High-mountain groundwater quality affected by natural acid drainage

Abstract

Groundwater in high-mountain areas like the Central Chilean Andes is a crucial freshwater source for downstream communities. However, its pristine reputation masks a hidden threat when metallogenic systems exist: Natural Acid Drainage (NAD). This study comprehensively investigates the hydrogeological systems and the impact of NAD on groundwater quality in this copper-rich high-altitude region from an interdisciplinary approach. Specifically, the study area lies in the El Arpa Valley, a site with minimal human influence. Isotopic and hydrogeochemical analyses of groundwater, surface water, and snow samples revealed a groundwater origin between 2900 and 3300 m a.s.l. and the influence of fractures and gullies on recharge mechanisms. Physicochemical parameters exhibit increasing mineralisation downstream (118 to 714 µS/cm) with a pH range of 3.86–7.01. SO₄^2--Ca²⁺ facies and elevated aluminium (4.59–6349.31 ppb), iron (1.00–7003.24 ppb), and manganese (1.25–1098.41 ppb) contents characterise groundwater composition. Rock geochemistry and mineralogy show that phyllic alteration overprinted by supergene processes contributes to NAD by dissolving pyrite and iron oxyhydroxides. Principal component analysis on Landsat 8 images allows for identifying potential NAD areas over 11.6 % of the high Andes. The widespread occurrence challenges the perception of pristine mountain water, emphasising the potential adverse effects on human health and infrastructure, mainly due to high manganese content (>80 ppb). Findings advance the knowledge on NAD occurrence in remote mountainous regions, urging a reassessment of water quality perceptions in the presence of geogenic pollution sources, particularly considering the current threat of climate change.