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 streamdoctor@aol.com.
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