Specific storage
The storage coefficient describes in hydrogeology the ability of an aquifer to store water or leave. The storage coefficient can be viewed as a measure of the compressibility of the aquifer. Responsible for the storage capacity is essentially the compressibility of air bubbles trapped in the pore space as well as the deformability of the soil skeleton. Density change of the grains and the water are negligible compared to the volume changes of soil skeleton and gas inclusions. The elastic deformations of fluid and soil skeleton allow for increase in pore pressure can be a water storage in the soil skeleton. This process is reversible, therefore initially carried out at a relief of the pore pressure (eg in wells promotion ) a discharge of stored water from the soil skeleton. Only when the water was discharged from the storage change is the change in pressure can spread as a result of lowering wells in the aquifer. This is significant when the rate of pressure change is rapid compared to the hydraulic permeability k of the aquifer, such as the drainage of reaching the groundwater pit. Due to the delayed response due to large capacity hydraulic gradients can build up that can lead the course of a subsequent Dissipationsvorgangs for the reduction of the effective stresses or even hydraulic base failure.
Formally, a distinction depending on the spatial consideration between the storage coefficient and the specific storage coefficient.
Specific storage coefficient
The specific storage coefficient describes the volume of water that can be (for example, an imaginary cube of side length 1 m ) stored in a unit volume of an aquifer or removed from the case of a change of the potential h [L ] by one unit.
According to the above definition of the specific storage coefficient is defined by the following formula:
With
At full water saturation of the pore space has the specific storage coefficient values between 10-5 and 10-6 m-1 m -1. In a m3 of groundwater Leites can thus by increasing the hydraulic potential by dh = 1 m, a volume of water dVw between 1 - 10 ml of water are stored.
Due to the significantly higher compressibility of gases, however, this value increases significantly as soon as the smallest amounts ( 1-2 %) is included in the air in the cavity. Air inclusions occur in nature due to fluctuating water table frequently. Due to the Druckzuname with depth results in a depth distribution of the gas volume, which directly is hardly measurable.
Storage coefficient
In the groundwater hydraulics is considered simplistic groundwater flow than over the entire thickness of an aquifer averaged, where one neglects the vertical flow component Dupuit after. In this approach, the storage coefficient S is used, stating the aquifer from an integration of the specific storage coefficient on the thickness [L]. The thus-obtained storage coefficient S is dimensionless. The storage coefficient is usually determined by means of pumping test. The orders of magnitude for storage coefficient for confined aquifer are between 10-5 and 10-3. The higher values occur only in air pockets in the soil skeleton. In unconfined porous aquifers corresponds to the storage of usable and porosity is in the range from 0.10 to 0.25.