A Regional Groundwater Evaluation Using Magnetotelluric Soundings For Monclova, Mexico

A regional groundwater evaluation and exploration project was completed for the city of Monclova, Mexico in<br>1998. Monclova is located in the desert (13.7 inches rainfall per year) and is dependent upon groundwater for its<br>municipal and industrial resources. Its population (332,000) is expected to grow to 1,200,OOO by 2046. The<br>municipal wells are completed in Cretaceous carbonate units up to 3,300 feet deep and 25 mgd of water is pumped<br>from the wells. Monclova has a deficiency of 9.4 mgd with a projected shortfall of 91 mgd in 50 years. In addition,<br>substantial dewatering of the aquifer is due to over pumpage.<br>The study began with an evaluation of regional water resources. Previous drilling programs encountered large<br>variations in production between closely spaced wells. The main aquifers are steeply dipping fractures and faults<br>generally located in the vicinity of the axis of large anticlinal structures forming the local mountain ranges in the<br>Coahuila folded belt of Northern Mexico.<br>A magnetotelhnic (MT) was conducted to assess new well sites to meet the long and short term shortfalls. A total of<br>77 MT soundings were conducted along nine profiles with the Stratagem system (Geometries). The profiles were<br>conducted to find favorable structural targets in the dolomite aquifer at depths up to 3,000 feet. The profiles were<br>located as the result of photo geologic interpretation of the structural geology of the area. Bostick transformations of<br>the profiles were constructed and the soundings were one-dimensional modeled and correlated into geologic crosssections.<br>Four wells were drilled on the basis of the MT data. The wells were all excellent producers and make a combined<br>total of Smgd.