water pollution and control

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Environmental Science: WaterPollution and Control

EV20001 – Autumn 2015

Brajesh K DubeyDepartment of Civil EngineeringIIT Kharagpur

The Story of bottled water

Oxygen Demand● Substances that diminish D.O. in water

Definitions Biochemical Oxygen Demand [BOD]

The amount of oxygen required by bacteria whilestabilizing decomposable organic matter under aerobicconditions

Chemical Oxygen Demand [COD] The conversion of organic matter to CO2 and H2O by

strong oxidizing agents under acidic conditions

Aerobic Decomposition

−−++

24

324O

dcbaNOHC

dcba

( )322

2

3dNHOH

dbCOa +

−+

Biochemical Oxygen Demand Application to wastewater and receiving waters; not appropriate

for drinking water Estimated by a laboratory method to simulate conditions in

wastes and receiving waters; controlled conditions BOD test is a bioassay test

Conditions include Temperature = 20oC in an incubator under dark conditions Seed = insure bacteria populations are present Oxygen = saturated at 20oC at start of test Add nutrients [optimize growth] and salts [pH and osmotic

pressure] in dilution water Dilute the sample to insure correct results of BOD test [Table 23.1] Control, remove, or mask effect of toxics on seed organisms [serial

dilution] Microbial process follows first order kinetics

- dC/dt = k’ C k’ = first order rate constant

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BOD Kinetics BOD5 = y = L (1 - e-k’t) k’ is lne base BOD5 = y = L (1 – 10-kt) k is log10 base

y = 5 day BOD or BOD5

L = “ultimate” or “maximum” BOD

k’ depends on microbial community and the type of wasteinvolved

BOD5 may or may not approximate L Different values of k’ result in different values of L at time, t L is needed for wasteload allocation modeling L can be calculated or estimated in the lab Effect of change in variables on BOD

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Biochemical Oxygen Demand(BOD, pronounced bee-oh-dee) Most commonly used parameter to define

the strength of a polluted water Used to determine the relative oxygen

requirements for polluted waters What threat dose BOD present to a

receiving water body? Reduction in dissolved oxygen (DO)

< 30 mg/L is the standard

BOD Laboratory Definition Quantity of oxygen utilized by a mixed

population of microorganisms in the aerobicoxidation (of the organic matter (OM) in a sampleof wastewater) at a temperature of 20°C ± 1 °Cin an air incubator or water bath

Assumes all biodegradable OM will be oxidizedto H2O and CO2

Reaction Mechanism Metabolism of OM & uptake of DO by

bacteria, releasing CO2 & producing asubstantial increase in bacterial pop.

OM CO2 + bact. cells CO2 + prot. Cells

If [OM] increases, oxygen consumption: Increases, therefore BOD test indirectly measures

[OM]

DO + bacteria DO + protozoans

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Calculation of BOD BOD (mg/L) = (D1-D2)/P

D1 = initial DO (after 15min. prep time)

D2 = final DO (after 5 day incubation) D1 and D2 both have units of mg/L

P = decimal fraction of wastewater = mL of wastewater/mL of BOD bottle

BOD Curves (1st order kinetics) BOD(t)=Ultimate BOD(1-10-Kt)

K = 0.1/day (common value for domestic waste)

Nitrifying lags behind (5 days) Replication should be 15%

Standardized BOD test using glucose & glutamic acid

Why is BOD test a 1st order reaction? Rate of O2 consumed is directly proportional to the

concentration of degradable OM (Figure)

Example Problem If the BOD3 of a waste is 75 mg/L and the

K is 0.150/day, what is the ultimate BOD?

Answer is 115 mg/L

BOD5 = y = L (1 - e-k’t) k’ is lne base

BOD5 = y = L (1 – 10-kt) k is log10 basey = 5 day BOD or BOD5

L = “ultimate” or “maximum” BOD

Testing BOD of Municipal Wastewater

Determine solution volume from Table orBOD dilution equation

For a valid test DO consumed > 2 mg/L,but DOf > 1 mg/L

Example, if BOD = 350 mg/L; DO of 5mg/L, what is volume of WW added?

Determination of BOD-K K = 2.61(B/A)

Formula derived from Thomas’ GraphicalMethod

K = rate constant/day

A = y-intercept

B = slope

What is the relationship b/wCOD and BOD? What is COD?

Chemical oxygen demand Oxidation or uptake of oxygen to decompose

nonbiodegradable OM

OM + O2 CO2 + H2O + others

So the relationship is?

