Where Can Freshwater Be Found?
Only a tiny fraction of the planet’s abundant water is available to us as freshwater. About 97.4% by volume is found in the oceans and is too salty for drinking, irrigation, or industry (except as a coolant) unless it is purified with Reverse Osmosis first to eliminate salts.
Most of the remaining 2.6% water is freshwater and locked up in ice layers or glaciers or it’s too deep underground to be reached, or too salty to be used.
Thus, only about 0.014% of the earth’s total volume of water is easily available to us as soil moisture, usable groundwater, water vapor, and lakes and streams.
Three countries—the United States, Germany, and Russia—with only 8% of the world’s population consume about 75% of the world’s most widely used metals. The United States, with 4.5% of the world’s population, uses about 20% of the world's metal population and 25% of the fossil fuels produced each year.
What Causes Pollutants In Freshwater Ecosystems
There have been listed some 1500 substances as pollutants in freshwater ecosystems, and each of them occurs in the following types of freshwater pollutants. Here is a generalized list of them:
Acids & alkalis
Anions
Detergents
Domestic sewage and farm manures
Food processing wastes (including processes taking place on the farm)
Gases (e.g. chlorine, ammonia)
Heat
Metals (e.g. cadmium, lead, mercury)
Nutrients (especially phosphates, nitrates)
Oil and oil dispersants
Organic toxic wastes (e.g. formaldehyde, phenols)
Pathogens
Pesticides
Polychlorinated biphenyls
Radionuclides
The different pollutants exert different problems to different freshwater waterways. Mostly expressed in the amount of oxygen that is available for fish and other species. This sometimes results in habitat destruction and extinction of local populations.
How Metals Get Into Freshwater
Metals are introduced in aquatic systems as a result of the weathering of soils and rocks, from volcanic eruptions, and from a variety of human activities involving the mining, processing, or use of metals and/or substances that contain metal pollutants.
The most common heavy metal pollutants are arsenic, cadmium, chromium, copper, nickel, lead and mercury. There are different types of sources of pollutants: point sources (localized pollution), where pollutants come from single, identifiable sources.
The second type of pollutant sources are nonpoint sources, where pollutants come from dispersed (and often difficult to identify) sources. There are only a few examples of localized metal pollution, like the natural weathering of ore bodies and the little metal particles coming from coal-burning power plants via smokestacks in air, water, and soils around the factory.
The most common metal pollution in freshwater comes from mining companies. They usually use an acid mine drainage system to release heavy metals from ores, because metals are very soluble in an acid solution. After the drainage process, they disperse the acid solution in the groundwater, containing high levels of metals. See also acids & alkalis.
What Happens When An Excess Of Metals Enters Freshwater Ecosystems?
When the pH in water falls, metal solubility increases and the metal particles become more mobile. That is why metals are more toxic in soft waters. Metals can become ‘locked up’ in bottom sediments, where they remain for many years. Streams coming from draining mining areas are often very acidic and contain high concentrations of dissolved metals with little aquatic life. Both localized and dispersed metal pollution cause environmental damage because metals are non-biodegradable.
Unlike some organic pesticides, metals cannot be broken down into less harmful components in the environment.
Campbell and Stokes (1985) described two contrasting responses of an organism to metal toxicity with declining pH:
- If there is little change in speciation and the metal binding is weak at the biological surface, a decrease in pH will decrease owning to competition for binding sites from hydrogen ions.
- Where there is a marked effect on speciation and strong binding of the metal at the biological surface, the dominant effect of a decrease in pH will be to increase the metal availability.
Generally, the ionic form of metal is more toxic because it can form toxic compounds with other ions. Electron transfer reactions that are connected with oxygen can lead to the production of toxic oxyradicals, a toxicity mechanism now known to be of considerable importance in both animals and plants. Some oxyradicals, such as superoxide anion (O2-) and the hydroxyl radical (OH-), can cause serious cellular damage.
Some inorganic pollutants are assimilated by organisms to a greater extent than others. This is reflected in the Bioconcentration Factor (BCF), which can be expressed as follows:
BCF = concentration of the chemical in the organism/concentration of the chemical in the ambient environment.
The ambient environment for aquatic organisms is usually water or sediments. With inorganic chemicals, the extent of long-term bioaccumulation depends on the rate of excretion. Toxic chemicals can be stored into tissues of species, especially fat tissues. Bioaccumulation of cadmium in animals is high compared to most of the other metals, as it is assimilated rapidly and excreted slowly. Also, the sensitivity of individuals of a particular species to a pollutant may be influenced by factors such as sex, age, or size. In general, the concentrations of metals in invertebrates is inversely related to their body mass. In fish, the embryonic and larval stages are usually the most sensitive to pollutants.
