groundwater ­­ notes.notebook
Water Beneath the Surface
• groundwater: the water that is beneath Earth's surface
> important source of freshwater in the United States
• aquifer: body of rock or sediment in which large amounts of water can flow and be stored
> for water to flow freely pores or fractures must be connected
> affected by porosity and permeability
• Porosity: the percentage of the total volume of a rock or sediment that consists of open spaces
> sorting, which affects porosity, is the amount of uniformity in the size of the rock or sediment particles
– well sorted: particles are about the same size
– poorly sorted: particles are various sizes
> particle packing affects porosity
– loosely packed particles= more open spaces that can store water= high porosity
– tightly packed particles = less open spaces that can store water = low porosity
• Permeability: the ability of a rock or sediment to let fluids pass through its open spaces, or pores
> open spaces must be connected for it to be permeable
> sorting affects permeability
– better sorted particles are, the more permeable the rock tends to be
– impermeable: water cannot flow through the rock/sediment
« ex. clay
groundwater ­­ notes.notebook
• Zones of Aquifers
> Zone of Saturation: pore space is completely filled with water
– water table: upper surface of zone of saturation
– lower of the two zones of groundwater
> Zone of Aeration: lies between the water table and Earth's surface
– composed of 3 regions
« uppermost region holds soil moisture
« bottom region is the capillary fringe
« between is a dry region except when it rains
« water is drawn up from the zone of saturation into the capillary fringe by capillary action
» caused by attraction of water molecules to other materials, such as soil
• WELL: hole that is dug to below the level of the water table and through which groundwater is brought to Earth's surface
• SPRING: natural flow of groundwater to Earth's surface in places where the ground surface dips below the water table
• wells and springs can be ordinary or artesian
> ordinary wells: work only if they penetrate highly permeable sediment or rock below the water table
– pumping water from a well lowers the water table around the well and forms a cone of depression; if too much water is taken the cone of depression may drop and the well could dry up
> ordinary springs: usually found in rugged terrain where the ground surface drops below the water table.
groundwater ­­ notes.notebook
• Artesian wells & springs
> many wells are supplied by groundwater from local precipitation, but some could come from hundreds of km away; could become part of an artesian formation
> artesian formation: a sloping layer of permeable rock sandwiched between two layers of impermeable rock and exposed at the surface
– permeable rock is the aquifer and the top layer of impermeable rock is the caprock
– water is under pressure so the water flows up through the well freely without being pumped
• Hot springs: groundwater is heated when it passes through rock that has been heated by magma, and the water rises to the surface before cooling
• Geysers: hot springs that periodically erupt from surface pools or through small vents
groundwater ­­ notes.notebook
Groundwater & Chemical Weathering
• as groundwater passes through permeable rock, minerals in the rock dissolve
> the warmer the rock is and the longer it is in contact with the water the greater the amount of dissolved minerals in the water
– hard water: water that has high concentrations of dissolved minerals (calcium, magnesium and iron)
– soft water: water that has relatively low concentrations of dissolved minerals
• Results of Weathering by groundwater: minerals become dissolved by chemical weathering by water combining with carbon dioxide to form carbonic acid
> Caverns: a natural cavity that forms in rock as a result of the dissolution of minerals; also a large cave that commonly contains many smaller, connecting chambers
– typically occurs in rocks rich with calcite (limestone)
– when water containing dissolved calcite grips from the ceiling of a limestone cavern, some of the calcite is deposited on the ceiling
« stalactite: cone­shaped deposit on the ceiling of the cavern
« stalagmite: upward­pointing cone that forms on the cavern ground due to the water falls onto the cavern floor
« column: forms when a stalagmite and stalactite meet
groundwater ­­ notes.notebook
• Sinkholes: circular depression that forms when rock dissolves, when overlying sediment fills an existing cavity, or when the roof of an underground cavern or mine collapses
> most form from dissolution, in which the limestone or other rock dissolves where weak areas in the rock (fractures) previously existed
– dissolved material is carried away from the surface and a small depression forms
– subsidence sinkholes: as the rock dissolves, overlying sediments settle into cracks in the rock and a depression forms
– collapse sinkholes: sediment below the surface is removed and an empty space forms within the sediment layer; the overlying sediments collapse into the empty space below
« can form during dry periods
« develop abruptly and cause extensive damage
• Natural bridges: when the roof of a cavern collapses in several places, a straight line of sinkholes form; the uncollapsed rock between each pair of sinkholes will form an arch­­could eventually collapse
groundwater ­­ notes.notebook
• Karst Topography: a type of irregular topography that is characterized by caverns, sinkholes, and underground drainage and that forms on limestone or other soluble rock
> in the US there is karst topography in Kentucky, Tennessee, souther Indiana, northern Florida and Puerto Rico
> generally forms in regions where the climate is humid and limestone formations exist at or near the surface