groundwater hydrology slides
TRANSCRIPT
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HYDROGEOLOGY
VIKRANT SHARMA
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The thickness of the capillary
fringe varies depending on
the pore sizes in the medium.
In a silt or clay, the capillary
fringe can be more than a
meter thick, while the
capillary fringe in a coarse
gravel would be less than amillimeter thick.
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An Aquifer can be defined as a geological formation in which water
accumulates and may circulate, via its pores and fissures, thus enabling
humans to make use of it in economically viable quantities. (Custodio andLlamas, 1996)
Sustainable Groundwater Development
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Lithology
Unconsolidated Sedimentary rocksConsolidated Sedimentary rocks
Volcanic
Igneous
Metamorphic
Hydrostatic P of GW
Unconfined Aquifers
Confined Aquifers
Semi-Confined Aquifers
Depth of Aquifer
ShallowDeep (>300m)
Scale of StudyLocal
Regional
Basic Aquifer Classifications
Porosity
Single Porosity
Double Porosity
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An aquifer is defined as a saturated permeable geological unit that is
permeable enough to yield economic quantities of water to wells. The
most common aquifers are unconsolidated sand and gravels, but
permeable sedimentary rocks such as sandstone and limestone, and
heavily fractured or weathered volcanic and crystalline rocks can also beclassified as aquifers.
An aquitard is a geological unit that is permeable enough to transmit
water in significant quantities when viewed over large areas and long
periods, but its permeability is not sufficient to justify production wellsbeing placed in it. Clays, loams and shale are typical aquitards.
An aquiclude is an impermeable geological unit that does not transmit
water at all. Dense unfractured igneous or metamorphic rocks are typical
aquicludes. In nature, truly impermeable geological units seldom occur;
all of them leak to some extent, and must therefore be classified as
aquitards. In practice, however, geological units can be classified as
aquicludes when their permeability is several orders of magnitude lower
than that of an overlying or underlying aquifer.
*** These definitions are imprecise with respect to permeability.
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Aquifer Types
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Aquifer Types
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Unconsolidated Semi-consolidated
Sandstone Carbonate
VolcanicPlutonic
Principle Aquifer Rock Types
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Unconsolidated AquifersUnconsolidated deposits are geological formations formed by the accumulation of
particles that are transported by gravity, water, wind or ice, in riverbeds, lakeside or
marine settings. They usually comprise of sands and gravels of varying geological
origin. Fluvial deposits are made up of the alluvial materials of rivers and their terraces.Deltaic deposits accumulate at river mouths. In general such deposits are recent in
geological time. Its porosity is due to voids or space between the rock particles, or
single porosity.
Aquifers that are mapped as unconsolidated
sand and gravel can be grouped into four
broad categories:
basin-fill or valley-fill aquifers
blanket sand and gravel aquifers
glacial-deposit aquifers
stream-valley aquifers
All four types have intergranular porosity
and all contain water primarily under unconfined or water-table conditions.
Determination of depth, thickness and extension of
permeable deposits and confining layers.
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Ground water in unconsolidatedaquifers flows along relatively shortflow paths typical of local flow systems
Basin-fill aquifers typically haveintermediate flow systems
Thick basin-fill aquifers may supportregional flow system.
Likewise, the thick blanket sandsaquifers and alluvial aquifers canrepresent regional flow systems.
Unconsolidated
Aquifer Properties
Unconsolidated
Aquifer Flow Systems
The hydraulic conductivity ofunconsolidated aquifers is variable,depending on the sorting of aquifermaterials and the amount of silt andclay present, but generally it is high.
Aquifer thickness ranges from a fewmeters or tens of meters in the blanketsands to several hundred meters in the
basin-fill aquifers.
Unconsolidated sand and gravelaquifers are susceptible to
contamination because of theirgenerally high hydraulic conductivity.
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Semi-consolidated aquifers consist of sand interbedded with silt, clay, andminor carbonate.
The aquifers are typically of fluvial, deltaic, and shallow marine origin.
The varied depositional environments of these sediments have causedcomplex interbedding of fine and coarse-grained materials.
Accordingly, some aquifers are thin and local whereas others are thick andmay extend over hundreds of square kilometres.
