groundwater supply dr. martin t. auer michigan tech department of civil & environmental...
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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
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