Geology 111G/Lecture 16

Hydrologic Cycle

Groundwater

Water table

Movement of Groundwater

Rates of Flow

Porosity

Permeability

Darcy's Law

Groundwater Phenomena

Springs

Wells

Confined Aquifers

Groundwater Recharge

Groundwater Contamination

Salt Water Intrusion

Storing Hazardous Waste

I. Hydrologic cycle: Circulation of water from world ocean to atmosphere, precipitation and runoff via surface and subsurface back to ocean. Because water in system is conserved, precipitation=runoff plus evapotranspiration.

A. Runoff: water that flows across surface or via groundwater following infiltration.

B. Evapotranspiration: Water returned as vapor to atmosphere directly or via metabolic processes of plants.

II. Groundwater: the mass of water stored beneath Earth's surface as a result of infiltration. Globally this amounts to about 22% of all the fresh water stored in lakes, rivers, glaciers, polar ice and atmosphere.

A. Water table (groundwater table): irregular surface or boundary that separates rock or sediment saturated with water from unsaturated zone above. Water table elevation fluctuates as a result of increased or decreased infiltration or withdrawal by pumping. Water table tends to generally parallel the ground surface

1. Zone of saturation: pore spaces and cracks in rock completely filled with groundwater. Water in this zone typically moves, albeit slowly.

2. Zone of aeration (vadose zone). Pore spaces and cracks filled partly with air, partly with water. Water in this zone does not move.

3. Modification by pumping. Removal of groundwater near a well faster than it can flow toward the well results in a cone of depression.

III. Movement of groundwater.

A. Rates of groundwater flow. Flow rates measured in cm/day. Flow in highly permeable gravel is about 15 cm/day. Surface flow rates in rivers are measured in meters/second. Difference is due to confined passages that water must move through.

1. Porosity. Percentage of total rock that consists of pore space, as opposed to framework grains. It is determined by shape, size and arrangement of framework grains, the presence or absence of mineral cement, and the shape of the pore system

a. Unconsolidated sand may have porosity as high as 50%.

b. Compacted sand may have 15% (this is still high).

2. Permeability: Ability of rock or sediment to transmit groundwater. Depends on total porosity and how well the pores are connected to form a system.

a. Aquifer: geologic formation that is permeable and transmits groundwater.

b. Aquiclude: impermable formation.

B. Controls on flow rates or discharge. This is determined by slope of water table, the permeability

1. Hydraulic gradient: literally, this is the slope of the water table. Determined by drilling two wells. It is the head (change in elevation, h₂-h₁) divided by the horizontal distance between the measured points (l): (h₂-h₁/l)

2. Darcy's Law: an equation that describes the discharge of groundwater.

a. Discharge (Q): flow rate, measured in volume/time.

b. Q=Ak(h₂-h₁)/l

Q=discharge

A=cross sectional area of pore network

K=coefficient of permeability, usually determined empirically by testing. It varies by material.

c. Note that water table slope and permeability are important factors in flow rate.

IV. Groundwater Phenomena.

A. Springs: places where groundwater flows out at the surface of the earth naturally; intersection of water table with the surface. May occur where impermeable layers force water laterally. Perched water table: local water table that lies above the regional water table.

B. Well: Opening cut from the surface into the zone of saturation. Artificial lift is required to withdraw water.

C. Confined aquifer. Groundwater restricted to a permeable layer between two impermable layers. Recharge of aquifer takes place at high elevation in mountains. Head developed by a system of tilted strata causes the potentiometric surface to lie above the surface.

1. Artesian well. A flowing well created by a confined aquifer.

V. Groundwater Recharge: replenishment of groundwater by infiltration to the water table. Main agents of recharge are rainfall, and in dry climates, surface flow in streams.

A. Recharge in southern New Mexico. RioGrande is an important recharge element in the Mesilla Valley

B. Recharge in Florida. Aquifer in Florida is limestone filled with fractures and caverns created by dissolution. Groundwater radiocarbon dating suggests that it takes 19,000 yr for water to travel 130 km. 130km/19x103 yr=.007 km/yr=7 m/yr=.02 m/day=2cm/day.

VI. Groundwater contamination.

A. Sewage: water quality degraded by presence of bacteria from septic systems, broken sewer lines, dairies.

1. Bacteria may travel long distances through gravel and limestone because these do not act as effective filters.

2. In contrast, sand (a) mechanically filters out bacteria (water goes through but bacteria do not); (b) allows oxidation of bacteria; (c) allows consumption of bacteria by other microorganisms.

3. Note that chemicals are not removed by these processes. Clays may

adsorb chemicals onto their surfaces or remove them by cation exchange.

VII. Salt water intrusion. Takes place in coastal communities where pumping exceeds influx of freshwater to coast.

VIII. Storing hazardous waste. Desirable conditions.

1. Enclosing rock of low permeability, no fractures. Salt is good.

2. Groundwater flow away from biosphere; minimal groundwater.

3. Thick unsaturated zone (arid climates).

4. Low rain fall.

5. Low seismic risk.