The Stream Doctor

December 2nd, 2002

Your questions and answers about everything stream related.

Q. From Steven in Minnesota: I have read in newspapers and magazines, that some people want to attempt to clean river bottoms of pollutants that have accumulated in the sediments, saying that it would be beneficial for the health of the stream. I have also read, that others think we should leave the pollutants alone, and let them stay undisturbed. The latter believe that more harm than good would result, in attempting to cleanup the pollutants, and that the vast amount of money required to do the projects would be better spent elsewhere. What are your thoughts on the matter?

A. Well, Steve, you've opened up a very complex can of worms with your question, one that has many aspects to be considered. These include: what is the nature of the stream, what is the contaminant of concern, what is the potential for the contaminant to adversely impact the stream's food web or humans, and what do you do with the contaminated sediments once they are removed? These are all ecologically related questions and don't address the question of cost/benefit which you refer to. It would take many pages to adequately address each of these in detail, but let me briefly talk a bit about each of the above points to give you a feeling of the complexity involved. Since each potential problem usually addresses a specific contaminant in a specific stream, most answers must come on a case-by-case basis. I'm going to try to answer your question with some generalities.

What is the nature of the stream? Here must be considered the ecological characteristics of the stream, in particular, the nature of the sediments and whether they are aggrading (accumulating), or degrading (getting moved downstream). Finer sediments, such as silt, accumulate most contaminants more readily than coarser sediments. The contaminants bind tighter to these fine particles and accumulate in higher concentrations because of the high surface to volume ratio. In relating to the aggrading/degrading feature, you have to consider whether contaminated sediments are being buried by accumulating sediments or whether they are subject to suspension and transport by floods or high water. The above considers only the physical aspects of the receiving stream; we'll get to ecology and toxicity below.

What is the contaminant of concern? This involves such things as whether the contaminant is one that is specifically toxic (e.g., mercury, some radionuclides) or whether they cause harm because of long-term bioaccumulation in tissues (e.g., DDT). An important factor here is the potential for the contaminant to chemically degrade over time into harmless chemicals. A good example of this is the accumulation of short- and long-lived radionuclides in the sediments of the Columbia River reservoirs below the Hanford Site. I'm familiar with this because I did research on this for 15 years. The chemical element of most radionuclides, per se, are not harmful; it is the radiation that they emit that can be dangerous (plutonium is an exception). Without going into detail of which ones and how much reach the Columbia, suffice it to say that several different radionuclides have accumulated in the river's sediments. Most of these are buried deeper and deeper in the sediments above the many downstream dams. Thus, they are largely precluded from entering aquatic food chains. Further, they decrease in radioactivity because of radioactive decay. Short half-life elements disappear fairly quickly; longer half-life elements will be there for some time, but are slowly, and surely, decreasing in concentration while getting buried deeper and deeper. Many of the toxic chemicals of concern in different rivers (PCBs) don't degrade. Now, do the radionuclides or other chemicals such as PCBs, DDT, etc. have potential to be harmful. Sure they do, but it depends on how much becomes available to food webs, how much is taken up by the bodies of the organisms, and how much of this is passed on to whomever feeds on them. Which leads us to:

What is the potential for the contaminant to adversely impact the stream's food web or humans? Again, this depends on the nature of the contaminant. Some are directly toxic and can kill aquatic organisms if present in high enough concentrations. If a particular stream has a fairly simple and straightforward food web, elimination of one item can have severe impacts on the organisms that feed on or are otherwise depending on the eliminated organism. Streams with more complex food webs can adjust, if one organism is eliminated, by finding alternate sources of food. That's the beauty of high biodiversity. The impact on humans is fairly straightforward. If we eat an organism, such as a particular species of fish that accumulates a toxic chemical in its muscle tissues, these chemicals accumulate in our tissues. Over time, if enough becomes present, then symptoms may appear than indicate toxic reactions.

What do you do with the contaminated sediments once they are removed? Now we're getting into the cost/benefit aspects of the problem, though they are not completely removed from ecological problems. Simply removing contaminated sediments by dredging and disposal only transfers the problem. You've removed the problem from one place and moved it to wherever the contaminated sediments will be disposed. They are still available for accumulation and movement through food webs just different ones. However, these food webs may be more manageable in terms of keeping the contaminants out of food webs leading to humans, but mother nature can be tricky. We've been fooled before when we thought we'd solved a disposal problem, only to have a different one appear. Also remember that the physical process of dredging contaminated sediments mobilizes these contaminants for a period of time. The question is, of course, which is better: to stir up and mobilize a small percentage (hopefully) of the contamination for a short period and eliminate the bulk of the contamination for the long term or let the contaminated sediments lie still and become buried?

Now we're back to the nature of the stream and what is the contaminant of concern I think I'll get off this merry-go-round here. Hope this is of help; let me know if you want more. ~ Stream Doctor

The 'Stream Doctor' is a retired professional stream ecologist and author, now living in the West and spending way too much time fly-fishing. You are invited to submit questions relating to anything stream related directly to him for use in this Q & A Feature at

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