Pdf/11/5/1438/3335054/1438.Pdf 1438 by Guest on 01 October 2021 Research Paper

Research Paper

GEOSPHERE Architecture of the aquifers of the Calama Basin, Loa catchment basin, northern Chile

1 2 3 4 GEOSPHERE; v. 11, no. 5, p. 1438–1474 Teresa Jordan *, Christian Herrera Lameli , Naomi Kirk-Lawlor , and Linda Godfrey 1Department of Earth & Atmospheric Sciences and Atkinson Center for a Sustainable Future, Snee Hall, Cornell University, Ithaca, New York 14853-1504, USA 2Departamento de Ciencias Geológicas, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile doi:10.1130/GES01176.1 3Department of Earth & Atmospheric Sciences, Snee Hall, Cornell University, Ithaca, New York 14853-1504, USA 4Earth and Planetary Sciences, Rutgers University, 610 Taylor Road, Piscataway, New Jersey 08854, USA 18 figures; 10 tables

CORRESPONDENCE: [email protected] ABSTRACT The Loa groundwater and surface water are inseparable resources; the Loa River is the discharge channel of the groundwater of a >34,000 km2 area CITATION: Jordan, T., Herrera L., C., Kirk-Lawlor, In the Loa water system of the Atacama Desert in northern Chile, careful basin. The coincidence of modern climate, topography, and geology leads to N., and Godfrey, L., 2015, Architecture of the aquifers of the Calama Basin, Loa catchment basin, north- management of groundwater is vital and data are sparse. Several key man- a geography in which recharge occurs far from where humans use the water ern Chile: Geosphere, v. 11, no. 5, p. 1438–1474, agement questions focus on aquifers that occur in the Calama sedimentary resources, and groundwater aquifers supply the vast majority of stream doi:10.1130/GES01176.1. basin, through which groundwater and Loa surface water flow to the west. water. The aridity of the region sharply restricts the number of human inhabi- The complexity of the two major aquifers and their discharge to wetlands tants and extent of native plants or animals. However, under different climate Received 11 February 2015 and rivers are governed by primary facies variations of the sedimentary rocks states during the past few millennia the water flux was greater than now (Rech Revision received 13 May 2015 Accepted 17 June 2015 as well as by faults and folds that create discontinuities in the strata. This et al., 2002; Latorre et al., 2006); this leads to great uncertainty in estimations Published online 5 August 2015 study integrates geological studies with groundwater hydrology data to docu- of how much of the current water flow is renewable versus fossil (Houston ment how the aquifers overlay the formations and facies. Neither the phreatic and Hart, 2004). aquifer nor the confined or semiconfined aquifer, each of which is identified Under the Chilean water code, water is treated as a property independent in most basin sectors, corresponds to a laterally persistent geological unit. of land, to which a permanent right is granted for a given production rate from The variable properties of low-permeability units sandwiched between units a given extraction site. Although the water code treats groundwater rights and of moderate to high permeability cause a patchwork pattern of areas in which surface-water rights separately, for the Loa system the regulatory body (Direc water is exchanged between the two aquifers and areas where the lower aqui- ción General de Aguas, hereafter DGA) treats them as strictly coupled. In cur- fer is confined. The westward termination of most of the sedimentary rocks rent practice, all requests for additional rights or for changes in the points of against a north-trending basement uplift at an old fault zone terminates the extraction are subject to an environmental impact review by an independent principal aquitard and the lower aquifer. That termination causes lower aquifer agency, the Ministerio del Medio Ambiente. water to flow into the upper aquifer or discharge to the rivers. The regionally Initial management efforts distributed rights to exploit surface and ground- important West fault juxtaposes formations with differing lithological and hy- water when only a limited understanding of the natural system was avail- draulic properties, resulting in some exchange of water between the upper able. Now management practices strive to minimize both economical and and lower aquifers across the fault. ecologi cal problems, yet to do so requires improved information about the natural system. The desire to better inform the management of this coupled OLD G natural-human resource system is a central motivator for this study. Further- INTRODUCTION more, because the Loa system is at the junction of a natural extreme (e.g., an arid to hyperarid climate) and an atypical water governance approach (e.g., pri- The Loa River water system of northern Chile’s Atacama Desert (Fig. 1), in vate water rights), lessons from the Loa system may be broadly useful to those OPEN ACCESS the Antofagasta region, exemplifies the high stakes involved in sustainable who are considering regional-scale groundwater management strategies. management of scarce water resources. The Loa surface and groundwater Because of extreme aridity at low elevations, precipitation that is likely to system supplies the great majority of water used in Antofagasta Region, and lead to direct recharge of aquifers occurs only in the eastern mountainous meets much of the municipal and agricultural demands. It is vital to Anto fringe of the hydrologic basin (Fig. 1B) (CORFO 1977, see Table 1; Houston, fagasta Region copper mining, which constitutes ~50% of Chile’s copper pro- 2009). At lower elevations there is extensive exchange of water between aqui- duction (Servicio Nacional de Geología y Minería, 2011), which in turn supplies fers and rivers (DGA 2001, see Table 1; Houston, 2006), but the locations and This paper is published under the terms of the one-third of the world’s copper needs. However, a key property of the Loa sys- net outcomes of those exchanges are not well documented and constitute CC‑BY license. tem is the scarcity of surface water. major themes for ongoing research.

