Groundwater Pollution Remediation (NOTE 2)

Joonhong Park

Yonsei CEE Department

2015. 10. 05.

CEE3330 Y2013 WEEK3

CEE3330-01 May 8, 2007 Joonhong Park Copy Right

Darcy’s Experiment (1856)

Flow of water in homogeneous sand filter under steady conditions

Datum

h1

h2Sand

Porous Medium L

A: cross area

Q = - K * A * (h2-h1)/L K= hydraulic conductivity

CEE3330-01 May 8, 2007 Joonhong Park Copy Right

Darcy’s Law

Q = - K * A * (Φ2 - Φ1)/L Φ piezometric head

In a 1-D differential form, Darcy’s law may be:

Darcy’s velocity: q = Q/A = dV/[A*dt] = - K * [dΦ/dL]Hydraulic Conductivity, K (L/T)

K Ξ k * ρ * g / μHere, k = intrinsic permeability (L2)

ρ: fluid density (M L-3); g: gravity (LT-2) μ: fluid dynamic viscosity (M L-1 T-1)

Modeling of Water Flow in Porous Media

- Micro-scale modeling: the Navier-Stokes equation (flow through the void spaces in aquifers; fluid elements are described by differential equations )

- Macro-scale modeling: the Darcy’s equation(Darcy’s velocity: a volume flux defined as the volume of discharge per unit of bulk area)

(What is seepage velocity? Velocity of a fluid element [v] vs Average v [q/n])

- Discussion(Differences? Advantages/Disadvantages?)

Forces on Fluids in Porous Media (I)Driving forces: pressure (p) and a body force due to gravity

Resistance forces (F) are involved in fluid motion in porous media

ρ:density of fluidg:gravity constantn:porosityp:pressure

ρ*g*n*dA*dl

dA

dl

z

l

dz

p*n*dA

(p+dl*dp/dl)*n*dA

F

p*n*dA -(p+dl*dp/dl)*n*dA = ρ*g*n*dA*dl * (dz/dl) + F (at Equilibrium)

F/(n*dA*dl) = - (dp/dl + ρ*g*dz/dl) Macro-scale

Forces on Fluids in Porous Media (II)

1) 8*μ*ave. v/R^2 = - (dp/dl + ρ*g*dz/dl) for a cylindrical tube of small radius R

Meanwhile, from Exact Solution of N-S Equation

2) 3*μ*ave v/d^2 = - (dp/dl + ρ*g*dz/dl) for a thin film of thickness d

3) 12*μ*ave v/b^2 = - (dp/dl + ρ*g*dz/dl) for between two plates spaced a distance b apart

Micro-scaleResistance forces per unit volume (F/[dA*dl])

Forces on Fluids in Porous Media (III)

F/(n*dA*dl) = (C*μ/[characteristic length^2])*q

Here: q= ave v/n

The effects of the tortuous path traversed by fluid elements in a porous medium are Included in the parameters of characteristic length and a dimensionless number (C). WHY?

q = - (characteristic length^2/ [C*μ]) * (dp/dl + ρ*g*dz/dl) = - (k/μ)*(dp/dl + ρ*g*dz/dl) = - (k ρ g/μ)*(dФ/dl)

Fundamental Background for the1-D Darcy’s Law

Effect of turbulence

q = - (k/μ)*(dp/dl + ρ*g*dz/dl)

QUESTION: When can the linearity maintain or when cannot?

(1) F/(n*dA*dl) = (μ/k)*q + ρ*q^2/([k/C]^0.5) = - (dФ/dl)(The Forchheimer’s equation) (q^2 is the inertial forces)

(2) -([k/C]^0.5/[ρ*q^2])*(dФ/dl) = μ/(ρ*q*([k*C]^0.5) + 1

(3) f = 1/Re + 1 (f=the friction factor)

when Re < 0.02 [<0.1], Darcy’s law is extremely exact [probably acceptable]

Effects of change in fluid density

q = - (k/μ)*(dp/dl + ρ*g*dz/dl) (Eq.3.10)

A rather general form of Darcy’s Law which applies for fluids with either constant or variable density contained in porous media whose intrinsic permeability may depend upon both direction and location.

Density of water is fairly constant. Therefore, the Eq.3.10 can be rewritten into the following equation.

q = - (k*ρ*g/μ)*d(p/ρ*g + z)/dl = - (k ρ g/μ)*(dh/dl) (Eq.3.15).

