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Sources of chemical contaminants and routes into the freshwater environment
Food and Chemical Toxicology 38 (2000) S21±S27
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Sources of Chemical Contaminants and Routes into the Freshwater Environment M. S. HOLT ECETOC, Av. E.Van Nieuwenhuyse 4, B-1160 Brussels, Belgium SummaryÐDrinking water is derived from either surface waters or groundwater. The latter is of enormous importance, with more than 65% of Europe's drinking water needs being supplied in this way. However, water from either source is rarely, if ever, pure. Industrialization and urbanization together with intensi®ed agricultural activity have led to increased demands for water on the one hand but to the potential for large scale release of contaminants on the other. The result is that surface water can be contaminated through direct or indirect emissions and groundwater can be contaminated by leaching from the soil. The diversity and number of existing and potential sources of chemical contamination are quite large. This paper reviews the major sources of chemical emissions and the routes by which contaminants can arise in surface waters and groundwaters intended for use as a supply of drinking water. It is estimated that there are between 90,000 and 100,000 chemicals in regular use but that as few as 3000 account for about 90% of the total mass used. Whether a substance may be found in the air, soil or aqueous environment depends on a number of factors, including how the chemical is released, the volume released, where the chemical is released, its release pattern and its physicochemical properties. Of the major routes of contamination for the aquatic environment, the most signi®cant are directly from treated and untreated waste waters, run-o and atmospheric deposition (including spray drift) and indirectly from leaching. The fate of emissions of contaminants in a particular water body will depend not only on the amount of the substance emitted but also on the transport, dispersion and transformation (biodegradation, hydrolysis, photolysis) processes in the receiving body. The preventative measures (biodegradation testing and sewage treatment) taken to minimize contamination are discussed. # 2000 Published by Elsevier Science Ltd. All rights reserved Keywords: aquatic; contamination; emissions; environmental fate; biodegradation.
Introduction The ever-increasing demands of modern day society in its quest for economic development and drive for improved standards of living are leading to increased use of raw materials. There is little doubt that the use of chemicals has played a key role in many of the major sectors such as agriculture, industry, housing, transport, textiles and health that have contributed signi®cantly to the rise in the standard of living among populations around the world. Their use, however, results in a continuous release of both naturally occurring and man-made substances including gases, heavy metals, volatile organic compounds, soluble organic compounds, suspended solids, colour, nitrogen and phosphorus compounds into the air, water and terrestrial environments. Although eorts are made to reduce these emissions and hence the impact of human activities, it is virtually impossible to prevent some contamination of the environment. This paper con®nes itself to the emissions that can ®nd themselves ultimately in the freshwater environment.
Water is rarely if ever pure. For the purpose of discussion in this paper, a distinction is made between contamination, where substances are present, and pollution which can be de®ned as the appearance of some environmental quality for which the exposed community is incapable of neutralizing the negative eects and where some demonstrable risk has been identi®ed. Indeed, some substances are essential for life, and exposure to these substances (e.g. Cu, Co, Mn, Ni, Se and Zn) is a prerequisite for the healthy functioning of ecosystems. Not all water quality problems are due solely to human impact. For example, metals present in the lithosphere may enter the environment either as a result of geological activity or human activity (Moore, 1991). The reports of reduced iron in Denmark, ¯uoride in Bavaria and Moldova, arsenic and strontium in some mountainous regions illustrate the fact that natural geochemical conditions may cause elevated concentrations of metals locally (EEA,1995). Geological processes, including volcanic action, related hydrothermal activity and weath-
0278-6915/00/$ - see front matter # 2000 Published by Elsevier Science Ltd. All rights reserved. Printed in Great Britain PII S0278-6915(99)00136-2
S22
M. S. Holt
ering, can also lead to local problems (Duke, 1996). The importance of these processes show a high spatial variability. Current estimates are that approximately 90,000± 100,000 chemicals are in daily use. The production, distribution, use and disposal of chemicals lead almost inevitably to their presence in the environment either in a localized way or in a widespread manner. The life cycle and use pattern can be used to predict the main emission sources and the probable receiving environmental compartment. Chemical release into the environment may occur at any stage, starting with extraction/winning, manufacture through formulation and use until end-user disposal or destruction. The emission pattern, the quantities released into the dierent environmental compartments and the number of emission points will vary widely from substance to substance. For example, in the case of an intermediate used for pharmaceutical production then only a very small fraction may be released into the environment, whereas in the case of household detergents almost 100% will be released into the public sewer. The major routes by which substances can enter the aquatic environment are illustrated in Fig. 1. The exact number of dierent sources from which they can enter the aquatic compartment are too numerous to list but they can, for the sake of simplicity and convenience, be separated into two major categories: . Urbanization and industrialization . Agriculture and forestry.
