Enclosure (1) TO COMDTINST 16478.12
aquatic areas easily accessible to humans (such as those in shallow water)
were studied.
An important selection criterion was the environmental or human health
sensitivity of the location. AtoNs deployed on land (0' of water) thereby
accessible on foot; those in recreational areas; those where bottom types are
particularly sensitive (such as coral reefs); or those that may promote the
entrapment of elemental mercury (such as fine particulate soils) or the
creation of methyl mercury are all potentially sensitive.
Based on the findings of the Morel-Mason study and other environmental
characterizations, four characteristics affect the likelihood of formation of
harmful forms of mercury in aquatic systems and can be used for site
selection: (1) the salinity of the water (fresh vs. salt), (2) the organic
content of sediment (high/low), higher organic content promoting the formation
of methyl mercury (Preston, 1989), (3) the sediment type or quality, ranging
from coarse to fine grained where fine grained sediments attract metallic
particles, and (4) the degree of mixing or "flushing" of the system.
Locations were chosen to represent a range of these four characteristics, and
to provide us with data on particularly sensitive environments. Estuarine
systems, such as those chosen for three of the four prototypes, were studied
because they exhibit variety in salinity zones, bottom types, biota, and
sediment quality.
Bottom types are often subdivided into a large number of categories, depending
on the rockiness, muddiness, or sandiness of the sediments. Sixty-two percent
of all fixed aids are located in some form of mud, and another 7 percent are
found in "earth" bottoms (ATONIC 1993). In this sense, selected AtoNs located
in "muddy" bottoms represent most of the bottom types of AtoNs in the field.
However, the national distribution of AtoN location bottom types is not easily
replicated at the state or even Coast Guard District level, since bottom types
are more specific to certain areas of the country than are water depths. In
addition, bottom types affect the likelihood of battery breakage, mercury
entrapment or dissolution, and the presence of biota likely to ingest any
released mercury. This makes it critical that a variety of bottom types be
represented in the study, even if the number of locations exhibiting those
characteristics was relatively small.
Thirty-six percent of all fixed aids are located on shore (i.e., in zero feet
of water), 29 percent are in water more than 20 feet deep, 20 percent are in 0
to 10 feet of water, and 15 percent are in 10 to 20 feet of water. Therefore,
AtoNs were selected from all water depths, where possible, in each of the
environments studied.
The locations chosen represent sensitive environments, such as estuaries, a
variety of salinity levels, ranging from fresh water to open marine
environments;
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