Evaluation of a Coastal Aquifer in Northern Peru Using Chemical and Isotopic Methods

The Mancora aquifer, located along the northern coast of Peru, is the primary source of fresh water for Mancora and surrounding areas. This dry region has undergone a rapid increase in tourism during the past 20 years, which is now the primary source of its economy. Because the aquifer is located adjacent to the ocean and susceptible to salt-water intrusion, an improved hydrogeologic understanding is needed for its sustainable development in providing future water for the community. The current study of the Mancora aquifer is being conducted under the International Atomic Energy Association Isotope Hydrology Division’s “Application of Isotopic Tools for the Integrated Management of Coastal Aquifers” program, with the principal objectives being the hydrogeologic characterization and evaluation of sources of salinity. The Mancora aquifer is an unconsolidated aquifer primarily composed of fluvial sand and gravel deposits covering an area of about 760 square kilometers in the Mancora Basin adjacent to the Quebrada Fernandez ephemeral stream. Surface geophysics and drilling conducted as part of this study indicate that the aquifer is up to 45 meters deep and as much as 200 meters wide. The current conceptual model of the aquifer is based upon geochemistry, isotopes, and dissolved gases collected during diverse hydrologic conditions in order to identify both the temporal dynamics of the system and the principal zones of recharge. Isotope and dissolved-gas samples included oxygen-18, deuterium, tritium/helium, noble gases, chloroflourocarbons, and sulfer hexaflouride. Preliminary findings suggest that during wet periods, recharge to the aquifer is dominated by seepage from the stream; during dry periods when the stream is dry, the primary source of recharge is from underlying and adjacent bedrock aquifers. Geochemical evidence suggests that seepage from these bedrock aquifers may be the primary source of salinity to the Mancora aquifer, rather than the initial hypothesis of seawater intrusion.