Hydrogeology

The occurrence, distribution, storage, and flow of groundwater beneath the surface are the subjects of hydrogeology. Groundwater, often known as subsurface water, is any water that is found below the surface of the Earth. Groundwater requires very little treatment to be used, in contrast to surface water. One element of the hydrologic cycle in nature is groundwater. Groundwater seeps through natural cavities and intergranular pores in rocks, known as primary openings, as well as discontinuities like fractures, joints, and solution cavities, known as secondary openings. While secondary openings can be found in most igneous and metamorphic rocks as well as in certain sedimentary rocks, primary openings are typically found in sedimentary rocks.

A rock mass's porosity, or the percentage of its volume made up of pore spaces, voids, and openings, determines how much groundwater it can hold. Permeability, which is the attribute of rock related to its capacity to transfer water, is quantified as the flow through unit area of a material in unit time under unit hydraulic head. Permeability, commonly known as hydraulic conductivity or coefficient of permeability, is measured in meters per second in the SI; for rocks, it is more conveniently expressed in meters per day. The porosity and permeability values of various common rocks, soils, and rock fracture zones are shown in the following table.

Source: Waltham, T., Foundations of Engineering Geology, Spon Press, London, 2002

Not all rocks that are porous are equally permeable. A rock's ability to absorb water is determined by the size and degree of interconnectivity of its pore spaces and apertures. While most rocks include discontinuities like joints and cracks and pores, most soils transfer water through their pores. Usually, joints and fractures transfer more water than pores. Reduced permeability can arise from the loads exerted by the ground-based constructions on pores and fractures. On the other hand, permeability can be enhanced by desiccation-induced shrinkage, which can enlarge voids in soluble rocks, and by dissolving, which can open cracks in clays. The following categories apply to geological formations based on their ability to yield and contain water:
Aquifers: Rocks and soils that are both porous and permeable.
Aquicludes: Rocks and soils that are porous but not permeable.
Aquitards: Rocks and soils that are porous but have limited permeability.
Aquifuges: Rocks and soils that are neither porous nor permeable.

Large volumes of groundwater are stored in aquifers, and their permeability allows regular or pumping wells or springs to always have access to a sufficient supply of water. Unconfined aquifers are those in which groundwater exists at air pressure. The water table is represented by the water level in a well that is drilled into an unconfined aquifer. Constrained aquifers, sometimes referred to as artesian or pressure aquifers, are aquifers that are situated between two comparatively impermeable strata (aquicludes or aquifuges). Because groundwater cannot pass through impermeable layers, the pressure inside the aquifer is higher than that of the surrounding atmosphere. The recharge region is the space where precipitation seeps into the limited aquifer. The term "piezometric surface" refers to an imaginary surface that coincides with the water's hydrostatic pressure level in the confined aquifer. A representative schematic cross-section of the confined and unconfined aquifers is presented in the figure below. Depending on the depth and location of the well, a drilled well may be classified as an artesian well (site C with the well bottom lying in the confined aquifer zone), a flowing well (site B with the well bottom lying in the confined aquifer zone), or a water table well (site A and site B with the well bottom lying in the unconfined aquifer zone). As a result, the location, size, and makeup of the aquifers in any given area determine the likelihood of having a sufficient supply of groundwater. Aquifers are common in river valleys, and the water table is typically close to the surface. Groundwater is typically easily accessed in these areas via both conventional and pumping wells.

Sand, gravel, sandstone, limestone, grit, conglomerate, and other minerals are examples of typical aquifers. Aquifers can also be found in igneous and metamorphic rocks at fault-, shear-, and joint zones, among other places. A rock or soil must have permeability greater than 1 m/day in order to qualify as an aquifer (Waltham, 2002).

Unconfined and confined aquifers

Aquicludes include rocks such as siltstones, mudstones, clays, and shales. Aquitards, sometimes known as leaky aquifers, are clays with limited silt content. Typical examples of aquifuges include massive, compact granite, syenite, gabbro, gneiss, and quartzite with no discontinuities. There is absolutely no way for groundwater to seep into these rocks. Permeability of impermeable rocks and soils, or aquicludes and aquifuges, is often less than 0.01 m/day (Waltham, 2002).