2-1. Fundamentals. To locate and evaluate water sources, engineers must know about the earth's topography and geologic formations. Surface sources of water, such as steams, lakes, and springs, are easy to find. Quartermaster personnel are responsible for locating surface-water sources and for providing adequate water supplies to troops in the field. Groundwater sources often require more time to locate. Geologic principles can help engineers locate groundwater supplies and eliminate areas where no large groundwater supplies are present. About 97 percent of the earth's fresh water (not counting the fresh water frozen in the polar ice caps and glaciers) is located underground. Most of the groundwater tapped by water wells is derived from precipitation on the earth's surface.
2-2. Hydrologic Cycle. The constant movement of water above, on, and below the earth's surface is the hydrologic cycle (Figure 2-1). The concept of the hydrologic cycle is that precipitation returns again to the atmosphere by evaporation and transpiration. Undemanding this cycle is basic to finding groundwater. Three-fourths of the earth's surface is covered by ocean water. Direct radiation from the sun causes water at the surface of the oceans to change from a liquid to a vapor (evaporation). Water vapor rises in the atmosphere and can accumulate as clouds. When the clouds accumulate enough moisture and conditions are right, the water is released in the form of rain, sleet, hail, or snow (precipitation).
a. Recharge and Discharge. Precipitation on land surfaces maybe stored on the surface. It also flows along the surface (runoff) or seeps into the ground (infiltration). Surface storage is in lakes, ponds, rivers, and streams and is snow and ice. Polar ice caps and glaciers store 85 percent of the earth's freshwater. Runoff provides the main source of water for streams and rivers. Water infiltrating into the soil is the major source of groundwater. Water that seeps into the ground recharges groundwater resources.
As groundwater moves from the recharge area, it may discharge back to the surface. Groundwater flows from recharge areas to discharge areas where the water is discharged to lakes, rivers, springs, and oceans (Figure 2-2). Another form of discharge is the consumption of water by plants and animals. Plants draw large quantities of water from the soil and return this water to the atmosphere (transpiration). Man also causes water discharge for consumption.
b. Water Storage. Water wells are constructed to produce water supplies from groundwater reserves. Developing a groundwater supply has many advantages over using surface water. Groundwater is more abundant than surface water, is cleaner, requires less treatment, and may be easier to protect than surface-water supplies. A water well is easy to seal from natural contamination and to protect from clandestine contamination. Short-term droughts have little effect on groundwater, which can be depended on when surface sources dry up.
2-3. Groundwater Occurrence. The hydrologic cycle exists on global and local levels. By understanding the local hydrologic cycle, it is possible to predict the directions and rates of groundwater flow, to identify groundwater resources, and to select development areas. An area drained by a stream or river is a drainage basin. For example, the Mississippi River Basin includes most of the area between the Rocky and Appalachian Mountains (Figure 2-3). Major river basins can be subdivided into smaller basins. The Missouri and Ohio River Basins are regional subdivisions of the Mississippi River Basin. These subdivisions can also be divided into local drainage basins (hydrographic basins) for each tributary (Figure 2-4). The boundaries of hydrographic basins are usually represented by mountains or hills, which restrict the flow of water, and by low areas where the water is discharged out of the basin.
The depth to groundwater may range from 10 feet to more than 1,000 feet. In most arid areas, the shallowest groundwater sources occur in recharge and discharge areas. Precipitation over mountainous areas results in a groundwater recharge at the base of the mountains into alluvial valleys. Precipitation over a mountainous area results in higher runoff and lower groundwater recharge ratios on the slopes of mountainous terrain. Even though groundwater may be shallow in these areas, the rough terrain and the rocks make mountainous areas difficult sitings for water wells. Even though mountainous areas are presumed to be recharge areas, military engineers will not drill tactical water-supply wells on mountain sides.
2-4. Geological Setting. The ability of soils and rocks to hold and transmit water varies, and the depth to groundwater varies in different geological settings. Physical properties of soils and rocks, such as degree of consolidation, cementation, and hardness, determine drilling methods and the potential for groundwater production. When drilling, consolidated rock is harder to penetrate but is more stable than unconsolidated rock. Well drillers usually use a down-hole air hammer on consolidated rock with no well casing for support. Drillers must support holes in unconsolidated rock to avoid cave-ins. Wells in unconsolidated rock frequently yield more water at a shallower depth than wells in consolidated rock.
- Cover the majority of the earth's surface.
- Range in thickness from a few inches to several thousand feet.
- Can underlie consolidated rocks.
- Consist of weathered rock particles of varying materials and sizes.
- Include clays, silts, sand, and gravel.
- May include salt deposits and fragments of shells of marine organisms.
See FM 5-410 for descriptions and detailed analyses of the engineering properties of unconsolidated deposits.
b. Consolidated Deposits (Rock). These rocks consist of mineral particles of different sizes and shapes. Heat and pressure or a chemical reaction has formed the rocks into a solid mass (often called bedrock). Geologists classify consolidated deposits into the following three categories, depending on origin:
(1) Igneous. These rocks form when hot molten material (magma) cools or solidifies either inside the earth's crust or on the earth's surface (lava). Basalt and granite are two common igneous rocks that military well drillers encounter.
(2) Sedimentary. These rocks are composed of sediments that are converted to rock through compaction, cementation, or crystallization. Sedimentary rocks cover about 75 percent of the earth's surface. Over 95 percent of these rocks are comprised of shale, sandstone, and limestone.
