UPLAND RECHARGE IN THE COAHUILAN EDWARDS AQUIFER

The northeastern corner of the Mexican state of Coahuila contains some of the most extensive limestone outcrops in the country, yet the karst and groundwater have seen relatively little investigation compared to other areas. The western (recharge) portion of the area contains ridge tops of Lower Cretaceous rocks over 1500 m in elevation, which slope down to the east and south to plunge underneath less karstic Upper Cretaceous outcrops. This represents a confined aquifer zone where wells can flow under artesian pressure. In this respect it resembles the Edwards Aquifer across the Rio Grande in Texas, with similar carbonate lithologies. Proportions of recharge between upland and drainage channel sources have been the subject of debate in the Texas portion of the Edwards.

Recent karst investigations to the northwest of Múzquiz have shown that upland caves and sinkholes play a significant role in aquifer recharge. Three north-south trending anticlinal ridges have horizontally-bedded mesas extending off of their east sides. Much of the rainfall on these mesas enters recharge features and never makes it to surface drainages. Over 125 of these mesa recharge features have been identified thus far, resulting in 30 mapped caves. Five of these are among the ten deepest in the state, dropping rapidly in vertical shaft series up to 240 m deep. Loose silt plugs at the bottom of these shafts likely represent a nearby base level where flow deceleration causes sediment loads to drop. Eventual exploration past these plugs may reveal horizontal conduits leading onward to springs and the artesian zone of the Coahuilan Edwards Aquifer.

In the lower part of the recharge zone close to the artesian zone, there are a number of caves which are estavelles. These have large funnel-shaped sinkhole entrances that slope down to pits, dropping up to 90 m to intersect flowing streams. These sinkholes normally take water, but during times of heavy rains in the recharge zone they can become springs. A number of large artesian springs occur to the east of the Sierra del Burro and the Sierra de Santa Rosa. These comprise a regionally important water source that is under increasing demand. The Coahuilan Edwards is home to the endangered Mexican Blindcat Prietella phreatophila, yet knowledge and regulation of aquifer conservation is lacking.