The big picture

What is the aquatic environment like where you live? Do you live in an area surrounded by rivers, lakes or the sea or do you live in a more arid place with few aquatic ecosystems present? How clean is this aquatic ecosystem? What are the signs you are using to make this judgment? How does the state of your surroundings make you feel?

The state of our aquatic ecosystems is important as they provide us with many different benefits:

• Source of water.
• Source of food.
• Aesthetic and recreational value that can contribute to human well-being.
• Ecological value e.g. habitat to many different species.
• Potential future value not yet discovered.
• Bequest value, passing the ecosystem onto future generations.

Figure 1. Recreational use of water.

In this subtopic we will be exploring the sources of water pollution, their environmental impacts and what can be done to reduce aquatic pollution.

Pollutants

Pollutants can have a detrimental effect on aquatic ecosystems. There are a wide variety of different aquatic pollutants which include:

• Chemical pollutants e.g. toxic metals.
• Physical pollutants e.g. noise and thermal pollution.
• Biological pollutants e.g. invasive species.

Figure 2. Lionfish an invasive species in Florida.

Once pollutants have entered the environment they can move between the atmosphere, lithosphere and hydrosphere.

Figure 3. Pollutants can move between the atmosphere, lithosphere and hydrosphere.

Pollution emissions can be transported by the atmospheric system prior to being deposited into surface waters e.g. emissions of nitrogen oxides and sulphur dioxide from fossil fuel combustion can form acid rain within the atmosphere and following precipitation cause lake acidification.

Pollutants released on land can be washed into nearby rivers and streams which flow into to the sea. Ocean currents can transport these pollutants large distances around the world. Hence pollutants such as polychorinated biphenyls (PCBs) can be found in remote regions of the world such as the Antarctic, where they may never have been used.

Rubber ducks and ocean currents

The ability of ocean currents to move material including pollutants around the globe was illustrated by the accidental release of ducks into the oceans. In January 1992 a container of 28,000 rubber ducks fell overboard on route from Hong Kong to the USA.

Over the years these ducks have travelled thousands of miles and been found across the world. Scientists have used this information to help determine the movement of ocean currents.

Figure 4. Rubber ducks lost in the Pacific Ocean in January 1992 have been found in Australia, Alaska, USA, Arctic, Scotland and Chile.

Sources of water pollution I

There are many different sources of pollutants that have a detrimental effect on aquatic ecosystems and on human use of the environment and its resources.

Inland and coastal pollution

Some key sources that effect inland and coastal waters include the following:

Domestic sewage

Domestic sewage is the most universal pollutant, found wherever people are. Domestic waste is mainly organic, consisting of carbohydrates, proteins, fats, ammonium compounds, as well as soaps, synthetic detergents and dissolved salts. Domestic sewage effluent is also a main source of pathogens such as bacteria and viruses which can cause human illness if consumed.

Figure 1. Bathroom illustrates a main source of domestic sewage effluent.

Industrial discharge

The type of industrial discharge varies in composition depending on the on-site processes and materials used. It can include organic matter, toxic metals or synthetic non-biodegradable compounds. For example:

• Processes within the pulp and paper industry use large amounts of water and produce wastewater containing high levels of organic matter, nutrients, sodium hydroxide, sodium carbonate and organochlorine compounds.
• Tanneries undertake various processes in the production of leather that lead to wastewater rich in organic matter, chromium salts, calcium hydroxide (lime), sodium chloride and polyphenolic compounds.

Figure 2. Tannery in Morocco using many different coloured chemicals.

Agricultural run-off

Run-off from farms may contain pesticides, fertilizers, manure, slurry or silage, particularly where intensive farming practices are used. There are a wide variety of pesticides with different degrees of toxicity to aquatic ecosystems. Fertilizers often contain nitrates that are highly soluble and easily leached from the land. Nitrates are important plant nutrients and therefore they can increase growth of water plants within the river. If manure, slurry or silage is washed into nearby streams or rivers, this can result in an increase in organic material and suspended solids to the water. As manure (i.e. animal dung) and slurry (i.e. semi liquid mixture of animal waste) are excrement products from animals they also contain pathogens.

Figure 3. Agriculture activity such as spraying of pesticides can contaminate nearby water systems.

Urban run-off

As rainwater flows through an urban area, it picks up pollutants from the catchment. As discussed in section 4.1.3, surface run-off can contain high levels of organic waste, suspended solids, oil and toxic metals.

Land development

Land clearance for building may involve forest clearance which can increase soil erosion and contribute to the suspended sediments entering aquatic habitats. Wash off from building works and hard surfaces can also result in high levels of suspended solids.

Figure 4. Construction activity can generate a lot of particulate matter that contribute to suspended solids within aquatic systems.

Landfill sites

Disposal of waste on land can lead to leachates entering groundwater or surface waters. The composition of the leachates will be dependent on the materials disposed. Leachates tend to be a cocktail of concentrated pollutants that can be highly toxic to aquatic organisms.

Accidental discharges

These can occur from storage sites or during site operations involving hazardous materials such as toxic metals or non-biodegradable synthetic compounds. Without adequate barriers to contain the spill, material can escape into nearby water systems.

