An Introduction to Environmental Chemistry

If the concentration of the measured component is, like salinity, controlled by

simple physical mixing, the relationship will be linear (Fig. 6.3). This is called

conservative behaviour and may occur with riverine concentrations higher than,

or lower than, those in seawater (Fig. 6.3). By contrast, if there is addition of the

component, unrelated to salinity change, the data will plot above the conserva-

tive mixing line (Fig. 6.3). Similarly, if there is removal of the component, the

data will plot below the conservative mixing line (Fig. 6.3). In most cases, removal

or input of a component will occur at low salinities and the data will approach

the conservative line at higher salinity (Fig. 6.4). Extrapolation of such a ‘quasi-

conservative line’ back to zero salinity can provide, by comparison with the mea-

sured zero salinity concentration, an estimate of the extent of removal (Fig. 6.4a)

or release (Fig. 6.4b) of the component.

The Oceans 185

Box 6.1 Salinity

Salinity is defined as the weight in grams of

inorganic ions dissolved in 1 kg of water.

Seven ions constitute more than 99% of the

ions in seawater and the ratios of these ions

are constant throughout the world oceans.

Consequently, the analysis of one ion can, by

proportion, give the concentration of all the

others and the salinity. The density of

seawater and light and sound transmission all

vary with salinity.

Salinity is measured by the conductance of

electrical currents through the water

(conductivity). Measured values are reported

relative to that of a known standard; thus

salinity has no units—although, in many older

texts, salinities are reported in units of parts

per thousand (ppt or ‰) or grams per litre.

Open-ocean waters have a narrow range

of salinities (32–37) and most are near 35. In

estuaries, values fall to less than 1

approaching the freshwater end-member. In

hypersaline environments salinities can

exceed those of seawater, reaching values

greater than 300.

(^03) Salinity 1
Concentration
CR
(^03) Salinity 1
ConcentrationRemoval fromsolution
Conservative
dilution line
(straight)
Addition to
solution
Addition to
solution
Removal from
solution
Conservative
dilution line
(straight)
CS
CR
(a) (b)
CS
Fig. 6.3Idealized plots of estuarine mixing illustrating conservative and non-conservative mixing. CR and CS are
the concentrations of the ions in river and seawater respectively. After Burton and Liss (1976), with permission
from Elsevier Science.