Evidences and characterization of groundwater circulation in dryland piedmonts+ impacts on aquifer recharge. Example of the Andean Piedmont between 19.5°S and 20°S (Northern Chile).

International audienceA piedmont acts as a transition zone, shaped by an alluvial cover overlying the bedrock, between i) amountain range area and its lowland+ ii) a high and low precipitation area which generally correlatedwith the relief in drylands. These transition zones can be associated to significant groundwatercirculation, from the mountain front recharge zones towards a downstream aquifer, generally locatedin the lowlands. Nevertheless, due to several constraints (rugged terrains, significant vadose zone) thedensity of observation points (e.g. well) in the piedmont is very restricted, whereas a large number ofwells (groundwater exploitation) are usually available in the lowlands. Consequently, both groundwatercirculation and aquifer recharge in the piedmont cover are poorly addressed and thus often neglected.The Andean Piedmont (between 19,5°S and 20°S - Northern Chile) is located in the Pampa delTamarugal (PdT) Central Depression. The piedmont cover and the lowlands are composed of late -Cenozoic alluvial deposits (hundreds of meters thick) which contain the Pampa del Tamarugal Aquifer(PTA). In this arid and hyperarid climatic context, the PTA is one of the main strategic groundwaterresources in Northern Chile. In order to determine both groundwater circulation and aquifer rechargewithin the piedmont, we used various approaches such as (i) Time Domain Electromagnetic (TDEM)method to assess resistivity variations with depth+ (ii) stream flow measurements+ (iii) analysis ofcurrent springs location and paleogeomorphic features of the piedmont cover and (iv) hydrogeologicalmodelling. This allowed to better constrain the aquifer boundaries with the geological structures+ toidentify preferential recharge areas and to highlight a regional groundwater flow patter n in the alluvialcover. This methodology, transposable to other dryland piedmonts, allowed providing a newconceptual model of the PTA