Environmental contaminants are substances introduced into the environment that can harm ecosystems and human health. These can range from chemicals released by industrial processes and agriculture to waste. Environmental pollution occurs when these contaminants reach concentrations that cause harm to living organisms or the environment. This article explores their movement and changes through environmental compartments and their ultimate fate.
Movement Through Environmental Compartments
Contaminants move through the environment’s interconnected compartments: air, water, and soil. Atmospheric transport involves the movement of airborne pollutants, influenced by wind speed, direction, and particle size. For instance, a polluted air mass from China can travel to the USA in as little as eight days. Pollutants released into the atmosphere can spread due to atmospheric diffusion, with their dispersal rate depending on the source type and release parameters.
In aquatic systems, contaminants move through surface water and groundwater via processes like advection, which is the movement with the bulk flow of water, and dispersion, which involves spreading due to turbulence and mixing. Contaminants can enter surface water from sources such as sewage treatment plant effluent or agricultural runoff. Once in surface water, they can infiltrate the ground, eventually reaching groundwater zones.
Soil infiltration is another pathway, where water carries contaminants from the surface downward through the soil profile. The movement of water and contaminants through soil is influenced by soil properties like porosity and permeability; permeable soils with larger pores allow easier flow and spread of pollutants. This interconnected movement highlights how pollution in one compartment can readily affect others, such as airborne pollutants depositing onto soil or water, or water carrying contaminants from soil into groundwater.
Transformation and Degradation Processes
Once released, contaminants undergo chemical changes and breakdowns within the environment. Photodegradation occurs when sunlight directly transforms contaminants or initiates reactions with other substances to break them down. The effectiveness of this process depends on factors like sunlight intensity, water depth, and the presence of dissolved organic carbon.
Biodegradation, another process, involves microorganisms like bacteria, fungi, and archaea breaking down pollutants. These microbes use metabolic pathways to transform harmful substances into less toxic or harmless compounds, often through oxidation, reduction, and hydrolysis reactions. Microorganisms can utilize pollutants as a carbon source, converting them into carbon dioxide and water. This process is influenced by environmental factors such as oxygen availability, pH, and temperature.
Chemical oxidation and reduction reactions also play a role in altering contaminants. Chemical oxidation involves the transfer of electrons from a contaminant to an oxidizing agent, breaking down the contaminant into simpler forms. Common oxidizing agents used in environmental cleanup include hydrogen peroxide and permanganate. Conversely, chemical reduction can remove elements like chlorine from compounds or stabilize metal pollutants, making them less mobile. Hydrolysis, the reaction of a substance with water, can also break down contaminants, particularly in wastewater treatment, by converting complex compounds into smaller molecules.
Accumulation and Bioaccumulation
Contaminants can build up in environmental reservoirs through a process known as environmental accumulation. This involves the progressive increase of harmful substances in air, water, soil, or sediments over time. For example, plastic marine debris accumulates in oceans, while pesticides can build up in agricultural soil. This occurs when pollutants enter a system faster than they are removed or broken down.
A related process, bioaccumulation, describes how pollutants accumulate in the tissues of individual living organisms. This happens when an organism absorbs a substance faster than it can eliminate it through metabolism or excretion. The longer a toxic substance remains in an organism’s body, the greater the risk of chronic poisoning. This process can occur in both aquatic and terrestrial environments, with aquatic organisms being particularly susceptible due to their constant exposure to contaminants in water and diet.
Biomagnification is the increase in concentration of a substance as it moves up through successive levels of a food chain. As smaller organisms absorb pollutants, and are then consumed by larger predators, the concentration of the contaminant becomes progressively higher in the tissues of organisms at higher trophic levels. For instance, mercury absorbed by plankton can become increasingly concentrated in small fish that eat the plankton, and then even more so in larger fish that consume many smaller fish. Persistent organic pollutants (POPs) are particularly prone to bioaccumulation and biomagnification due to their resistance to degradation and their ability to accumulate in fatty tissues. This can lead to health issues for top predators, including humans, who consume contaminated food sources.
Ultimate Environmental Fate
The ultimate fate of contaminants in the environment varies depending on their chemical properties and the prevailing environmental conditions. Some contaminants are removed from active environmental cycles through processes such as sequestration. For instance, in deep ocean sediments, substances can become permanently bound, especially if they are hydrophobic, reducing their mobility and bioavailability.
Mineralization is a process where organic pollutants are broken down into simpler, harmless inorganic compounds like carbon dioxide and water. Microorganisms often facilitate this process, utilizing the pollutants as a food source. For example, some organic molecules are extensively mineralized during composting.
Natural attenuation refers to the use of natural processes to reduce the concentration and toxicity of contaminants. This can involve biodegradation by microbes, sorption where contaminants stick to soil particles, dilution as pollutants mix with clean water, and evaporation. Human interventions, such as remediation efforts, can also manage or alter the ultimate fate of pollutants. These include bioremediation, which uses microorganisms or plants to break down or remove contaminants, and chemical oxidation, which injects strong oxidants to destroy pollutants. These methods aim to either transform contaminants into less harmful forms or physically remove them from the environment.