Page 111 - Environmental Control in Petroleum Engineering
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98 Environmental Control in Petroleum Engineering
The most common impact of brine on plants is that it increases the
osmotic pressure of the soil solution. Osmosis is a process that controls
the movement of water between solutions, with water flowing from
lower to higher osmotic pressure. Plants have an osmotic pressure in
their cells, which varies from species to species. If the osmotic
pressure in the soil solution outside the plant exceeds that inside the
cell, water cannot flow into the plant. High osmotic pressure produced
by soluble salts also retards water imbibition by seeds, resulting in
decreased germination and slower seedling emergence rates, and
disrupts the uptake of nutrients in plants.
The osmotic pressure (OP) is related to the EC through the follow-
ing equation (Deuel, 1990):
OP = 0.36*EC (3-2)
In this equation, the osmotic pressure is in atmospheres and the
electrical conductivity is in mmho/cm.
The capacity of a soil to adsorb positively charged ions (cations)
is called the cation exchange capacity (CEC). The exchange-
able cations in a soil are those held on surface exchange sites and
are in equilibrium with the soil solution. The measure of the degree
that the exchange sites are saturated with sodium is called the
exchangeable sodium percentage (ESP) and is calculated through
the following equation:
ESP(%) = * 100 (3-3)
CEC
where NaX is the amount of exchangeable sodium. Both the CEC and
NaX are expressed in units of meq/100 g. In fertile soils, the most common
exchangeable cations are calcium and magnesium. These ions are less
soluble than sodium and do not affect plant growth to the same degree.
For ESP greater than 15%, some soils can lose their structure and
disperse in water. Dispersive soils are devastating to plant life because
they limit the free exchange of air and infiltration of water (American
Petroleum Institute, 1989b).
The sodium adsorption ratio (SAR) is an empirical mathematical
expression used to characterize the detrimental effects of sodium on
soils. It is calculated through the following equation:
