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Abstract
Science affects the way we think together.
Surface Water Sediments
Science affects the way we think together.
—Lewis Thomas, scientist
Introduction
Before providing information on surface water sediments in general, it is important to define surface water itself, at least for the purposes of this book. Keep in mind that the focus herein is on freshwater surface systems. Saltwater systems and deposited sediments are important and involve discussions very similar to those presented here for freshwater surface water sediments, but our focus here is on surface water sediments.
Surface Water
Approximately 40 million cubic miles of water cover or reside within the Earth. The oceans contain about 97% of all water on Earth. The other 3% is freshwater: (1) snow and ice on the surface of the Earth which contain about 2.25% of the water; (2) usable groundwater, which accounts for approximately 0.3%; and (3) surface freshwater, which is less than 0.5%. In the United States, for example, average rainfall is approximately 2.6 feet (a volume of 5900 km3). Of this amount, approximately 71% evaporates (about 4200 cm3), and 29% goes to stream flow (about 1700 cubic km3).
Beneficial freshwater uses include manufacturing, food production, domestic and public needs, recreation, hydroelectric power production, and flood control. Total U.S. water withdrawals in 2005 were 410,000 million gallons per day; 85% of that was freshwater withdrawals, and surface water supplied 80% of all water withdrawals. Withdrawals for thermoelectric power generation, primarily coal, nuclear, and natural gas, were 201,000 million gallons per day. The remainder went toward domestic use, irrigation, livestock, aquaculture, mining, and various industrial uses (USGS, 2009). Historically, in the United States, water usage increased through 1980 but has declined somewhat. For example, in 1975, just under 350 billion gallons of freshwater were used per day. By 1980, about 375 billion gallons of freshwater were used each day. In 2010, it was estimated that just over 300 billion gallons of freshwater were used per day (Donnelly and Cooley, 2015).
The primary sources of freshwater include the following:
- Surface water from lakes, rivers, and streams
- Groundwater from springs, artesian wells, and drilled or dug wells
- Captured and stored rainfall in cisterns and water jars
- Desalinized seawater or brackish groundwater
- Reclaimed wastewater
Current federal drinking water regulations actually define three distinct and separate sources of freshwater: (1) surface water, (2) groundwater, and (3) groundwater under the direct influence of surface water (GUDISW). This last classification is the result of the Surface Water Treatment Rule (SWTR). The definition of what conditions constitute GUDISW, while specific, are not obvious.
As stated earlier, the focus in this book is on surface sources of freshwater, specifically because sediment deposition within surface waters, sediment transport within surface waters, and the contamination of sediments within surface waters are important issues for the overall environmental health of Earth. It is important to point out that surface waters are not uniformly distributed over the Earth’s surface. In the United States, for example, only about 4% of the landmass is covered by rivers, lakes, and streams. The volumes of these freshwater sources depend on geographic, landscape, and temporal variations, as well as on the impact of human activities. Surface water is water that is open to the atmosphere and results from overland flow (i.e., runoff that has not yet reached a definite stream channel). Put a different way, surface water is the result of surface runoff. For the most part, however, surface (as used in the context of this text) refers to water flowing in streams and rivers, as well as water stored in natural or artificial lakes, manmade impoundments such as lakes made by damming a stream or river, springs that are affected by a change in level or quantity, shallow wells that are affected by precipitation, wells drilled next to or in a stream or river, rain catchments, and muskeg and tundra ponds.
Advantages and Disadvantage of Surface Water Supplies
The biggest advantage of using a surface water supply as a water source is that these sources are readily located; finding surface water sources does not demand sophisticated training or equipment. Many surface water sources have been used for decades and even centuries (in the United States, for example), and considerable data are available on the quantity and quality of the existing water supply. Surface water is also generally softer (i.e., not mineral laden), which makes its treatment much simpler.