terns and the relative contribution of the fate processes. Continuous emissions are characterized by a practically constant emission over a long period (e.g. sewage treatment euent or discharge from a continuous production process). Many substances are released to the environment as a result of batch processes as opposed to continuous processes. Discontinuous emissions show variations in both volume and concentration with time. They can be peak emissions or block emissions. Peak emissions are characterized by relatively large discharges over a short time. Furthermore, the time between peaks and the peak height can vary greatly. Block emissions are characterized by a reasonably constant ¯ow rate over a certain time, but with regular intervals of low or zero emission. Surface water is the main source of water abstractions by all utilization sectors in Europe. On average 70% of the abstraction is from this source (EEA, 1995) but with large variations between countries (Spain, Belgium, The Netherlands >90%, Cyprus, Switzerland, Denmark <25%). Surface water can be contaminated through direct or indirect emission. Drinking water can also be derived from groundwater. EEA (1995) estimate that more than 65% of the potable water supply for Europe is provided this way. Contamination of groundwater occurs through leaching. Urbanization and industry Wastewaters
Types of emissions and sources Emissions to the air, water and terrestrial compartments arise from human activities and natural processes. The simplest classi®cation of water pollution sources is that they are either point sources or non-point sources (Table 1). The characteristics of these sources can vary widely from well-de®ned point sources (which in themselves can be single or multiple) to diuse releases from large numbers of small point sources (e.g. automobile exhaust gases or houses) or line sources (e.g. motorway run-o). A typical example of a point source is a wastewater treatment plant, while pesticides applied to land demonstrate a diuse release pattern. Point and non-point sources also dier in the way that the pollutants are released to the receiving water body. Point sources usually result in direct discharges to water courses, whereas the route for non-point sources may involve a path which results in partial deposition before reaching the water course. Furthermore, contaminant concentrations arising from non-point sources may vary signi®cantly with time and space re¯ecting both the seasonal use pat-
The most important sources of organic waste entering surface waters are from domestic and industrial sewage. Various processes are possible for treating domestic and industrial wastewaters, all of which are essentially controlled variations of the processes that nature uses to purify the aqueous or terrestrial compartments. However, in a treatment plant the processes of ¯occulation, sedimentation and biological removal (aerobic and/or anaerobic) are optimized. Historically, municipal wastewater treatment plants have been designed to deal with conventional organic pollutants found in domestic and industrial wastes. Conventional wastewater plants consist of primary, secondary and tertiary levels of treatment. The number of stages and the processes required depend on the type of waste and the desired euent quality. The capacity of many plants has recently been extended from organic carbon removal to include nitrogen removal by nitri®cation and denitri®cation, as well as the removal of phosphates and heavy metals. The con®guration of the plants has consequently increased in complexity and the number of physical, biological and chemical processes
Routes into freshwater
S23
Table 1. Examples of types of emission source Point sources Non-point sources Line sources
Domestic sewage networks, industrial euents, accidental spills, mining, storm over¯ow Agricultural practices, scattered dwellings, atmospheric deposition, construction work, land run-o, river transportation Highway run-o
in¯uencing euent quality has been expanded. Nevertheless, at many waste water treatment plants, improvements in removal eciency can only be achieved by reducing the suspended solids concentrations. This can often be done without the additional cost of euent ®ltration equipment but rather through simple improvements in secondary settling tank design and operation to improve ¯occulation. Chemical treatment of wastewater discharge In addition to the physicochemical and biological processes, a number of chemical processes are used during the treatment of wastewaters which can have an impact on the quality of the euent. For example, sodium hypochlorite is used to oxidize inorganic and organic compounds and as a disinfectant to treat wastewaters. This practice can result in the production of by-products in the ®nal euent discharged into the receiving water. A second example, which leads to one of the major sources of aluminium in surface waters, is the discharge of alum sludge from municipal waste water treatment plants. Alum sludges are produced when alum or aluminium sulfate is used for coagu-
lation and ¯occulation of raw water supplies to remove turbidity and/or colour. Water supply Treatment of potable water supplies and the distribution systems themselves can also lead to contaminants in drinking water. For example, the disinfection processes produce a range of by-products. The use of chlorine results in the formation of chloroform and other bromo and chloro haloforms. Ozonation of drinking waters containing bromide ions produces a number of brominated organic compounds such as bromoform and bromohydrins (IPCS, 1997). Chemicals such as lead, organtins and polynuclear aromatic hydrocarbons (PAHs) can enter the water supply as leachates. The detection of lead in drinking water generally re¯ects its dissolution from solder joints, brass ®ttings and other materials in the distribution network. Organotins can leach into drinking water from certain types of polyvinyl chloride pipes and PAHs (particularly ¯uoranthene) can leach from the older type of pipes which were lined with coal tar pitch.
Fig. 1. Major routes of entry into surface waters.
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M. S. Holt
Dissolution In addition to the problems outlined above, dissolution is the route by which many other substances can enter a water course. For example, the main source of organotins is from the dissolution of tributyl tin and related compounds, used as antifouling agents. Run-o Storm water. Increasing attention is now being directed towards the problems of urban drainage. Until recently rainwater and the resultant storm water were considered to be relatively clean, but it has now been shown that the actual and potential contaminants of rain water represent a major route for contaminants into surface waters. Storm water run-o can give rise not only to local pollution incidents, but when collected in combined sewerage systems (a very common practice in Europe), it is also the reason for hydraulic overload and often the reason for sewage plant malfunctioning. Highways. Contaminants contained in highway run-o can be generated by a wide variety of sources which can for convenience be separated into three main groups: . trac . maintenance (e.g. de-icing, weed and pest control) . accidental (accidents and spills). The impact of urban storm-water discharges has been studied recently (Luker and Montague, 1994) and the indications are that an initial peak ¯ushing of contaminants can be attributed to the removal of dry weather deposits. Factors such as population density, trac density and farming intensity are known to have an impact on the contaminant composition of run-o. Of the many types of urban surface contributing directly to storm run-o the most important in terms of metals and sediment contributions are roads. Snow scavenges and absorbs more atmospheric pollutants than rain and clearance of snow may involve greater than normal applications of de-icing material. Mineral extraction Mining is second only to agriculture as the most important land use industry (Solbe, 1984). Discharges from the mining industry can result in increased levels of metals which may aect water quality. For example, the discharge of saline waters from coal mines to surface waters results in highly corrosive waters. While working mines constitute an obvious problem, the closure of mines can be a double-edged sword. Besides being a source of contamination in their own right, mine waters also serve as diluent for sewage euents in many catchments. If pumping of these mine waters ceases, local pollution may occur from that loss of dilution.
In time, the underground waters may rise in the closed mines and the water will eventually surface, creating the potential for contamination elsewhere. The further extraction of metals by the smelting industry accounts for much of the anthropogenic input of many metals to the aqueous environment either directly or indirectly via air emissions. Atmospheric deposition Atmospheric deposition is a major diuse source of contaminants. Substances released into the atmosphere are present in the gaseous and the aerosol phases or are adsorbed to particles. One of the dominant deposition mechanisms to the ground is wet removal due to the scavenging of particles by, and partitioning of organic vapour into rain and snow. The extent of this process depends on the distribution of the chemical between the gaseous and aerosol phases, particle size distribution and Henry's Law constant. In addition, pollutants can be removed from the atmosphere by deposition of particles to which the chemical is adsorbed (dry deposition). Atmospheric deposition is dependent on climate, particularly wind direction, sunlight (photodegradation) and rain/snowfall. Combustion of fossil fuels, chemicals manufacturing and incineration of waste all contribute to the release of chemicals into the air compartment. Acidi®cation of soils are well described for northern and central Europe (Last and Watling, 1991). It is caused primarily by atmospheric deposition of acidifying compounds released into the atmosphere as a result of human activities. As a result increased concentrations of aluminium, sulfate and hydrogen ions have been reported in groundwaters below sandy soils. The most serious consequence of acidi®cation of groundwater is the increased mobilization of trace elements, especially aluminium.