Why do you think milk would be considered a hazardous material, if large quantities were spilt into a river?

Acid mine drainage

Acid mine drainage occurs as a result of rainwater percolating through either disused or active mines. As the water flows through, it reacts with iron pyrite within the rock forming sulphuric acid. The resulting acidic water dissolves toxic metals (e.g cadmium, lead and copper) which are then transported by the flow of water into nearby water systems causing pollution.

Figure 5. Pollution from a copper mine.

Atmospheric input

Emissions from domestic and industrial flues, vehicle exhaust, evaporation or spraying of chemicals can be washed out of the atmosphere by rain into aquatic systems. Pollutants which enter aquatic systems in this manner include toxic metals (e.g. lead and cadmium) and synthetic compounds (e.g. polychlorinated biphenyls (PCBs) and dichlorodiphenyl trichloroethane (DDT)).

Figure 6. Air emissions from industry contributes to pollution of aquatic systems.

Sources of water pollution II

Marine based sources

Rivers are a major route by which pollutants enter the marine environment. Hence activities inland that contribute to freshwater pollution may also have an impact on coastal and marine ecosystems. Globally about 80% of marine pollution is from land based sources discussed in the previous section, sources of water pollution. Further sources which contribute to marine pollution include:

Outfall pipes

Outfall pipes can be used to discharge material directly from the land to the sea e.g. sewage effluent (either treated or non treated sewage) and cooling waters from power stations.

Figure 1. Outfall pipe discharging into the sea.

Materials dumped at sea

Material may be directly dumped out at sea e.g.:

• Sewage sludge from sewage treatment plants.
• Radioactive waste.
• Military waste such as ammunitions and other chemicals.
• Disused platforms e.g. oil platforms.
• Fly ash from power stations.
• Dredging spoils from widening shipping channels.

Shipping activities

Shipping activities can contribute to marine pollution via:

• Disposal of litter and other waste at sea.
• Accidental discharges, such as oil spills.
• Discharge of ballast waters that contain oil residues.

Figure 2. Ballast waters used to provide ship stability can transport organisms thousands of kilometers from one location to another.

Exploitation of resources

Extraction of materials such as oil or gravel beneath the sea bed may cause marine pollution.One of the most famous incidents was the BP Deepwater Horizon in the Gulf of Mexico in 2010 that caused extensive oil pollution of the environment. The following video ‘BP Oil Spill’ by Fox10News provides a brief overview of what happened.

Your aquatic environment

How many different sources of potential water pollution can you identify in your locality? Do you know what is being done to address these problems?

International-mindedness

A source of pollution in one country can cause pollution problems in another country (e.g. impact may be downstream or downwind of the source), which potentially can lead to international conflict.

Effects of water pollution I

Each type of pollution has particular effects on the aquatic environment. The impacts of some of the main categories of pollution are discussed here.

Organic pollution

There are many sources of organic pollution including sewage effluent, farm runoff and waste from the food and drink industry (e.g. diaries, food processing, brewing and distilleries).

Figure 1. Contamination of river with blood from slaughterhouse in Narobi, Kenya.

Organic waste is biodegradable and is a food source for micro-organisms naturally found in the environment. When organic material enters the water, micro-organisms break down the complex organic compounds (e.g. fats, proteins and carbohydrates) into smaller simpler molecules utilizing oxygen in the process. This aerobic breakdown results in the production of nitrates, phosphates, sulphates and carbon dioxide.

If organic waste is discharged from an outfall pipe, it creates turbulence in the water, maintaining oxygen levels. As the material flows downstream and is degraded, oxygen levels fall. Some aquatic organisms may be deprived of sufficient oxygen levels and die. If conditions become anoxic, anaerobic bacteria breakdown the organic matter into methane, ammonia and hydrogen sulphide. The later that has a distinct smell of rotten eggs. These gases are toxic and result in fish kills. Once the organic material is broken down oxygen levels begin to recover.

Figure 2. Organic degradation.

The level of oxygen depletion will depend on the concentration of the organic discharge, rate of dilution and rate of aeration (including turbulence). The greater the concentration of the discharge the more oxygen is required. If the receiving waters are large and fast flowing, it will reduce the impact of oxygen utilization.

Oxygen levels in the water are also determined by:

• Temperature: water holds more oxygen at lower temperatures.
• Pressure: pressure decreases with altitude and at lower pressure the water holds less oxygen.
• Turbulence: the movement and agitation of the water, the greater the aeration increasing dissolved oxygen levels in the water.
• Photosynthesis: during the day, the process of photosynthesis produces oxygen which enhances dissolved oxygen levels.
• Respiration: this process utilizes oxygen and can lower oxygen levels.

Inorganic plant nutrients

Nitrates and phosphates are essential plant nutrients and when their levels in the water are increased there is usually enhanced primary production. Enrichment of waters with plant nutrients is referred to as eutrophication. There are many sources of nitrates and phosphates and include the breakdown products of organic pollution (described above), sewage effluent (potentially rich in phosphate based detergents), farm run-off and uric acid from birds and wildfowl.

