GROUNDWATER AND SURFACE WATER IN YUCATAN AND QUINTANA ROO, MEXICO: WHY DOES ITS ISOTOPIC COMPOSITION DIFFER FROM THAT IN BELIZE?

We have collected water samples from public water supplies, open and covered cenotes (karst sink holes), lakes, and rain in Yucatan and northern Quintana Roo, Mexico for a decade (since 1998). These have been analyzed for oxygen and hydrogen isotopes in several laboratories with the objective of using the distinctive tropical storm isotopic signature reported by Lawrence (1998) as a groundwater tracer. Almost all of our water samples fall on or somewhat below the global meteoric water line, as would be expected in a dry tropical climate. Excluding large lakes such as Coba and Bacalar, which follow an evaporation trajectory, the median δ¹⁸O for our sample suite is -4.2‰; δD is -25.5‰, and the median deuterium excess is 7.9‰. Recently (2004), Marfia et al (Applied Geochemistry, 19, 937-946) have reported oxygen and hydrogen isotope values from Belize that correspond closely to our oxygen isotope values δ¹⁸O_groundwater = -4.4‰, δ¹⁸O_{surface water} = -3.9‰) but which differ significantly in hydrogen isotopic composition (δD_groundwater = -13.8‰, δD_{surface water} = -12.4‰); they also reported deuterium excesses from 10.0 to 40.8‰, which they attributed to processes related to evaporation. We are extending our study southward along the eastern Yucatan Peninsula to further evaluate this interesting effect.