The Ravenscrag Formation is a Saskatchewan example of a semi-consolidated aquifer.
Semi-consolidated Aquifers
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Semi-consolidated
Aquifer Properties
Semi-consolidated
Aquifer Flow Systems
Porosity is intergranular, and thehydraulic conductivity of the aquifers ismoderate to high.
The aquifers form thick extensivewedges of sediment.
Wedges tend to dip away fromtopographically high erosional sourceareas.
Aquifer thicknesses can reach severalhundred metres.
Numerous local aquifers can be grouped intoa few regional aquifer systems that containgroundwater flow systems of local,intermediate, and regional scale.
In topographically high recharge areasaquifers are unconfined but become
confined in the downdip direction. Discharge is by upward leakage to shallower
aquifers or to saltwater bodies in coastalareas.
Because flow is sluggish near the ends ofregional flow paths, the aquifers commonly
contain unflushed saline water in theirdeeply buried, downdip extremities.
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Consolidated Sedimentary RocksConsolidated sedimentary rocks are made of sediments that have
become consolidated by compaction or diagenesis processes, which
reduce the space occupied by the voids. On the basis of their porositythey can be classified as double porosity aquifers as is the case of
sandstones with primary and or interstitial porosity and secodary
porosity mainly due to fracturing in karstic aquifers as limestones
and dolomites, secondary porosity is due to fracturing and chemical
dilution processes.
Origin (Detritic)
Conglomerates
Sandstones
Clays
Chemical
Limestone (Carbonates)
Dolomites (Carbonates)
Chalk (Carbonates)
Marls
Organic
Carbons
Natural
Hydrocarbons
One of the main targets in the study of such aquifers is to localize the
fractured and voided areas.
S d t A if
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Sandstone Aquifers
Secondary openings, such as joints and fractures, along with beddingplanes, typically transmit most of the groundwater in bedrocksandstone aquifers.
Sandstone retains only a small part of the intergranular pore space that
was present before the rock was consolidated.
Compaction and cementation greatly reduce the primary pore space.
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Sandstone Aquifer
Properties
Sandstone Aquifer
Flow Systems
The hydraulic conductivity ofcemented sandstone aquifers is lowto moderate.
Transmission is primarily throughfractures although primary porosity
may continue to contribute tostorage.
Because bedrock sandstones extendover large areas, these aquifers canoften provide large amounts of
water.
Sandstone aquifers in the Prairies arehorizontal to gently dipping.
Because they are commonly interbedded withsiltstone or shale, most of the water in theseaquifers is under confined conditions.
Groundwater flow systems in relatively thinsandstone aquifers are local to intermediate.
Regional, intermediate, and local flow arepresent in the sandstone aquifers westernCanada.
Many extensive sandstone aquifers containhighly mineralized water at depths of only afew hundred meters.
C b t A if
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Carbonate Aquifers
The most important are the carbonate rocks among the consolidated
sedimentary rocks. Chalk, limestone and dolomites they vary considerably
in density, porosity and permiability - some are considered to be confining
units, whereas others are among the most productive aquifers known. Most of the carbonate-rock aquifers consist of limestone, but dolomite and
marble locally yield water.
Carbonate rocks originate as sedimentary deposits in marine environments.
Compaction, cementation, and dolomitization processes act on the depositsas they undergo lithification and greatly change their porosity and
permeability.
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Carbonate Aquifer
Properties
Carbonate Aquifer
Flow Systems
The principal post-depositional change incarbonate rocks is the dissolution of partof the rock by circulating, slightly acidicgroundwater.
Solution openings in carbonate rocksrange from small tubes and widenedjoints to caverns that may be tens of
meters wide and hundreds to thousandsof meters in length.
Where they are saturated, carbonaterocks with well-connected networks ofsolution openings yield large amounts ofwater to wells that penetrate the openings
The undissolved rock between the largeo enin s ma be almost im ermeable.
Where carbonate rocks are exposed at landsurface, solution creates karst topography,characterized by little surface drainage,sinkholes, blind valleys, sinking streams, andkarst towers (mogotes).
Because water enters the carbonate rocks
rapidly through sinkholes and other largeopenings, any contaminants in the water can
spread rapidly through the aquifers.
