A - Basics

In the hydrologic cycle

which means that the water that falls to the ground in the form of rain, snow, etc will either soak into the groundwater, runoff into surface streams, or be evaporated from the surface or transpired through plant leaves.

In the U.S. the average precipitation is on the order of 30 inches per year; of which 8.9" runs off into streams, 0.1" infiltrates the ground, and 21" is returned to the atmosphere via evapotranspiration.

The water that infiltrates the ground will percolate (seep) downward through porous and permeable soil, sediment, and rock until it reaches an impermeable unit.

Porous means having void spaces between grains. Permeable means the voids are connected so water can pass through.

Porous and permeable materials include soil (if not too clay rich), sand, sandstone, limestone, fractured igneous and metamorphic rock, vesicular basalt and scoria.

Impermeable and/or non-porous materials include clay, shale, non-fractured igneous and metamorphic rocks.

Porous/permeable layers are called aquifers; impermeable layers called aquicludes.

In an unconfined aquiferthe zone of saturation (all voids filled with water) lies above an aquiclude; the top of the zone of saturation is the water table; above this is the zone of aeration (voids filled with air, though grains may be wet - coated with water).

Pumping a well in an unconfined aquifer can lower the water table in a cone-shaped pattern around the well because it takes time for water to seep between grains; the total amount the water level drops in the well is called the drawdown; the area affected by the pumping is called the cone of depression.

Water table elevations generally follow the topography (lay of the land); water tables are high where the land is high and low where the land is low.

Water seeps into the ground in recharge areas, which are usually higher areas, and water is removed from the ground in low-lying discharge areas, such as lakes, streams, and springs.

In a confined aquifer aquicludes or confining units lie above and below the permeable aquifer units. The water to which level rises in a well tapping a confined aquifer is called the potentiometric surface (pressure surface). In most confined aquifers the water is under pressure (water rises above the top of the aquifer in a well). This condition is known as artesian. A flowing artesian aquifer (well) is one in which the water in a well flows to the surface (the potentiometric surface is above the land surface).

Groundwater flows from areas with a higher water table or potentiometric surface to areas with a lower water table or potentiometric surface.

Near the coast a lens of fresh groundwater lies above more dense saltwater.

Saltwater intrusion occurs where too much freshwater is pumped out of the ground and is replaced by brackish and eventually saltwater

Groundwater pollution may occur where toxic materials are dumped (eg. at a landfill). Rainwater leaches toxic chemicals from the dumped materials and percolate down to the water table. The toxic laden groundwater may contaminate local wells. Proper landfills are now designed with impermeable liners and caps.

B - Aquifer Characteristics

There are two basic characteristics of an aquifer in terms of its potential as a water resource.

I) First is various measurements as to how much water is held in an aquifer including such quantities as:

II) The other is measures of how easily the water flows through the aquifer:

I) Quantity of Available Water in an Aquifer

In the zone of saturation all of the void spaces are filled with water. The space available for water storage is

However, not all of the water in the pore spaces is available. Some water is held by capillary forces (capillary water) and some by electrostatic forces (hygroscopic water). The available water is called gravitational water because it is able to drain out of the rock under the force of gravity.

The effective porosity is less than the total porosity because of these forces that retain water in the pores spaces.

The term specific yield (Sy) is akin to effective porosity. Specific retention (Sr) refers to the portion of the total porosity that is not available due to capillary and electrostatic forces.

Fine sediments like clay and silt have a high porosity but a high specific retention and low specific yield. Capillary forces become very important in fine sediments where the pores spaces are very small. Clay minerals have electrostatic forces that attract large amounts of water (dipolar molecule).

Coarse sediments like sand and gravel have a lower porosity but high specific yield and low specific retention because the void spaces are larger and electrostatic attractions are much less.

See figure 7.15, p. 231 (Rahn, 1996)

Knowing the average specific yield in an aquifer and the volume of the aquifer, one can calculate the total amount of water held in storage that could be extracted.

Storativity (S), or the storage coefficient, or storage, relates the amount of water that can be removed from an aquifer to the resulting drop in the water table or potentiometric surface (per unit area).

S is a ratio (no units).

In high pressure, artesian, confined aquifers:

S ranges from .001 to .00001
Small withdrawals result in large reductions of the potentiometric surface.

In unconfined, water table aquifers:

S ranges from .3 to .01

Much more water can be removed without seriously affecting water table levels.

II) Groundwater Flow (Permeability - Hydraulic Conductivity (k) - Transmissivity)

Darcy's Law

In a permeable medium the flow velocity is:

- directly proportional to the head loss

- inversely proportional to the length (distance)

where V_d is the Darcy velocity, H is the head loss or difference in the height of the water table or potentiometric surface, and L is the length or distance between observation points. This is the "rise over the run" or the slope of the water table or potentiometric surface. This slope is also known as the hydraulic gradient. So the groundwater flow velocity is proportional to the slope of the water table or potentiometric surface.

The constant of proportionality is called the hydraulic conductivity (K), which is analagous to the permeability.

The hydraulic conductivity has units of distance per time (like velocity). The hydraulic gradient is a ratio and has no units (length divided by length cancels units).

Since the discharge (Q; volume of water flowing through a medium in a given amount of time) is equal to the flow velocity times the cross sectional area through which it flows (Q = VA), Darcy's Law becomes:

Because not all of the groundwater is flowing (some is held by capillary and electrostatic forces) the true velocity (V_t) or seepage velocity is greater than the Darcy velocity (V_d). The true velocity is equal to the Darcy velocity divided by the effective porosity

Additionally, groundwater does not flow in straight paths but rather flows around grains through connected void spaces. The velocity of the individual water molecules in their sinuous paths is faster yet. A complete treatment of V_t would ideally include the sinuosity, but the sinuosity is difficult to quantify.

Transmissivity (T) is the hydraulic conductivity of a vertical strip of aquifer one meter wide. It is the hydraulic conductivity times the saturated thickness (b) of the aquifer. Units for transmissivity are area per time.

Darcy's Law becomes:

where W is the width of the aquifer.