Sludges having over that amount must be incorporated into the soil. Crop contamination by cadmium. Although no immediate health problems result from a slightly elevated cadmium level in crops, Cd can accumulate in the human body over a period of years. Because of this, U.S. EPA regulations define the maximum amount of Cd that can be applied on agricultural soils used for growing food-chain crops such as corn. It should be noted, however, that the cadmium levels found in corn have no adverse affects on yields, and that transfer of Cd into the grain is minimal, making corn an ideal crop for sludge-treated soils.
All sewage sludges contain the heavy metals copper, zinc, nickel, and lead.
If added to soils in relatively large amounts, copper, zinc, and nickel can cause a plant toxicity that reduces yields. An upper limit for lead is based on minimizing intake by grazing animals, since plant uptake is minimal. Various analyses from across the U.S. have also shown that sludges can sometimes contain unusually high concentrations of other metals or elements. Again, by keeping apprised of the chemicals added to its system by industry, a sewage treatment plant should know when to test for certain elements to see whether levels are high enough to pose a plant growth or crop contamination problem. Surface water contamination by runoff.