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.
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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.
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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.
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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 possibili-
ties 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 dril- led 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. Formerly there v.ere springs in various parts of the valley, but they have ce sed to flow ar- the water table declined. Periodic v.ater-level measurements in selected observation wells have shown that the water table is rising at C buza, although still recli ning downstream. The rise can be atributed to two causes, although their relative importance cannot be judged. Most of the rise probably is due to recharge from the exeptionully high flows of the pest two years, in the intermittunt as veil os in the perennial flow portion o: the streams, lart of the rise may be the result of the arrival of the Lauca water and the accompanying decrease in 'withdrawals in the Cabuz r rea.
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The waiter table slopes toward the Pacific Ocean and fresh ground water is being discharged into the ocean at Arica. However» the lowering cf the water levels could cause a reversal of the water-table gradient and cause the incursion of salt water into the aquifer at Arica. The salt water could enter only that portion of the a iuifer that is below sea level. For example» the new well at the Municipal
Stadium (Corfo rica N°14) is approximately 30 meters above sea level and the static water level is 18.4 meters below the surface, or approx-' imatly 1.1.6 meters above ee.'i level. If the water level wore to drop below see level salt vwter could enter that portion of the aquifer which is below ser. level (more th¿in JO.meters below; the surface).
Lluta /alley.
Three wells drilled by fiego in 1955 ?;nd one drilled by CC 1FC in 1962, inthe Lluta Valley failed to encounter under-ground water. The CORFO.well was terminated in rock (probably Jurassic), at 520 meters. Although swampy areas slong the water course show that the river is influent in some portions and effluent in others, there if no aquifer present in the alluvial sediments filling the valley. mnilBÎO OE IWIMIDIIEJ 6M0G1MS CASILLA 411 - TELEFONO 2010 ARICA
La Concordia. In the Concordia Peruana, 7 kilometers north of the border, several water wells were drilled about 10 ye°rfi ago, the ground-water being used for irrigation* The concordia Chilena being geographic ally, and geologically, the sane, the CCRFO decided to start test drilling program, ac a result of which 4 wells have been completed between the Uuebradu Pscritos end the border. In addition, a well supplies water for the Chacalluta rirport and the Carabineros Retén at the border. The four test wells, Concordia numbers 1, 2, 3 and 4, encountered artesian water in sand and gravel strata between 100 and 130 meters deep in wells îï° 1, 2 and 5, and between 75 and 83 and 110 and 125 meters in well 4. The water rose to 1. .8 meters below the surface in well N® 1* .ell N° 2 v.**s pumped r.t a rz,te of 150 liters a second for 120 hours with a. drawdown of 8.5 metGrs, giving a specific capacity of 17*6 liters/second per meter of drawdown. At the end of the test the water contained 835 milligrams per liter of total solids, 247 of chlor ides, 172 of sulfates and 368 total hardness. The source of recharge to the aquifer is unknown and there is a possibility that there is no recharge, the r. .ter representing "fossil water", or rrter that entered the .^uifer when the climatic f ctors uere more favorable. On the other hand, several factors indie: te that there is recharge. The f area of higher potential to one of lower potential. Also, the quality of the we ter shows that the aquifer was flushed of much of its salt content following deposition of the sediments. The source and amount of recharge are unknown and a planned development could not. be car ried out lacking this information. There are 4 possibilities for sources of recharge: from the Lluta vlley, rainfall in the area, the Azapa v^lney, or from the Peruvian Concordia. The results of the test drilling in the Lluts valley indicate that it is not one of the sources of recharge. K'infall in the drainage basins Of the Gallinazos and "scritos rivers might furnish some recharge, but rainfall in the area could not be recharging the artesian aquifer. The elevations of the voter table and the depth of the aquifer in the ¿zapa Valley show that w. ter moving into the sediments of the Con cordia from the valley could not supply the artesian head found in the test wells that have been completed The remaining possibility is prob bly the correct one: water is moving from the Peruvian Concordia into the Chilean part. The nature of the sediments is such that the a jiifer being exploited, in the Peru is the same s the one that is pre-rent in Chile. The slope of the pie some trie surface indie tes that the water is r.oving in a southwest erly direction; in other words, coming fro- Peru. The Tacna river has a perennial flow and a w ter-t¿ble ¡uifer is present in the valley, and recharge to the artesian aquifer could bo t aking place at eleva tions high enough to supply the rtesian he d found in the Concordia. 