Figure 3. Application of fertilizer to rice fields which can lead to contamination of water systems.

An increase in primary production can lead to further growth of marcrophytes or to cyanobacteria blooms. The additional plant growth can block out light penetrating the water and reaching submerged plants. This can result in a loss of overall biodiversity. The plants may also impede navigation and other uses of the water. When the macrophytes die, they sink to the bottom of the river, lake or seabed and decomposition occurs. As plants are composed of organic compounds the same process occurs as described above and illustrated in Figure 2. Oxygen levels may be depleted resulting in anoxic areas, sometimes also referred to as dead zones as few species (including fish) can survive these adverse conditions. Eutrophication demonstrates examples of both positive and negative feedback mechanisms as illustrated in the figure below.

Figure 4. Examples of positive and negative feedback within eutrophication.

If cyanobacteria blooms occur, some species can produce toxins that are harmful to human health (e.g.Microcystis; Anabena and Melosira). Symptoms can range from skin rashes, vomiting, diarrhoea to extreme cases of paralysis and death. They can appear as a ‘scum’ on the surface of the water. Some species are nitrogen fixing and therefore not limited by nitrate levels.

Figure 5. Cyanobacteria bloom.

In marine waters algal blooms may be reported as red tidal blooms, although these can vary in colour depending on the composition of the phytoplankton.

Problems associated with eutrophication include:

• Water can become unsuitable for drinking unless expensive treatment methods are used. The excessive plants can clog filters and decomposing algae can have a detrimental effect on taste and odour.
• Reduced recreation use of the water (e.g. boating, swimming and fishing) due to excessive plant growth or cyanobacterial blooms.
• Reduced commercial value of the aquatic ecosystem e.g. due to loss of fisheries or loss of navigation routes used for trade.
• Increase in water related diseases. The excessive plants can provide a habitat for insects and other organisms that spread disease.

Figure 6. Water hyacinth is an invasive macrophyte that does well under eutrophic conditions.

Toxic metals

At very low levels most metals are not a problem and many are present at natural background levels without detrimental effects. Some metals such as zinc and copper are required as micro-nutrients by many organisms. However, at higher levels these metals can be toxic by interfering with essential cellular processes.Bioaccumulation can occur, where on continual exposure the levels of the metals build up within the organism over time. Bio-magnification can also occur in which the levels of the metal then build up though the food chain.

Figure 7. Location of Minamata and Minamata Bay, Japan

One of the first recognized cases of environmental metal poisoning occurred in Minamata, Japan during the 1950s. Initial signs were observed with cats that displayed hyperactive behaviour. Hundreds of people in Minamata, Japan were seriously affected from eating seafood contaminated with mercury compounds. Symptoms included convulsions, loss of neuromuscular co-ordination, slurred speech and memory loss.

Concentrations of mercury were found in the seawater within the bay, the plankton and shellfish. Mercury compounds were found to bioaccumulate within the organisms and levels were biomagnified through the food chain. This resulted in levels within the shellfish being consumed by humans containing on average 30 times the amount in the seawater.

The source of mercury was traced back to a factory producing plastic vinyl chloride that used mercury sulphate as a catalyst. Effluent containing the catalyst was being discharge into Minamata Bay, which within the sediments was converted by bacteria to methyl mercury. This organic form of mercury can be readily absorbed by phytoplankton and is able to cross the blood brain barrier in humans causing brain damage.

Figure 8. Bacterial action within sediments converts inorganic metal mercury into organic methyl mercury and dimethyl mercury.

In the 1970s the factory was closed based on economic reasons. In 1989 its two former executives received a two year jail sentence. Only 3,000 out of the 13,000 people that applied for compensation were officially recognized as suffering from mercury poisoning until 1996, when 1,500 more victims were acknowledged and also awarded compensation.

Effects of water pollution II

Following on from the previous section which considered the effects of organic pollution, inorganic plant nutrients and toxic metals, in this section we look at more aquatic pollutants ranging from synthetic non-biodegradable compounds to invasive species.

Synthetic compounds

Synthetic compounds are man-made and have only been prevalent since the 1940s. They cover a wide variety of different compounds but of particular concern are the organochlorine compounds. They are non-biodegradable and can bioaccumulate e.g. polychlorinated biphenyls (PCBs) and the insecticide dichloro diphenyl trichloroethane DDT.

Figure 1. Generalised structure of PCBs.

PCBs are a group of compounds historically used widely in electrical equipment, hydraulic fluid, antifouling paints and printing inks. They are found throughout the aquatic environment due to discharge of industrial effluent, fall out of industrial emissions, leakage from landfill sites and dumping of waste containing PCBs.

PCBs are able to bioaccumulate within organisms and through the process of biomagnification, levels increase further up the food chain.

Biological effects include:

• Inhibition of phytoplankton growth.
• Inhibition of oyster shell growth.
• Adverse effect on fish reproduction.
• Suppression of the immune system in birds resulting in death.
• Adverse effect on the immune system and endocrine system of mammals linked to reproductive failure.
• Yusho illness in humans which includes acne, darkening of the skin and respiratory problems. PCBs have also been found to cause birth defects and cancer.

