KKA 4106 Fate and Transport of Contaminants Lecture 4

1
KKA 4106 Fate and Transport of
ContaminantsLecture 4

• Dr Robiah Yunus
• Dept. of Chemical and Environmental Eng.
• Universiti Putra Malaysia

2
FATE AND TRANSPORT OF CONTAMINANTS

• It deals with key release mechanisms, contaminate
  transport and the fate of contaminants in the
  environment.
• Contaminant Release
• Three phases liquid, solid and/or gas
• Air Emission
• Water release
• Transport of Contaminants in the Subsurface
• Fate of Contaminants in the Subsurface

3
1. CONTAMINANT RELEASE

• Air Emission
• Water release

4
AIR EMISSIONS

• Types of Air Emmsions
• Point Landfill vent or incinerator stack
• Line Dust from road and/or vehicle
• Area Lagoon or construction site
• Volume Building with open windows
• Puff Volatile emissions from an accidental spill

5
AIR EMISSIONS

• Volatilization
• Transfer of a chemical substance from a liquid to
  a gaseous phase. Predominant source of
  atmospheric emissions at most uncontrolled hw
  sites.
• Function of temperature, vapor pressure,
  difference in concentrations.
• Can be measured or modeled using Fick's Law.
• Usually insignificant at undisturbed, inactive
  sites. Remediation activities, however, may
  substantially increase emissions perhaps several
  thousand fold.

6
AIR EMISSIONS

• Particulate Emissions
• Originate from materials handling and surface
  areas.
• The Soil Conservation Service has a graphical
  method for predicting annual average wind and
  erosion and resulting soil loss.
• Transportable and suspendible amount is that
  portion of the total soil loss represented by
  particles 100 m in diameter or smaller.
• Inhalable size 10 m in diameter or smaller.

7
AIR EMISSIONS

• Particulate Emissions
• The greatest source of fugitive dust is
  remediation involving soil handling.
• E k(.0032)
• The emission factor, E, may be multiplied by the
  tons of material handled to achieve an estimate
  of total emissions.

8
AIR EMISSIONS

• Example
• Given A site to be remediated has the following
  soil characteristics
• The average particle size lt 15mm
• The mean wind speed in the worst season in the
  Spring is 22 mph
• The moisture content is only 60 can be raised to
  85 by judicious spraying. Overspraying is not a
  good idea as a hw leachate may be formed.
• The soil that needs to be moved occupies a vol.
  of 346' x 123' x 56' of which 1000 tons can be
  move per day.
• The average soil density is 75 lb/ft3
• Find Estimate the particulate emissions on a
  daily and total site basis.

9
AIR EMISSIONS

• Example
• Given A site to be remediated has the following
  soil characteristics
• The average particle size lt 15mm
• The mean wind speed in the worst season in the
  Spring is 22 mph
• The moisture content is only 60 can be raised to
  85 by judicious spraying. Overspraying is not a
  good idea as a hw leachate may be formed.
• The soil that needs to be moved occupies a vol.
  of 346' x 123' x 56' of which 1000 tons can be
  move per day.
• The average soil density is 75 lb/ft3
• Find Estimate the particulate emissions on a
  daily and total site basis.

10
AIR EMISSIONS

• Solution
• 1.) Particulate Emission per Day
• From p. 156 for particle size lt 15 mm, k.48
• E k(.0032) .48(.0032) .001536 x 6.86/190.4
• E 5.53 x 10-5 lb of particulate/ton of soil
  moved
• Daily emissions Daily tons of soil moved x E
• Daily emissions 1000 tons/day x 5.53 x 10-5 lb
  of particulate/ton of soil moved
• Daily emissions .055 lbs of particulates / day

11
AIR EMISSIONS

• Solution
• 2) Emissions for Entire Site
• Total tonnage 346' x 123' x 56' x 75 lb/ft3 x 1
  ton/2000lbs
• Total tonnage 89,372 tons
• Total emissions Total tons of soil moved x E
• Total emissions 89,372 tons/day x 5.53 x 10-5
  lb of particulate/ton of soil moved
• Total emissions 4.94 lbs of particulates /
  total site

12
WATER RELEASES

• Controlled releases
• Virtually every industrial and commercial
  facility generates a wastewater and it not
  possible to clean the water to 100 standards.
• Landfill leachate.

13
2. TRANSPORT OF CONTAMINANTS IN THE SUBSURFACE

• Hydrologic Cycle
• Ground Water Flow
• Hydraulic Conductivity of Geologic Materials
• Flow in the Unsaturated Zone
• Contaminant Transport Mechanisms
• Real-World Contaminant Transport
• Flow and Transport Equations
• Ground Water Modeling

14
HYDROLOGIC CYCLE

• The hydrological cycle is the continuous
  circulation of water in the atmosphere, surface
  and subsurface.
• Waters reaching the saturated zone in the
  subsurface will flow from areas of high hydraulic
  head to areas of low hydraulic head.
• If the stratum readily permits this flow it is
  know as an aquifer and if it does not, it is an
  aquitard or confining layer. If the aquifer is
  free to move up and down, it is an unconfined
  aquifer. The stratum is homogeneous if the
  properties of the soil do not vary with location.
  
