Water Pollution PDF

Water Pollution 15 Abstract Water pollution results from many anthropogenic activities and occurs in various forms. Soil erosion leads to contamination of natural waters with suspended soil particles and ca...

Water Pollution 15 Abstract Water pollution results from many anthropogenic activities and occurs in various forms. Soil erosion leads to contamination of natural waters with suspended soil particles and can be the cause of excessive sedimentation. Organic wastes impart a high oxygen demand often culminating in low dissolved oxygen concentrations in water bodies. Nutrient pollution of streams and lakes—primarily from nitrogen and phosphorus—results in eutrophication, deterioration of water quality and loss of biodiversity. Pesticides, synthetic organic chemicals and heavy metals from industry, and pharmaceutical compounds and their degradation products can be toxic to aquatic animals or have other adverse effects on them. Toxins in drinking water can lead to several serious illnesses to include cancer in humans. Water bodies also may be contaminated with biological agents that cause aquatic animal and human diseases. Elevated sulfur dioxide and carbon dioxide concentrations in the atmosphere as a result of air pollution can influence water quality. Wetland destruction must be considered in a discussion of water pollution, because functional wetlands are important for natural water purification. Control of water pollution from all sources is important for protecting natural resources that are becoming increasingly more scarce. Keywords Types of pollution Biochemical oxygen demand Eutrophication Toxicity evaluation Wetland destruction Introduction The human population grew slowly for many millennia and other than for areas that were most suitable for human habitation, the earth was not heavily populated. Natural ecosystems were capable of supplying the resources and services necessary © Springer International Publishing Switzerland 2015 313 C.E. Boyd, Water Quality, DOI 10.1007/978-3-319-17446-4_15 314 15 Water Pollution to support society in a sustainable manner. The demand placed on ecosystems by humans did not cause significant damage to overall ecosystem structure and function other than in isolated, highly populated areas. Humans initially had relatively little control over the environment, and their population was basically limited by the same factors controlling the populations of other species. The population was around ten million when agriculture was invented around 10,000 BC. This increased food availability, but population reached only 750 million by 1750 AD. However, beginning in the 1500 and 1600s, mankind began to flourish because of increasing knowledge of science and technology, and a “critical mass” was reached in the mid-eighteenth century resulting in the industrial revolution. Since the industrial revolution, the global population has increased at a rapid rate with exponential growth since the mid-1800s (Fig. 15.1). The expanding human population has placed a huge demand on the world’s ecosystems for water and other resources as well as taxing their waste assimilation capacity. One of the major impacts of the growing human population has been an increasing pollution load that has greatly deteriorated the quality of aquatic ecosystems and water supplies. In this chapter, the major sources of pollution and their effects on natural aquatic ecosystems and water use by humans will be discussed. Fig. 15.1 World population 1750–2014 with projection to 2100 Overview of Water Pollution 315 Overview of Water Pollution Water pollution is separated into two broad categories—point sources and nonpoint sources. Well-defined effluent streams discharged in both wet and dry weather via a pipe, channel, or other conduit are point sources. Common point sources of pollution are industrial operations and municipal wastewater treatment plants. Storm runoff enters sewers and other effluent conduits and contributes to discharge during wet weather. Storm runoff accumulates pollutants from a broad area and is considered a nonpoint source. Runoff from urban areas, farmland, construction sites, etc., is known as nonpoint source pollution. Acidic deposition in rainfall and dry fallout also is nonpoint source of pollution. Pollutants vary widely in their properties. Organic wastes that demand oxygen when decomposed by microorganisms are major contaminants in domestic and municipal wastewater, animal feedlot effluents, and discharges from food processing and paper manufacturing. Suspended solids are important pollutants consisting of suspended mineral and organic particles. Solids do not settle immediately, and they can create turbidity plumes that are not aesthetically