Regional, intermediate, and local ground-
water flow systems are present in carbonate
aquifers but most near-surface carbonates tend
to provide only local and intermediate systems.
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P t A if Ch t i ti
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Parameters: Aquifer Characterization
Three conditions are necessary for the existence and use of an aquifer:1: The rock must be able to store water (Aquifer Storage Property)
2: The water has to be able to circulate through the rock (Aquifer Flow Property)3: There must be water replenishment (Aquifer Recharge)
*** Water removal
The amount of water held in a rock depends upon itsporosity. Porosity is
controlled by the grain size and shape, the degree of sorting, the extent of
chemical cementation and the amount of fracturing.
Solid density: s = Msolids/Vsolids
Bulk density: b= Msolids/Vtotal
Simple Cubic Body-Centered Cubic Face-CenteredCubicn = 0.48 n = 0. 26 n = 0.26
total
pores
V
Vn
total
solidstotal
V
VVn
total
solids
V
Vn 1
s
bn
1
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Porosity that has developed after the rocks have formed is termed
secondary porosity to distinguish it from intergranular or primary
porosity. Secondary porosity typically results from two main causes:fractures associated with joints, along bedding-planes, tectonic joints and
faulting (although where fault gouge has been produced or secondary
mineralization has occurred along the fault plane, groundwater
movement will be restricted rather than enhanced), and karst processesthat dissolve the limestone aquifers.
Dolomitization of limestones (i.e. the replacement of calcium ions with
magnesium) also increases porosity because the magnesium ion is
smaller than the calcium ion that it replaces by as much as 13 per cent.However, the dolomite crystals are usually very small, producing tiny
pore spaces, and are unevenly distributed through the rock, resulting in
only small increases in the hydraulic conductivity.
Aquifer Storage Property: Porosity
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Aquifer Storage Property: Porosity
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Aquifer Storage Property: Porosity
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Aquifer Storage Property: Porosity
E l ti f A if P ti
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Evaluation of Aquifer PropertiesPorosity (Grain Size)In unconsolidated materials, the size of the mineral grains is a key
characteristic of the material. The distribution of grain sizes determineshow much pore space is available to hold water, and how easily water is
transmitted through the material.
E l ti f A if P ti
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Evaluation of Aquifer PropertiesGrain Size Distribution
Sieve Analysis
Evaluation of Aquifer Properties
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Evaluation of Aquifer PropertiesGrain Size DistributionHydrometer Analysis
L1
L2
L
Evaluation of Aquifer Properties
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Evaluation of Aquifer PropertiesGrain Size Distribution
Evaluation of Aquifer Properties
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Evaluation of Aquifer PropertiesParticle ShapeThe shape of particles present in a soil mass is equally as important asthe particle size distribution because it has significant influence on the
physical properties of a given soil. However, not much attention is paid
to particle shape because it is more difficult to measure.
Bulky: Bulky particles are mostly formed by mechanical weatheringof rock and minerals. Geologists use such terms as angular, sub-angular,
rounded and sub-rounded.
Flaky: Flaky particles have very low sphericity usually 0.01 or less. These particles are predominantly clay minerals.
Needle Shaped: Needle-shaped particles are much less commonthan the other two particle types. Examples of soils containing needle-
shaped particles are some coral deposits and attapulgite clays.
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Parameters: Aquifer Storage
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The proportion of voids in relation to the total volume of rock considered
is evaluated by the total porosity n. But some voids may not be
connected with other voids, and the fluid inside that is trapped; itsvolume proportion is named trapped porosity nt. Trapped porosity is
abundant in karstic and hard-rock aquifers. In unconsolidated sediments
and in sandstones , the role of nt is negligible and can be disregarded
usually.
Parameters: Aquifer Storage
How much of the stored water of the
aquifer is available for pumping?
Real Question
Parameters: Aquifer Storage
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The amount of interconnected pore space that is available for fluid flow
and advective transport is termed the effective porosity ne. The main
application field of ne is contaminant-transport modeling according toadvective-dispersive equation. ne is high in well-sorted sands and/or
gravels; and in clayey rock types. ne is low in poorly sorted deposits such
as glacial tills.
