Environmental Engineering Reference
In-Depth Information
from the watershed and flow to the lowest point. Water then accumulates
until the water body is large enough for evaporation to equal inflow. As
the water evaporates, it leaves behind the salts. The relative proportion of
ions in the inflowing water and varied solubility of different ion pairs as
the salts are concentrated determine the chemistry of the lake.
Most modern nonmarine lake brines are dominated by the anion chlo-
ride, followed by carbonate and sulfate. Sodium and potassium are the
most common cations, but some lakes are dominated by magnesium or cal-
cium instead (Hardie, 1984). Such lakes can have concentrations of salts
far in excess of those found in marine waters (oceans are approximately
3.5% salt by weight). The actual concentration of the constituents also
changes as the lakes become more saline, and certain combinations of ions
precipitate before others. The known series of deposition can be used by
saltworks that have a series of evaporation ponds and “harvest” different
salts as they are concentrated by evaporation.
One interesting feature of some salt lakes is the formation of salt
columns. These are formed as saline water moves up through elevated
columns of salt by capillary action and evaporates off of the top, leaving
the salt behind. Carbonate deposition by photosynthetic organisms en-
hances the process. Such a process forms the bizarre landscape on the shore
of parts of Mono Lake (Fig. 15.1) and similar pillars of salt along the Dead
Sea may have been mistaken for Lot's wife.
Depth of salt lakes varies considerably over the years because their lev-
els depend on a balance between evaporation and inflow. During wet years,
the depth of the lake increases until the surface area is great enough to al-
low evaporation to equal inflow. In these cases, salinity decreases and this
can have ecological effects on the aquatic community. Thus, a series of wet
years led to flooding of the Great Salt Lake in Utah in the 1980s and led
to shifts in the lake's food web. Decreases in salinity allowed the preda-
ceous insect Trichocorixa verticalis to invade the pelagic zone of the lake.
This predator caused decreases in the brine shrimp Artemia franciscana
and subsequent increases in protozoa and three microcrustacean zoo-
plankton (Wurtsbaugh, 1992).
There is a general decrease of diversity of animals and plants as salin-
ity increases and the upper tolerance limit of various organisms is exceeded
(Table 15.1). As with other extreme environments, the Bacteria and Ar-
chaea dominate in the harshest habitats. Some animals such as brine shrimp
(the anostracan Artemia salina ) can withstand more than 30% salt. The
upper salinity limit of some animals may actually be a lower limit for O 2 ;
solubility of O 2 decreases with increased salinity. However, the majority of
freshwater species disappear under only moderate salinity, presumably be-
cause of an inability to osmoregulate (Bayly, 1972).
Salt lakes have scientific, economic, cultural, recreational, and ecolog-
ical values. These lakes are very sensitive to decreases in flow. Human ap-
propriation of freshwaters in the dry regions where they are located can
have devastating effects. For example, prior to 1960, the Aral Sea in Rus-
sia was the fourth largest lake in the world, with a surface area of 68,000
km 2 and volume of 1090 km 3 . By 1993 the lake area had decreased almost
by half and the volume decreased to 340 km 3 . Agricultural uses reduced
water inflow from 50 to 7 km 3 year 1 . The former lake bed is a source of
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