Botswana is one of very few countries in the world that still possesses an impressive diversity and abundance of wild fauna and flora. This is reflected by the occurrence of more than 150 different species of mammals, over 500 species of birds, numerous species of reptiles, amphibians, insects and plants. The Government of Botswana has demonstrated considerable commitment to maintaining the country's rich heritage by setting aside 18% of the country as National Parks and Game reserves. An additional 21% has been dedicated to Wildlife Management Areas where the primary land use is wildlife utilisation. In recent years, declines have been observed in the numbers of many wildlife species and a number of factors have been evoked to explain this alarming trend (National Research Council, 2002).
Nature development often is planned on contaminated soils and sediments of former agricultural land and in floodplain areas; however, this contamination may present a risk to wildlife species desired at those nature development sites. Specific risk assessment methods are needed, because toxicological information is lacking for most wildlife species. The vulnerability of a species is a combination of its potential exposure, sensitivity to the type of pollutant, and recovery capacity.
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Many industries use large volumes of water for various purposes. Once used, the water is sometimes placed in lagoons, ponds, or impoundments (Arands et al., 1991, Krueger et al., 1991, Rebhun and Galil, 1994 and (Reeves and Falivene, 1992)). These waste management units are often built and integrated into a wastewater management system to remove contaminants, control temperature, facilitate groundwater infiltration, or hold and condition water for reuse (Tchobanoglous and Burton, 1991). While surface impoundments have been instrumental in eliminating pollutants from wastewater prior to its discharge into surface water, there have also been long standing concerns that management of contaminated wastewater in surface impoundments may lead to the release of harmful substances into the environment and human or ecological exposures ( Freeman, 1997 and LaGrega et al., 1994).
Wildlife observations provide considerable evidence that environmental contaminants can play a critical role in reproductive and developmental dysfunction. Early evidence leading to a widespread awareness of the impact of environmental chemicals on surrounding wildlife was observed in the Laurentian Great Lakes. A suite of reproductive and congenital defects was identified in birds, reptiles, and mammals alike that were attributed to high concentrations of organochlorine pesticides and industrial chemicals. Due to the ubiquitous and persistent nature of many anthropogenic chemicals, these defects, including thyroid dysfunction, hatching success, egg shell thinning, and gross birth deformities, have since been identified in numerous wildlife populations across the world. Certain wildlife taxa such as amphibians are especially vulnerable to chemical perturbation and are suffering alarming population declines. It is well documented that a large variety of chemical compounds, including plant-based phytoestrogens, can function as hormone mimics and alter endocrine signalling in wildlife. Regretfully, no studies to date have been performed to determine the absolute aqueous concentration of phytoestrogens where one would expect to see negative physiological impacts on aquatic organisms. Thus, a potential environmental threshold must be estimated based on existing research.
In wastewater treatment systems, the common water quality variables of concern are pH, biological oxygen demand (BOD),chemical oxygen demand (COD), dissolved oxygen (DO),suspended solids, nitrate, nitrite and ammonia nitrogen, phosphate, salinity and a range of other nutrients and trace metals (DeCico, 1979; Brooks, 1996). The presence of high concentrations of these pollutants above the critical values stipulated by national and international regulatory bodies is considered unacceptable in receiving water bodies.
Physiological and molecular responses to chemical insult are often conserved across vertebrates, alerting scientists and medical professionals alike that greater attention needs to be paid to the roles environmental contaminants play in the etiology of congenital disorders in both humans and wildlife.
The physico-chemical characteristics of wastewater that are of special concern are pH, dissolved oxygen (DO), oxygen demand (chemical and biological), solids (suspended and dissolved), nitrogen (nitrite, nitrate and ammonia), phosphate, and metals (DeCicco, 1979; Larsdotter, 2006). The hydrogen-ion concentration is an important quality parameter of both natural and waste waters. It is used to describe the acid or base properties of wastewater. A pH less than 7 in wastewater influent is an indication of septic conditions while values less than 5 and greater than 10 indicate the presence of industrial wastes and noncompatibility with biological operations. The pH concentration range for the existence of biological life is quite narrow (typically 6-9). An indication of extreme pH is known to damage biological processes in biological treatment units (EPA, 1996; Gray, 2002). Another parameter that has significant effect on the characteristics of water is dissolved oxygen. It is required for the respiration of aerobic microorganisms as well as all other aerobic life forms. The actual quantity of oxygen that can be present in solution is governed by the solubility, temperature, partial pressure of the atmosphere and the concentration of impurities such as salinity and suspended solids in the water (EPA 1996; Metcalf and Eddy, 2003). Oxygen demand, which may be in the form of BOD or COD, is the amount of oxygen used by microorganisms as they feed upon the organic solids in wastewater (Water Environmental Federation, 1996; Gray, 2002; FAO, 2007).
