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# Modflow - modpath

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I have a one grid layer steady state modflow model in gms 10 . When I put one particle at a distance of 100 meters away from the intake well, it entered the well in 70 days with default porosity value of 0.3. Material K value is 0.002 m/s. Then I changed the porosity value to value 0.01 and after running modflow again, GMS informs that particle enters the well in two and a half days. I"m "confused", when there is less amount of effective pores available for ground water to move then GMS tells that water moves faster (from one place to other) ?

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• 2 months later...

Arto, hydraulic conductivity determines how much volume flows through fro a given aquifer for a given area an gradient. Imagine a block of aquifer and a block of water being extruded out of the side as it flows. If the aquifer was all space (porosity of 1) a particle of water would flow through the aquifer at the same rate is is extruded out the side. If the porosity is only half (0.5) and everything else remains the same, the velocity of a particle in water in the aquifer has to flow twice as fast to keep the block of water moving out. If your porosity is 0.01 for the same K, it has to move 100 times as fast. This is expressed in  the Darcy flow velocity equation

V=K*(dh/dl)/Pe

where V = velocity

K= hydraulic conductivity

Pe = effective porosity.

Particle flow velocity is inversely proportional to effective porosity, if all other parameters remain the same.

This may seem counter intuitive, because you are thinking that less pore space means lower flow, but remember, you are keeping hydraulic conductivity the same, so the volumetric water flow stays the same.

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• 6 months later...

Thank You very much Rob ! Because I´m a geologist then I´m thinking effective porosity "purely" as a material dependent phenomena (eg. gravel 0.3, clay 0.0001). You "opened my eyes" to figure out "what´s going on" (= keep hydraulic conductivity the same), Thank You.

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