11 IIJHIÏ0IO Di miGIIWJ CEOlBCiriS CASILLA 411 - TELEFONO 2010 A*R I C A Alsor precipitation in the drainage area of the river reportedly is greater than further south. Therefore, if the artesian aquifer in the Concordia Chilena is being recharged, the water,is probably moving into the Concordia Peruana portion of the aquifer and then passing into the Chilean part. The possibilities of salt-water intrusion would have to be con sidered in any ground-water development in the Concordia. If salt J water is present in down dip portion of the aquifer, or if sea water has access to the aquifers, a decline in artesian pressure as a result of pumpage could result in a reversal of the gradient and the encroach ment of salt water. ~ A shallow water-table aquifer was encountered in test wells Con- 1 cordia l, 2 and 3 at 15-to 20 meters below the surface. The aquifer is probably recharged by the infrequent rains in the area and could not be used for large, sustained withdrawals Additional test drilling is being carried out by the CORFO in the Arica area. A well was completed to 395 meters near the mouth of the Lluta Valley, but the results were inconclusive. Other wells.are «Í now being drilled to determine, if possible, the depth of the Pleis tocene sediments, as well as to locate any deeper aquifers. The pro gram will also establish whether or not there is a hydrologic connection between the sediments of the Azapa Valley and those of the Concordia. ÍÜHIÍÍI0 D[ Iflfmmu fiEOLOOKH CASILLA 41 1 - TELEFONO 2010 ARICA ADDITIONAL INVESTIGATION In order to completly utilize thé ground-water resources of the Arica area it will be neceseaiy to continue the test-drilling program and to carry out additional» technical studies. A quantitative study should’be made in the Azapa valley to determine the maximum quantity of water that can be withdrawn without endangering the supply. To accomplish this it would be necessary to conduct aquifer-performance tests to determine the aquifer characteristics, to make seepage measurements, to determine the recharge In the *upper part of the val ley as fcell as. from irrigation in the lower part, to determine the effect of* the Lauca water, both as recharge and to ascertain the rate movement of water through the aquifer.- For the artesian aquifer in the Concordia, it would be necessary to determine the geological continuity of the aquifer, the aquifer characteristics, the direction and rate of movement of the water, the source (or sources) of recharge, the location of any fresh-water-salt water interface and the maximum safe yield of the aquifer ÎMTITBÎ8 BE 6E0L06IUS CASILLA 411 - TELEFONO 2010 ARICA SUMMARY AND CONCLUSIONS The ground-water investigations being carried out by the IIG and the CORRO has shown that the additional water supplies necessary for the development of the Arica area probably can be obtained from two ground-water reservoirs; the presently exploited unconfined aquifer in the Azapa Valley and the artesian aquifer in the Concordia Chilena. À detailed scientific study would lead to a better utilization of the water from the unconfined aquifer. The water table is declin> ing because of overpumpage and will continue to decline unless the maximum safe yield of the aquifer is determined and utilized. The recharge increments from upstream, downstream and the Lauca water should be determined and the possibility of artificial recharge studied The test-drilling program has shown that an artesian aquifer underlies the Concordia Chilena and that it is probably contiguous with the aquifer being exploited in the Concordia Peruana. It is not known if the aquifer is being recharged, but if so the recharge probably is coming from the Peruvian highland to the northeast and from the Tacna river valley. More information is necessary, but results of the in-- vestigation to the present indicate that the aquifer is capable of supplying much of the water requirements of the city of Arica. The possibility of salt-water encroachment in both the water-t&bl* . and artesian aquifers as a result of withdrawals should be studied. Test wells have been drilled in the lower part of the Lluta Valle but no ground-wátér ‘supplies were encountered. - 14. -