HYDROGEOLOGY OF A SHIELD VOLCANO IN THE TRANS-MEXICAN VOLCANIC BELT

Tamaula, a village of 400 residents in Guanajuato, Mexico, is one of many poor communities living on extinct shield volcanoes within the Trans-Mexican Volcanic Belt without sufficient sources of potable water. Prior to the summer of 2009, the residents of Tamaula relied on two muddy reservoirs, several rooftop catchment systems, six springs yielding only 120 L/d for the most productive spring, and 4000 L of water per week delivered by the municipality of Irapuato. In August 2009 a site for a 50-m-deep well was chosen based on a dream. The resulting well yields 50 L/s, making it as productive as the most productive wells in the valley surrounding the volcano and completely transforming the life of the village. The objective of this study is to determine why this particular well site was so successful and whether the current well production is sustainable so that the knowledge can be used to aid similar poor communities on shield volcanoes. Twelve springs were located on the volcano and water samples were collected from the springs, reservoirs and well. Electrical conductivity, pH and temperature were measured on site. Concentrations of As, Fe, Cu, Ni, Co, Mn, Zn, Cr and F were measured with a spectrophotometer. The size of the aquifers will be estimated by applying cluster analysis to the spring and well chemistry. A comparison of spring locations with fracture traces seen on aerial photos shows that two springs lie along a shorter fracture trace, while 11 springs lie along a longer fracture trace that includes the well site. Slug tests were carried out during several stages of the drilling. The slug test results will be combined with description of the rock chips collected every meter of drilling to determine the probable thickness of the aquifer. The discharge from the well and the resulting long-term change in head are being monitored by the local high school students so that the sustainability of the well can be determined. Results will be reported at the meeting.