Specific storage, storativity, specific capacity, and specific yield are storage properties of an aquifer, referring to the quantity of water that can be released. Storage definitions and some values for the Sparta aquifer in Louisiana are presented in Endnote 4. ‘Storativity ‘ is the subject of sections that follow in this paper.
Release of Water from a Confined Aquifer, like the Sparta, as opposed to an Unconfined Aquifer
An aquifer’s pores make up the space between the aquifer’s sand. In an unconfined aquifer, withdrawing water results in a decline in water table; aquifer pores are drained of water, leaving air to fill in. In a confined aquifer, where water is compressed, withdrawing water results in a pressure drop (decline in potentiometric surface), but aquifer pores remain filled with water. From equivalent unit areas with equivalent change in head, the volume of water released from a confined aquifer is much less than that released from an unconfined aquifer. The concept of storativity is illustrated in Figure 8.
From equivalent unit areas with equivalent change in head, the volume of water released from storage in the Sparta aquifer has been reported as generally about 1,000 times less than that released from the unconfined Mississippi River Valley alluvial aquifer. (1) The alluvial aquifer has greater storativity (Figure 8), as well as greater porosity and hydraulic conductivity. (11)
Importance of the Sparta Aquifer’s Small Storativity
While the Sparta aquifer can produce high quality water, its relatively small storativity, as a confined aquifer, means that large water-level declines over extensive areas are required to achieve the water yields of an unconfined aquifer over a much smaller area. This means that:
Decreased pumping in one location will result in water level rises that become relatively widespread over time.
Yield is a measure of the pumping rate as gallons per minute of a specific well. It is influenced by aquifer characteristics, pumping time, and well construction. Individual wells in the Sparta (excluding those wells located within areas of large drawdowns) generally yield 100 to 500 gallons per minute, with less common rates up to 1,200 gallons per minute. (8)
Specific Capacity is the pumping rate per unit foot of drawdown, tracked over time. The measure is used to identify well and aquifer performance problems. For Sparta wells, the value has been estimated as five to ten gallons per minute per foot of drawdown. (10)