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WFM 5103 Hydrogeology and Groundwater Lectures 3-4. WFM 5103 Hydrogeology and Groundwater . Subsurface environment Water bearing properties of rocks and soils Principles of groundwater movement Recharge Groundwater withdrawal Groundwater Quality Groundwater in Coastal zones

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Presentation Transcript
slide1

WFM 5103

Hydrogeology and Groundwater

Lectures 3-4

slide2

WFM 5103

Hydrogeology and Groundwater

Subsurface environment

Water bearing properties of rocks and soils

Principles of groundwater movement

Recharge

Groundwater withdrawal

Groundwater Quality

Groundwater in Coastal zones

Hydrogeological mapping

Groundwater management

Conjunctive use

Groundwater Models

Groundwater development in Bangladesh

slide4

RECAP

piezometer

Observation well

slide6

Aquifer properties/parameters

Water transmitting parameter

Permeability or

Hydraulic Conductivity

Mean pore velocity:

slide7

Pores, Porosity and Permeability

Pores: The spaces between particles within geological material (rock or sediment) occupied by water and/or air.

Porosity:is defined as the ratio of the volume of voids to the volume of aquifer material. It refers to the degree to which the aquifer material possesses pores or cavities which contain air or water.

Permeability:The capacity of a porous rock, sediment, or soil to transmit ground water. It is a measure of the inter-connectedness of a material's pore spaces and the relative ease of fluid flow under unequal pressure.

slide13

Perched Aquifers

An aquifer in which a ground water body is separated from the main ground water below it by an impermeable layer (which is relatively small laterally) and an unsaturated zone. Water moving downward through the unsaturated zone will be intercepted and accumulate on top of the lens before it moves laterally to the edge of the lens and seeps downward to the regional water table or forms a spring on the side of a hillslope.

slide15

Specific retention

-Water that is retained as a film on rock surfaces and in very small openings. The physical forces that control specific retention are the same forces involved in the thickness and moisture content of capillary fringe

Specific yield

-Water that will drain under the influence of gravity

slide16

Groundwater exploration

Geologic methods

Groundwater withdrawal

Groundwater exploration

Geologic methods

slide17

Relation between K and grain-size distribution

Water transmitting

Parameter….contd.

(a) General relationship

(b) Empirical formulas

(i) Hazen

A = 1.0 for K in [cm/sec] and d10 in [mm]

(ii) Krumbein and Monk

dg= geometric mean grain diameter [mm]; k in [mm];

(i) Kozeny-Carman

slide18

Water transmitting

Parameter….contd.

slide19

Transmissivity

Water transmitting

Parameter….contd.

T = Kb

slide20

Storage parameter

Unconfined aquifer

Specific yield

-Water that will drain under the influence of gravity

Confined aquifer

Storage coefficient/storativity

-Water that is released or taken into storage per unit surface area of aquifer per unit change in head

 = bulk modulus of compression of matrix

 = bulk modulus of compression of water

slide21

Head gradient

From surrounding

aquifer to well

Convergent flow

into the well

Decline in WL in well

Pumping

Cone of Depression

slide22

Cone of Depression

  • Unconfined aquifer
  • Cone of depression expands very slowly (drainage through gravity)
  • Increased drawdown in wells and in aquifer (dewatering of aquifer)
  • Confined aquifer
  • Cone of depression expands very rapidly (why??)
  • No dewatering takes place

Mutual interference of expanding cones around adjacent wells occurs more rapidly in confined aquifers

slide25

1. 1 Exploration of groundwater

Objective:

to locate aquifers capable of yielding water of suitable quality, in economic quantities, for drinking, irrigation, agricultural and industrial purposes, by employing, as required, geological, geophysical, drilling and other techniques.

Assessments of ground water resources range in scope and complexity from simple, qualitative, and relatively inexpensive approaches to rigorous, quantitative, and costly assessments.

Tradeoffs must be carefully considered among the competing influences of the cost of an assessment, the scientific defensibility, and the amount of acceptable uncertainty in meeting the objectives of the water-resource decision maker.

slide26

Groundwater exploration

Exploration of Groundwater

1.1.1 Surface exploration

- “non-invasive" ways to map the subsurface.

-less costly than subsurface investigations

1. Geologic methods

2. Remote Sensing

3. Surface Geophysical Methods

(a) Electric Resistivity Method

(b) Seismic Refraction Method

(c) Seismic Reflection Method

(d) Gravimetric Method

(e) Magnetic Method

(f) Electromagnetic Method

(g) Ground Penetrating Radar

and others

slide27

Groundwater exploration

Exploration of Groundwater

1.1.2 Subsurface exploration

1. Test drilling

geologic log

drilling time log

Water level measurement

2. Geophysical logging/borehole geophysics

Resistivity logging

Spontaneous potential logging

Radiation logging

Temperature logging

Caliper Logging

Fluid Conductivity logging

Fluid velocity logging

3. Tracer tests

and others

slide28

Groundwater exploration

Exploration of Groundwater

1.1.1 Surface exploration

- “non-invasive" ways to map the subsurface.