None

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Conclusions, What is BOD5? BOD measured after 5 days and used as the

standard procedure Should you always use BOD5?

If comparing 2 wastes, only if K is the samefor both

Again, BOD is the quantity of O2 used bybugs in the aerobic stabilization of pollutedH2O!

What is Potable Water?

Potable Water can be costumed in any desiredamount without concern for adverse health effectsbut does not necessarily taste good

Palatable Water is pleasing to drink but notnecessarily safe

We need Potable Water that is also Palatable.

Inorganic compounds such as Antimony, Arsenic, Asbestos,Cyanide, Lead, Mercury, etc.

Organic CompoundsVolatile Organic Chemicals: Benzene, Carbon tetrachloride,

Chlorobenzene, Methylene Chloride, etc.Synthetic Organic Chemicals: Acrylamide, Xylene, Ethyl

Benzene, Styrene, etc.

Microbial contaminants such as bacteria, virus

Radioactive contaminants

What prevents water from being potable? In what form/state are these impurities present in water ?

• Suspended: Particles above the size of 0.1 µm such asalgae, bacteria, fungi, organic material, sand.

• Colloidal: Particles smaller than 0.1 µm such as viruses,complex molecules, ions.

• Dissolved: volatile and synthetic organic chemicals.

No sharp divisions between the various categories

Measurement of the amount of Impurities in Water

• Suspended : Total Particle Count (TPC)(Visible under a microscope)

• Colloidal : Turbidity, Color, TPC

• Dissolved : Hardness, Alkalinity, pH, Color

Turbidity

Turbidity is a measure of the degree to which a colloidalsuspension reflects light at a 90o angle to the entrance beam.

There are various standards against which a sample is comparedto obtain the turbidity. A turbidity higher than 5 TU (turbidityunit) is physically unaesthetic.

The colloidal particles that cause turbidity also give color to thewater.

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Hardness

Hardness is the sum of all polyvalent cations such as Ca2+,Mg2+ (in units of mg/l as CaCO3)

Hard water does not form lather with soap and is bad forcleaning. Ca and Mg in hard water reacts with soap to forminsoluble calcium and magnesium carboxylates (the “ring”in the bath tub)

Water Quality Goals

Turbidity < 0.1 TUColor < 3 color unitsTaste None objectionableOdor NoneTotal Dissolved Solid (TDS) < 200 mg/l (1 mg/l ~ 1ppm)Hardness 80 mg/l of CaCO3

Toxic Inorganic Substances

Synthetic Organic Chemicals

Volatile Organic Chemicals

Microorganisms (Coliforms and E. coli.) 0 in at least 80% of tests

Sources of Water

More than 50% of world population relies ongroundwater as a source of drinking water

GroundConstant compositionHigh mineralizationLittle turbidityLow or no colorBacteriologically safeNo dissolved oxygenHigh hardnessH2S, Fe, Mn

Surface (Reservoir, Lake,River)

Varying compositionLow mineralizationHigh turbidityColorMicroorganisms presentDissolved oxygenLow hardnessTastes, odors

How to purify water from these sources?

Removal of Suspended Matter

Sedimentation: Settling of the large suspended particles due togravity.

Filtration: Removal of large particles by passing water through aporous membrane or a packed bed.

Centrifugation: Removal of the particles by centrifugal forces.

Removal of Dissolved Matter

Processes that cause a phase changes such as

Precipitation: Formation of a product that doesn’t dissolvein water leading to its separation into suspended form.

Distillation: Removal of non-volatile liquids by evaporatingwater followed by condensation.

Adsorption: Removal of organic liquids utilizing theiraffinity for activated carbon.

Extraction: Removal of liquids by exposing water to othermediums in which the liquids have a higher solubility.

Removal of Colloidal Particles

High Speed Centrifugation: Very high speed rotation leadingto accumulation of the particles towards the periphery of thecentrifuge.

Filtration through membranes with very small pores

Flocculation: Entrapping the coagulated colloidal particles insettling flocs of alumina.

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In order to understand purificationprocesses such as precipitation,

flocculation, we need to understandwater chemistry

∆Gr=cGc + dGd - aGA - bGb

aA + bB cC + dDk1

k-1

At equilibrium: ba

dc

rBA

DClnRTKlnRTGΔ

Example: H2O H+ + OH- 14w 10OHHK

Reaction Equilibrium

Acid/Base Reactions

Acids are H+ donors and bases are H+ receptors

Examples:

HCl H+ + Cl-

NaOH Na+ + OH-

HW

WHK a

CH3COOH H+ + CH3COO-

HW H+ + W-

pKa=4.75

Normality and Molarity and pH

Molarity is equal to the number of moles/liter andNormality is the number of equivalents/liter

Example: If we dissolve 10 g of NaCl in 500 ml of water whatis the molarity and normality of the solution?