Here (p/ρ*g + z) is a scalar force potential or piezometric head (h).

3-D Differential Form of Darcy’s Equation

q = - (k ρ g/μ) * ∇h (Eq.3.17)

∇ = ∂/∂x * i + ∂/∂y * j + ∂/∂z * k (the gradient operator)i, j, and k are the unit vectors in the x, y, and z coordinate directions, respectively.Piezometric head is a scalar. Its negative gradient is a vector representing the force per unit weight acting on the fluid. (force potential)

q = - (k ρ g/μ) * ∇h = -K * ∇h (Eq.3.20)Barotropic fluids (ρ = function of p). However, constant density of water in most of groundwater is a good assumption. Of course, there are often exceptions.Suppose K is constant (homogeneous). Then it is permissible to define Ф = K*h

q = -∇ Ф (Eq.3.21)

Laboratory Determination of K

The Fair-Hatch formula Eq.3-25 at p.81. k = 1/{A*[(1-n)^2/n^3]*[(B/100)* ∑(F/dm)]^2}n:porosityA: a dimensionless packing factor (~5)B: a particle shape factor (ex. 6 for spherical particles and 7.7 for highly angular ones)F: the percent by weight of the sample between two arbitrary particle sizesdm:the geometric mean of the particle sizes corresponding to F.

Harleman et al.’ formula: k = (6.54 x 0.0001) * d^2d:characteristic grain sizeThe formula is nearly valid for materials of very uniform particle size and shape.

Carman-Kozeny Equation

-k = Co * [n3/(1-n)2] * (1/SS2)

n: porosity

SS: specific surface area

or empirically,

-k = [n3/(1-n)2] * (dM2/180)

dM: grain size for 50 percentile

Reading assignments

Please read Darcy’s Law and the Equations of Groundwater Motion, p.65-82 including

Example 3-1Example 3-2Example 3-3Example 3-4Example 3-5Example 3-6Example 3-7

Non-Homogeneity

Homogeneous: K is a scalar

Heterotrophic: K is a function of positions at x, y, and z.

See p. 84-87

∇

∇

dl

q-a

q-b

K-a

K-b

α-a

α-b

K-a/K-b = tan (α-a) / tan (α-b)

Anisotropy

-∇h

q-x

q-y

q

Reading assignments

Please read Darcy’s Law and the Equations of Groundwater Motion, p.82-90 including

-Flow parallel to the layers in a stratified aquifer-Flow through beds in series-Figure 3-12 and Eq.3.39 to 3.44

-See p. 70-71 in the reading material

3D Generalization of Darcy’s Law

Heterotrophic Isotropic: q = - K (x,y,z) ∇ h

For homogenous case, can rewrite as

q =

~ ~

- ∇ [K * h] = - ~

Anisotropic: q = - K ∇h

~ ~=

K ==

Kxx Kxy Kxz

Kyx Kyy Kyz

Kzx Kzy Kzz

~ ∇ Φ

~

General form of Darcy’s lawValid for multi-dimensions, all Newtonian fluids – incompressible or compressible.

q = - k / µ . [ P - ρ g ]= ~~~

Flow in Aquifer

A Differential Mass Balance

△ X

△ Y

△Z

(x,y,z)

QxQx+dx

Qy

Qy+dy

Qz

Qz+dz

Reading assignments

Please read p.58-63 in the reading material

-Governing Equation for Confined Aquifers-Governing Equation for Unconfined Aquifers-Governing Equation for Aquitards-The Duipuit-Forchheimer Approximation-The Boussinesq Equation

Also read p. 72

GW Flow Eq: Confined aquifer with leakage

X

ΔX

qz-t

qz-b

Z

Assumptions:

Horizontal flow

Constant width into paper, W (a fixed y-value)

Aquifer thinkness at a point: B(X)

B(X)

GW Flow Eq: Confined aquifer with leakage

Aquitard

Impermeable rockx

Assumptions: Homogeneous formation

Steady-state

Constant thickness

Φ = Φ A at left boundary, Φ = Φo in overlying formation

Semi-infinite system

A

ФAb’: thickness of aquitard

b: the thickness of aquifer

K’: hydraulic conductivity for aquitard

K: hydraulic conductivity for aquifer