Agriculture and forestry Diuse pollution from land use practices can aect water quality in four ways. The ®rst is the direct release of agrochemicals either during application or disposal. The second is due to the discharge of water from agricultural land as a result of irrigation and land drainage. The third is the result of leaching and the fourth results from farming practices that are carried out in water (e.g. watercress or ®sh farming). Agricultural chemicals There are currently more than 600 dierent pesticides used in agriculture, forestry and horticulture. Pesticides and fertilizers are usually applied to ®eld crops using a boom sprayer with hydraulic spray nozzles. Application to orchards is often using an air blast mist blower. The produced spray cloud
Routes into freshwater
contains droplets of various sizes. Drift cannot be avoided and some of the pesticide may reach surface water directly. A number of factors will play in role in how much and how far the pesticide may drift namely: . droplet size (surface tension, nozzle size and liquid pressure) . dosage, formulation, spray volume and the solvents used to prepare the spray . environmental conditions (wind speed and direction, humidity, temperature) . crop height (drift increases with crop height) and spray height. Drainage, either natural or man-made, from agricultural land may end up in surface waters. To regulate the groundwater level in areas where the groundwater table is high, the fate of in®ltrate water is often controlled via tile drains. Pesticide leachate, produced as rain water migrates through the unsaturated zone of the soil column, will be transported due to gravity and capillary/adsorptive into groundwater. Most leaching in Europe occurs between November and April. In the case of nitrate, for example the main source is mineralization of soil organic matter, and following an initial ¯ush, concentrations fall as the winter progresses and soil temperature decreases. As spring approaches, increased crop uptake of water (and nitrate) results in almost zero loading in receiving waters. Surface run-o and erosion are additional sources of contamination. Run-o is the excess rainfall that cannot in®ltrate the soil. Land form, soil type and vegetation are the major factors in¯uencing run-o from a given area. Most impact is generally during heavy rainfall shortly after the application of the chemical. Other sources of contamination of waters from agricultural practices include overspray, disposal of excess spray liquid and back-siphoning of tank liquid into the surface waters used for ®lling the tank. For many years the priority was to ensure maximum agricultural productivity, but the emphasis is shifting from agronomic to environmental priorities. For example, historically most emphasis on controlling phosphorus in surface waters was directed point sources by controlling phosphate in detergents and wastewater. The phosphorus balance is now starting to be redressed and the impact of diuse sources questioned (Moss et al., 1996). Within the European Union it has been estimated that 50% of the phosphorus in surface waters comes from diffuse agricultural sources, 41% from waste waters (24% from human sources, 10% from detergents and 7% from industry) and 9% from background sources (Morse et al., 1993). In addition to chemicals applied to crops, other agrochemicals used in animal husbandry such as
S25
sheep dip chemicals have been identi®ed as major contaminants of surface waters. Organic manure (a combination of dung and urine from housed livestock and plant waste) and municipal sewage treatment works sludges are applied to farmland to replenish the nutrients and as soil conditioners. The aim of the current legislation is to ensure animal and human health are not put at risk by potentially toxic chemicals and pathogens which may be present in sludge. They do not impose any limits for the maximum allowable concentration of organic contaminants in sludge. Surface run-o following application of manures and sewage sludges, together with leaks and spills of silage liquor, slurry and other farm wastes, are a major source of freshwater contamination resulting from current agricultural practices. Contamination of groundwaters The routes to groundwater are generally dierent to those for surface waters. The chemical nature of rocks through which groundwater moves in¯uences the chemistry of the water. For example, igneous rocks composed mainly of silicate materials of low solubility give rise to waters low in dissolved minerals, whereas sedimentary rocks consisting of rock materials accumulated from various sources yield waters with assorted solute content. The main routes and sources of groundwater contamination are: . waste disposal including a wide range of inorganic and organic contaminants from point sources in municipal, industrial or military land®ll, mining, leaking underground storage tanks and pipelines, and spills . non-disposal use of chemicals on the surface of the land, for example leaching of nitrates and pesticides . overpumping (for example salt water intrusion) or depletion . acidi®cation. Much has been written about the presence of nitrates and pesticides in groundwater, and while land®lls, for example in the UK, account for most incidents of point-source contamination, Bishop et al. (1998) have recently identi®ed leaking sewers in urban areas as sources of contamination of groundwater. A recent development in the gasoline industry is the use of methyl tert-butyl ether (MTBE) as the major fuel oxygenate. It is a good example of a substance that can arise from either point or nonpoint sources. Gasoline spills to land and releases from storage tanks are examples of point sources. Potential non-point sources are rainfall, urban runo and power boats. The environmental fate and behaviour of MTBE mean that it can aect both surface and groundwater quality. Of the 60 volatile organic chemicals detected in shallow groundwaters
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M. S. Holt
Fig. 2. Possible fates of chemicals.