pleasing. Solids settle from water creating sedi- ment deposits that may suffocate benthic organisms. Sediment also reduces water depth, and shallow water favors aquatic macrophyte infestations. The oxygen demand of sediment with a high organic matter content can cause anaerobic conditions in shallow areas. Suspended solids in municipal, industrial, and feedlot effluents tend to be highly organic, while effluents from agricultural land, logging operations, construction sites, and surface mining have a high proportion of inor- ganic, suspended solids. Nutrient pollution results primarily from nitrogen and phosphorus in runoff or effluents. Municipal wastewater and other effluents with high concentrations of organic matter also tend to have large concentrations of nitrogen and phosphorus. Runoff from residential areas with lawns and from cropland and pastureland also contain elevated concentrations of nitrogen and phosphorus. The main concern over nitrogen and phosphorus additions in water bodies is eutrophication characterized by excessive phytoplankton productivity, and low dissolved oxygen concentration that may cause fish kills, loss of sensitive species, unsightly algal blooms, and bad odors. A variety of chemicals used for domestic, industrial, and agricultural purposes find their way into water bodies. Toxic chemicals may be contained in effluents from normal operations or they may leak or seep from storage depots or waste storage sites. Toxic substances may be directly harmful to aquatic life or they may accumulate in the food chain (bioconcentration) and be toxic to organisms in the food web. Bioconcentration also presents a potential food safety hazard for consumers of aquatic products. Toxins also can be hazardous in domestic drinking water or in waters used for agricultural purposes. 316 15 Water Pollution Runoff from surface mining and seepage from underground mines are well known sources of acidification in natural waters. Combustion of fossil fuels contaminates the air with sulfur dioxide, nitrous oxide, and other compounds that oxidize to form strong, mineral acids. Rainfall in heavily populated or industrialized areas is a major source of acidification. Nitrification also can be a significant source of acidity in surface waters. On the other hand, some effluents may be alkaline and cause an excessive pH in receiving waters. Recent research has revealed a new water pollution concern. Many natural waters contain residues and degradation products of pharmaceutical chemicals. Some are possibly directly toxic to aquatic life, while others may have more subtle, but negative effects on the physiology of organisms. Of course, these substances also can enter the water supply for humans. Contamination of waters with disease organisms of human origin is still a major concern in many developing nations. If human fecal material enters water, the risk of disease spread through drinking water is greatly increased. Many industrial processes generate waste heat that may be disposed of by transfer to water. Thermally-enriched effluents may raise temperatures in streams or other water bodies to cause serious ecological perturbations. The major contributors of several pollutants are summarized in Table 15.1 for the United States. Ten major sectors contributed more than half of the total quantities of the point sources for each pollutant. Municipal sewage plants were a leading pointsource of pollution. Nonpoint sources provided greater quantities of pollutants than point sources, and agriculture was responsible for more than half of the nonpoint pollution. Desalination of seawater necessary to supplement water supply in some countries also causes pollution. The discharge water from reverse osmosis plants is of higher salinity than coastal waters. Distillation plants discharge thermally polluted cooling water, and metals from heat exchangers enter the cooling water. Not all pollution results from contaminants contained in runoff or effluents. Sometimes, accidents may result in sudden spills of potentially toxic chemicals or other substances. For example, a highway or rail accident can result in a cargo being inadvertently spilled into a watercourse, or a ship accident can spill crude oil or other substances into the ocean or into coastal and inland waters. The most famous cases of crude oil pollution probably are the Exxon Valdez oil spill that occurred when an Exxon tanker struck a reef in Prince William Sound, Alaska in 1989, and the BP Deepwater Horizon oil spill that resulted from an accident on an off-shore oil drilling platform in the Gulf of Mexico in 2010. Water quality can be impaired through natural processes without human intervention as illustrated by the following examples. In