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The 5-day BOD (BOD5) is the most widely organic pollution parameter applied to wastewater. It involves the measurement of dissolved oxygen used by microorganisms in the biochemical oxidation of organic matter. The presence of sufficient oxygen promotes the aerobic biological decomposition of an organic waste (Metcalf and Eddy, 2003). Although BOD test is widely used, it has a number of limitations, which include the requirement of a high concentration of active acclimated microorganisms and the need for treatment when dealing with toxic wastes, thus reducing the effects of nitrifying organisms. The BOD measures only the biodegradable organics and requires a relatively long time to obtain test results (Gray, 2002; Metcalf and Eddy, 2003). Similarly, the COD test measures the oxygen equivalent of the organic material in wastewater that can be oxidized chemically. The COD will always be higher than the BOD. This is because the COD measures substances that are both chemically and biologically oxidized. The ratio of COD: BOD provides a useful guide to the proportion of organic material present in wastewaters, although some polysaccharides, such as cellulose, can only be degraded anaerobically and so will not be included in the BOD estimation. One of the main advantages of the COD test is that it can be completed in about 2.5 h, compared to the 5-day BOD test (Eckenfelder and Grau, 1992; Gray, 2002; Metcalf and Eddy, 2003). The amount of solids in drinking water systems has significant effects on the total solids concentration in the raw sewage. Although wastewater is normally 99.9 % water, 0.1 % of it comprises of solids. Discharges from industrial and domestic sources also add solids to the plant influent. Although there are different ways of classifying solids in wastewater, the most common types are total dissolved solids (TDS), total suspended solids (TSS), settleable, floatable and colloidal solids, and organic and inorganic solids (EPA, 1996).Normally, wastewater processes using settling or flotation are designed to remove solids but cannot remove dissolved solids. Biological treatment units such as trickling filters and activated sludge plants convert some of these dissolved solids into settleable solids that are removed by sedimentation tanks (Eckenfelder and Grau, 1992).
Heavy and trace metals are also of importance in water. The metals of importance in wastewater treatment are As, Cd, Ca, Cr, Co, Cu, Fe, Pb, Mg, Mn, Hg, Mo, Ni,K, Se, Na, V and Zn. Living organisms require varying amounts of some of these metals (Ca, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni and Zn) as nutrients (macro or micro) for proper growth. Other metals (Ag, Al, Cd, Au, Pb and Hg) have no biological role and hence are non-essential (Metcalf and Eddy, 2003; Hussein et al., 2005). Heavy metals are one of the most persistent pollutants in wastewater. Unlike organic pollutants, they cannot be degraded, but accumulate throughout the food chain, producing potential human health risks and ecological disturbances. Their presence in wastewater is due to discharges from residential dwellings, groundwater infiltration, and industrial discharges. The accumulation of these metals in wastewater depends on many local factors, such as the type of industries in the region, way of life and awareness of the impact on the environment through the careless disposal of wastes (Hussein et al., 2005; Silvia et al., 2006). The danger of heavy and trace metal pollutants in water lies in two aspects of their impact. Firstly, heavy metals have the ability to persist in natural ecosystems for an extended period, and, secondly, they have the ability to accumulate in successive levels of the biological food chain (Fuggle, 1983). Although heavymetals are naturally present in small quantities in all aquatic environments, it is almost exclusively through human activities that these levels are increased to toxic levels (Nelson and Campbell, 1991).
The methods for determining the concentrations of these metals vary in complexity according to the interfering substances that may be present. Typical methods of determining their concentrations include flame atomic absorption, electrothermal atomic absorption, and inductively coupled plasma (ICP)/mass spectrometry (APHA, 2001).
