Glossary

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 Darcy's Law

an empirical relationship between hydraulic gradient and the viscous flow of water in the saturated zone of a porous medium under conditions of laminar flow. The flux of vapors through the voids of the vadose zone can be related to a pressure gradient through the air permeability by Darcy's Law.

 degradation potential

the degree to which a substance is likely to be reduced to a simpler form by bacterial activity.

 denitrification

bacterial reduction of nitrite to gaseous nitrogen under anaerobic conditions.

 density

the amount of mass per unit volume.

 diffusion

the process by which molecules in a single phase equilibrate to a zero concentration gradient by random molecular motion (Brownian motion). The flux of molecules is from regions of high concentration to low concentration and is governed by Fick's Second Law.

 dispersion

the process by which a substance or chemical spreads and dilutes in flowing groundwater or soil gas.

 dissolution

dissolving of a substance in a liquid solvent (e.g., water).

 downgradient

in the direction of decreasing static head (potential).

 Downstream Operations

Operations concerned with oil refining, transportation and gasoline marketing.

 drawdown

lowering the water table due to withdrawal of groundwater as from a well.

 DTW

Dealer Tank Wagon-Wholesale price of gasoline delivered to a retail outlet (transportation costs are included)

 dynamic viscosity

a measure of a fluid's resistance to tangential or shear stress.