Hydrogeochemistry and stable isotopes of ground and surface waters from two adjacent closed basins, Atacama Desert, northern Chile

Abstract

The geochemistry and stable isotopes of groundwaters, surface waters, and precipitation indicate different sources of some dissolved constituents, but a common source of recharge and other constituents in two adjacent closed basins in the Atacama Desert region of northern Chile (24°15′–24°45′S). Waters from artesian wells, trenches, and ephemeral streams in the Punta Negra Basin are characterized by concentrations of Na>Ca>Mg and Cl≥SO 4,with TDS<10 g/l. Values of δD and δ 18O for Punta Negra Basin waters follow an evaporitic trend typical of closed basin waters in northern Chile and elsewhere. In contrast, ground waters in the Hamburgo Basin, located about 25 km NW of the Punta Negra Basin, have concentrations of Na > Mg≥Ca and SO 4 >Cl, with TDS also <10 g/l. Aqueous speciation calculations indicate that Hamburgo Basin groundwaters are close to saturation with respect to gypsum. The relatively high SO 4 and low Ca in Hamburgo Basin waters result from SO 4 influx and subsequent gypsum precipitation related to weathering at La Escondida, a large porphyry copper deposit located near to the center of the basin. Deep mine waters from 130 m below the water table at La Escondida also have Na>Mg≥Ca and SO 4 >Cl, but with TDS up to 40 g/l. The deep mine waters have pH between 3.2 and 3.9, and are high in dissolved CO 2 (δ 13C= −4.8%PDB), indicating probable interaction with oxidizing sulfides. The deep mine waters have δ 18O values of ≈−1.8%.compared with values < −3.5‰ for other Hamburgo Basin waters; thus the mine waters may represent a mixture of meteoric waters with deeper “metamorphic” waters, which had interacted with rocks and exchanged oxygen isotopes at elevated temperatures. Alternatively, the deep mine waters may represent fossil meteoric waters which evolved isotopically along an evaporative trend starting from values quite depleted in δ 18O and °Dd relative to either precipitation or shallow groundwaters. High I/Br ratios in the Hamburgo Basin waters and La Escondida mine waters are consistent with regionally high I in surficial deposits in the Atacama Desert region and may represent dissolution of a wind-blown evaporite component. Rain and snow collected during June 1984, indicate systematic δ 18O and δD fractionation with increasing elevation between 3150 and 4180 m a.s.l. (−0.21ℵ.δ 18O and−1.7ℵ.δD per 100 m). Excluding the deep mine waters from La Escondida, the waters from the Hamburgo and Punta Negra Basins have similar δD and δ 18O values and together show a distinct evaporative trend (δD = 5.0 δ 18O− 20.2). Snowmelt from the central Andes Cordillera to the east is the most likely source of recharge to both basins. Some of the waters in the Hamburgo Basin may have been recharged during late Pleistocene, when the climate was wetter and a lake filled the intervening Punta Negra Basin, as suggested by recent archaeological and geomorphological studies.