Page 110 - Environmental Control in Petroleum Engineering

The Impact of Drilling and Production Operations i?

plants "(Kaszuba and Buys, 1993). On the other hand, the addition of
clays from drilling muds can increase the water holding capacity of
sandy/coarse-textured soils, improving plant growth.
A number of ways to measure the salinity of soils has been developed,
These measurements include directly measuring the electrical conductivity
of the soil and various measurements of sodium concentration.
The electrical conductivity (EC) of a solution is a measure of the
total amount of cations and anions dissolved in water. These ions can
include sodium (Na), calcium (Ca), magnesium (Mg), potassium (K),
chloride (Cl), sulfate (SO 4), bicarbonate (HCO 3), carbonate (CO 3), and
hydroxide (OH). The electrical conductivity is the reciprocal resistance
of the solution. Table 3-6 summarizes the effects of different EC values
on crops (U.S. Salinity Staff, 1954). A level of salinity that will not
adversely impact most vegetation, land, or groundwater resources from
the one-time discharge is one at which the electrical conductivity of
the discharged brine is less than 4 mmho/cm. This level will limit the
reduction of crop yields to less than 15% (Deuel, 1990).
The electrical conductivity is related to the total dissolved solids
(TDS) concentration in the water. TheTDS is the weight of residue
after all of the water has been evaporated. The TDS has units of mass/
volume of solution. The relationship between EC and TDS is given
as follows:

TDS = A*EC (3-1)

where A is an empirical constant equal to about 640 (Tchobanoglous
and Burton, 1991). The units of the constant are cm-mg/mmho/liter.

Table 3-6
Effect of Electrical Conductivity (EC) on Crops

EC Range
(mmhos/cm) Effect

0-2 Negligible
2-4 Yield of very sensitive crops impacted
4-8 Yield of many crops impacted
8-16 Only tolerant crops still produce
>16 Only very few tolerant crops still produce

Source: U.S. Salinity Staff, 1954.