Abstract

Farmers from the Pica Oasis localities, foothills of northern Chile, take advantage of groundwater at different depths in the irrigation of citrus crops, tropical fruits and vegetables. In order to establish a monitoring network and provide recommendations to the irrigators, the physicochemical properties of these waters were characterized, their annual temporal behavior and their quality for irrigation was determined, as well as to interpret factors that affect the composition. 29 wells at three depths (deep, intermediate and superficial aquifer) were managed for the network and the temperature, CE and pH were recorded in four campaigns between 2016 and 2017. The parameters quantified at <0.45 µm were TDS, major ions, nitrate, trace elements Cr-Mn-Fe-Co-Ni-Cu-Zn-Mo-Al-Cd-Pb-Hg-Li-Ba-B-As and the indicators % Na, SSP, SAR, RCS and PS. There are spatial variations, but not temporary, of the quality of the thermal groundwater, neutral to slightly alkaline. The vast majority are waters suitable for irrigation according to CE, RAS and CSR and there is no risk from metals and metalloids; except for % Na, PSS, SP, boron and lithium (citrus). The deep aquifer (80-100 m) has fresh waters of low EC, bicarbonated and/or sulphated sodium, but the level of % Na and PSS affects the soil; the intermediate (24-39 m) has fresh and brackish water, sulphated sodium and % Na, PSS and harmful SP. The surface aquifer (17-21 m) contains higher salinity (brackish), sodium-calcium and is distributed in the boundary between sulfated bicarbonated or sulphated chloridated, are in breach of regulations for chloride, sulfate and nitrate. The increase of dissolved ions in the surface aquifer is attributed to the dissolution of sulphated by alluvium in the area, bad practices in irrigation-fertilization processes and the aridity condition.