LOCATION USING HYDROGEOLOGIC AND THERMONIC METHODS IN THE VICINITY OF LEÓN, GUANAJUATO,
William M. Turner, Ph.D.
The water well field serving the city of León 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.
The León, 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.
We measured temperature logs in existing wells in the area south of León and north of
Lomita de San Angel. In addition, the temperature of pumped water was measured from
a number of pumping wells in the León area.
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 León 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
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.
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.