ISOTOPIC EVOLUTION OF RIVER WATER IN THE NORTHERN CHILE REGION

Two main northern Chilean rivers, the Loa and Tarapaca, were investigated regarding their isotopic characteristics. Groundwater associated with various recharge zones, and the input of tributaries along their courses, mainly control their (18O, 2H) isotopic composition of the rivers. In the Loa river, carbon isotopic exchange between dissolved inorganic carbon (DIC) and atmospheric CO2 plays a minor role in the inorganic carbon cycle of the Loa river. The carbon isotopic composition (14C, 13C) of this river is probably controlled by carbon source associated with volcanic and geothermal activities and by the deposition of travertines. For the Tarapaca river the carbon isotope content of the DIC reflects the input of recirculated water from irrigated areas along the river course and carbon isotopic exchange. These findings imply that the input of volcanic and/or geothermal CO2 into the DIC pool has to be evaluated in order to use carbon isotopes as a dating tool for groundwater in the Loa basin and that modern conditions are not analogous to the paleohydrology of the Tarapaca river. Copyright 1990 by the American Geophysical Union.