18
STANLEY N. DAVIS
Even so, the total effect of upward capillary transport over large areas
cannot be ignored, particularly if the depth to water is less than 5 meters.
The presence of brine at the surface will also add an osmotic potential
which could have an effect greater than the capillary potential in bringing
water to the land surface where it will be evaporated.
VII. Water Quality
For human occupation of desert areas, water quality is probably a more
severe limitation than the scarcity of water itself. Modern technology has
overcome part of this difficulty by desalinization devices, but investment
in equipment and cost of power still make most large-scale schemes to
produce fresh water of questionable economic value.
Tolerance of organisms to salt varies widely according to the species,
the health of the organism, its total water intake, and the specific constituents in the water. Most humans can tolerate up to 2.5 and 3.5 gm/liter
of total dissolved solids provided specific constituents such as arsenic,
selenium, and lead are virtually absent. Sheep and beef cattle can generally tolerate 4.0-10.0 gm/liter or even higher amounts under certain
circumstances.
Most useful plants, on the other hand, have lower tolerances to salty
water than do humans and stock animals. Citrus trees, peaches, pears, and
most legumes are particularly sensitive. Date palms, suger beets, asparagus,
and a few other plants, nevertheless, rival sheep and cattle in their tolerance of saline water.
An important key to irrigation is to supply excess water which is removed by drainage systems, thus preventing harmful salt accumulation
through évapotranspiration. If copius amounts of water are used in welldrained sandy soils, some salt-tolerant crops can be grown with water containing more than 10 gm/liter of dissolved solids.
The bulk of all dissolved constituents in normal groundwater falls into
seven main categories, namely, Na
+
, Ca
2+
, Mg
2+
, HC0 3 ~, Cl", S0 4
2
~, and
H 4 Si0 4 . Ions which may be of secondary importance are K
+
, C0 3
2
", Fe
2+
,
and F". Under reducing conditions and a low pH, Fe
2+
may become a
dominant ion. Where contamination from ancient or modern animals persists, NO3- may become a dominant ion. Actual ionic species which are
present in brines are not understood very well. As concentrations increase,
dissolved constituents such as Mg(OH)
+
, CaSO t , and MgC0 3 undoubtedly become important. By convention, however, analyses of brines are
given in the form of simple hypothetical ions found in waters of low concentration of dissolved solids. Analyses of some typical desert waters are
shown in Table I.
STANLEY N. DAVIS
Even so, the total effect of upward capillary transport over large areas
cannot be ignored, particularly if the depth to water is less than 5 meters.
The presence of brine at the surface will also add an osmotic potential
which could have an effect greater than the capillary potential in bringing
water to the land surface where it will be evaporated.
VII. Water Quality
For human occupation of desert areas, water quality is probably a more
severe limitation than the scarcity of water itself. Modern technology has
overcome part of this difficulty by desalinization devices, but investment
in equipment and cost of power still make most large-scale schemes to
produce fresh water of questionable economic value.
Tolerance of organisms to salt varies widely according to the species,
the health of the organism, its total water intake, and the specific constituents in the water. Most humans can tolerate up to 2.5 and 3.5 gm/liter
of total dissolved solids provided specific constituents such as arsenic,
selenium, and lead are virtually absent. Sheep and beef cattle can generally tolerate 4.0-10.0 gm/liter or even higher amounts under certain
circumstances.
Most useful plants, on the other hand, have lower tolerances to salty
water than do humans and stock animals. Citrus trees, peaches, pears, and
most legumes are particularly sensitive. Date palms, suger beets, asparagus,
and a few other plants, nevertheless, rival sheep and cattle in their tolerance of saline water.
An important key to irrigation is to supply excess water which is removed by drainage systems, thus preventing harmful salt accumulation
through évapotranspiration. If copius amounts of water are used in welldrained sandy soils, some salt-tolerant crops can be grown with water containing more than 10 gm/liter of dissolved solids.
The bulk of all dissolved constituents in normal groundwater falls into
seven main categories, namely, Na
+
, Ca
2+
, Mg
2+
, HC0 3 ~, Cl", S0 4
2
~, and
H 4 Si0 4 . Ions which may be of secondary importance are K
+
, C0 3
2
", Fe
2+
,
and F". Under reducing conditions and a low pH, Fe
2+
may become a
dominant ion. Where contamination from ancient or modern animals persists, NO3- may become a dominant ion. Actual ionic species which are
present in brines are not understood very well. As concentrations increase,
dissolved constituents such as Mg(OH)
+
, CaSO t , and MgC0 3 undoubtedly become important. By convention, however, analyses of brines are
given in the form of simple hypothetical ions found in waters of low concentration of dissolved solids. Analyses of some typical desert waters are
shown in Table I.