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GROUNDWATER QUALITY
Riddhi Singh Lecture 10
Email: riddhi@civil.iitb.ac.in
CE 626
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Today we will learn about…
• Water quality issues in groundwater
• Solute transport processes
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GROUNDWATER QUALITY
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For a given application, both quantity and quality of available
water are critical
4https://www.telegraph.co.uk/news/worldnews/asia/india/9705777/Delhis-Yamuna-River-to-be-revived-with-help-from-London.html
Two thirds of Delhi's 600 million gallons of sewage is dumped in the river
every day, but few of the capital's water treatment plants are in operation,
which means much of the waste flows slowly and fragrantly through the
city.
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Saha, D., Singh, B., Srivastava, S., Dwivedi, S. and Mukherjee, R., 2014. Concept note on geogenic
contamination of ground water in India (with a special note on Nitrate). Central Ground Water Board
(CGWB), Bujal Bhawan, NH-IV, Faridabad, Haryana.
Water quality is expressed by its:
• Physical state
• Chemical composition
– organic/ inorganic composition
– Isotopic composition
• Biological composition
• Radiological composition
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Quality requirements are determined by purpose: drinking,
industrial or irrigation.
Geogenic (naturally occurring) vs. Anthropogenic (due to human
activities)
All groundwater contains salts in solution that are derived
from the location and past movement of water.
• Source of dissolved solids: aquifer gases, minerals, and salts
• Bicarbonate: carbon dioxide released by organic
decomposition in soils
• Salinity: varies with specific surface area of aquifer materials,
solubility of minerals, and contact time, increases with depth
(why?)
• Groundwater quality is influenced by: local geology, land use,
climatic conditions such as pattern and frequency of rainfall,
anthropogenic activities such as:
– use of fertilizers and pesticides,
– disposal of domestic sewage,
– industrial effluents, etc.
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Geochemical cycle of
surface water and
groundwater from USGS
water supply paper. From:
Todd.
• Precipitation contains only
small amounts of dissolved
mineral matter
• On earth, water reacts with
minerals in soil and rocks
• Dissolution depends upon
rock type and pH and
redox potential (Eh) of
water
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10
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12
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Chemical analysis
• Concentration by weight (mg/l) or chemical equivalence (meq/l)
– (Concentration in mg/l)/combining weight, combining weight =
formula weight/charge
• Total dissolved solids (electrical conductance)
– Specific electrical conductance defines the conductance of a
cubic centimetre of water at a standard temperature (25 ֯C),
measured in microsiemens/cm (μS/cm)
• Hardness: divalent metallic cations (Mg2+, Ca2+), react with soap to
form precipitates, not satisfactory for household purposes.
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Presenting water quality data
15Vertical bar graphs.
Vector diagrams
Presenting water quality data
16Trilinear diagramCircular diagram
Physical and biological analysis
• Physical analysis:
– Temperature
– Color: due to mineral or organic matter
– Turbidity: measure of suspended and colloidal matter
(clay, silt, organic matter, microscopic organisms)
– Taste and odour
• Biological analysis: coliform group bacteria, reported as
most probable number (MPN) of coliform group
organisms in a given volume of water
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Groundwater chemistry: major controls
• Presence of carbon dioxide in the unsaturated zone
• Dissolution of calcite (CaCO3) and dolomite (CaMg(CO3)2), precipitation of calcite
• Cation exchange
• Oxidation of pyrite (FeS2) and organic matter
• Reduction of oxygen, nitrate (NO3−), sulphate (SO4
2−) with
production of sulphide (S2−)
• Reductive production of methane (CH4)
• Dissolution of gypsum (CaSO4·2H2O), anhydrite (CaSO4) and halite (NaCl)
• Incongruent dissolution of primary silicates (SiO44−) with formation
of clays
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Sources of groundwater contamination: septic tanks
and cesspool
19Images: http://www.simplifydiy.com/plumbing-and-heating/mains-water-systems/septic-tanks (right)
https://en.wikipedia.org/wiki/Septic_tank (left)
Septic tank
Cesspool
Landfills
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A plume of groundwater contaminated with high sulphate leaching from a
fly ash landfill located below the water table. From: Fetter
SOLUTE TRANSPORT PROCESSES
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Solute transport processes
• Diffusion: process by which both ionic and molecular species
dissolved in water move from areas of higher concentration to
areas of lower concentration
• Advection: is the process by which moving groundwater carries
with it dissolved solutes.
