Clearing Up Oregon’s Definition of Turbidity

Marten Law published an article earlier this week describing the challenges facing Oregon’s Department of Environmental Quality (DEQ) as they revise their 30-year-old definition of “turbidity.”

Photo by John Kratz. Some rights reserved.

What is turbidity? Turbidity is a measure of water clarity or cloudiness, caused by material that is suspended in the water. The material can range from soil particles from eroding watersheds to decaying plant matter to industrial waste discharges. High turbidity can be evidence of pathogens in drinking water, can increase water temperatures, impair photosynthesis, clog fish gills and otherwise harm aquatic life.

While increases in turbidity can often be attributed to human activities such as logging, agricultural practices, or construction, there is also substantial natural variability in a given water body’s turbidity from season to season (as big rains wash excessive sediment into the water) or in mountainous areas (where erosion and glacial flow can cause large changes in turbidity).

Turbidity, as a measure of water quality, is covered by the Clean Water Act (CWA). Under Section 303(c) of CWA, the EPA mandates individual states to develop water quality standards. While the EPA produces publications such as Quality Criteria for Water from time to time, these documents are not regulations in and of themselves, but rather offer data and guidance on which states can develop their own standards.

These standards are used in the evaluation of NPDES permits under CWA Section 402 and dredge and fill permits under CWA Section 404, in determining whether a body of water should be listed as “impaired” under CWA Section 303(d), and also in determining pollutant loading allocations under TMDL programs.

Ideally, states can come up with cold, hard numerical standards – limiting turbidity to a certain number and size of Nephelometric Turbidity Units (NTUs). Of course, actually determining useful absolute standards is much easier said than done. In addition to the inherent variability in a given water body, one must also establish a baseline “background” turbidity for each body of water, as well as grapple with the limitations of differences in instruments and techniques for gathering turbidity data.

Many states, like Oregon, apply a more “relative” approach for turbidity standards. Their current rule (OAR 340-041-0036) states that “No more than a ten percent cumulative increase in natural stream turbidities may be allowed, as measured relative to a control point immediately upstream of the turbidity causing activity.” According to Marten Law, DEQ hasn’t changed the water quality standard in more than 30 years! DEQ’s draft issue paper on the proposed revisions explains that the purpose of the review is to “incorporate best available science regarding the effects of increased turbidity levels.” For a clear explanation of the anticipated changes, don’t forget to check out the Marten Law article.

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