Heavy metal accumulation in plants and soil irrigated with wastewater from Mexico city

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Abstract

Analysis by PIXE has been successfully employed in the study of environmental samples since it is possible to detect in a simple and fast way a high number of elements (heavier than Na) with a high sensitivity. In this work the PIXE method has been applied to study the extent of heavy metal contamination in soils and plants from Mixquiahuala, Hidalgo and Tláhuac, D.F. These two agricultural sites have been irrigated with wastewater from Mexico City for more than 50 years. The use of wastewater increased the crop productivity; however, in the areas irrigated for long periods of time, deterioration is now observed and only certain plant species can be grown. In this work, we present the range of variation that was observed for metals such as Fe, Cr, Co, Mn, Ni, Cu, Zn and Pb. Some of these elements were found at concentrations above the potentially hazardous levels in soils; however, they accumulated in plants to a lower extent, probably because of the physical and chemical properties of soils that prevent their translocation to plants.

Introduction

Analysis by PIXE has been successfully employed in the detection of trace elements in soils and plants because its multielemental character, high sensitivity and high sample throughput. In this study we present the accumulation of trace metals in plants and soils from Tláhuac, D.F. and Mixquiahuala, Hidalgo in the Mezquital Valley. These two agricultural sites have been irrigated with wastewater from Mexico City Metropolitan Area (MCMA) for more than 50 years. The Mezquital Valley is located at about 50 km from MCMA and receives most of its (MCMA) wastewater and rainwater. This valley is one of the largest areas in Latin America irrigated with wastewaters since 1890 [1]. Tláhuac is located in the Chinampa zone, in the southern part of Mexico City. The Chinampa agriculture production is a pre-Columbian method still practiced today. The Chinampas are small rectangular plots with an area about 500–1000 m2 irrigated by a channel network. The potable water extracted from wells is recycled and returned after passing through three treatment plants to the channels and reused for agriculture.

The objective of this study was to asses the accumulation of metals such as Mn, Fe, Co, Cu and Zn and heavy metals such as Ni, Pb and Cr in soils irrigated for different periods of time and their absorption by crops and fresh vegetables. Crop plants such as alfalfa and soils irrigated for 50 and 100 years respectively were collected from Mixquiahuala, Hidalgo. Fresh vegetables and soils irrigated for approximately 50 years with partially treated wastewaters were collected from Tláhuac.

Section snippets

Materials and methods

The two sites selected are shown in Fig. 1. Mixquiahuala is located 50 km north of Mexico City. Tháhuac is in the Southern portion of MCMA. Soil samples were collected from the top soil (first 15 cm). The main crops grown in Mixquiahuala include corn and alfalfa. Vegetables that grow close to the soil are rarely grown. In Tláhuac, in contrast, over 40 agriculture species are produced (vegetables, cereals, flowers) [2]. For this study, several edible vegetables were collected including greens,

Results and discussion

The average concentration and variation range of heavy elements measured in superficial soils from five sites irrigated for 50 years and 5 sites irrigated for 100 years in Mixquiahuala are shown in Fig. 2. Variation ranges of elements such as Cr, Mn, Fe, Ni, Cu, Zn and Pb are shown. Eventual appearances of As, Br, Rb, Sr, Cd, Zr and Hg were also detected. The toxic levels are included for comparison. The average levels in soils are very similar for most elements in spite of a difference of 50

Conclusion

The results from this study show that some elements such as Cr, Co and Cu are present at levels considered potentially hazardous in soils from Tláhuac and Mixquiahuala. In spite of the different origin soil properties and irrigation periods, all studied soils present similar average values of all elements.

Some elements such as Cu are present in soils as well as in plants at levels that exceed those considered as toxic. However, most of the other studied trace metals do not seem to be

Acknowledgments

The authors acknowledge the technical assistance of K. López and F.J. Jaimes y Rubén Cuellar for maintenance and operation of the accelerators. Work supported in part by UC MEXUS-CONACYT and DGAPA UNAM IN 228603-3. Alibech Mireles receives a fellowship from DGEP, UNAM.

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