in the US MTBE was the second most common after chloroform (Squillace et al., 1996). Environmental fate and behaviour When released into the environment, substances are subjected to any one, or combination, of a number processes that may aect their fate and behaviour (Fig. 2). The eect of each of these processes on the concentration of a chemical in any given environmental compartment (such as water, air, soil, sediment, biomass) depends on the chemical's physicochemical properties, environmental conditions and the discharge pattern. The major processes are: . transport (volatilization, advection, dispersion, adsorption, wet deposition, sedimentation, resuspension, soil or sediment mixing and diusion). Transport processes determine the variation in spatial and temporal distribution of a chemical in the environment. Rates of advection and dispersion are determined solely by environmental parameters such as current or wind speed. In air, rates are usually very fast while in water they may vary from very rapid in fast ¯owing rivers to very slow in stagnant lakes or ponds. In soil and sediment these rates may be insigni®cant. . transformation (biodegradation, hydrolysis, photolysis, speciation). Transformation is of major importance in determining the persistence of a chemical. The mechanism of these processes and their rates may vary greatly between and within an environmental compartment depending on the reactivity of the substance and environmental
parameters such as temperature, light intensity and numbers of competent bacteria. . uptake (bioaccumulation, bioconcentration). Two dierent modes of uptake can be distinguished, Passive uptake and active uptake. Passive uptake occurs via the skin and or gills of the truly soluble fraction while active uptake occurs via the digestive tract. For aquatic fauna, the principle mechanism of uptake is passive uptake via the skin and gills. For organisms living in sediment and soil, both mechanisms are involved. For plants, uptake may occur through passive uptake from water or air via the leaves but also through passive or active uptake from pore water via the roots. Sorption to the outside surface of root crops has also been shown to be signi®cant for crops with a high lipid content. Uptake and subsequent concentrations in biomass depend on the bioavailability of the substance. The measured total environmental concentration of a substance does not necessarily represent the actual concentration to which individual species will be exposed. The fraction of the total chemical that is available for uptake is de®ned as the bioavailable fraction. For example, the bioavailability of metals is complicated by the variety of physical and chemical forms in which they occur, including free ions, organic and inorganic complexes and particles. Water hardness, acidity and salinity have a strong in¯uence on metal speciation and in general free ions are the most bioavailable. The most important physicochemical properties of a substance that impact on its fate and distribution in the environment are boiling point, melting
Routes into freshwater
point, octanol/water partition coecient, vapour pressure. EU regulatory legislation requires information on these properties to be collected in the `base set' in support of classi®cation, packaging and labelling of dangerous substances (67/548/EEC and subsequent amendments). Biodegradation is, however, the key intrinsic property that determines the fate and distribution of substances. Biodegradation is the process by which organic materials are broken down, ultimately into carbon dioxide and water, by the enzymatic activity of microorganisms. Not surprisingly, therefore, EU legislation also requires data on a substance's biodegradability to support classi®cation and labelling of dangerous substances, risk assessment and other environmental management practices such as discharge permits. One of the most visual examples of the importance of the use of biodegradable substances comes from the detergent industry. The ®rst synthetic surfactants for use in detergent products were introduced in the 1950 s but resulted in extensive foaming in receiving waters. The detergent industry subsequently changed to biodegradable surfactants and such sights are no longer to be seen. A three-tier hierarchical approach to biodegradation testing has been developed. At the base level (level 0) a stringent screening test with low nutrients and low bacterial numbers are used to determine `ready' biodegradability. A pass in this test implies that the substance will rapidly biodegrade in the aquatic environment. At level 1 the inherent biodegradability (i.e. potential to biodegrade) is determined using higher microbial populations and allowing time to adapt. At the highest tier (level 2) a substance's behaviour during sewage treatment is simulated. These tests cover biodegradation in aerobic environments. Tests are also available to measure the biodegradability of substances under anaerobic conditions. Conclusions Chemical contaminants in water bodies intended for use as a source of drinking water can come from both natural and anthropogenic origins. The production, distribution, use and disposal of products leads almost inevitably to the release of substances into the environment in either a localized way or, in many instances, in a widespread manner which consequently may ®nd their way into water bodies intended as a source of drinking water. The fate and distribution of all emissions depend on both the amount and physicochemical properties of the emission, on the hydrology, geochemistry and biological characteristics of the receiving environment. Although the numerical values ascribed may vary slightly, many countries have already
S27
established or are now in the process of establishing, surface water quality standards and objectives for the protection of water resources. While greater understanding of the fate and behaviour of substances in the environment has resulted in signi®cant reductions in emissions, there is still an enormous amount of work to be done. Over the last few years there has been an active policy in many European countries to reduce point source emissions from both the domestic and industrial sectors. In addition, improvements in wastewater treatment processes coupled with the use of biodegradable substances in products intended for release into the environment have and will continue to improve the quality of the aquatic environment. Addressing the issue of non-point sources is more complex and the relative importance, of these two types of emission, is gradually changing.
REFERENCES
Bishop P. K., Misstear B. A., White M. and Harding N. J. (1998) Impacts of sewers on groundwater quality. Water and Environment Management 12, 16±223. Duke M. (1996) Geological sources of metals in the environment. Report of the international workshop on risk assessment of metals and their inorganic compounds. Angers, 13±15 November 1996, pp. 127. The International Council on Metals and the Environment, Ottawa, Canada. EEA (1995) Europe's environment. In The Dobris Report, ed. D. Stanners and P. Bourdeau. Earthscan Publications, London. IPCS (1997) International Programme on Chemical Safety: Environmental health criteria for disinfectants and disinfectant by-products. Draft report, World Health Organization, Geneva. Last F. T., Watling R., (Eds) (1991) Acidic deposition: its nature and impacts. Proceedings of the international symposium held in Glasgow, Scotland, 16±21 September 1991. Royal Society of Edinburgh. Luker M. and Montague K. (1994) Control of pollution from highway drainage discharges. Construction Industry Research and Information Association, Project 473. Moore J. W. (1991) Inorganic Contaminants of Surface Water: Research and Monitoring Priorities. Springer± Verlag, New York. Morse G. K., Lester J. N. and Perry R. (1993) The Economic and Environmental Impact of Phosporus Removal from Wastewater in the European Community. Selper Publications, London. Moss B., Johnes P. and Phillips G. (1996) The monitoring of ecological quality and the classi®cation of standing waters in temperate regions: A review based on a worked scheme for British waters. Biological Reviews 71, 301±339. Solbe J. F. (1984) Eects of Land use on Fresh Waters, ed. J. Solbe. Ellis Horwood, Chichester. Squillace P. J., Zogorski J. S., Wilber W. G. and Price C. V. (1996) Preliminary assessment of the occurrence and possible sources of MTBE in groundwater in the United States, 1993±1994. Environmental Science and Technology 30, 1721±1730.
