What is Assimilative Capacity?

Legal Definition
Assimilative capacity refers to the ability of a body of water to cleanse itself; its capacity to receive waste waters or toxic substances without deleterious effects and without damage to aquatic life or humans who consume the water. It is level to which water body or nature control the toxicity without effecting the aquatic life.

Human health and welfare, food security, industrial development and the ecosystem on which they are all at risk, unless the resources are managed more effectively during the present decade and beyond, than they have to be in the past. The well being of people at present and future demands immediate and effective action. Concerted action is needed to reverse the present trend of inequitable resource consumption, and over increasing shortages and environmental degradation. A proper management plan is essential for judicious utilization of resources for sustainable development.
The socio-economic roots of present environmental crisis lie in the paradigm of scientific materialism and economic determination that fail to recognize the physical limits imposed by ecological system on economic activity. The economies must expand within ecosystems that have regenerative capacities. Contrary to neoclassical theory of continuous material growth, economic activities directly determine the potential for development through over exploitation of natural resources and indirectly compromise future production through discharge of residuals. There is a limit to the capability of ecological systems in accepting the residuals without discernable changes in the quality of recipient bodies.
-- Wikipedia
Legal Definition
The amount of waste a natural body of water can handle without any harm being done.