OCEANS AND ESTUARIES - Water Quality Engineering in Natural Systems - page 329

concentrations and nutrient overenrichment, which in
turn can be related to diurnal DO depressions due to
algal respiration. The nutrients of greatest concern in
an estuary are nitrogen and phosphorus. Typically, the
control of phosphorus levels can limit algal growth
near the head of the estuary, while the control of nitro-
gen levels can limit algal growth near the mouth of
the estuary; however, these relationships depend on
factors such as nitrogen-phosphorus (N/P) ratios and
light penetration potential. Excessive phytoplankton
concentrations can cause wide diurnal variations in
surface DO. Excessive chlorophyll a levels also result
in shading, which reduces light penetration for sub-
merged aquatic vegetation (SAV). The prevention of
nutrient overenrichment is probably the most important
water-quality

be related to the chloride concentration by the follow-
ing approximation

S = 1 8 655 c . 0 c (9.74)

where S is in ppt, and c c is the chloride concentration in
ppt or g/l. Salinity is typically in the range of 0-35 ppt
in estuaries and is approximately equal to 35 ppt in
open oceans. The density of water, ρ , depends on both
salinity, S , and temperature, T , and this relationship can
be approximated by (Millero and Poisson, 1981)

ρ ρ = + aS bS +

3 2 /

+ cS

2

(9.75)

o

requirement for a healthy SAV

where ρ is in kg/m 3 , S is in ppt, ρ o is the density of fresh
water in kg/m 3 , and a , b , and c are constants. The param-
eters in Equation (9.75) are given in terms of the tem-
perature, T (°C) by

community.
Sewage treatment plants are typically the major
point source of nutrients, particularly phosphorus, to
estuaries that are adjacent to urban areas. Agricultural
land uses and urban land uses represent significant non-
point sources of nutrients, particularly nitrogen. Point
sources of nutrients are typically much more amenable
to control than are nonpoint sources. Because phospho-
rus removal from municipal wastewater discharges is
typically less expensive than nitrogen removal, the
control of phosphorus discharges is often the method of
choice for the prevention or reversal of use impairment
in the upper estuary. However, nutrient control in the
upper reaches of the estuary may cause algal blooms in
the lower reaches. This occurs because phosphorus
control in the upper reaches may reduce algal blooms
there, but in so doing, increases the amount of nitrogen
transported to the lower reaches, where nitrogen is the
limiting nutrient causing a bloom there.
Potential impacts from toxic substances, such as pes-
ticides, herbicides, heavy metals, and chlorinated efflu-
ents, must generally be considered, since the presence
of certain toxicants in excessive concentrations within
bottom sediments of the water column may prevent the
attainment of designated uses (particularly fisheries
propagation and sea grass habitat uses) in estuaries that
satisfy water-quality criteria for DO and chlorophyll a .
Pathogenic bacteria and viruses do not generally pose
a threat to estuarine aquatic life; however, shellfish can
accumulate pathogens, causing disease when harvested
and consumed by humans.

ρ o = 999 84 6 7939 1 . + . × 0

− 2

T − 9 953 1 . 0 × 0

− 3

T

2

+ 1 . 00 17

× 1 0

− 4

T

3

− 1 12 1 . 0 × 1 0

− 6

T

4

+ 6 5363 1 . × 0

− 9

T

5

(9.76)

a = 8 2449 1 . × 0

− 1

− 4 899 1 . 0 × 0

− 3

T + 7 6438 1 . × 0

− 5

T

2

− 7 3 0 T 9 4

− 8 2467 1 . × 0 T + 5 3 . 875 1 ×

(9.77)

(9.78)

b = − 5 7246 1 . × 0

− 3

+ 1 227 1 . 0 × 0

− 4

T − 1 6546 1 . × 0

− 6

T

2

4

(9.79)

−

c = 4 8314 . × 1 0

The density of pure water at 20°C is approximately
998 kg/m 3 , and the density of seawater at the same tem-
perature is approximately 1026 kg/m 3 ; therefore, under
stable isothermal conditions, lower salinity water will
occur above higher salinity water. In estuaries, this con-
dition is further reinforced by ocean water typically
being colder than stream inflow.
The saturation concentration of dissolved oxygen in
salt water, c s , also depends on the salinity, S , and tem-
perature, T , and this relationship can be approximated
by (APHA, 1992)

10.754 2140.7

2

c = c exp − S 0.017674 − +

(9.80)

s o

T T

a a

9.3.3 Salinity Distribution

where c s is in mg/l, c o is the saturation concentration of
dissolved oxygen in fresh water in mg/l, S is the salinity
in ppt, and T a is the temperature in K. The value of c o
(at 1 atm) can be estimated by the relation (APHA,
1992)

Salinity is the mass of salt per unit mass of water and is
often expressed in either parts per thousand (ppt) or
g/l (when the density of water is factored in); the salin-
ity is approximately the same in both units. Salinity can