Hydrologic properties of rocks – A presentation by ACWADAM

This presentation by ACWADAM deals with hydrologic properties of the rock framework in which groundwater occurs and through which it moves. In watershed programmes, it is necessary to develop a correct understanding of the storage and transmission of water within such channels because the shape of openings in a rock or sediment, their size, volume and interconnection play a vital part in the hydraulic characteristics of an aquifer.

The two important properties of an aquifer that are related to the storage function are the “porosity” and “specific yield”. The porosity of a water bearing formation is determined by that part of its volume consisting of openings or pores. Porosity is an index of how much groundwater can be stored in a saturated medium and is usually expressed as a percentage of the bulk volume of the material.

The types of rock openings (intergranular/intragranular) that attribute porosity to rocks are described in the presentation. Porosity in alluvium i.e., gravel, sand & clay is different from porosity due to fractures in hard rock. The types of porosity such as primary/secondary are described. Molecular forces of cohesion, adhesion and capillarity that are under the influence of gravity (that is under gravity drainage) are explained.

Although the volume of water contained in a part of the aquifer is of interest to us, it is more important to know how much of this volume can actually be released by the aquifer for use. This release of water per unit area of the aquifer is generally expressed as volume per unit change in head, i.e., change in the water level. The presentation thereafter, deals with hydraulic conductivity, the property of the rock material that relates to its function allowing the flow of water.

This presentation is part of the training modules on planning, development and management of groundwater with special reference to watershed management programmes by ACWADAM. Please write to ACWADAM at acwadam@vsnl.net for sourcing these presentations.