The most significant disadvantage of using surface water as a water source is pollution. Surface waters are easily contaminated with microorganisms that cause waterborne diseases and chemicals that enter the river or stream from surface runoff and upstream discharges. Another problem with many surface water sources is turbidity, which fluctuates with the amount of precipitation. Increases in turbidity increase treatment costs and operator time. Surface water temperatures can be a problem because they fluctuate with ambient temperature, making consistent water quality production at a waterworks plant difficult. Drawing water from a surface water supply might also present problems; for example, intake structures may clog or become damaged from winter ice, or the source may be so shallow that it completely freezes in the winter. Water rights are another issue, in that removing surface water from a stream, lake, or spring requires a legal right. The lingering, seemingly unanswerable, question is who owns the water? Using surface water as a source means that the purveyor is obligated to meet the requirements of the Surface Water Treatment Rule and Interim Enhanced Surface Water Treatment Rule (IESWTR), which applies only to large public water systems (PWSs) serving more than 10,000 people. The IESWTR tightened controls on disinfection byproducts and turbidity and regulates Cryptosporidium.
Surface Water Hydrology
To properly manage and operate water systems, it is important to have a basic understanding of the movement of water and the factors that affect water quality and quantity—in other words, hydrology. A discipline of applied science, hydrology includes several components, such as the physical configuration of the watershed, the geology, soils, vegetation, nutrients, energy, wildlife, and the water itself. The area from which surface water flows is a drainage basin or catchment area. With a surface water source, this drainage basin is most often referred to, in nontechnical terms, as a watershed (when dealing with groundwater, we call this area a recharge area).
Note: The area that directly influences the quantity and quality of surface water is called the drainage basin or watershed.
Figure 3.1 Typical watershed.
When we trace on a map the course of a major river from its meager beginnings along its seaward path, it is readily apparent that its flow becomes larger and larger. Every tributary adds to its size, and between tributaries the river grows gradually due to overland flow entering it directly (see Figure 3.1). Not only does the river grow, but its entire watershed or drainage basin, basically the land it drains into, also grows in the sense that it embraces an ever-larger area. The area of the watershed is commonly measured in square miles, sections, or acres. When taking water from a surface water source, knowing the size of the watershed is desirable.
Surface Water Quality
Surface waters should be of adequate quality to support aquatic life and be aesthetically pleasing, and waters used as sources of supply should be treatable by conventional processes to provide potable supplies that can meet the drinking water standards. Many lakes, reservoirs, and rivers are maintained at a quality suitable for swimming, water skiing, and boating as well as for drinking water. Whether the surface water supply is taken from a river, stream, lake, spring, impoundment, reservoir, or dam, surface water quality varies widely, especially in rivers, streams, and small lakes. These water bodies are not only susceptible to waste discharge contamination but also to “flash” contamination (can occur almost immediately and not necessarily over time). Lakes are subject to summer/winter stratification (turnover) and to algal blooms. Pollution sources range from runoff (agricultural, residential, and urban) to spills, municipal and industrial wastewater discharges, and recreational users, as well as from natural occurrences. Surface water supplies are difficult to protect from contamination and must always be treated.
Soil vs. Dirt
Soils are crucial to life on earth … soil quality determines the nature of plant ecosystems and the capacity of land to support animal life and society. As human societies become increasingly urbanized, fewer people have intimate contact with the soil, and individuals tend to lose sight of the many ways in which they depend upon soils for their prosperity and survival. The degree to which we are dependent on soils is likely to increase, not decrease, in the future. Of course, soils will continue to supply us with nearly all of our food and much of our fiber. On a hot day, would you rather wear a cotton shirt or one made of polyester? In addition, biomass grown on soils is likely to become an increasingly important source of energy and industrial feedstocks, as the world’s finite supplies of petroleum are depleted over the coming century. The early signs of this trend can be seen in the soybean oil-based inks, the cornstarch plastics, and the wood alcohol fuels that are becoming increasingly important on the market.
