    down riverside. E.g. Hoover Dam on the Colorado
    River

28
Water
  • Surface water
  • Dams Environmental effects
  • Change of land use. Generally agriculture to
    reservoir. In the case of Aswan 6000 km2 needed.
    Kielder covers around 10.5 km2 and is the largest
    in Europe.
  • Ecology. Ecological changes happen not only near
    the reservoir but also upstream and downstream.
    Down stream sedimentation is also changed and
    annual flooding may cease.
  • Dam Failure. Dams may collapse releasing large
    volumes of water downstream onto habituated or
    cultivated land.
  • Sediment filling. The lifetime of reservoirs can
    vary greatly many last over 1000 years, more
    usually 50. This is due to sedimentation build up
    and the reduction of water capacity.

29
Water
  • Surface water
  • Dams Environmental effects
  • Sediment filling cont... Lake Mead, Colorado has
    had its capacity halved since 1935. This is less
    important in Britain as rivers are smaller and
    carry less sediment.
  • Derwent reservoir has 1 capacity reduction in
    60 years.
  • Sediment loss to agriculture. Trapped sediment
    behind the dams is effectively lost to
    agriculture use. E.g. Aswan
  • Soil salination. Perennial irrigation can cause
    soil salination as salts normally present in the
    river water accumulate in the soils. These salts
    were previously washed away by annual river
    flooding. This causes a decline in crop yields
    and eventually the soil becomes useless for
    agriculture

30
Water
  • Surface water
  • Dams Environmental effects
  • Inducted earth quakes. Rare but have been
    recorded in China. Some of these are over 100m
    deep, often earthquake-affected areas L Marathon
    Greece, L. Mead USA, Kariba Zimbabwe, L. Nasser
    Egypt.

31
Water
  • Water quality
  • Natural waters
  • Total dissolved solids (TDS) composition
  • Rainwater, seawater, river water and groundwater
    differ widely in their concentrations of TDS and
    chemical composition as shown below. N.B. the
    different scales used!

32
Water
  • Water quality
  • Natural waters
  • Total dissolved solids (TDS) composition
  • Groundwater limestone granite

33
Water
  • Water quality
  • Natural waters
  • Water Hardness
  • Hardness in water is mainly due to the presence
    of ions of the elements Ca, Mg Fe. They cause
    scum scaling, and react with soap forming
    insoluble compounds and preventing the soap from
    lathering.
  • The extent of hard water in Britain tends to
    follow a North to South-East gradient the
    softest water is in Scotland, N England Wales
    and the hardest in E Anglia and SE England.
    There is a similar trend for mortality from
    cardiovascular disease! Soft water High CV
    rate. EU has set level of minimum hardness for
    drinking water.

34
Water
  • Water quality
  • Water Pollution
  • Water quality in England Wales. Water may be
    polluted by discharges (sewage works), overflow
    (farmland) or incidents (rupture of farm slurry
    tank).

35
Water
  • Water quality
  • Water Treatment
  • The quality of water required depends on the
    intended use. Drinking water needs quality
    standards different from those for industrial or
    irrigation water. The World Health Organisation
    and the European Community have set water quality
    standards.
  • In recent years, the nitrate concentration in
    surface and groundwater is some areas of the UK
    has been increasing and is now above EU
    acceptable levels with associated health risks.
    To treat would be too expensive, so new boreholes
    have been sunk, water of high levels is blended
    with low-levels, and more control on the use of
    fertilizers in farmland to control the nitrates
    getting into the water.
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