Assimilative capacity

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 the level of dissolved oxygen in the stream to fall; but the decrease in dissolved oxygen causes additional oxygen to enter the stream from the atmosphere , a process referred to as reaeration. A stream can assimilate a certain amount of waste and still maintain a dissolved oxygen level high enough to support a healthy population of fish and other aquatic organisms. However, if the assimilative capacity is exceeded, the concentration of dissolved oxygen will fall below the level required to protect the organisms in the stream.

Two other concepts are closely related: 1) critical load; and 2) self purification. The term critical load is synonymous with assimilative capacity and is commonly used to refer to the concentration or mass of a substance which, if exceeded, will result in adverse effects, i.e., pollution. Self purification refers to the natural process by which the environment cleanses itself of waste materials discharged into it. Examples include biodegradation of wastes by natural bacterial populations in water or soil, oxidation of organic chemicals by photochemical reactions in the atmosphere, and natural dieoff of disease causing organisms.

Determining assimilative capacity may be quite difficult, since a substance may potentially affect many different organisms in a variety of ways. In some cases, there is simply not enough information to establish a valid assimilative capacity for a pollutant. If the assimilative capacity for a substance can be determined, reasonable standards can be set to protect the environment and the allowable waste load can be allocated among the various dischargers of the waste. If the assimilative capacity is not known with certainty, then more stringent standards can be set, which is analogous to buying insurance (i.e., paying an additional sum of money to protect against potential future losses). Alternatively, if the cost of control appears high relative to the potential benefits to the environment, a society may decide to accept a certain level of risk.

The Federal Water Pollution Control Amendments of 1972 established the elimination of discharges of pollution into navigable waters as a national goal. More recently, pollution prevention has been heavily promoted as an appropriate goal for all segments of society. Proper interpretation of these goals requires a basic understanding of the concept of assimilative capacity. The intent of Congress was to prohibit the discharge of substances in amounts that would cause pollution, not to require a concentration of zero. Similarly, Congress voted to ban the discharge of toxic substances in concentrations high enough to cause harm to organisms.

Well meaning individuals and organizations sometimes exert pressure on regulatory agencies and other public and private entities to protect the environment by ignoring the concept of assimilative capacity and reducing waste discharges to zero or as close to zero as possible. Failure to utilize the natural assimilative capacity of the environment not only increases the cost of pollution control (the cost to the discharger and the cost to society as a whole); more importantly, it results in the inefficient use of limited resources and, by expending materials and energy for something that nature provides free of charge, results in an overall increase in pollution.

[Stephen J. Randtke ]