some coastal areas, soils contain iron pyrite. Iron pyrite oxidation in the dry season produces sulfuric acid that may leach out in the rainy season to cause acidification of surface water. Some groundwaters may be unfit for domestic or other uses because of high iron or manganese concentrations. The infamous poisoning of many inhabitants of areas in Bangladesh and India by naturally-occurring arsenic in groundwater has already been discussed (Chap. 14). Salinization has made many freshwaters too salty for most purposes. Major Types of Pollution 317 Table 15.1 Annual contributions of selected pollutants (millions of kilograms) from each of 24 major point sources and the contribution of agriculture to total nonpoint sources in the United States as modified from van der Leedon et al. (1990) Sector TSS BOD N P Dissolved metals Point sources Municipal sewage plants 1,746 1,723 369 33.5 4.2 Power plants 529 11.1 Pulp and paper mills 355 240 Feedlots 191 44 18 9.9 Iron and steel mills 113 17 3.4 Organic chemicals 65 49 19 0.6 1.6 Miscellaneous food and 42 25 5 2.1 beverages Textiles 28 11 Mineral mining 24 1.1 Seafood processing 23 39 5 0.6 Cane sugar mills 23 Miscellaneous chemicals 18 Pharmaceuticals 40 Meat packing 16 1.5 Petroleum refining 7 0.7 2.7 Pesticide manufacturing 4 Leather tanning 3 Laundries 1.5 Fertilizer manufacturing 1.2 Poultry production 0.5 Electroplating 0.2 Machinery manufacturing 0.2 Oil and gas extraction 0.2 Foundries 0.1 Major sources 3,116 2,192 486 52.1 24.8 Total 6,234 3,149 561 86.6 26.8 Nonpoint sources Agriculture 2,800,000 12,698 6,168 2,395 N/A Total 4,432,000 18,730 9,107 3,519 N/A Major Types of Pollution Inorganic Solids and Turbidity Inorganic solids contribute the largest weight of pollutants entering water bodies; they cause two major problems: turbidity from suspended particles and sediment accumulation when particles settle. The major source of sediment in water bodies 318 15 Water Pollution is soil particles eroded from the land by rainfall and runoff. Falling raindrops dislodge soil particles, and the energy of flowing water further erodes the land surface and keeps the particles in suspension during transport. Factors opposing erosion are the resistance of soil to dispersion and movement, slow moving runoff because of gentle slope, vegetation that intercepts rainfall, vegetative cover to shield soil from direct raindrop impacts, roots to hold the soil in place, and organic litter from vegetation to protect the soil from direct contact with flowing water. Erosion usually is considered to be one of three types: raindrop erosion, sheet erosion, or gully erosion. Raindrop erosion dislodges soil particles and splashes them into the air. Usually, the dislodged particles are splashed into the air many times, and because they are separated from the soil mass, they can be readily transported in runoff. Sheet erosion refers to the removal of a thin layer of soil from the surface of gently sloping land. True sheet erosion does not occur, but flowing water erodes many tiny rills in surface soil to cause more or less uniform erosion of the land surface. These rills are not seen in cultivated fields because they are removed by tillage. Gully erosion produces much larger channels than rills and these channels are visible on the landscape. Estimating Soil Erosion Rates The universal soil loss equation (USLE) is used widely to estimate soil loss by erosion. The initial efforts to predict soil erosion by mathematic procedures of Zingg (1940) and Smith (1941) lead to further research on the topic, and the first complete version of the USLE was published in 1965 (Wischeier and Smith 1965). The equation has been slightly revised over time, and the present form is A ¼ ðRÞðKÞðLSÞðCÞðPÞ ð15:1Þ where A = soil loss, R = a rainfall and runoff factor, K = soil erodability factor, LS = slope factor (length and steepness), C = crop and cover management factor, and P = conservation practice factor. Presentation of the instructions and tabular and graphical information for obtaining the various factors are too lengthy to include here, but there are many online sources including calculators for solving the equation. Land surface disruption facilitates erosion; construction, logging and mining sites typically have very high rates of soil loss. Deforestation is a major concern both because of reduction in forest area and because of the serious erosion that follows. Row cropland also has a high erosion potential. The lowest rates of erosion are for watersheds that are forested or completely covered with grass. Erosion of streambeds and shorelines also can be important sources of suspended solids in water bodies. Effects of Soil Erosion on Water Bodies A portion of the soil particles