Groundwater origin and recharge in the hyperarid Cordillera de la Costa, Atacama Desert, northern Chile

Abstract

The Cordillera de la Costa is located along the coastline of northern Chile, in the hyperarid Atacama Desert area. Chemical and isotopic analyses of several small coastal springs and groundwater reservoirs between 22.5 °S and 25.5 °S allow understanding groundwater origin, renewal time and the probable timing of recharge. The aquifers are mostly in old volcanic rocks and alluvial deposits. All spring waters are brackish, of the sodium chloride type due to intensive concentration of precipitation due aridity and for deep groundwater to additional water-rock interaction in slowly renewed groundwater and mixing with deep seated brines. The heavy δO and δH values in spring water are explained by recharge by the arrival of moist air masses from the Pacific Ocean and the originally lighter values in the deep wells can be associated to past recharge by air masses coming from the Atlantic Ocean. Current recharge is assumed almost nil but it was significant in past wetter-than-present periods, increasing groundwater reserves, which are not yet exhausted. To explain the observed chloride content and radiocarbon (C) activity, a well-mixed (exponential) flow model has been considered for aquifer recharge. The average residence time of groundwater feeding the springs has been estimated between 1 and 2kyr, up to 5kyr and between 7 and 13kyr for deep well water, assuming that current recharge is much less than during the previous wetter period. The recharge period feeding the coastal springs could have been produced 1 to 5kyr BP, when the area was already inhabited, and recharge in the Michilla mine was produced during the 10 to 14.5kyr BP CAPE (Central Andean Pluvial Event) pluvial events of the central Andes. The approximate coincidence of turnover time with the past wet periods, as revealed by paleoclimate data, points to significant recharge during them.

Global Isotope Hydrogeology―Review

“…The inset in the lower left shows two processes thought to influence many groundwater dissolved inorganic carbon isotope compositions: (i) the aging of groundwater during storage in aquifer systems (decreasing 14 C with no change in δ 13 C) and (ii) the dissolution of marine carbonates (mixing trend towards δ 13 C ≈ 0 ‰ PDB and 14 C ≈ 0 pmC). The global groundwater dissolved inorganic δ 13 C versus 14 C trend lies intermediate to values expectedly arising from each of these two processes, implying they may be dominant controls on global groundwater dissolved inorganic carbon isotope compositions (e.g., see figures by Azzaz et al, , and Herrera et al, ).…”

Isotopic characteristics and paleoclimate implications of the extreme precipitation event of March 2015 in northern Chile

“…Groundwater isotopic compositions reflect a time-integrated amount-weighted average of precipitation (Aravena, 1995;West et al, 2014). In some areas of the Atacama Desert groundwater recharge continued during the latter half of the 20th century (e.g., Houston, 2007), whereas other aquifers primarily recharged hundreds to thousands of years ago (Aravena, 1995;Herrera and Custodio, 2014;Herrera et al, 2018). Within that spectrum of recharge are insights to both recent climate and paleoclimate states.…”

“…Within that spectrum of recharge are insights to both recent climate and paleoclimate states. For example, spring water for coastal Antofagasta province (climate zone II) of apparent mid-Holocene age (Herrera and Custodio, 2014;Herrera et al, 2018) (Fig. 3A) has δ 18 O near zero (+0.2‰ to -2.2‰), evidence that those coastal springs recharged by precipitation derived from the adjacent Pacific Ocean.…”

Reassessing Hydrological Processes That Control Stable Isotope Tracers in Groundwater of the Atacama Desert (Northern Chile)

“…[25]. Vapor masses that contribute to precipitation in the Andes can originate from east, north-east and west-north-west (most significant sources: Amazon basin and Atlantic Ocean) [23,27,28]. However, the PdT itself which is situated in the Andean rain shadow did probably not experience notable amounts of precipitation in the Late Quaternary [29,30].…”