Solution: 10g = 10/58 moles10g/500 ml = 20/58 mol/l =0.344 M = 0.344 N

pH=-log[H+] where [ ] denotes concentration in mol/l

Alkalinity is the sum of all titrable bases down to pH 4.5

Example: What is the pH of 0.01 M concentrated H2SO4 solution?Solution: Assuming complete dissociation, [H+]= 0.02 m/lpH=-log[.002]=2.7

If 100 mg of H2SO4 are added to a liter of water, what is the final pH?Solution: 100 mg = 100/98 mM = 100/98 x 10-3 m/lAssuming complete dissociation, [H+]= 2 x 100/98 x 10-3 m/lpH=-log[2 x 100/98 x 10-3]=2.699

Example: Add 100 mg of 100% pure sulfuric acid and 100 mgof Sodium hydroxide to a liter of water. What is the final pH?Mol wt of sulfuric acid = 98Mol wt of Sodium hydroxide = 40

Solution: mMoles of Sulfuric acid = 100/98 = 1.02mMoles of Sodium hydroxide = 100/40 = 2.5

Net mili-moles of Sodium hydoxide = 2.5 – 2(1.02) =0.46/liter

Assuming complete dissociation and neglecting hydroxyl ionsfrom water, [OH-]=0.46x10-3

[H+]=10-14/[OH-]=2.17x10-11

pH=10.6

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Example: If 15mg/l of HOCl is added to potable water fordisinfection and the final pH is 7.0, what % of HOCl isdissociated?Ka=2.9x10-8

Solution:

22.5%100/4.45ddissociatenot%

45.4

1

][][

][

][45.3][/10][

10

7

7

HOClOCl

OCl

OClOClKHOCl

HOCl

OCl

HOCl

OClHK

a

a

Precipitation

AaBb(s) aA+ + bB-

Ca3(PO4)2 (s) 3Ca2+ + 2PO43-

bas BAK pKs=-log Ks

If the product of the ionic concentrations exceed thecorresponding Ks, precipitation occurs.

Example

Substance Equilibrium Equation pKs

1 Aluminum Hydroxide Al(OH)3 Al3+ + 3 OH- 32.9

2 Calcium Carbonate CaCO3 Ca2+ + CO32- 8.32

3 Ferric Hydroxide Fe(OH)3 Fe3+ + 3 OH- 38.04

4 Ferric Phosphate FePO4 Fe3+ + PO43- 21.9

5 Magnesium Hydroxide Mg(OH)2 Mg2+ + 2 OH- 10.74

Coagulation and Flocculation

Coagulation is the process by which small colloidal particlesapproach each other and adhere to form larger flocs so thatthey could be subsequently separated in a settler.

Most colloids are negatively charged so coagulants such as Al 3+ orFe3+ need to be added to neutralize the charge resulting inaggregation of colloids.

Alum (Al2(SO4)3.14H2O) when added to water releases Al ionthat destabilizes colloids and also reacts with water to form alarge complex such as [Al(OH)20.28H2O]4+ that enmeshescolloids as it settles.

Adsorption

Use of activated carbon or silica to adsorb taste and odor causingsubstances and to remove synthetic and volatile organicchemicals.

Disinfection

Killing the microorganisms by UV Light,Chlorination, Ozone(O3) or Chlorine Dioxide (ClO2).

Jar Tests

Determine the optimum alum dosage and pH for turbidity removal

•In one set of Jars keep constant pH and vary alumdosage from 10-60 mg/l.

•In another set, keep alum dosage constant and vary pHby adding dilute sulfuric acid from 5 to 7.5.

Measure the change in turbidity in both sets todetermine the optimal pH and alum dosage.

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Thermodynamics vs. Kinetics

Each physical process progresses at rates thatdepend on a number of variables and eventuallyachieves thermodynamic equilibrium.

Equilibrium vs. Steady State

Steady state implies no change in state variables with time.Steady state in a closed system implies equilibrium

Reaction Rates

• Zeroth order: -rA = k

• First order: -rA = kcA

• Second order: -rA = kcA2

SDWA (1974) – federalmandate to protect publichealth~ 40% of the world’spopulation does not haveadequate access to safe water

Potable Water TreatmentWhat do we need to remove? All impurities that could cause death,

disease, or adverse health effects?

