Tracing the impact of glacial melt on the hydrology of the Andean Nevado Coropuna glacier and adjacent drainages

Abstract:

Glacial retreat in the tropical Andes has been accelerating in recent decades. The Nevado Coropuna glacier has lost 24% of its mass over the past thirty years and the effect of this retreat on water resources in adjacent watersheds is unknown. In order to evaluate the impact of rapid glacial retreat locally, hydrochemistry and water stable isotopes are used to quantify the contribution of meltwater to four drainages surrounding the glacier. Spring and surface waters were sampled during March and October 2019 in order to obtain representative samples of the wet and dry season changes. Hydroisotopic signatures of glacial ice, springs and surface waters collected in the north versus south facing slopes are different due to the altitude effect and the rain-shadowing effect on precipitation. The recharge elevation is determined to be between 6000 and 3000 masl using d¹⁸O-values of local precipitation collected from each side of the glacier. Radiocarbon age-dates of springs reflect the “inherited age” of the glacial meltwater and various degrees of mixing with modern recharge. As a result, the oldest groundwaters appear to be located nearer the glacier and get younger with increasing distance from the glacier as modern recharge is added. The lithium-magnesium geothermometer corroborates the geochemical evolution of groundwater along flow paths from bicarbonate type waters with a glacial meltwater end-member to sulphate type waters with a geothermal hot-spring end-member Sr²⁺ concentrations increase with distance from the recharge zone. The trend in Sr²⁺ indicates that large-scale groundwater flow systems with longer residence times occur along the southern slopes while the northern slope has smaller-scale groundwater flow systems with shorter residence times. These differences in flow path manifest in the proportion of glacial meltwater versus precipitation in groundwaters within each drainage as calculated based on an isotopic end-member mixing model. The southern drainages with longer residence times have a higher proportion of glacial meltwater contribution (avg. 73% glacial) than the northern slope (47% glacial). These data demonstrate the importance of glacial meltwater in providing dry-season discharge and sustaining the water supply during the long dry season in this region.