Review Questions and Answers; Groundwater
1. What percentage of freshwater is groundwater? If glacial ice is excluded and only liquid freshwater is considered, about percentage is groundwater?
According to Table 11.1, groundwater comprises about 14 % of all freshwater. This quantity significantly exceeds water contained in rivers, lakes, unsaturated soils, and the atmosphere. In as much as water stored in glaciers and ice caps accounts for 85 % of all freshwater, groundwater comprises about 94 % of all liquid freshwater.
2. Geologically groundwater is important as an erosional agent. Name another significant geological role for groundwater.
Groundwater inflow sustains flow in perennial streams and accounts for most, if not all stream discharge during extended time intervals between precipitation events. Thus groundwater contributes to the geological work of streams.
3. Compare and contrast the zones of aeration and saturation. Which of these zones contains groundwater?
The aeration and saturation zones (Fig. 11.2) are defined by the status of their pore space. In the saturated zone, water completely fills all pore space over an indefinitely long period of time; this water constitutes the groundwater. In the aerated zone, pores are normally filled or partly filled with air (aerated) and soil gases; temporary saturation may follow heavy rains or snowmelt. For unconfined groundwater conditions, the water table marks the upper, boundary surface of the saturated zone. Water in the aerated zone is commonly referred to as soil moisture to differentiate it from groundwater in the saturated zone.
4. Although we usually think of tables as being flat, the water table generally is not. Explain.
The water table (the upper boundary of the saturated zone) is a two-dimensional feature (surface) but it is rarely flat. For unconfined aquifer conditions in humid areas (Figs. 11.3, 11.4, 11.6, & 11.11), the water table mimics the surface topography. In dry lands, the water table domes upward beneath an influent stream (Fig. 11.4B). Relative highs in the water table indicate recharge, and lows associated with effluent streams and pumping wells (cones of depression, Fig. 11.11) indicate that water is being discharged from the groundwater system.
5. What is an effluent stream? How does an influent stream differ?
The terms describe the movements of water from groundwater into streams and vice versa (Fig. 11.3). Effluent describes a stream receiving an inflow of groundwater because the water table elevation locally exceeds the elevation of the bottom of the stream channel. Influent describes the situation where the water table is below the channel bottom, and stream water is seeping downward into the groundwater system. Effluent streams are generally perennial. Unless they are sustained by discharge from upstream, influent streams are usually intermittent, flowing only after a precipitation event.
6. Distinguish between porosity and permeability.
Both describe important, hydraulic characteristics of soil and rock. Porosity is defined as the volume percentage of open space (voids, pores, cracks, etc.) in a given volume of soil or rock. Highly porous materials can hold abundant water when saturated; low porosity materials can hold only small amounts of water. Permeability refers to how easily water will flow from opening to opening through a porous material. To be permeable, a porous material must have openings and cracks (pore spaces) that connect with one another and are large enough for water to flow freely between pores.
7. What is the difference between an aquitard and an aquifer?
Both terms describe bedrock or unconsolidated deposits in terms of their hydraulic properties. An aquitard is composed of impermeable material (water will not flow through it); thus an aquitard (an impermeable stratum or layer) can stop water percolating downward from the surface or prevent water from moving upward or downward from a saturated zone (an aquifer or aquifers). An aquifer is a general term to describe any saturated, water-bearing, subsurface, geologic stratum or deposit of porous, permeable bedrock or unconsolidated material.
8. Under what circumstances can a material have a high porosity but not be a good aquifer?
If the pore spaces and interpore connections are very small, the material will have a low permeability despite having a high porosity. A water-saturated, mud layer would be a good example. It has a substantial water content (porosity) but the pores and connections are very small; thus water moves with great difficulty and the mud has a very low permeability.
9. As illustrated in Figure 11.4, groundwater moves in looping curves. What factors cause the water to follow such paths?
Figure 11.4 shows the flow paths (streamlines) in an isotropic, unconfined aquifer. The water always moves toward regions of lower pressure (the downslope direction of the water table), and the slope and orientation of a tangent line to any point on a streamline indicates the direction and magnitude of the pressure-gradient (hydraulic gradient) force pushing the water through the saturated media. The average magnitude of the hydraulic gradient is found by dividing the elevation difference between the initial (recharge) and final (discharge) points by the path length of the streamline. Note that recharge points are on the water table at elevations above the common elevations of the discharge points (the surface of the effluent stream). Although the local, upward flow of groundwater beneath the effluent stream might at first glance appear to defy the law of gravity, the water is being pushed "uphill" by the weight of water laterally above it along the same streamline.
