Historical trace metal fluxes in the Mexico City Metropolitan Zone as evidenced by a sedimentary record from the Espejo de los Lirios lake

Abstract

The accumulation of selected trace metals (Ag, Cd, Co, Cr, Cu, Hg, Ni, Pb, V and Zn) was studied in a sediment core collected at Espejo de los Lirios lake, a precipitation-dominated seepage lake in Northern Mexico City Metropolitan Zone (MCMZ). A ²¹⁰Pb-derived chronology, obtained from the same core, was used to reconstruct the historical metal fluxes at the site, allowing evaluation of the impact of environmental changes promoted by the development of the City during the last ∼125 years. The highest levels of metal enrichment above natural concentration levels (NCL) in the sediments from Espejo de los Lirios lake were found for Ag and Pb (∼250%) as well as a slight enrichment for Cd (55%), Cr (84%), Co (20%), Cu (60%), Hg (47%), Ni (45%), V (59%) and Zn (66%). Fluxes of trace metals appeared to have noticeably increased from the last 45 years showing the maximum increments for Cd, Co, Cr, Ni, V and Zn during the 1980's (9 to 13 fold natural fluxes), for Ag and Cu (17 and 12 fold, respectively) during the 1990's and for Hg and Pb (2 and 13 fold) during the middle 1970's. Low levels of metal enrichment observed have evidenced that the most conspicuous consequences of the expansive growth of this area of the MCMZ, are mostly related to deforestation and erosion of the surrounding areas, rather than to trace metal pollution. Based on PCA, it can be assumed that atmospheric deposition, weathering of bedrock and soil within the watershed and authigenic production, are the most important processes that explain the trace metal distribution in the site.