The major microorganisms found in wastewater influents are viruses, bacteria, fungi, protozoa and helminthes. Although various microorganisms in water are considered to be critical factors in contributing to numerous waterborne outbreaks, they play many beneficial roles in wastewater influents (Kris, 2007). Traditionally, microorganisms are used in the secondary treatment of wastewater to remove dissolved organic matter. The microbes are used in fixed film systems, suspended film systems or lagoon systems, depending on the preference of the treatment plant. Their presence during the different treatment phases can enhance the degradation of solids, resulting in less sludge production (Ward-Paige et al., 2005a). Apart from solid reduction, wastewater microbes are also involved in nutrient recycling, such as phosphate, nitrogen and heavy metals.
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If nutrients that are trapped in dead materials are not broken down by microbes, they will never become available to help sustain the life of other organisms. Microorganisms are also responsible for the detoxification of acid mine drainage and other toxins in wastewater (Ward-Paige et al., 2005b).Microbial pollutants can also serve as indicators of water quality. The detection, isolation and identification of the different types of microbial pollutants in wastewater are always difficult, expensive and time consuming. To avoid this, indicator organisms are always used to determine the relative risk of the possible presence of a particular pathogen in wastewater (Paillard et al., 2005). For instance, enteric bacteria, such as coliforms, Escherichia coli, and faecal streptococci are used as indicators of faecal contamination in water sources (DWAF 1996; Momba and Mfenyana, 2005).To indicate viral contamination, bacteriophages (somatic and F-RNA coliphages) are used. Also, Clostridium perfringens, a faecal spore-forming bacterium, which is known to live longer in the environment and reported to be resistant to chlorine, is used as an indicator for the presence of viruses, protozoa or even helminthes eggs(Payment and Franco, 1993; Grabow et al., 1997).Furthermore, diatoms are used to indicate the general quality of water with respect to nutrient enrichment, and they provide valuable interpretations with respect to changes in water quality, such as turbidity, conductivity, COD, BOD and chloride (Dela et al., 2002).
The impacts of such degradation may result in decreased levels of dissolved oxygen, physical changes to receiving waters, release of toxic substances, bioaccumulation or biomagnifications in aquatic life, and increased nutrient loads (Environmental Canada, 1997). Wastewater is a complex resource, with both advantages and inconveniences for its use. Wastewater and its nutrient contents can be used for crop production, thus providing significant benefits to the farming communities and society in general. However, wastewater use can also impose negative impacts on communities and on ecosystems. The widespread use of wastewater containing toxic wastes and the lack of adequate finances for treatment is likely to cause an increase in the incidence of wastewater borne diseases as well as more rapid degradation of the environment.
Eutrophication due to excessive amounts of nutrients contributes to the depletion of dissolved oxygen. It is important to note that other constituents of wastewater effluents also play an important role in the depletion of DO. The bacterial breakdown of organic solids present in wastewater and the oxidation of chemicals in it can consume much of the dissolved oxygen in the receiving water bodies (Borchardt and Statzner, 1990). These effects may be immediate and short-term or may extend over months or years as a result of the buildup of oxygen-consuming
material in the bottom sediments (Environmental Canada,1999).The impacts of low dissolved oxygen levels include an effect on the survival of fish by increasing their susceptibility to diseases, retardation in growth, hampered swimming ability, alteration in feeding and migration, and, when extreme, lead to rapid death. Long-term reductions in dissolved oxygen concentrations can result in changes in species composition (Welch, 1992; Chambers and Mills, 1996; Environmental Canada, 1997). Poorly treated wastewater effluent can also lead to physical changes to receiving water bodies. All aquatic life forms have characteristic temperature preference and tolerance limits. Any increase in the average temperature of a water body can have ecological impacts. Because wastewater effluents are warmer than receiving water bodies, they are a source of thermal enhancement (Welch, 1992; Horner et al., 1994). Also, the release of suspended solids into receiving waters can have a number of direct and indirect environmental effects, including reduced sunlight penetration (reduced photosynthesis), physical harm to fish, and toxic effects from contaminants attached to suspended particles (Horner et al., 1994). Another environmental impact of untreated wastewater effluent, which at times can be linked to health, is the phenomenon of bioaccumulation and biomagnifications of contaminants. Due to the phenomenon of bioaccumulation, certain substances which are in low concentrations or barely measurable in water can sometimes be found in high concentrations in the tissues of plants and animals. These substances tend to be stable, live long chemically, and are not easily broken down by digestive processes (Environmental Canada, 1997; 1999).