• An isotropic stratum does not vary with
  orientation or direction. An anisotropic stratum
  has a higher hydraulic conductivity in the
  horizontal direction and less in the vertical.
  Most soils are anisotropic and heterogeneous, but
  they are usually considered homogeneous.

15
GROUND WATER FLOW

• Darcy's Law
• Darcy's Law is the basis of our understanding of
  subsurface ground water flow
• Q kiA k hydraulic conductivity
• k 1x10-2 cm/sec for medium to fine sand
• The hydraulic gradient, i, describes the rate of
  change in which the head is lost as water flows
  through the porous materials
• i (h1 - h2)/l (hydraulic gradient)
• Darcy's Law is empirical
• Rearranging the first equation
• Q/A ki,
• Q/A specific discharge, Darcy flux, or Darcy
  velocity, v
• v ki
• Flow is in direction of declining hydraulic head
  requiring a negative sign. In differential form
• v -k dh/dl (dh/dl hydraulic conductivity)

16
HYDRAULIC CONDUCTIVITY OF GEOLOGIC MATERIALS

• Impermeable soil such as a tight clay may have a
  hydraulic conductivity of 1x10-9 cm/sec the most
  permeable material such as a clean gravel may be
  1x105 cm/sec.
• Two things should be remembered regarding
  hydraulic conductivity
• Order of magnitude refinement is appropriate
• The value given for a specific material type may
  be very different than the hydraulic conductivity
  for the entire formation.
• Transmissivity is the hydraulic conductivity
  times the thickness of the formation.
• T kt
• The specific yield is the volume of water that
  drains from the saturated soil pores as the water
  table drops. For unconfined conditions,
  storativity is synonymous with specific yield.

17
REAL-WORLD CONTAMINANT TRANSPORT

• Mechanisms that influence and change idealized
  flow predictions.
• Fractured Media Flow. Flow will occur in the
  fractures which behave like pipes as opposed to
  the bulk media.
• Heterogeneity. If the formation is not
  homogenous, the contaminant may transport a great
  deal more quickly through a highly conductive
  lens.
• Nonaqueous Phase Liquid (NAPL). Immiscible
  organic liquid compounds exist as a separate
  liquid phase in the subsurface.

18
CONTAMINANT TRANSPORT MECHANISMS

• Contaminants that are dissolved in water are
  solutes and the water is the solvent and the
  combination is the solution. As the water flows,
  the contaminants are transported with the water a
  process known as advection.
• As the water flows around the soil particles, it
  is mixed, a process known as mechanical
  dispersion. The result is dilution or reduction
  in the contaminant concentration.
• A one time introduction of pollutants is termed a
  slug and opposed to a continuous source. If the
  pollutant is introduced at a discrete location,
  it is known as a point source as opposed to a
  non-point source.

19
CONTAMINANT TRANSPORT MECHANISMS

• The distribution and extent of contaminants
  migrating in the subsurface is known as the
  plume..
• Contaminates, particularly the ionic and
  molecular constituents, will move from areas of
  high concentration to areas of low concentration
  and this process is termed diffusion.
• In general, advective transport and associated
  mechanical dispersion dominate the contaminant
  transport in formations of medium to high
  hydraulic conductivity. In formation of low
  hydraulic conductivity, including clay liners,
  diffusive transport is the controlling mechanism.

20
FLOW AND TRANSPORT EQUATIONS

• 1.) Laplace equation for isotropic flow in two
  dimensions
• 2h/x2 2h/y2 0
• May be solved by finite elements and finite
  difference mathematical methods.
• 2.) Flow nets.
• Flow nets. Flow nets are two-dimensional
  graphical representations of hydraulic head
  conditions in the subsurface. Lines of equal
  total hydraulic head are know as equipotential
  lines and the potential energy at any point on
  one of these lines is the same. Flow lines are
  perpendicular to the equipotentials and represent
  the average path a particle of water takes as it
  flows in the subsurface. Flow nets give a feel
  for the flow patterns and are alternate
  methodology, independent verification, to other
  methods.

21
FLOW AND TRANSPORT EQUATIONS

• 2) Flow nets
• Fundamental properties of flow nets
• The head difference between any pair of adjacent
  equipotentials is the same as any other pair.
• Flow lines intersect equipotentials at right
  angles.
• Figures enclosed by adjacent pairs of
  equipotentials and flow lines are essentially
  square.
• The spacing of equipotentials is inversely
  proportional to the hydraulic gradient and to the
  Darcy velocity.
• Every flow channel transmits the same quantity of
  seepage.
• The impervious boundary is a flow line the free
  water boundary is an equipotential. .

