Groundwater Review

Groundwater comes from precipitation. Precipitated water must filter down through the vadose zone to reach the zone of saturation, where groundwater flow occurs. The rate of infiltration is a function of soil type, rock type, antecedent water, and time.

The vadose zone includes all the material between the Earth’s surface and the zone of saturation. Near  the upper boundary of the zone of saturation where water pressure equals atmospheric presure, is the water table. The capillary fringe is a layer of variable thickness that directly overlies the water table. Water is drawn up into this layer by capillary action.

The vadose zone has an important environmental role in groundwater systems. As with water, surface pollutants must filter through the vadose zone before entering the zone of saturation.

Large amounts of water are stored beneath the earth’s surface in aquifers. To be an aquifer, the stored water must be accessible at a usable rate. Aquifers consist of porous material such as sand, gravel, and fractured rock.

Aquifers can be confined or unconfined. Confined aquifers have non-porous layers above and below the aquifer zone. The non-porous layers hold water and restrict water movement. Such layers are referred to as aquitards or aquicludes. Clay soils, shales, and non-fractured, weakly porous igneous and metamorphic rocks are examples of aquitards. Sometimes a lens of non-porous material will be found in material that is more permeable. Water percolating through the unsaturated zone will be intercepted by this layer and will accumulate on top of the lens. This water is a perched aquifer. An unconfined aquifer does not have confining layers that retard water movement.

Some aquifers are confined under pressure. These aquifers are called artesian systems. Sufficient pressure results in free-flowing water, either from a spring or from a well.

Water is continually recycled through aquifer systems. Groundwater recharge is any water added to the aquifer zone. Processes that contribute to groundwater recharge include precipitation, streamflow, leakage (reservoirs, lakes, aqueducts), and artificial means (injection wells). Groundwater discharge is any process that removes water from an aquifer system. Natural springs and artificial wells are examples of discharge processes.

Pumping water from a well causes a cone of depression to form in the water table at the well site. Overpumping can have two effects. It can cause a change in the groundwater flow direction. It also lowers the water table, making it necessary to dig a deeper well.

Groundwater Movement
Movement of groundwater depends on rock and sediment properties and the groundwater’s flow potential. Porosity, permeability, specific yield and specific retention are important properties of groundwater flow. Porosity is the volume of pore space relative to the total volume (rock and/or sediment + pore space). Primary porosity (% pore space) is the initial void space present (intergranular) when the rock formed. Secondary porosity (% added openings) develops later. It is the result of fracturing, faulting, or dissolution. Grain shape and cementation also affect porosity.

Permeability is the capability of a rock to allow the passage of fluids. Permeability is dependent on the size of pore spaces and to what degree the pore spaces are connected. Grain shape, grain packing, and cementation affect permeability.

Specific yield (Sy) is the ratio of the volume of water that drained from a rock (due to gravity) to the total rock volume. Grain size has a definite effect on specific yield. Smaller grains have larger surface areas.  Larger surface areas mean more surface tension.  Fine-grained sediment will have a lower specific yield than more coarsely-grained sediment. Sorting of material affects groundwater movement.Poorly sorted material is less porous than well-sorted material.

Specific retention (Sr) is the ratio of the volume of water a rock can retain (in spite of gravity) to the total volume of rock.

Specific yield plus specific retention equals porosity (often designated with the Greek letter phi).

Porosity, permeability, specific yield, and specific retention are all components of hydraulic conductivity. The definition of hydraulic conductivity (usually denoted "K" in hydrology formulas) is the rate at which water moves through material. Internal friction and the various paths water takes are factors affecting hydraulic conductivity. Hydraulic conductivity is generally expressed in meters per day.

Hydraulic head (denoted "h" in hydrology formulas) is the name given to the driving force that moves groundwater. The hydraulic head combines fluid pressure and gradient, and can be though of as the standing elevation that water will rise to in a well allowed to come to equilibrium with the subsurface. Groundwater always moves from an area of higher hydraulic head to an area of lower hydraulic head. Therefore, groundwater not only flows downward, it can also flow laterally or upward. Direction of flow is dependent on local conditions.

The hydraulic gradient (I) is approximately the slope of the water table—in a simple unconfined water system (remember however, that confined aquifer systems can be more complex, in such systems, fluid pressure must also be considered).

The Water Table
Water table contour lines are similar to topographic lines on a map.  They essentially represent "elevations" in the subsurface.   These elevations are called the hydraulic head mentioned above.

Water table contour lines can be used to tell which way groundwater will flow in a given region. Lots of wells are drillled and hydraulic head is measured in each one. Water table contours are drawn that join areas of equal head (like "connect-the-dots"!). These water table contours lines are also called equipotential lines.

Darcy’s law: Q = KIA
In 1856, Henry Darcy studied the movement of water through porous material. He determined an equation that described groundwater flow.  The following description tell how Darcy determined his equation:

A horizontal pipe filled with sand is used to demonstrate Darcy’s experiment. Water is applied under pressure through end A, flows through the pipe, and discharges at end B. Water pressure is measured using piezometer tubes (thin vertical pipes installed at each end of the horizontal pipe). The difference in hydraulic head (between points A and B) is dh (change in height). Divide this by the flow length (i.e. the distance between the two tubes), dl, and you get the hydraulic gradient ( I ).

The velocity of groundwater is based on hydraulic conductivity (K), as well as the hydraulic head (I). Therefore, the equation determined by Darcy to describe the basic relationship between subsurface materials and the movement of water through them is Q = KIA   where Q is the volumetric flow rate (or discharge) and A is the area that the groundwater is flowing through.  This relationship is known as Darcy’s law. In summary the components of Darcy's Law include:

- Discharge
*symbol - Q
*units - volume/time EX. (m^3/day)
*volume of water flowing through an aquifer per unit time


- Area of flow
*symbol - A
*units - distance squared EX. (m^2)
*Cross-sectional area of flow. (i.e. aquifer width x thickness)

Now, rearrange the equation to Q/A = KI, which is known as the flux (v), which is an apparent velocity. Actual groundwater velocity is lower than that determined by Darcy, and is called Darcy Flux (vx).

- Flux
*symbol - v
* units - distance/time EX. (m/sec)
*v = Q/A = KI
*this is a velocity measure and gives the IDEAL velocity of groundwater (assumes that water molecules can flow in a straight line through the subsurface).
*this is ideal because it doesn't account for tortuosity of flow paths (this means that the water molecules actually follow a very windy path in and out of the pore spaces and so travel quite a bit slower in reality than the flux would indicate).

- Darcy Flux
*symbol - vx
*units - distance/time EX. (m/sec)
*vx = Q/An = KI/n
*This is the ACTUAL velocity of groundwater and DOES account for tortuosity of flow paths by including porosity in its calculation.

Darcy’s law is  used extensively in groundwater studies.  It can help answer important questions such as what direction an aquifer pollution plume is moving in, and how fast it is travelling. Click here for an explanatory figure.

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