The irrigation fields of Paris have been used for 100 years. Their soils mainly contain heavy metals in the topmost layer. Metals come from raw sewage as well as from digested sludge of biological treatment plants which have been diluted for years in raw water. Vegetables that are cultivated in the irrigation fields concentrate metals but their average contents, however, are lower than the recommended limit values. Some vegetables concentrate more specifically one type of metal. Corn seeds accumulate less metal than green vegetables. The SIAAP keeps operating irrigation fields by delivering clariflocculated water with a low metal content from the new Seine Centre plant, with the purpose of keeping some 2,000 ha of green zone in an otherwise heavily constructed area and to prevent a metal release from the soil should irrigation be interrupted. Maintaining irrigation fields also relieves the biological treatment plant and then contributes to preserve the quality of the Seine river, especially in summer.

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