Peru Figure 1. (A) Inset map shows location of Bol. the Loa hydrologic system in west-central South America. (B) Map of the three types Chile of basin pertinent to the hydrology of the Loa system. Base is a digital elevation C

Precordiller Arg. Depression entral model in which tan colors show elevations W below ~2500 m, gray indicates higher ele- e ste vation, and blue is the Pacific Ocean. The surface catchment basin is outlined in red r n Cordillera n (solid where persistent surface drainage R. uncertai to the Loa is clear; dashed where ground-

a Amargo Altiplano water flow is a major intermediate step). B The maximum plausible extent of the groundwater basin is outlined in black (solid line for borders defined where there is little possibility for recharge; dash-dot line where there is possible recharge but N there are no direct constraints on the validity of this selection of the ground water basin border). The Calama sedimen-

R.Loa tary rock basin is outlined by the evenly D R.San Pedro spaced dashed black line. Blue lines mark the Loa River and its main tributaries. ?? Rivers (R., river name) and mountains S.San (S., mountain name) mentioned in the n R.San Lorenzo U text are labeled. Only areas above 4000 m R.Salado M Salvador C (patterned regions) have a combination Calama of sufficient precipitation and soil prop-

Coastal Cordillera erties suitable to significant infiltration S. Valley Guacate of rainfall and snowmelt and thus direct recharge (Houston, 2009). The formally de- S. Limon a Pacific Ocea Verde Bolivia fined boundaries of the Loa surface water basin are located at U (dividing upper and middle Loa) and M (dividing middle and Argentin Chile lower Loa). C—Calama city (at the western margin of the ~50 km × 50 km Calama Val- 0 50 ley); D—Conchi Dam. Boxed area shown in km Figure 2. plausible Calama surface >4000 m major streams groundwater sedimentary catchment system basin elevation

This paper focuses on the rocks through which the groundwater flows in sparse, the combined use of knowledge of the sedimentary architecture of the the central sector of the Loa system, within and adjacent to the Calama Valley Calama sedimentary basin and of piezometric head enables informed extrap- (Fig. 1B), in which an upper phreatic aquifer and a lower confined aquifer are olation of the hydraulic data laterally and vertically. For the first time for the routinely described (Figs. 2 and 3) (CORFO 1977, see Table 1; Houston, 2004). Loa system, we analyze the controls on the spatial variability of major aquifers There are three primary purposes of this paper: to clarify the spatial distribu- imposed by the complex stratigraphic architecture, and the uncertainties that tion of the rocks with hydraulic conductivity favorable to function as aquifers; remain. Examination of the state of knowledge reveals sectors of the ground- to identify where the lower aquifers discharge to the surface water system; and water basin for which it is most critical to obtain hydrochemical, geophysical, to identify the most likely sectors in which water is exchanged between upper and hydrological data with which to monitor the impacts of water extraction or and lower aquifers. Although the data available for hydraulic properties are to constrain parameters in a numerical model.

