Environmental Engineering Reference
In-Depth Information
in the pore will diffuse out and be decomposed. Diffusion is a slow process and therefore
the decomposition process will appear to occur at a slow rate. A component's
concentration cannot continuously increase or decrease exponentially. Eventually inputs
will cease or equal losses or the component will completely disappear from the
environment. Sensitive chemical instrumentation indicates that complete loss is not
likely.
A concrete example of looking at the changes in a component in soil is soil phosphate.
In soil it increases as organic matter decomposes and decreases as plants take it up and as
it reacts to form insoluble phosphates. Changes in phosphate in soil could be described by
a simple mathematical relationship:
Future levels of phosphate=previous levels of P+additions of P −losses of P
or
P(
t
+∆
t
)=P
t
+P
i
−P
o
where P(
t
+∆
t
) is the phosphate content at some time in the future (∆
t
is a change in
t
), P
t
is the phosphate at the present time
t,
P
i
the inputs of phosphate, and P
o
the outputs or
loss of phosphate. This whole relationship can be represented by a simple flow diagram,
shown in Figure 7.11. In Figure 7.11 the central rectangle represents the pool of available
phosphate
FIGURE 7.11
A simple flow diagram for the input and loss of phosphate from
soil (triangles represent P
i
(left) and P
o
(right).
while the arrows indicate the direction of flow. The circles represent the activity while
the oblong circles represent the rate of the activity happening. The rates are connected to
the process, indicating that the total flow over time is determined by the rate at which the
activity happens.
In developing a dynamic model inputs would be separated into those from fertilizer
application, organic matter breakdown, and release from minerals. In a like fashion losses
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