This presentation by ACWADAM deals with the structure of the sub-surface. The lithology and structure of rocks together define the geometric framework for the movement of groundwater. To some extent, these factors also govern the movement of surface water too. In other words, the physical environment of rocks and rock material hosts water, and hence, controls its accumulation and flow in the watershed.
The geology of a watershed, therefore, forms the basic science in the study of water resources in a watershed. At the moment, watershed professionals are more concerned with the topography of the watershed than the geology. In fact understanding the geology also helps in understanding the topography in a watershed, these two factors being quite closely connected to each other.
The presentation details the three types of rocks – igneous, sedimentary and metamorphic. Igneous and metamorphic rocks do not show primary porosity (pore spaces between grains). Metamorphic rocks have similar characteristics as igneous rocks but have structures similar to the sedimentary rocks. They show a prominent schistosity similar to the stratification in sedimentary rocks. However, under certain circumstances they help store and allow movement of groundwater. The attitude of joints and fractures determine the direction in which the water will move. The size of openings made by joints and fractures often would decide the ease with which water would move in the rock.
Fractures may be associated with movement of groundwater or they may not be associated with movement. Irrespective to their association with movement they may be opened by decay, thereby making them channels, through which water may move and may be stored. Discordant bodies of igneous rocks such as dykes often form barriers which do not allow movement of groundwater and tend to accumulate water in underground reservoirs.
Secondary porosity developed during the decay and structural deformation may allow water to move and get stored in metamorphic rocks. Sedimentary rocks exhibit a variety of textures and structures which allow storage and movement of groundwater. A study of these structures will indicate the direction of the movement of groundwater in the sub-surface.
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.