TABLE 1. UNPUBLISHED REPORTS CONTAINING DATA CITED IN TEXT Report identiﬁcation, year Organization Title Report identiﬁcation or URL English translation title CORFO 1973 Corporación de Fomento de la Estudio de los Recursos Hídricos de http://catalogo.corfo.cl Study of the Water Resources of Producción, Santiago, Chile la Cuenca del Río Loa. V. 2 Anexos /cgi-bin/koha/opac-detail.pl the Loa River Basin. Volume 2, ?biblionumber=5041 Appendices. CORFO 1977 Corporación de Fomento de la Hidrogeología de la Segunda Región CHI-69/535 Hydrogeology of the Second region Producción, Santiago, Chile con referencia especial a las with reference to investigated zones, zonas investigadas, in Recursos in Hydraulic Resources of the Grand hidráulicos del Norte Grande, North DGA 2001 Dirección General de Aguas, Actualización delimitación de Informe Técnico, S.I.T. 76 Updated delimitation of the aquifers Ministerio de Obras Públicas, acuíferos que alimentan vegas y which supply springs and wetlands, Santiago, Chile bofedales, Región de Antofagasta Antofagasta region Mayco 2013 Mayco Consultores, report Informe Final: Levantamiento http://documentos.dga.cl Final Report: Hydrogeological prepared for Dirección General información hidrogeológica Región /SUB5493.pdf information for Antofagasta region de Aguas, Ministerio de Obras de Antofagasta Públicas, Santiago, Chile Montgomery 2009 Montgomery & associates for Informe Annual 2009 Monitoreo Minera El Tesoro, report to Hidrogeológico, Sector Campo Dirección General de Aguas de Pozos Minera El Tesora Montgomery 2010 Montgomery & associates for Anexo C: Diagramas esquemáticos Provided by Dirección General Appendix C: Schematic diagrams of Minera El Tesoro, report to de la construcción de los pozos, de Aguas construction of wells, Calama, Chile, Dirección General de Aguas Minera El Tesoro, Calama, Chile El Tesoro Mining Company Minera Leonor 2007 Aquaconsult for Minera Leonor Informe Annual Monitoreo Provided by Dirección General Annual Monitoring Report Promised to Comprometido con DGA Período: de Aguas DGA for 2007 Año 2007 EIA 2005 Knight Piésold S.A. report CODELCO Chile División CODELCO http://seia.sea.gob.cl/archivos Environmental impact study for Mansa to Servicio de Evaluación Norte Proyecto Mansa Mina /EIA/2013102201/EIA_6313 Mina project of CODELCO Norte Ambiental (Environmental Estudio de Impacto Ambiental _DOC_2128727783_-1.pdf Division of CODELCO Chile Evaluation Agency), Santiago, Chile EIA 2011 Aquaconsult report to Servicio Informe Final para Estudio de http://seia.sea.gob.cl/archivos Final report on study of environmental de Evaluación Ambiental Impacto Ambiental: Estudio /Anexo_2-1__1_de_6_.pdf impacts of the Quetena Project: (Environmental Evaluation Modelamiento Hidrogeológico Hydrogeological model study for Agency), Santiago, Chile Subcuenca Cluster Toki para theToki cluster subbasin Proyecto Quetena Matraz 2012 Matraz Consultores Asociados, Estudio Acuífero de Calama, Sector http://documentos.dga.cl Study of the Calama aquifer, middle Universidad Politécnica de Medio del Río Loa, Región de /SUB5431v1.pdf sector of the Loa River, Antofagasta Cataluña, report prepared for Antofagasta region Dirección General de Aguas, Santiago, Chile GAC 2012 Gestión Ambiental Consultores Anexo 8. Componente Hidrogeología Codigo BIP: 20191503-0Appendix 8. Hydrogeology component: DIA construccion paseo Rio Loa, DIA construction of Loa River tourist Calama: Región de Antofagasta, route, Calama: Antofagasta region, Chile, prepared for City of Calama Chile

A broader objective is to advance appreciation that knowledge of sedimen- large-scale architecture of the strata likely plays a major role in determining tary basin architecture is valuable for regional hydrogeology research. Barthel the continuity of hydraulic properties. This paper demonstrates an application (2014) drew attention to the need for improved fundamental regional hydro- of knowledge of sedimentary basin architecture to a major sector of a coupled geological approaches. The hydrocarbon resource industry routinely utilizes groundwater–surface-water basin. the architecture of entire sedimentary basins as a tool to predict reservoir flow After describing the existing management premises and the physical properties. Likewise, for groundwater systems within sedimentary basins, the context of the study area, we describe the available piezometric framework

