Groundwater

SupplyDr. Martin T. Auer

Michigan Tech Department of Civil & Environmental Engineering

Approximately two-thirds of the population of the U.S. receives its supply from surface waters. However, the number of communities supplied by groundwater is four times that supplied by surface water. This is because large cities are typically supplied by surface waters and smaller communities use groundwater.

Drinking Water Sources

Domestic Wells

water table

An Aquifer

water table wellvadose zonecapillary fringeunconfined aquiferwater tableimpermeable layer

unconfinedaquifer

water table

unconfinedaquiferre-charge water

table well

Unconfined Aquifer

water table – piezometric surface where water pressure equals atmospheric pressure

UnconfinedAquifer

manometer

ConfinedAquifer

piezometricsurface

aquiclude

Confined Aquifer

confinedaquiferre-charge

confinedaquifer

confininglayer

piezometricsurface

= f (K)

confinedaquiferre-charge piezometric

surface

= f (K)

Confined Aquifer

confinedaquifer

confininglayer

Unconfined aquifer• zone of aeration • zone of saturation• vadose zone• capillary fringe• water table• water table well• re-charge zone

unconfinedaquifer

water table

unconfinedaquiferre-charge water

table well

Confined Aquifer

Confined aquifer • piezometric surface• confining layer• artesian well• re-charge zone

Cone of Depression

cone of depressionaquaclude = impermeable layer

Effect of Pumping Rate

drawdownradius of influence

Effect of Multiple Wells

Effect of Pumping Rate

Small soil particles pack together more closely than large particles, leaving many small pores.

Large soil particles pack together less closely, leaving fewer, but larger, pores.

Most soils are a mixture of particle sizes. Poorly sorted soils (greater range of particle sizes) will have a lower porosity, because the smaller particles fill in the "gaps“.

A given volume of spherical solids will have the same porosity, regardless of the size of the particles. The significance of porosity lies in role of surface tension (higher for small pores) in retaining water and frictional losses in transmitting water.

Porosity and Packing

Clays are small soil particles and thus one would expect tight packing. However, the net negative charge of clay particles separates them, resulting in a higher porosity than for a sphere of equivalent volume.

Sands are large particles, more regular in shape than silts and thus having a porosity similar to that expected for spherical particles.

Silts are intermediate in size between clays and sands and are irregular in shape. This irregularity leads to poorer packing than for spherical particles of similar volume and thus a higher than expected porosity.

Porosity of Specific Soils

Material Porosity (%) CommentClay 55 negative

chargeLoam (silts) 35 irregular shapeCoarse sand 30 regular shape

soil particles

pores

The net effect of the physicochemical properties of clay, silt and sand particles is that the porosity and thus water content tends to decrease as particle size increases.

Porosity Values

This is the amount of water, expressed as a %, that will freely drain from an aquifer

Specific Yield

Having a lot of water does not mean that an aquifer will yield water. Surface tension effects, most significant in soils with small pores, tend to retain water reducing the specific yield.

Material Porosity (%) Sp. Yield (%)Clay 55 3Loam 35 5Coarse sand 30 25

A better expression of the water available for development in an aquifer is the ratio of specific yield to porosity.

Material Ratio of Specific Yield : Porosity

Clay 0.05Loam 0.14Sand/Gravel 0.83

Specific Yield

Hydraulic Gradient

dhQ K Adx

Darcy’s Law

hydraulicconductivity

Hydraulic Conductivity (a.k.a. coefficient of permeability)

K = m3·m-2·d-1 = m·d-1

H

M2M1E

S1

S2

h1h2

h = H - s

r1

r2

An extraction well (E) is pumped at a constant rate (Q) and the drawdown (S) is observed in two monitoring wells (M) located at a distance (r) from the extraction well.

Determination of Hydraulic Conductivity

Hydraulic conductivity (m3∙m-2∙d-1) is then calculated by solving Darcy’s Law to yield:

DeterminingHydraulic Conductivity

dhQ K Adx

)(

ln

21

22

1

2

hhrrQ

K

At maximum drawdown, conditions at r1 (the well radius) are s1 = H and h1 = 0 and conditions at r2 (the edge of the cone of depression) are s2 = 0 and h2 = H.

H

E

S1

S2

h1 h2

h = H - s

r1

r2

And the maximum pumping rate (m3∙d-1) is calculated using the equation below:

1

2

21

22

lnrr

hhKQ

EstimatingWell Production