Groundwater transport

Remaining issues from last class

storativity [L^-1]/specific storativity [dimensionless]
transmissivity T = K*thickness of the aquifer
movies unsturated/saturated flow (sdbox & capbar)

Prediction of transport in the GW model (fig)

Advection

groundwater flow velocity v can be calculated from Darcy's law
according to Darcy's law, a contaminated parcel of water would not change its shape during transport in a horizontal flow regime (Fig)
However, if you take a liter of a salt solution for example and release it in the aquifer, after a while the contaminant will have spread out and not any more confined to 1 liter

Dispersion

spreading of a contaminant can be explained by `dispersion'
variability of the flow velocity on the pore/grain scale (Fig) and on the aquifer scale (Fig) can explain the spreading of the plume

Adsorption

certain solutes interact with the solid phase and are temporarily adsorbed on the surface
results in a retardation of the contaminant
example: certain organic contaminants, Cs-137
demonstration with the model: separation of yellow and blue plume

Decay

radioactive decay or microbial activities remove contaminants
example: conversion from nitrate to nitrogen gas

Mathematical description of groundwater transport

D = av+ Dm, a is dispersivity [L], Dm: molecular diffusion
R_d: retardation coefficient due to adsorption
scale dependence of the dispersivity (Fig)

Everything working together

random walk model

all above processes affect the spreading of contaminants in groundwater (Fig)
transport of contaminants is more complicated when it does not dissolve in groundwater (Fig)
example: hydrocarbons floating on the watertable in Brooklyn/Queens
groundwater remediation (model demonstrations)

remove the source of the contaminant
let the aquifer flush out the contaminant naturally
pump and treat
bioremediation

Discussion: the 'groundwater model' (Fig)

water quality can vary in the aquifers
pollutants travel with the groundwater
dispersion, inhomogeneities, adsorption
wells can cause groundwater pollution
contaminated groundwater can pollute surface water and vice versa
leakage through confining beds
capillary action may cause upward movement of water