Heavy metal risks to human health in farmland of wastewater-irrigated areas have long been recognized. It remains to be shown whether farmland heavy metals from wastewater irrigation can migrate to deeper soil at a regional scale.
A study by YANG Shushen from Center for Agricultural Resources Research, Institute of Genetics and Developmental Biology of the Chinese Academy of Sciences showed that heavy metals from wastewater irrigation can migrate downward to deep soil of 30 m, and wheat grains across the whole sampling area are of health risks in terms of total metal concentrations. The study was published in Environmental Pollution.
The researchers analysed a total of 400 soil layer samples, 41 composite wheat samples and 41 composite topsoil samples which are from deep soil sampling of nine soil cores down to 30 m and a regional sampling of farmland topsoils and wheat grains.
Considerably high contents of heavy metals in both total and soluble forms were detected in deep soils, especially for one transect where total As of 73.0 mg·kg-1 at 29 m, Cd of 3.80 mg·kg-1 at 13 m and Pb of 214 mg·kg-1 at 30 m were detected.
Across the studied area, 19.5%–34.1% of the topsoil samples were contaminated by As, Cd, Cu, Pb and Zn, and 34.1% and 19.5% of the wheat grains were contaminated by Cd and Pb, respectively.
Conventionally, it is thought that downward migration of farmland heavy metals are normally less than 2 m, while this study revealed a depth of 30 m of heavy metal downward migration, and the topsoil and wheat grain sampling covered more than 10 km2.
The findings of this study revealed that long-term wastewater irrigation has a wider and deeper impact than expected, and suggest that measures are urgently in need for the metal pollution control, and the remediation of deep soil down to groundwater table.
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