Figure 2. Effect of PCBs on phytoplankton.

Inert suspended solids

Suspended solids enter aquatic ecosystems from domestic and industrial effluent and as run-off. The suspended solids suppress plant life by preventing light penetration. They can clog feeding and respiratory structures and smother benthic organisms living on the river, lake or seabed.

Hot water

A major source of hot water is the cooling water discharged from electricity generating power stations. The warmer water discharged elevates the local water temperature. If subtropical species have been introduced to the aquatic ecosystem, they may find the conditions favourable and out-compete native species. A higher water temperature will lead to lower concentrations of oxygen in the water which may result in an increase in the level of stress experienced by the aquatic organisms.

Figure 3. Relationship between temperature and dissolved oxygen levels.

Oil

Oil covers the water forming a surface film that prevents gaseous exchange and therefore can result in oxygen depletion within the water. In addition the oil film blocks out light and prevents photosynthesis. Some fractions of the oil are readily biodegradable and overtime bacteria will break up the oil.

Figure 4. Oil pollution is often associated with images of oil contaminated birds.

Accidental oil spills at sea often receive significant media coverage due to the high concentration of oil within an area. Impacts include:

• High loss of plankton in the immediate area, although recovery can be fairly rapid as a result of migration of plankton from unaffected areas.
• Death of organisms living on the seabed as a result of sedimentation of some of the oil. It can take years for the population to recover.
• Although the fish tend to try to avoid polluted areas, they may suffer from tumours, fin erosion, reproductive organ damage and the flavour of the fish may also be adversely affected.
• Birds may die from drowning or hyperthermia as a result of plumage losing its ability to repel water.
• Mammals such as sea otters if covered with oil may also die from drowning as their fur loses the ability to repel water.
• Birds and mammals may also die as a result of the toxic effect of oil. The animals try and clean themselves by preening and ingest the oils which can cause intestinal disorders, renal and liver failure.

Figure 5. Physical impact of oil on seabirds.

The following video 'Exxon Valdez Oil Spill (1989): In the Wake of Disaster - Retro Report' by The New York Times looks at one of the largest oil spill accidents. Make notes on what lessons were learnt from this accident.

Pathogens

A variety of pathogens that include bacteria and viruses are contained in sewage effluent discharged into inland and coastal waters. These waters are a potential hazard to health and are of particular concern if they are used for recreation (e.g. swimming) or are shellfish collection sites. Shellfish grown in contaminated waters accumulate pathogens posing a significant health hazard if consumed.

Plastic debris

Figure 6. Plastic debris can be mistaken as food and ingested by some animals.

Aquatic animals can be affected by plastic waste by:

• Becoming entangled in the plastics and drown.
• Ingesting the plastics which block their digestive system, which reduces feeding and can also cause internal injury and death. Plastics also release polychlorinated biphenyls which can alter hormone levels, lead to reproductive problems, increase risk of disease and cause death.

Light pollution

Artificial lights along coastal areas can have devastating effects on sea turtle populations. Artificial lights disorientates hatchlings as they try to find their way to the ocean and may wander further inland instead increasing risk of death from predators, from dehydration or accidental death on roads.

Figure 7. Artificial lights from human habitations can disorientate turtle hatchling reducing their survival rate.

Noise pollution

Noise such as underwater sonar is considered to be a contributing factor to the beaching of whales and dolphins.

Invasive species

Invasive species are categorized by some scientists as a biological pollution. They can be introduced accidentally, for example:

• Escape from nearby domestic use such as garden ponds.
• Through discharge of ballast waters.
• Escape from aquaculture.

Some species may migrate via ocean currents and with the effects of global warming may acclimatize well to its new environment. Examples include introduction of water hyacinth in freshwater systems and introduction of Chinese mitten crab E.sinensis to NW Europe.

Figure 8. Chinese mitten crab found in the River Thames, London.

Assessing water quality I

Figure 1. A pristine environment - Tuolumne River in Yosemite National Park, USA.

How can you determine water quality?

Water within aquatic systems is not pure. It contains an array of different minerals and nutrients that support a community of life. The precise composition varies with time and location. For example, a stream in a chalk area contains more calcium based minerals reflecting the geology of the area compared to a stream running through a peat dominated region which is likely to more acidic.

How to examine the environment was discussed in subtopic 2.5 Investigating ecosystems. You may find it useful here to have a recap of the section on abiotic factors.

A wide range of different physical and chemical parameters can be measured to help determine water quality. However, the amount of resources available for monitoring aquatic systems is usually limited. Some key parameters often tested for include pH, temperature, suspended solids, total dissolved solids, dissolved oxygen, biochemical oxygen demand, nitrates, phosphates and metals. If a problem is identified more parameters may be tested for.

