Characteristics of Aquifers

Groundwater refers to water located beneath the surface in soil pore spaces and in fractures of rock (lithologic) formations. An aquifer is an underground layer of water bearing, permeable rock or unconsolidated materials such as gravel, sand, silt, or clay. The characteristics of aquifers changes based on the geology and structure of the substrate and topography. Productive aquifers typically occur in sedimentary geologic formations. Weathered and fractured crystalline rocks produce smaller amounts of groundwater in most environments. Unconsolidated to poorly cemented sediment materials that accumulate in river valleys and geologically subsiding basins are often the most productive water bearing aquifers. Groundwater maintains a relatively constant temperature due to the specific heat capacity of water and the insulating effect of soil and rock.

Characteristics of an Aquifer

The transmissivity T (m2/sec) is a hydraulic property which measures the ability of  an aquifer to transmit ground water through its entire saturated thickness. It is defined as the product of the hydraulic conductivity K (m/sec) and the saturated thickness B (m), in the direction normal to the base of the aquifer:

T = KB

Hydraulic Conductivity

Hydraulic conductivity (K) defines the rate of movement of water through a porous medium such as an aquifer. It is the constant of proportionality in Darcy’s Law and as such is defined as the flow volume per unit cross-sectional area of porous medium under the influence of a unit hydraulic gradient. This translates to SI units of m³/m²/day or m/d, but other measurement units are commonly used as well.

Commonly used units for hydraulic conductivity (K)

Description	metres/day (m/d)	metres/second (m/s)	millimetres/day (mm/d)	millimetres/hour (mm/hr)
Extremely slow	0.000001	1.5741x10-11	0.001	0.000041667
Very Slow	0.0001	1.5741x10-9	0.1	0.0041667
Slow	0.01	1.5741x10-7	10	0.41667
Moderate	1	1.5741x10-5	1000	41.667
Fast	10	1.5741x10-4	10000	416.667
Very Fast	100	1.5741x10-3	100000	4166.667

Measurement of hydraulic conductivity is problematic, considering the parameter can differ over several orders of magnitude across the spectrum of sediments and rock types, as indicated in. The parameter can also vary greatly in space, even with apparent minor changes in sediment characteristics. Hydraulic conductivity is influenced by the properties of the fluid being transmitted, such as viscosity, as well as the porous medium. Hydraulic conductivity is also scale dependent, so that measurements taken at the core sample level may not be directly extrapolated to the aquifer scale. It is also direction dependent, so that hydraulic conductivity can be markedly different in the vertical from the horizontal. Hydraulic conductivity cannot be directly measured but inferred from field, laboratory or modelled data.

Specific Capacity

The Specific Capacity of a well is simply the pumping rate, or yield, divided by the drawdown. It is a critical number that can be used to provide the design pumping rate or maximum yield for the well. It can be used to identify potential well, pump, or aquifer problems, and accordingly to develop a proper well maintenance schedule. It can also be used to estimate the transmissivity of the aquifers tapped by the well's perforations. Transmissivity is the rate water is transmitted through an aquifer under a unit width and a unit hydraulic gradient. It equals the aquifer's hydraulic conductivity. or permeability, times the aquifer thickness. The higher the transmissivity, the more prolific the aquifer and the less drawdown observed in the well.