www.elsevier.com/locate/foodchemtox
Sources of Chemical Contaminants and Routes into the Freshwater Environment M. S. HOLT ECETOC, Av. E.Van Nieuwenhuyse 4, B-1160 Brussels, Belgium SummaryÐDrinking water is derived from either surface waters or groundwater. The latter is of enormous importance, with more than 65% of Europe's drinking water needs being supplied in this way. However, water from either source is rarely, if ever, pure. Industrialization and urbanization together with intensi®ed agricultural activity have led to increased demands for water on the one hand but to the potential for large scale release of contaminants on the other. The result is that surface water can be contaminated through direct or indirect emissions and groundwater can be contaminated by leaching from the soil. The diversity and number of existing and potential sources of chemical contamination are quite large. This paper reviews the major sources of chemical emissions and the routes by which contaminants can arise in surface waters and groundwaters intended for use as a supply of drinking water. It is estimated that there are between 90,000 and 100,000 chemicals in regular use but that as few as 3000 account for about 90% of the total mass used. Whether a substance may be found in the air, soil or aqueous environment depends on a number of factors, including how the chemical is released, the volume released, where the chemical is released, its release pattern and its physicochemical properties. Of the major routes of contamination for the aquatic environment, the most signi®cant are directly from treated and untreated waste waters, run-o and atmospheric deposition (including spray drift) and indirectly from leaching. The fate of emissions of contaminants in a particular water body will depend not only on the amount of the substance emitted but also on the transport, dispersion and transformation (biodegradation, hydrolysis, photolysis) processes in the receiving body. The preventative measures (biodegradation testing and sewage treatment) taken to minimize contamination are discussed. # 2000 Published by Elsevier Science Ltd. All rights reserved Keywords: aquatic; contamination; emissions; environmental fate; biodegradation.
Introduction The ever-increasing demands of modern day society in its quest for economic development and drive for improved standards of living are leading to increased use of raw materials. There is little doubt that the use of chemicals has played a key role in many of the major sectors such as agriculture, industry, housing, transport, textiles and health that have contributed signi®cantly to the rise in the standard of living among populations around the world. Their use, however, results in a continuous release of both naturally occurring and man-made substances including gases, heavy metals, volatile organic compounds, soluble organic compounds, suspended solids, colour, nitrogen and phosphorus compounds into the air, water and terrestrial environments. Although eorts are made to reduce these emissions and hence the impact of human activities, it is virtually impossible to prevent some contamination of the environment. This paper con®nes itself to the emissions that can ®nd themselves ultimately in the freshwater environment.
Water is rarely if ever pure. For the purpose of discussion in this paper, a distinction is made between contamination, where substances are present, and pollution which can be de®ned as the appearance of some environmental quality for which the exposed community is incapable of neutralizing the negative eects and where some demonstrable risk has been identi®ed. Indeed, some substances are essential for life, and exposure to these substances (e.g. Cu, Co, Mn, Ni, Se and Zn) is a prerequisite for the healthy functioning of ecosystems. Not all water quality problems are due solely to human impact. For example, metals present in the lithosphere may enter the environment either as a result of geological activity or human activity (Moore, 1991). The reports of reduced iron in Denmark, ¯uoride in Bavaria and Moldova, arsenic and strontium in some mountainous regions illustrate the fact that natural geochemical conditions may cause elevated concentrations of metals locally (EEA,1995). Geological processes, including volcanic action, related hydrothermal activity and weath-
0278-6915/00/$ - see front matter # 2000 Published by Elsevier Science Ltd. All rights reserved. Printed in Great Britain PII S0278-6915(99)00136-2
S22
M. S. Holt
ering, can also lead to local problems (Duke, 1996). The importance of these processes show a high spatial variability. Current estimates are that approximately 90,000± 100,000 chemicals are in daily use. The production, distribution, use and disposal of chemicals lead almost inevitably to their presence in the environment either in a localized way or in a widespread manner. The life cycle and use pattern can be used to predict the main emission sources and the probable receiving environmental compartment. Chemical release into the environment may occur at any stage, starting with extraction/winning, manufacture through formulation and use until end-user disposal or destruction. The emission pattern, the quantities released into the dierent environmental compartments and the number of emission points will vary widely from substance to substance. For example, in the case of an intermediate used for pharmaceutical production then only a very small fraction may be released into the environment, whereas in the case of household detergents almost 100% will be released into the public sewer. The major routes by which substances can enter the aquatic environment are illustrated in Fig. 1. The exact number of dierent sources from which they can enter the aquatic compartment are too numerous to list but they can, for the sake of simplicity and convenience, be separated into two major categories: . Urbanization and industrialization . Agriculture and forestry.