Physical and chemical parameters

pH

pH often reflects the local geology and soil. The presence of certain plants in the area is also a good indicator of the expected pH of the water. Water discharged into natural water bodies that alter the pH can affect the organisms that live there. Some species have a narrow pH range in which they can survive. Also changes in the water pH can affect reproduction and overall population growth rates. The pH is commonly measured using a calibrated pH probe, although a rough guide can be provided using litmus paper.

Figure 2. pH probe.

Temperature

Temperature normally reflects changes in ambient temperature (the surrounding environmental temperature). It affects the amount of dissolved gases present in the water. At colder temperatures, the water holds more oxygen than at warmer temperatures. Discharge of warmer water that elevates the overall water temperature reduces oxygen levels and can stress organisms living in the water. Under extreme conditions, anoxic conditions may prevail leading to death of many organisms. Temperature can be measured in-situ using a thermometer.

Figure 3. Thermometer to measure temperature.

Suspended solids

Suspended solids are small particles that can block sunlight penetrating through the water reducing photosynthesis. These small particles can also block the feeding and respiratory systems of some organisms. If the water is calm, the suspended solids may precipitate out, potentially killing organisms living on the river, lake or sea bottom. The amount of suspended material in the water is determined by:

1. Filtering a known volume of the water sample using pre-weighed filter paper (A).
2. Drying out the filter paper and collected residue.
3. Weighing the dried filter paper and residue (B).
4. Calculating the weight of the dried residue = B-A in micrograms/litre (mg/l).

Figure 4. Filter water sample and collect residue on filter paper.

Alternatively an indirect measurement can be taken using either a turbidity meter or secchi disk:

• A turbidity meter determines the amount of light scattered by the particles in the water. The greater the amount of suspended solids present, the higher the turbidity readings.
• A secchi disk, as previously discussed is used to measure water transparency. The greater the amount of suspended solids the lower the light transparency in the water.

Total dissolved solids and conductivity

Measurement of the total dissolved solids (TDS) provides an indication of the amount of salts present. The TDS can be an indication of the geology or the type of effluent discharged into the water. It is measured indirectly using a conductivity meter. With increasing levels of TDS the higher the conductivity readings.

Dissolved oxygen

Dissolved oxygen (DO2) is often used as indication of the quality of the water. Well oxygenated water allows for aerobic respiration and provides a suitable environment for many organisms. Water with low amounts or no oxygen can result in loss of many species including fish. The levels of oxygen can be effected by temperate (as discussed above) or the amount of organic matter added to the water that is broken down by bacteria present in the water utilizing oxygen. DO2 can be measured using an oxygen meter on site or by chemical analysis which is based on the Winkler method. The Winkler method involves:

1. Filling a bottle completely with the water sample.
2. Then adding reagents to the sample to ‘fix’ the oxygen by converting it into an acid, prior to transportation to the laboratory.
3. Titrating the sample in which a reagent is gradually added to neutralize the acid until there is a colour change indicating the ‘end point’ (i.e. all the acid has been neutralized).
4. The amount of reagent used to reach the end point is used to calculate the amount of dissolved oxygen levels (mg/l) which was present in the water sample.

Figure 5. Titration of sample to determine oxygen levels.

Biochemical oxygen demand

Biochemical oxygen demand (BOD) is the measure of the amount of oxygen used by organisms present in the water sample. It provides an indirect measure of the amount organic material that can be oxidised.

Process to measure BOD:

1. The initial dissolved oxygen reading of the sample is taken in mg/l.
2. One litre bottle is filled with the sample and sealed.
3. The bottle is incubated in the dark at 20°C for five days.
4. The dissolved oxygen levels are measured again.
5. The difference between the initial and final oxygen readings is the BOD5.

As a general guide for BOD5, waters with a value of 7mg/l or less are considered to be pristine. Whereas a BOD5value of 20mg/l would indicate a badly polluted site.

Figure 6. A litre water bottle is typically used that excludes light to prevent photosynthesis.

Nutrients

Key plant nutrients include nitrates and phosphates, with silica also being important in marine environments. Aspreviously discussed, if nutrient levels are enhanced through water discharges it can lead to excessive plant growth and algal blooms. When the plants die, biodegradation leads to consumption of oxygen which in turn can lead to anoxic waters. Without sufficient oxygen many aquatic species will die. In addition, some algal blooms may include cyanobacteria species that produce toxins.

Figure 7. Water sampling.

The water can be analysed for nutrient either on site using test kits or taken back to the laboratory and measured using chemical analytical methods (e.g. Inductively Coupled Plasma – Optical Emissions Spectrometry which rely on transfer of light through the sample that is dependent on the concentration of pollutants present). On site kits tend to come with their own specific directions and usually rely on nutrients reacting with a reagent to produce a color change that is used to determine the concentration of nitrates or phosphates in mg/l.

Metals

Water may contain metals which over a certain concentration have an adverse effect on the aquatic ecosystem or on humans if consumed. The high levels of metals may be due to the geology or due to discharges from human activities:

• In many areas around the world the level of arsenic in groundwater is high due to the underlying geology. When this water has been abstracted and used by humans it has led to many adverse health effects.
• In Japan, human activity led to the discharge of factory effluents into Minamata Bay, resulting in the bioaccumulation and biomagnification of mercury in the fish and shellfish eaten by the local population.

