Groundwater transport

Take away ideas and understanding

• Darcy's law is not sufficient for understanding contaminant or solute transport.
• Contaminant transport is controlled by advection, dispersion, adsorption and nth order decay reactions.
• The consequence of these effects is that contaminants spread a lot more and often move faster or slower than predicyted by Darcy's law.

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

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