Examples Colloidal, dissolved, and suspended material

Pathogens, carcinogens, tastes, odors, color(atrazine standard)!

How are we going to purify H2O?

Typical unit process train for purifying surface water (e.g.,lakes, reservoirs, rivers).

Combination of chemical and physical treatment, althoughincluding biology is becoming more popular.

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Mix the chemicals such as Alum(Al2(SO4)3) or FeCl3 for coagulation; canalso add Cl2, PAC, KMnO4.Blender – thoroughly andinstantaneously mix the chemicals

Purpose for Rapid Mix Basins

Source Dose (mg/L)Reservoir 16Lake 22River 29

Alum Dose as a function of WaterSource

Design VariablesResidence timeVolume (shape, dimensions)Velocity gradient, GType of Impeller and PowerBaffle design

Rapid Mix Basin

Full-scale vs. Pilot Scale The design equations we will discuss are

intended for full-scale

You must address their applicability forpilot-scale!

Basic equation: T = V/Q

Choose Detention time, t0 , between 10 and 30 secondsV < 8 m3; V=t0 Q where Q is the given flowrateLiquid depth: 0.5 - 1.1 times basin diameter or widthG(t0), velocity gradient of 600 - 1000 s-1

Turbine or Axial Flow Impeller, P=µVG2

Impeller diameter, Di 0.3 - 0.5 times the tank diameteror widthBaffles extend 10% of tank diameter or width

Design of a Rapid Mix Basin

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to

0.5 (in-line blending)10 – 2020 – 3030 – 40> 40

G

3,5001,000900800700

G values for Rapid Mixing Bring small precipitates and colloidal particlesinto contact so that they collide, stick andgrow to a size that readily settles.

Create big settling flocs such as[Al(OH)20.28H2O]4+ that enmesh colloids as itsettles.

Purpose for Coagulation andFlocculation

Add a coagulant such as Al2(OH)3 – alum or FeCl3– ferric chloride that provides positively chargedions to neutralize the negative charge of colloidalparticles resulting in aggregationDesired properties of a coagulant

Trivalent cationNon-toxicInsoluble in the neutral pH range (i.e.,coagulant must precipitate out)

How do Coagulation andFlocculation work When added to water, it dissociates releasing

an aluminum ion that develops water clustersaround itThis large particle precipitate (e.g., Al-OH-H2O, floc) settles enmeshing colloids with itOptimum pH range is b/w 5.5 – 6.5

Coagulant aids: pH adjusters, activatedsilica, clay, and polymers

Goal is to create larger, denser floc

How Does Alum Work?

Design VariablesResidence timeVolume (shape, dimensions)Velocity gradient, G (large enough for mixingbut avoid shearingTapered Flocculation 3 G zones

G decreases from beginning to end

Average G is the design value

Baffle designImpeller design

Flocculation

Type

Low turbidity, colorremoval, coagulation

High turbidity, solidsremoval, coagulation

Softening, 10% solids

Softening, 39% solids

G (s-1) Gt0 (unitless)

20 – 70 60000-200000

50 – 150 90000-180000

130 – 200 200000-250000

150 – 300 390000-400000

G and t0 Values for Flocculation

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Choose Velocity gradient, G (i.e., what kind of H2O?)

Get t0(G)

Get V=t0 Q where Q is the given flowrate

Liquid depth: 0.5 - 1.1 times basin diameter or width

Turbine or Axial Flow Impeller, P=µVG2

Impeller diameter, Di 0.2 - 0.5 times the tank diameteror width (Max impeller diameter = 3 m)

Rules of thumb:minimum v = Length/ t 0 0.5 – 1.5 ft/min

t = 30 min

Design of Flocculator

Removal of particulate matter, chemicalfloc, and precipitates from suspensionthrough gravity settling

Types of Settlers/ClarifiersUpflow ClarifierHorizontal flow Clarifier

Purpose of Sedimentation Upflow Clarifier

Design Variables: Length (L), Height (h) and Width (W)Perforated Baffle Design to distribute the incoming fluidEffluent Weir Design to remove the effluentSlope of the Sludge zone: 1 to 10% slope

1% for mechanically cleaned5 to 10% for manually cleaned

Horizontal Flow Clarifier

Inlet – evenly distribute the flow acrossx-sectional area

~ 25% of tank in theory

Settling – gravity settlingOutlet – remove effluent

Sedimentation Basin Zones

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Stoke’s Law for spherical particles

vs = g (s - ) d2

18 µg = acceleration due to gravity, m/s2

s = density of particles, kg/m3

= density of fluid, kg/m3

d = diameter of sphereµ = dynamic viscosity, Pa•s

Weight

Buoyancy

Drag

Why do particles Settle?

