Effects of land application of waste water from Mexico City on soil fertility and heavy metal accumulation: a bibliographical review

Abstract

Waste water from Mexico City metropolitan area has been used to irrigate the Mezquital Valley since 1912. This zone is nowadays the largest area under this situation. Mexico City's drainage is a water mixture emanating from different sources: household, industries, commerce, rain, and runoff. Waste waters used in the valley have greatly increased crop yields. Owing to the increase in the country's industrial development, since the seventies several studies have been carried out in this area, especially related to metal content in water, soils, and crops. The aim of this paper is to present a bibliographical review of the effects of Mexico City's waste water land application on soil fertility and heavy metal accumulation. Pb, Cd, Cu, and Zn levels in soils and crops at present do not represent risks, but there is a clear tendency for these metals to accumulate in the upper layers of soils. Salt content in waste water is intensifying soil salinization in impeded drainage areas. The local conditions of soil, such as high levels of organic matter and high pH values, seem to be preventing the solubility of heavy metals, and hence their transportation or absorption by crops.Key words: heavy metals, waste water irrigation, Mexico City, nutrients, environment, pollutants.