Metals are not usually measured on site and samples are collected and taken back to the laboratory to be analyzed using Inductively Coupled Plasma by Optical Emissions Spectrometry.

Figure 8. Water sampler used in lakes and reservoirs.

Limitations to testing for physical and chemical parameters

Monitoring physical and chemical parameters within a water ecosystem only provides a ‘snap shot in time’. They provide information for that specific sample at that particular time and chemical pollution can be quickly washed away. If toxic material is discharged into an aquatic ecosystem it may be dispersed before sampling has occurred. However, the ecosystem may have been severely damaged by the pollution resulting in death of species including fish and loss of biodiversity.

Assessing water quality II

An examination of only physical and chemical parameters is not sufficient to determine if the water is safe to use.Pathogens may be present in the water that are not detected by physical or chemical test parameters. In addition pollutants may be washed away before sampling has occurred, even though they may have severely damaged the ecosystem.

Biological monitoring

Together with chemical monitoring, biological monitoring is frequently used. Biological organisms can indicate whether the water quality has declined and whether there have been episodes of pollution between periods of sampling.

If individual indicator species are used, we need to understand their ecology and which types of pollution effect their population and growth. The common mussel, Mytilus edilus has been used widely in marine waters as it bioaccumulates many pollutants from its environment. In the US the Mussel Watch Program has been running since 1986 to assess coastal contamination and monitor for a wide range of pollutants including DDT and PCBs.

Figure 1. Mussels can be used to monitor pollution.

More commonly used are communities of species. Use of macro-invertebrates is very popular as they are:

• Stationary and therefore representative of the environment they are found in.
• Have a relative long life history providing a summary of conditions over time.
• Abundant
• Relatively easy to sample.
• Consists of diverse groups of organism which increases the chances of at least one species or group reacting to the pollutant.

Macro-invertebrate samples can be taken from shallow waters using a hand-net by a process called kick-sampling in which the bottom of the stream is agitated by kicking the riverbed and the organisms are swept into the net for analysis.

In deeper waters, sediments containing organisms from the river, lake or seabed may be taken. The animals are then isolated and identified.

Figure 2. Kick-sampling.

Disadvantages include:

• Potential problem of organisms drifting into the area and being accidentally sampled.
• The difficulty of identifying some species.
• Absence of some species due to natural life cycle.

Biotic index

Biotic indices are used to determine water quality using aquatic organism. Different biotic indices are used around the world and are determined by species that are specific to the region. The underlying principles are:

• The presence of a wide range of species (i.e. high biodiversity) indicates there is little if any pollution; whereas if there is a reduction in species diversity or a low diversity it would suggest pollution has occurred.
• Some species are more sensitive to pollution whereas other species are more tolerant of pollution. They are referred to as indicator species and reflect specific conditions.

In freshwaters marco-invertebrate groups can be broadly divided into the following categories:

• Highly sensitive to oxygen depletion e.g. damselfly nymphs and freshwater mussels require over 5mg/l of oxygen levels. Their presence indicates good to excellent quality water.
• Moderately sensitive to oxygen depletion e.g. water beetles, freshwater shrimp and freshwater snails.
• Little sensitivity to oxygen depletion e.g. mosquito larvae and hoverfly maggots. If only these species are found in the water, it would indicate poor quality water that is highly polluted.

Figure 3. Invertebrates used as indicators of water quality. E.g. presence of stonefly larvae and mayfly nymphs indicates good water quality whereas high abundance of bloodworms and leeches indicates poor water quality.

The following video ‘Biotic index’ by Wisconsin University Extension demonstrates how biotic index can be measured including sampling technique, identification of macroinvertebrate groups and calculation of biotic index score:

Microbial test

Additionally if the water is to be used for recreation or drinking purposes, it will be tested for pathogens of faecal origin. Indicator species such as Escherichia coli and Faecal Streptococci are used which have the following characteristics:

• Occur when pathogens are present. Hence they are absent in unpolluted water.
• Occur in numbers greater than pathogens to allow for reliable isolation.
• Easy to isolate, enumerate and identify using conventional microbiological methods.

Disadvantage of using bacteria as indicators are:

• It is an expensive process requiring a large amount of equipment.
• Sterile laboratory conditions are necessary.
• There is a time delay in growing bacteria colonies prior to identification (e.g. incubation period of 48 hours).

Figure 4. Agar plate used to grow E.coli is inoculated with water sample.

Theory of Knowledge

There are many different test that can be used to assess water quality. How can you determine the reliability of the knowledge these tests are based on?

International-mindedness

To improve access to safe drinking water, monitoring systems are necessary. International cooperation and sharing of knowledge has allowed development of some cheap and efficient methods of water monitoring that can be adapted to be used in different environments.

Water pollution management I

As discussed in Humans and pollution, pollution can be managed at various levels:

• Stopping human activities that lead to pollution e.g. through education, legislation and economic measures the demand for products that produce pollutants can be prevented.
• The amount of pollutant used and released into the environment can be controlled e.g. through legislation and treatment processes.
• Pollutants can be removed from the environment and damaged systems can be restored e.g. clean up methods removing the pollutants and reintroducing aquatic plants and animals.