(3) Metamorphic. These rocks are igneous, sedimentary, or preexisting metamorphic rocks that undergo further transformation by temperature, pressure, or chemical changes. The transformation usually occurs very deep in the earth's crust. Schist and gneiss are common metamorphic rocks.
a. Porosity. Soil and rock are composed of solids and voids (pores). Groundwater can fill up and flow through pores. Pores formed at the same time as the rock such as in sand, gravel, and lava tubes in basalt, are called primary openings. Pores formed after the rock was formed are called secondary openings. Examples are fractures in massive igneous rocks like granite and the caves and caverns in limestone. The pore sizes vary and mayor may not be filled with water. The ratio of volume of the pore space to the total volume of the soil or rock is called porosity. Porosity is normally expressed as a percentage (Table 2-1). Figure 2-5 shows primary and secondary openings.
b. Permeability. The property of permeability is related to porosity. In qualitative terms, permeability is expressed as the capacity of a porous rock or soil to transmit a fluid. Large interconnected pore openings are associated with high permeability, while very small unconnected pore openings are associated with low permeability. Sand and grovel with large interconnected pore openings have high porosity and permeability. Clay tends to have high porosity, but the very small openings tend to inhibit the passage of water. Therefore, clay displays low permeability.
Hydraulic conductivity is a measurement of the capacity of rock or soil to transmit water. The higher the hydraulic conductivity, the faster the water will flow at a given pressure (Figure 2-6). Hydraulic conductivity is related to the size and spacing of particles or groins in soils or to the number and size of fractures in rocks (Figure 2-7). The hydraulic conductivity of rocks and soil can be measured by field or laboratory tests. Findings are recorded as volume of water flowing per unit area per unit of time and expressed as foot per day or centimeter per second.
c. Specific Yield and Retention. Table 2-2 shows specific yield and retention percentages. Only a portion of the water stored in pores can be pumped out of a well. Specific retention (Sr) is the water that cannot be pumped out of a well and is left as a film on the rock surfaces. Specific yield (Sy) is the water that can be pumped from a well and is that part of the water that would drain under the influence of gravity (Figure 2-8). The Sr and Sy percentages compared to porosity indicate how much water can be developed from a rock formation.
d. Transmissivity. Although hydraulic conductivity measures the relative flow of water through a subsurface material, the results may not be an accurate measurement of the yield that can be obtained from the material. The inaccuracy exists because hydraulic conductivity does not account for the thickness of the water-bearing unit (aquifer). For example, a well in a 100-foot-thick aquifer will produce more than a well in a 10-foot-thick aquifer provided both aquifers have the same permeability. Hydraulic conductivity is the water-transmitting capacity of a unit of an aquifer (see Figure 2-9, A). Transmissivity is the water-transmitting capacity of a unit prism (the saturated thickness) of the aquifer and usually is expressed in units of gallons per day per foot of aquifer width (see Figure 2-9, B). Wells in high transmissivity areas will produce high well yields. Wells in low transmissivity areas will produce low well yields. Appendix D details the Electrical Logging System you can use to log shallow wells in various rock formations.
e. Yield and Drawdown. Yield is the volume of water discharged from the well per unit of time when water is being pumped or is flowing freely. Yield is commonly measured in units of gallons per minute (GPM) and gallons per hour (GPH) for small yields or in cubic feet per second (cfs) for large yields. Yield is a measure of how readily an aquifer gives up its supply of groundwater.
Drawdown is a measure of how much the water level near the well is lowered when the well is pumped. Drawdown is the difference (in feet) between the static water level and the water level when pumping (Figure 2-10). See Chapter 8 for methods of measuring yield and drawdown.
f. Hydraulic Gradient. Darcy's Law describes the flow of groundwater and is applied to evaluate aquifer and aquifer material hydraulic characteristics. The hydraulic gradient is the change in head (water elevation) with distance. The hydraulic gradient determines the direction of groundwater flow. To calculate the hydraulic gradient, use the following formula:
i = hydraulic gradient, in feet
h = hydraulic head, in feet.
L = horizontal distance from h1 to h2, in feet (Figure 2-11)
Darcy's experiments show that the flow of water through a column of saturated sand is proportional to the difference in the hydraulic head at the ends of the column. Darcy's Law is still used as the basic principle that describes the flow of groundwater and is expressed as follows:
Q = KiA
Q = quantity of water discharged, in cfs.
K = hydraulic conductivity (constant factor).
i = hydraulic gradient, in feet.
A = cross-sectional area, in square feet.
g. Aquifer Tests. Groundwater aquifer performance and well efficiency are tested by placing boreholes through the aquifer. Testing and recording the yield and the drawdown can provide useful information to select the best pumping equipment and well screens for the actual well. Measurements of drawdown in observation wells (near the pumping well) and accurate pumping rates of the actual well will provide useful information on the hydraulic characteristics of the aquifer.
Aquifer tests provide values of hydraulic conductivity and transmissivity that better represent the actual aquifer than laboratory permeability tests conducted on small intact samples. The much greater volume of aquifer material tested in a field aquifer test takes into account secondary porosity in fractures and connected voids as well as the primary porosity of the materials, Field aquifer tests may also reveal the presence of boundary zones, which are zones of greater or lesser permeability or recharge zones that define the limits of the aquifer. Aquifer test data are useful in determining well yield (discharge) for groundwater supply and in engineering projects requiring dewatering of excavation sites and subsurface excavation. Field testing of aquifers is described in Chapter 7 and Appendix C of Technical Manual (TM) 5-818-5.
