Fflstitoïû DÉ Ihfbtiíatíom 6MI«Î PRELIMINARY REI ORT on THE

fflSTiTOïû DÉ IHfBTIíAtíOm 6MI«Î CASILLA 411 - TELEFONO 2010 !:v ' , :<;3 fomento ARICA ¡ r-r’. ‘ -Mirso,-. ’ /¿Micos Cfr.a. Archivo ! fo. PRELIMINARY REI ORT ON THE GROUnO*'..âTI® RZiSOUHCES OF THE ARICA ARPA CHILE by Filliao W. Doyel Santiago, Chile mniTOIO DÍ IHÏESTIfifiaOBES GEOLOGICAS CASILLA 411 . TELEFONO 2010 ARICA Contents Introduction Topography Resume of geology Ground-water possibilities A zapa valley Lluta valley La Concordia Additional Investigation Summary and Conclusions Illustrations Figure 1. Map of the Arica area. mimio oí iRvraionn geolim CASILLA 411 - TELEFONO 2010 AR I C A PRELIMINARY REPORT OF THE CROUND-MER RESOURCES OF THE ARICA AREA, CHILE by "William Tí. Doyel INTRODUCTION Water supplies in the Arica area come from two sources, wells in the Asapa valley and the Lluta river (fig. 1). There being prac- «■ ^to»w — —«w ■——««> Figure i near here. tically no rainfall west of the pre-cordillera the populace is en tirely dependent on the water from these spurces The San.José river as a peremial flow in its upper reaches but flows to the océan only in the. form of summer flood-stage flow which lasts for a few days or at most a few weeks and which occurs only every 4 or 5 years. Some o he flood water is used to a limited extent for irrigation but most of it is lost into the ocean. The Lluta river has a peremial flow, approximately 2 mJ in the-.winter and increasing during the summer months, but the water is of poor quality end" has an adverse effect on certain types of soil when used for irrigation. Tráten levels in wells in. the Azapá valley have been declining for the past few years and some wells in the lower part of the val ley, near Arica, have bean abandoned. The increase in the number of drilled wells and in the withdrawals of.ground water have been re sponsible for the déclines. mnmio di iMHïi&fltioBn gemims CASILLA 411 - TELEFONO 2010 ARICA water from the Lauca river on the Altiplano, approximately 100 kilometers east of ?.rica, is being diverted into the Azapa Valley and used for irrigation. Although this will help alleviate the water shortage that exists in the area, the development of additional sup plies is essential for the continued economic growth of the city. As part of an effort to solve the problem an investigation was initi ated cooperatively by the Corporación de Fomento, lección Aguas Subterráneas, and the Instituto de Investigaciones Geológicas in 1959 to explore the possibility of supplementing the present supplies with ground-water from new sources and to secure sufficient data in the Azapa valley to assure the maximum beneficial use of the ground-water supplies. - 2 - INSTITUTO DE INVESTIGACIONES GEOIOGIEM SILLA 411 - TELEFONO 2010 ARICA TOPOGRAPHY Arica lies at the northern end of the Cordillera de la Costa and at the mouth of the San José river (Azapa valley). Beginning at Arica and extending northward into Perú is a plain,, the Concordia, which is hounded on the eastern side by a north-south scarp and on the western side by the Pacific Ocean. The plain increases in width northward, being approximately 8 kilometers wide at the Peruvian border, and slopes gently from the scarp to the ocean. The scarp marks the western edge of a westward-dipping, flat plain extending from the pre-cordillera. The San José, LIuta, Galli nazos, and Escritos rivers have dissected the hi hland, although the Gallinazos and Escritos rivers now have no flow other than at widely separated intervals. The rivers issue onto the Concordia from steep walled valleys in the scarp. The surface of the Concordia, which was formed as a depositional terrace, is now composed of st least 3 erosional terraces, marking stages in the last recession of the sea. The Gallinazos and San José rivers have incised steep-walled channels in the surface and the Llu- ta river has cut a valley containing a serie* of terraces showing the progressive lowering of the base level of the river. The Lluta and Gallinazos rivers have built alluvial f'ins- at their mouths and the San José is building a delta. The present coast line is marked by a wave-cut scarp 1 to 2 meters in height and a steeply sloping sandy beach. 