Dependence of bioavailability of lead compounds in soils on the type and source of contamination (the example of Mexico City and mining area of Parral, Mexico)

Abstract

Evaluation and monitoring of soil contamination with heavy metals relies in most cases on their bulk concentrations whereas the risks of negative effects on human health depend upon the content of their bioavailable forms. The paper presents the study of the dependence of lead bioavailability in the surface soil horizons upon the contamination type and source. Two territories in Mexico with sharp difference of the source of pollution: Mexico City megapolis where the lead in soils originates from the ethyl gasoline burning and mining district in the south of Chihuahua State where the mine tailings are the source of the toxic elements including lead. Bulk lead concentrations were measured by the X-ray fluorescence method, bioavailability was estimated by the method imitating the effect of the stomach and intestine solutions. The research has shown that both in Mexico City (especially in its central part) and in the mining district in the Chihuahua State bulk lead concentrations are several times higher than the background values. The soils of the mining district have average bulk lead concentrations 787.5 mg/kg and show the average bioavailability values 64 % whereas in the Mexico City megapolis average bulk concentrations of lead are lower — 324.7 mg/kg but bioavailability reaches 83 % and in some samples even 100 %. These results support the conclusion that despite lower bulk lead concentrations in Mexico City the risk of its negative effect on humans is higher than in the mining district polluted with the wastes of mining industry. The explanation of the observed tendency is that the chemical form of lead has a major influence on its bioavailability. Dependence of bioavailability upon the bulk concentrations within each study territory has not been established.