1. Introduction to GW contamination and
Lecture 22 to 23
Introduction to GW contamination and
Solute partitioning

2. 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

3. 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)

4. Some Characteristics Mobility Time since release Solubility in water
Lead and other metals are immobile (soil contaminants)
Time since release
Solubility in water
Aqueous or non-aqueous phase
LNAPL and DNAPL

5. 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

6. 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

7. 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

8. Mathematical expressions for solute mass flux

9. Combined flux
Often the two processes are combined using a combined coefficent
D called the hydrodynamic dispersion coefficient
If we include advection then:

10. Mass balance equation J in J out Δ x
Rate of mass accumulation in C.V. = - net rate of mass flux out + source
Consider saturated porous media with dissolved conservative solute
With steady 1-d flow in x-direction
J in
J out
Δ x

11. Mass balance equation cnt
For 1-d dissolved solute transport the Advective-dispersive equation is :

12. General form of the advection dispersion equation for solute transport in saturated porous media
Homogeneous steady uniform velocity and constant dispersion coefficients
The equation becomes:

14. Multiphase contamination in porous media

15. Solute partitioning

16. 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.

17. 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

18. 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

19. 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

20. Partitioning in a multiphase system

21. 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

22. 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

23. 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

24. Partitioning between free product and water phase
Where:
ώ = molecular weight
S = solubility
C = concentration in OIL

25. Bulk concentration for a constituent is the sum of the four phases
m = mass per bulk volume for a constituent say k
Bw = bulk water partitioning coefficient

26. General hydrocarbon contamination
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) that
Can dissolve or attach to the four phases
Are the constituent concentrations in the various phases related?
We may assume linear equilibrium

27. P.C. Characteristics of fluid mixtures
Ideal mixture

28. Mole fraction
Molecular weight of mixture
Molar volume
Partial molar volume
Mixture density

29. Problems

38. 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:
It can be shown to be:

39. Compositional model
Here we take care of changes in aqueous concentration due to the change
in composition due to the loss of mass leached
combining
With the following relations

40. Example-Compositional model
Solubility
TCE = 1.1 g/l, PCE = 0.15 g/l, CTC= g/l