Geology Reference
In-Depth Information
cycle and the diurnal nature of the tides. Together these influences
determine the geographic extent of sediments experiencing a variable
salinity regime. Variations in the river discharge affect the mass loading
of the discharge, both in terms of suspended sediment and bed load
material. The hydrodynamic regime in the estuary influences the depo-
sition of the riverine sedimentary material and the mixing of dissolved
material.
The estuary is a mixing zone for river water and seawater, the
characteristics of which differ considerably. River water is slightly acidic
and of low ionic strength with a salt matrix predominantly of Ca(H-
CO
3
)
2
(
120 mg L
1
). In contrast, seawater has a higher pH (
B
B
8),
higher ionic strength (
35 g
L
1
). As a consequence, the salt matrix within the estuary is dominated
by the sea salt end member throughout the mixing zone except for a
small proportion at the dilute extreme. Salinity can be used as a
conservative index, although conductivity is better, not being subject
to systematic conversion errors in the initial mixing region.
In a plot of concentration vs. some conservative index (i.e.,S%,Cl% or
conductivity), the theoretical dilution curve would comprise a straight line
between the river and seawater end members. A dissolved constituent that
exhibits such a distribution is said to behave conservatively in the estuary.
Whereas a negative slope shows that the riverine end member is progres-
sively diluted during mixing with seawater, a positive slope indicates that
the seawater end member has the greater concentration. Conservative
behaviour is exhibited, for example, by Na
1
,K
1
and SO
4
2
.Reactive
silica may at times behave conservatively. Non-conservative behaviour
can result from an additional supply of material (i.e., causing positive
deviations from the theoretical dilution curve). Elements that may show a
maximum concentration at some intermediate salinity are Mn and Ba.
Alternatively, the removal of dissolved material during mixing (i.e.,
negative deviation from the theoretical dilution curve) can be caused by
biological activity or by dissolved to particulate transformations. Biolog-
ical activity can cause non-conservative behaviour for nutrient elements,
including reactive silica. Dissolved constituents typically transformed to
the particulate phase include Al, Mn and Fe (see Figure 14) in some
estuaries. The pH distribution is usually characterised by a pH minimum
in the initial mixing zone, resulting from the non-linear salinity depend-
ence of the first and second dissociation constants of H
2
CO
3
.Notwith-
standing the obvious utility of component-conservative index plots, they
can be applied and interpreted only with caution. Often it is difficult to
define the exact composition of the end members. Hence, a plot that
apparently denotes non-conservative behaviour could arise if temporal
B
0.7) and consists primarily of NaCl (
B
