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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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.