Figure 2. Piezometric map of the phreatic aquifer in the middle Calama groundwater system, superimposed on a digital elevation model. Filled circles mark locations of wells used in creat- ing map; red circles distinguish wells with time-series reports of water levels (Table 8). Black numbers and thin lines are piezometric contours; gray numbers and lines are land surface contours. Heavy black lines are faults. Elevations are in meters above sea level. A single italicized gray number notes the elevation of the bed of the Loa River north of the region with piezometric data. Solid piezometric contours indicate nearby well control, whereas dashed lines indicate long-distance extrapolation of sparse data. Data largely correspond to before 2005. Dashed lines in the southern sector (Llalqui area, L, and farther to the east) are from Houston (2006). The piezometric contours of previous reports (Fuentes Carrasco, 2009; EIA 2011, Matraz 2012, and Mayco 2013, see Table 1) were modified using wells and exploration boreholes consulted for this project (Table 7), which are identified with black dots. Hills in the midst of the Calama Basin sedimentary fill that expose deformed Eocene and older rocks are marked by cross-hachured zones. Blue diamonds are locations with stream flow data noted in Table 2. Locations: Ch— Chintoraste hills; O—Ojos de Opache region; C—Calama (star); H—Calama Hill; T—Talabre area; LC—La Cascada waterfall. The rock units in contact with a thin alluvial fill below the Loa and Salado Rivers are indicated by colors. The rectangle marks the extent of Figure 3.

A B

∆∼

Figure 3. (A) Piezometric map of the lower aquifers for region in box in Figure 2, superimposed on a digital elevation model. Data largely correspond to before 2005. Labels and symbols for elevation and topographic contours, piezometric contours, landforms, and locations are as in Figure 2 (tr—extensive tufa carbonate deposits). Wells and exploration boreholes consulted for this project are identified by dots; red dots distinguish wells with time-series reports of water levels (Table 8). Hills in the midst of the Calama Basin sedimentary fill that expose deformed Eocene and older rocks are marked by cross-hachured zones. Heavy black lines mark faults. The piezometric contours of previous reports (EIA 2005, EIA 2011, Matraz 2012, and Mayco 2013, see Table 1) are modified based on available well data (Table 7). (B) Differential head of lower and upper aquifers. Green shows regions where head of lower aquifer exceeds that of upper aquifer (Lo>Up). Red shows regions where head of upper aquifer is higher than that of lower aquifer (Up>Lo). Blue shows areas with near equality of the two (Δ~0). Sectors with question marks are regions in which the contours in either A (or Fig. 2) are poorly constrained by data. Red circles indicate wells for which time series of head data are available that show a minimum human impact (Table 9); large diameter circles are for upper aquifer and small diameter circles are for lower aquifer. Stars indicate locations at which the impact on head levels of the variability of nearby well pairs is analyzed (Table 9). In the green areas there is a tendency for the lower aquifer to recharge the upper aquifer. In the red regions there is a tendency for the upper aquifer to recharge the lower aquifer.