Parameters: Aquifer StorageA small part of the water can be attached by molecular forces to the walls
of the grains (within 0.0002 mm from surface), and its proportion to the
total volume of rock is called bound waterb; the rest of the water iscalledfree waterf.
For saturated conditions soil moisture content (theta) is equal to porosity,
theta(m) is mobile water theta(MRS) is soil moisture deciphered by MRS.
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Methods for Obtaining Effective Porosity
Tables
Aquifer Flow Property
Usually, hydraulic conductivity, and to a much lesser extent dispersivity,
are the focus of field and laboratory data-collection efforts for models
that are based on the advectiondispersion equation (ADE). A third
hydraulic parameter required for transport modeling is effective porosity.
For aquifer simulations, it has become common practice to estimate
effective porosity from ones experience or the literature. Effective
porosity is not often evaluated because it has a small range of variability
compared with hydraulic conductivity and dispersivity.
Values from the tables can vary substantially from the actual values.
Hence we can use the tables for an initial guess, however these values
could be inefficient and/or inaccurate.
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Effective porosity is sometimes obtained from other measured parameters, such as
specific yield, or total porosity minus specific retention or residual water content. This
is not correct. Specific yield and effective porosity are two different parameters. Thesetwo parameters have in fact comparable values for coarse rock materials (because
specific retention is small) where Sy ne. However in fine grained rocks and
particularly in clayey materials, Sy is low while ne is high so the Sy differs substantially
from ne.
Effective porosity defined in context of transport is different from effective porosity that
pertains to drainage and capillary processes.
Methods for Obtaining Effective Porosity
Tables
Aquifer Flow Property
Texture
class
Sampl
e size
Total
porosity
Residual
(bound)
Effective
porosity
Pore-size
Distribution
Specific
retention (Sr)
Wilting
point
Specific
yield
Saturated
hydraulic
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class e size
[cm3]
porosity
n
[cm3/cm3]
(bound)
water
content b
[cm3/cm3]
porosity
ne
[cm3/cm3]
Distribution
arithmetic
mean
retention (Sr)
water retained
at 33 kPa
[cm3/cm3]
point
water
retained at
1500 kPa
[cm3/cm3]
yield
estimated
from
Sy = n Sr
[cm3/cm3]
hydraulic
conductivity
K [cm/h]
Sand 762 0.437(0/374-0/500)
0.020
(0.01-0.039)
0.417
(0.354-0.480)
0.694
(0.298-1.090)
0.091
(0.018-0.164)
0.033
(0.007-0.059)
0.346 23.56
Loamy
sand
338 0.437(0.368-0.506)
0.035(0.003-0.067)
0.401(0.329-0.473)
0.553(0.234-0.872)
0.125(0.060-0.190)
0.055(0.019-0.091)
0.312 5.98
Sandy
loam
666 0.453
(0.351-0.555)
0.041
(-0.024-0.106)
0.412
(0.283-0.541)
0.378
(0.140-0.616)
0.207
(0.126-0.288)
0.095
(0.031-0.159)
0.246 2.18
Loam 383 0.463(0.375-0.551)
0.027
(-0.020-0.074)
0.434
(0.334-0.534)
0.252
(0.086-0.418)
0.207
(0.195-0.345)
0.117
(0.069-0.165)
0.193 1.32
Silt loam 1206 0.501(0.420-0.582)
0.015
(-0.028-0.058)
0.486
(0.394-0.578)
0.234
(0.105-0.363)
0.330
(0.258-0.402)
0.133
(0.078-0.188)
0.171 0.68
Sandy
clay
loam
498 0.398
(0.332-0.464)
0.068
(-0.001-0.137)
0.330
(0.235-0.425)
0.319
(0.079-0.559)
0.255
(0.186-0.324)
0.148
(0.085-0.211)
0.143 0.30
Clay
loam
366 0.464
(0.409-0.519)
0.075
(-0.024-0.174)
0.390
(0.279-0.501)
0.242
(0.070-0.414)
0.318
(0.250-0.386)
0.197
(0.115-0.279)
0.146 0.20
Silty
clay
loam
689 0.471
(0.418-0.524)
0.040
(-0.038-0.118)
0.432
(0.347-0.517)
0.177
(0.039-0.315)
0.366
(0.304-0.428)
0.208
(0.138-0.278)
0.105 0.20
Sandy
clay
45 0.430
(0.370-0.490)
0.109
(0.013-0.205)
0.321
(0.207-0.435)
0.223
(0.048-0.398)
0.339
(0.245-0.433)
0.239
(0.162-0.316)
0.091 0.12
Silty
clay
127 0.479
(0.425-0.533)
0.056
(-0.024-0.136)
0.423
(0.334-0.512)
0.150
(0.040-0.260)
0.387
(0.332-0.442)
0.250
(0.193-0.307)
0.092 0.10
Clay 291 0.475(0.427-0.523)
0.090
