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Assimilative capacity is the ability for pollutants to be absorbed by an environment without detrimental effects to the environment or those who use of it. Natural absorption into an environment is achieved through dilution, dispersion and removal through chemical or biological processes. The term assimilative capacity has been used interchangeably with environmental capacity, receiving capacity and absorptive capacity. It is used as a measurement perimeter in hydrology, meteorology and pedology for a variety of environments examples consist of: lakes, rivers, oceans, cities and soils. Assimilative capacity is a subjective measurement that is quantified by governments and institutions such as Environmental Protection Agency (EPA) of environments into guidelines. Using assimilative capacity as a guideline can help the allocation of resources while reducing the impact on organisms in an environment. This concept is paired with carrying capacity in order to facilitate sustainable development of city regions. Assimilative capacity has been critiqued as to its effectiveness due to ambiguity in its definition that can confuses readers and false assumptions that a small amount of pollutants has no harmful effect on an environment.

Hydrosphere


Assimilative capacity in hydrology is defined as the maximum amount of contaminating pollutants that a body of water can naturally absorb without exceeding the water quality guidelines and criteria, that determine the amount of pollutants needed to cause detrimental effects on aquatic life and humans that use it. Self-purification and dilution are the main factors effecting level of assimilative capacity a waterbody has. Estimations of breaches of assimilative capacity focus on the health of aquatic organisms in order to predict an excess of pollutants in a waterbody. Dilution is the main way that waterbodies reduce the concentration of contaminants to levels under their assimilative capacity. This means that waterbodies that move rapidly or have a large volume of water will have larger assimilative capacities then a slow-moving stream.

Costal and Marine
Costal and Marine environments will have much greater assimilative capacity due to the large volumes of water creating a much greater dilution factor. contaminants added into areas would be needed in much greater volumes in order to exceed the assimilative capacity and create harmful negative effects on aquatic life. However, oceans are a sink for many pollutants it is estimated that “270 tonnes of nitrogen enter the sea annually” in Western Australia.

Rivers
Rivers have a large focus on being monitored as they are the primary place for runoff from agricultural industries. Agricultural runoff is high in contaminants including Phosphorus and Nitrogen. When phosphorus is added to a river eutrophication occurs a rapid production of algae that’s production was previously limited by the amount of phosphorus in the water. These algae have a high biochemical oxygen demand and reduce the available oxygen for other aquatic organisms. Close monitoring of the assimilative capacity or rivers is needed in order to stop eutrophication which can result in the loss of many aquatic organism.

Atmosphere
Assimilative capacity of the atmosphere is defined as the maximum load of pollutants that can be added without compromise of its resources. Meteorologists calculate assimilative capacity through the atmosphere using ventilation coefficient or through the pollution potential. The ventilation coefficient and is calculated by multiply the mixing height (the height at which vigorous mixing of gasses occur) with the average wind speed. Atmospheric concentrations change rapidly as gasses move due to winds, convection current and dispersion of gasses. The pollution potential is determined by calculating the concentration of pollutants and comparing that to the acceptable limits. This way of calculating takes into consideration the current level of pollutants and assesses how much more can be added in order to reach the assimilative capacity. Sulphur dioxide SO2, Nitrogen monoxide and Nitrogen dioxide (NO and NO2) and suspended particulate matter (SPM) are important pollutants to measure. High concentrations SO2 can cause acid rain which damages structures and increases acidity of soils and waterbodies. High concentrations NO and NO2 can cause photochemical smog which has adverse effects on those with compromised lungs. High concentrations of SPM can be absorbed by the lungs into the bloodstream can cause pneumonia.

Uses to determine management of environments
Assimilative capacity is used as a monitoring guideline for sustainable growth of city regions. Assimilative capacity allows governments to understand how much pressure region is under. Working within the assimilative capacity means that regions will be constructed with future stability in mind. “An assimilative capacity study (ACS) develops specific scientific modelling to support and assist municipalities and other legislative authorities in predicting the impacts of land use”.

United States
In the United States legislation on assimilative capacity as a guideline for the maximum amount of pollutants to be added to water bodies comes from each individual state and from the environmental protection agency.

India
India uses assimilative capacity in management of land, water and air. Though each have largely varying assimilative capacities due to variations in type of pollutants and the difference in dilution dispersion and chemical and biological breakdown in differing environments.

Comparison to accommodative capacity
Assimilative capacity has been critiqued as to the value it adds as a tool for creating guidelines in hydrology. There is a large amount of ambiguity to the definition as it is subjective what harmful to aquatic organism means “death of individual organisms, elimination of food chains, or a change in energy flow patterns”. Inconsistency in assimilative capacity has led to the term to be restricted by the national oceanic and atmospheric administration and environmental protection agency. Accommodative capacity is used to mean “the rate at which waste material can be added to a body of water in such a way that the ambient concentration of contaminants is maintained below levels that produce unacceptable biological impact”.