Environmental Engineering Reference
In-Depth Information
Table 8.6 Mathematical expressions for in-stream water quality processes
Water quality variables
Transformation equation
Particulate organic N
dON
dt
¼ð
a
1
k
r
A
p
b
3
ON
r
4
ON
Þþ
a
9
k
db
A
b
h
F
b
F
2
a
9
l
b
¼
b
3
ON
b
1
NH4
þ
r
h
F
1
a
1
l
p
A
p
A
b
h
F
b
Ammonium N
dNH4
dt
A
b
h
F
b
Nitrate N
dNO3
dt
¼ð
b
2
NO2
ð
1
F
1
Þ
a
1
l
p
A
p
Þð
1
F
2
Þ
a
9
l
b
Particulate organic P
dOP
dt
¼ð
a
2
k
r
A
p
b
4
OP
r
5
OP
Þþ
a
10
k
db
A
b
h
F
b
a
10
l
b
Dissolved inorganic P
dDIP
dt
¼
b
4
OP
þ
r
h
a
2
l
p
A
p
A
b
h
F
b
dDO
dt
¼ð
k
2
ð
DO
s
DO
Þþð
a
3
l
p
a
4
k
r
Þ
A
p
k
1
CBOD
k
4
Dissolved oxygen
h
a
5
b
1
NH4
a
6
b
2
NO3
Þþð
a
7
l
b
a
8
k
rb
Þ
A
b
h
F
b
A
p
Algae (phytoplankton)
dA
p
dt
¼
l
p
k
r
r
h
Algae (bottom algae)
dA
b
dt
¼
l
b
k
rb
k
db
ð
Þ
A
b
8.3.4 Nutrient and Interaction with Flow
and Sediment Transport
Surface runoff can remove large quantities of nutrients from the soil in both
dissolved and particulate forms. The loss of dissolved nutrients in surface runoff is
the result of rainfall mixing with the dissolved nutrients in the upper portion of the
soil. Dissolved nutrients interact with surface runoff and once in water, they are
transported. Suspended nutrients, which are assumed to be either organic or
adsorbed inorganic components, attach to eroded sediment material derived from
erosion, and are transported with water. The process of erosion is selective for finer
particles. The finer particles, particularly clay, have larger surfaces of adsorption,
and the clay fraction contains much of the organic matter and hydrous oxides (iron
and aluminum) that can bind nitrogen and phosphorus (Nelson and Logan
1983
).
Runoff has two roles in the transport of nutrients: particle detachment and trans-
port. Most of the nitrogen leaving watersheds through surface runoff is attached to
finer soil particles. Novotny and Chester (
1981
) reported enrichment ratios for
organic nitrogen or nitrogen adsorbed onto organic matter ranging from 2 to 4.
Surface runoff also contains dissolved forms of nitrogen including NH
4
+
and
NO
3
-
. Phosphorus is most commonly assumed to be transported predominantly in
particulate forms through soil erosion by surface runoff. Particulate phosphorus is
attached to mineral and organic sediment as it moves with the runoff. The
enrichment ratio for particulate phosphorus varies from 1 to 10 depending on
watershed size and soil characteristics. However, where soil erosion is limited, the
majority of phosphorus transported by surface runoff may be in dissolved forms.
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