PRELIMINARY HYDROLOGIC SURVEY OF THE SIERRA TARAHUMARA, CHIHUAHUA, MEXICO: WATER CHEMISTRY

The objective of this study was to carry out a hydrologic survey of 185 km² of the Sierra Tarahumara volcanic tuff region near the village of Creel, Chihuahua, Mexico. This work was done in cooperation with a Jesuit mission that drills wells for the Tarahumara Indians and has been drilling an excessive number of dry wells at a cost of $US 3500 per well. Water samples were collected from 11 wells, six springs, and eight streams. Wells were purged before sampling. It was not possible to filter samples due to the abundance of clay and samples were not preserved with acid. Electrical conductivity (EC), pH and temperature were measured on site. The Hach DR-2700 Spectrophotometer was used to measure As, Fe, Cu, Mn, Co, Ni, Zn, K, Cl^-, SO₄^-2, Ca as CaCO₃, and Mg as MgCO₃. Average As concentrations were As = (0.08 ± 0.02) mg/L for groundwater and As = (0.17 ± 0.07) mg/L for surface water with the highest concentrations being As = 0.403 mg/L for groundwater and As = 0.619 mg/L for surface water. All As concentrations exceeded the WHO standard of As = 0.01 mg/L and the Mexican standard of As = 0.025 mg/L. Elevated average concentrations were also measured for the heavy metals that are normally associated with As, which were Fe ((0.20 ± 0.03) mg/L for streams and (0.3 ± 0.1) mg/L for groundwater), Cu ((0.021 ± 0.005) mg/L for streams and (0.09 ± 0.05) mg/L for groundwater), Mn ((0.33 ± 0.06) mg/L for streams and (0.4 ± 0.1) mg/L for groundwater), Co ((0.034 ± 0.008) mg/L for streams and (0.06 ± 0.02) mg/L for groundwater), Ni ((0.013 ± 0.005) mg/L for streams and (0.04 ± 0.02) mg/L for groundwater), and Zn ((0.23 ± 0.08) mg/L for streams and (1.8 ± 0.5) mg/L for groundwater). For groundwater there was a good positive correlation (R² = 0.79) between As and Fe, which suggests that elevated As concentrations result from oxidation of pyrite or arsenopyrite. For surface water there was an excellent positive correlation (R² = 0.93) between As and EC, which suggests that streams that are high in As are fed by groundwater. There do not appear to be any abandoned mine tailings upstream of any sampling sites. Although related work identified four aquifers, there were no statistically significant differences among aquifers for any measured parameters, implying that confined aquifers are connected by leaky confining layers. Future work will focus on appropriate water treatment.