Introduction to Ground Water Contamination

1
Introduction to Ground Water Contamination

• CIVE 7332
• Lecture 2

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Darcy allows an estimate of

• the velocity or flow rate moving within the
  aquifer
• the average time of travel from the head of the
  aquifer to a point located
  downstream

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Darcys Law

• Darcys law provides an accurate description of
  the flow of ground water in almost all
  hydrogeologic environments.

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Who Was Darcy?

• Henry Philibert Gaspard Darcy was born June 10,
  1803 in Dijon, France.
• Admitted to the French School of Bridges and
  Roads in Paris, part of the Corps of Bridges and
  Roads. After graduation, he was eventually
  assigned by the Corps to a position in Dijon.
• In 1828, Darcy designed a 12.7 km system of
  aqueducts to supply the city of Dijon with
  surface water. The system included 28,000 m of
  pressurized surface lines and required no pumps
  or filters.
• Made important contributions to flow and friction
  loss in pipes, created an improved pitot tube
  design, and was the first to postulate the
  existance of a boundary layer in fluid flow.
• In 1856, carried out experiments while
  researching sand filters that lead to Darcys
  Law.
• Died unexpectedly January 3, 1858 from pneumonia
  during a trip to Paris.

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Darcys Legacy
Place Darcy, Dijon, France.
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Flow in Aquifers
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Darcys Experiment (1856)
Flow rate determined by Head loss dh h1 - h2
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Darcys Law

• Henri Darcy established empirically that the flux
  of water through a permeable formation is
  proportional to the distance between top and
  bottom of the soil column.
• The constant of proportionality is called the
  hydraulic conductivity (K).
• V Q/A, V ? ?h, and V ? 1/?L

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Darcys Law

• V K (?h/?L) and since
• Q VA (A total area)
• Q KA (dh/dL)

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Hydraulic Conductivity

• K represents a measure of the ability for flow
  through porous media
• Gravels - 0.1 to 1 cm/sec
• Sands - 10-2 to 10-3 cm/sec
• Silts - 10-4 to 10-5 cm/sec
• Clays - 10-7 to 10-9 cm/sec

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Conditions

• Darcys Law holds for
• 1. Saturated flow and unsaturated flow 2.
  Steady-state and transient flow 3. Flow in
  aquifers and aquitards 4. Flow in homogeneous
  and heterogeneous systems 5. Flow in
  isotropic or anisotropic media 6. Flow in rocks
  and granular media

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Darcy Velocity

• V is the specific discharge (Darcy velocity).
• () indicates that V occurs in the direction of
  the decreasing head.
• Specific discharge has units of velocity.
• The specific discharge is a macroscopic concept,
  and is easily measured. It should be noted that
  Darcys velocity is different .

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Darcy Velocity

• ...from the microscopic velocities associated
  with the actual paths if individual particles of
  water as they wind their way through the grains
  of sand.
• The microscopic velocities are real, but are
  probably impossible to measure.

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Darcy Seepage Velocity

• Darcy velocity is a fictitious velocity since it
  assumes that flow occurs across the entire
  cross-section of the soil sample. Flow actually
  takes place only through interconnected pore
  channels.

Av voids
A total area
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Darcy Seepage Velocity

• From the Continuity Eqn
• Q A vD AV Vs
• Where Q flow rate A total
  cross-sectional area of        material A
  V area of voids Vs seepage
  velocity VD Darcy velocity

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Darcy Seepage Velocity

• Therefore VS VD ( A/AV)
• Multiplying both sides by the length of the
  medium (L) VS VD ( AL / AVL ) VD ( VT /
  VV )
• Where VT total volume VV void
  volume
• By Definition, Vv / VT n, the soil porosity
• Thus VS VD / n

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Equations of Groundwater Flow

• Description of ground water flow is based
  on Darcys Law Continuity
  Equation - describes conservation of
  fluid mass during flow through a
  porous medium results in a partial
  differential equation of flow.
• Laplaces Eqn - most important in math

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Derivation of 3-D GW Flow Equation from Darcys
Law
z
Fluid density
mass/area/time
y

• Mass In - Mass Out Change in Storage

Steady State
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Derivation of 3-D GW Flow Equation from Darcys
Law

• Replace Vx, Vy, and Vz with Darcy using Kx, Ky,
  and Kz

Divide out constant ?, and assume Kx Ky Kz
K
incompressible fluid, isotropic, homogeneous mediu
m
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Permeameters
Constant Head
Falling Head
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Constant head Permeameter

• Apply Darcys Law to find K V/t Q
  KA(h/L) or K (VL) / (Ath)
• Where V volume flowing in time t A
  cross-sectional area of the sample L length of
  sample h constant head
• t time of flow

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Pressure and Elevation Heads - Laboratory
? pressure head z elevation head h ? z
total head
Freeze and Cherry, 1979.
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Pressure and Elevation Heads - Field
? pressure head z elevation head h total
head
Freeze and Cherry, 1979.
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Horizontal and Vertical Head Gradients
Freeze and Cherry, 1979.
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Two Confined Aquifers with Different Heads
Groundwater will tend to flow from the top
aquifer to the bottom aquifer. (Assuming that
horizontal distance between piezometers is small)
Charbeneau, 2000.
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Hydraulic Head is a Potential Field
Hubbert (1940) potential a physical quantity,
capable of measurement at every point in a flow
system, whose properties are such that flow
always occurs from regions in which the quantity
has a higher values of those in which it has
lower, regardless of the direction in space.
Potential fields and associated physical laws
Fluid Flux
Head (Darcys Law) Temperature (Fouriers Law)
Conduction of heat in solids Concentration
(Ficks Law) Diffusion of chemicals
Heat Flux
Mass Flux
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Horizontal and Vertical Head Gradients
Freeze and Cherry, 1979.
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Potentiometric Surface Dakota Sandstone
Domenico and Schwartz, 1992.
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Drinking Water Standards
Primary Standards

• Maximum Contaminant Level (MCL)
• Microorganisms
• Disinfectants
• Disinfection by-products
• Inorganic chemicals
• Organic chemicals
• Radionuclides

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List of National Secondary Drinking Water
Regulations
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BTEX-Related Compounds
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Chlorinated Solvents
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Chlorinated Solvents (contd)
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Superfund - CERCLA
The Comprehensive Environmental Response,
Compensation, and Liability Act (CERCLA),
commonly known as Superfund, was enacted by
Congress on December 11, 1980. This law created a
tax on the chemical and petroleum industries and
provided broad Federal authority to respond
directly to releases or threatened releases of
hazardous substances that may endanger public
health or the environment. Over 5 years, 1.6
billion was collected, and the tax went to a
trust fund for cleaning up abandoned or
uncontrolled hazardous waste sites. Superfund
National Priorities List (NPL) sites are the most
serious uncontrolled or abandoned hazardous waste
sites that have been identified for possible
long-term remedial action under Superfund. The
list is based primarily on the score a site
receives from the Hazard Ranking System. The U.S.
Environmental Protection Agency (EPA) is required
to update the NPL at least once a year. A site
must be on the NPL to receive money from
Superfund for long-term remedial action.
Long-term remedial action is defined as action
that stops or substantially reduces a release or
threat of a release of hazardous substances,
where such a threat is serious but not an
immediate threat to public health.The EPA
administers the Superfund program in cooperation
with individual States and tribal governments.
Source EPA website
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National Priorities List Sites in Texas
(Superfund)
Triangle proposed (2) Circle approved
(43) Square deleted (9)
The NPL is the list of national priorities among
the known releases or threatened releases of
hazardous substances, pollutants, or contaminants
throughout the United States and its territories