Geology Reference
In-Depth Information
removal from the atmosphere. Thus, for trace substance A,
d
½
A
dt
¼
L
½
A
where L is the washout coefficient, with units of s
1
. A typical value of L
for a soluble substance is 10
4
s
1
although actual values are difficult to
measure and are highly dependent upon factors such as rainfall inten-
sity.
7.3 TRANSFER IN AQUATIC SYSTEMS
When rain falls over land some drain off the surface directly into surface
water courses in surface runoff. A further part of the incoming rainwater
percolates into the soil and passes more slowly into either surface waters
or underground reservoirs. Water held in rock below the surface is
termed groundwater, and a rock formation that stores and transmits
water in useful quantities is termed an aquifer. Water that passes
through soil or rock on its way to a river is chemically modified during
transit, generally by addition of soluble and colloidal substances washed
out of the ground. Some substances are removed from the water; for
example, river water often contains less lead than rainwater; one mech-
anism of removal is uptake by soil.
River waters carry both dissolved and suspended substances to the
sea. The concentrations and absolute fluxes vary tremendously. The
suspended solids load is largely a function of the flow in the river, which
influences the degree of turbulence and thus the extent to which solids
are held in suspension and resuspended from the bed, once deposited.
Table 3 shows a comparison of 'average' riverine suspended particulate
Table 3 A comparison of the concentration of major elements in 'average'
riverine particulate material and surficial rocks
Concentrations (g kg
1
)
Riverine particulate material
Element
Surface rocks
Al
94.0
69.3
Ca
21.5
45.0
Fe
48.0
35.9
K
20.0
24.4
Mg
11.8
16.4
Mn
1.1
0.7
Na
7.1
14.2
P
1.2
0.6
Si
285.0
275.0
Ti
5.6
3.8