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Hydrogeological conceptual model of the Camiña aquifer Matias Taucare1, Elisabeth Lictevout2, Claudio Moya2, Sonia Amaro2 1Universidad Católica del Norte, Antofagasta, Chile 2Centro de Investigación y Desarrollo en Recursos Hídricos (CIDERH), Vivar 493, Tercer Piso, Iquique, Chile Contact email: matias.taucare@gmail.com Abstract. Agriculture is the main activity for inhabitants in the Camiña quebrada. All the water utilized for irrigation comes from the Camiña River that flows westward across the quebrada. Although the importance of water resources in the area, it is still not clear how the hydrological system works. There is little information about geology in the area and nil about aquifers or groundwater flow. In order to characterize the hydrogeological system geological study, an inventory of springs and a TEM survey including nine points were done, to understand the aquifer behavior and its relationship with the river. After the elaboration of the geological map and the TEM data inversion and modeling, a hydrogeological conceptual model was developed and three different hydrogeological systems were identified, the Northern Flank, the Southern Flank and the Fluvial channel systems. Most of the differences among the three systems are based in structural and geomorphological factors. Two aquifers are recognized, the main aquifer in the Camiña Member of the Latagualla Formation and an aquifer composed of fluvial sediments in the river channel. Despite the fact that the two aquifers seem to be connected, the data of this study is not enough to properly constrain this finding. Keywords: Camiña quebrada, aquifer, groundwater, TEM, hydrogeology, springs. 1 Introduction The study area is located in the Atacama Desert, particularly in the Tarapacá region, one of the most arid areas in the world. Because of this, water is an extremely valuable resource. The Camiña quebrada is an east-west elongated basin running from the Altiplano to the Coastal Plain. It is an exoreic basin with a permanent regime in the upper part, including also several springs along its course. The base flow does not reach the ocean regularly but it does during extreme weather events occurring in summer. The Camiña quebrada forms part of the Camiña-Camarones zone, which is one of five hydrographic zones in the Tarapacá region. The definition of these zones was developed by the Dirección General de Aguas (DGA) in 1978 being a significant work at the moment. Nowadays these limits are recognised to be merely administrative as they do not match throughout with hydrologic parameters. Because of this, the Centro de Investigación y Desarrollo en

Recursos Hídricos (CIDERH, 2015) did a new definition of surface hydrographic units in the region based in hydrological parameters. Consequently, the CIDERH basin limit definition is used in this study instead of the DGA limits. One of the biggest challenges of working in this area is the lack of data as most of the previous studies are mostly focused in the Pampa del Tamarugal and Altiplano, but not in the Camiña quebrada area. The little available information is, mainly about geology and there is a complete lack in the understanding of groundwater flow, aquifer distribution, dynamics, etc.  Most of the Andean Piedmont is covered by volcano-sedimentary sequences from the Cenozoic (particularly Upper Oligocene to Miocene; Naranjo and Paskoff, 1985). These sequences have been named differently throughout the northernmost part of Chile depending of the stratotype location. For instance the Azapa, Oxaya and El Diablo formations are recognised in the Arica region (Salas et al. 1966; Montecinos, 1963; Tobar et al. 1968) whereas the Latagualla Formation was defined to the south of the previous (including the Camiña quebrada area; Pinto et al. 2004), and the Altos de Pica Formation (composed of five members) is described between the Aromas quebrada and the homonymous locality in the Tarapacá region. Altogether these sequences form a big volcano-sedimentary unit produced by a compensation of the Andean uplift in a favourable climate environment in the piedmont produced by a detritus accumulation during the Oligocene-Neogene (Naranjo and Paskoff, 1985). A correlation scheme of these sequences is observed in Figure 1 that is based in the data by Garcia (2002), Pinto et al. (2004), Victor et al. (2004) and Farias et al. (2005). The main known structural feature in the Camiña area in the Moquella Flexure which is a monoclonal fold inclined to the west and orientated N30ºW (Pinto, 1999; Pinto et al. 2004). According to Pinto (1999) the flexure would be related to a blind reverse fault with active periods, the first of them between 25 to 16 My and the second one from 9 to 8 My. Furthermore the Moquella Flexure, there is also a large reverse fault (the Cuisama Fault) uplifting the pre-Oligocene rocks above the Latagualla Formation to the east from the flexure. 2 Methodology

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A detail geologic survey of the Camiña quebrada, from Altusa up to Chillayza, was carried out, with an inventory and description of all springs, which can be found in Taucare (2015). Then a Transient Electromagnetic Method (TEM) survey was carried out in July 2014 to obtain subsurface resistivity values as there are not groundwater bores available with useful stratigraphic data. Nine points were selected for this survey (Table 1), separating them into three sections of three points each at different levels of the quebrada. The survey was done using a FAST-TEM equipment made by Applied Electromagnetic Research in Holland. The selected loop size varied according to the limitations in the field and ranged from a 12.5 m loop to a 100 m loop, but the most commonly used was the 50 m loop. One advantage of the of the FAST-TEM equipment is the necessity of using one loop only, which is called “coinciding loop” where the same loop acts firstly as transmitter and later as receptor (Barsukov et al. 2007). After the TEM survey finished each station was processed by separate to observe their associated error. Then, tables were done including time (s) v/s resistivity (Ω×m) data measured in the field and graphs were produced with these data in order to visualize it.