2-6. Aquifers. Saturated rock or soil units that have sufficient hydraulic conductivity to supply water for a well or spring are aquifers. Aquifers transmit water from recharge areas to discharge areas, such as springs, lakes, and rivers. Typical aquifers are gravel, sand, sandstone, limestone, and fractured igneous and metamorphic rock. Those subsurface rock or soil units that do not transmit water readily and cannot be used as sources of water supplies are called aquicludes. Typical aquicludes are clay, shale, and unfractured igneous and metamorphic rock. Aquicludes that exist between aquifers are confining beds; the water moves only within the aquifers.
a. Unconfined. These are aquifers that are partly filled with water, have fluctuating water levels, and can receive direct recharge from percolating surface water. Wells drilled into an unconfined aquifer are called water-table wells (Figure 2-12).
b. Confined. These are aquifers that are completely filled with water and are overlaid by a confining bed. The water level in a well supplied by a confined aquifer will stand at some height above the top of the aquifer. Water that flows out of the well is called flowing artesian (Figure 2-13). Water rises because of the pressure that the overlying materials exert on the water and the height of the column of water driving the water through the interconnecting pores of the aquifer. The height of the column of water that is driving water through the aquifer is the head. The height that the water will rise to inside a tightly cased artesian well is the potentiometric surface and represents the total head of the aquifer.
c. Perched. These aquifers lie above an unconfined aquifer and are separated from the surrounding groundwater table by a confining layer (Figure 2-14). The aquifers are formed by trapping infiltrating water above the confining layer and are limited in extent and development. In some arid environments, perched aquifers form sources of shallow and easily developed groundwater.
d. Catchment. This is formed where impervious rock underlies a zone of fractured rock or alluvium that serves as a reservoir for infiltrated water (Figure 2-15). A catchment can be a special type of perched aquifer. Catchments cannot provide large quantities of water, but they may provide easily developed groundwater for small demands or temporary supplies for drilling operations.
(1) Soil. These aquifers occur in unconsolidated deposits. Examples of sediment deposits and their sources are--
- Alluvium, which comes from running water.
- Glacial drift, which comes from flowing ice.
- Sand dunes, which come from blowing winds.
Sand and gravel deposits are the primary materials of unconsolidated aquifers. Alluvial deposits are prevalent in river valleys in humid environments and in dry wadis in arid environments. In many desert areas, alluvial deposits may be the primary source of groundwater. These deposits may be unconfined, confined, or perched.
(2) Rock. This aquifer is a mass of rock that can store and transmit groundwater.
(a) Limestone and dolomites. These are carbonate rocks that dissolve when carbon dioxide from the atmosphere and groundwater mix to form carbonic acid. The acid dissolves and widens the fractures and bedding planes into larger rock openings. These openings may eventually develop into caves. Limestone with fractured zones develop solution channels. Caves can be productive aquifers that yield appreciable quantities of groundwater (Figure 2-16).
(b) Basalt. This is an igneous rock and can be a very productive water bearer. The water flows through openings that include lava tubes, shrinkage cracks, joints, and broken or brecciated zones at the top of cooled lava flows (Figure 2-17).
(c) Sandstone. This is consolidated or cemented sand. In unfractured sandstone, groundwater can be stored in the pores between individual sand grains and can be recovered through pumping. Water in sandstone may flow through bedding planes and joints.
2-7. Groundwater Exploration. In groundwater exploration, it is possible to preedict the location of an unconfined aquifer within alluvial sediments. However, identifying these unconfined alluvial aquifers requires a detailed knowledge of sediments in the area. Usually, this information can only be obtained from existing water- or oil-well-drilling records or from an exploratory drilling program. For development of water supplies in support of tactical operations, the unconfined aquifer will be the preferable target zone. Deeper confined aquifers should be investigated if an unconfined aquifer cannot provide an adequate water supply or if the unconfined aquifer is contaminated.
Rock aquifers should be considered for exploration only when soil aquifers are not present or when soil aquifers cannot provide a sufficient water supply. Identifying suitable well sites in rock aquifers is more difficult than in soil aquifers. Water development in rock aquifers is more time-consuming and costly and has a higher risk factor. However, in some areas, rock aquifers may be the only source of groundwater.
Indicators of groundwater resources are those conditions or characteristics that indicate the occurrence of groundwater. No indicator is 100 percent reliable for detecting groundwater, but the presence or absence of certain indicators can be used as detection possibilities. Indicators useful in identifying groundwater resources are called hydrogeologic indicators (Table 2-3).
a. Reservoir Indicators. These indicators are characteristics in soils, rocks, and landforms that define the ability of the area to store and transmit groundwater but do not directly indicate the presence of groundwater. One of the first steps in groundwater exploration is to identify and evaluate reservoir indicators.
The size, shape, and water-bearing characteristics of a hydragraphic basin are important in evaluating water resources. The characteristics could be more useful in selecting sites for ground-level study or drilling than knowing the indicators that indicate the presence of groundwater. Plants around a dry lake bed are good indicators of groundwater. However, dry lake beds contain very fine-grained sediments; wells in these areas usually produce low water yields and contain poor quality groundwater. The rock or soil type present could be the most important reservoir indicator as it usually defines the type of aquifer and its water-producing characteristics. For reconnaissance, it is only necessary to recognize three types of rock and one type of soil.
(1) Igneous Rocks. These are poor aquifers except where the rocks have been disturbed by faulting or fracturing (Table 2-4). In many cases, the rocks are not capable of storing or transmitting groundwater and will act as a barrier to groundwater flow. In other cases, the stresses and movements of mountain building may have resulted in fracturing of the rock. groundwater can accumulate in fractures and move through the rock if the fractures are connected. Most groundwater-bearing fractures are within 500 feet of the surface, and drilling deeper to find water in igneous rocks is not advised. Wells in these aquifers are often poor producers and are seldom developed unless no other water source is available.
(2) Metamorphic Rocks. These areas rarely produce sufficient groundwater and are considered an effective barrier to groundwater flow (Table 2-4). Metamorphic rocks have poor potential for groundwater development.
(3) Sedimentary Rocks. These areas have the greatest potential for groundwater development (Table 2-4). Sedimentary rocks are capable of supplying low yields if unfractured and moderate to high well yields if fractured. Sandstone, limestone, shale, and evaporites are the four common types of sedimentary rocks.
