1. Figure 12.1: Water on earth is stored in various reservoirs which include the oceans, glaciers/polar ice, groundwater, lakes, rivers, atmosphere and biosphere.

2. Figure 12.2: Water is continuously cycled through these various reservoirs on earth. This cyclical movement of water through these various reservoirs can be represented by means of the hydrologic cycle.

3. Within the hydrologic cycle, water is transferred from one reservoir to another via the processes of evaporation, precipitation, and runoff.

1. Some of the water that falls on land soaks into the ground. Water penetrates the ground through joints or small pore spaces between particles via a process known as infiltration.

2. The water contained in spaces within bedrock and regolith is referred to as Groundwater.

(a) constitutes ~1% of water on Earth

(b)) 40x more voluminous than that of freshwater lakes & streams

(c) 1/3 volume of water contained in glaciers and polar ice

(d) mostly originates as rainfall

(e) mostly occurs within ~750 m of the Earth's surface

(f) once underground, migrates slowly towards ocean

1. Figure 12.8: The distribution of water within soil and regolith can be represented as zones:

2. The uppermost layer is the unsaturated zone that may contain some water but is not saturated. This is known as the zone of aeration.

3. The saturated zone lies below the zone of aeration and is the layer where the pores of the soil or rock are completely filled with water.

4. The groundwater table is the upper surface of the saturated zone.

5. Figure 12.9: The groundwater table:

(a) intersects the land surface at lakes, marshes and streams

(b) somewhat imitates the topography of the overlying land surface

(c) may drop during drought and rise after heavy rainfalls.

1. Groundwater passes through soil, regolith and rock by way of open pore spaces between grains or fractures.

2. Figure 12.7 & Table 12.2: Porosity is the percentage of open spaces within a total volume of a body of regolith or bedrock. Porosity in most rocks usually < 20% but can be as high as 50%.

3. Permeability is a measure of how easily a rock allows fluids to pass through it. Sandstone typically more permeable than clays.

4. The Specific Yield refers to that portion of groundwater that will drain under the influence of gravity and therefore is available for use.

5. The Specific Retention refers to that portion of the groundwater that remains bound in the material and is unable to pass through it.

6. The movement of groundwater in the saturated zone is termed percolation. Water moves slowly by percolation through very small pores along parallel, threadlike paths.

7. Figure 12.9: Water percolates from areas where the water table is high towards areas where it is lowest (towards surface streams or lakes).

8. Figure 12.9: Much of groundwater flow is along long, curving paths under influence of gravity. Upward flow beneath streams or lakes is due to water being under greater pressure beneath a hill than beneath a stream. Water flows towards areas of lower pressure.

1. Recharge of groundwater occurs in areas of the landscape where precipitation seeps downward beneath the surface and reaches the saturation zone. Water flows to discharge areas.

2. The time water takes to move through the ground from recharge to discharge depends on permeability, travel distance and path of flow. Ranges from days to thousands of years.

3. Figure 12.10a: In humid regions, recharge areas encompass nearly all the landscape beyond streams. Effluent streams are supplied by water from the zone of saturation.

4. Figure 12.10b: In arid regions, recharge occurs mainly in mountains and alluvial fans. Recharge also occurs beneath Influent streams underlain by permeable alluvium.

1. Figure 12.16: For a given permeability, the velocity of ground water increases as the slope of the water table increases.

2. The hydraulic gradient [= (h1-h2)/l] is a measure of the slope of the water table.

V = velocity of groundwater

V is proportional to (h1 - h2) / l

(h1-h2) = difference in altitude of two points on the water table

l = horizontal distance between the two points

V = K(h1-h2) / l

K = coefficient of permeability is a measure of the ease with which water moves through rock or sediment. Function of permeability, acceleration due to gravity and viscosity of the water.

Q = AV

Q = discharge of groundwater (volume per unit time)

A = cross-sectional area

substituting V = Q/A for V

Q = AK(h1-h2) / l (Darcy's Law)

3. Velocities of groundwater typically range between 1/2 m per day to several meters per year.

1. Figure 12.9 and 12.12: A spring is a natural flow of water that occurs when the groundwater table intersects the land surface. The water emerges at the ground surface. Most large springs issue from fractured lava, limestone or gravel.

2. An aquiclude is a body of impermeable or distinctly less permeable rock adjacent to a permeable one.

1. Figure 12.13: A well supplies water when it intersects the water table.

2. The rate of water withdrawal from a new well may initially exceed the rate of local groundwater flow, creating a cone of depression around the well. With time, the rate of inflow balances the rate of withdrawal.

1. An aquifer is a highly permeable rock or regolith lying within the zone of saturation.

2. Aquifers can be unconfined or confined. Confined aquifers are bounded by aquicludes.

3. Figure 12.11: Water that percolates into a confined aquifer flows downward under the pull of gravity and comes under increasing hydrostatic pressure.

4. Hydrostatic pressure is the weight an overlying column of water would exert at a particular depth.

5. The water under pressure seeks to rise to the same height as the water table in the recharge area minus an amount determined by the loss of energy due to frictional resistance during percolation.

6. Figure 12.11: Water within an artesian aquifer is under pressure and will flow out of an artesian well without pumping.

1. In regions where withdrawal exceeds recharge, the water table will gradually drop.

2. Figure 12.14: If the water table is allowed to drop significantly, then land subsidence will result.

(a) spreading biodegradable liquid wastes over land surface. Water perolates down to water table.

(b) Channeling of runoff in urban areas to basins where water will seep down and recharge the aquifer.

(c) Figure 12.22: Groundwater that is withdrawn by industries can be pumped back into the ground with injection wells.

(a) Significant salt is dissolved from surrounding bedrock.

(b) Groundwater passes through sulfur-rich rocks (dissolved H₂S).

(c) Figure 12.22: Drainage from septic tanks, broken sewers, and barnyards. Sewage-contaminated groundwater can be purified in some cases when it percolates through sand or sandstone.

(d) Agricultural poisons.

(e) Underground storage of hazardous wastes.

(f) Figure 12.15: Contamination by seawater in coastal areas.

1. Figure 12.21: Carbonate rocks are readily attacked through dissolution by groundwater because many ground waters are slightly acidic due to reactions with CO₂.

2. Figure 12.20: Caves are mostly carved within the shallowest portions of the saturation zone. The reaction between acidic groundwater and carbonate rocks results in dissolution of the carbonate rocks, leaving behind underground cavities called caves.

3. Figure 12.18: Precipitation of calcite from dripping water within caves can lead to formation of various cavern features including stalactites, stalagmites, columns and flowstone. Stalactites are icicle-like features of calcium carbonate that suspend from the cavern ceiling whereas stalagmites protrude upward from the cavern floor.

4. Figure 12.19: Sinkholes are large dissolution cavities evident on the land surface. Sinkholes may form when the surface of a cave collapses. Sinkholes may also form at the surface along intersections of joints within the bedrock.

5. Figure 12.20: Karst topography refers to terrain containing caves, sinkholes, karst towers and underground streams. Karst Towers form in areas of thick, well-jointed limestone that separate into isolated blocks.

6. Figure 12.24: The circulation of groundwater near a magma body can result in geysers and hot springs.