146 Marin and A/cocer Karst Frontiers Karst Waters lnstitute Special Publication 7 THE CHICXULUB IMP ACT CRA TER AND THE REGIONAL HYDROGEOLOGYOFNORTHWESTYUCATAN Luis E. Marin Sección de Hidrogeologia y SistemasHidrotermales Departamento de Recursos Naturales Instituto de Geofisica Universidad Nacional Autónoma de México Mexico City, Mexico CP 04510 Javier Alcocer Laboratorio de Limnología, Proyecto CyMA, UIICSE Facultad de Estudios Profesionales-Iztacala Universidad Nacional Autónoma de México Mexico City, Mexico CP 54090 Introduction water lens, there is sa1inewater. The sa1ine water has been mapped more than 110 km from the coast (Marín, 1990; Perryeta1., 1995; SteinichandMarín, 1996; Steinich, 1996). This aquifer is high1y vu1nerab1e to contamination both from above, from anthropogenic activites, and from be1ow, due to the presence ofthe saline water (Marin and Perry, 1994). Perry etal, (1995) and Velázquez Oliman (1996) have shown that this sa1ine water has two components: sa1t water in1rusion and disso1ution of evaporites. An overview of the geochemistry ofthe Peninsu1a ofYucatan is given by Perry eta1 (inreview). Steinich (1996) mapped over6,000 cenotes (sink holes) and aguadas in Northwest Yucatan. Schmitter- Soto et al., (in press) present a geochemical classification of the cenotes for the Peninsula of Yucatan as well as a description of the biota found in them. In this extended abstract, we present a short overview ofthe Chicxulub Impact Crater, the hydrogeology ofthe Nothwest Yucatan Peninsula followed by the influence that the Chicxulub Impact Crater exertsover the regional tlow system and a short description of someof the environmenta1 concems current1y facing the Peninsula ofYucatan. Chicxulub Impact Crater Undemeath the carbonate and evaporite rocks of northwest Yucatan lies one ofthe largest impact crater found on Earth, namely, the Chicxulub Impact Crater. Sharpton et al., (1992) have demonstratedample evidenceof impact metamorphism including shocked quartz, impact melt rock, etc. According to some authors, this crater may be as large as 300 km in diameter (Sharpton et al., 1993), although other authors have suggested that the crater is on the order of200 km in diameter (Morgan et al., 1997). Sharpton et al., (1992) and Marin et al., (2002) have shown that the age ofthe impact crater is 65 mil1ion years linking it to the massextinctions that occurred at the K-Tboundary. Figure 1 shows a3-D representation of the gravity anomaly over the northwest Yucatan Peninsula. A summary of the Chicxulub Impact Crater is given in Sharpton et a1.,(1996). The aquifer is unconfined, except for a narrow band parallel to the coast,where it is confmed. The confming bed is found approximately five kilometers from the coast at Dzilam de Bravo to 22 kilometers at Celestún. The confming layer extends approximately five kilometers into the sea (Perry et al., 1989, 1990). A fIrst order survey conducted by the Mexican Mapping Agency, INEGI (Instituto Nacional de Estadística, Geografía, e Informática) allowed the measurement of the extremely low hydraulíc gradient. It is on the order of 7-10 mm/km (Marín et al., 1987; Marin, 1990). The water table map for northwestem Yucatan is shown in Figure 1. Regional ground water flow directions are from south to north. Marin (1990), Steinich and Marín (1996), and Marínetal., (2001) have shown that the Ghyben- Herzberg relation holds for Northwest Yucatan with a ratio of 1:40. Figure 2 shows a map of the thickness of the freshwater lens using a Ghyben- Herzberg relation of 1:40. Regional hydrogeology The Yucatan Peninsula obtains its only source ofwater from a sole-sourceaquifer (Ward et al., 1985). The aquifer consists of a mature karstic system. This aquifer is a thin fresh water lens that has a thickness of only 60 meters underneath the City ofMérida with a population greater than 600,000 plus inhabitants (Escolero et al., 2000). Underneath this fresh