dislodged from watersheds by erosion remain suspended in runoff when it enters streams and other water bodies. Suspended solids in waters create turbidity making the water less appealing to the eye, and less Major Types of Pollution 319 enjoyable for watersports. Turbidity also reduces light penetration into the water, and diminishes primary productivity. Moreover, suspended solids often must be removed from water to allow its use for human and industrial water supply adding to the cost of water treatment. When turbulence in water carrying suspended solids is reduced, sedimentation occurs. Sediment creates deposits of coarse particles in areas where turbid water enters water bodies and finer particles over the entire bottom. Elevated sedimenta- tion rates have several undesirable consequences. They make water bodies shallower, and this may lead to greater growth of rooted aquatic macrophytes. Shallower water bodies have less volume, and this may have negative ecological effects as well as reducing the volume of water that can be stored for flood control or human uses. Sediment also destroys breeding areas for fish and other species, and it can smother benthic communities. Erosion and sedimentation are, of course, natural processes that have been operating since the earth began. The morphology of the earth’s surface is the result of eons of erosion and sedimentation and other geological processes. However, natural processes tend to operate slowly allowing living organisms time to adapt. The problem today is that rates of erosion and sedimentation have been greatly accelerated by human activities, and many negative impacts are resulting. Organic Pollution Bacteria and other saprophytes in aquatic ecosystems remove dissolved oxygen for use in decomposing organic matter. The effect of addition of organic matter in pollutants on dissolved oxygen concentration depends upon the capacity of a water body to assimilate organic matter relative to the amount of organic matter introduced. A given organic matter load might not influence dissolved oxygen concentrations in a large body of water, but the same load might cause oxygen depletion in a smaller body of water. Likewise a rapidly flowing stream reaerates more rapidly than a sluggish stream of the same cross-sectional area, and therefore can asssimilate a greater organic matter input than the sluggish stream. Biochemical Oxygen Demand Assessment of the oxygen demand of a wastewater is a critical factor in water pollution control. The biochemical oxygen demand (BOD) is a measure of the amount of dissolved oxygen consumed by microscopic organisms while decomposing organic matter in a confined sample of water. Standard 5-Day Measurement The standard 5-day BOD determination or BOD5 normally is measured to provide an estimate of the pollutional strength of a wastewater (Eaton et al. 2005). In the BOD5 procedure, an aliquot of wastewater is diluted with inorganic nutrient 320 15 Water Pollution solution and a bacterial seed added. The inorganic nutrients and bacterial seed are necessary to prevent a shortage of bacteria and inorganic nutrients that might result from dilution. Because of the possibility of oxygen demand from the bacterial seed, a blank consisting of the same quantity of bacterial seed used in the sample is introduced into nutrient solution and carried through the same incubation as the sample. To prevent photosynthetic oxygen production, samples are incubated in the dark. The incubation is continued in the dark for 5 days at 20 C. At the beginning and end of incubation, the dissolved oxygen concentration is measured in blank and sample to permit estimation of BOD5 as illustrated in Ex. 15.1. Ex. 15.1: In a BOD5 analysis, the sample is diluted 20 times. The initial dissolved oxygen concentration is 9.01 mg/L in sample and blank. After 5 days of incubation, the dissolved oxygen concentration is 8.80 mg/L in the blank and 4.25 mg/L in the sample. The BOD will be calculated. Solution: The oxygen loss caused by the bacterial seed is the blank BOD, Blank BOD ¼ Initial DO Blank DO or ð9:01 8:80Þmg=L ¼ 0:21 mg=L: The oxygen consumption by the sample is ðInitial DO Final DOÞ Blank BOD or ð9:01 4:25Þ 0:21 ¼ 4:55 mg=L: The sample BOD is the oxygen consumption by the sample multiplied by a correction factor equal to the number of times the sample was diluted—20 times in this case. BOD ¼ 4:55 20 ¼ 91 mg L: A formula for estimating BOD is BOD ðmg=LÞ ¼ ðIDO FDO Þs ðIDO FDO Þb D ð15:2Þ where IDO and FDO = initial and final DO concentrations in sample bottle and blank bottle, respectively, and subscript s = sample, subscript b = blank, and D = the dilution factor. The BOD of a sample represents the amount of dissolved oxygen that will be used up in