(a) Sandstone. Sandstone's ability to stem and tramsmit groundwater varies. If the sand grains are small and tightly cemented together, a well will have a low yield, but if the rock has been fractured extensively, a well could have a high yield. If the sand grains are relatively large and poorly cemented, moderate to high well yields may be possible.
(b) Limestone and dolomites. Undisturbed limestone and dolomites are poor aquifers but limestone can bean excellent water source, where fractured. In some areas of the world, limestone areas are the principal groundwater sources. Fractures in limestone are more important than in other fractured rocks because when groundwater moves through the fractures, it can dissolve the rock and enlarge the fractures. This process is called dissolution. Dissolution allows the rock to store and transmit greater volumes of water than other types of fractured rocks. In most cases, limestone has been fractured and is considered the highest potential source of groundwater from rock.
(c) Shale. Shale is fine-grained, does not usually store much groundwater, and does not transmit large quantities of ground water. Where fractured, shale generally can only produce a few gallons per minute. However, identifying shale units is important because shales could indicate that artesian conditions exist in more productive water-bearing units below or between shale layers.
(d) Evaporites. Evaporites are generally capable of storing and transmitting groundwater but tend to dissolve in the water. Groundwater from evaporites is often unfit for human consumption and often not fit for any use. Because of the poor water quality, evaporites are generally considered to have poor potential for development. Because evaporites can dissolve in the water during drilling, immediate surface areas could collapse and men and equipment could be lost. Special protective measures must be taken when developing a well in evaporites.
(4) Alluvium. Groundwater is most readily available in areas under soils (unconsolidated sediments). This is largely because uncemented or slightly cemented and compacted materials have maximum pore space, are relatively shallow, and are easily penetrated. Soils deposited by running water are called alluvium. Alluvium has been formed in relatively recent geologic time and is generally restricted to lowland terrain features, such as alluvial valleys, terraces along rivers, alluvial fans, glacial outwash plains, and alluvial basins and between mountains and coastal terraces. Factors that have an important bearing on groundwater yield in soils are particle size, cleanliness (percent of frees), and degree of sorting or gradation. Clay yields almost no water; silt yields some, but very slowly. A well-sorted, clean, coarse sand or gravel yields water freely.
(a) Valleys. Alluvial valleys are one of the most productive terrains for recovering groundwater (Figure 2-18). Normally, sand and gravel form a large part of the stream alluvium so that wells located in the alluvium are likely to tap a good aquifer or series of aquifers Individual aquifers do not usually extend far, and the number and depth of water-bearing sands and gravels change rapidly from place to place.
Alluvium tends to become progressively finer downstream as the stream gradient decreases and the distance from outcrops of rock increases. In lower stream courses far from hard-rock highlands, alluvium is mostly silt and clay, with a few sand stringers. A shallow well has a small chance of striking such a sand stringer. The sands may be so fine-grained that most wells will have small or moderate yields. For large supplies, several wells or deeper wells may be needed.
(b) Stream and coastal terraces. Stream and coastal terraces usually are underlaid by grovel or sand deposits similar to floodplain alluvium. If the terraces are fairly bread and the deposits thick, they may be good water sources (Figure 2-19). In someplace, terraces are so deeply trenched by stream erosion that most of the groundwater rapidly drains out of the terrace gravels through the stream-cut slopes. Well drillers must be swam of the possibility of saltwater intrusion problems when drilling in coastal areas.
(c) Fans. Alluvial fans can be found where steep mountain slopes rise abruptly from adjacent plains (Figure 2-20). The streams coming from the mountains drop coarse material near the apex of the fan and progressively finer material down the slope. At the toe of a large fan, the deposits are mostly silt and clay, with few sand stringers. Alluvial fans often produce groundwater the water depth is greater than the depth in alluvial valleys. The aquifers often have a braided pattern and individual beds are limited in extent. Aquifers are abundant near mountains. However, to reach water may require drilling several hundred feet. Large boulders, which are common at the apex of the fan, can make drilling difficult. Down the slope of a fan, aquifers get progressively thinner (pinch out). In the lower part of a large fan, test drilling may be needed to locate a productive bed.
(d) Basins. Alluvial basins are in regions where mountains alternate with structural troughs (Figure 2-21). Erosional products from the mountains partly fill the basins with alluvium laid down as a series of coalescing alluvial fans. The upper alluvial slopes form piedmont plains or alluvial aprons that gradually decrease in slope toward the interior of the basin until they merge with the interior flats. Lakes or playas occupying part of the central flat are usually saline. Lake-bottom deposits are largely clay. Many alluvial basins are good sources of groundwater. Good wells yield several hunched to a thousand or more GPM.
(e) Glaciated regions. Glaciated and postglaciated areas can yield water from glacial deposits (Figure 2-22). Large quantities of alluvium laid down by steams emerging from glaciers (glacial outwash) contain a higher percentage of gravel and coarse sand than clay contains. Such areas are good sources of groundwater. Extensive deposits occur in all glaciated regions of the earth, especially in the northern United States, northern Europe, and areas bordering high mountains. Many large cities, including some in the upper Mississippi basin, get their water from glacial outwash sediments. Glacial materials deposited directly by ice (glacial till) are poor water bearers, yielding barely enough for farm wells. In places where outwash sands and gravels are interbedded or associated with the till, good aquifers form. Some of these aquifers may carry water under pressure.
