Chemistry Reference
In-Depth Information
CHCl:CCl
2
and CCl
2
:CCl
2
and, although there has
been a decrease in usage over the past 25 years, most
still are found frequently in drinking water.
Pesticides found in UK drinking water include
atrasine, chlortoluon, isoproturon, mecoprop and
simazine. A major source of insecticide in fresh-
water is from sheep dipping. When organochlorine
pesticides such as lindane were banned, they were
replaced by the less persistent organophosphorus
groups. These, however, are now coming under sus-
picion of causing health problems and the use of syn-
thetic pyrethroids has increased. However, although
the latter have low environmental persistence and
are largely non-toxic to mammals, they are at least
100 times more toxic to invertebrates in the aquatic
environment than organophosphates and are result-
ing in the death of significant numbers of aquatic
organisms in UK rivers.
Table 3.
8
Concentrations of the principal ions in seawater
(basis salinity
=
35)
Element
Chemical species
(mol l
-
1
)
(g kg
-
1
)
Sodium
Na
+
4.79
¥
10
-
1
10.77
Magnesium
Mg
2
+
5.44
¥
10
-
2
1.29
Calcium
Ca
2
+
1.05
¥
10
-
2
0.4123
Potassium
K
+
1.05
¥
10
-
2
0.3991
Strontium
Sr
2
+
9.51
¥
10
-
5
0.00
8
14
5.59
¥
10
-
1
Chlorine
Cl
-
19.353
SO
2
-
, NaSO
-
2.
8
9
¥
10
-
2
Sulfur
0.905
HCO
-
, CO
2
-
2.35
¥
10
-
3
Carbon
0.276
Br
-
8
.62
¥
10
-
4
Bromine
0.673
B(OH)
3
, B(OH)
-
4.21
¥
10
-
4
Boron
0.0445
F
-
, MgF
+
7.51
¥
10
-
5
Fluorine
0.00139
Source: Refs 21 and 22.
last 900 million years comes from evaporites: salts
deposited in constant sequences and thicknesses
from evaporating seawater basins cut off from the
open ocean.
The residence times of ions in seawater reflect the
way in which biogeochemical cycling is operating
and influence their ecotoxicity. Most residence times
are very long (10
4
-10
8
years) compared with that of
the water itself (3.8 ¥ 10
4
years). Ions with the
highest solubility generally have the longest resi-
dence times. Alkali and alkaline-earth metals, halo-
gens and oxo-anions thus tend to remain in solution
for a long time. Elements that readily form hydrox-
ides, such as Al and Fe, reside only for a relatively
short time in solution. The ecotoxicity of a pollutant
is generally higher when in solution than when
adsorbed on sediments.
Inputs of chemicals take place from the atmos-
phere, rivers, glaciers, hydrothermal sources, sedi-
ments and basement rock. Sinks are principally
sediments, precipitates and biological removal but
also include sea to air fluxes, evaporites and geologi-
cal subduction of the seabed.
Residence times for the major ions based on
steady-state assumptions are given in Table 3.9.
The ionic strength of seawater is approximately
0.7 mol l
-1
and the acidity/alkalinity is buffered by
carbonate/bicarbonate to pH 8.
In addition to dissolved salts the oceans contain
dissolved gases and a large number of organic com-
pounds in solution and particles in suspension.
The dissolved oxygen concentration varies from
4 Chemistry of the Oceans
4.1 Chemistry of the open ocean
'How inappropriate to call this planet Earth when
it is quite clearly ocean.'
Arthur C. Clarke (1917- )
The oceans and seas of the world cover 71% of the
planet's surface, i.e. 3.6 ¥ 10
8
km
2
, with a depth of
0-10 km, averaging 3.7 km. They contain 97% of all
the water on the planet and until recently were con-
sidered infinite sinks for anthropological pollution.
However, the fragility of the marine environment,
especially estuaries, coastal waters and enclosed
seas such as the Baltic and Mediterranean, is now
recognised. Millions of years of constant conditions
have led to marine organisms becoming highly spe-
cialised and consequently intolerant to environmen-
tal change.
Seawater is saline. Salinity is defined as the weight
in grammes of inorganic ions dissolved in one kilo-
gramme of water. However, because it is normally
measured in terms of conductance and quoted rela-
tive to known standards, it has no units. Open ocean
waters are well mixed and have a narrow range of
salinities between 32 and 37. Most are around 35.
The main inorganic chemical species in seawater
with a basis salinity of 35 are given in Table 3.8. Evi-
dence that the salinity and ionic composition of sea-
water have remained more or less constant over the
