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Transport Processes
helps in understanding of groundwater contamination problems
| Question | Answer |
|---|---|
| what are sources of contamination? | point sources distributed (non-point) sources linearly distributed sources |
| what are some examples of point sources? | • leaky storage tanks (petroleum or other chemicals) • accidental spills and historical releases • waste lagoons, landfills and dumps • septic systems • mine tailings/waste rock • graveyards/animal burial • injection wells |
| what are some examples of distributed (non-point) sources? | • land applied manure/sewage/sludge • fertilizers and pesticides • air pollution fallout • urban runoff • military firing ranges |
| what are some examples of linearly distributed sources? | • pipeline leakage (many leakage points) • saltwater intrusion (coastal areas) • road salt • losing streams • canal leakage |
| what are the 2 contaminant types? | chemical biological |
| what are some examples of chemical contaminants? | Inorganic - nitrate, chloride, phosphorous Organic - pesticides, petroleum products Metals - arsenic, lead, chromium, selenium Radionuclides - radon-222 |
| what are some examples of biological contaminants? | Bacteria - E. coli, Campylobacter jejuni Viruses - rotavirus, norwalk, poliovirus |
| what are some physical properties of contaminants that can drastically influence the distribution of said contaminant in the subsurface? | miscible/dissolved - fully dissolved in water (cations & anions & organic compounds) immiscible - oily liquids; have low h2o sol & remain as separate liquid phase in subsurface; unique migration patterns |
| what are transport processes important in? | • migration of contaminants • nutrient cycling in catchments • formation of hydrothermal ore deposits |
| how are most contaminants in gw transported? what can this result in? | in dissolved phase (solutes) plumes of contaminated gw traveling long distances in the subsurface |
| what is a gw plume? | concentrated form of liquid contaminants moving in gw in the subsurface |
| What are the 3 key solute transport mechanisms? | advection diffusion mechanical dispersion |
| what is advection? | transport due to bulk gw flow |
| what is diffusion? | molecular transport due to solute concentration gradients (no h2o movement) |
| what is mechanical dispersion? | spreading of solute mass due to gw velocity variations at the pore scale and larger scales advective process that results from veolicty variations that we cannot account for hydraulic mixing process = 3D solute spreading |
| what is hydrodynamic dispersion? | diffusion + mech dispersion |
| what is volumetric flux (q)? | Specific discharge, q, is the volume of water crossing a unit area per unit time |
| what is transport flux (J)? | rate of mass transport per unit area (kg s-1 m-2) |
| what is the advective mass flux? | Advective Mass Flux = JA = q C in porous medium = nevC C = solute concentration (M/L^3) |
| what is the diffusive mass flux? | Diffusive Mass Flux = Jd = -Dd dC/dx for porous media = - neD* dC/dx |
| does diffusion in porous media differ from free water diffusion? | yes; value of diffusion coefficient is smaller than Dd because of tortuous flow paths that molecules have to travel |
| what is the mechanical dispersion mass flux? | Jm = -ne alpha v dC/dx |
| what is alpha? | dispersivity, found from mechanical dispersion coefficient and average groundwater velocity |
| what is longitudinal dispersion? | • spreading parallel to the direction of groundwater flow |
| what is transverse dispersion? | • spreading in the direction normal to groundwater flow • much smaller than longitudinal dispersion • can be different horizontally than vertically |
| is dispersivity directional? | yes; follows same trend as mechanical dispersion rule of thumb; transverse dispersivity is much smaller than longitudinal typically by factor of 10 or 100 |
| is dispersivity scale dependent? | yes geologic heterogeneity has a major influence on solute dispersion as size of flow system increases = more geologic heterogeneity (velocity variations) = increase in dispersion (macrodispersion) |
| what is the hydrodynamic dispersion mass flux? | Jh = -nDi dC/dx = -n(alphaiv + D*_dC/dx |
| what dominates the dispersive flux under low flow velocities? | diffusion dominates |
| what dominates the dispersive flux under high flow velocities? | mech dispersion dominates |
| Largr Dh results in more mixing. In what situations would you expect to see more dispersion? Less dispersion? | more dispersion = high K materials less dispersion = low K materials |
| what is sorption? | the process where solute molecules attach to the surface of solid particles in porous medium slows the advection of the contaminant and reduces the dispersion of the contaminant |
| what is an isotherm? | the relationship between the concentration of solute in aqueous soluton (C) to the mass sorbed on the solid surfaces (S) |
| Distribution coefficient (Kd) | Kd = dS/dC = slope of linear portion of isotherm |
| what is the retardation factor? | retardation factor provides a measure of the mean velocity of a reactive contaminant, vc, relative to the mean velocity of a non reactive contaminant (or average groundwater velocity), v |
| Application of Kd Model | assumes sorption is reversible; when concentrations decrease, contaminants attached to soil particles will desrob & come back into solution sorption only delays arrival of contaminants |
| what are the effects of sorption on plumes? | • Sorption can be reversible • Contaminants undergoing sorption will travel more slowly than non-reactive contaminants in a plume • this is termed retardation. • Strong sorption= contaminant to be immobile over decades or centuries |
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