Table 1: TEM station locations, loop size and maximum reached depth. 2.1 Data Inversion The TEM data modelling was done using the ZondTEM 1D software (Demo version) that allows TEM data inversion by adjusting manually the modelled curve with the curve obtained in the previous step. The closer the adjustment line the more reliable the resistivity that characterises a specific depth interval. At the end of the inversion edition several intervals are obtained that represent rocks with similar resistivity values and, hence, water content character. The same inversion process must is done for every TEM station obtaining resistivity intervals in all of them. 2.2 Data Modelling Once the resistivity intervals in each TEM station are known they were grouped in three sections according to their location. Two sections were modelled perpendicular to the Camiña quebrada orientation and the other along

the quebrada. Each section was modelled using the Leapfrog Geo software (Aranz Geo Limited version 1.4.2) with the input of the resistivity data. 3 Results Overall all TEM points showed three to four distinctive resistivity intervals. All resistivity values above 100 Ω×m were considered as dry formations, the values between 40 to 100 Ω×m were considered as an humid horizon but no saturated and values below 40 Ω×m were considered as saturated corresponding to the main aquifer. The first cross section (Quistagama) included the TEM points CAM-1, CAM-2 and CAM-3 (Table 1). This is the easternmost cross section and is located in the northern flank of the Camiña quebrada. The section is perpendicular to the quebrada and it does not include the river. Four resistivity levels were identified with the two upper levels characterising a dry sector, the third level an humid horizon and the bottom level is the aquifer, which is approximately 255 m below ground surface (mbgs). The influence of the river is not observed as no TEM point is positioned on its surroundings. The second cross section (Camiña River at Quistagama) included the TEM points CAM-4, CAM-5 and CAM-6 (Table 1). This section is located along the Camiña river just south of Quistagama. The aquifer is identified below the river and they are connected at the CAM-4 point as the first interval showed evidence of water saturation with an aquifer thickness of 30 m. A similar situation occurs at the other two TEM points but with smaller aquifer thicknesses. This situation is of particular interest as it evidences a connection between the Camiña River and the aquifer. The last cross section (Chillayza) included the TEM points CAM-7, CAM-8 and CAM-9 (Table 1). This is the westernmost cross section and is located perpendicular to the Camiña quebrada from the Camiña River to the northern flank of the quebrada. The scenario in this section is similar to section one, recorgnising a thick dry section in the northern part of the valley with an aquifer at an approximate level of 280 mbgs. Conversely, in this section is possible to see an aquifer continuity from the Camiñan River to the valley. This situation was not observed in cross section one because there were no TEM points located onto the river. 3.1 Hydrogeological Conceptual Model Three hydrogeological systems can be differentiated in the conceptual model, the Northern Flank, the Southern Flank and the Fluvial Channel. The Northern Flank was better characterised compared to the other two hydrological systems owe to the data distribution and its conceptual model is presented in Figure 2. Precipitation infiltrates through the Tana Lavas unit, which is highly fractured generating secondary porosity but it does not present water storage capacity.

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During summer months is possible to see sporadic surface run-off caused by seasonal extreme weather events. The aquifer is composed by an intercalation of ignimbrites and conglomerates that form part of the Latagualla Formation Member one. Only members one and four (Camiña and Pacagua, respectively) of the Latagualla Formation crops out in this area. Several springs were identified within this unit being hosted mostly in the ignimbrites of the Camiña Member and discharging near the fluvial channel. The Camiña Member presents two domains separated by the Moquella Flexure. To the east of the Flexure is positioned at a higher elevation than the river level, the Member is overlaid by the Tana Lavas and is underlaid by the pre-Oligocene basement. Water presence in the Member is confirmed by resistivity values of the TEM survey and the presence of springs. The second domain to the west of the Flexure the Tana Lavas are absent being replaced by the Pacagua Member which overlays the Camiña Member. Groundwater flow is mostly westwards and it can be observed in Figure 2. The Southern Flank is slightly different and the Latagualla Formation distribution is smaller but the aquifer is also restricted to the Camiña Member of this Formation. The hydrogeologic system is restricted to the north-east part of the study area as a large reverse fault (the Cuisama Fault) uplift the pre-Oligocene basement above the Latagualla Formation producing a lateral discontinuity in this formation. The Cuisama Fault acts as an impermeable barrier to groundwater flow. In the Fluvial Channel the aquifer is composed by fluvial sediments. The river recharges this aquifer in summer months as river flow increases by precipitation events but the aquifer discharges into the river the rest of the year when there is no rainfall. Although connection between the fluvial sediments aquifer and the Camiña Member aquifer can be observed in cross section three this cannot be assured yet as more data is still needed to confirm this because of the large distance between the TEM points in this cross section. 4 Conclusions  The flanks of the Camiña quebrada shows a different hydrogeological behaviour based on their structural and geomorphological settings. The geology in the Northern Flank hydrogeological system is defined by two domains separated by the Moquella Flexure. In both cases the aquifer (Camiña Member) is present. The geology in the Southern Flank hydrogeological system is represented by ignimbrites covering the pre-Oligocene basement and by mas wasting. This system is