(5) Stratigraphic Sequence. The stratigraphic sequence of geologic strata that occurs in an area can give clues to the types and depths of aquifers present. Figure 2-23 shows a stratigraphic column for a portion of the arid Great Basin. The Great Basin contains a number of individual rock units. Some of these units, such as the Ely Limestone, can readily transmit water, but other units, such as the Eureka Quartzite, are relatively impermeable and cannot store or transmit exploitable quantities of groundwater. Relatively impermeable units are aquitards or aquicludes. Aquitards are units that retard or slow the passage of water.
In the stratigraphic sequence, there are five discrete aquifers and six discrete aquitards. By knowing the stratigraphic sequences in a region, it is possible to predict the type of aquifer present at a given depth, For example, the Ely Springs Dolomite is not highly fractured, but the dolomite is a productive aquifer where it is fractured. Because this unit is not overlain in the area by an aquitard, it could be an unconfined aquifer. The stratigraphic sequence indicates that the Eureka Quartzite underlies the aquifers and is an impermeable barrier or aquitard. This unit is underlain by the Pogonip Group that could provide suitable quantities of groundwater. Because this unit is overlain and underlain by aquitards, it is a confined aquifer. By using the geologic map, it is possible to estimate the thickness of each unit and the anticipated well depth. If data exists for this aquifer in other areas, it may also be possible to predict the expected well yield and groundwater quality.
In soils, the stratigraphic sequence is usually less extensive, more variable, and not as well known as rock. Alluvial deposits often consist of interbedded gravel, sand, silt, clay, mixtures of these materials, and, possibly, interbedded evaporite deposits. These sediments often occur in complex stratigraphic sequences with some units discontinuous and other units grading into different soil types vertically and horizontally. Unless sufficient existing well data is available, it is impractical to define the stratigraphic sequence of such sediments.
(6) Structure Density and Orientation. Geologic structures, such as folds, fractures, joints, and faults, are features that disrupt the continuity of rock units. For groundwater exploration, identifying folds has a limited use but identifying faults and fractures is important, especially in rock aquifers. The ability of rock aquifers to transmit groundwater is related to the number and size of fractures in the rock. Density of fractures is an important consideration in locating well sites in rock terrain. Figure 2-24 shows a structure map of part of western Iran. The best potential well sites are located at the intersection of fracture zones. Secondary sites are along individual fracture zones. Areas with no fractures are probably low permeability areas and poor sites for water wells.
Fracture orientation is an important characteristic of rock aquifers and of soil aquifers Faults may act as either barriers to or conduits for groundwater flow. To distinguish between fault conduits and fault barriers, vegetative indicators are important. If a fault zone is acting as a barrier to groundwater flow, groundwater accumulates behind the barrier and often becomes shallow enough to support springs or dense stands of vegetation, or it forms wetlands (Figure 2-25).
(7) Dissolution Potential. Dissolution potential is the potential for the development of high secondary permeability in a soluble rock because of the dissolution of the rock through contact with groundwater. Unfractured soluble rocks, such as limestone, have a low permeability referred to as the primary permeability of the rock. Where fractured, the rock has a secondary permeability that is related to the size and density of the fractures. If the rock is soluble and saturated, the contact between the groundwater stored or moving through the fractures and the rock may result in dissolution of the rock.
The dissolution process increases the size of fractures and can result in an increased secondary permeability. Many of the world's caves and sinkholes (karst topography) result from limestone dissolving in groundwater and are indicators of high dissolution potential. However, if such features are not present, dissolution potential must be estimated on the basis of rock type and structure density. The highest dissolution potential occurs in heavily fractured carbonates (limestone and dolomite) and evaporites. Other rock types generally do not dissolve in groundwater and identifying dissolution potential is of little use.
(8) Grain Size and Sorting. The grain size and sorting of an aquifer are related to the porosity and permeability of the aquifer and the production capability of the aquifer. Fine-grained materials (clay) have a high porosity but a very low permeability and are poor aquifers. Sands have porosity (about half that of clay) and high permeability and are usually productive aquifers. Generally, well production capacity is directly promotional to grain size. Areas of fine-grained sediments (playas and lake beds) have poor water-production potential. Areas of coarse-grained sediments (alluvial fans) have a higher potential.
(9) Lithification. This is the process by which sediments are converted to rock. Lithification includes compaction, consolidation, cementation, and desiccation. The degree of compaction or consolidation affects the porosity and permeability of an aquifer (Figure 2-26). Porosity and permeability of unconsolidated materials (subjected to little or no overburden pressure) are related to grain size. With compaction and consolidation, the pore spaces between grains are reduced and the porosity and permeability of the aquifer are decreased.
Another source of lithification is the cementing of groins by the precipitation of minerals from solution in the groundwater. Many fragmented sedimentary rocks are cemented by silica or calcium carbonate precipitation from the waters they are deposited in or from waters introduced after they are deposited. Cementation often occurs along fault zones where deep mineral-rich water migrates upward. When the minerals and water mix, they precipitate travertine or other minerals along the fault zone. This can result in barriers to groundwater flow in areas selected as potential well sites on the basis of the fault zones.
(10) Drainage Basin Size. Because most groundwater is derived from the infiltration of precipitation over an area, the size of individual drainage basins can help define the overall groundwater potential. Large drainage basins may receive more precipitation and have a larger groundwater supply than smaller basins. This is true where precipitation is the same over a region; however, it may not apply in areas of high relief or variable climate.
Perhaps the most useful areas where drainage basin size can give an indication of groundwater potential is in mountainous terrain near coastal areas. In such areas, the larger drainage basins receive more recharge from precipitation. This recharge flows toward the coast and the areas of highest groundwater potential occur along the coastal plains adjacent to the larger basins. In arid environments, recharge usually coincides with areas where surface water drains from the mountainous areas and infiltrates into the groundwater system. The magnitude of this infiltration usually depends on drainage basin size and on the groundwater resource potential of the area.