-less costly than subsurface investigations

1. Geologic methods

2. Remote Sensing

3. Surface Geophysical Methods

(a) Electric Resistivity Method

(b) Seismic Refraction Method

(c) Seismic Reflection Method

(d) Gravimetric Method

(e) Magnetic Method

(f) Electromagnetic Method

(g) Ground Penetrating Radar

and others

1.1.2 Subsurface exploration

1. Test drilling

geologic log

drilling time log

Water level measurement

2. Geophysical logging/borehole geophysics

Resistivity logging

Spontaneous potential logging

Radiation logging

Temperature logging

Caliper Logging

Fluid Conductivity logging

Fluid velocity logging

3. Tracer tests

and others

slide31

Groundwater exploration

Geologic methods

Groundwater withdrawal

Groundwater exploration

Geologic methods

  • 1.1.1.1 Geologic Methods
  • an important first step in any groundwater investigation
  • involves collection, analysis and hydrogeologic interpretation of existing geologic data/maps, topographic maps, aerial photographs and other pertinent records.
  • should be supplemented, when possible, by geologic field reconnaissance and by evaluation of available hydrologic data on stream flow and springs, well yields, groundwater recharge and discharge, groundwater levels and quality.
  • - nature and thickness of overlying beds as well as the dip of water bearing formations will enable estimates of drilling depths to be made.
slide32

Groundwater exploration

Geologic methods

Groundwater withdrawal

Groundwater exploration

Geologic methods

Relationship between

geology and groundwater

 The type of rock formation will suggest the magnitude of water yield to be expected.

it is the perviousness or permeability and not porosity which is significant in water yielding capacity of rocks.

Igneous rocks have a porosity of 1% and may yield all water while some clays have a pososity as high as 50% but are practically impervious.

Porosity = f (grainsize, shape, grading, sorting, amount and distribution of cementing materials)

Permeability = f (interconnectedness, fissures, joints, bedding planes, faults, shear zones and cleavages, vesicles )

slide33

Groundwater exploration

Geologic methods

alluvial aquifers :90% of all developed

Aquifers are alluvial aquifers, consisting

of unconsolidated alluvial deposits,

chiefly gravels and sands.

Limestone aquifer varies in density, porosity and permeability depending on degree of consolidation and development of permeable zones after deposition. Original rock materials offer important aquifers.

Volcanic rock can form highly permeable aquifers. Basalts form a good source of water; easily susceptible to weathering.

Sandstones are cemented forms of sands and gravels; yields are reduced by the cements. Some may form good aquifers depending on shape and arrangement of constituent particles and cementation and compaction.

Igneous and metamorphic rocks, in solid state, are relatively impermeable and hence serve as poor aquifers. Under weathered conditions, however, the presence of joints, fractures, cleavages and faults form good water bearing zones, and small wells may be developed in these zones for domestic water supply.

Relationship between

geology and groundwater

slide34

Selection of site for a well

Factors to be considered are:

(i) Topography: Valley regions are more favorable than the slopes and the top of the hillocks.

(ii) Climate(annual rainfall, sunlight intensity, max. temperature, humidity):

heavy to moderate rainfall -- more deep percolation – good aquifer. Intense summer weather -- evaporates and depletes GW through direct

evaporation from shallow depths and evapotranspiration through plants.

slide35

Selection of site for a well

Groundwater exploration

Geologic methods

(iii) Vegetation: can flourish where GW is available at shallow depths.

Phreatophytes, plants that draw the required water directly from the zone of saturation indicate large storage of groundwater at shallow

slide36

Selection of site for a well

Groundwater exploration

Geologic methods

(iii) Vegetation: can flourish where GW is available at shallow depths.

Phreatophytes, plants that draw the required water directly from the zone of saturation indicate large storage of groundwater at shallow depths.

Xerophytes, plants that exist under arid conditions by absorbing the soil moisture (intermediate or vadose water), indicate the scarcity of groundwater at shallow depths.

slide37

Selection of site for a well

Groundwater exploration

Geologic methods

(iii) Vegetation: can flourish where GW is available at shallow depths.

Phreatophytes, plants that draw the required water directly from the zone of saturation indicate large storage of groundwater at shallow depths.

