Ground-penetrating radar, resistivity and spontaneous potential investigations of a contaminated aquifer near Cancún, Mexico NCKRI Symposium 2: Proceedings of the Thirteenth Multidisciplinary Conference on Sinkholes and the Engineering and Environmental Impacts of Karst

pg(s) 231-237 Geophysical surveys were made over portions of the Cancún municipal well field in the Yucatán Peninsula of Mexico, approximately 20 km southwest of the city of Cancún, in order to identify karst conduits that channel contaminated surface waters into the main aquifer. Specifically, ground-penetrating radar (GPR), vertical electrical soundings (VES) and spontaneous potential (SP) surveys were employed to identify these conduits and detect water movement through them. Cancún s municipal water supply has frequently been affected by fecal coliform bacteria and other contaminants. Water supplies are largely derived from highly permeable fractured karst limestone aquifers characterized by rapid transport of microbial and chemical contaminants from the surface to subsurface unconfined and confined aquifers. Quaternary and Tertiary limestone bedrock outcrops across this entire area, which exhibits less than 3 m of local relief. Schlumberger array VES were made at two locations.. One sounding revealed a 3-layered structure consisting of a 177 ohm-m layer 2.1 m thick, (probably weathered limestone), overlying a high resistivity layer 8.2 m thick (massive limestone with some small caves), overlying saturated limestone (45 ohm-m). The other sounding could not be successfully inverted due to lateral resistivity variations. Twenty-one GPR profiles were also made with 50- and 100-MHz antennas along roads passing through the well field. In the upper 5 m these profiles reveal cut-and-fill structures and a myriad of diffractions that may represent collapsed and filled sinkholes or solution-enlarged fractures. A major interface delineated by GPR at about 6-8 m depth probably represents the water table. An unusual transparent zone (absence of GPR reflections) was also visible in one GPR profile made near a surface conduit. This transparent zone was at least 1.5 m wide and extended over several meters depth. SP measurements near this conduit during a rainstorm revealed a peak-to-peak variation of 16 mV, suggesting SP may also be a viable method for mapping subsurface water movement in this well field. The overall implication of this work is that geophysical methods are valuable in delineating recharge points and shallow contaminant pathways, and should be used more extensively in this part of the Yucatán Peninsula to support groundwater investigations. Open Access - Permission by Publisher See Extended description for more information