Brady and Weil (1996, p. 2)
In any discussion about sediments and their eventual contamination, we must initially describe, explain, and define exactly what soil is and why soil is as important to us as air and water. First, we must also clear up a major misconception about soil. People often confuse soil with dirt. Soil is not dirt. Dirt is misplaced soil—soil where we don’t want it, contaminating our hands or clothes, tracked in on the floor. Dirt we try to clean up and keep out of our environment. But soil is special—mysterious, critical to our survival, and, whether we realize it or not, essential to our existence. We have relegated soil to an ignoble position. We commonly degrade it—we consider only feces to be a worse substance, but soil deserves better.
Before we move on, let’s take another look at that handful of “dirt”. What do we really have in hand when we reach down and grab a handful of “dirt”? The point is that it isn’t actually dirt but soil. What is soil, really? Perhaps no word causes more confusion in communications among various groups of laypersons and professionals—environmental scientists, environmental engineers, specialized groups of earth scientists, and engineers in general—than the word “soil.” Why? From the professional’s perspective, the problem lies in the reasons why different groups study soils.
Pedologists (soil scientists) are interested in soils as a medium for plant growth. Representing a corresponding branch of engineering soils specialists, soil engineers look at soil as a medium that can be excavated with tools. A geologist’s view of soil falls somewhere between that of pedologists and soil engineers—they are interested in soils and the weathering processes as past indicators of climatic conditions and in relation to the geologic formation of useful materials ranging from clay deposits to metallic ores.
To clear up this confusion, let’s view that handful of soil from a different—but much more basic and revealing—perspective. Consider the following descriptions of soil to better understand what soil is and why it is critically important to us all:
- A handful of soil is alive, a delicate living organism—as lively as an army of migrating caribou and as fascinating as a flock of egrets. Literally teeming with life of incomparable forms, soil deserves to be classified as an independent ecosystem or, more correctly stated, as many ecosystems.
- When we reach down and pick up an handful of soil, exposing the stark bedrock surface, it should remind us, maybe startle some of us, that without its thin living soil layer Earth is a planet as lifeless as our own moon.
If you still prefer to call soil dirt, that’s okay. Maybe you view dirt in the same way as E.L. Konigsburg’s character Ethan does (Konigsburg, 1966, p. 64):
The way I see it, the difference between farmers and suburbanites is the difference in the way we feel about dirt. To them, the earth is something to be respected and preserved, but dirt gets no respect. A farmer likes dirt. Suburbanites like to get rid of it. Dirt is the working layer of the earth, and dealing with dirt is as much a part of farm life as dealing with manure: neither is user-friendly, but both are necessary.
Soil Basics
Soil is the layer of bonded particles of sand, silt, and clay that covers the land surface of the Earth. Most soils develop multiple layers. The topmost layer (topsoil) is the layer in which plants grow. This topmost layer is actually an ecosystem composed of both biotic and abiotic components—inorganic chemicals, air, water, decaying organic material that provides vital nutrients for plant photosynthesis, and living organisms. Below the topmost layer (usually no more than a meter in thickness), is the subsoil, which is much less productive, partly because it contains much less organic matter. Below that is the parent material, the bedrock or other geologic material from which the soil is ultimately formed. The general rule of thumb is that it takes about 30 years to form one inch of topsoil from subsoil; it takes much longer than that for subsoil to be formed from parent material, the length of time depending on the nature of the underlying matter (Franck and Brownstone, 1992).
Soil Properties
From the environmental scientist’s view (with regard to land conservation and remediation methodologies for contaminated soil remediation through reuse and recycling), four major properties of soil are of interest: soil texture, slope, structure, and organic matter. Soil texture (see Figure 3.2), or the relative proportions of the various soil separates in a soil, is a given and cannot be easily or practically changed significantly. It is determined by the size of the rock particles (sand, silt, and clay particles) or the soil separates within the soil. The largest soil particles are gravel, which consists of fragments larger than 2.0 mm in diameter. Particles between 0.05 and 2.0 mm are classified as sand. Silt particles range from 0.002 to 0.05 mm in diameter, and the smallest particles (clay particles) are less than 0.002 mm in diameter. Clays are composed of the smallest particles, but these particles have stronger bonds than silt or sand; once broken apart, though, they erode more readily. Particle size has a direct impact on erosion. Rarely does a soil consist of only one single size of particle; most are a mixture of various sizes. The slope (or steepness of the soil layer) is another given, important because the erosive power of runoff increases with the steepness of the slope. Slope also allows runoff to exert increased force on soil particles, which breaks them apart more readily and carries them farther away.