decomposing the readily-oxidizable organic matter. Of course, if there is a lot of phytoplankton in the sample, a large portion of the BOD will represent phytoplankton respiration, because all the phytoplankton may not die during 5 days Major Types of Pollution 321 in the dark. By knowing the volume of an effluent and its BOD, the oxygen demand of the effluent can be estimated as shown in Ex. 15.2. Ex. 15.2: A wastewater effluent has a discharge rate of 75 m3/h and a BOD of 265 mg/L. The daily BOD load to the receiving water will be calculated. Solution: The volume of effluent per day is 75 m3 =h 24 h=day ¼ 1, 800 m3 =day: The daily BOD load is 1, 800 m3 =day 265 g=m3 10 3 kg=g ¼ 477 kg=day: Thus, a BOD equal to 477 kg of dissolved oxygen will be discharged into the receiving water each day. All of the organic matter in a sample does not decompose in 5 days as shown in Fig. 15.2. It would require many years for the complete degradation of all the organic matter in a sample. However, the rate of oxygen loss from a sample (expression of BOD) usually is exceedingly slow after 30 days, and the BOD30 is a good indicator of the ultimate BOD (BODu) of a sample. Fig. 15.2 A typical expression of carbonaceous biochemical oxygen demand (BOD) over a 30- day period 322 15 Water Pollution Effects of Nitrification Many wastewaters contain appreciable ammonia nitrogen. The oxidation of ammonia to nitrate by bacteria (nitrification) consumes two moles of oxygen for each mole of ammonia nitrogen (Chap. 11) contributing to oxygen demand. Thus, the oxygen demand of ammonia nitrogen in a sample—the NOD—can be estimated from the total ammonia nitrogen concentration (Ex. 15.3). Ex. 15.3: A water sample contains 15 mg/L ammonia nitrogen. The BOD30 of the sample is determined to be 550 mg/L. The possible contribution of nitrification to the BOD will be estimated. Solution: From (11.9), it can be seen that 15 mg=L X N ¼ 2O2 14 g=mol 64 g=mol and X ¼ 68:6 mg=L: Thus, nitrification could have accounted for 68.6 mg/L of BOD or 12.5 % of the BOD. By repeating Ex. 15.3 for 1 mg/L of ammonia nitrogen, it can be seen that each milligram per liter of ammonia nitrogen nitrified by bacteria will require 4.57 mg/L O2 or have a potential NOD of 4.57 mg/L. In samples that are diluted several fold for BOD analysis, the abundance of nitrifying organisms is greatly diluted, and it takes more than 5 days for the nitrifiers to build up a population great enough to cause significant nitrification. The typical influence of nitrification on BOD in a highly diluted sample is illustrated (Fig. 15.3). However, in a sample that is not diluted or only diluted a few times, nitrification can be a factor in BOD. In order to obtain the BOD resulting from organic matter decomposition only (carbonaceous BOD), a nitrification inhibitor such as 2-chloro-6-(trichloromethyl) pyridine (TCMP) may be added to the sample. If it is desired to determine both carbonaceous BOD (CBOD) and nitrification BOD (NOD), then one portion of the sample is treated with nitrification inhibitor and another portion is not. The NOD is obtained by subtracting the results of the nitrification-inhibited portion from the uninhibited one. Of course, the NOD can be calculated from total ammonia nitrogen concentration as mentioned above. BOD Concentrations Concentrations of standard BOD 5 can range from less than 5 mg/L in natural, unpolluted water bodies to more than 20,000 mg/L in certain industrial effluents. Domestic sewage usually has a BOD 5 of 100–300 mg/L. Typical BOD5 values for effluents from selected industries are as follows: pond aquaculture, 10–30 mg/L (Boyd and Tucker 2014); beet sugar refining, 450–2,000 mg/L; brewery, 500–1,200 mg/L; laundry, 300–1,000 mg/L; milk processing, 300–2,000 mg/L; Major Types of Pollution 323 Fig. 15.3 Illustration of the expression of biochemical oxygen demand (BOD) over time in water samples that were either greatly diluted or slightly diluted with nutrient solution meat packing, 600–2,000 mg/L; canneries, 300–4,000 mg/L; grain distilling, 15,000–20,000 mg/L (van der Leeden et al. 1990). Effects of Wastewater Oxygen Demand The typical response of streams to BOD loads is a dissolved oxygen sag downstream from the effluent outfall (Fig. 15.4). If the BOD load is extremely high, there may be a reach of the stream where anaerobic conditions exist. The distance downstream before dissolved oxygen concentration returns to normal depends upon the amount of BOD and the rate of stream reaeration—a topic already discussed in Chap. 6. The rate of change of the oxygen deficit with time at a location in a stream is equal to the rate of deoxygenation caused by the BOD load minus the rate of stream reaeration (Vesilind et al. 1994). Mathematical