Recharge to groundwater in the Turi Basin, northern Chile: An evaluation based on tritium and chloride mass balance techniques

The Turi Basin contains an unconfined phreatic aquifer within the Toconce Formation volcano-sedimentary sequence that passes eastwards under the Quaternary volcanic chain and outcrops in the Altiplano. The phreatic aquifer is floored by impermeable welded Miocene ignimbrites. Precipitation captured by the volcanic edifices infiltrates through high permeability soils to recharge the phreatic aquifer. The hydrochemistry of the groundwater in the phreatic aquifer shows little evidence of mixing with geothermal water and therefore approximates to a closed system. Tritium values demonstrate that recharge is occurring under current climatic conditions and takes around 40 years to pass 10+ km from recharge areas on the slopes of the volcanoes and the Altiplano to the center of the basin. The chloride mass balance technique has been used to quantify precipitation recharge at 15,500 m^3 d^-1. This is significantly less than the flow through the phreatic aquifer (58,000-86,000 m^3 d^-1) calculated from its physical properties and the difference is interpreted to be due to subsurface inflow through the Toconce Formation from the Altiplano. The results of this study have wider application throughout the Andean hydrological environment of northern Chile.