Problems in using Stokes EquationParticles are not spherical (Use equivalent diameter)Particle diameter changes with settling (Use theminimum diameter : Conservative Design)

Settling properties of particles are oftencategorized into one of three classes:

Type I - particles settle discretely @ a constantsettling velocity (i.e., no flocculation)Type II - particles flocculate duringsedimentation (since they flocculate their size isconstantly changing (i.e., vs is )Type III - particles settle as a mass (i.e., limesoftening)

Obtain vs experimentally

Treatment vo (m3/d•m2)

Alum or iron floc 14.5 - 22.3Lime softening floc 22.3 - 82.1

Typical Sedimentation Tank vo

vl

vs

l

h

Condition for Settling: 0s

ssls

vA

Q

V

Qhv

Q

V

v

h

v

l

v

h

V

Ql

t

lv

0l

0s vv

Percentage Removal, P =0

s

v

v100

Design of Horizontal Flow Clarifier

Q is the given flowrate

Calculate vs (d)

Choose v0 = 0.8 vs

Obtain As using v0 = Q/As

Choose L/W~5; As = l W; Use these to obtain l,W

Choose horizontal velocity, vl 0.5 ft/min

Obtain Detention time, t0 = l/vl

Obtain V using t0 = V/Q

Obtain h using V = l W h

Design of Horizontal Flow Clarifier

Removal of flocs that do not settle in theClarifier due to small size.Reduce settled water turbidity of ~ 5 TU tobelow 0.3 TU.

Most common design Granular-media gravityfilter

Purpose of Filtration

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Loading RateThe flow rate of water applied per unit area of filter

va = Q / As

va = loading rate, gpm/ft2

Q = flow rate onto filter, gal/dayAs = filter surface area, ft2

Design rates are between 2 – 10 gpm/ft25 gpm/ft2 is most common (slow, rapid, and high-ratefilters)

Choose va and calculate As for a given Q.

Filter Design

Filter Depth

Typical depth ~ 9 feetFilter Bed Breakdown

Granular media ~ 2 feetSand ~ 6 feetCourse gravel ~ 1 foot

Underdrain

Filter Design

Head loss will increase overtime as filtercollects impuritiesFilter is back-washed (reverse flow) toremove impurities

~ 15 gpm/ft2 for ~ 10 – 15 minTypically, filters backwash every 24hours or when headloss is between 6 –9 feet

Head loss

Choose media to promote straining,flocculation, and sedimentationGrain size – desired to retain large quantitiesof floc, but prevent passing of small particlesDual-media filter

AnthraciteSand

Media specified by effective size anduniformity coefficient

Filter Media

Definition: the 10 percent diameter,which means that 10% of the filtergrains by weight are smaller than thediameterSuggested range –

0.35 – 0.55 mm for sand

Effective Size (E)

Definition: ratio of the 60-percentilediameter to the 10-percentile diameterTypically < 1.7 for sand and anthracite

Uniformity Coefficient (U)

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Determine the size distribution of asample via a sieve analysisPlot the sieve size versus thepercentage of material retained on eachsieve (log-probability paper)

How does one determine E and U?

Modelling Contaminant Transport

Overview:

Hydrologic Cycle

Definitions

Groundwater Flow ProcessesHeads and Gradients, Darcy’s Law, Hydraulic Conductivity,Aquifer Properties, More Definitions

Solving Groundwater Flow EquationsGoverning Equations, Flow Nets, Finite-Difference Solutions

Model SpecificationsPhysical Dimensions, Grid Size, Hydraulic Properties, InitialConditions, Boundary Conditions

Hydrologic Cycle

Groundwater: Underground water in formations where void space is completelyfilled with water

Hydrologic Cycle

Saline water inoceans: 97.2%

Ice caps and glaciers: 2.14%

Surfacewater: 0.009%Soil moisture: 0.005%Atmosphere: 0.001%

Groundwater: 0.61%

Distribution of World’sWater Supply

(Source: Heath, R.C., 1984. “Basic Ground-Water Hydrology,” United States Geological Survey Water-Supply Paper2220)

Underground Formations

UnconsolidatedRocks

ConsolidatedRocks


Definitions


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DefinitionsAquifer: A geologic formation that contains water

and permits significant amounts of water tomove through it under ordinary field conditions.

