November 2013

Al-Azhar University-Gaza Master Program of Water and Environmental

Science

Properties of aquifer

Porosity Porosity: the percentage of rock or sediment that

consists of voids or openings.

porous: a rock that holds much water

Porosity

Effective Porosity •ne Effective porosity is the fraction of the porosity that is

available for transporting water (excludes fraction of pores too small to hold water, or those that are not inter-connected

- can be measured in the lab directly by saturating a dried sample of known volume and measuring water uptake in a sealed chamber over time

- for unconsolidated coarse-grained sediments there is no significant difference

Porosity of Sedimentary Rocks •Primary porosity

–Pores between grains

•Secondary porosity

–Fractures

Permeability permeability: the capacity of a rock to transmit a fluid

such as water or petroleum through pores and fractures

permeable: a rock that allows water to flow easily through it

impermeable: a rock that does not allow water to flow through it easily

Specific Yield (Sy) •Specific Yield Sy the ratio of the volume of water that

drains from a saturated geomaterialowing to the attraction of gravity to the total volume of the geomaterial

Specific yield & material

Specific Retention (Sr) •Specific retention Sr the ratio of the volume of water a

geomaterial can retain against gravity drainage to the total volume of the geomaterial.

n = Sy+ Sr

Hydraulic Conductivity

Darcy’s Experiments

Darcy’s Experiments Flow through sand filters

Discharge (Q) proportional to

Area, A

Head drop, h1-h2

Inverse of length, L

Hydraulic Conductivity K hydraulic conductivity (L/T)

A function of properties of both porous media and the fluid passing through it

A function of porous media only =Ki intrinsic permeability

ρ = density µ = dynamic viscosity

g = gravitational constant

Factors Affecting Measurement of K

Effective stress

Hydraulic gradient

Volume of Flow

Ground water

the water that lies beneath the ground surface, filling the pore space between grains in bodies of sediment and clastic sedimentary rock, and filling cracks and crevices in all types of rock

The Water table water table: the upper surface of the zone of saturation

and pore water pressure is equal to atmospheric pressure.

saturated zone: the subsurface zone in which all rock openings are filled with water

vadose zone: a subsurface zone in which rock openings are generally unsaturated and filled partly with air and partly with water; above the saturated zone.

capillary fringe: a transition zone with higher moisture content at the base of the vadose zone just above the water table

The Water table The following generalizations are valid:

1. In the absence of flow the water table will be flat

2. A sloping water table indicates flow

3. Ground-water discharge occurs in low zones

4. The water table has the same general shape as the surface topography (but less relief change)

5. Ground water generally flows from topographic highs to lows

Aquifers

Aquifer: A formation, that contains sufficient saturated, permeable material to yield significant quantities of water to wells and springs.

Types of Aquifers •Unconfined aquifer(Water-table aquifer)

•Confined aquifer(Artesian aquifer)

Semi confined aquifer.

1. Unconfined Aquifer •An aquifer that is close to the ground surface, with

continuous layers of materials of high intrinsic permeability extending from the land surface to the base of the aquifer.

•Recharge from downward seepage through the unsaturated zone, lateral ground water flow, or upward seepage from underlying strata.

2.Confined Aquifer •An aquifer that are overlain by a confining layer.

•Recharge occurs in recharge area, where the aquifer crops out, or by slow downward leakage through a leaky confining layer.

•Potentiometric surfaceis the surface representative of the level to which water will rise in a well cased to the aquifer

3.Semi confined aquifer

Are overlain by a semi confining layer.

Unconfined & confined Aquifer

Artesian Wells •A well whose source of water is a confined aquifer

• The water level in artesian wells is at some height above the water table due to the pressure of the aquifer

Water Table Well

Artesian Well

Flowing Well

Pot. Surface

Confined Aquifer and

Confining Unit Above

Bedrock Zone

artesian well: a well in which water rises above the aquifer

Perched Aquifer Perched Aquifer - aquifer in the vadose zone because

of a lens of impermeable material

- common in glacial outwash (clay from ponds), volcanic deposits .

the top of a body of ground water separated from the main water table beneath it by a zone that is not saturated

Perched Aquifer

Confining layer - layer having low or no peremeability

- whether a layer is considered “confining” or not will depend on main aquifer material

- usually confining layers have some permeability, just very low

permeability –less than about 10-2darcy.

Types of confining layers:

Aquiclude - layer of low permeability that can store and transmit groundwater slowly between aquifers (now more commonly referred to as “leaky confining layer”)

Aquifuge - absolutely impermeable and contains no water

Pumping in confined aquifer Pumping lowering the aquifer surface in a cone of

depression.

cone of depression: a depression of the water table formed around a well when water is pumped out; it is shaped like an inverted cone

Pumping in confined aquifer

Storativity Storativity (S) or Storage coefficient

•The volume of water that a permeable unit will absorb or expel from storage per unit surface area per unit change in head.

Specific Storage

pw density of water

g the acceleration of gravity

α compressibility of aquifer skeleton (1/(M/LT2))

βcompressibility of water (1/(M/LT2))

n porosity (L3/L3)

Storativity Confined aquifer

S = B Ss

Unconfined aquifer

S = Sy+ h Ss

Homogenous unit Is one that has the same properties at all location.

For a sank stone this would indicate that the graine size ,porosity, thickness are variable in a small unit , so the value of transmissivity and storativity of the unit would be the same wherever present.

Heterogeneous unit Formations , hydraulic properties change.

Isotropic & anisotropic

If a porous medium has equal intrinsic permeability in all directions it is said to be isotropic

If the pattern of voids allows for a path of least resistance (i.e. direction in which Ki is higher) the unit is said to be anisotropic

- fractured rocks, basalts often highly anisotropic

- sedimentary rocks may have many homogenous units

Grain shape and orientation can affect the isotropy or anisotropy of a sediment

Anisotropy of fractured rock units due to directional nature of fracturing