Properties of Aquifers

Physical Properties

Density of Fluid - mass per unit volume (e.g., kg/m3 or g/cm3).

Dynamic Viscosity of Fluid - resistance to relative flow of a Newtonian fluid (Pa-s or poise (g/s-cm))

Bulk Modulus - Proportionality constant between density and pressure. Inverse of compressibility (Pa or lb/ft2)

Porosity of Earth Materials
Porosity = percentage of rock or soil that is void of solid material.

Effective Porosity = porosity available for fluid flow (interconnected and large enough pore throats)

Porosity depends on packing of grains (cubic or rhombohedral), grain size distribution (sorting), and shape of grains.

Specific Yield = ratio of the volume of water that drains from a saturated rock owing to the attraction of gravity to the total volume of the rock.

Specific Retention = ratio of the volume of water a rock can retain against gravity drainage to the total volume of the rock.

n = Sy + Sr

Maximum specific yield occurs in medium to coarse sand size sediments.

Hydraulic Conductivity of Earth Materials
Darcy's Experiment - rate of water flow through a sediment sample is proportional to the difference in height of the water between the two ends and inversely proportional to the length of the flow path. The quantity of flow is proportional to the hydraulic conductivity.

Q = discharge

K = hydraulic conductivity

A = Area

L = Length

h = hydraulic head

Hydraulic Conductivity (m/s or ft/day)

Hydraulic conductivity is a function of both the porous medium and the fluid passing through it.

Intrinsic Permeability (m2 or Darcy)

Where  is fluid density and  is fluid dynamic viscosity. Fluid density and viscosity depend on temperature, salinity and pressure.

Intrinsic Permeability is representative of the porous medium alone. It depends on size of openings, degree of interconnection, and amount of open space. Thus, permeability depends on grain size and sorting. Coarse grained and well-sorted sediments have higher permeabilities.

Measuring Hydraulic Conductivity
Constant Head Permeameter

Falling Head Permeameter (measuring low permeability sediments)

Water Table
Water table is the undulating surface at which pore pressure is equal to atmospheric pressure.

A sloping water table indicates ground water is flowing

Ground water discharge occurs in topographic low spots

Water table has same general shape as surface topography

Ground water general flows away from topographical high spots and toward topographic lows.

Aquifer - a geologic unit that can store and transmit water at rates fast enough to supply reasonable amounts to wells (e.g., unconsolidated sands and gravels, sandstones, limestones and dolomites, basalt flows, and fractured igneous and metamorphic rocks).

Confining Layer - a geologic unit having little or no intrinsic permeability (less than 10 md). Salt, shales and unfractured igneous and metamorphic rocks.

Unconfined Aquifer - connected to the surface (meteoric recharge)

Confined Aquifer - overlain by a confining layer. Limited or no direct access to recharge from surface. Potentiometric surface of a confined aquifer may be above the land surface (i.e, an artesian well).

Aquifer Characteristic
Transmissivity (m2/s) - saturated thickness times hydraulic conductivity

T = bK

where b = saturated thickness of aquifer

Specific Storage (m-1) - amount of water per unit volume stored or released from aquifer per unit change in head.

Storativity - volume of water that an aquifer will absorb or expel from storage per unit surface area per unit change in head.

S = bSs (confined aquifer) or S = Sy + hSs (unconfined aquifer)

where Sy = specific yield

Effective Stress - stress supported by aquifer skeleton

Thus if fluid pressure decrease, effective stress increases and the aquifer may compact

Compressibility (m2/N) - coefficient relating stress (pressure) to strain (change in aquifer thickness)

where db = change in aquifer thickness

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