terns and the relative contribution of the fate processes. Continuous emissions are characterized by a practically constant emission over a long period (e.g. sewage treatment euent or discharge from a continuous production process). Many substances are released to the environment as a result of batch processes as opposed to continuous processes. Discontinuous emissions show variations in both volume and concentration with time. They can be peak emissions or block emissions. Peak emissions are characterized by relatively large discharges over a short time. Furthermore, the time between peaks and the peak height can vary greatly. Block emissions are characterized by a reasonably constant ¯ow rate over a certain time, but with regular intervals of low or zero emission. Surface water is the main source of water abstractions by all utilization sectors in Europe. On average 70% of the abstraction is from this source (EEA, 1995) but with large variations between countries (Spain, Belgium, The Netherlands >90%, Cyprus, Switzerland, Denmark <25%). Surface water can be contaminated through direct or indirect emission. Drinking water can also be derived from groundwater. EEA (1995) estimate that more than 65% of the potable water supply for Europe is provided this way. Contamination of groundwater occurs through leaching. Urbanization and industry Wastewaters
Types of emissions and sources Emissions to the air, water and terrestrial compartments arise from human activities and natural processes. The simplest classi®cation of water pollution sources is that they are either point sources or non-point sources (Table 1). The characteristics of these sources can vary widely from well-de®ned point sources (which in themselves can be single or multiple) to diuse releases from large numbers of small point sources (e.g. automobile exhaust gases or houses) or line sources (e.g. motorway run-o). A typical example of a point source is a wastewater treatment plant, while pesticides applied to land demonstrate a diuse release pattern. Point and non-point sources also dier in the way that the pollutants are released to the receiving water body. Point sources usually result in direct discharges to water courses, whereas the route for non-point sources may involve a path which results in partial deposition before reaching the water course. Furthermore, contaminant concentrations arising from non-point sources may vary signi®cantly with time and space re¯ecting both the seasonal use pat-
The most important sources of organic waste entering surface waters are from domestic and industrial sewage. Various processes are possible for treating domestic and industrial wastewaters, all of which are essentially controlled variations of the processes that nature uses to purify the aqueous or terrestrial compartments. However, in a treatment plant the processes of ¯occulation, sedimentation and biological removal (aerobic and/or anaerobic) are optimized. Historically, municipal wastewater treatment plants have been designed to deal with conventional organic pollutants found in domestic and industrial wastes. Conventional wastewater plants consist of primary, secondary and tertiary levels of treatment. The number of stages and the processes required depend on the type of waste and the desired euent quality. The capacity of many plants has recently been extended from organic carbon removal to include nitrogen removal by nitri®cation and denitri®cation, as well as the removal of phosphates and heavy metals. The con®guration of the plants has consequently increased in complexity and the number of physical, biological and chemical processes
Routes into freshwater
S23
Table 1. Examples of types of emission source Point sources Non-point sources Line sources
Domestic sewage networks, industrial euents, accidental spills, mining, storm over¯ow Agricultural practices, scattered dwellings, atmospheric deposition, construction work, land run-o, river transportation Highway run-o
in¯uencing euent quality has been expanded. Nevertheless, at many waste water treatment plants, improvements in removal eciency can only be achieved by reducing the suspended solids concentrations. This can often be done without the additional cost of euent ®ltration equipment but rather through simple improvements in secondary settling tank design and operation to improve ¯occulation. Chemical treatment of wastewater discharge In addition to the physicochemical and biological processes, a number of chemical processes are used during the treatment of wastewaters which can have an impact on the quality of the euent. For example, sodium hypochlorite is used to oxidize inorganic and organic compounds and as a disinfectant to treat wastewaters. This practice can result in the production of by-products in the ®nal euent discharged into the receiving water. A second example, which leads to one of the major sources of aluminium in surface waters, is the discharge of alum sludge from municipal waste water treatment plants. Alum sludges are produced when alum or aluminium sulfate is used for coagu-
lation and ¯occulation of raw water supplies to remove turbidity and/or colour. Water supply Treatment of potable water supplies and the distribution systems themselves can also lead to contaminants in drinking water. For example, the disinfection processes produce a range of by-products. The use of chlorine results in the formation of chloroform and other bromo and chloro haloforms. Ozonation of drinking waters containing bromide ions produces a number of brominated organic compounds such as bromoform and bromohydrins (IPCS, 1997). Chemicals such as lead, organtins and polynuclear aromatic hydrocarbons (PAHs) can enter the water supply as leachates. The detection of lead in drinking water generally re¯ects its dissolution from solder joints, brass ®ttings and other materials in the distribution network. Organotins can leach into drinking water from certain types of polyvinyl chloride pipes and PAHs (particularly ¯uoranthene) can leach from the older type of pipes which were lined with coal tar pitch.
Fig. 1. Major routes of entry into surface waters.
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Dissolution In addition to the problems outlined above, dissolution is the route by which many other substances can enter a water course. For example, the main source of organotins is from the dissolution of tributyl tin and related compounds, used as antifouling agents. Run-o Storm water. Increasing attention is now being directed towards the problems of urban drainage. Until recently rainwater and the resultant storm water were considered to be relatively clean, but it has now been shown that the actual and potential contaminants of rain water represent a major route for contaminants into surface waters. Storm water run-o can give rise not only to local pollution incidents, but when collected in combined sewerage systems (a very common practice in Europe), it is also the reason for hydraulic overload and often the reason for sewage plant malfunctioning. Highways. Contaminants contained in highway run-o can be generated by a wide variety of sources which can for convenience be separated into three main groups: . trac . maintenance (e.g. de-icing, weed and pest control) . accidental (accidents and spills). The impact of urban storm-water discharges has been studied recently (Luker and Montague, 1994) and the indications are that an initial peak ¯ushing of contaminants can be attributed to the removal of dry weather deposits. Factors such as population density, trac density and farming intensity are known to have an impact on the contaminant composition of run-o. Of the many types of urban surface contributing directly to storm run-o the most important in terms of metals and sediment contributions are roads. Snow scavenges and absorbs more atmospheric pollutants than rain and clearance of snow may involve greater than normal applications of de-icing material. Mineral extraction Mining is second only to agriculture as the most important land use industry (Solbe, 1984). Discharges from the mining industry can result in increased levels of metals which may aect water quality. For example, the discharge of saline waters from coal mines to surface waters results in highly corrosive waters. While working mines constitute an obvious problem, the closure of mines can be a double-edged sword. Besides being a source of contamination in their own right, mine waters also serve as diluent for sewage euents in many catchments. If pumping of these mine waters ceases, local pollution may occur from that loss of dilution.
In time, the underground waters may rise in the closed mines and the water will eventually surface, creating the potential for contamination elsewhere. The further extraction of metals by the smelting industry accounts for much of the anthropogenic input of many metals to the aqueous environment either directly or indirectly via air emissions. Atmospheric deposition Atmospheric deposition is a major diuse source of contaminants. Substances released into the atmosphere are present in the gaseous and the aerosol phases or are adsorbed to particles. One of the dominant deposition mechanisms to the ground is wet removal due to the scavenging of particles by, and partitioning of organic vapour into rain and snow. The extent of this process depends on the distribution of the chemical between the gaseous and aerosol phases, particle size distribution and Henry's Law constant. In addition, pollutants can be removed from the atmosphere by deposition of particles to which the chemical is adsorbed (dry deposition). Atmospheric deposition is dependent on climate, particularly wind direction, sunlight (photodegradation) and rain/snowfall. Combustion of fossil fuels, chemicals manufacturing and incineration of waste all contribute to the release of chemicals into the air compartment. Acidi®cation of soils are well described for northern and central Europe (Last and Watling, 1991). It is caused primarily by atmospheric deposition of acidifying compounds released into the atmosphere as a result of human activities. As a result increased concentrations of aluminium, sulfate and hydrogen ions have been reported in groundwaters below sandy soils. The most serious consequence of acidi®cation of groundwater is the increased mobilization of trace elements, especially aluminium.