(-0.015-0.195)
0.385
(0.269-0.501)
0.165
(0.037-0.293)
0.396
(0.326-0.466)
0.272
(0.208-0.336)
0.079 0.06
Lubczynski, M.W. and Roy, J., 2007. Use of MRS for hydrogeological system parametrization and modeling. Table 1 Boletin Geologico y Minero pp 514
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Methods for Obtaining Effective Porosity
Lab Measurements I Tracer Column Testing
Aquifer Flow PropertyFor traditional solute-transport modeling, effective porosity (ne) can be defined as the
ratio between Darcy flux and seepage velocity, where q is experimental Darcy flux(specific discharge) and v is seepage velocity (or velocity of a conservative tracer).
Advective and dispersive processes are active
within the pore spaces designated as effective
porosity.
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Methods for Obtaining Effective Porosity
Lab Measurements I Tracer Column Testing
Aquifer Flow Property
Assuming it is a chemical and physical
equilibrium transport, we use this ADE
(Advection Dispersion Equation).
Assuming no retardation, the traditional column
testing approach can
utilize the analytical
solution of a one-
dimensional version of
the above ADE.
The relative concentration point (c/co=0.5) describes solute moving at
the average velocity and for a nonreactive tracer c/co=0.5 should occur
when one pore volume of solution has flowed from the column. Using
the measured elapse time, t0.5 at c/co=0.5, the known column length, L,
and experimental Darcy flux, q, the effective porosity can be calculated.
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Methods for Obtaining Effective PorosityLab Measurements II CMR Approach to Column Testing
Aquifer Flow Property
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Tables
Lab Experiments
Tracer Tests (CXTFIT)
MRS (50 m 60000; 100 m 90000; 150 m 130000)
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Evaluation of Aquifer Properties
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Storage
Storativity
Evaluation of Aquifer PropertiesHydraulic Properties
The amount of water held in a rock depends upon itsporosity. Porosity is
controlled by the grain size and shape, the degree of sorting, the extent ofchemical cementation and the amount of fracturing.
The hydraulic properties can be measured in the field or laboratory but can also be assessed in
general terms by consideration of the overall aquifer geology.
The amount of interconnected pore space that is available for fluid flow
is termed the effective porosity.
Porosity does not provide a direct measure of the amount of water thatwill drain out of the aquifer because some of the water will remain in the
rock, retained around individual grains by surface-tension forces. That
part of the groundwater that will drain from the aquifer is termed the
specific yield, andthe part that is held in the aquifer is called thespecific
retention.
Evaluation of Aquifer Properties
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Evaluation of Aquifer PropertiesHydraulic Properties
Groundwater Flow
The hydraulic properties can be measured in the field or laboratory but can also be assessed in
general terms by consideration of the overall aquifer geology.
Hydraulic Conductivity
Hydraulic conductivity depends on both the properties of the aquifer and the
density and viscosity of the water. - concentrations of dissolved minerals andtemperature - increase in water temperature from 5C to about 30C, will
double the hydraulic conductivity and will double the rate of groundwater flow.
Not a problem in deep aquifers. In some shallow aquifers in areas of climate
extremes or in particular situations involving waste hot water and industrial
effluent the flow rates may be affected by the temperature. Hence always takethe temperature of pumped water in the field tests.
The property of a rock that controls the hydraulic conductivity is its intrinsic
permeability (k), and is constant for an aquifer regardless of the fluids flowing
through it, applying equally well to oil, gas and water. Intrinsic permeability