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

The chemical and isotopic compositions of rainwater were monitored at Los Azufres geothermal ®eld (88 MWe) and its surroundings during May ± September 1995, which is the rainy season. Samples were collected from eight sites: three within the ®eld, three in its surroundings and two suciently far from the ®eld such that they have no geothermal input. The concentrations of Cl ÿ , SO4 2ÿ and NO3 ÿ 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 ÿ , SO4 2ÿ and d34S also suggests an industrial source for the rainwater sulfur. The determination of pH was found to be necessary, but is not sucient to characterize rainwater acidity. The Gran titration method was used to determine alkalinity with respect to equivalence point of H2CO3 . Values of alkalinity were found to range from 10 ÿ4 to 10 ÿ6 eq/L, and were negative only for some samples from Vivero and Guadalajara. Thus, SO4 2ÿ and NO3 ÿ are in general not in acidic form (i.e. balanced by Na+, Ca2+, etc. rather than H+). Sulfate d34S values were about ÿ1.5- in Los Azufres and its surroundings, and in Morelia, but di€ered from the value of ÿ0.2- for Guadalajara. The d34S values for H2S from the Los Azufres geothermal wells are in the range ÿ3.4 to 0.0-. The d34S ranges for 0375-6505/00/$20.00 7 2000 Published by Elsevier Science Ltd on behalf of CNR. PII: S0 37 5 -6 50 5 (0 0) 00 0 24 -9 Geothermics 29 (2000) 593±608 www.elsevier.com/locate/geothermics * Corresponding author. Tel.: +52-73-183-811; fax: +52-73-182-526. E-mail address: mahendra@iie.org.mx (M.P. Verma). the natural and anthropogenic sources for environmental sulfur overlap, making it dicult to di€erentiate between the contribution of di€erent sources. However, a similarity of values of d34S at Los Azufres and Morelia (85 km distant) suggest a regional source of sulfate that is not associated with geothermal emissions from Los Azufres. 7 2000 Published by Elsevier Science Ltd on behalf of CNR.