19 basics of mass transport advection, diffusion, and dispersion
TRANSCRIPT
19Basics of Mass Transport
Advection, Diffusion, and Dispersion
Introduction
Mass Transport: Delivery/distribution problem
Dissolved mass: ions + molecules Natural or contaminated: SOURCE
Iron in gw comes from iron silicate mineral or old car in a landfill
Mass moves and distributes by Physical process (advection, dispersion) Chemical process (reactions) Biological process (redistribution of mass forms)
Key Elements of Mass Transfer
Physical and Chemical Mass Transport Processes operating in a groundwater flow system
Conceptualization of mass transport in a groundwater flow system
Introduction
Topics:
19.1 Advection
19.2 Diffusion
19.3 Dispersion
19.8 Tracer and Tracer Tests
19.1 Advection
Advection:
mass transport due simpley to the flow of water in which the mass is dissolved
It’s The main process
Direction and rate of transport = direction and rate of groundwater flow
19.1 Advection
(a) advection alone (b) advection + dispersion
Mass spreading by advection in a shallow unconfined aquifer
Advection
Velocity of advective transport (Darcy):
v: linear groundwater velocity
Kxx, Kyy, Kzz: Hydraulic conductivity along x,y, z
n: effective porosity dh/dx, dh/dy, dh/dz: hydraulic gradient
v v v vK
n
h
x
K
n
h
y
K
n
h
z
K
ngradx y z
xx yy zz
( , , ) ( , , ) h
Advection velocity
cases where velocity of groundwater and transported mass are different:
1- negatively charged ions vm>vgw
2-small voids (medium works as membrane)3-Retardation
e
K hv
n l
Advection, example
A small plume of tracer is added to an unconfined aquifer that has a hydraulic conductivity of 1 m/d and a porosity of 0.35. the hydraulic gradient is 0.07.
Calculate how far the center of mass of the tracer will move in one year.
Solution: Assume advection only, use previous equation v = -K/n * grad (h) = (1 m/d/0.35) x 0.07 = 0.2 m/d Distance = d = v x t = 0.2 m/d x 365 d = 73 m
19.2 Diffusion
Fick’s Law:relates mass flux to gradient in concentration (similar to Darcy’s)
Jdif: chemical mass flux [L2/T]dC/dx concentration gradient [C: moles/L3]
Dm molecular diffusion coefficient [L2/T
Molecular diffusion: mixing caused by random molecular motions due to thermal kinetic energy of the solute
Coefficient is larger in gases than in liquids, in liquids than solids
J DdC
dxdif m
19.3 Dispersion
Dispersion:
is a process of mixing that causes a zone of mixing to develop between a fluid of one composition that is adjacent to or
being displaced by a fluid with a different composition
Dispersion spreads mass beyond the region it normally would occupy due to advection alone
Mixing caused by local variations in velocity Advective process Variations in K
Dispersion with time
mixing zone size increases with time
Longitudinal & transverse dispersion
19.3 Dispersion
Dispersion:
Occurs because of two processes
1. Diffusion:
mass transport by concentration gradient
2. Molecular Dispersion:
mixing due to local differences around some mean velocity
of flow
“Rubber duckies” released in a river from the circle at point “a” will end up highly dispersed due to local variability in the flow velocity
(a) horizontal transverse dispersion(b) vertical transverse dispersion
Map view of Cl- ion distribution in a tracer test after 462 days
Mixing in Fractured Media
19.8 Tracer and Tracer tests
Tracers:1. Ions occuring naturally in groundwater
systems: Br-, Cl-
2. Environmental isotopes: 2H, 3H, 18O3. Contaminants of all kinds in the flow
systems: radioisotopes 3H, 131I, 82Br…organic compounds
Ideal tracers: no reaction (conservative tracers)
Reactive tracers
19.8 Tracer and Tracer tests
dfkd
Natural Gradient Test Single well pulse test Two well tracer test Single well injection or withdrawal with multiple observation
wells
Tracers and Tracer Tests
Cl- concentration distribution at various times after injection