Excavation under water table
Introduction
Groundwater is water present under the earth's surface, in rocky spaces or porous soils or in the voids of rock formations. About 30 percent of all fresh water available in the world is groundwater. An accumulation of groundwater is considered an aquifer when it can provide a sufficient amount of water for use by man. The depth at which spaces of porous soil and cracks or voids in rocks are filled with water is known as the water table. Groundwater is recharged from the surface; It can also come to the surface naturally through springs and seeps, and can form oases or swamps. On the other hand, groundwater is often extracted for use in agriculture, industry, and human consumption by constructing wells. The study of the distribution and movement of groundwater is called hydrogeology.
Groundwater represents an important fraction of the mass of water present on the continents, under the Earth's surface, both in the soil and in the subsoil. The volume of groundwater is much more important than the mass of water retained in lakes or circulating, and although smaller than that of the largest glaciers, the largest masses can reach a million or more km² (such as the Guaraní Aquifer). Underground water is an important resource, and it supplies a third of the world's population,[1] but it is difficult to manage, due to its sensitivity to pollution and overexploitation. Groundwater is part of the precipitation that percolates through the soil until it reaches rock material that is saturated with water. Groundwater moves slowly toward low levels, usually at steep angles (due to gravity), and eventually reaches streams, lakes, and oceans.
It is a common belief that groundwater fills cavities and circulates through galleries. However, this is not always the case, as it can be found occupying the interstices "Interstice (mineralogy)") (pores and cracks) of the soil, rocky substrate or unconsolidated sediment, which contain it like a sponge "Sponge (utensil)"). The only significant exception is offered by soluble rocks, such as limestone and gypsum "Gypsum (mineral)"), susceptible to undergoing the process called karstification, in which water excavates chasms, caverns and other circulation routes, the model that best fits popular belief.
An aquifer is a clearly differentiated accumulation of water in the pores or cracks of a mass of permeable rocks that allows its circulation. Storage rocks can be made of very varied materials such as poorly cemented porous gravels and sandstones (ancient marine, river, beach, and wind sediments), silt, certain types of clay, cracked limestone, and even volcanic formations. The upper level of groundwater is called the water table, which in the case of a free aquifer corresponds to the water table.