(11) Landforms. Identifying landforms with drainage patterns can help identify rock types in areas without geologic maps. Landforms can provide information related to water depth, well-production potential, and water quality. Table 2-5 lists landform classifications with hydrogeologic conditions that may be based on the presence of the landforms.
(12) Elevation and Relief. Elevation and relief provide an idea of the amount of groundwater replenishment within a drainage basin and its groundwater potential. In a region of about the same latitude, precipitation distribution is related to the elevation of the area with zones of higher elevation receiving more precipitation than lower areas. A small drainage basin at a high elevation may receive appreciably more recharge from precipitation than a much larger basin at a lower elevation. Relief also has some effect on groundwater recharge. Areas with high relief (valleys bounded by prominent mountain ranges) have well-defined and easily identifiable recharge areas. In broad plains or plateaus, the moderate relief does not indicate recharge areas, and other indicators (grain size or drainage density) must be used.
(13) Drainage Pattern and Density. Recognizing drainage patterns can help define rock types, recharge areas and potential, and general hydrologic conditions of an area. Without geologic maps or other information on rock types, classification of the drainage pattern and landforms can provide an accurate interpretation of rock types and recognition of the area's structure. Because most groundwater recharge occurs as infiltration of surface-water drainages, areas with high drainage densities receive more recharge than areas with low drainage densities. Recognizing drainage patterns and density can provide indications of the type of aquifers the magnitude of recharge in an area, and directions of groundwater flow. Figure 2-27 shows some of the more common drainage patterns. Rock has widely spaced rectangular or dendritic patterns and alluvium has medium to widely spaced parallel drainage patterns along alluvial fans and dendritic patterns along valley axes and floodplains.
b. Boundary Indicators. These are characteristics that are indicative of local or regional groundwater flow systems. By identifying the boundaries of flow systems, it is possible to define directions of groundwater flow and to estimate the depth and quality of groundwater within an area. Boundary indicators do not directly indicate the presence of groundwater in an area.
(1) Recharge Areas. These are areas where the groundwater reservoir is replenished. Recharge may be derived from the runoff of precipitation into rock fractures in mountainous areas, leakage along streambeds or under lakes, or the flow of groundwater from upgradient areas. Figure 2-28 shows a sketch map of a hydrographic basin with identified recharge areas. Some recharge occurs in mountain areas as direct infiltration. The precipitation that does not infiltrate runs off into the local drainage network. Some recharge occurs along the streambeds and the remaining runoff discharges on the alluvial fans where it infiltrates. Precipitation over the valley floor is channeled, and quantities of recharge occur along the valley drainage system. Various amounts of recharge are derived from lakes, ponds, or channels that may occur within the basin.
The recharge of groundwater from surface water sources usually results in a mound (bulge) in the surface of the groundwater (Figure 2-29). Groundwater in such areas flows away from the recharge sources. If the source is a lake, flow is radial away from the source. If the source is a linear source (mountain range or a stream), the groundwater divides and flow is primarily in two directions from the source. Areas recharged by direct infiltration or precipitation usually contain good quality groundwater. Groundwater under lakes may exhibit poor water quality because evaporation of lake water may increase the concentration of chemicals in the lake and any recharge derived from the lake. Groundwater recharged from streams is usually intermediate quality between groundwater recharged by precipitation and lakes.
Subsurface recharge from adjacent basins is often difficult to assess. By knowing the elevation of groundwater in adjacent areas, the transmissivity of the aquifer, and the width of the recharge area, it is possible to estimate the amount of recharge from subsurface flow. This estimate usually is not possible in areas with limited data. It is possible to infer that such flow is occurring on the basis of differences in elevation and the location of barriers or conduits between hydrographic basins.
(2) Discharge Areas. Recharge to and discharge from a hydrographic basin must be equal. Figure 2-30 shows a sketch map of a hydrographic basin with identified discharge areas. Figure 2-31 shows the directions of groundwater flow from the recharge to the discharge areas. Groundwater discharge can occur in streams and lakes through consumption by plants or man and by subsurface flow to adjacent down gradient basins.
The location of discharge areas can help identify areas of shallow groundwater. Streams sustained by groundwater seepage, wetlands, and certain types of vegetation indicate discharge areas where groundwater is close to land surface. Some types of vegetation are capable of sending tap roots to depths of over 100 feet and are not indicative of shallow groundwater. The location of subsurface discharge areas requires more detailed knowledge of the hydrologic balance of the area.
(3) Impermeable and Semipermeable Barriers. The quantity and rate of groundwater flow from recharge areas to discharge areas are controlled by the transmissivity of the aquifers. Impermeable barriers are those features (solid rock masses) through which groundwater cannot flow. Semipermeable barriers are those features (faults or fractured rock masses) that restrict flow but do not act as a complete barrier. Such features should be recognized because they usually form the boundaries of groundwater flow systems and, when located within a flow system, can result in areas of shallow groundwater.
(4) Surface-Water Divides. Surface-water divides can form boundaries between groundwater flow systems. The mounding of groundwater under areas that receive recharge from the infiltration of precipitation causes groundwater to flow away from the recharge area. Similarly, surface water flows away from topographic highs that often correlate with groundwater recharge areas so that surface-water flow patterns usually coincide with groundwater flow patterns. The identification of surface-water divides can help define groundwater flow systems.
c. Surface Indicators. Surface indicators are those features that suggest the presence of groundwater. These indicators can provide information about the depth, quantity, and quality of the groundwater resources in an area; however, they do not positively indicate the presence of groundwater. The resource potential of an area is an inference on the basis of the presence (or absence) of certain indicators and particularly the association of these indicators.
