An Introduction to Environmental Chemistry

Evaporation also causes CaCO 3 to precipitate from water before NaCl, the latter

being a more soluble salt. The formation of CaCO 3 removes calcium ions from

the water, increasing the Na+: (Na++Ca^2 +) value. Data for rivers influenced by

evaporation plot along a diagonal B to B¢on Fig. 5.3, evolving toward a sea-

148 Chapter Five

Total dissolved salts (mg l

–1)

Ionic strength (mol l

–1

)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Weight ratio of Na/(Na+Ca)

10

100

1000

10 000

0.7

2 ¥ 10 –3

4 ¥ 10 –5

+= Seawater data

= Lake data

= River data

++

++

+

+

+

+

Mississippi B

Ganges

Lena

Niger

Orinoco

Weathering

dominance

Rainfall

dominance

Mobile

Congo

Lake Superior

Negro

A

A¢

Onyx

Ob Columbia

Yukon

Volga

Rio Grande

Colorado

Pecos

Pecos

Jordan

Caspian

Black

Major oceans

Salton Sea

Baltic

Evaporation

B¢

Fig. 5.3Variation of the weight ratio of Na/(Na+Ca) as a function of the total dissolved salts

and ionic strength for surface waters. Arrows represent chemical evolution of rivers from

source downstream. Modified from Science 170 , 1088–1090, Gibbs, R.J. Copyright (1970) by

the AAAS.