Some examples of ways of managing water pollution from domestic sewage effluent and industrial effluent are discussed below.

Domestic sewage effluent

Sewage effluent can contaminate groundwater, surface waters and coastal waters with pathogens, organic waste and nutrients. This can cause the spread of disease such as cholera, typhoid and polio. Waters can become anoxic as the organic material breaks down and nutrients can lead to eutrophication.

Figure 1. Extensive networks of underground sewers are often used to collect and transport sewage to wastewater treatment works.

Collection and treatment of sewage effluent can be used to reduce its impact on the environment. Sewage effluent can be routed away from sensitive areas e.g. groundwater and waters that are sensitive to eutrophication. The effluent can be treated to breakdown the organic material and therefore reduce the BOD and also the amount of suspended solids. During treatment the amount of a nutrients and pathogens are reduced. The level of sewage effluent treatment can vary and will typically dependent on:

• Amount of money and resources available to build collection systems and sewage treatment works.
• Legislation determining minimum quality of effluent discharged into receiving waters.
• Population size i.e. amount of effluent produced.
• Amount of dilution and dispersion within receiving waters. E.g. effluents discharged into coastal turbulent waters will have greater dilution and dispersal compared to discharges entering an inland enclosed lake system. Hence discharges into coastal levels may receive a lower level of treatment than discharges into inland waters without a significant increase in risk of detrimental effects.

Levels of sewage treatment can involve various processes.

Preliminary treatment which involves: (i) screens to remove large objects that may otherwise damage the mechanical equipment or cause blockages and (ii) grit removal to prevent abrasion and wear of equipment and deposition in pipes and channels.

Primary treatment in which the piped sewage is allowed to settle within primary sedimentation tanks during which time any settlable solids are removed. This reduces the suspended solids and BOD levels.

Figure 2. Primary treatment involving sedimentation tanks that relies on physical process of particulates in the sewage precipitating out.

Figure 3. Aerial view of primary sedimentation tanks.

Secondary treatment is a biological process in which micro-organisms use the unsettled organic waste as a food source. If a process called activated sludge is used, the mixture is constantly aerated (using paddles which physically agitate the water or direct pumping of air into the water) to encourage aerobic decomposition of the organic matter. This further reduces the levels of suspended solids and BOD.

Figure 4. Secondary treatment involving biological breakdown of organic material present.

Tertiary treatment is less common than primary and secondary treatment. It can involve a variety of different processes. For example:

• Nitrate removal involves biological processes in which ammonium ions are oxidised to nitrates and then using denitrifying bacteria the nitrates are converted to nitrogen gas which can be lost to the atmosphere.
  • Ammonium → nitrites → nitrates (nitrosomonas bacteria and nitrobacter).
  • Nitrates → nitrogen gas (denitrifying bacteria).
• Phosphate removal involves use of chemicals such as iron and aluminium salts which react with the phosphates and precipitate it out.
• Macrophyte beds can be used to treat effluent from primary of secondary treatment. The effluent is passed through the beds of growing macrophytes e.g. Phragmites australis to remove suspended solids, nitrates, phosphates, metals and pathogens.

Figure 5. Phragmites australis - common reed.

• Disinfection can be used to kill pathogens in the effluent prior to discharge. Ultra-violet light is often favoured over the use of chlorine and ozone due to less by-products.

Once effluent is discharged, DO2 levels within the receiving waters can be increased by use of weirs, steps or waterfalls to aerate the water as it flows.

Figure 6. Weirs help to aerate the water and increase oxygen levels.

Within the European Union the level of required treatment is determined by population size within the area and classification of receiving waters. The general assumption is that domestic sewage effluent should receive secondary treatment and more stringent treatment (e.g. nitrate and phosphate removal) if discharged to sensitive waters.

Watch the following video ‘The Sewage Treatment Process’ by Severn Trent Water which provides an overview and considers biological filters and activated sludge as secondary treatment processes:

Industrial discharge

Pollution from industrial discharge can involve replacing the chemical causing pollution with an alternative. For example, PCBs within electrical transformers have now been replaced with silicone and mineral oils. Under the Stockholm Convention on Persistent Organic Pollutants and PCBs the production and use of PCBs is banned. More information about PCBs can be found at this website.

The amount of pollutant discharged into the environment can be controlled through legislation. Consent licenses providing permission to release effluent into surface or coastal water are usually required. These often have specific requirements in terms of quantity and quality of the discharge including maximum levels of potential pollutants. Hence, companies may need to adopt onsite treatment processes to reduce the amount of pollutants within their wastewater.

Figure 7. Large amounts of industrial wastewater may be released into aquatic ecosystems.

In some industries, materials such as metals may be extracted from the waste stream by chemical or biological processes and either reused or recycled. This has the additional advantages of reducing the amount of resources used and waste generated; each with associated reduced costs.

