Constraining water sources and hydrologic processes from the isotopic analysis of water and dissolved strontium, Lake Junin, Peru

This paper presents the results of a hydrogeochemical study of the Lake Junin watershed, Peru, to evaluate the controls over chemical composition of water in this remote Andean site. Stream, spring, and lake waters in the Lake Junin basin are all Ca²⁺ and HCO₃^- type. Evaporation has enriched the δ¹⁸O of the lake water by about 6‰ compared to that of input water. The ⁸⁷Sr/⁸⁶Sr ratio of dissolved strontium varies from 0.70777 to 0.72242 in the watershed because of water interactions with limestone and silicate rocks, respectively, in the watershed. The ⁸⁷Sr/⁸⁶Sr ratios of strontium in water from streams that flow through clastic sedimentary rocks east of the lake is more radiogenic (>0.7100) than elsewhere in the watershed, where the ⁸⁷Sr/⁸⁶Sr ratios of surface waters range between 0.7076 and 0.7090. These lower ratios are consistent with marine limestone sources found in these watersheds. Synoptic discharge measurements, combined with the strontium isotopic data, show that that three rivers provide over 90% of the surface water to the lake: the Río Chacachimpa, the Río San Juan, and the Río Palcamayo and about 60% of the total water input to the lake, the rest coming from direct precipitation.