Natural background conditions exist when there is no measurable difference between the quality of water now and the quality of water that would exist if there were no human-caused changes in the watershed.  Since human-caused changes don't always affect all aspects of water quality, it is possible for water to be considered natural for one parameter but not another.  For instance, it would be possible for a water body's temperature to be unaltered by humans, but at the same time be polluted with a humanmade toxic substance. 

Water quality standards in most states include a provision that allows for water quality to exceed numeric criteria due to natural background conditions of the water body. Inclusion of such provisions is recognition that water quality laws are about control of pollution, and that pollution is a human-caused alteration of water quality. Idaho has had such general provisions in its water quality rules since April 2000.

When pollutants are humanmade chemicals, the evidence of pollution is clear. But in controlling water quality and protecting beneficial uses we also pay attention to natural characteristics of water, such as sediment, nutrients, dissolved oxygen, and temperature, which can be adversely altered by human activity. Even some toxic materials, such as metals and ammonia, occur naturally. This complicates pollution control, because for these natural constituents of water it becomes necessary to distinguish between the natural background concentration and the increase in concentration due to pollution.

Federal rules for total maximum daily loads (TMDLs) have recognized this since 1991, and require TMDL load allocations to distinguish between natural background loads and pollutant loads from human sources. This is important because regulators have no legal authority to require polluters to correct natural loads they are not responsible for. Similarly, the federal rules governing state adoption of water quality standards have always recognized that not all waters can be fishable and swimmable, and that one reason may be that naturally occurring pollution concentrations prevent the attainment of the use. This is a basic acknowledgement that nature is not always benign or favorable.

It then becomes difficult to set water quality criteria for natural characteristics of water. If criteria are set high enough to allow for the naturally unfavorable conditions that can sometimes occur, they will be under-protective. Instead, criteria are set at more favorable levels for the designated uses. The consequence of this is that these criteria can and will be exceeded naturally, which is then allowed for in rules by a provision excepting natural background concentrations from being violations of criteria.

Because of the great variability in some natural characteristics (e.g., temperature) and the general lack of data on natural concentrations in areas that may have been altered for a century of more (e.g., metals in mining districts), the use of a natural background allowance is difficult and subject to controversy even though it is plainly sensible.

An acknowledgement that water quality can naturally exceed criteria is a common sense notion that has been in Idaho's water quality rules in some form since 1997 (the language was originally limited to toxics).

In 2000, Idaho removed the limitation to toxics, generalizing the application to all pollutants. This was an outgrowth of the Henry's Lake Clean Lakes Project in which naturally low dissolved oxygen levels were problematic because there was no way, at the time, to excuse them from meeting criteria. During that change Idaho also added the words "site-specific" to the language to guard against people that might want to extrapolate what was naturally adverse in one water to all other waters.

The natural background language in Idaho's rules was further revised in 2002. Among other issues, the revision removed the requirement for rulemaking to occur to apply natural background provisions. An allowance was also added for a limited human-caused increase in water temperatures already naturally warmer than criteria, on principal that it is not the intent of the Clean Water Act that there be no room for human use of water. Thus, Idaho's treatment requirements allow for a small 0.3°C increase in temperature when the natural background temperatures exceed numeric criteria. When waters are cooler than criteria, the allowable increase is greater and depends on the use designation. The choice of 0.3°C is consistent with allowances in water quality standards for the state of Washington and the Colville Tribe. The EPA approved the 2002 rule changes on July 20, 2004.

The EPA had pressed Idaho to see implementation guidance from the state on how it will determine what is natural before acting to approve or disapprove the natural background language in Idaho's rules. While the state is not obligated to provide EPA such guidance, in April 2003, DEQ transmitted to EPA a document titled Concepts and Recommendations for Using the "Natural Conditions" Provisions of the Idaho Water Quality Standards. (DEQ Publication, April 2003: pdf 1 mb, 44 pages)