separated by the Cuisama Fault which acts as an impermeable barrier. The Camiña River forms a hydrogeologic system with the fluvial deposits. The river feeds the aquifer during summer and the aquifer feeds the river during the other seasons. References Centro de Investigación y Desarrollo en Recursos Hídricos (CIDERH). 2015. Estudio hidrográfico de la región de Tarapacá, norte de Chile. (En proceso) Farías, M.; Charrier, R.; Comte, D.; Martinod, J.; Hérail, G. 2005. Late Cenozoic deformation and uplift of the western flank of the Altiplano: Evidence from the depositional, tectonic, and geomorphologic evolution and shallow seismic activity (northern Chile at 19-30°S). Tectonics, Vol. 24, TC4001. García, M. 2002. Évolution oligo-néogene del’Altiplano Occidental (Arc et Avant-Arc du Nord du Chili, Arica): Tectonique, volcanisme, sédimentation, géomorphologie et bilan érosion sédimentation. Tesis de Doctorado, Univ. Joseph Fourier, Grenoble, Francia. 117 p. Montecinos, F. 1963. Observaciones de geología en el cuadrángulo Campanani, Departamento de Arica, Provincia de Tarapacá. Unpub. Thesis, Universidad de Chile, p. 106. Naranjo, J.A.; Paskoff, R. 1985. Evolución cenozoica del piedemonte andino en la Pampa del Tamarugal, norte de Chile (18°–21° S). Actas IV Congreso Geológico Chileno, Antofagasta, Vol. 5, p. 149–164.      Pinto, L. 1999. Evolución tectónica y geomorfológica de la deformación cenozoica del borde occidental del Altiplano y su registro sedimentario entre los 19°08'-19°27'S (Región de Tarapacá, Chile). Thesis, Departamento de Geología, Universidad de Chile, Santiago, 125 p. Pinto, L.; Hérail, G.; Charrier, R. 2004. Sedimentación sintectónica asociada a las estructuras neógenas en la Precordillera de la zona de Moquella (19°15’S, norte de Chile). Revista Geológica de Chile, Vol. 31(1), p. 19-44.      Salas, R.; Kast, R.; Monteciones, F.; Salas, I. 1966. Geología y recursos minerales del departamento de Arica y Parinacota, Provincia de Tarapacá. Inst. Invest. Geol., Vol. 21, p. 114. Taucare, M. 2015.   Hidrogeologías y aplicación de perfiles TEM para el reconocimiento de acuíferos en la Quebrada Camiña, Región de Tarapacá, Chile. Unpub. Thesis, Universidad Católica del Norte, p. 142.      Tobar, A.; Salas, I.; Kast, R. 1968. Cuadrángulos Camaraca y Azapa, Provincia de Tarapacá. Inst. Invest. Geol., Carta Geol. Chile, escala 1:50.000, Nºs. 19 y 20, p. 20.      Victor, P.; Oncken, O.; Glodny, J. 2004. Uplift of the western Altiplano plateau: Evidence from the Precordillera between 20° and 21°S (northern Chile). Tectonics, Vol. 23, TC4004.

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Figure 1: Stratigraphic correlations scheme of the Andean Piedmont sequence in northern Chile (18º15’S – 21ºS). Abbreviations used are: AF, Azapa Formation; OF, Oxaya Formation; EDF, El Diablo Formation; LF, Latagualla Formation; APF, Altos de Pica Formation; CEF, Cerro Empexa Formation; M1, Member one; M2, Member two; M3, Member three; M4, Member four; M5, Member five.

Figure 2: Conceptual model of the Camiña quebrada northern flank and its interaction with the river. It covers from the Precordillera to the Coastal Range and highlights the no precipitation threshold.