1/2) Intro and Porosity

what does evaporation rate depend on?

availability of water

heat/energy (easier to evaporate if it’s moving more)

strength of surface to air vapour gradient

turbulent air to bring in more dry air

what are groundwater inputs and outputs?

inputs: recharge, from rain or river, lateral, or upward flow in
GW recharge: downward mass flux entering an aquifer (at water table)

outputs: discharge, evapotranspiration (shallow aquifers), interaquifer flow out, pumping
GW discharge: the mass flux leaving an aquifer at the surface

details on water ages

younger water at the surface

slightly younger water in the fractures of bedrock, and macropores where roots have allowed movement of younger water down

discharge out of system is a mixture of these ages

what is porosity

porous media’s ability to store water
doesn’t control ability to transmit water 

what is REV

representative elementary volume

what volume do we need to get representative sample of the things we are looking for

property values change depending on scale of measurement

if sample is homogeneous, can use large representative volume

using a too small REV results in too much variation, same with too big, so pick in between

what are geological controls on porosity?

grain-size distribution
grain shape
grain arrangement and packing
geochemical reactions
material type

how does grain size affect porosity?

grain size can be different but does not alone dictate porosity

same shape and packing = same porosity

differences arise with distribution

poorly sorted = lower porosity because small grains fill larger gaps = well graded (lots of grain sizes)

explain why each line is this way

uniform sand has bigger pore sizes = green line

silty sand, higher silt content = wider distribution = lower porosity

steeper line = more uniform grain size = higher porosity

how does grain shape influence porosity?

fine grains have higher porosity due to disk shaped grains creating “house of cards” arrangement

how does arrangement of grains/packing influence porosity?

cubic packing has porosity =0.48, rhombohedral packing = 0.26

how do geochemical reactions influence porosity?

mineral dissolution = increase
secondary mineral precipitation = decrease (lithification, cementation)

depends on chemical makeup of fluid running through sediment

how does material type influence porosity

unconsolidated sediments have higher porosity than rock
exceptions are sandstone and vesicular basalt

definition of void ratio

e = Vv / Vs

void volume / solid volume

definition of porosity

ne = Vv / VT

void volume / volume porous media (total volume)

details on measuring porosity

directly measuring void volume is difficult

can take known volume and saturate it, record mass, then dry and record mass again to infer VV. (fully saturating sample is difficult

definition of particle density

mass of solids / volume of solids

ps = Ms / Vs

weighted average density of mineral grains making up a soil (mass per unit volume of soil solids

definition of dry bulk density

mass of solids (dry) / volume porous medium (total volume)

Pbd = Ms / VT

dry density of the soil, usually considered constant over time, but increases with depth due to compaction by weight of overlying soils

definition of wet bulk density

mass of solids + fluid / volume porous medium (total volume)

Pbw = (Ms + Mf) / VT

equation for porosity

as porosity decreases ___

ø = 1 - (pb/ps)

1 - bulk density / particle density

as porosity decreases, bulk density increases

difference between primary and secondary porosity

primary: caused during deposition of unconsolidated material or original formation of rock (magma cooling), intrinsic porosity
voids between individual grains
very little in igneous and metamorphic
more storage

secondary: additional porosity caused by fracturing or dissolution
more connectivity
porosity of fractured rock is quite low

reasonable porosities for various sediments

sand: 0.20-0.5

silt: 0.35-0.5

clay: 0.25-0.7

total (n) vs effective porosity (ne)

some pores are dead end or isolated, this means the ability to transmit water is decreased

disconnected pores won’t contribute to flow or storage of groundwater

effective porosity is always less than or equal to total porosity. sand and gravel its close or equal

clays have high total but low effective porosity: lots of saturated void space, very small pores
vesicular basalts, many of the vesicles are not connected

definition for volumetric water content

volume of water / total volume
mass wet - mass dry / density*total volume

theta = Vw / VT

theta = Mwet - Mdry / pw*VT

definition of water saturation

volume of water / void volume
volumetric water content / porosity

Sw = Vw / VT = Vw / (Vair + Vw) = theta / n