Stable isotope evidence for the origins of waters in the Guayas estuary and Gulf of Guayaquil

The Guayas estuary in Ecuador is the largest estuary system along the western coast of South America. It is formed by interactions of fresh waters from the Guayas River and numerous smaller rivers, and marine waters flowing through the Gulf of Guayaquil from the Pacific Ocean. To better understand the sources of waters and their distribution in the Guayas estuary, we collected a suite of water samples at the end of consecutive dry (December) and rainy seasons (May) from locations distributed across whole estuary system plus a limited amount of historical information about precipitation. The measured salinities together with oxygen (δ 18 O) and hydrogen (δ 2 H) isotopic ratios of these samples were used to characterize the end-member compositions of waters flowing into the Guayas estuary from the ocean and primary riverine sources, and to consider environmental factors that affect end-member compositions. Inverse relationships exist between the isotope ratios of river waters and the mean elevations of the drainage basins of the rivers, which are situated along the western flank of the Andes Mountains; δ 18 O and δ 2 H values of river waters were more positive at the end of the dry season than the end of the rainy season. Observed δ 18 O–salinity and δ 18 O–δ 2 H relationships confirm the Babahoyo River, and to a lesser extent, the Daule River (e.g., Salado sub-estuary), are principal sources of fresh water for the entire Guayas estuary. Spatial and temporal variations of the δ 18 O–salinity relationships of waters within five estuary regions indicate how river influx, tidal circulation, resultant mixing of waters from different sources, and evaporation affect these regions during dry and rainy seasons. Estuarine Circulations, Water Cycle