Environmental Engineering Reference
In-Depth Information
case, the relevant questions are about the quantities of material that move and the mechan-
isms that move them. The vectors that move materials may be physical, chemical, or bio-
logical. Movement may occur down physical or chemical gradients (e.g., insect frass
falling out of trees onto soil or diffusion of carbon dioxide away from areas of respiration
in the soil). Biological vectors sometimes move materials against physical or chemical gra-
dients (e.g., migrating salmon carry large amounts of nutrients upstream, against the cur-
rent;
Helfield and Naiman 2001
). Biological vectors can also result in the increase in the
concentration of toxins as they are transferred up trophic levels (e.g., bioaccumulation of
mercury in small fish and its transfer to predatory fish or birds).
When an element sticks it is temporarily held in place somewhere within an ecosystem—
for example, in the soil, a stream, an organism, or even the atmosphere. Since there is usually
some kind of resistance to movement out of such a place, sticking is important in limiting
losses from systems. Retention may occur on time scales ranging from fractions of seconds to
millennia or more. Examples include sedimentation of organic material in a lake, precipita-
tion of iron and sulfur as iron sulfide at the bottom of a lake, or retention of allochthonous
carbon from a watershed through consumption by in-lake bacteria that are themselves
consumed and eventually become part of fish biomass. As these examples illustrate, sticking,
like movement, can be driven by physical, chemical, or biological mechanisms.
In the case of sticking, however, we are also often interested in how long something
will remain (i.e., its residence time) and its availability within a system. For example, ele-
mental mercury (Hg
0
) has a residence time between five months and a year in the atmo-
sphere (i.e., the average Hg
0
atom emitted will remain in the atmosphere for the better
part of a year) and can be globally distributed. In contrast, reactive gaseous mercury (in
the Hg
2
1
form) is much more soluble. It has a residence time in the atmosphere of less
than five days and usually is deposited within 300 km (and often much less) of its emis-
sion source. In a terrestrial example, a calcium ion associated with charged soil particles
(i.e., the soil exchange complex;
Figure 5.2
) is more available for plant uptake than one
that precipitates as calcium carbonate, so the former is more likely to cycle within the
system or be lost sooner than the latter. In general, properties such as solubility, charge,
size, reactivity, and specific gravity influence the stickiness of a substance.
Transformations—changes in materials from one chemical state or form to another—are
important since an entity's state will often determine whether it moves or sticks, as well as
its availability to the biota. Some of these changes are phase changes driven by energetic
gains or losses from a system—for example, when snow melts. In other cases, there are
changes in how an element is chemically bound—for example, whether iron is bound with
phosphate or is free in solution at the sediment-water interface at the bottom of a lake.
Some of these chemical changes may involve a switch between biotic and abiotic binding,
say when potassium is released from plant roots and binds on the soil exchange complex.
Alternatively, changes from one chemical form to another (either with or without a phase
change) can be biologically mediated—for example, when sulfate is reduced to hydrogen
sulfide by microbes using sulfur as an electron acceptor in metabolic processes (see appen-
dix). Such changes in form are often coupled with changes in mobility. For example, liquid
water is more easily lost from a watershed than snow, and calcium precipitated with phos-
phorus is much less likely to leach from the soil than when loosely bound on the soil
exchange complex. These changes also can be important in storage or release of energy in
