Assimilative capacity refers to the ability of the environment or a portion of the environment (such as a stream, lake, air mass, or soil layer) to carry waste material without adverse effects on the environment or on users of its resources. Pollution occurs only when the assimilative capacity is exceeded. Some environmentalists argue that the concept of assimilative capacity involves a substantial element of value judgement, i.e., pollution discharge may alter the flora and fauna of a body of water, but if it does not effect organisms we value (e.g., fish) it is acceptable and within the assimilative capacity of the body of water.

A classical example of assimilative capacity is the ability of a stream to accept modest amounts of biodegradable waste. Bacteria in a stream utilize oxygen to degrade the organic matter (or biochemical oxygen demand) present in such a waste, causing...