Soil structure (tilth) should not be confused with soil texture—they are different. In fact, in the field, the properties determined by soil texture may be considerably modified by soil structure. Soil structure refers to the combination or arrangement of primary soil particles into secondary particles (units or peds). Simply stated, soil structure refers to the way various soil particles clump together. Clusters of soil particles, called aggregates, can vary in size, shape, and arrangement; they combine naturally to form larger clumps called peds. Sand particles do not clump because sandy soils lack structure. Clay soils tend to stick together in large clumps. Good soil develops small friable (easily crumbled) clumps. Soil develops a unique, fairly stable structure in undisturbed landscapes, but agricultural practices break down the aggregates and peds, lessening erosion resistance.
The presence of decomposed or decomposing remains of plants and animals (organic matter) in soil improves not only fertility but also soil structure—especially the ability of soil to store water. Live organisms such as protozoa, nematodes, earthworms, insects, fungi, and bacteria are typical inhabitants of soil. These organisms work to either control the population of organisms in the soil or to aid in the recycling of dead organic matter. All soil organisms, in one way or another, work to release nutrients from the organic matter, changing complex organic materials into products that can be used by plants.
Figure 3.2 (A) Textural triangle similar to U.S. Department of Agriculture model; (B) broad groups of textural classes.
Soil Formation
Soil is formed as a result of physical, chemical, and biological interactions in specific locations. Just as vegetation varies among biomes, so do the soil types that support that vegetation. The vegetation of the tundra and that of the rain forest differ vastly from each other and from vegetation of the prairie and coniferous forest; soils differ in similar ways. In the soil-forming process, two related, but fundamentally different, processes are occurring simultaneously. The first is the formation of soil parent materials by weathering of rocks, rock fragments, and sediments. This set of processes is carried out in the zone of weathering. The end point is producing parent material for the soil to develop in and is referred to as C horizon material. It applies in the same way for glacial deposits as for rocks. The second set of processes is the formation of the soil profile by soil-forming processes, which gradually change the C horizon material into A, E, and B horizons. Figure 3.3 illustrates two soil profiles, one on hard granite and one on a glacial deposit.
Figure 3.3 Soil profiles on residual and transported parent materials.
Soil development takes time and is the result of two major processes: weathering and morphogenesis. Weathering (the breaking down of bedrock and other sediments that have been deposited on the bedrock by wind, water, volcanic eruptions, or melting glaciers) happens physically, chemically, or a combination of both. Physical weathering involves the breaking down of rock primarily by temperature changes and the physical action of water, ice, and wind. When a geographical location is characterized as having an arid desert biome, the repeated exposure to very high temperatures during the day followed by low temperatures at night causes rocks to expand and contract and eventually to crack and shatter. At the other extreme, in cold climates rock can crack and break as a result of repeated cycles of expansion of water in cracks and pores during freezing and contraction during thawing. Figure 3.4 shows another example of physical weathering in which a slot canyon is carved down to various-sized rocks that, with time, are reduced to soil particles in which various vegetation types spread their roots and grow; roots can exert enough pressure to enlarge cracks in solid rock, eventually splitting the rock. Plants such as mosses and lichens also penetrate rock and loosen particles.
Figure 3.4 Slot canyon carved by the Virgin River in Zion National Park, Utah.
