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Dear Mr. Combs,
I don't know if you accept comments on papers published in
Contaminated Soil Sediment and Water as I haven't seen any
comments in the magazines. Nevertheless, I wanted to dispell a
popular misconception.
In the Soil and Sediment paper in the August 2001 issue,
Professor Conklin states that sediments "by their vary nature
are always saturated with water". While it it obvious that
Professor Conklin refers to recent sediments in streams,
lakes, ponds, etc., there are many more types of sediments in
the natural environment, some of which are not saturated with
water nor were they originally transported by water in the
past.
In the broader context of the term (i.e., the geologic),
sediments include those that are being or were transported by
wind or ice as well as water. These sediments may now be
either above or below the water table (or sea level). Examples
include sand dunes and loess (aeolian), beaches, barrier
islands, and glacial moraines as well as fluvial and deltaic
sediments which may or may not now be in river or stream beds
(e.g., the Mississippi delta). In fact, coastal plains
bordering the coasts of continents are comprised of
unconsolidated sediments, most of which are not now being
transported by water. Sand in deserts is also a type of
unconsolidated sediment which is transported primarily by wind
but may also be tranported by water during storms. Much of
this sand is unsaturated (there may be deep groundwater).
To go one step further, once you dig below the soil horizons,
you typically encounter unconsolidated sediments, which may or
may not be saturated, below which is bedrock.
Sincerely,
Jeff Story, P.G.
Response from Dr.
Alfred Conklin:
I want to thank
Mr. Story for his insightful observations about the nature of
sediments. It is my intention to make my articles as
informative as possible to the broadest audience. In looking a
various dictionaries it would seem that the most common usage
of the word sediment is to refer to material deposited by
water. However it is true that any material deposited by any
fluid can be considered as sediment whether or not it is now
or has ever been saturated with water. I would argue that
sediments saturated with water are generally of more
environmental concern than those, which are not. Water is very
active, collecting material from wide areas and concentrating
suspended material in sediments. Also these sediments are
labile in that a heavy rainfall event may dislodge them
resulting in their being redeposited, sometimes at great
distances from their original position.
Sediments deposited by wind and ice are both qualitative and
quantitatively different from those deposited by water.
Sediments from wind often are global in that soil picked up in
the great plains of the United States can be deposited in
Africa. However, on a worldwide bases the quantity of material
transported and deposited by wind is often small. On the other
hand in local areas, such as the sand dunes in Michigan, large
quantities of materials are moved but the area involved is not
extensive. Ice in the form of glaciers produces large amounts
of sediments and these sediments in the United States are very
productive agriculturally. However, these are not very active
in that glaciers are slow moving. It is always important to
keep all sources of movement and sedimentation in mind
however, I would argue that it is not good to lump them all
together in the same category.
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