Module 9: The Hydrosphere: Groundwater

water table

the two-dimensional surface beneath Earth’s surface that divides the saturated zone, in which pore spaces in the rock are filled with groundwater, from the overlying unsaturated zone

groundwater

underground water below the level at which all open spaces in rock and sediment are saturated

porosity

the ratio of pore volume in a rock to its total volume expressed as a percentage

primary porosity

pores develop at the same time the rock itself forms

secondary porosity

develops after a rock is formed

surface tension

the attraction between molecules at the surface of a liquid

specific yield

the ratio of the volume of water that is drained from an aquifer by gravity to its total volume

specific retention

the ratio of groundwater retained by surface tension to total volume of water

permeability

the ability of a rock to allow the flow of groundwater through its pores

hydraulic conductivity

the volume of groundwater that moves through a unit of area perpendicular to the direction of flow in a unit of time

aquifers

a permeable body of rock or sediment that stores and transmits enough water to supply wells or springs

confined aquifers

an aquifer in which the water is held under pressure between strata that are impermeable or have very low permeability

unconfined aquifers

an aquifer that is not held under pressure by and impermeable layer

hydraulic gradient

the change in water pressure over horizontal distance in an aquifer

potentiometric surface

a hypothetical plane located above the top of a confined aquifer to which water would rise if a well were drilled into the aquifer

artesian spring

a natural flow of groundwater at Earth’s surface that occurs when water under high pressure in a confined aquifer is able to reach the surface, ex: along a fault

artesian wells

a well that taps water under high pressure in a confined aquifer and needs little or no pumping

Darcy’s law

a formula used to quantify the flow of fluids through a porous medium, such as an aquifer; according to this law, the rate of groundwater flow is proportional to the hydraulic gradient of the fluid and the hydraulic conductivity of the porous medium

cone of depression

the cone-shape dip in the water table that forms around a well when the amount of water being removed from the well exceeds the flow of water into the well

land subsidence

the slow sinking of an area of crust without major deformation (ex: water is removed from an aquifer, pressure on the inner walls of the pores decreases and the porous structure deflates)

water spreading

the practice of spreading runoff water over a large land area to infiltrate permeable regions where the water table is not close to the surface

recharge basins

basins that have been constructed to hold water in order to resupply groundwater by allowing the water to infiltrate over time

injection wells

wells through which water is pumped into the ground to maintain the pressure of the reservoir or to dispose of chemical wastes

leachate

a liquid mixture of water, chemicals and metals produced in water that has infiltrated a landfill come into contact with waste, and moved into the zone of saturated groundwater

miscible contaminants

in groundwater, contaminants capable of dissolving in water

immiscible contaminants

in groundwater, contaminants that are insoluble