Water pollution • Water pollution almost always means that some damage has been done to an ocean, river, lake, or other water source. • Surface water bodies are often stratified in that they have layers composed of different • temperatures and densities & • with different chemical compositions • Pollutant release into a body of water may or may not uniformly contaminate all the water within the lake.
Effect of water pollution • Terrestrial and aquatic life forms, are sensitive to changes in the quality of the fresh water supply. • Changes in properties such as total dissolved solids, pH, and dissolved oxygen may affect the mortality of aquatic life. • In addition, the presence of contaminants can be hazardous to organisms.
Mechanisms in water pollution Diffusion is the result of the tendency of molecules to move from high to low concentration zones. Advection is the result of water moving in response to gravity or pressure forces. • The aquatic environment is especially vulnerable to pollutants. • Two mechanisms are primarily responsible for the transport and subsequent contamination of water bodies
Water pollution • Water pollution is all about quantities: • how much of a polluting substance is released and • how big a volume of water it is released into. • A small quantity of a toxic chemical may have little impact if it is spilled into the ocean from a ship. BUT • the same amount of the same chemical can have a much bigger impact pumped into a lake or river, where there is less clean water to disperse it.
Water pollutants Water pollutants are categorised as point sourceor non pointsources. i) Point source is identified as all dry weather pollutants that enter watercourses through pipes or channels. ii) Storm drainage, even though the water may enter water courses by way of pipes or channels, is considered nonpoint sourcepollution.
Non point sources • Runoff due to rainfall or melting snow that dissolves and transports pollutants as it moves. • Industrial diffuse or fugitive releases and leaks. • Accidental spills. • Atmospheric wet and dry deposition. • Local air contamination sources (e.g., forest fires and highly transited highways or streets)
Point sources • It comes mainly from industrial facilities and municipal wastewater treatment plants. • The range of pollutants is vast, depending on what gets ‘thrown down the drain’. • When point-source pollution enters the environment, the place most affected is usually the area immediately around the source. E.g when a tanker accident occurs, the oil slick is concentrated around the tanker itself
Causes of water pollution • Most water pollution doesn't begin in the water itself. • About 80 percent of ocean pollution enters our seas from the land. • Virtually any human activity can have an effect on the quality of our water environment. E.gWhen farmers fertilize the fields, the chemicals they use are gradually washed by rain into the groundwater or surface waters nearby. • Water pollution has many different causes and this is one of the reasons why it is such a difficult problem to solve.
Sewage • With billions of people on the planet, disposing of sewage waste is a major problem. • Sewage disposal affects people's immediate environments and leads to water-related illnesses such as diarrhoea. • Sometimes sewage waste is pumped untreated into the sea. • 90 percent of sewage is water but it contains all kinds of other chemicals
Waste water • Around half of all ocean pollution is caused by sewage and waste water. • Each year, the world generates 400 billion tons of industrial waste, much of which is pumped untreated into rivers, oceans, and other waterways. • Factories are point sources of water pollution, but a lot of water is polluted by ordinary people from nonpoint sources.
Waste water • Detergents and pesticides eventually end up in rivers and oceans. • A lot of toxic pollution also enters waste water from highway runoff. • Some highway runoff • runs away into drains • others can pollute groundwater or • accumulate in the land next to a road making it increasingly toxic as the years go by.
Chemical waste • Detergents are relatively mild substances. • At the opposite end of the spectrum are highly toxic chemicals such as polychlorinated biphenyls (PCBs). • An estimated half million tons of PCBs were discharged into the environment during the 20th C. • Although PCBs are widely banned, their effects will be felt for many decades because they last a long time in the environment without breaking down.
Heavy metals pollution • Pb was once commonly used in petrol, Hg and Cd are still used in batteries. • Minamana Bay- Discharged Hg contaminated fish stocks • Hundreds of people were left dead or disabled • The presence of metal ions in water can bring about side effects such as bad taste, increase in turbidity and colouration, staining of fixtures in contact with water.
Oil and grease spills • Oil spills represent an important hydrocarbon input into oceans, surface water, and groundwater. • They are produced by accidents, E.g • ship, train, or truck wreckages, • lack of maintenance (e.g., corroded pipelines), • neglect (e.g. car oil changes that end up in the sewage). • Generally, the oil will spread in the bodies of water in dissolved, film, emulsion, or sorbed fractions.
Oil and grease spills • The lighter fractions (VOCs) will tend to volatilize, & the most persistent will be the heavy high-molecular weight PAHs. • Besides physically affecting aquatic organisms, many of these substances are toxic and carcinogenic. • Formation of oil emulsions can block the entrance of light into bodies of water, with the concomitant decrease in biological activity and cleansing action.
Plastics • They are light and float easily so they can travel enormous distances across the oceans; Most plastics are not biodegradable which means they can survive in the marine environment for long. While plastics are not toxic in the same way as poisonous chemicals, they present a major hazard to seabirds, fish, and other marine creatures. E.g plastic fishing lines and other debris can strangle or choke fish.
