Variations of Pb in a mine-impacted tropical river, Taxco, Mexico: Use of geochemical, isotopic and statistical tools

The potential environmental threat from Pb in Mexican rivers impacted by historic mining activities was studied using geochemical, isotopic and statistical methods. Lead geochemical fractionation and factor analysis of fractionated and total Pb indicate that anthropogenic sources have contributed significantly to Pb concentrations, while natural sources have contributed only small amounts. The analyses also indicate that two main processes are controlling the total Pb variation throughout the year in both rivers: erosion with discharge processes, and proportional dilution related to differences in grain-size distribution processes. Bio-available Pb in riverbed sediments was greater than 50% in 80% of the sampling stations indicating a high potential environmental risk, according to the risk assessment criteria (RAC). Nevertheless, based on the environmental chemistry of Pb and on multivariate statistical analysis, these criteria did not apply in this particular case. Significant differences (p<0.05) in total Pb concentrations (from 50 to 5820mgkg−1) and in the geochemical fractionation were observed as a function of seasonality and location along the river flow path. In the Cacalotenango and Taxco rivers, the highest concentrations of total Pb were found at stations close to tailings during the rainy and post-rainy seasons. The geochemistry of Pb was mainly controlled, during the dry and post-rainy seasons by the organic matter and carbonate content, and in the rainy season by hydrological conditions (e.g., the increase in river flux), hydrological basin erosion, and the suspended solids concentration. Isotopic analyses of the 210Pb/214Pb ratio showed three processes in the Cacalotenango and Taxco rivers. First, the accumulation of atmospheric excess 210Pb, favoured during calmer hydrodynamic conditions in the river basin commonly during dry periods, is recorded by a 210Pb/214Pb ratio of >1. In the case of the Cacalotenango river, 210Pb did show preferential accumulation in sediments. Second, a 210Pb/214Pb ratio of <1 in some samples might be indicating (a) the presence of eroded material from weathered tailings with similar 210Pb depletion (probably by secular disequilibrium caused by weathering or mining processes, which was observed in both rivers), or (b) preferential transport of Pb and sediments during high energy events (e.g., flow increase, as is the case of Taxco river). Third, no significant changes in the 210Pb/214Pb ratio might be reflecting a situation where mining material is not entering the system, or where hydrodynamic changes throughout the year of equal magnitude allow the system to reach a new equilibrium for the 210Pb/214Pb ratio. Finally, based on these results it is recommended that inhabitants of the studied area avoid using water from the Cacalotenango river in the rainy and post-rainy seasons, and to take precautions for its use in the dry season, such as allowing suspended material to settle before use, and that they should avoid use of Taxco river water at all times.