of the aquifers, then illuminate the spatial distribution of the geological units Calama city (Salazar, 2003). Consequently, the official measure of the water in in which the aquifers and aquitards occur across the middle part of the Loa the system available for ecosystem use, and potentially for additional human groundwater basin. The paper concludes with identification of hydrogeologi- use, is given by the discharge in the Loa and San Salvador Rivers west of this cal trends and unresolved problems. assumed final location of transfer from aquifers to surface streams. The water balance model that underpins water management decisions is informed by a set of long-term stream gauging stations that exist within Water Management Hydrological Premises and Uncertainties the central Loa basin as well as by a small set of monitoring wells (Fig. 2; Table 2). However, there are long reaches of the Loa River where flow is not The Loa hydrologic system is located on the western flank of the Andes measured, or where only single-year gauging campaigns have been reported Mountains in northern Chile and extends westward to the Pacific Ocean coast (Matraz 2012, see Table 1). There has not been a previous analysis of the poten- (Fig. 1). The generally accepted premise is that the natural water basin is at tial for hydraulic interconnections among the rocks that contain the aquifers, steady state such that recharge equals combined flows out of the basin plus although extensive monitoring plans have been developed to try to demon- extraction plus evapotranspiration. An alternative conceptual model holds that strate the presence or absence of hydraulic connections. Although recently some flow results from head decay established during times of wetter climate published geological and stratigraphic studies illuminate the stratigraphic and (Houston and Hart, 2004). spatial positions of units that may function as aquitards or aquifers, the result- Based on empirical relationships between precipitation and elevation as ing insight into the likely complexity of the groundwater system has not been well as temperature and elevation, combined with the topography of the basin, integrated into basin-scale water management assessments that are important the DGA (2003) estimated that the total annual available recharge of the Loa to the integrated management of the groundwater and surface water system. surface and groundwater hydrologic basin is 6.4 m3/s. However, the Loa River The lack of understanding of the architecture of the sedimentary-hosted discharges only 0.6 m3/s to the Pacific Ocean (Salazar, 2003) (Table 2). The dif- aquifers within the Calama Basin contributes to a lack of understanding of ference, 5.8 m3/s, is attributed to evapotranspiration and consumptive water where groundwater exits the middle sector of the Loa groundwater basin. This use. Large uncertainties exist with this steady-state model. A trend is found gap in knowledge is particularly relevant to deriving, let alone monitoring, a for water to flow predominantly in the subsurface in the upper parts of the water budget. Absent data regarding the western distribution or terminations basin, in a combination of surface channels and subsurface flow in the middle of the aquifers, an arbitrary location of where discharge to the Loa is mea- Loa basin, and in surface channels in the lower Loa basin. It is thought that the sured could produce misleading information, especially for monitoring of the final significant transfer from subsurface to surface flow occurs just west of impacts of operating well fields. An outcome from this paper, a data-based

TABLE 2. REPRESENTATIVE LOW-FLOW MEASUREMENTS FOR THE MIDDLE AND LOWER SECTORS OF THE LOA RIVER Pre-1979: mean of 1–7 single-date Post-1979 inauguration measurements (from Corporación of Conchi Dam; means of de Fomento de la Producción, 1973*) DGA monthly means† Station September– August June–July June-July Mean of July values number in October 1916 1918 1961 1969 1990–2000 unless noted Figure 2 Descriptive location (L/s) (L/s) (L/s) (L/s) (L/s) 1 Below future position of the Conchi Dam220019002100 880 2 After junction of Salado and Loa Rivers 3200 3100 240 3 At Angostura 3500 3200 4 Northeast of Calama Hill 4200 3600 1200 5 Near La Cascada§ 1300 3700 2400 640 Loa River before junction with San Salvador River1200 Loa River after junction San Salvador River2200 690 A.D.1993–2000 San Salvador River before junction with Loa River 600 Loa River at shore of Paciﬁc Ocean 3600 600Salazar (2003) *Corporación de Fomento de la Producción (CORFO 1973; see Table 1) Estudio de Los Recursos Hídricos de la Cuenca del Río Loa, Anexos (Studies of the Water Resources of the Loa River Basin, Appendices). Universidad de Chile, Departamento de Recursos Hidraúlicos. †Data reported by Dirección General de Aguas (DGA), http://snia.dga.cl/BNAConsultas/reports. §Pre-1973 reports cite station “Loa en Chintoraste”; post-1990 data reported by DGA at location “Loa en La Finca”; see Table 1.

