Environmental Engineering Reference
In-Depth Information
highest concern is the mobility of the toxic metals to ground and surface water. Table
10.3 shows the maximum permissible values of selected metals in wastewater reused for
irrigation with corresponding adverse impacts. The recommended values with respect to
long-term and short-term applications are presented in table 10.4.
Cd is a relatively rare metal and natural concentrations in soils are very low. Man-
made sources of Cd are usually associated with industrial activities. It forms part of
automotive batteries and pigments, and could be released to the environment due to the
improper management of waste products. It forms part of some P fertilizers as well. The
uptake of Cd by plants is greatest in acid soils of high Cd concentration.
Table 10.3. Threshold values of trace elements in
water for crop production.
Element Recommended
maximum
concentration (mg/l)
Remarks
Al
5.0
Causes non-productivity in acid soils (pH<5.5) but no
problems at alkaline pH>7
Cd
0.01
Toxic to beans, beets, turnips in a concentration as low as 0.1
mg/l in nutrient solution. Conservative limits recommended
due to the potential to accumulate in plant tissues to
concentration that may be harmful to humans.
Cr
0.1
Not an essential element. The conservative limit is due to lack
of knowledge on toxicity.
Fe
-
Not toxic to plants in aerated soils but can increase acidity
and reduce the availability of phosphorus and molybdenum.
Sprinkling can cause unsightly deposits on plants, buildings
and equipment
Mn
0.2
Toxic to a number of crops at a few tenths to a few mg/l only
in acid soils pH<5.5
Ni
0.2
Toxic to a number of plants at 0.5 mg/l to 1 mg/l. Reduced
toxicity at neutral or alkaline pH.
Pb
5.0
Can inhibit plant cell growth at a high concentration
Zn
2.0
Toxic to many plants at widely varying concentration.
Reduced toxicity at pH>6.0 and in fine textured and organic
soils
After Pescod (1992)
Table 10.4. Concentration of trace elements in
wastewater suitable for irrigation (mg/l).
Element
Long-term*
Short-term*
Long-term**
Short-term**
Aluminum
-
-
5
20
