WELL-SITE LOCATION USING HYDROGEOLOGIC AND THERMONIC METHODS IN THE VICINITY OF LE?N, GUANAJUATO, MEXICO
William M. Turner, Ph.D.
The water well field serving the city of Le?n in the State of Guanajuato in Mexico is mining the aquifer. The Secretaria Recursos Hidraulicos predecessor to CONAGUA decided to explore for ground water in areas distant from the existing well field. The primary area of exploration selected is south of the community of Lomita de San Angel. The general area of the study is shown in Figure 1.
Past experience with costly water wells has been that their yields vary over a wide range of 10 to 158 l/s. To ensure that any new water wells are high production wells, AGW was contracted to locate optimum well sites using its Thermonic methods.
HYDROGEOLOGICAL SETTING
The Le?n, Gto. area is characterized by thick sequence of extrusive volcanic rocks of Tertiary and Quaternary age. Rhyolite, basalt, andesite and volcanic ash and tuff deposits are present. Overlying the volcanic rock and possibly interbedded with it are thick deposits of clastic lacustrine rock of Quaternary age. Associated with the Rio Turbio, the major river in the area, are clastic deposits of Holocene age.
The distribution of good wells and poor wells south of Purisma del Rincon and the lithologic logs from these wells suggests that the Purisma del Rincon - Lomita de San Angel part of the Rio Turbio Valley may be a down-faulted crustal block on which clastic sediment and basalt flows accumulated to a great thickness.
The geology of the area and water-level data suggest that the Rio Turbio is a line source of recharge. Water running in the river percolates rapidly into the ground and moves downward to the saturated zone.
AGW scientists conducted the work reported here during heavy rains and thousands of acre feet (millions of cubic meters) of water recharged in a period of several days along the Rio Turbio.
PROCEDURE
We measured temperature logs in existing wells in the area south of Le?n and north of Lomita de San Angel. In addition, the temperature of pumped water was measured from a number of pumping wells in the Le?n area.
THERMONICS
An isobathythermonic map showing isothermonic contours at a depth of 33 meters beneath the land surface was prepared. The map indicates that a zone of rapid ground-water flow and high aquifer transmissivity extends southwest from Le?n towards Lomita de San Angel. Of note, however, is the fact that the zone of high transmissivity is not co-linear with the Rio Turbio but is oblique to it. The zone of high transmissivity may correspond to the thickest or best sorted saturated sequence of basalt and clastic sediment.
RESULTS
The best well sites are within the Rio Turbio Valley. The optimum well sites are located in the zone of high transmissivity determined from the Thermonic survey. The zone of high transmissivity extends south of Lomita de San Angel. This area was inaccessible because of poor weather and flooding.
DISCUSSION
The most important observation made in this study is the fact that major zones of ground-water flow and the best well sites in alluvial valleys do not necessarily coincide with the course of the surface stream. On consideration, this is reasonable because we must remember that the alluvial aquifer is the result of a three-dimensional construction process by a river that frequently changed course within an aggrading valley over time. It is the vertical section of saturated material that has the best-sorted, clastic material that will have the highest transmissivity under a hydraulic gradient that is equivalent to the grade of the valley.
? 1999 2016 AGW Consultants. All Rights Reserved.