Environmental Geology - geol 406/506

GROUNDWATER - PROCESS, SUPPLY & USE

Module 7 - Part 2

By Scott Hughes

• Reading:  Chapters 10, Keller, 2000

• Review of topics covered in this module in pdf format or powerpoint format.

Recommended Reading: The textbook Rocks, Rails and Trails found on the Digital Atlas of Idaho Web Site has great information about Southern Idaho. Take a look at Rocks, Rails and Trails, Chapter 8

E-mail assignments to: hughscot@isu.edu

INTRODUCTION

The major source of all fresh water drinking supplies in the United States is groundwater. Groundwater is stored underground in aquifers, and is highly vulnerable to pollution. Understanding groundwater processes and aquifers is crucial to the management and protection of this precious resource.

GROUNDWATER

Groundwater comes from precipitation. Precipitated water must filter down through the vadose zone, or unsaturated zone, to reach the zone of saturation, where more "horizontal" 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. The upper boundary of the zone of saturation is called 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. Subsurface monitoring of the vadose zone can locate plumes of contaminated water, tracking the direction and rate of plume movement.

Visit this link for the basics of groundwater sponsered by "The Groundwater Foundation" - http://www.groundwater.org/kc/kc.html for more groundwater information.

AQUIFERS

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 flow in an artesian well rises above the height of the recharge zone .

Study figure 10.9 on p. 268 in Keller's Environmental Geology

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.

Click here for the NAWQUA FACT SHEET on the Snake River Plain aquifer

GROUNDWATER MOVEMENT

Movement of groundwater depends on rock and sediment properties and the groundwater’s flow potential. Porosity, permeability, specific yield and specificretention are important properties of groundwater flow.

Porosity

* symbol - n * units -% * pecent of void space (empty space) in soil or rock * represents the path water molecules can follow in the subsurface * primary porosity-intergranular * secondary porosity-fractures, faults

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):

SrSyPhi.gif (1597 bytes)

Porosity, permeability, specific yield, and specific retention are all components of hydraulic conductivity.

Hydraulic Conductivity

* symbol-K * units-length/time EX. (m/day) * ability of a particular material to allow water to pass through it

The definition of hydraulic conductivity (denoted "K" or "P" in hydrology formulas) is 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/Fluid Potential

* symbol-h * units-length EX. (m) * a measure of energy potential (essentially is a measure of the elevational/gravitational potential energy) * is the driving force for groundwater flow * WATER ALWAYS FLOWS FROM AREA OF HIGH HEAD TO AREA OF LOW HEAD (EVEN IF THIS MEANS IT MAY GO "UPHILL"!) * measure head by sinking a well then measuring the level (elevation) to which the water rises in the well in relation to a reference poing which is taken as zero meters (usually sea level) * hydraulic head determines hydraulic gradient

Note: Hydraulic Head = dh (i.e., the difference in elevation of water table between two points.)

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.

HYDRAULIC GRADIENT

* symol-I * units - unitless (why? because length divided by length cancels out the units!) * this is essentially the slope of the water table, and groundwater flow will be "down" this slope * sink two wells and measure head. Then find the difference between them and divide this by the flow length (distance between the two wells) * EXAMPLE: head in well one = 100 feet. Head in well two = 10 feet. Distance between the two wells is 10 feet. So the hydraulic gradient is: 100 feet-10 feet/10feet = 9

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).

REMEMBER: JUST BECAUSE POROSITY(n) IS HIGH DOESN'T MEAN HYDRAULIC CONDUCTIVITY (K) WILL BE HIGH! For example, clay has a high n, but a low K (because it has very small pores).

THE WATER TABLE

Water table contour lines (or flow lines) are similar to topographic lines on a map. They essentially represent "elevations" in the subsurface. These elevations are 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 drilled 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. The map of contour lines is called a flow net. Remember: groundwater always moves from an area of higher hydraulic head to an area of lower hydraulic head, and perpendicular to equipotential lines.

TAKE A LOOK at some flow net sketches that will help clarify the relationships between aquifer matrix, and groundwater movement.

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.

DISCHARGE

* symbol-Q * units-volume/time EX. (m^3/day) * volume of water flowing through an aquifer per unit time * FIND WITH DARCY'S LAW Q = KIA

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 the 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 an 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 qroundwater and DOES account for tortousity 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 traveling. (SEE Module 8 - Water Pollution)

INTERACTION AND BETWEEN SURFACE WATER AND GROUNDWATER

Surface streams have an effect on the groundwater table. Influent streams recharge groundwater supplies. Influent streams, located above the groundwater table, flow in direct response to precipitation. Water percolates down through the vadose zone to the water table, forming a recharge mound.

Effluent streams are discharge zones for groundwater. Effluent streams are generally perennial (flow year round). Groundwater seeps into stream channels, maintaining water flow during dry seasons.

The Big Lost River in Idaho is a good example of an intermittent, ephemeral influent stream. Natural flow of the Big Lost River terminates in the Big Lost River Sinks, located on the INEEL. But, local irrigation now diverts the Big Lost River from its natural terminus.

MODULE OVERVIEW

Terms to know:

Do you have indoor plumbing and a water heater (Think about the implications of this question)? If so, note where the water heater is located. Consider where the source of your water is. Is your bathtub or shower located near the water heater or at the opposite end of the house? The water pressure will be greater near the source. You will also get hot flowing water more quickly. The water’s pathway has a major effect on how fast it moves and how soon it is usable. Hopefully these examples help you understand Darcy’s law and the Darcian velocity.

End of Module # 7 - Part 2

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