http://www.ctecomputer.com/fountains/

Groundwater

All water found underground, frequently in:

Joints and cracks in rocks

Open space between sediment grains

Cave systems

Porosity – volume of open space within a body of rock or sediment (all space not occupied by solid material)

Permeability – how easily fluid can move through the rock or sediment, which is frequently dependent on how much connectivity there is between pores.

Porosity and Permeability

http://www.epa.gov/

PorosityVolume of open space within a body of rock or sediment. Usually expressed as a percentage of the total volume of rock or sediment.

http://www.gly.uga.edu/railsback/GeologicalDiagrams1.html

PermeabilityEase with which fluid moves through the rock or sediment.

Rock and sediment can have high porosity, but very low permeability (e.g., mudstone), and vice versa (e.g., lava tubes).

AquifersBodies of rock and/or sediment with high permeability through which groundwater flows.

Bodies of rock with low permeability are called

aquicludes or aquitards.

Unconfined aquifer - aquifer with open to the surface

Confined aquifer - aquifer confined at the top by an aquiclude or aquitard

Unsaturated zone

saturated zone

Unconfined aquifers

Groundwater in unconfined aquifers is found in two underground zones:

Unsaturated zone - pore space primarily filled with air

saturated zone - pore space filled with water

The top of the saturated zone is a surface called the water table.

This is an unconfined aquifer (open to air at top)

Water Flow in an Unconfined Aquifer

Groundwater flows from high pressure to low pressure.

high pressure

low pressure low pressure

http://www.horsleywitten.com/evergreen/mod-1-aquifers.html

Confined Aquifers

Confined aquifers get their water from precipitation in a recharge area – an area where the confined aquifer comes to the surface and becomes an unconfined aquifer.

The water flows down into the confined part of the aquifer, where it is under pressure from the mass of the overlying water. Flow through confined aquifers is determined solely by pressure. The water flows away from areas of high pressure.

If the confining layer is punctured by a well, the water will rise inside the well to a level determined by the amount of pressure within the confined aquifer.

The weight of the overlying rocks puts pressure on the water in a confined aquifer.

Potentiometric surface - level to which water in a confined aquifer will rise if given the chance.

Artesian well - a well in which water rises without pumping due to a high potentiometric surface.

If the potentiometric surface is

above the well head (the top of the well), water will flow to the surface, making a flowing artesian well.

A successful water well is a hole in the ground that penetrates the water table of an unconfined aquifer or the confining layer of a confined aquifer.

Wells are pumped by creating a zone of relatively low pressure, which pulls water up and out of the surrounding sediment or rock.

Pumping creates a cone of depression in the surrounding water table, and changes the local flow system.

unconfined aquifer

confined aquifer

aquicude

Water Flow Through Model

underwater storage tankwell 1 well 2 stream or lake

Well 1: taps the upper, unconfined aquifer

Well 2: taps the lower, confined aquifer

Water Table and Potentiometric Surface

Water table - in unconfined aquifer, the border between the saturated and unsaturated zonesPotentiometric Surface - level to which water from a confined aquifer will rise if given a chance. Determined by pressure in confined aquifer.

unconfined aquifer

confined aquifer

aquicude

Both the surfaces are dynamic, changing with the amount of water flow through the system

Water Table and Potentiometric Surface

Groundwater Contamination

contaminant in underground tank

Contaminant - any material that can be moved through groundwater

Groundwater Contamination

In groundwater with no flow, the contaminant will move through water by diffusion (from high concentration to low concentration)

Groundwater Contamination

In more realistic model (e.g., in which water flows out at stream) the contaminant will flow with the water, making a pollution plume.

Groundwater Contamination

Pumping of Well 1 results in a cone of depression in water table, and thus the pumping of the contaminant up the well.

Well 1 cone of depression