Hydrogeochemical characterization of glacierized watersheds in the subhumid inner tropics

Abstract

Mountain glaciers in the tropics are particularly vulnerable to climate change, asthe fastest rates of warming are predicted in high altitude, low latitude regions. Current observations indicate a decrease in streamflow in many tropical glacierized watersheds in the Andes, where 99% of tropical glaciers are located. I investigate two Ecuadorian watersheds in the subhumid inner tropics, Gavilan Machay on Volcán Chimborazo and Río La Dormida on Volcán Cayambe, where seasonal changes in precipitation are mild, leading to year-round ablation. I characterize these watersheds using a hydrochemical mixing model to distinguish glacier melt and groundwater contributions to stream discharge. Using XRD analysis, feldspar and pyroxene minerals were identified in soil and bedrock in both watersheds, confirming an andesitic lithology and informing groundwater tracer selection for the mixing model. Six synoptic water sample sets were collected from the Gavilan Machay (August 2018, January 2019, June 2019, and January 2020) and La Dormida watersheds (January 2019 and January 2020), which were analyzed for naturally occurring tracers and stable water isotopes. Isotopes and ions were used as tracers in a hydrochemical mixing model, which when combined with past work, provided estimates of varying glacier contributions to total stream discharge from 14- 65% in Gavilan Machay and 3-9% in La Dormida. Absolute glacier melt contributions varied from 0.02 to 0.06 m3/s in Gavilan Machay and 0.02 to 0.05 m3/s in La Dormida. After comparison with a direct measurement of melt contribution to discharge at the glacier terminus, we suspect these estimates may underrepresent the amount of total glacier melt that reaches the watersheds’ outlets. It is possible some glacier melt infiltrates and travels in the subsurface, picking up a groundwater hydrochemical signature before discharging to the stream. The spatial distribution of ion concentrations illuminates contrasting patterns in the watersheds, with a concentrating effect of stream chemistry as we move downgradient in the watershed in Gavilan Machay and a diluting effect in La Dormida, due to the presence of a hydrothermal spring. Future work should constrain uncertainties is discharge measurements and provide additional sample periods, particularly in La Dormida.