(1) Springs. Springs are effluences of groundwater occurring where the water table intercepts the ground surface. Springs are usually good indicators of the presence of shallow groundwater occurrences. However, the presence of shallow groundwater may not be indicative of a good area for well construction. Springs occur where groundwater discharges to the earth's surface. Figure 2-32 shows several types of springs. Faults, valley-depressions, and alluvial-fan springs may discharge appreciable quantities of groundwater.
(2) Vegetation Type. Certain types of vegetation (or vegetative assemblages) are associated with specific hydrogeologic environments. Some plants (phreatophytes) can only exist if their root systems are in direct contact with groundwater. Phreatophytes, such as mesquite tress, have tap roots that go down more than 100 feet. Shrubs, such as saltbush, have roots that descend only a few feet, making them excellent indicators of shallow groundwater. Table 2-6 lists several plants that indicate the presence of shallow groundwater. The density of vegetation can help define the location of recharge and discharge areas. Dense stands of vegetation along stream channels are riparian vegetation. Riparian vegetation along streams that discharge mountainous watersheds indicates that surface water is infiltrating the streambed and recharging the groundwater. In many cases, the vegetation will decrease in density after the stream reaches the valley floor. Somewhere along the streambed, the riparian vegetation assemblage will give way to the typical valley-floor vegetation.
(3) Playas. These are dry lake beds composed mainly of clay and located in intermountain valleys (Figure 2-33). During rainy seasons, playas may store large quantities of surface water.
(4) Wetlands. Wetlands such as marshes, bogs, and swamps are indicative of very shallow groundwater. Although wetlands are not typical of arid environments, they have been observed in arid flow systems where groundwater accumulates behind flow barriers. Wetlands can also occur in low-lying areas where the discharge of regional spring water accumulates. The presence of wetlands is an excellent indicator of groundwater. However, wetlands generally are not suitable for water-well locations because of low permeability of wetland soils, marginal water quality from the evapotranspiration processes of wetland vegetation, and severe mobility constraints. Wetlands are important in groundwater detection because they usually represent regional discharge points. Areas upgradient of wetlands are usually favorable targets for groundwater development.
(5) Streams and Rivers. Streams and rivers (including dry streambeds and riverbeds) are usually recharge areas in arid regions and may be recharge or discharge areas in temperate climates, depending on seasonal rainfall. This type of recharge is especially true of major streams that drain the central portions of most valleys. Because recharge occurs along stream courses and streams occur in lower elevation areas in the valley, the areas adjacent to streams are considered good locations for wells, especially near the intersection of major streams. However, such locations are not always the best available areas for water wells.
Streams often migrate over large areas of the valley floor and deposit mixtures of gravel, sand, silt, and clay. Often these deposits are discontinuous and result in a vertical sequence of poorly sorted materials with low overall permeability and low to moderate well yields. Older, buried stream channels may be better aquifers because they are composed of coarser subgrade materials and much of the surface contamination has been filtered out.
(6) Snow-Melt Patterns. Snow-melt patterns can provide evidence of recharge areas and directions of groundwater flow. Snow packed in mountainous areas is usually a good source of recharge because slowly melting snow produces more infiltration than rainfall.
(7) Karst Topography. Karst topography results from the dissolution of carbonate rocks by groundwater and is characterized by caves, sinkholes, closed depressions, and disappearing streams (Figure 2-16). These features indicate that the rock has a very high dissolution potential and that groundwater is present. Collapse-type sinkholes (irregular, debris-filled sinkholes) usually indicate the presence of shallow groundwater because they result from the collapse of the surface materials into a dissolution cave. In some cases, the water provides evidence of the depth to groundwater.
(8) Soil Moisture. Soil moisture content can provide some indication of recharge and discharge areas. Areas with high soil moisture are not necessarily areas with high groundwater potential and good water-well locations. Soil moisture content is related to local rainfall and to groin size. The smaller the grain size, the higher the soil moisture. Playas and lake deposits often exhibit high soil moisture but very poor groundwater potential, resulting in low well yields.
(9) Salt Encrustation. Salt encrustations often occur in playas and are indicative of saline groundwater (Figure 2-33). Often, salt buildups result from the evaporation of surface water and can cover many acres. Certain salt-tolerant plants may grow in such areas, indicating shallow groundwater containing high concentrations of sodium, potassium, and other soluable salts. Although salt encrustations indicate shallow groundwater, drilling for groundwater should be avoided even if water treatment equipment is available. Surface salt deposits usually indicate deep evaporite deposits. Subsurface evaporite deposits are very susceptible to collapse and should be avoided.
(10) Wells. One of the best indicators of groundwater is groundwater development with well systems. Water wells are difficult to detect, especially from imagery. It is possible to detect wells indirectly from irrigation patterns. Pivot irrigation patterns (Figure 2-34) are distinctive and are usually supplied by centrally located water-supply wells. Such features are good indications that quality groundwater is present at economic pumping depths.
(11) Reservoirs and Lakes. Surface water bodies can be groundwater recharge, discharge, or both artificial surface-water reservoirs usually capture surface water and represent areas of recharge, as do natural reservoirs created by the damming of streams. Natural reservoirs in lowland areas are often formed by the discharge of groundwater from seeps along the lake bed or from springs.
(12) Crop Irrigation. Crop irrigation indicates the use of surface or groundwater for agriculture. In most arid environments, surface-water-based irrigation is located adjacent to streams and rivers. Beyond the river floodplains, agriculture is negligible. Agricultural development in areas without surface water is a good indicator of the presence of groundwater at relatively shallow depths. The leaching action of irrigated water and the use of chemical fertilizers may impair the groundwater quality in such areas.
(13) Population Distribution. Population distribution in arid regions or sparsely populated areas is closely related to water availability. Because of the lack of resources and technical capabilities for wide-scale groundwater development projects, population centers in arid environments without surface water are usually good indicators of groundwater supplies. These centers are often located on perched aquifers with limited capabilities; the population sizes are directly proportional to the production capacities of the aquifers.