models based on this concept are used to predict dissolved oxygen concentrations at different distances downstream from effluent outfalls. Discharge of effluents into lakes, estuaries, or the ocean also can depress oxygen concentrations in the vicinity of the outfall. The severity of this effect depends both on the BOD load and the extent to which the effluent is transported away from the outfall by water currents. Organic wastes typically contain nitrogen and phosphorus, resulting in ammonia and phosphate being released along with carbon dioxide during decomposition. Carbon dioxide, ammonia, and phosphorus concentrations tend to increase in streams below effluent outfalls, or in the vicinity of outfalls into lakes, estuaries, and the sea. Solids in effluents settle in the vicinity of outfalls and oxygen depletion may occur in sediments with abundant organic matter. 324 15 Water Pollution Fig. 15.4 Oxygen sag curve below an effluent outfall in a stream There is a typical pattern in nitrogen concentrations downstream from outfalls. First, there is a large increase in organic nitrogen. Next, organic nitrogen declines and total ammonia nitrogen increases as a result of decomposition. Nitrite may also increase because of low dissolved oxygen concentration. Finally, downstream of the oxygen sag, nitrate increases and ammonia nitrogen decreases because of nitrification. Nutrient Pollution Nutrient pollution stimulates the growth of phytoplankton in water bodies and can lead to eutrophication. Heavy blooms of phytoplankton have a number of adverse effects. In lakes and reservoirs they can cause shallow thermal stratification with an oxygen-deficient hypolimnion. Excessive plant growth causes wide daily fluctuations in dissolved oxygen, and low dissolved oxygen concentrations that may occur at night may be harmful to aquatic animals. Blue-green algae often are abundant in eutrophic waters, and these algae are subject to sudden die-offs that can lead to dissolved oxygen depletion. Some species of blue-green algae may be toxic to other organisms, and other species may impart off-flavor to fish or crustaceans or cause taste and odor problems in drinking water. Dense algal blooms in waters used for recreational purposes are undesirable because they limit visibility into the water Major Types of Pollution 325 and can cause bad odors. Dense scums of blue-green algae can accumulate on the surface of eutrophic water bodies and detract from aesthetic value. Large inputs of nutrients to streams can cause filamentous algae mats and encourage the growth of rooted plants in shallow water areas. Of course, dense phytoplankton blooms may occur in slow-moving streams. Nutrients released into coastal waters are mixed into seawater by currents and greatly diluted. However, in the mixing zones in estuaries, water pollution can cause eutrophic conditions with undesirable phytoplankton blooms or infestations of macrophytes in shallow water. Certain blue-green algae, dinoflagellates, and diatom blooms in areas for mulluscan shellfish production may produce toxins that can be absorbed and stored in shell fish and later cause serious illnesses in humans. There has been much disagreement over the actual concentrations of nitrogen and phosphorus required for eutrophication and excessive phytoplankton. Van der Leeden et al. (1990) presented data on nitrogen and phosphorus concentrations for selected lakes heavily impacted by human activity. Concentrations of total phos- phorus ranged from 0.006 to 0.29 mg/L with an average of 0.043 mg/L. Total nitrogen concentrations had a range of 0.047–7.11 mg/L (average = 1.26 mg/L). Van der Leeden et al. (1990) also cited a study by the U. S. Environmental Protection Agency which indicated nearly half of 23,236 lakes surveyed in the United States were eutrophic. In eutrophic ecosystems, the combination of widely-fluctuating, daily dissolved oxygen concentrations in the water column, low dissolved oxygen concentration in the sediment, and high ammonia concentration lessens the growth and survival of many environmentally-sensitive species of plants and animals. Eutrophic water bodies have a high abundance of a few species, and many sensitive species disappear. Thus, eutrophication tends to reduce biological diversity. Diversity is an index of how the individuals in a community are distributed among the species. There are many equations for diversity, and a common one suggested for assessing phytoplankton species diversity (Margalef 1958) is S 1 H ¼ ð15:3Þ lnðNÞ where S = the number of species and N = the total number of individuals. The diversity of two phytoplankton communities is calculated in Ex. 15.4. Ex. 15.4: Community A contains 25 species of phytoplankton and 1,000 individuals/mL