The following video ‘Industrial wastewater treatment plant’ explains how heavy metals can be removed from industrial effluent:

Food and beverage industries have effluent that is organic in nature. These industries often employ the same or similar processes to that used at sewage treatment plants to meet the standards set for effluent discharged e.g. within the European Union the guideline is 25mg/l for BOD5 and 35mg/l for suspended solids.

If factories do pollute waters, they may need to pay for the associated clean up, habitat restoration and compensation to communities adversely affected by the pollution. This will dependent on national and international legislation and the level of policing and enforcement.

Water pollution management II

Following the previous section that considered ways of managing water pollution from domestic sewage effluent and industrial discharge, here we consider the management of water pollution from agricultural runoff and eutrophication.

Agricultural run-off

Appropriate farm management can reduce the amount of pesticides, fertilizers and organic matter causing water pollution.

Pesticides

The amount of pesticide entering aquatic systems used can be reduced by:

• Using alternative approaches to reduce pest such as biological control i.e. the use of natural predators.
• Only applying pesticides when and where required rather than blanket spraying on a regular basis.
• Using biodegradable pesticides which do not bioaccumulate or biomagnify through the food chain.
• Using pesticides that are target specific and do not harm other species (non-target species). Some pesticides have been banned due to their toxicity on non-target organisms such as phytoplankton and fish.
• Storing pesticides in impermeable containers and within areas that can contain any accidental spill e.g. with bunds.

Figure 1. Ladybirds feed on black aphids.

Fertilizers

The amount of nutrients from fertilizers entering aquatic systems can be reduced by:

• Replacing soluble nitrate fertilizers with ammonium fertilizers.
• Using organic fertilizers that release nitrates more slowly than most artificial fertilizers.
• Only applying fertilizers where and when required by plant growth.
• Only applying fertilizers at the rate necessary for plant growth.
• Only applying fertilizers during dry weather to avoid it being washed away into nearby water systems.
• Not applying fertilizers near any aquatic systems.

Watch the following video on ‘Nutrient run-off’ by Jim Toomey on the effects of nutrient run-off on marine ecosystems and what action can be taken to reduce the problem:

Organic waste

The amount of slurry, manure and silage effluent entering aquatic systems can be reduced by:

• Avoiding spreading this organic and nutrient rich matter (when used as fertilizers) near water courses.
• Only applying this matter as a fertilizer to land during dry weather.
• Ensuring the slurry, manure and silage is contained and run-off collected and treated prior to discharge into the water body.

Figure 2. Application of slurry on a farm.

Run-off

Reducing the amount of run-off can decrease the amount of pollution entering water systems from agriculture land. Some of these techniques include:

• Reducing the amount of water used by employing more efficient irrigation systems.
• Use of contours and terraces that impede the flow of the water and potential pollutants.
• Planting cover crops to intercept the rain and reduce run-off.
• Using buffer zones to remove pollutants from agricultural run-off before it enters nearby aquatic ecosystems. Buffer zones are areas of vegetation that intercepts the run-off. It helps to improve water quality by trapping sediments, organic matter, nutrients, pathogen and pesticides. In addition buffer zones contribute to preventing soil erosion and provide a habitat for wildlife.

Figure 3. Buffer zone – vegetation that intercepts run-off

Managing eutrophication

As discussed previously eutrophication of waters is often caused by domestic or industrial effluent discharges and run-off from farms. Action to reduce nutrients which cause eutrophication entering aquatic ecosystems includes:

• Substituting phosphates in detergents with an alternative such as Zeolite A.
• Removing nitrates and phosphates from sewage effluent (as discussed above this is typically part of tertiary treatment of sewage effluent).
• Diverting sewage effluent away from water systems that are vulnerable to eutrophication (e.g. may have low degree of dilution and dispersal properties).
• More efficient use of fertilizers and appropriate methods of dealing with animal manure, slurry and silage effluent (as discussed above).
• Using buffer zones to intercept runoff and absorb the nutrients.
• Restricting access of livestock to aquatic ecosystems

Figure 4. Cattle can pollute aquatic ecosystems and should be kept away from drinking water supplies.

Once the nutrients have entered the waters, the following approaches can be taken:

• Use macrophyte channels to absorb the nutrients from the water. The macrophytes would need to be harvested to prevent nutrients re-entering the water. In some regions, macrophyte growth is seasonal and therefore its use is limited to certain times of the year.
• Mix the water to aerate it and prevent anoxic conditions that will kill many aquatic organisms.
• Dredge the bottom to remove sediments that contain nutrients and enhance eutrophication.
• Use herbicides to control algal blooms, although this could be problematic if the aquatic system is a source of drinking water.
• Mechanically remove the macrophytes and use e.g. as a fertilizer on land.
• Use biological control e.g. fish such as Tilapia that feed off the algal bloom. However, the introduction of non-native species may cause changes in the community composition and threaten other species.
• Following action to remove nutrients and algal blooms reintroduce native species back into the aquatic ecosystem.

Figure 5. Dredging of sediments to reduce nutrient levels.

Theory of Knowledge

How do cultural and historical values influence the management choices we make?