Groundwater

Groundwater Hydrology

Active groundwater is conventionally distinguished as the portion that circulates in contact and exchange with Earth's surface hydrological cycle; this accounts for approximately half of the total groundwater. The remaining deep groundwater portion occurs within Earth's crust and is largely inaccessible to surface hydrologic interaction or human use. Geological conditions, specifically geochemical processes and residence times, influence the quality of groundwater. Much deep groundwater is considered to be of a quality that is not suitable for ecological or human purposes; for example, it frequently has high dissolved mineral content as a result of prolonged chemical interaction or deposition from saline ocean water over geological timescales. Shallow groundwater extensively interacts with surface water over timescales that vary from days to millennia.

Aquifers are the spatial units of rock, alluvium, and other subsurface materials where groundwater occurs, and they are separated from other aquifers by nonporous geological formations that impede the subsurface flow of water. Aquifer properties include porosity, rock fractures, conductivity (groundwater movement that a pressure gradient produces), and transmissivity (conductivity over the depth of the water-bearing strata). These properties, coupled with groundwater pressure that results from elevation differences of water levels in the aquifer, determine the rates of groundwater extraction that pumping will produce. Drawdown and cone of depression refer to localized groundwater-level effects resulting from extraction.

Groundwater is recharged vertically from the surface infiltration of rainfall, other atmospheric precipitation, or water bodies, including lakes, rivers, wetlands, and estuaries. Under certain aquifer conditions, recharge may occur laterally from surface water bodies. Water occurring in the immediate near-surface, weathered soil layer, often coinciding with the depth that plant roots will penetrate, is referred to as soil moisture. Below this, water percolates in the unsaturated vadose zone (the area between the surface and the saturated zone) and deeper to saturated layers of the aquifer, commonly referred to as the water table. Alluvial deposits often result in unconfined aquifer conditions. In contrast, impervious strata, for example, solid or low-porosity rock or clay layers, can result in confined or semiconfined aquifers and, depending on the groundwater pressure, may cause water to flow freely to the surface in artesian wells.

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