Xerophytes, plants that exist under arid conditions by absorbing the soil moisture (intermediate or vadose water), indicate the scarcity of groundwater at shallow depths.

Halophytes, plants with a high tolerance of soluble salts, and white efflorescence of salt at ground surface indicate the presence of shallow brackish or saline groundwater.

slide38

Selection of site for a well

Groundwater exploration

Geologic methods

(iv) Geology of the area:thick soil or alluvium cover, highly weathered, fractured, jointed or sheared and porous rocks indicate good storage of groundwater, whereas massive igneous and metamorphic rocks or impermeable shales indicate paucity of groundwater.

(v) Porosity, permeability: highly porous, permeable zones of dense rocks encourage storage of groundwater. Massive rocks do not permit the water to sink.

(vi) Joints and faults in rocks: Wells sunk into rocks with interconnected joints, fractures, fissures and cracks yield copious supply of water.

(vii) Proximity of rivers: Streams and rivers serve as sources of recharge and water is stored in the pervious layers.

slide39

Groundwater exploration

Remote sensing

Source

A physical quantity

(screen)

(light/radiation)

Sensor

Processor

signal

(records data and interprets information)

(eyes)

1.1.1.2 Remote sensing

slide40

Groundwater exploration

Remote sensing

  • Remote sensing
  • an increasingly valuable tool for understanding GW conditions.
  • information on an object on the earth is acquired by remote registration/ sensing from aircraft or satellite at various wavelengths of the electromagnetic energy reflected and emitted.
  • difference in reflectance properties of objects produce varying signatures on the photos or images, which can be interpreted for a variety of purposes of which application of hydrogeology is one.
  • stereoscopic airphotos (color, black and white, infrared), oblique air photos and high resolution satellite imageries taken from GMS, APT, NOAA, AVHRR, SPOT and Landsat, ERS-SAR, RADARSAT, open up new possibilities for the assessment of groundwater resources.
  • - observable patterns, colors, and relief makes it possible to distinguish differences in geology, soils, soil moisture, vegetation and land-use (hence areas of groundwater recharge and discharge).
slide41

Groundwater exploration

Remote sensing

  • RS applications
    • forest cover mapping and monitoring;
    • land use and land cover mapping;
    •  mapping of water resources;
    • Others: agriculture; fisheries; coastal zone; marine environment.
slide43

Groundwater exploration

Remote sensing

Advantages of remote sensing technique in general:

- speed of operation

- survey of inaccessible areas

- possibility of repetitive coverage of changing landform, land use, vegetal cover, water spread in reservoirs, soil salinity, water logged areas, etc.

- permits mapping and preliminary evaluation at lesser cost.

“The remote sensing technique is only an additional tool in the quest of groundwater and not a substitute for other methods. For a meaningful interpretation, there should be adequate ground check in the field”.

slide44

Groundwater withdrawal

Groundwater exploration

Surface geophysical methods

  • 1.1.1.3. Surface Geophysical
  • Methods
  • - scientific measurement of physical properties and parameters of the earth’s subsurface formations and contained fluids by instruments located on the surface for investigation of mineral deposits or geologic structure.
  • provide only indirect indication of groundwater
  • -success depends on how best the physical parameters are interpreted in terms of hydrogeological language.
  • - Accurate interpretation requires supplemental data from subsurface investigations to substantiate surface findings.
slide45

Groundwater exploration

Surface geophysical methods

Electric resistivity

Groundwater withdrawal

Groundwater exploration

Surface geophysical methods

1.1.1.3 (a) Electric Resistivity Method

♦Electrical resistivity is the resistance of a volume of material to the flow of electrical current.

♦current is introduced into the ground through a pair of current electrodes

♦resulting potential difference is measured between another pair of potential electrodes

♦Apparent resistivity is then calculated as:

V is the measured Potential difference (in Volts)

and I is the current introduced (in Amperes).

slide46

Groundwater exploration

Surface geophysical methods

Electric resistivity

Wenner arrangement

Schlumber configuration

1.1.1.3 (a) Electric Resistivity Method

slide47

Groundwater exploration

Surface geophysical methods

Electric resistivity

The measured potential difference is a weighted value over a subsurface region controlled by the shape of the region, and yields an apparent resistivity over an unspecified depth.

  • Vertical electrical Sounding (VES)

Changing the spacing of electrodes changes the depth of penetration of the current. So it is possible to obtain field curve of apparent resistivity vs depth.

For a single homogeneous, isotropic layer of infinite thickness, resistivity curve will be a straight line.

True/actual resistivity - if formation is homogeneous and isotropic.

Apparent resistivity

if formation is anisotropic consisting of two or more layers of different materials.