while community B has 11 species and 14,000 individuals/mL. The diversity index for the two communities will be estimated by (15.3). Solution: 25 1 24 A. H ¼ ¼ ¼ 3:47 ln ð1; 000Þ 6:91 11 1 10 B. H ¼ ¼ ¼ 1:05: ln ð14; 000Þ 9:55 326 15 Water Pollution In Ex. 15.4, the community with the most species relative to the total number of individuals (community A) has a much higher diversity than the other community with fewer species relative to the number of individuals. As a rule, the greater the diversity, the more stable an ecosystem (Odum 1971). Eutrophic aquatic ecosystems tend to be less stable than oligotrophic ones. This lack of stability may be reflected in sudden shifts in the abundance of species and also in sudden changes in dissolved oxygen concentrations and other water quality variables. Toxins Many of the chemicals used to improve human life can be toxic to humans and other organisms. There is a large number of potentially toxic chemicals, but the major classes of chemicals are petroleum products, inorganic compounds such as ammo- nia, heavy metals, and cyanide, pesticides and other agricultural chemicals, syn- thetic organic compounds used in industry, and pharmaceutical compounds that may be discarded into sewage systems or enter via human wastes. Evaluation of the toxicity of waterborne toxins to aquatic organisms is difficult. There can be different degrees of response to a toxin. A toxin may kill aquatic organisms directly. The mortality may be rapid (acute) if a high concentration of the toxin is introduced or slow (chronic) if a lower concentration is maintained in the water. The lowest concentration at which mortality can be detected is the threshold toxic concentration. The threshold concentration also may be defined as the lowest concentration necessary to elicit some response other than death. Responses may include failure to reproduce, lesions, aberrant physiological activity, susceptibility to disease, or behavioral changes. The exposure time necessary for a toxin to produce some undesirable effect on organisms decreases with increasing concentration. Organisms usually must absorb a certain amount of a toxin before the toxic effect occurs. The total body burden of a toxin may be calculated as illustrated in Ex. 15.5 using the following equation: TBB ¼ ðDI þ RÞ DL ð15:4Þ where TBB = total body burden, DI = daily intake of toxin, R = residual of toxin in body before exposure, and DL = daily loss of toxin from body by metabolism or excretion. Ex. 15.5: Suppose a 1-kg fish has a residual concentration of toxin X of 5 mg, the daily intake is 0.1 mg, and the daily loss is 0.08 mg. The total body burden will be estimated after 30 days. Solution: From (15.4), TBB ¼ ½30ð0:1Þ þ 5 30ð0:08Þ ¼ 5:6 g: Major Types of Pollution 327 Toxicity occurs when the body burden reaches the threshold level. If the toxin disappears from the water, organisms will eliminate the toxin, but the rate of loss usually declines as the total body burden decreases. Bioaccumulation occurs when an organism accumulates a toxin in specific organs or tissues. For example, many pesticides are fat-soluble and tend to accu- mulate in fatty tissues. The term bioconcentration is used to describe the phenome- non in which a toxic substance accumulates at greater and greater concentrations as it passes through the food chain. A toxin introduced into water may be bioaccumulated by plankton. Fish eating the plankton may store this toxin in their fat and have higher body burdens than did the plankton. Birds feeding on the fish may further concentrate the toxin until a toxic body burden is reached. Obviously, bioaccumulation and bioconcentration of toxic substances by aquatic food organisms is a human food safety concern. In synergism, the toxicity of a compound may increase as a result of the presence of another compound; the mixture of two compounds is more toxic than either compound alone. Toxicity normally increases with increasing water temperature. In the case of metals, the free ion usually is the most toxic form, and the toxicity of a metal will be less in water with high concentrations of humic substances that complex metals than in clear water. Other water quality factors such as pH, alkalinity, hardness, and dissolved oxygen concentration can affect the toxicity of substances. Toxicity Tests Toxicity tests are important tools of aquatic toxicology for determining the effects, including death, of different concentrations of toxins on various species. From such tests, threshold concentrations for different responses can be estimated, and the influence of exposure time and water quality conditions on toxicity can be evaluated. Based on toxicity tests, safe concentrations of pollutants can be recommended for