Such curved, "looping" streamlines and orthogonal, equipotential lines (lines of equal hydraulic gradient, not shown in Fig. 11.4) are forms of solutions to potential-flow problems, groundwater being but one example. In such cases, matter or energy moving through some physical media are driven by potential-field gradients (forces) and scaled by a media property (permeability in the case of porous media flow). Thus Darcy's Law is formulated as V = K(h/l) (V is velocity, a vector; K is the permeability, a property of the porous media; and h/l is the hydraulic gradient force, a vector).
10. Briefly describe the important contribution to our understanding of groundwater movement made by Henry Darcy.
Henry Darcy was a nineteenth century French engineer and hydrologist who, in 1856, formulated the basic equation describing groundwater flow on the basis of his theoretical and observational studies of groundwater in the area around Dijon, France. This equation, V = K(h/l), is now known as Darcy's Law.
11. When an aquitard is situated above the main water table, a localized saturated zone may be created. What term is applied to such a situation?
This situation results in a perched water table. Water seeping downward from the surface is stopped at the top of the aquitard and accumulates, forming a gently sloping, mound-shaped, local, saturated zone in an aquifer above the aquitard. This saturated zone has its own water table "perched" above the elevation of the regional water table (Fig. 11.6).
12. What is the source of heat for most hot springs and geysers? How is this reflected in the distribution of these features?
Most geothermal waters are heated by geologically young, hot, igneous bodies at depth; thus they are concentrated in areas of active or recent volcanism in the western states (Fig. 11.7). Warm springs also occur in nonvolcanic areas, such as those in the Appalachian Mountains (Fig. 11.7). In these situations, the groundwater circulates deep below the surface and is heated by the warmer rocks at depth; being less dense than cold water, it then rises back to the surface as a warm spring.
13. Two neighbors each dig a well. Although both wells penetrate to the same depth, one neighbor is successful and the other is not. Describe a circumstance that might explain what happened.
This situation could arise for many different reasons. First, a perched water table (Fig. 11.6) may be intersected by one well and not the other. In other areas, the natural slope of the water table or a cone of depression from another well could be involved (Fig. 11.11). In karst areas (Fig. 11.24), solution cavities, collapse breccias, or other highly porous zones may alternate locally with relatively impermeable, non-porous bedrock, resulting in a prolific well in one location and a dry hole nearby. In areas of complex bedrock or regolith geology, neighboring wells drilled to the same depths may penetrate units with greatly differing porosities and permeabilities. In areas underlain by massive, non-porous bedrock such as granite and gneiss, a single, fortuitous fracture intersection may make the difference between a productive well and a dud.
14. What is meant by the term artesian?
Under unconfined conditions, the water in a well rises to the exact level of the local water table. In artesian aquifers, the groundwater is confined and under pressure. In a well drilled into such an aquifer, the water will rise above the elevation of the top of the saturated zone, and the excess pressure may be high enough for the well to flow freely at the surface (no pumping). An artesian aquifer must be sealed by an overlying aquitard and saturated laterally to elevations above the aquifer-aquitard boundary where the well penetrates into the aquifer; lateral saturation at higher elevations and confined hydraulic conditions are necessary to generate the excess pressure. This typically involves inclined strata such as a porous and permeable sandstone with shale aquitards above and below (Fig. 11.13).
15. In order for artesian wells to exist, two conditions must be present. List these conditions.
Artesian aquifers are typically inclined, distinctive strata or lithologic units. First, they must be bounded above and below by impervious strata. Second, the aquifer must be saturated below its unconfined water table in the recharge area, typically along a mountain front (Figs. 11.13 & 11.15). At any point in an artesian aquifer, the water is under a pressure generated by the weight of the water in the overlying, saturated part of the aquifer (note the pressure surface in Figs. 11.13). If a well penetrates the aquifer, the water rises to the elevation of the pressure surface, but the well will flow freely (without pumping) only if the elevation of the pressure surface exceeds the elevation of the well head (Fig. 11.14).
