Discrimination between diffuse and point sources of arsenic at Zimapán, Hidalgo state, Mexico

Abstract

There are two principal sources of arsenic in Zimapán. Point sources are linked to mining and smelting activities and especially to mine tailings. Diffuse sources are not well defined and are linked to regional flow systems in carbonate rocks. Both sources are caused by the oxidation of arsenic-rich sulfidic mineralization. Point sources are characterized by Ca–SO₄–HCO₃ ground water type and relatively enriched values of δD, δ¹⁸O, and δ³⁴S(SO₄). Diffuse sources are characterized by Ca–Na–HCO₃ type of ground water and more depleted values of δD, δ¹⁸O, and δ³⁴S(SO₄). Values of δD and δ¹⁸O indicate similar altitude of recharge for both arsenic sources and stronger impact of evaporation for point sources in mine tailings. There are also different values of δ³⁴S(SO₄) for both sources, presumably due to different types of mineralization or isotopic zonality in deposits. In Principal Component Analysis (PCA), the principal component 1 (PC1), which describes the impact of sulfide oxidation and neutralization by the dissolution of carbonates, has higher values in samples from point sources. In spite of similar concentrations of As in ground water affected by diffuse sources and point sources (mean values 0.21 mg L⁻¹ and 0.31 mg L⁻¹, respectively, in the years from 2003 to 2008), the diffuse sources have more impact on the health of population in Zimapán. This is caused by the extraction of ground water from wells tapping regional flow system. In contrast, wells located in the proximity of mine tailings are not generally used for water supply.