Bare rocks are also subjected to chemical weathering, which involves chemical attack and dissolution of rock. Accomplished primarily through oxidation via exposure to oxygen gas in the atmosphere, acidic precipitation (after having dissolved small amounts of carbon dioxide gas from the atmosphere), and acidic secretions of microorganisms (bacteria, fungi, and lichens), chemical weathering speeds up in warm climates and slows down in cold ones. Physical weathering and chemical weathering do not always (if ever) occur independently of each other; instead, they normally work in combination, and the results can be striking.
A classic example of the effect and power of their simultaneous actions can be seen in the ecological process known as bare rock succession, explained earlier in Chapter 1. An example of bare rock succession that can be seen today demonstrates just how effective and dramatic this weathering process can be. The Natural Bridge in Virginia (see Sidebar 3.1) illustrates the awesome power of physical and chemical processes, working in tandem, reshaping the Earth, and producing and transporting the particle material (eventually sediment particles) upon which and from which soil will eventually form.
Sidebar 3.1 Natural Bridge of Virginia
Thomas Jefferson stated that the Natural Bridge of Virginia (see Figure SB3.1.1) is “the most sublime of Nature’s Works.” The great stone causeway, situated a few miles west of the Blue Ridge Mountains in the heart of the great Appalachian Valley of western Virginia, has been proclaimed one of the natural wonders of the world. The proportions of the Natural Bridge are enormous. It is 90 feet long, and the width varies from 150 feet at one end to 50 feet at the other. The Natural Bridge is taller than Niagara Falls. The span contains approximately 450,00 cubic feet of rock. If we could weigh it, the mass would probably come in at about 36,000 tons (72,000,000 pounds). At its feet flows Cedar Creek, now only a small trickle in comparison to the massive, roaring flow of water it once was.
Figure SB3.1.1 Natural Bridge in Virginia.
The usual question asked about the Natural Bridge is “How was it formed?” Many theories have been suggested as to the exact origin of the Natural Bridge. Thomas Jefferson held the theory that the Natural Bridge was formed by some sort of cataclysmic event (what he called “some great convulsion”) and that its formation was relatively recent. (At the time, though, the Earth was believed to be only several thousands of years old. To Jefferson, that such an event could have occurred over the course of millions of years was inconceivable.)
Today we know that ideas about natural features such as the Natural Bridge change as we gain more knowledge. Spencer (1985) observed that, when talking about the exact age of the Natural Bridge, we must be careful to distinguish several important events; for example, the rocks that compose the bridge are early Ordovician (about 500 million years old). Toward the end of the Paleozoic Era (about 200+ million years ago), the internal forms of these rocks (the folds and breaks in the layers) were imposed during the Appalachian Mountain building process. Probably no more than a few million years ago, the formation of the stream drainage and the carving out of the bridge began.
With input from others, Jefferson later modified his “great convulsion” theory as the cause of the formation of the bridge. Jefferson, an astute student of science, was aware that other natural bridges on Earth had been formed by the work of water—the wearing action of water running through them—rather than by a convulsion of nature. He came to believe that these same wearing actions might have formed the Natural Bridge. One of Jefferson’s friends, Francis W. Gilmer, put forward a detailed description of the origin of the Natural Bridge in 1816. Gilmer outlined his thinking on the subject in a paper he presented to the American Philosophical Society:
… instead of its being the effect of a sudden convulsion, or an extraordinary deviation from the ordinary laws of nature, it will be found to have been produced by the very slow operation of causes which have always, and must ever continue, to act in the same manner. … the country above the bridge … is calcareous. … This rock is soluble in water to such a degree, as to be found in solution with all the waters of the country, and is so soft as to yield not only to its chemical agency, but also to its mechanical attrition. … Here, as in calcareous countries generally, there are frequent and large fissures in the earth, which are sometimes conduits for subterraneous streams, called “sinking rivers.” … It is probable, then, that the water of Cedar Creek originally found a subterraneous passage beneath the arch of the present bridge … The stream has gradually widened, and deepened this ravine to its present situation. Fragments of its sides also yielding to the expansion and contraction of heat and cold, tumbled down even above the height of the water. … The stone and earth composing the arch of the bridge, remained there and nowhere else; because, the hill being of rock, the depth of rock was greatest above the surface of the water where the hill was highest, and this part being very thick, and the strata horizontal, the arch was strong enough to rest on such a base. … Indeed, the very process by which the natural bridge was formed is still visibly going on; the water … is excavating the rock, and widening the channel, which, after a long lapse of time, may become too wide to support the arch, and this wonder of our country will disappear.