Benthic organisms are likely to be the most directly affected by metal concentrations in the sediments because the benthos is the ultimate repository of the particulate materials that are washed into aquatic systems.
Metal tolerance
Some metals, such as manganese, iron, copper, and zinc are essential micronutrients. They are essential to life in the right concentrations, but in excess, these chemicals can be poisonous. At the same time, chronic low exposures to heavy metals can have serious health effects in the long run.
Tolerance to metals has also been recorded in invertebrates and in fish. After exposure for 24 hours to a copper concentration of 0.55 mg/l, rainbow trout showed a 55 percent inhibition of sodium uptake and a 4 percent reduction in affinity for sodium, which resulted in an overall decrease in total sodium concentration of sulphydryl-rich protein (Lauren and McDonald 1987a,b). The protein was considered to be metallothionein. These low molecular weight proteins contain many sulfur-rich amino acids which bind and detoxify some metals. The pretreatment of an organism with low doses of metal may stimulate metallothionein synthesis and provide tolerance during a subsequent exposure (Pascoe and Beattie, 1979).
Many rivers are polluted with heavy metals from old mine workings and some species of algae become very tolerant to polluted conditions. A survey of 47 sites with different concentration of zinc found the filamentous green alga 'Hormidium rivulare' to be abundant everywhere, tolerating zinc concentrations as high as 30.2 mg Zn/l.
Toxicity Of Metals
For the protection of human health, the maximum permissible concentrations for metals in natural waters that are recommended by the Environmental Protection Agency (EPA), are listed below:
Maximum Permissible Concentrations (MPC) of Various Metals in Natural Waters For the Protection of Human Health
| Metal | Chemical Symbol | mg m-3 |
|
Mercury |
Hg |
0.144 |
|
Lead |
Pb |
5 |
|
Cadmium |
Cd |
10 |
|
Selenium |
Se |
10 |
|
Thallium |
Tl |
13 |
|
Nickel |
Ni |
13.4 |
|
Silver |
Ag |
50 |
|
Manganese |
Mn |
50 |
|
Chromium |
Cr |
50 |
|
Iron |
Fe |
300 |
|
Barium |
Ba |
1000 |
Source: EPA (1987); Federal Register 56 (110): 26460-26564 (1991).
This table gives an idea of the relative toxicity of various metals. Mercury, lead and cadmium are not required even in small amounts by any organism.
Because metals are rather insoluble in neutral or basic pH, pHs of 7 or above give a highly misleading picture of the degree of metal pollution. So in some cases, it may underestimate significantly the total of metal concentrations in natural waters.
Definitions According To Freshwater Pollution
Abiotic: nonliving. Compare biotic.
Acid deposition: the falling of acids and acid-forming compounds from the atmosphere to the earth’s surface. Acid deposition is commonly known as acid rain, a term that refers only to wet deposition of droplets of acids and acid-forming compounds.
Acid solution:Â any water solution that has more hydrogen ions (H+) than hydroxide ions (OH-); any water solution with a pH less than 7.
Acute toxicity:Â the short-term toxicity of a product in a single dose. Can be divided into oral, cutaneous, and respiratory toxicities.Â
Adsorption:Â adsorption is a surface phenomenon that some products exhibit, whereby they form a physicochemical bond with substances. Not to be confused with absorption.
Aerobic respiration: a complex process that occurs in the cells of most living organisms, in which nutrient organic molecules such as glucose (C6H12O6) combine with oxygen (O2) and produce carbon dioxide (CO2), water (H2O), and energy. Compare photosynthesis.
Alkaline:Â sometimes water or soils contain an amount of alkali (strongly basic) substances sufficient to raise the pH value above 7.0 and be harmful to the growth of crops.
Ammonia: a pungent colorless gaseous compound of nitrogen and hydrogen that is very soluble in water and can easily be condensed into a liquid by cold and pressure. Ammonia reacts with NOx to form ammonium nitrate.
Anion:Â atom that bear a net negative charge, because it contains more electrons than protons; a negatively charged ion.
Artificial eutrophication: Cultural eutrophication occurs when human water pollution speeds up the aging process by introducing sewage, detergents, fertilizers, and other nutrient sources into the ecosystem.
Benthic:Â located on the bottom of a water body or in the bottom sediments, or pertaining to bottom-dwelling organisms
Bioaccumulation:Â an increase of a chemical in specific organs or tissues at a higher level than would normally be expected.Â
Bioconcentration:Â the increase in concentration of a chemical in an organism resulting from tissue absorption levels exceeding the rate of metabolism and excretion.
Bioconcentration Factor (BCF):Â the ratio of chemical concentration in the organism to that in surrounding water. Used to describe the accumulation of chemicals in organisms, primarily aquatic, that live in contaminated environments.
Biodegradability:Â the susceptibility of a substance to be decomposed by microorganisms; specifically, the rate at which bacteria and/or natural environmental factors may chemically break down compounds.