22
FLOW AND TRANSPORT EQUATIONS

• 2) Flow nets

23
FLOW AND TRANSPORT EQUATIONS

• 3) Finite Difference Method
• A two-dimensional grid is constructed in which
  each nodal point is modeled to represent the
  average head for the area enclosed by the square.
  Also it is assumed that the head at a particular
  node is the average head at the four surrounding
  nodes.
• Boundaries conditions must be established either
  as constant head or no-flow boundaries.
• Numerical head values are assigned.
• The heads are recalculated using the finite
  difference equation using the trial and error
  relaxation technique until a specified tolerance
  is achieved.

24
GROUND WATER MODELING

• Once the model is created, "what-if" scenarios
  may be easily evaluated.
• Most models are mathematical, although analog,
  electrical etc. are possible. Once the numerical
  model is selected, it must be digitized so that
  the data can be fed into a computer. Calibration
  or adjusting the model by comparing the model
  data to real data comes next.
• Once calibrated, the model can be used to
• Guide the placement of monitoring wells.
• Predict contaminant concentrations.
• Assess remedial alternatives.
• Predict residual contaminants

25
3. FATE OF CONTAMINANTS IN THE SUBSURFACE

• Three processes
• Retardation Processes
• Attenuation Processes
• Mobility Enhancement
• Soil is a mixture of different inorganic and
  organic materials. The predominant inorganic
  elements are silicon, aluminum and iron.
  Hazardous material is naturally occurring such as
  arsenic and cadmium. which are generally
  insoluble.
• Organic matter consists of decomposed plant
  matter known as humus. Typical organic content
  is .2-3. The organic matter acts as a stabilizer
  to bind inorganic particles together as
  aggregates. Much of the retardation and
  attenuation takes place in the aggregate
  microscale.

26
RETARDATION PROCESS

• It refers to processes that impede the transport
  of contaminants by removing or immobilizing them
  from a free state.
• The contaminants are NOT transformed and the
  process is reversible.

27
RETARDATION PROCESS

• Sorption
• The accumulation of organic chemicals at soil
  surfaces for example the adherence of organic
  molecules to naturally occurring humic matter in
  soil. Desorption is also possible which may limit
  the use of "pump and treat" cleanup technology.
  The potential retention capability can be
  estimated by saturating undisturbed soil with a
  liquid contaminant and allowing the sample to
  drain. The retained contamination is termed
  residual saturation or retention capacity.

28
RETARDATION PROCESS

• Linear Sorption Model.
• S KdC
• For saturated conditions and non-polar organics.
  Kd can be related to organic content
• Kd Kocfoc
• A reasonable estimate is forthcoming if the
  following conditions are met Minimum Greater
  than .1 few tenths of a percent 1 (different
  sources) Maximum 20

29
RETARDATION PROCESS

• Example (Sorption)
• Given A phenol underground storage tank is
  leaking such that the immediate surrounding
  ground water contains 7.3 mg/l of phenol. The
  soil contains 2.5 organic matter, a fact
  ascertained by analyzing the soil.
• Find Assuming a linear sorption model, what is
  the concentration of phenol sorbed in to the
  soil? Calculate the sorbed concentration, S.
• 1.) Kd
• From App. B, p.1113, Koc for phenol 14.2 ml/g
  Note units.
• Kd Kocfoc units p.187 14.2 ml/g x (.025)
• Kd .355 ml/g
• 2.) S
• S KdC units p.187 .355 ml/g x 7.3 mg/l
• S 2.59 mg/kg

30
RETARDATION PROCESS

• Ion Exchange
• Ion exchange involves the sorption of ions in
  solution onto oppositely charged, discrete sites
  on the surface of a soil particle. Ion exchange
  applies to metals while sorption applies to
  organics, in general.
• The capacity of a soil to retain and exchange
  cations is quantified as the cation exchange
  capacity e.g. 150 meg/100grams. Clay has a much
  higher cation exchange capacity than other
  inorganic particles because of its extremely
  large surface area that contains many negative
  sites.
• Cations are replaced in the following order
• Na lt Li lt K lt Rb lt Cs lt Mg lt Ca lt Ba lt
  Cu lt Al lt Fe lt Th

31
RETARDATION PROCESS

• Precipitation
• Precipitation occurs when the concentration of a
  solution exceeds the solubility of that compound
  and any excess solute changes to a solid and
  falls out of the solution. Reversible.
• Particularly applicable to heavy metals.

32
ATTENUATION PROCESSES

• It refers to 2 processes
• irreversible removal
• transformation
• Chemical Oxidation-Reduction. Redox involves the
  gain or loss of electrons.
• Hydrolysis. Chemical substances reacting with
  water molecules.
• Volatilization. The conversion of volatile
  chemical constituents in ground water to vapor
  the vapor ending up in the atmosphere.

33
MOBILITY ENHANCEMENT

• Opposite of retardation and attenuation i.e. the
  contaminant is sped along.
• Cosolvation. The presence of bulk solvents
  promotes increased interaction between a solute
  and the solvent that would not occur in water and
  can dramatically increase the mobility of
  contaminants.
• Ionization. Increased solubility
• Dissolution. Dissolving of chemical substances
  such as a leachate.
• Complexation. Also chelation is the formation of
  a coordinate bond between a metal ion and an
  anion know as a ligand.