Since the time of Gilmer’s original theory about the origin of the Natural Bridge, all geologists who subsequently studied the Natural Bridge have agreed with his view that the bridge was formed by the action of running water diverted from the surface of the ground into a subterranean passage beneath the arch. They differ only in the details of this diversion. In 1893, for example, C.D. Walcott “suggested the bridge was once the site of a waterfall and at that time the valley floor of Cedar Creek was at a level close to the top of the canyon. Walcott postulated that the water somehow diverted just upstream from the waterfall into an underground passage that emptied out of the base of the falls. This would have left the span of dolomite between the diversion and edge of the waterfalls intact” (Spencer, 1985, p. 44).
Most of the others who have studied the origin of the Natural Bridge (e.g., Malott and Shrock, 1930; Woodward, 1936) favor ideas much closer to Gilmer’s—that a surface stream was diverted into an opening in the Earth (a cave) from which water issued farther downstream. This underground flow formed a long, natural tunnel. Over time, the roof of this tunnel collapsed, leaving only the span of Natural Bridge (see Figure SB3.1.2).
Cedar Creek still flows beneath the Natural Bridge (see Figure SB3.1.3). It originates in the Allegheny Mountains and empties into the James River. The structure of the rock of which the Natural Bridge is made determined its location. The arch of the bridge, which is massive and compact, is formed from dolomite, and its structural integrity seems sound. However, in time, the bridge will fall into Cedar Creek; it will be gone, but just as Nature works to modify and eventually destroy the Natural Bridge, she also works to form other natural wonders of the world, as she must.
Figure SB3.1.2 How the Natural Bridge was formed over time: Beginning with a shallow hole, Cedar Creek formed a natural tunnel (as shown). Later, the tunnel roof caved in. Because the Natural Bridge was formed of hard dolomite, it remained in place.
Figure SB3.1.3 Cedar Creek as it appears today. Notice the inclined layers of limestone in the creek bed, rising up the slope.
The final stages of soil formation include the process of morphogenesis, or the production of a distinctive soil profile with its constituent layers, or soil horizons (see Figure 3.3). The soil profile (the vertical section of the soil from the surface through all its horizons, including C horizons) gives environmental scientists critical information. When properly interpreted, soil horizons can warn about potential problems with using the land and can tell much about the environment and history of a region. The soil profile allows us to describe, sample, and map soils.
Soil horizons are distinct layers, roughly parallel to the surface, which differ in color, texture, structure, and content of organic matter (see Figure 3.3). The clarity with which horizons can be recognized depends on the relative balance of the migration, stratification, aggregation, and mixing processes that take place in the soil during morphogenesis. In podzol-type soils, striking horizonation is quite apparent; in vertisol-type soils, the horizons are less distinct. When horizons have been evaluated, they are each assigned a letter symbol to reflect the genesis of the horizon (see Figure 3.3).
Certain processes work to create and destroy clear soil horizons. Various formations of soil horizons that tend to create clear horizons by vertical redistribution of soil materials include the leaching of ions in the soil solutions, movement of clay-sized particles, upward movement of water by capillary action, and surface deposition of dust and aerosols. Clear soil horizons are destroyed by mixing processes that occur because of organisms, cultivation practices, creep processes on slopes, frost heave, and swelling and shrinkage of clays—all part of the natural soil formation process.
The bottom line: For our purposes in this text, whether you call it dirt or soil, we call it sediment.
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