Abstract
Abstract. Improving our understanding of hydrogeological processes on the western flank of the central Andes is critical to communities living in this arid region. Groundwater emerging as springs at low elevations provides water for drinking, agriculture, and baseflow. However, the high-elevation sources of recharge and groundwater flow paths that convey groundwater to lower elevations where the springs emerge remain poorly quantified in the volcanic mountain terrain of southern Peru. In this study, we identified recharge zones and groundwater flow paths supporting springs east of the city of Arequipa and the potential for recharge within the high-elevation closed-basin Lagunas Salinas salar. We used general chemistry and isotopic tracers (δ18O, δ2H, and 3H) in springs, surface waters (rivers and the salar), and precipitation (rain and snow) sampled from March 2019 through February 2020 to investigate these processes. We obtained monthly samples from six springs, bimonthly samples from four rivers, and various samples from high-elevation springs during the dry season. The monthly isotopic composition of spring water was invariable seasonally in this study and compared to published values from a decade prior, suggesting that the source of recharge and groundwater flow paths that support spring flow is relatively stable with time. The chemistry of springs in the low-elevations and mid-elevations (2500 to 2900 m a.s.l.) point towards a mix of recharge from the salar basin (4300 m a.s.l.) and mountain-block recharge (MBR) in or above a queñuales forest ecosystem at ∼4000 m a.s.l. on the adjacent Pichu Pichu volcano. Springs that clustered along the Río Andamayo, including those at 2900 m a.s.l., had higher chloride concentrations, indicating higher proportions of interbasin groundwater flow from the salar basin likely facilitated by a high degree of faulting along the Río Andamayo valley compared to springs further away from that fault network. A separate groundwater flow path was identified by higher sulfate concentrations (and lower Cl-/SO4-2 ratios) within the Pichu Pichu volcanic mountain range separating the city from the salar. We conclude that the salar basin is not a hydrologic dead end. Instead, it is a local topographic low where surface runoff during the wet season, groundwater from springs, and subsurface groundwater flow paths from the surrounding mountains converge in the basin, and some mixture of this water supports groundwater flow out of the salar basin via interbasin groundwater flow. In this arid location, high-elevation forests and the closed-basin salar are important sources of recharge supporting low-elevation springs. These features should be carefully managed to prevent impacts on the down-valley water quality and quantity.
Highlights
Predicted climate change in the tropical Andes includes increases in temperature and evapotranspiration and changes in precipitation patterns (Urrutia and Vuille, 2009; Somers et al, 2019)
Spring samples collected at the queñuales forest zone have an isotopic composition similar to springs in the Characato District and the weighted average of rain sampled in Characato in 2019 (δ18O = −9.5 ‰±0.7 ‰, δ2H = −66 ‰±5 ‰) (Fig. 3)
Samples from Laguna Salinas surface water (LSA) obtained in the dry season (October 2019) had a highly enriched isotopic composition, while LSA sampled during the wet season (April 2019) had an isotopic composition of −16.3 ‰ δ18O and −122 ‰ δ2H, similar to springs in that area (Fig. 3)
Summary
Predicted climate change in the tropical Andes includes increases in temperature and evapotranspiration and changes in precipitation patterns (Urrutia and Vuille, 2009; Somers et al, 2019). A ∼ 10 %–30 % reduction in precipitation is projected for the western Andes in the coming century (Minvielle and Garreaud, 2011; Neukom et al, 2015). These changes are projected to have a negative impact on the long-term sustainability of critical groundwater resources in southern Peru (Vuille et al, 2018). High-elevation zones of groundwater recharge in the Andes are extremely sensitive to the interplay between evapotranspiration and precipitation due to climate and anthropogenic land-use change altering vegetative land cover, energy balance, and water balance. The importance of MBR on groundwater resources in the Andes and how they might respond to changing precipitation and temperature and melting glaciers is poorly understood (Somers et al, 2018, 2019)
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