natural waters, effluent standards for pollutants can be established, and the risk of toxicity from waterborne pollutants can be greatly reduced. Of course, some species are more sensitive than others, and the overall risk of waterborne toxins to ecosystems is extremely difficult to establish. In acute toxicity tests, aquatic organisms are exposed for specific time periods to a concentration range of a toxicant under carefully controlled and standardized conditions in the laboratory. The mortality at each concentration is determined, and the resulting data are helpful in assessing toxicity under field conditions. Toxicity studies may be conducted as static tests in which water with toxicant is placed in chambers and organisms introduced. There may or may not be water or toxicant renewal during the exposure period. The duration of static tests seldom exceed 96 h and sometimes is shorter. Toxicity studies also may be conducted as flow-through trials in which fresh toxicant solution is continuously flushed through the test chambers. Animals may be fed in flow-through tests, and animals may be exposed to a toxicant for weeks or months. There are many sources of information on toxicity test methodology; one of the best is the Standard Methods for the Exami- nation of Water and Wastewater (Eaton et al. 2005). 328 15 Water Pollution Fig. 15.5 Graphical estimation of the LC50 The most common way of analyzing results of acute toxicity tests is to calculate the percentage survival (or mortality) at each test concentration, and plot percent- age survival on the ordinate against toxicant concentrations on the abscissa. Semi- log paper is normally used for preparing the graph of concentration versus mortality because the relationship is logarithmic. The concentration of toxicant that caused 50 % mortality can be estimated from the graph. The concentration of the toxicant necessary to kill 50 % of the test animals during the time that organisms were exposed to the toxicant (exposure time) is called the LC50. The exposure time of animals to toxicants usually is specified by placing the number of hours before LC50, e.g., 24-h LC50, 48 h-LC50, or 96-h LC50. The graphical estimation of the LC50 from the results of a toxicity test is illustrated (Fig. 15.5). In addition to providing the LC50, toxicity testing can reveal the lowest concen- tration of a substance that causes toxicity or the highest concentration that causes no toxicity. Sometimes tests may be conducted in which the endpoint is some response other than toxicity. For example, in long-term tests, the concentration that inhibits reproduction could be measured, the concentration that produces a particular lesion, or the concentration that elicits a particular physiological or behavioral change might be ascertained. Selected Toxicity Data The 96-h LC50 concentrations of selected inorganic elements, pesticides, and organic chemicals for freshwater fish are provided in Tables 15.2, 15.3, and 15.4. Major Types of Pollution 329 Table 15.2 Ranges in 96-h LC50 values for various species of fish exposed to selected inorganic elements Inorganic substance 96-h LC50 (mg/L) Inorganic substance 96-h LC50 (mg/L) Aluminum 0.05–0.2 Iron 1–2 Antimony 0.3–5 Lead 0.8–542 Arsenic 0.5–0.8 Manganese 16–2,400 Barium 50–100 Mercury 0.01–0.04 Beryllium 0.16–16 Nickel 4–42 Cadmium 0.92–9 Selenium 2.1–28.5 Chromium 56–135 Silver 3.9–13.0 Copper 0.05–2 Zinc 0.43–9.2 Table 15.3 Acute toxicities of representative compounds of several classes of pesticides to fish Trade name 96-h LC50 (μg/L) Trade name 96-h LC50 (μg/L) Chlorinated hydrocarbon insecticides Pyrethum insecticides DDT 8.6 Permethrin (synthetic 5.2 pyrethroid) Endrin 0.61 Natural pyrethroid 58 Heptachlor 13 Miscellaneous insecticides Lindane 68 Diflubenzuron >100,000 Toxaphene 2.4 Dinitrocresol 360 Aldrin 6.2 Methoprene 2,900 Organophosphate insecticides Mirex >100,000 Diazinon 168 Dimethoate 6,000 Ethion 210 Herbicides Malathion 103 Dicambia >50,000 Methyl 4,380 Dichlobenil 120,000 parathion Ethyl parathion 24 Diquat 245,000 Guthion 1.1 2,4-D (phenoxy herbicide) 7,500 TEPP 640 2,4,5-T (phenoxy herbicide) 45,000 Carbamate insecticides Paraquat 13,000 Carbofuran 240 Simazine 100,000 Carbaryl (Sevin) 6,760 Fungicides Aminocarb 100 Fenaminosulf 85,000 Propoxur 4,800 Triphenyltin hydroxide 23 Thiobencarb 1,700 Anilazine 320 Dithianon 130 Sulfenimide 59 330 15 Water Pollution Table 15.4 Acute toxicities of some representative industrial chemicals to fish Compound 96-h LC50 (mg/L) Acrylonitrile 7.55 Benzidine 2.5 Linear alkylate sulfonates and alkyl benzene sulfonates 0.2–10 Oil dispersants >1,000 Dichlorobenzidine 0.5 Diphenylhydrazine 0.027–4.10 Hexachlorobutadiene 0.009–0.326 Hexachlorocyclopentadiene 0.007 Benzene