In some cases, through the process of biomagnification, the concentrations of some of the contaminants may be increased dramatically through passage in the food chain that is prey to predators (Chambers and Mills, 1996). Because of the processes of bioaccumulation and biomagnification, very low concentrations of certain substances in wastewater are of concern. Examples of such substances include organochlorine pesticides, mercury and heavy metals. Although there are several other sources of persistent bioaccumulatives (such as toxic substances in the environment), including industrial discharges and deposition of atmospheric contaminants (Environmental Canada, 1997). Also, the release of toxic substances from wastewater into receiving water bodies has direct toxic impacts on terrestrial plants and animals.
The toxic impacts may be acute or cumulative. Acute impacts from wastewater effluents are generally due to high levels of ammonia and chlorine, high loads of oxygen-demanding materials, or toxic concentrations of heavy metals and organic contaminants. Cumulative impacts are due to the gradual buildup of pollutants in receiving water, which only become apparent when a certain threshold is exceeded (Welch, 1992; Chambers et al., 1997).In addition, eutrophication of water sources can lead to nutrient enrichment effects. Nutrient-induced production of aquatic plants in receiving water bodies has the following detrimental consequences: (i) Algal clumps, odours and decolouration of the water, thus interfering with recreational and aesthetic water use; (ii) extensive growth of rooted aquatic life interferes with navigation, aeration and channel capacity; (iii) dead macrophytes and phytoplankton settle to the bottom of a water body, stimulating microbial breakdown processes that require oxygen, thus causing oxygen depletion; (iv) extreme oxygen depletion can lead to the death of desirable aquatic life; (v) siliceous diatoms and filamentous algae may clog water treatment plant filters and result in reduced backwashing, and (vi) algal blooms may shade and submerge aquatic vegetation, thus reducing or eliminating photosynthesis and productivity (Atlas and Batha,1987; Ratsak et al., 1996; Kurosu, 2001; Alm, 2003;Mbewele, 2006; McCasland et al., 2008). Although nitrogen and phosphorus are beneficial to aquatic life in small amounts, when in excess they contribute to eutrophication. Eutrophication leads to algal blooms and plant growth in streams, ponds, lakes, reservoirs and estuaries and along shorelines (EPA, 2000; Eynard et al., 2000).
In lakes, rivers, streams and coastal waters where large algal blooms are present, the death of the vast numbers of phytoplankton that make up the blooms may smother the lake bottom with organic material. The decay of this material can consume most or all of the dissolved oxygen in the surrounding water, thus threatening the survival of many species of fish and other aquatic life (Environmental Canada, 1997; 2003; Eynardet al., 2000). The net effect of eutrophication on an ecosystem is usually an increase of a few plant types and a decline in the number and variety of other plant and animal species in the system (Environmental Canada, 1999; 2003). In most surface waters, total ammonia concentrations greater than 2 mg/L are toxic to aquatic life, although this varies between species and life stages. Studies that have been carried out on the toxicity of ammonia to freshwater vegetation have shown that concentrations greater than 2.4 mg/L inhibit photosynthesis (Chambers et al., 1997; WHO, 1997). Nitrate is believed to cause a reduction in amphibian populations. Adverse effects are reported to be poor larval growth, reduced body size, and impaired swimming ability (Environmental Canada, 1999).
Diseases caused by bacteria, viruses and protozoa are the most common health hazards associated with untreated drinking and recreational waters. The main sources of these microbial contaminants in wastewater are human and animal wastes (WHO, 1997; Environmental Canada, 1998; 2003 EPA, 2000; WHO, 2006).These contain a wide variety of viruses, bacteria, and protozoa that may get washed into drinking water supplies or receiving water bodies (Kris, 2007). Microbial pathogens are considered to be critical factors contributing to numerous waterborne outbreaks. Many microbial pathogens in wastewater can cause chronic diseases with costly long-term effects, such as degenerative heart disease and stomach ulcer. The density and diversity of these pollutants can vary depending on the intensity and prevalence of infection. The detection, isolation and identification of the different types of microbial pollutants in wastewater are always difficult, expensive and time consuming. To avoid this, indicator organisms are always used to determine the relative risk of the possible presence of a particular pathogen in wastewater (Paillard et al., 2005). Viruses are among the most important and potentially most hazardous pollutants in wastewater. They are generally more resistant to treatment, more infectious, more difficult to detect and require smaller doses to cause infections (Toze, 1997; Okoh, et al., 2007).
