OCEANS AND ESTUARIES - Water Quality Engineering in Natural Systems - page 333

Based on these calculations, the maximum oxygen
deficit is 4.9 mg/l, which will occur approximately
3.04 km downstream of the wastewater discharge.

throughout the estuary except in the immediate vicinity
of the head. Such an estuary is commonly referred to as
a well-mixed estuary . Undoubtedly, many estuaries are
best classified as having circulation patterns intermedi-
ate between those of typical salt wedge and well-mixed
estuaries. The important point to keep in mind is that
there is a net outflow of water near the surface and a
net inflow near the bottom at the mouth of all estuaries,
regardless of whether the details of the circulation
pattern correspond most closely to those of a salt wedge,
well-mixed, or intermediate-type situation. Since there
is a net inflow and upward mixing of seawater at the
bottom of an estuary, detritus that sinks out of the mixed
layer at the surface and regenerated nutrients from the
deeper portions of the estuary are constantly being
carried back into the estuary and mixed up into the
surface waters. Suspended organic matter that drifts out
of the estuary on the surface current, and that subse-
quently sinks offshore or is eaten and then excreted
offshore, tends to be swept back into the estuary by the
net influx of bottom water. Figure 9.18 depicts the
cycling of nutrients and organic matter in an estuary as
influenced by the estuarine circulation pattern. The
physical circulation pattern in estuaries provides a
natural mechanism for recycling food and inorganic
nutrients, and thereby maintains a high level of produc-
tion in the estuarine system. In a similar manner, pollut-
ants introduced into an estuary tend to be recycled in
the same manner as nutrients. Consequently, estuaries
are undesirable receiving waters for pollutants, since the
pollutants tend not to be directly washed out to sea and
dispersed. If an effluent is discharged into the bottom
water layer of a stratified estuary and has neutral buoy-
ancy, it may move upstream until it reaches the end of
the saltwater wedge and comes under the influence of
the seaward-moving upper layers of water. If it has neg-
ative buoyancy, it may never travel seaward at all. As a
general rule, estuarine circulation patterns can be

9.3.5 Flow and Circulation

Regardless of their mode of formation, estuaries tend
to be highly productive ecosystems because of the
nature of the estuarine circulation pattern that charac-
terizes these systems. Since freshwater is less dense than
salt water of comparable temperature, there is a natural
tendency in estuarine systems for the freshwater from
land runoff to flow from the head (entrance) to the
mouth (exit) of the estuary along the surface, whereas
seawater moves in and out with the tides along the
bottom. This typical pattern of water movement in an
estuary is illustrated in Figure 9.17. As indicated by the
curved arrows in Figure 9.17, there is typically some
upward mixing of seawater into the freshwater, so that
some of the seawater that enters the estuary near the
bottom flows back out near the surface. As a result,
there is a net outflow of water (freshwater mixed with
some salt water) at the mouth of the estuary in the
upper part of the water column and a net inflow of
seawater in the lower part of the water column. If the
flux of freshwater into the estuary at the head is large
compared with the tidal in-and-out flux of seawater,
there is generally a sharp demarcation between the
freshwater at the top of the water column and the salt
water below. Due to the mixing of saltwater and fresh-
water, this sharp transition region gradually blurs as one
approaches the mouth of the estuary. Such an estuary is
commonly referred to as a salt wedge estuary because of
the shape of the saltwater “wedge” in the lower part of
the water column when the estuary is viewed in longi-
tudinal profile. If the in-and-out flux of the tides is large
compared with the flux of freshwater, then freshwater
and seawater tend to be thoroughly mixed together

mouth of estuary head of estuary

Outward transport of plankton and detritus

River inflow

river inflow

freshwater

Sinking of
ditritus

Inward and upward transport
of nutrients and detritus

Bottom of estuary

saltwater

bottom of estuary

Figure 9.18. Cycling of nutrients in an estuary. Source : laws
(2000).

Figure 9.17. Typical pattern of water movement in an estuary.