if you do some investigation you can observe the quality of water in ponds or rivers in your school neighborhood. And you have also proved whether the water is poisonous for plants and animals in the neighborhood. Water considered to be ‘good’ for living organisms in the rivers or ponds, may not be “good’ for other living organisms, or it may even cause death.
The poisonous substances coming into the environment is a form of contamination (pollution). Pollutants are wastes that cause pollution. This understanding comes in handy to explain some changes that happen in the environment. But how can you tell what pollutant is? some nitrogen that is decomposed from waste and is essential nitrogen for all organisms, to a certain extent may turn to become pollutant.
Sometimes, it is easy to find things that become pollutant in our ecosystem. For instance, when we see many dead fish in riverbanks, scientists can do a research to check the quality of the water there. They can check the oxygen in the water, acidity (pH), or the poisonous substances that can kill the fish. At other time, a complex chemical analysis is needed to check the water quality. Scientists use a kind of gas chromatography like machine to detect and identify the chemical substances in the ecosystem. The existence of chemical substances in our ecosystem does not indicate the cause of destruction. To distinguish the dangerous chemical substances from the ones which are not, it is necessary that experiments be conducted to determine the toxicities of those substances. Toxicity is a measurement of how much a substance is needed to poison or kill organisms.
The result of the above experiment will be considered when we need to decide whether one substance is a pollutant or not. As an example, cyanide is a toxic substance used in some industrial processes. Waste containing cyanide is very dangerous. An adult will be sick or even die if he/she consumes cyanide more than 300 mg. Naturally, cyanide is also found in apples and other fruits.
Scientists are more worried on the quantity of chemical substances that pollute the ecosystem than their degree of toxicity. Some substances in big amount can cause pollution. Salt (NaCl) is not toxic, but if it presents in a huge quantity in the environment, it can cause serious damage to the ecosystem, infiltration of salty water to fresh water, the increase of salt content in the ground causes some plants difficult to grow in that place. Another damaging chemical substance is pesticide. Pesticide is a chemical substance used to kill pest and microbes that infect plants. The rule of using that substance is developed based on the environmental study. The effect of biotic factors is more difficult to explain. When chemical substances enter into food chain, organisms in the system might keep these substances in their bodies. The substance will accumulate in theor bodies. This process is called biomagnification; some organisms have the ability to keep the toxic substance in their bodies due to their eating ways. Organisms like shellfish and other animals with shells, are potential as food filters.