WATER SUPPLY OPTIONS FOR A VILLAGE ON A PLEISTOCENE SHIELD VOLCANO IN THE TRANS-MEXICAN VOLCANIC BELT

Tamaula, a village of approximately 400 residents in the state of Guanajuato, Mexico, is located near the top of a Pleistocene shield volcano in the Trans-Mexican Volcanic Belt. In May 2008 geology students and faculty from Utah Valley University assessed the water supply options for Tamaula. Tamaula's population relies on three local sources for water: 1) six natural springs, 2) two reservoirs, and 3) rooftop catchment systems. These sources are insufficient and additional water must be trucked over 10 miles of dirt roads. The village council has considered three options for providing additional local water: 1) development of additional springs in a perched aquifer, 2) expansion of the catchment area of reservoirs, and 3) development of a deep (250-650 m) well to access the main water table, as recommended by the State Water Commission based on time-domain EM soundings. The viability of the perched aquifer was assessed by performing a bail-down test in which approximately 2500 L was removed from one spring that had been developed into a shallow well. The head was observed over a period of nine days. Using the Bouwer-Rice slug test and assuming the depth of the aquifer equals the depth of the well, the hydraulic conductivity K of the aquifer was found to be K = 2.1×10^-7 m/s, which is typical for shallow fractured basalt. The recharge of the spring was equivalent to a yield of 120 L/day. A drop in water level from a second spring 150 m away indicated that all springs utilized by residents may be tapping the same perched aquifer and that further development of the aquifer is unlikely to satisfy the village's estimated culinary water need of 20 L/person/day. The village council did not wish to pursue the development of surface water, while the proposed deep well to be drilled beneath the volcano has the potential to yield highly mineralized water besides being expensive to drill, operate and maintain. We are evaluating the fourth option of an intermediate-depth (~50 m) well that could access the water from multiple perched aquifers. This option is based upon the model of a succession of gently dipping lava flows with the upper portion of each lava flow acting as an aquifer and the lower portion acting as a leaky confining layer. The likely yield of such a well will be evaluated using Visual ModFlow with published data on hydraulic properties of lava flows.