hypothesis for the locations of aquifer discharge west of Calama city, should exploitation (Fig. 2). All three classes of basin are important, and it is necessary be considered when planning monitoring stations. to clarify which basin (surface catchment, groundwater, or sedimentary) is the The extraction of water has had significant impact on stream flow. Stream focus of various parts of the analysis. gauge measurements from the early twentieth century provide data least af- fected by extraction (Table 2; 1916 data). Some water would have been di- verted then for agricultural use, and the first sluice, built in 1915, supplied the Surface Catchment Basin early copper industry. By the 1960s there was important extraction of water for mining purposes, and in 1979 the high Conchi Dam was built to reduce the im- The surface catchment basin (33,570 km2; Tables 2 and 3) is limited at the pacts of the rare floods. The result (Table 2) was a decrease between the early topographic crestline in the Andes Mountains on the east, the Pacific coast on 1960s to the 1990s by >50% in stream flow. By the 2000s, the regulatory agency the west, and ~lat 21°S and 23°S on the north and south, respectively. The Loa DGA began to tighten evaluations of petitions for additional extractions, in River main stem measures 440 km in length, with a series of four orthogonal response to the assessment that the water system was in deficit. reaches, each 50–150 km long (Fig. 1). The first broad valley through which the surface drainage system passes is the 50 km by 50 km Calama Valley, the focus here, whose low-relief floor is ~2200–2800 m above sea level. The two Basic Components: Surface Water, Groundwater, main tributaries, the upper Loa River and the Salado River, join within the and Sedimentary Rocks Calama Valley. The general attributes of the topography, characterized by mountains with Three distinct types of basin are important to the Loa system hydrology. elevations >4000 m in the east and lowlands toward the west, set the bound- From a hydrological perspective, the Loa basin is tightly coupled to conti- ary conditions for groundwater recharge and water flow. In the Atacama Des- nental-scale landforms and is extensive, whether one considers its surface ert precipitation increases with elevation (Table 3) and evaporation is intense. catchment area (first type of basin, the surface-water basin) or its groundwater Precipitation currently capable of recharging the aquifers only occurs above recharge-discharge footprint (second type of basin, the groundwater basin) 3500 m above sea level (asl) but, after consideration of evapotranspiration, (Fig. 1). The third basin type is geological, and pertains directly to the physi recharge is most likely above 4000 m elevation (Fig. 1) (DGA, 2003; Houston, cal properties of the aquifer rocks. The Calama sedimentary basin forms the 2009). The area in the Loa catchment with widespread potential for modern subsurface rocks of a central sector of the Loa hydrological basin (~2400 km2; recharge is the eastern mountains (Western Cordillera and Altiplano; Fig. 1), sedimentary basin) and constitutes one of two major regions for groundwater although there may have been recharge at elevations below 4000 m during

TABLE 3. SURFACE-WATER DRAINAGE BASIN OF THE LOA RIVER Divisions Boundaries and dimensions PropertiesReferences East: crest line, Andean peaks (> 6000 m asl) DGA (2005) West: Pacific Ocean coastline Berenguer et al. (2005) North: lat ~21°S South: lat 23°S Area: 33,570 km2 Upper Headwaters in Western Cordillera to junction of Arid to hyperarid, precipitation 5–200 mm/yr; sparse vegetation on Houston and Hartley (2003) Loa River with Salado River hillslopes Middle Juncture Salado River to junction San Salvador Hyperarid, precipitation 2–5 mm/yr; steep topographic gradient (from Houston and Hartley (2003) River (location M in Fig. 1) >2900 m elevation below Conchi Dam (location D) to ~2300 m at Calama city (location C) to 1200 m at juncture with San Salvador River (location M in Fig. 1); except ~20 km reach, passes through narrow canyon incised 20–200 m into either consolidated sedimentary rocks, volcanic rocks, or crystalline basement Lower Junction San Salvador River to Pacific Ocean Hyperarid, precipitation <1 mm/yr; flows in canyon entrenched tens to Houston and Hartley (2003) hundreds of meters depth below the average surface of the central depression; gains water only from aquifer discharge focused at single tributary (Amargo River) Note: asl—above sea level; DGA—Dirección General de Aguas.

