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

Mp. Verma et al., Origin of rainwater acidity near the Los Azufres geothermal field, Mexico, GEOTHERMICS, 29(4-5), 2000, pp. 593-608
Citations number
Categorie Soggetti
Earth Sciences
Journal title
ISSN journal
0375-6505 → ACNP
Year of publication
593 - 608
SICI code
The chemical and isotopic compositions of rainwater were monitored at Los A zufres geothermal field (88 MWe) and its surroundings during May - Septembe r 1995, which is the rainy season. Samples were collected from eight sites: three within the held, three in its surroundings and two sufficiently far from the field such that they have no geothermal input. The concentrations of Cl-, SO42- and NO3- were measured in about 350 samples and found to be g enerally <5 ppm. Chloride concentrations remained constant with time, but s ulfate and nitrate concentrations decreased, which suggests a nearby indust rial source for the sulfate and nitrate. A mixing model for Cl-, SO42- and delta(34)S 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 dete rmine alkalinity with respect to equivalence point of H2CO3*. Values of alk alinity were found to range from 10(-4) to 10(-6) eq/L, and were negative o nly for some samples from Vivero and Guadalajara. Thus, SO42- and NO3- are in general not in acidic form (i.e. balanced by Na+, Ca2+, etc. rather than H+). Sulfate delta(34)S values were about -1.5 parts per thousand in Los Azufres and its surroundings, and in Morelia, but differed from the value of -0.2 parts per thousand for Guadalajara. The delta(34)S values for H2S from the Los Azufres geothermal wells are in the range -3.4 to 0.0 parts per thousan d. The delta(34)S ranges for the natural and anthropogenic sources for envi ronmental sulfur overlap, making it difficult to differentiate between the contribution of different sources. However, a similarity of values of delta (34)S at Los Azufres and Morelia (85 km distant) suggest a regional source of sulfate that is not associated with geothermal emissions from Los Azufre s. (C) 2000 Published by Elsevier Science Ltd on behalf of CNR.