3 the varying inputs from weathering reactions and organic matter decompo-
sition in soil and rocks;
4 differential uptake by biological processes in soils.
Where crystalline rock or highly weathered tropical soils are present (i.e. where
weathering inputs are low or exhausted), dissolved freshwater chemistry is most
influenced by natural atmospheric inputs, for example sea spray and dust, as well
as by anthropogenic gases, for example SO 2.
Sea-salt inputs are common in coastal regions. These salts have been introduced
into the marine atmosphere from bubble bursting and breaking waves and are
deposited on land with rain and dust fall. Small amounts of sea-salts are, however,
also present in rainwater of central continental areas, thousands of miles from the
sea. Sea-salt inputs have broadly similar, predominantly sodium chloride (NaCl),
chemistry to the seawater from which they were derived. Thus, sodium or chlo-
ride ions can be used as a measure of sea-salt inputs to rainwater. Chloride con-
centrations in rain falling on oceanic islands are around 200mmol l-^1 , rain within
100 km of coastal continental areas contains around 10–100mmol l-^1 , while further
inland chloride concentrations fall below 10mmol l-^1 , but not to zero.
The importance of a seawater source of ions other than sodium and chloride
in rainwater can be assessed by computing their relative abundance with respect
to sodium and comparing this with the same ratio in seawater. This comparison
can be extended to freshwater, although here there is the complication that some
of the ions could be derived from weathering. If we overlook this complication
initially, then, where rainwater inputs make a large contribution to the chemistry
of freshwater, the dominant cation is likely to be Na+. If weathering reactions are
important, then the major dissolved cations will be those soluble elements derived
from local rock and soil. In the absence of evaporite minerals, which are a minor
component of continental crust (see Fig. 4.1), the most weatherable rocks are
limestones (CaCO 3 ). The calcium ion, liberated by limestone dissolution, is an
indicator of this weathering process. This is clearly demonstrated by comparing
the Ca^2 +concentration in groundwater from a limestone aquifer, with ground-
water from granites or metamorphic schists (Table 5.3).146 Chapter Five
Table 5.3Chemical analyses of US groundwater from various rock types (mmol l-^1 ). Adapted
from Todd (1980). This material is used by permission of John Wiley & Sons, Inc.
Granite Metamorphic schist Limestone
South Carolina Georgia Texas
Ca^2 + 0.3 0.7 1.8
Mg^2 + 0.2 0.2 0.4
Na+ 0.4 0.7 1.0
K+ 0.1 0.2 0.2
Cl- 0.1 0.1 0.7
SO 42 - 0.1 0.1 0.2
HCO 3 - 1.2 2.3 4.5
Al 0.004 0 —
Fe 0.003 0.002 0.001
SiO 2 0.6 0.35 0.18
Na+/(Ca^2 ++Na+) 0.57 0.50 0.36