wetter times of the Pleistocene or Holocene (Latorre et al., 2002; e.g., Rech Groundwater Basin et al., 2002). Outside of the eastern highlands, the Loa system surface water bodies are fed by aquifers (DGA, 2003). The boundaries of the groundwater basin are not well known (Table 4), The San Salvador River rises at springs just north and west of Calama city both in the western region (Coastal Cordillera, Fig. 1) and in the eastern high- (Fig. 2). The Loa and San Salvador Rivers are ~5 km apart and are parallel as lands, where the border may coincide with the surface-water basin or may the south and north canyon boundaries of a 65-km-long east-trending valley extend east of the surface catchment, beneath the Altiplano Plateau. The areal that has a table-like planar surface. With the exception of ~20 km distance of extent, 34,000–65,000 km2, is very uncertain (Fig. 1). the Loa main stem, the Loa and San Salvador Rivers pass through narrow The Loa groundwater basin can be considered to include three geograph- canyons incised 20–200 m into either consolidated sedimentary rocks, volcanic ical sectors, upper, middle, and lower; the lower groundwater basin is not rocks, or crystalline basement. discussed herein. The eastern limit of the upper, or eastern, groundwater River flow data from the early austral spring season in a year prior to ex- basin likely occurs among the volcanic centers that form the Western Cor- tensive consumptive use of the Loa River system (September 1916) indicate dillera and that cover broadly the southwestern Altiplano Plateau (Fig. 1). that natural flow of the middle Loa River where it entered the Calama Val- Those volcanic peaks overlie laterally extensive Miocene and Pliocene pyro ley (~2200 L/s) was increased substantially by influx from the Salado River clastic volcanic deposits and interbedded epiclastic sands and gravels (de (~1000 L/s) (Table 2). The San Salvador River carried ~600 L/s. Between the Silva, 1989; Montgomery et al., 2003; Houston, 2007) that compose aquifers junctions of the Salado and the San Salvador, the Loa River flows through the in some of the upland basins (Mardones Perez, 1998; Montgomery et al., hyperarid Calama Valley and the Calama sedimentary basin. Exchanges be- 2003; Houston, 2007; DGA, 2003). A reasonable but unproven extrapolation tween surface water and groundwater are suggested by both downstream in- is that some water recharged east of the surface catchment divide flows as creases and decreases in Loa River flow in reaches where there are no tributary groundwater westward into the surface water Loa catchment (Pourrut and streams (Table 2; e.g., gains between stations 2–4; losses between stations 4–5). Covarrubias, 1995; Houston, 2007).

TABLE 4. GROUNDWATER BASIN OF THE LOA RIVER Divisions Boundaries and dimensions Geological setting of aquifers Climate and recharge potentialReferences Overall East: within Andes Mountains Precipitation increases with elevation: DGA (2003); Houston and West: Pacific Ocean coastline areas from the Calama Valley Hartley(2003) North: lat ~21°S westward receive <5 mm/yr on South: lat 23°S average; mountain regions to the Area: ~34,000 km2 (similar to north and east receive 60–200 mm/yr surface drainage) to 65,000 km2 (speculative maximum) Upper At latitudes of the Calama Valley, Hydraulically conductive volcanic- Widespread potential for modern DGA (2003); Houston (2009); western margin is the border of related deposits, probably inclusive of recharge at elevations >4000 m Rech et al. (2002a); Latorre the Calama sedimentary basin porous volcanic ash deposits, detrital (western Cordillera and Altiplano); et al. (2002) sedimentary deposits, and fractured potential recharge during Pleistocene dense volcanic deposits and/or Holocene wet intervals at elevations <4000 m Middle Calama Valley and eastern ~10 km Sedimentary rocks of Calama Basin Evapotranspiration exceeds precipitation DGA (2003); Montgomery 2009, of Salvador-Loa valley; western 2010; El Tesoro 2010; EIA 2011; limit defined by the emergence at Matraz 2012 (see Table 1) surface of the rocks that, to east, compose lower aquifer and a major confining unit; south margin poorly constrained Lower Western ~50 km of valley between Combination of thin sedimentary Evapotranspiration exceeds precipitation San Salvador and Loa Rivers, units near Batea-María Elena, thick and entire region farther west and sedimentary units near Quillagua, farther downstream and impermeable basement rock everywhere else Note: DGA—Dirección General de Aguas.

The middle sector of the groundwater basin (Table 4) is strongly linked unit encompasses the confined aquifers in some sectors of the groundwater to the Calama sedimentary basin. Across the Calama topographic valley the basin. Second, Oligocene to lowermost Miocene strata reach ~2000 m in thick- water table drops nearly 500 m, from ~2700 m asl to 2240 m asl at Calama ness beneath the north-central part of the valley (Fig. 6C). Third, the overlying city, with a further drop of >100 m to the western limit of the aquifers (Fig. 2). lower and middle Miocene strata spread widely across what is now the Calama Groundwater exploration, monitoring, and production in some parts of the Valley (Fig. 6B). In the northern part of the basin some facies of these units Calama Valley have demonstrated that aquifers extend to several hundred host both the lower and upper aquifers, but in the southern and western part meters depth. A phreatic aquifer extends locally to a depth as great as 100 m of the basin the fine-grained facies that corresponds to this time slice forms an (Houston, 2006), and lower aquifers occur in a depth range of 100–300 m (Fig. important confining layer. Fourth, the upper Miocene and Pliocene Opache and 3A) (inclusive of DGA, 2003; EIA 2011, Matraz 2012, and Mayco 2013 in Table 1). Chiquinaputo Formations extend across most of the Calama Valley as well as These depths greatly exceed the thickness of unconsolidated sediment (Blanco northward along the lower reach of the surface water upper Loa basin and west- and Tomlinson, 2009; Tomlinson et al., 2010), and are within the compacted ward across the mesa-like surface between the San Salvador and Loa Rivers Cenozoic sedimentary rock. To date, no aquifers are known within the Paleo- (Figs. 4 and 6A). Parts of the middle Miocene strata and much of the upper zoic or Mesozoic highly indurated strata, and therefore these units plus plu- Miocene–lower Pliocene strata constitute the phreatic aquifers. tonic rocks, metamorphic rocks, and lava flows are treated as the hydraulic basement. Fracture flow may be possible within the rock units treated as hydraulic basement, but is not discussed here. Geohydrological Framework

