Protecting Public Health and the Environment.

Natural Background Conditions

Natural background conditions describe the quality of water that would exist without human-caused changes in the watershed. In some settings, this natural water quality may be poorer than the criteria set to protect uses. Since human-caused changes don't always affect all aspects of water quality, it is possible for water quality to be considered natural for one parameter but not another. For instance, a water body may have temperature conditions unaltered by humans while at the same time being polluted with a human-made toxic substance. 

Water quality standards in most states include a provision that allows for water quality to exceed numeric criteria absent human causation (i.e., naturally). Such a provision recognizes that water quality laws are about controlling 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 human-made chemicals, the evidence of pollution is clear. But in controlling water quality and protecting beneficial uses, DEQ also pays attention to natural characteristics of water―such as sediment, nutrients, dissolved oxygen, and temperature―that are not really “pollutants” but can be adversely altered by human activity. Even some toxic materials, such as metals and ammonia, occur naturally. These natural constituents of water complicate pollution control efforts because it becomes necessary to distinguish between the natural background concentration and the increase in concentration due to pollution (i.e., human-caused).

Federal rules for total maximum daily loads (TMDLs) have recognized this distinction since 1991 and require TMDL load allocations to distinguish between natural background loads and pollutant loads from human sources. This step is important because regulators have no legal authority to require polluters to correct natural loads. Similarly, the federal rules governing state adoption of water quality standards have always recognized that not all waters can be fishable and swimmable. Sometimes, naturally occurring pollutant concentrations prevent attaining human-desired beneficial uses.

It is difficult to set water quality criteria for natural characteristics of water. If numeric criteria are set high enough to allow for naturally unfavorable conditions that can sometimes occur, then they may be under-protective. Instead, criteria are set at more favorable levels clearly protective of beneficial uses. As a result, these protective criteria can and will be exceeded naturally at times. The rules balance this by allowing for water quality that does not meet criteria due to natural background conditions.

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 or more (e.g., metals in mining districts), the use of a natural background allowance is difficult. It is subject to controversy even though it is logical.

History of Natural Background Provisions in Idaho

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 1997 language was originally limited to toxics and then expanded to other parameters in 2000.

The 2000 revision 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 who might want to extrapolate what was naturally adverse in one water body 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. The revisions also addressed issues specific to temperature criteria. An allowance was added for a limited human-caused increase in water temperatures already naturally warmer than criteria, on the principle 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 even when the natural background temperatures exceed numeric criteria. When waters are cooler than criteria, this provision does not apply; instead, the human-caused increase is limited by the numeric criteria and may be greater than 0.3 °C. The choice of 0.3 °C is consistent with allowances in water quality standards for the state of Washington and the Colville Tribe and was roughly based on accuracy of field temperature measurement at the time.

EPA requested that Idaho provide implementation guidance on how it will determine what is natural before the agency approved or disapproved the natural background language in Idaho's rules. DEQ developed and transmitted such guidance to EPA in 2003. EPA approved the 2002 rule changes on July 20, 2004.

Idaho's Rule Language

There are three separate sections of Idaho’s water quality standards (IDAPA 58.01.02) that together acknowledge natural background conditions and how they apply to water quality criteria:

  • Natural background conditions are defined in section 010.63.
  • The narrative criterion is described in section 200.09.
  • An allowable increase in water temperature above natural background conditions specific to wastewater discharges is addressed in section 401.01.c. This allowable increase was further clarified in a letter to EPA dated February 5, 2004.

Staff Contacts

Water Quality Standards Lead
DEQ State Office
Water Quality Division
1410 N. Hilton
Boise, ID 83706
(208) 373-0502

Related Study

Hem Creek 2009 Field Investigations (June 2012)
The Hem Creek field investigations were prompted by EPA’s February 2009 disapproval of Idaho’s removal of the Hem Creek assessment unit from Idaho’s 2008 Clean Water Act Section 303(d) list as water-quality impaired for temperature. The report supports DEQ’s pursuit of temperature impairment delisting in the 2012 Integrated Report on the basis of natural conditions.