Because of the difficulty in detecting viruses, due to their low numbers, bacterial viruses (bacteriophages) have been examined for use in faecal pollution and the effectiveness of treatment processes to remove enteric viruses (Okoh, et al., 2007).Bacteria are the most common microbial pollutants in wastewater. They cause a wide range of infections, such as diarrhea, dysentery, skin and tissue infections, etc. Disease-causing bacteria found in water include different types of bacteria, such as E. coli O157:H7; Listeria, Salmonella, Leptosporosis, Vibrio, Campylobacter, etc (CDC, 1997; Absar, 2005). Wastewater consists of vast quantities of bacteria, most of which are harmless to man. However, pathogenic forms that cause diseases, such as typhoid, dysentery, and other intestinal disorders may be present in wastewater. The tests for total coliform and faecal coliform nonpathogenic bacteria are used to indicate the presence of pathogenic bacteria (EPA, 1996; APHA, 2001).
Because it is easier to test for coliforms, faecal coliform testing has been accepted as the best indicator of faecal contamination. Faecal coliform counts of 100 million per100 millilitres may be found in raw domestic sewage. Detectable health effects have been found at levels of2300 to 2400 total coliforms per 100 milliliters in recreational waters. Disinfection, usually chlorination, is generally used to reduce these pathogens (EPA, 1996; Absar, 2005). Waterborne gastroenteritis of unknown cause is frequently reported, with the susceptible agent being bacterial. Some potential sources of this disease are E. coli and certain strains of Pseudomonas, which may affect the newborn and have also been implicated in gastrointestinal disease outbreaks (Metcalf and Eddy, 2003). Also, highly adaptable, protozoa are widely distributed in natural waters, although only a few aquatic protozoa are pathogenic.
STATEMENT OF THE PROBLEM
Ways of determining the potential ecological consequences of wastewater provision for wildlife ranges from testing for coliforms, using faecal coliform testing which has been accepted as the best indicator of faecal contamination. Faecal coliform counts of 100 million per100 millilitres may be found in raw domestic sewage. Detectable health effects have been found at levels of2300 to 2400 total coliforms per 100 milliliters in recreational waters (EPA, 1996; Absar, 2005). Enzyme assays which assess microbial activities to simple methods such as plate counts of soil microorganisms, determining the heavy metals concentrations which include flame atomic absorption, electrothermal atomic absorption, and inductively coupled plasma (ICP)/mass spectrometry (APHA, 2001).
AIMS AND OBJECTIVES
The goal of ecological risk assessment is to estimate the likelihood of adverse effects of chemicals on populations and ecosystems.
Determine the potential effects of disease organisms in wastewater (viruses, bacteria and protozoans) on human, wildlife and plant health.
Determine the potential impacts of increased ecological stress and overloading caused by elevated levels of nutrients, trace metals, chlorine, and other pollutants present in wastewater in wildlife, its habitat and humans.
SIGNIFICANCE OF THE STUDY
Tourism development has become a major policy of the government of Botswana to increase employment and economic growth. Tourism is now, according to the World Trade Organization, the world's biggest industry. Globally, tourism has a gross output of over US $7 trillion, is responsible for 11.5% of global gross domestic product (GDP), and employs 200 million people, which is 11% of the world's workforce. With 760 million international tourist arrivals recorded worldwide in 2004, tourism is a major global activity that has grown by 25% in the past 10 years. The sheer size of the industry makes it important to consider its environmental impacts.
It is important for the industry to understand its impacts, because its products often depend on the appeal of attractive natural capital - clean beaches and oceans, pleasant climate, and wildlife. Tourism may therefore be vulnerable to its local impacts; for example, degradation of beaches or biodiversity loss. In addition, tourism also contributes to global environmental issues. For example, traveling by airplane requires considerable amounts of fossil fuels and releases greenhouse gases into the atmosphere.