Origin of rainwater acidity near the Los Azufres geothermal field, Mexico

Abstract

The chemical and isotopic compositions of rainwater were monitored at Los Azufres geothermal field (88 MWe) and its surroundings during May – September 1995, which is the rainy season. Samples were collected from eight sites: three within the field, three in its surroundings and two sufficiently far from the field such that they have no geothermal input. The concentrations of Cl −, SO 4 2− and NO 3 − were measured in about 350 samples and found to be generally <5 ppm. Chloride concentrations remained constant with time, but sulfate and nitrate concentrations decreased, which suggests a nearby industrial source for the sulfate and nitrate. A mixing model for Cl −, SO 4 2− and δ 34S also suggests an industrial source for the rainwater sulfur. The determination of pH was found to be necessary, but is not sufficient to characterize rainwater acidity. The Gran titration method was used to determine alkalinity with respect to equivalence point of H 2CO 3 ∗. Values of alkalinity were found to range from 10 −4 to 10 −6 eq/L, and were negative only for some samples from Vivero and Guadalajara. Thus, SO 4 2− and NO 3 − are in general not in acidic form (i.e. balanced by Na +, Ca 2+, etc. rather than H +). Sulfate δ 34S values were about −1.5‰ in Los Azufres and its surroundings, and in Morelia, but differed from the value of −0.2‰ for Guadalajara. The δ 34S values for H 2S from the Los Azufres geothermal wells are in the range −3.4 to 0.0‰. The δ 34S ranges for the natural and anthropogenic sources for environmental sulfur overlap, making it difficult to differentiate between the contribution of different sources. However, a similarity of values of δ 34S at Los Azufres and Morelia (85 km distant) suggest a regional source of sulfate that is not associated with geothermal emissions from Los Azufres.