Agriculture and forestry Diuse pollution from land use practices can aect water quality in four ways. The ®rst is the direct release of agrochemicals either during application or disposal. The second is due to the discharge of water from agricultural land as a result of irrigation and land drainage. The third is the result of leaching and the fourth results from farming practices that are carried out in water (e.g. watercress or ®sh farming). Agricultural chemicals There are currently more than 600 dierent pesticides used in agriculture, forestry and horticulture. Pesticides and fertilizers are usually applied to ®eld crops using a boom sprayer with hydraulic spray nozzles. Application to orchards is often using an air blast mist blower. The produced spray cloud
Routes into freshwater
contains droplets of various sizes. Drift cannot be avoided and some of the pesticide may reach surface water directly. A number of factors will play in role in how much and how far the pesticide may drift namely: . droplet size (surface tension, nozzle size and liquid pressure) . dosage, formulation, spray volume and the solvents used to prepare the spray . environmental conditions (wind speed and direction, humidity, temperature) . crop height (drift increases with crop height) and spray height. Drainage, either natural or man-made, from agricultural land may end up in surface waters. To regulate the groundwater level in areas where the groundwater table is high, the fate of in®ltrate water is often controlled via tile drains. Pesticide leachate, produced as rain water migrates through the unsaturated zone of the soil column, will be transported due to gravity and capillary/adsorptive into groundwater. Most leaching in Europe occurs between November and April. In the case of nitrate, for example the main source is mineralization of soil organic matter, and following an initial ¯ush, concentrations fall as the winter progresses and soil temperature decreases. As spring approaches, increased crop uptake of water (and nitrate) results in almost zero loading in receiving waters. Surface run-o and erosion are additional sources of contamination. Run-o is the excess rainfall that cannot in®ltrate the soil. Land form, soil type and vegetation are the major factors in¯uencing run-o from a given area. Most impact is generally during heavy rainfall shortly after the application of the chemical. Other sources of contamination of waters from agricultural practices include overspray, disposal of excess spray liquid and back-siphoning of tank liquid into the surface waters used for ®lling the tank. For many years the priority was to ensure maximum agricultural productivity, but the emphasis is shifting from agronomic to environmental priorities. For example, historically most emphasis on controlling phosphorus in surface waters was directed point sources by controlling phosphate in detergents and wastewater. The phosphorus balance is now starting to be redressed and the impact of diuse sources questioned (Moss et al., 1996). Within the European Union it has been estimated that 50% of the phosphorus in surface waters comes from diffuse agricultural sources, 41% from waste waters (24% from human sources, 10% from detergents and 7% from industry) and 9% from background sources (Morse et al., 1993). In addition to chemicals applied to crops, other agrochemicals used in animal husbandry such as
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sheep dip chemicals have been identi®ed as major contaminants of surface waters. Organic manure (a combination of dung and urine from housed livestock and plant waste) and municipal sewage treatment works sludges are applied to farmland to replenish the nutrients and as soil conditioners. The aim of the current legislation is to ensure animal and human health are not put at risk by potentially toxic chemicals and pathogens which may be present in sludge. They do not impose any limits for the maximum allowable concentration of organic contaminants in sludge. Surface run-o following application of manures and sewage sludges, together with leaks and spills of silage liquor, slurry and other farm wastes, are a major source of freshwater contamination resulting from current agricultural practices. Contamination of groundwaters The routes to groundwater are generally dierent to those for surface waters. The chemical nature of rocks through which groundwater moves in¯uences the chemistry of the water. For example, igneous rocks composed mainly of silicate materials of low solubility give rise to waters low in dissolved minerals, whereas sedimentary rocks consisting of rock materials accumulated from various sources yield waters with assorted solute content. The main routes and sources of groundwater contamination are: . waste disposal including a wide range of inorganic and organic contaminants from point sources in municipal, industrial or military land®ll, mining, leaking underground storage tanks and pipelines, and spills . non-disposal use of chemicals on the surface of the land, for example leaching of nitrates and pesticides . overpumping (for example salt water intrusion) or depletion . acidi®cation. Much has been written about the presence of nitrates and pesticides in groundwater, and while land®lls, for example in the UK, account for most incidents of point-source contamination, Bishop et al. (1998) have recently identi®ed leaking sewers in urban areas as sources of contamination of groundwater. A recent development in the gasoline industry is the use of methyl tert-butyl ether (MTBE) as the major fuel oxygenate. It is a good example of a substance that can arise from either point or nonpoint sources. Gasoline spills to land and releases from storage tanks are examples of point sources. Potential non-point sources are rainfall, urban runo and power boats. The environmental fate and behaviour of MTBE mean that it can aect both surface and groundwater quality. Of the 60 volatile organic chemicals detected in shallow groundwaters
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Fig. 2. Possible fates of chemicals.
