Architecture of the aquifers of the Calama Basin, Loa catchment basin, northern Chile

Abstract

In the Loa water system of the Atacama Desert in northern Chile, careful management of groundwater is vital and data are sparse. Several key management questions focus on aquifers that occur in the Calama sedimentary basin, through which groundwater and Loa surface water flow to the west. The complexity of the two major aquifers and their discharge to wetlands and rivers are governed by primary facies variations of the sedimentary rocks as well as by faults and folds that create discontinuities in the strata. This study integrates geological studies with groundwater hydrology data to document how the aquifers overlay the formations and facies. Neither the phreatic aquifer nor the confined or semiconfined aquifer, each of which is identified in most basin sectors, corresponds to a laterally persistent geological unit. The variable properties of low-permeability units sandwiched between units of moderate to high permeability cause a patchwork pattern of areas in which water is exchanged between the two aquifers and areas where the lower aquifer is confined. The westward termination of most of the sedimentary rocks against a north-trending basement uplift at an old fault zone terminates the principal aquitard and the lower aquifer. That termination causes lower aquifer water to flow into the upper aquifer or discharge to the rivers. The regionally important West fault juxtaposes formations with differing lithological and hydraulic properties, resulting in some exchange of water between the upper and lower aquifers across the fault.