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 delta O-18 of the lake water by about 6% compared to that of input water. The Sr-87/Sr-86 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 Sr-87/Sr-86 ratios of strontium in water from streams that flow through elastic sedimentary rocks east of the lake is more radiogenic (> 0.7100) than elsewhere in the watershed, where the Sr-87/Sr-86 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 Rio Chacachimpa, the Rio San Juan, and the Rio Palcamayo and about 60% of the total water input to the lake, the rest coming from direct precipitation. (c) 2005 Elsevier B.V. All rights reserved.