lecture 22 to 23 introduction to gw contamination and solute partitioning
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Lecture 22 to 23
Introduction to GW contamination and
Solute partitioning
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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
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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)
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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
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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
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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
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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
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Mathematical expressions for solute mass flux
cqJ wadvection
^^
cDJ wdiffusive
^
*^
xc
vaJw
ldispersive
directionallongitudin)(
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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:
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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
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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 :
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General form of the advection dispersion equation for solute transport in
saturated porous media
Homogeneous steady uniform velocity and constant dispersion coefficientsThe equation becomes:
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Multiphase contamination in porous media
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Solute partitioning
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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.
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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
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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
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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
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Partitioning in a multiphase system
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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
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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
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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
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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
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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
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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
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P.C. Characteristics of fluid mixtures
Ideal mixture
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Mole fraction
Molecular weight of mixture
Molar volume
Partial molar volume
Mixture density
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Problems
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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:
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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
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Example-Compositional model
Solubility TCE = 1.1 g/l, PCE = 0.15 g/l, CTC= 0.825 g/l
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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
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0 100 200 300 400
tim e, days
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TC E
P C E
C TC
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0 100 200 300 400
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0 100 200 300 400
tim e, days
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ss r
em
ain
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TC E
PC E
C TC
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0 100 200 300 400
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in L
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, g/l
TC E
PC E
C TC