Aquitard: A saturated but poorly permeablestratum that impedes groundwater movement.Does not yield water freely to wells, but maytransmit appreciable water between adjacentaquifers.

Definitions

(Source: Todd,1980)

Definitions

Unconfined Aquifers water table is at atmospheric pressure water table takes on form depending on recharge,

discharge, pumpage from wells, and permeability

Confined Aquifers groundwater confined under pressure greater than

atmospheric by overlying relatively impermeable strata

Leaky Aquifer neither completely confined nor completely unconfined permeable stratum is overlain by a semipervious aquitard

Definitions

i) Porosity (n)

3

3

0.3 mn 0.30

1.0 m

v

t

Volume of voids V

Total volume V


-Slide 101-

ii) Specific Yield &Specific Retention

d ry r

t t

y r

V VS S

V V

n S S

Definitions

(Source: Heath, R.C., 1984. “Basic Ground-WaterHydrology,” United States Geological Survey Water-Supply Paper 2220)

Groundwater Flow Processes

Head & Gradient

t pelevation head

pressure head

ph z z h

g

L100 m 15 m 98 m 18 mh 85 m 80 m 5 m

iL 780 m 780 m 780 m

(Source: Heath, R.C., 1984. “Basic Ground-WaterHydrology,” United States Geological Survey Water-Supply Paper 2220)

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Darcy’s Law:

AA

1) Q A

13) Q

L

a bh h hQ KA KA KAi

L L

a b2) Q h h

a bh hQq K

A L

nAnA

Where, q = Darcy velocity or Darcy flux (L/T), vp = average pore velocity (L/T), K = hydraulicconductivity (L/T), n = soil porosity, dh/dl = hydraulic conductivity (L/L)

dhq K

dl p

K dhv

n dl

Not applicable for: High velocity flow (Reynolds number >1) in some gravels and in secondary pores (eg, Karst).

3 1

2

md mQdl mK

Adh dm m

AquiferProperties–HydraulicConductivity

Aquifer Properties - Transmissivity

Soils

hQ KA

L

Aquifers

h h hQ KA K bw Kb w

L L L

Transmissivity, T = Kb (m2d-1)

T represents flow per unit widthof the aquifer under unit gradient.

K represents flow per unit area ofthe sample under unit gradient.

Hydraulic Conductivity, K (md-1)

-Slide106-

Aquifer Properties -- Homogeneity

Heterogeneity K = f(Location)

Homogeneous

Heterogeneous

Aquifer Properties – Isotropy

Anisotropy K = f(Direction)

Isotropic AnisotropicAquifer Properties – Storage Coefficient

3 3

32

volume of water m mS

unit area unit head change mm m

Storage coefficient (or storativity), S [dimensionless], is the volume of waterproduced from a confined aquifer per unit aquifer area per unit decline in thepotentiometric surface.

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Storativity/Specific Yield – Unconfined Aquifer

Lowering of water table results in: Draining of water from the pores Lowering of the unsaturated water distribution in soils above the water table Water drained equals the volume of water corresponding to the shaded area in

the above figure Typical values of 0.1 – 0.3 Same as specific yield

Storativity/Storage Coefficient – Confined Aquifer

In a confined aquifer with a porosity of 0.2 , expansion of water alone releases 3x10-7 m3

of water per m3 of aquifer per meter of decline in head. Thus, if only the expansion ofwater is considered, the storage coefficient of a 100 m thick aquifer is 3 x 10-5. Thestorage coefficient of most confined aquifers is 10-5 to 10-3. The difference in the valuesis attributed to compression of the aquifer.

Lowering of potentiometric surface results in: Decrease in fluid pressure and expansion of fluid slightly A part of the weight of the overlying material is transferred from the fluid to

the solid - compressing the aquifer material (similar to squeezing a sponge)

Unconfined

Governing Equations

For Homogeneous andIsotropic Conditions: y

hhy hx

K K Sx y t

2 2y

2 2

Sh h h

x y K t

Diffusion Equation used for unsteady (or transient) flows

2 2

2 2

h h0

x y

Laplace Equation used for steady (or constant) flows

2 2y

2 2

S bh h h S h

x y Kb t T t

Valid for Unconfined+ Confined Aquifers

(S = Storage Coefficient or Storativity)

water pollution and control

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