Lecture 22 to 23

Introduction to GW contamination and

Solute partitioning

Sources of GW contamination

• What do we mean by a contaminant

• Quality impacted by– Natural processes– Runoff from agricultural & urban watersheds– Waste disposal practices– Accidental spills and leaks

Groundwater contaminants

The contaminants of concern can be :

• Organic chemicals• Metals• Radionuclides• Inorganic chemicals (cl, so4m na..)

Dealing with the different contaminants will depend on distribution and behavior (characteristics)

Some Characteristics

• Mobility – Lead and other metals are immobile (soil

contaminants)

• Time since release

• Solubility in water– Aqueous or non-aqueous phase– LNAPL and DNAPL

How humans are affected

• Soil contaminants may reach through skin or breathing vapors (children)

• Dissolved contaminants can reach drinking water for humans or food chain

• Significant pollution of water resources

Nature of Groundwater Contamination

• Most of the time late discovery (hidden nature)

• Clean up is difficult, requires long time, high cost (heterogeneity)

• Often problem is made worse

Mechanisms of mass transport

• Advection: movement of contaminants with flowing water

• Diffusion: movement of contaminants due to concentration gradients

• Mechanical dispersion: movement of contaminants due to the complex nature of flow in porous media

• Hydrodynamic dispersion combines the last two

Mathematical expressions for solute mass flux

cqJ wadvection

^^

cDJ wdiffusive

^

*^

xc

vaJw

ldispersive

directionallongitudin)(

Combined flux

xc

Dxc

qaJww

ldispersionicHydrodynam

*

xc

DJw

dispersionicHydrodynam

Often the two processes are combined using a combined coefficentD called the hydrodynamic dispersion coefficient

xc

DcqJw

w

If we include advection then:

Mass balance equation

Rate of mass accumulation in C.V. = - net rate of mass flux out + sourceConsider saturated porous media with dissolved conservative solute With steady 1-d flow in x-direction

xAreacM w )(J in J out

Δ x xxJ

JJ ininout

xxJ

tM

Mass balance equation cnt

xAreaxAreaxJ

tcw

xc

DcqJw

w

xcDx

cvt

cwww

2

2

For 1-d dissolved solute transport the Advective-dispersive equation is :

General form of the advection dispersion equation for solute transport in

saturated porous media

Homogeneous steady uniform velocity and constant dispersion coefficientsThe equation becomes:

Multiphase contamination in porous media

Solute partitioning

terminology

• A solute is a chemical substance dissolved in a given solution (i.e. water, air, OIL) cw, ca, co

• a phase is a separate, homogeneous part of a heterogeneous system (w,a,o,soil)

• A physical interface exists between each of the phases in contact, which is a dividing surface between the phases that compounds can migrate across.

Study transport and fate of contaminants

• deal with a multiphase system consisting of water, air, and soil and in some cases OIL

• Individual chemical constituents partition themselves among the various phases according to thermodynamic equilibrium principles and mass transfer kinetic factors

• models are needed to describe the mass transport processes, and solute partitioning among the various phases that are present must be quantified

Continued

• Most petroleum products are mixtures of many individual constituents. The physical characteristics of a mixture may be estimated from the characteristics of the individual constituents that form the mixture

• A petroleum hydrocarbon consists of more than one hundred chemical constituents. These constituents may dissolve in or attach to any or all of the phases present

• When considering the transport of constituent within the multiphase system how the concentrations of constituents within the various phases relate to each other

Assumption

• The local equilibrium assumption assumes that the problem is separable – even though a solute can exist in anyone of four

phases, at any point where two of these phases touch each other, the equilibrium set up at that interface is assumed to hold independent of the presence of the other phases

– the presence of NAPL does not affect the water-soil partitioning properties of a medium; the total amount of material just gets shared

– If the constituent of interest is lost from one phase, then the other phases serve as a contaminant reservoir that supplies the phase that is losing mass while maintaining equilibrium partitioning

Partitioning in a multiphase system

Partitioning between air and water phase

• Henry's law states that water-vapor partitioning is described by a linear relashon under equilibrium conditions. This relationship is

ca = KH cw

Where KH is the Henry's law constant

KH =K’H /RT

Partitioning between solid and water phase

• Assuming linearsorption isotherm under equilibrium conditions. This relationship is

cs = Kd cw

Where Kd (volume/mass) is the distribution coefficient which depends on organic carbon in soil and the properties of the organic compound.

High for hydrophobic organics

Partitioning between free product and water phase

• Raoult’s law states that water concentration is equal to the constituent solubility (pure) multiplied by the mole fraction of the constituent This relationship is

c0 = Ko cw = Ko Sk Xk

Where Ko is NLAPL water partitioning coefficient

ktconstituenforS

c

Kk

N

j j

oj

k

o

1

Partitioning between free product and water phase

Where:ώ = molecular weightS = solubilityC = concentration in OIL

Bulk concentration for a constituent is the sum of the four phases

m = mass per bulk volume for a constituent say kBw = bulk water partitioning coefficient

General hydrocarbon contamination

ccccm sbohohaaww

After a leak there will be a contamination zone. This zone will in general contain four phases (air, water, soil, NAPL)The NAPL consists of many constituents (see table for gasoline) thatCan dissolve or attach to the four phases

Are the constituent concentrations in the various phases related?

We may assume linear equilibrium

P.C. Characteristics of fluid mixtures

Ideal mixture

Mole fraction

Molecular weight of mixture

Molar volume

Partial molar volume

Mixture density

Problems

Short note about half-life

In this example we calculated the time required ro reduce the concentration to half and called it half life.This is used in dealing with first order reactions governed by an equation like:

CtdCd

)2ln(

2/1T

It can be shown to be:

Compositional model

Here we take care of changes in aqueous concentration due to the changein composition due to the loss of mass leached

combining

With the following relations

Example-Compositional model

Solubility TCE = 1.1 g/l, PCE = 0.15 g/l, CTC= 0.825 g/l

0 100 200 300 400

tim e, days

0

0.2

0.4

0.6

aq

ue

ou

s co

nce

ntr

atio

n, g

/l

TC E

P C E

C TC

0 100 200 300 400

tim e, days

0

0.2

0.4

0.6

aq

ue

ou

s co

nce

ntr

atio

n, g

/l

TC E

P C E

C TC

0 100 200 300 400

tim e, days

0

0.02

0.04

0.06

0.08

volu

me

re

ma

inin

g

0 100 200 300 400

tim e, days

0

10

20

30

40

50

ma

ss r

em

ain

ing

TC E

PC E

C TC

0 100 200 300 400

tim e, days

0

400

800

1200

1600

con

cen

tra

tion

in L

NA

PL

, g/l

TC E

PC E

C TC