16. When the Dakota Sandstone was first tapped, water poured freely from many artesian wells. Today these wells must be pumped. Explain.
Early wells (late 1800s) were strongly pressurized; some were gushers (Fig. 11.14). After over a century of continuous withdrawals, aquifer pressures have substantially declined; and many wells that once flowed freely now require pumping.
The Dakota Sandstone (Cretaceous) is a very important source of water in western and central South Dakota. Recharge begins in the Black Hills. Water from streams and snowmelt infiltrates an inclined, highly porous, Mississippian limestone unit stratigraphically below the Dakota. East of the Black Hills, the tilted strata flatten beneath the western plains. In central South Dakota, the aquitard between the two units is breached; water is recharged upward into the Dakota aquifer and spreads laterally beneath the central and western parts of the state.
17. What problem is associated with the pumping of groundwater for irrigation in the southern part of the High Plains?
The area is fairly dry and there is little natural recharge to the aquifer. Thus continued pumping depletes the groundwater and causes the water table to drop. In some areas, the water table in the Ogallala aquifer has declined over 200 feet since large-scale pumping for agricultural irrigation was started (Box 11.2).
18. Briefly explain what happened in the San Joaquin Valley of California as the result of excessive groundwater withdrawal. (Box 11.3.)
The aquifer here is composed of unconsolidated sands and silts that shrink or compact when dewatered (when they change from a water-saturated to an unsaturated condition). Compaction is accomplished by permanent closing of some original pore space in the aquifer; thus the land surface subsides.
19. In a particular coastal area the water table is 4 meters above sea level. Approximately how far below sea level does the freshwater reach?
Freshwater floats on the denser, salty water. The rule is that the freshwater extends downward a distance below sea level equal to 40 times the distance that the water table is above sea level. Thus the freshwater lens extends to a depth of 160 meters below sea level and 164 meters below the water table. This analysis assume that a reasonably permeable unconfined aquifer extends indefinitely downward from the water table.
20. Why does the rate of natural groundwater recharge decrease as urban areas develop?
With urbanization come pavements, roofs, storm sewers, concrete-lined stream channels, and other impermeable ground coverings that intensify runoff and prevent water from infiltrating into the subsurface soil and bedrock; thus natural recharge is reduced.
21. Which aquifer would be most effective in purifying polluted groundwater: coarse gravel, sand, or cavernous limestone?
The sand aquifer would be most effective. The water would move more slowly, and the pollutants would be more likely to contact grain surfaces where they could be adsorbed or chemically degraded.
22. What is meant when a groundwater pollutant is classified as hazardous?
Toxic, flammable, explosive, and corrosive substances are classified as hazardous. These would include pesticides, gasoline, jet fuel, and chemicals such as sulfuric acid and benzene.
23. Name two common speleothems and distinguish between them.
Two common speleothems (dripstone features) are stalactites and stalagmites. Both are composed of calcium carbonate precipitated from water dripping from the roofs of caverns. Stalactites grow (hang) down from the ceiling; they are slender and pointed like icicles. Stalagmites grow up from the floor; they are stout with blunt tips and rippled surfaces.
Speleothems grow only when the cavern is aerated and above the water table. Water dripping from the cave roof is moving downward through the unsaturated zone; obviously if the cave roof were below the water table, the cave would be filled with water!
24. Speleothems form in the zone of saturation. True or False? Briefly explain your answer.
As noted in response to Review Question 23, speleothems are deposited from dripping water, thus they form in the zone of aeration. If the cave were in the saturated zone, it would be filled with water.
25. Areas whose landscapes largely reflect the erosional work of groundwater are said to exhibit what kind of topography?
The term was coined in reference to the distinctive landforms developed on limestone bedrock in Slovenia, a small country that was once the northeasternmost province of the former Yugoslavia.
26. Describe two ways in which sinkholes are created.
Sinkholes develop only in areas underlain by soluble bedrock such as limestone, anhydrite, and gypsum. When a cavern suddenly collapses, a circular to elliptical, closed depression forms as the rocks and soil above the cavern subside (Box 11.4). Also, sinkholes may slowly subside and enlarge as intersecting vertical fractures are gradually widened and enlarged to a pipelike channelway by solution and removal of the soluble bedrock.