5 - mîlïfiîO DE IWEÍMDIIEJ GEOfilíM CASILLA 411 - TELEFONO 2010 ARICA Due to the lack of precipitation in the krica area the noly ero sive agents at work, outside of the river valleys, are the wind, earthquakes and gravity, as reflected by the present-day topography. Numerous blocks have fallen from the steep walls of the river valleys as the result of egrth movements. Kost of the blocks apparently fell during Pleistocene times, but there is some evidence, in the form of cracks, that present-day movements ere having an effect* nniiiîo de ibîmeioub deoiek CASILLA 411 - TELEFONO 2010 ARICA RESUEE OF GEOLOGY The geology of the Arica quadrangle has been described in detail by Doyel and Henriquez (1962) and of the area from Arica to Visviri, ón the Bolivian border, by Henriquez (1962). The following Is a sum mary of the geology of the area as related to the occurrence of ground water. The oldest rocks in the area outcrop in El Eorro and consist of lavas, lutites and sands of Jurassic Age. They represent the northern end of the coast range. Overlying the Jurassic rocks is the Arica formation composed of interbelded sinds and clays of probable early Tertiary age. The forma tion crops out in the baae of the scarp at the northeast edge of Arica (near the pipe-line terminal of Sicfe-'ica), and has surface expression scuth of the Azapa Valley. The Lluta formation, which overlies the Arica formation uncon- formably, forms thé platform extending eastward to the pre-Cordillera. The formation consists of conglomerate, sands and pyroclastic materials and is of late. Tertiary-early Pleistocene age. The Chuiio conglomerate of Pleistocene age, is present in the form of erosional romta3nts on the surface of the platform roamed by the Llu ta formation. The conglomerate is composed of rounded fragments of andesite and sand. The Pleistocene terrace extending northward from Arica is wade up of interbedded sediments of both curine and continental origin. These sediments, which have been penetrated to a depth or 200 meters in COREO te3t well Concordia number 1, compose the Concordia formation. - 5 - [BiïlTBIO OE lOmîIGMIOHES 6EOLOGIHS CASILLA 411 • TELEFONO 2010 ARICA Hecent sediments are precent in the area in the form of colluvium at the base of the scarp and the Trails of the river valleys, as allu vial deposits of the rivers and as rind-blown ssnd covering much of the area. There are apparently no recent sediments deposited by the Escritos river. However, the Gallinazos river has recent deposita near its mouth ^nd the Lluta river has thin flood pljiin, swamp and alluvinl fen deposits, representing Recent sedimentation. The Can José river* 1rs filled its rater course with corree unsorted allu vial materials and ic building a delta at its mouth. The f .ce of the scarp .'nd the surface of the platform are covered with a layer of salt-encrusted materials, either sandy or tuffaceous, that io as hard as if a lrcyer of concrete had been laid over the area. - 6 - IKÎIÎIÏO DE 6E0L06IEM CASILLA 411 - TELEFONO 2010 ARIC GROUND-ÎÎATER POSSIBILITIES The only possibilities of developing ground-water supplies in the area are in the A zapa valley, which is presently overdeveloped. The Lluta Valley'and the Concordia; or, in other words, in the Pleis- tocene sediments. The sands'and conglomerate pt the base of■the Lluta formation might contain water, but their depth and lack of recharge possibilities preclude their being an aquifer of importance. The Arica for mation contains sands that might serve as aquifers but has been subjected to diaetrophic forces and, dipping generally from south to north, has no recharge area. Azapa Valley. The Azapa valley is filled with less than 100 meters of unconso lidated sediments of Pleistocene and Recent age in its lower portions A test well drilled by COREO near Altos de Ramirez (COREO, Arica N°9) encountered tuffaceous materials, probably of the Lluta formation, at approximately 70 meters and "black rock", which probably corresponds to the Jurassic rocks, at 167 meters. An unconfined aquifer is present in the ulluvial deposits filling the valley and has been exploited for many years. Formerly wells were dug by hand to the water table and the water was lifted by hand; but in recent.years motorized pumps have been installed and numerous drilled wells have been completed. The increase in withdrawals of water from the aquifer and in the concentration of pumpage have resulted in a general lowering of the water table in the lower portion of the val ley, near Arica. Numerous shallow wells have been abandoned, having gone dry as the water t^ble dropped.? mniTOTO oc inianMiioHB smimi CASILLA 411 - TELEFONO 2010 ARICA Recharge to the rater table consists of seepage from the Sen José river, which h?s a perennial flow in its upper reachest from the floods that occur periodically in the lo er p rt of the valley and from irri“ gation water. Little recharge apparently is taking place in tho area of withdrawal, from Jabuza to ».rica; The effects of the ©venidas are unknown, but because of the high velocities, sediment lead and short duration, the floods probably are not an important source of recharge in the lower part of the valley. Cater levels in veils in the A a apa '¿alley vary from 58 meters below the surface at Cabuza to 20 meters below the surface near the south of the valley.

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