in the US MTBE was the second most common after chloroform (Squillace et al., 1996). Environmental fate and behaviour When released into the environment, substances are subjected to any one, or combination, of a number processes that may aect their fate and behaviour (Fig. 2). The eect of each of these processes on the concentration of a chemical in any given environmental compartment (such as water, air, soil, sediment, biomass) depends on the chemical's physicochemical properties, environmental conditions and the discharge pattern. The major processes are: . transport (volatilization, advection, dispersion, adsorption, wet deposition, sedimentation, resuspension, soil or sediment mixing and diusion). Transport processes determine the variation in spatial and temporal distribution of a chemical in the environment. Rates of advection and dispersion are determined solely by environmental parameters such as current or wind speed. In air, rates are usually very fast while in water they may vary from very rapid in fast ¯owing rivers to very slow in stagnant lakes or ponds. In soil and sediment these rates may be insigni®cant. . transformation (biodegradation, hydrolysis, photolysis, speciation). Transformation is of major importance in determining the persistence of a chemical. The mechanism of these processes and their rates may vary greatly between and within an environmental compartment depending on the reactivity of the substance and environmental
parameters such as temperature, light intensity and numbers of competent bacteria. . uptake (bioaccumulation, bioconcentration). Two dierent modes of uptake can be distinguished, Passive uptake and active uptake. Passive uptake occurs via the skin and or gills of the truly soluble fraction while active uptake occurs via the digestive tract. For aquatic fauna, the principle mechanism of uptake is passive uptake via the skin and gills. For organisms living in sediment and soil, both mechanisms are involved. For plants, uptake may occur through passive uptake from water or air via the leaves but also through passive or active uptake from pore water via the roots. Sorption to the outside surface of root crops has also been shown to be signi®cant for crops with a high lipid content. Uptake and subsequent concentrations in biomass depend on the bioavailability of the substance. The measured total environmental concentration of a substance does not necessarily represent the actual concentration to which individual species will be exposed. The fraction of the total chemical that is available for uptake is de®ned as the bioavailable fraction. For example, the bioavailability of metals is complicated by the variety of physical and chemical forms in which they occur, including free ions, organic and inorganic complexes and particles. Water hardness, acidity and salinity have a strong in¯uence on metal speciation and in general free ions are the most bioavailable. The most important physicochemical properties of a substance that impact on its fate and distribution in the environment are boiling point, melting
Routes into freshwater
point, octanol/water partition coecient, vapour pressure. EU regulatory legislation requires information on these properties to be collected in the `base set' in support of classi®cation, packaging and labelling of dangerous substances (67/548/EEC and subsequent amendments). Biodegradation is, however, the key intrinsic property that determines the fate and distribution of substances. Biodegradation is the process by which organic materials are broken down, ultimately into carbon dioxide and water, by the enzymatic activity of microorganisms. Not surprisingly, therefore, EU legislation also requires data on a substance's biodegradability to support classi®cation and labelling of dangerous substances, risk assessment and other environmental management practices such as discharge permits. One of the most visual examples of the importance of the use of biodegradable substances comes from the detergent industry. The ®rst synthetic surfactants for use in detergent products were introduced in the 1950 s but resulted in extensive foaming in receiving waters. The detergent industry subsequently changed to biodegradable surfactants and such sights are no longer to be seen. A three-tier hierarchical approach to biodegradation testing has been developed. At the base level (level 0) a stringent screening test with low nutrients and low bacterial numbers are used to determine `ready' biodegradability. A pass in this test implies that the substance will rapidly biodegrade in the aquatic environment. At level 1 the inherent biodegradability (i.e. potential to biodegrade) is determined using higher microbial populations and allowing time to adapt. At the highest tier (level 2) a substance's behaviour during sewage treatment is simulated. These tests cover biodegradation in aerobic environments. Tests are also available to measure the biodegradability of substances under anaerobic conditions. Conclusions Chemical contaminants in water bodies intended for use as a source of drinking water can come from both natural and anthropogenic origins. The production, distribution, use and disposal of products leads almost inevitably to the release of substances into the environment in either a localized way or, in many instances, in a widespread manner which consequently may ®nd their way into water bodies intended as a source of drinking water. The fate and distribution of all emissions depend on both the amount and physicochemical properties of the emission, on the hydrology, geochemistry and biological characteristics of the receiving environment. Although the numerical values ascribed may vary slightly, many countries have already
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established or are now in the process of establishing, surface water quality standards and objectives for the protection of water resources. While greater understanding of the fate and behaviour of substances in the environment has resulted in signi®cant reductions in emissions, there is still an enormous amount of work to be done. Over the last few years there has been an active policy in many European countries to reduce point source emissions from both the domestic and industrial sectors. In addition, improvements in wastewater treatment processes coupled with the use of biodegradable substances in products intended for release into the environment have and will continue to improve the quality of the aquatic environment. Addressing the issue of non-point sources is more complex and the relative importance, of these two types of emission, is gradually changing.
REFERENCES
Bishop P. K., Misstear B. A., White M. and Harding N. J. (1998) Impacts of sewers on groundwater quality. Water and Environment Management 12, 16±223. Duke M. (1996) Geological sources of metals in the environment. Report of the international workshop on risk assessment of metals and their inorganic compounds. Angers, 13±15 November 1996, pp. 127. The International Council on Metals and the Environment, Ottawa, Canada. EEA (1995) Europe's environment. In The Dobris Report, ed. D. Stanners and P. Bourdeau. Earthscan Publications, London. IPCS (1997) International Programme on Chemical Safety: Environmental health criteria for disinfectants and disinfectant by-products. Draft report, World Health Organization, Geneva. Last F. T., Watling R., (Eds) (1991) Acidic deposition: its nature and impacts. Proceedings of the international symposium held in Glasgow, Scotland, 16±21 September 1991. Royal Society of Edinburgh. Luker M. and Montague K. (1994) Control of pollution from highway drainage discharges. Construction Industry Research and Information Association, Project 473. Moore J. W. (1991) Inorganic Contaminants of Surface Water: Research and Monitoring Priorities. Springer± Verlag, New York. Morse G. K., Lester J. N. and Perry R. (1993) The Economic and Environmental Impact of Phosporus Removal from Wastewater in the European Community. Selper Publications, London. Moss B., Johnes P. and Phillips G. (1996) The monitoring of ecological quality and the classi®cation of standing waters in temperate regions: A review based on a worked scheme for British waters. Biological Reviews 71, 301±339. Solbe J. F. (1984) Eects of Land use on Fresh Waters, ed. J. Solbe. Ellis Horwood, Chichester. Squillace P. J., Zogorski J. S., Wilber W. G. and Price C. V. (1996) Preliminary assessment of the occurrence and possible sources of MTBE in groundwater in the United States, 1993±1994. Environmental Science and Technology 30, 1721±1730.