 `aquifer"; Section 4.3.3 discusses methods for determining ground water flow direction and`
`hydraulic gradient; Section 4.3.4 discusses methods for determining hydraulic conductivity;`
`and Section 4.3.5 discusses determining ground water flow rate. The special case of ground `
`water flow in aquifers dominated by conduit flow is discussed in Sections 4.3.1 and 5.2.`
`Most of the discussions provided in Sections 4.3.3, 4.3.4, and 4.3.5 do not apply to this`
`special case.`
`4.3.1`
`Introduction`
`Conventional ground water hydrology considers aquifers to be porous granular media`
`(either unconsolidated granular deposits or rock) having a well defined water table or`
`potentiometric surface. The flow of ground water in these types of aquifers is described by`
`Darcy's law:`
`Q =  KiA`
`where:`
`Q =`
`quantity of flow per unit of time, in (volume/time)`
`K =`
`hydraulic conductivity, in (length/time)`
`i =`
`hydraulic gradient, in (length/length)`
`A =`
`cross sectional area through which the flow occurs, in (length`
`2`
`)`
`Darcy's law assumes laminar flow of individual particles of water moving parallel to`
`the direction of flow, with no mixing or transverse component in their motion. The right `
`hand side of the Darcy equation is preceded by a negative sign because ground water flows`
`from high head to low head.`
`There are two types of ground water systems where the relationship expressed by`
`Darcy's law does not apply. These are systems where ground water flows through materials`
`with low hydraulic conductivities under extremely low gradients, and systems in which a`
`large amount of flow passes through materials with very high hydraulic conductivities`
`(turbulent flow). These two situations can be considered, respectively, as the lower and upper`
`limits of the validity of Darcy's law (Freeze and Cherry, 1979).`
`The Reynolds number (the ratio of inertial to viscous fluid forces) is a dimensionless`
`number used to define the limits of the validity of Darcy's law. The Reynolds number (R ) is`
`e`
`defined as:`
`v d`
`R =`
`e`
`November 1992`
`4 36`