2-8. Desert Environments. For contingency, water-well drillers must become familiar with drilling operations in an arid or desert environment. An arid environment usually has less than 300 millimeters (mm) of rainfall per year and high average daily temperatures. Most of the soils are coarse-grained with high porosity and permeability. Because of low rainfall, only deep water tables may exist. Therefore, any well-drilling units and equipment or kits deployed to a desert AO should be capable of achieving maximum depth of 1,500 feet.
Generally, the more arid the region, the greater the controls the host nation will place on welldrilling activities because of the potential impact of drilled wells depleting scarce aquifers. The host nation may also require more detailed documentation on the water sources. Because of the amount of water consumed during desert operations (up to 20 gallons daily per person), the challenge may not be in locating and drilling for water, but in finding who can identify water sources and give permission for using the sources.
In desert mountain areas, wells should be sited on the alluvial fans that extend from the mountains to the desert (Figure 2-35). Mountain areas usually receive more rainfall, and the streams draining away from the mountains carry coarse gravels and sands that, when deposited, produce the fans (Figure 2-36). At moderate depths, these fans may yield water. In deserts such as in Egypt, Jordan, and Saudi Arabia, deep confined aquifers are capable of producing extensive quantities of water. Locally, shallow confined aquifers come to the surface at an oasis, characterized by more extensive vegetation than the surrounding areas (Figure 2-37). Wells may be sited in the vicinity of an oasis to tap the confined aquifer.
Qanats are good indicators of groundwater in some desert areas like Iran. A qanat is a manmade, gently inclined underground channel that allows groundwater to flow from alluvial gravels at the base of hills to a dry lowland (Figure 2-38). In effect, qanats are horizontal wells. On aerial photographs, qanats appear as a series of ant-mound-like openings that run in a straight line and act as air shafts for the channel. They may be found in arid regions of Southwest Asia and North Mica. Qanats may be up to 30 kilometers in length.
a. Aquifer Contamination. Military engineers must be aware of possible aquifer contamination. As groundwater is transmitted from recharge to discharge areas, it contacts soils and rocks of the earth's crust. Contact causes some dissolution of soil and rocks into the water and alters the chemistry of the groundwater. Discharge areas that appear to be good well sites because of shallow groundwater may have poor water quality and may be poor sites for groundwater development. Discharge areas often correspond to zones of poor water quality. Generally, the longer the distance between the recharge and discharge areas, the poorer the water quality at the discharge area. The water-quality reduction is from the contact between the groundwater and the aquifer material during flow. Water consumption by plants often results in decreased water quality by the concentration of soluble salts in the groundwater.
In very shallow groundwater (less than 10 feet below land surface), water quality can be decreased by direct evaporation of water from the soil. Vegetation type can be used to infer groundwater quality characteristics. Saltbush around playas indicates not only the presence of shallow groundwater but also the probable occurrence of saline water, which would require treatment. The high evaporation rate in arid environments concentrates chemicals in the water, resulting in brackish or saline water quality.
b. Saltwater Intrusion. Saltwater intrusion into fresh groundwater is a problem in coastal areas and on islands. Saltwater is unfit for most human use and is harmful to automotive cooling systems, boilers, and other types of machinery. Chemical analysis determines the accuracy of contamination and salt levels. The average concentration of dissolved solids in sea water is about 35,000 parts per million (ppm) (3.5 percent). Most salts are chlorides.
When salt water and fresh water are present in sediments, fresh water floats on salt water. Contact between the two is determined by the head of the fresh water above sea level and by the relatively greater specific gravity of the salt water. The average specific gravity of sea water is about 1.025 (taking pure water as 1.000). For every foot of fresh water above sea level about 40 feet of freshwater is below sea level in homogeneous soils. The condition is best exhibited by small islands and peninsulas composed of permeable sands surrounded and underlain by salt water (Figure 2-39). The head of fresh water and resistance of the pores in sand prevents saltwater from entering the middle zone and mixing with fresh water. The diffusion zone (contact) between fresh water and salt water is narrow (less than 100 feet wide) unless affected by heavy pumping.
The amount of fresh water that can be pumped without intrusion of salt water depends on local conditions, type of well, rate of pumping, and the rate of recharge by fresh water. Any decrease in the head of fresh water by pumping or decrease in rainfall raises the saltwater level (Figure 2-40). The cone of depression (drawdown) produced in the freshwater level around a well allows a corresponding rise in the underlying salt water. Pumping a well should be restricted because salt water will enter the well if drawdown is maintained substantially below sea level for extended periods. The pumping rate should not exceed the rate of recharge. Saltwater intrusion is a potential problem when drilling in coastal-plain environments. Salt water may move into zones previously occupied by fresh water, this is called saltwater encroachment. The fresh water and the salt water migrate toward the well screen until a new balance between the waters is established (Figure 2-41).
c. Groundwater Contamination. Possible aquifer contamination from human activities should be considered when evaluating potential supplies of groundwater. Waste products are sources of groundwater contamination in some areas. Sources of waste products include agricultural activities; domestic, municipal, and industrial waste disposal operations; mine spoil piles and tailings; and animal feedlots. Figure 2-42 shows how waste contamination enters the groundwater. Nonwaste pollutants include leaks from buried pipelines, highway deicing (salting), pesticide and herbicide applications, and accidental spills from surface transportation and manufacturing activities. Supplemental information on WRDB overlays often indicate the severity of man-induced pollution. Especially important are aspects of bacterial contamination that will preclude the use of the groundwaters prevalent in many developing countries. Well drillers should consider the proper location of water wells shown in Table 2-7.
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