• Dispersion (additional process due to porous media): acts to
dilute the solute and lower its concentration
• Retardation: chemical and physical processes that slow down
solute movement so that it does not move as fast as the advection
rate would indicate (due to adsorption process)
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Diffusion is described by Fick’s law
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dCF D
dx= −
F: mass flux of solute per unit area per unit time
D: diffusion coefficient (area/time)
C: solute concentration (mass/volume)
dC/dx: concentration gradient (mass/volume/distance)
Negative sign indicates that the movement is from greater to lesser
concentrations.
D ranges from 1x10-9 to 2x10-9 m2/s for major cations and anions in water.
If concentration change with time, Fick’s 2nd law is applied:
In porous media , diffusion cannot proceed as fast as it can in water, because
ions follow longer pathways and are blocked by mineral grains. To take this into
account, an effective diffusion coefficient is used:
w is experimentally determined, ranges from 0.01 to 0.5
2
2
C CD
t x
=
*D wD=
Advection is described by Darcy’s law
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x
e
K dhv
dl=
vx: average linear
velocity
K: hydraulic conductivity
ηe: effective porosity
dh/dl: hydraulic gradient
Contaminants that are advecting are traveling at the same rate as the
average linear velocity of groundwater.
Dispersion: mechanical
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m L xD a v=
Mixing that occurs along the streamline of fluid flow is called longitudinal dispersion.
Dispersion that occurs normal to the pathway of the fluid flow is lateral dispersion.
Causes: (1) faster motion in centre of pores, (2) longer pathways for some fluid, (3) fluid travels faster in larger pores
aL: dynamic dispersivity
Longitudinal dispersion Lateral dispersion
Hydrodynamic dispersion
• Cannot separate molecular diffusion and mechanical dispersivity in
flowing groundwater
• Longitudinal coefficient of hydrodynamic dispersion (DL) accounts
for both mechanical mixing and diffusion
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*L L xD a v D= +
Breakthrough curve for a solute with 1% saline solution. Initially
distilled water is passed through a tube filled with sand.
One dimensional equation for hydrodynamic dispersion
The concentration ‘C’ at some distance ‘l’ from the source at concentration ‘Co’ at
time ‘t’ is given by Ogata (1970) as:
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2
2L x
C C CD v
x x t
− =
exp2 2 2
o x x x
LL L
C L v t v L L v tC erfc erfc
DD t D t
− += +
C: solute concentration (mg/L)
Co: initial solute concentration (mg/L)\
L: flow path (ft or m)
vx: average linear groundwater velocity (ft/day or m/day)
t is the time since the release of the solute (day)
DL: is the longitudinal dispersion coefficient (ft2/s or m2/s)
Erfc: complementary error function
( )22
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2 2
t
x
normal
erfc x e dt
xCDF erf
−=
− = +
( )
( ) ( )
2
0
2
1
xterf x e dt
erfc x erf x
−=
= −
Features of the solution to the advection dispersion
equation
• Centre of mass of the solute travels at the same rate as average linear
groundwater velocity
• Hydrodynamic dispersion causes the solute to spread out both ahead and behind
the centre of mass in a pattern that follows a normal distribution
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Transport and spreading of a solute slug with time due to advection and dispersion. A
slug of solute was injected at x=0+a at time to with resulting concentration Co.
Groundwater flows to the right. From: Fetter.
A contaminant plume from a continuous point source (Fetter)
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Contaminant slug from a one-time point source.
Density of dots indicates solute concentration. From: Fetter.
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