Biodegradable:Â a product in wastewater is biodegradable if it can easily be broken down or digested by, for example, sewage treatment.
Biodiversity:Â the variety and variability of living organisms and the ecosystems in which they occur. Biodiversity includes the number of different items and their relative frequencies; these items are organized at many levels, ranging from complete ecosystems to the biochemical structures that are the molecular basis of heredity. Thus, biodiversity encompasses expressions of the relative abundances of different ecosystems, species, and genes.
Biocide:Â an additive that destroys biological contaminants.
Biomagnification: increase of concentration of DDT, PCB’s, and other slowly degradable, fat-soluble chemicals in organisms at successively higher trophic levels of a food chain or web.Â
Biomarker:Â any parameter that can be used to measure an interaction between a biological system and an environment agent, which may be chemical, physical or biological (WHO 1993).
Biomonitoring:Â the use of living organisms to test the suitability of an effluent to be discharged into receiving waters and to test the quality of such waters downstream from a discharge.
Biotic: living organisms make up the biotic parts of ecosystems. Compare abiotic.
BOD:Â Biochemical Oxygen Demand, the amount of oxygen consumed by bacteria and other microorganisms.
Buffer:Â a solution that minimizes changes in hydrogen ion concentration that would otherwise occur as a result of a chemical reaction.
Buffering agent:Â drives an acidic or alkaline solution to neutral.
Chemosynthesis: process in which certain organisms (mostly specialized bacteria) extract inorganic compounds from their environment and convert them into organic nutrient compounds without the presence of sunlight. Compare photosynthesis.
Chronic toxicity:Â the long-term toxicity of a product in small, repeated doses. Chronic toxicity can often take many years to determine.
COD:Â Chemical Oxygen Demand, a test that measures the potential capacity for a microorganism to react with oxygen.
Compound:Â combination of atoms, or oppositely charged ions, of two or more different elements held together by attractive forces called chemical bonds.
Cultural (or artificial) eutrophication:Â eutrophication of lakes caused by humans.
Degradable:Â that which can be reduced, broken down or chemically separated.
Desalinization:Â the removal of salts from saline water to produce freshwater. This method is becoming a popular way of providing freshwater to populations.
Discharge:Â the volume of water that passes a given location within a given period of time. Usually expressed in cubic feet per second.
Dissolved oxygen (DO): the oxygen dissolved in sewage, water, or other liquid, usually expressed in milligrams per liter or percent of saturation. It is the test used in BOD determination.
Dissolved solids:Â the total amount of dissolved material, organic and inorganic, contained in water or wastewater. Excessive dissolved solids make water unpalatable for drinking and unsuitable for industrial use. Measurements are expressed as ppm or mg/L.
Domestic water use:Â water used for household purposes, such as drinking, food preparation, bathing, washing clothes, dishes, and dogs, flushing toilets, and watering lawns and gardens. About 85% of domestic water is delivered to homes by a public-supply facility, such as a country water department. About 15% of the Nation's population supply their own water, mainly from wells.
Ecological niche: total way of life or role of a species in an ecosystem. It includes all physical, chemical, and biological conditions a species needs to live and reproduce in an ecosystem. See fundamental niche, realized niche.
Ecosystem:Â a self-contained interacting community of organisms, considered together with the environment in which these organisms live and react.
Effluent:Â a liquid that has passed through a processing operation.
Endocrine disruptors:Â substances that stop the production or block the transmission of hormones in the body.
Epilimnion:Â an upper layer of warm water with high levels of dissolved oxygen.
Eutrophication:Â the enrichment of waters by inorganic plant nutrients. These nutrients are especially nitrogen and phosphorus and are a result of an increase in nutrients due to human activities.
Freshwater:Â water that contains less than 1,000 milligrams per liter (mg/L) of dissolved solids; generally, more than 500 mg/L of dissolved solids is undesirable for drinking and many industrial uses.
Fundamental niche: the full potential range of the physical, chemical, and the biological factors a species can use if there is no competition from other species. See ecological niche. Compare with realized niche.
Fungicide:Â chemical that kills fungi.
Habitat: place or type where an organism or the organisms of a population lives. Compare with ecological niche.
Heavy metals:Â a general term given to the ions of metallic elements such as copper, zinc, chromium, and aluminum. They are removed from wastewater by forming an insoluble precipitate (usually a metallic hydroxide).
Herbicide:Â chemical that kills a plant or inhibits its growth.
Influent:Â sewage, water or other liquid, either raw or partly treated, flowing into a reservoir basin, or treatment plant or any part thereof.
Inland wetlands:Â lands covered with water all or part of the time (excluding lakes, reservoirs and streams) and located away from coastal areas.
Insecticide:Â chemical that kills insects.
