Présentations Colloques

Oral Presentation
Session 8.04: Isotopic and residence time tracers
Ouysse Samira
Combined use of physico-chemical parameters and isotopic composition to characterize groundwater flow systems and their response to intensive extraction
The objective of this study is to develop a methodology which emphasizes the combined use of isotopes, geochemical analyses, geological features, soil and vegetation characteristics in assessing the effect of intensive groundwater extraction in San Luis Potosi Catchment, Mexico. The study area is located at the eastern boundary of the Central Alluvial Basins+ it has been subject to intensive groundwater extraction since late 1980’s. Reported drawdown values under current extraction (≈+4.0 m3 s) range from 0.5 to 4.0 m year. During the last two decades, studies proved Cl, Li, F and temperature to be used as potential residence time indicators+ increase in temperature accompanied by increase in Cl, Li and F was found indicative of long residence time. However, during a recent investigation Li and F contents registered a slight decrease with Cl and temperature increase in some sampled wells. Adapting appropriate sampling techniques+ the challenge lies in understanding how the change in chemical age proxies relates to stable and radioactive isotopes dating tools and how their combined use with regional parameters could identify different groundwater flow systems and their vulnerability to intensive extraction. This study proved effective, regarding the use of chemical and isotope dating tools to determine groundwater origin+ thus, groundwater is represented by an intermediate flow characterized by relatively cold water (22-25°C), with low Li (0.01 mg l) and F (0.3 mg l) contents. The δ+18O shows depleted values (-10.09 to -11.09 ‰)+ and the corrected groundwater age ranges from 1,000 to 3,300 years BP. Comparatively, induced regional groundwater flow system following intensive extraction, shows high temperature (30-34°C), high Li (0.18 mg l) and F (3.9 mg l) contents+ and more enriched δ+18O values (-9.2 to -10.4 ‰). The corrected regional groundwater age ranges from 1,300 to 6,500 years BP, suggesting longer residence time and deeper travelling conditions. **