Aquifer Storativity (NT)
AQUIFER STORATIVITY, NON-TECHNICAL
By Darrel Dunn, Ph.D., PG, Consulting hydrogeologist (Professional Synopsis)
This is a non-technical page on aquifer storativity. To go to the technical page, press here.
Aquifer storativity is a term used by hydrogeologists to indicate the amount of water that an aquifer yields to wells due to the compression of the aquifer. A confined aquifer is a layer of permeable material such as sand or gravel that is full of water and is separated from the land surface by low permeable material such as layers of clay or silt (aquitards). When water wells are operated they reduce the pressure in the groundwater in the vicinity. When the pressure is reduced the aquifer compresses. A crude analogy is the compression of an automobile tire when the air pressure in the tire is reduced. The pressure was helping hold up the weight of the material above it.
Storativity is important in water resources management for a couple of reasons:
- It represents water that is obtained from the aquifer due to aquifer compression. This groundwater is sometimes said to be mined because it is water that was originally in the aquifer and is extracted by the wells. The remainder of the water from the aquifer comes from recharge from outside sources such as infiltration of rainfall from the land surface downward through the overlying aquitards.
- It affects the rate that the water levels in the wells decline in the early stage of their operation and is involved in predicting the sustainable pumping rate, which is needed for proper management of groundwater resources and for selecting the proper size of pumps for the water wells.
Specific storage is an important and useful term that is related to storativity. Whereas storativity refers to water stored in the entire thickness of an aquifer, specific storage refers to water stored in a specified small volume of an aquifer. The term specific storage is also applied to water stored in low-permeable material associated with an aquifer (silt and clay, for example). Commonly, groundwater is not produced from simple layers of permeable material such as sand and gravel, but is actually produced from more complex aquifer systems that contain layers and lenses of clayey and silty material. Such fine grained material is more compressible than the more permeable material that conveys the water to the wells. Therefore it yields more water as pressure in the aquifer system declines. Also, since its permeability is low, it releases the water slowly. Hydrogeologists must take this delayed yield of water into account when predicting sustainable pumping rates.
The specific storage of clay and silt is also important because it is related to their relatively high compressibility. As layers of clay and silt compress during pumping, the ground surface subsides. This land subsidence may be great if a lot of clay and silt is present in the aquifer system. For example, in the San Joaquin Valley of California subsidence has exceeded 28 feet, primarily due to agricultural use of groundwater for irrigation. Such land subsidence cannot be reversed by stopping pumping or artificially recharging the aquifer because it involves permanent changes in the components of the aquifer system. In granular materials these permanent changes may be caused by compression due to rotation and closer packing of grains, deformation of soft grains, breakage of brittle grains, and solution and redeposition of minerals due to extreme pressure at grain contacts. Land subsidence may cause expensive damage to structures such as canals.
However, when the water pressure in an aquifer system is fluctuating within a range that is greater than the minimum pressure that has ever occurred, the deformation is elastic and compression is not permanent. This is because the permanent irreversible compression has already occurred. Elastic deformation is caused by the compression of the water and the solids in the aquifer, rather than a readjustment of the packing of the solids as described above. In this range of pressure, specific storage values are much smaller than when pressure is being reduced to values less than previously experienced.
The technical page on storativity contains published values of specific storage and a more detailed discussion of the subject. The published values of specific storage are derived from (1) laboratory tests on subsurface materials recovered when wells are constructed, (2) field tests involving measuring fluctuation of the vertical distance between two anchor points in the aquifer system, (3) interpreting water level fluctuations in monitoring wells due to pumping (aquifer pumping tests), (4) interpreting water level fluctuations in monitoring wells due to changes in atmospheric pressure, and (5) calibration of computer models of aquifer systems with measured land subsidence and measured water pressure in the aquifer system.
Hydrogeologists involved in water management must have a thorough understanding of the concept of specific storage.
Laypersons involved in water management should not confuse storativity with safe yield (aka sustainable yield and sustainability). Safe yield involves the balance between groundwater extraction, aquifer drawdown due to pumping, and aquifer recharge, Aquifer recharge includes infiltration of precipitation, infiltration from surface water (streams, lakes, wetlands), and infiltration from artificial recharge facilities. Once extraction and recharge are balanced and the groundwater system is in a steady state, there is no change in the amount of water stored in the aquifer. It is no longer compressing and storativity is not involved. Some investigators calculate total potential groundwater production from a confined aquifer by assuming no leakage from overlying or underlying low permeable materials, but this is seldom the case. This calculation does indicate how much water can be produced solely due to compression of the aquifer (mined), and sometimes in practice this mined water is initially produced at a rate that is not sustainable. Safe yield is a complex issue that may involve changes in groundwater quality and legal considerations. A qualified hydrogeologist should be consulted when it must be addressed.
Posted March 26, 2012