Environmental Engineering Reference
In-Depth Information
gradually losing its butyl groups over time, reducing its tox-
icity as it eventually becomes dibutyltin, then monobutyltin
and then inorganic tin, which is not toxic at all. Organotins are
very toxic, but inorganic tin is not toxic at all.
Where do metals concentrate in the environment?
Metals do not generally reach high concentrations in water
but bind to sediment particles, from which they are available
to varying degrees to marine organisms, particularly ben-
thic (bottom-dwelling) ones, from which the metals move up
the food chain. Bioavailability of sediment-bound metals is a
critical issue for their toxicity. Since smaller sediment particles
have more surface area for binding metals, the fine particles
of silt and clay in estuaries and marshes bind more metal
than sand, resulting in high contaminant levels in the fine
sediments that are consumed by some benthic animals. These
bottom-dwellers can also absorb the metal from the water sur-
rounding the sediment particles, known as pore water, while
others directly eat the sediments. Acid volatile sulfide (AVS)
in sediments binds metals and has been used to predict the
toxicity in sediments of some metals, including copper (Cu),
cadmium (Cd), nickel (Ni), lead (Pb), mercury (Hg), and zinc
(Zn). The rationale is that the AVS in sediment reacts with
the metal to form an insoluble metal sulfide that is relatively
unavailable for uptake. Estuarine sediments tend to have high
levels of sulfide (as in Berry's Creek), and thus relatively low
bioavailability of sediment-bound metals. Ironically, elevating
the oxygen in overlying water decreases AVS, thereby increas-
ing metal availability. Thus, increased oxygen from water
quality improvements can increase the availability of metals
and may cause metals that had been bound to sediments to
leach into the water.
Hg can be readily taken up by worms living in the sediments.
Polychaete worms are abundant and diverse in intertidal mud-
flats and are an important source for Hg biomagnification in
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