Intercropping: growing two or more different crops at the same time at a plot. For example, a carbohydrate-rich grain that deplets soil nitrogen and a protein-rich legume that adds nitrogen to the soil may be intercropped. Compare with monoculture, polyculture, polyvarietal cultivation.
LC50:Â the concentration of a material in air that will kill 50 per cent of a group of test animals with a single exposure (usually 1 to 4 hours). The LC50Â is expressed as parts of material per million parts of air, by volume (ppm) for gases and vapors, or as micrograms of material per liter of air (g/l) or miligrams of material per cubic meter of air (mg/m3) for dusts and mists, as well as for gases and vapors.
LD50:Â a single dose of a material expected to kill 50 per cent of a group of test animals. The LD50Â dose is usually expressed as miligrams or grams of material per kilogram of animal body weight (mg/kg or g/kg). The material may be administered by the mouth or applied to the skin.
Lentic:Â pertaining to standing (not flowing) waters such as lakes, reservoirs, ponds, and swamps.
Lotic:Â flowing waters, including creeks, streams, and rivers.
Mesotrophic: lake that falls between the two extremes of nutrient enrichment level (oligotrophic and eutrophic). Also described as lakes with intermediate characteristics.
Metal: an element which has a characteristic lustrous appearance, is a good conductor of electricity and generally enters chemical reactions as a positive ion or cation. (E.g. nickel, copper, cobalt, zinc, cadmium, lead, mercury)
Monoculture: cultivation of a single crop, usually on a large area of land. Compare with polyculture, polyvarietal cultivation.
Natural eutrophication:Â results from an increase caused by a nonhuman
Nonpoint source: large or dispersed land areas such as cropfields, streets, and lawns that discharge pollutants into the environment over a large area. Compare with point source.
Nutrients:Â materials that are considered essential to the support of biological life.
Oligotrophic:Â lentic water body with a small supply of plant nutrients and having a large amount of dissolved oxygen throughout.
Ore:Â part of a metal-yielding material that can be economically and legally extracted at a given time. An ore typically contains two parts: the ore mineral, which contains the desired metal, and waste mineral material (gangue).
Oxyradical:Â an unstable form of oxygen-containing an unpaired electron.
Pathogen:Â an organism that produces disease.
Pest:Â an unwanted organism that directly or indirectly interferes with human activities.
Pesticide: any chemical designed to kill or inhibit the growth of an organism that people consider to be undesirable. See fungicide, herbicide, and insecticide.
pH:Â positive Hydrogen ion concentration. The pH scale is used to express the concentration of hydrogen ions in a liquid. Solutions with a pH less than 7 are acid, and solutions with a pH greater than 7 are alkaline. Thus, the lower the pH, the greater the acidity of the solution. Each whole number change on the pH scale represents a tenfold change in the concentration hydrogen ions in a water solution.
Photosynthesis: complex process that takes place in cells of green plants. Radiant energy from the sun is used to combine carbon dioxide (CO2) and water (H2O) to produce oxygen (O2) and carbohydrates (such as glucose, C6H12C6) and other nutrient molecules. Compare aerobic respiration, chemosynthesis.
Point source: single identifiable source that discharges pollutants into the environment. Examples are the smokestack of a power plant or an industrial, drainpipe of a meatpacking plant, chimney of a house, or exhaust pipe of an automobile. Compare with nonpoint source.
Pollution:Â the introduction by man into the environment of substances or energy liable to cause hazards to human health, harm to living resources and ecological systems, damage to structure or amenity, or interference with legitimate uses of the environment.
Polychlorinated biphenyls (PCB’s): group of 209 different toxic, oily, synthetic chlorinated hydrocarbon compounds that can be biologically amplified in food chains and webs.
Polyculture: complex form of intercropping in which a large number of different plants maturing at different times are planted together. See also intercropping. Compare monoculture, polyvarietal cultivation.
Polyvarietal cultivation: planting a plot of land with several varieties of the same crop. Compare intercropping, monoculture, polyculture.
Realized niche: parts of the fundamental niche of a species that are actually used by that species. See ecological niche, fundamental niche.
Sewage:Â the total of organic waste and wastewater generated by residential and commercial establishments.
Speciation:Â the creation of a species through the splitting of one species into two or more, through descent.
Surfactant:Â a surface-active substance, such as a detergent or soap, that lowers the surface tension of a solvent (usually water).
Thermal pollution:Â harm to lakes and rivers resulting from the release of excessive waste heat into them.
Thermocline:Â zone of gradual temperature decrease between warm surface water and colder deep water in a lake, reservoir or ocean.
Toxic substances/material:Â chemical compounds that are poisonous, carcinogenic, or otherwise directly harmful to plants and animals.
Water pollution:Â any physical or chemical change in surface water or groundwater that can harm living organisms or make water unfit for certain uses.