Alien species • Alien species (invasive species) are animals or plants from one region that have been introduced into a different ecosystem where they do not belong. • Outside their normal environment, they have no natural predators, so they rapidly run wild, crowding out the usual animals or plants that thrive there. • In the Black Sea, an alien jellyfish reduced fish stocks by 90% after arriving in ballast water. • In Kenya, water hyacinth has invaded fresh water lakes like Victoria and Naivasha.
Other forms of pollution • Disruption of sediments (fine-grained powders) that flow from rivers into the sea is a form of pollution. • Sediment consists mostly of inorganic material washed into a stream as a result of land cultivation, demolition construction and mining operations. • Sediment interferes with fish spawning because it can cover gravel beds and block light penetration, making food harder to find. • The extra sediment can block the gills of fish, effectively suffocating them.
Oxygen demanding substances • Anything that can be oxidised in the receiving water with the consumption of dissolved molecular oxygen is termed oxygen-demanding material. • This usually consists of biodegradable organic matter, but it may also include certain inorganic compounds. • The consumption of dissolved oxygen (DO) poses a threat to fish and other higher forms of aquatic life that require it for respiration.
Oxygen demanding substances • Oxygen-demanding materials in domestic sewage come primarily from human waste and food residues. • Many industries that produce oxygen-demanding wastes are the food processing and paper industries. • Almost any naturally-occurring organic matter such as animal droppings, crop residues contribute to DO depletion. • If oxidation of the compounds in wastewater is carried out by microorganisms, the oxygen consumed is known as the biochemical oxygen demand (BOD).
Suspended solids • Suspended solids also contribute to oxygen depletion and create unsightly conditions. • Wastewater contains a wide variety of suspended solid materials. • Total solids are obtained by evaporating a sample of wastewater to dryness at 103-105°C. • Filtration separates the total suspended solids (TSS), which remain on the filter, from the total dissolved solids (TDS), which pass in the filtrate.
Suspended solids • The residue that remains after the TSare ignited at 500°C is called the total fixed solids (TFS), and they represent the inorganic salts in the sample. • The solids that are volatilized and burned off are the total volatile solids (TVS) and they represent the organic matter, including microorganisms. • Settleable solids are suspended solids that settle under quiescent conditions because of the influence of gravity. • Colloidal particles do not settle readily and cause the turbidity found in many surface waters
Nutrients • N and P are considered pollutants because they are essential nutrients for living organisms to grow. • They must be present in rivers and lakes to support the natural food chain. • When these nutrient levels become excessive, the food web is grossly disturbed, causing some organisms to proliferate at the expense of others. • Excessive nutrient concentrations often lead to eutrophication.
Nutrients • A more productive lake will have a higher fish population, but water quality is reduced as algal death and its decomposition depletes oxygen. • Cultural eutrophication is caused when human activity speeds natural processes by increasing the rate at which sediments and nutrients are added to the lake. • Major sources of nutrients include phosphorus-based detergents, fertilizers, and food-processing wastes.
Pathogenic organisms • Untreated sewage is a major public health hazard because it is a major pathway in the spread of many infectious diseases. • Waste from humans and animals infected with pathogens contains large numbers of these organisms or their eggs. • An infected person or animal may harbour low populations of the pathogen, thus continuing to act as a disease carrier. • If wastes from carriers contaminate drinking, swimming, or bathing water, the pathogens can infect others.
Heat • Heat is also an industrial waste that is discharged into water. • Heated discharges may drastically alter the ecology of a stream or lake. • It lowers the solubility of O2 in the H2O and thereby reducing the amount of dissolved oxygen (DO) available to gill-breathing species. • As the level of DO decreases, metabolic activity of aerobic aquatic species increases, thus increasing oxygen demand.
Acid mine drainage • Acidification of soilwater occurs if the rate of displacement of soil cations by H+ exceeds the rate of cation supply from weathering. • Rainwater is naturally acidic and soilwaters are further acidified by the production of H+ from the decomposition of organic matter. • While acid rain is a widespread phenomenon, acidified freshwaters are less common and are controlled both by rates of atmospheric input and by rock types.
Acid mine drainage • When mineral and coal deposits are mined, sulphide is left behind in the waste rock, which is piled in heaps. • These waste heaps have large surface areas exposed to the atmosphere, allowing extensive and rapid oxidation of the sulphide. • The oxidation reaction is a natural one but mining activities increase the scale and rate of the reaction. • The resulting H2SO4 makes drainage from abandoned mines strongly acidic (pH as low as 1 or 2). • This acidity increases the solubility of aluminium and other metals, causing toxicity in aquatic ecosystems.
Read on reaction that occurs in underground coal and metal (especially copper) mines with reference to FeS2 Further reading