Water in the Loa catchment north and east of the focus region enters per- Calama Sedimentary Basin meable units of the subsurface (Houston, 2007), but those units are not laterally continuous with possible host rocks of the Calama Valley focus area. The The Cenozoic sedimentary rocks of the Calama Basin compose the third cat- possible aquifer rock units within the Calama sedimentary basin are not tabu- egory of basin (Table 5) and host the major aquifers of the middle Loa ground- lar or continuous sheets. Thus water must pass from one set of stratigraphic water basin. Both the Calama Valley and the mesa-like surface between the San units to another set, in order to exit the middle section of the Loa system. A Salvador and Loa Rivers (Fig. 4) are the modern expressions of this long-lived comparison of the large amount of mapping data added by recent surface and sedimentary basin. The Calama Basin is composed of moderately consolidated subsurface geological studies (e.g., Jordan et al., 2006; Blanco, 2008; Blanco sedimentary rocks of Eocene–Pliocene age (May, 1997; May et al., 1999, 2005; and Tomlinson, 2009; Tomlinson et al., 2010) to the hydrogeological reports, Blanco et al., 2003; Jordan et al., 2006; Blanco, 2008; Blanco and Tomlinson, both published (Houston, 2004) and unpublished (EIA 2005 in Table 1), reveals 2009; Tomlinson et al., 2010) (Fig. 5). The depocenter of the Cenozoic sedimen- that the stated stratigraphic position of the gravels that serve as confined aqui- tary basin has shifted in location at least four times (Fig. 6). First, Eocene-age fers is commonly inconsistent with the recent geological mapping. conglomerates and volcanic-associated strata reach ~1000 m thickness and Although the Calama sedimentary basin strata are little deformed com- occur mostly in the southern third of the valley (Fig. 6D). This first stratigraphic pared to other Cenozoic basins of northern Chile, deformation by faults and

TABLE 5. SEDIMENTARY BASINS WITHIN THE LOA SYSTEM Divisions Boundaries and dimensions Relationships to surface and groundwater PropertiesReferences basins Calama ~2400 km2 Salado and Loa Rivers join in basin; Eocene–Quaternary strata; Naranjo and Paskoff (1981); May 21.8°–22.6°S San Salvador River rises from springs maximum thickness >2000 m (1997); May et al. (1999, 2005); 68.3°–69.2°W in basin; overlaps with middle Loa Jordan et al. (2006); Blanco groundwater basin (2008); Blanco and Tomlinson (2009); Tomlinson et al. (2010) Batea area Within Central Depression valley Junction of San Salvador and Loa Miocene and younger strata; Naranjo and Paskoff (1982); 22.2°–22.7°(?)S Rivers within basin maximum known thickness May(1997) 69.2°–69.6°(?)W ~100 m Pampa del Tamarugal Within Central Depression valley; Loa River passes through basin before Miocene and younger strata; Sáez et al. (1999, 2012); Jordan (Quillagua) 21.8° to farther north than limit of ﬁnal west-directed reach near path of Loa, maximum et al. (2010); Nester andJordan surface water basin (~21°S) thickness ~150 m (2012) 69°–69.7°S

A′

Figure 4. Simplified geological map of Calama Valley and eastern sector of the San Salvador–Loa Valley, showing only the surface distribution of units discussed in this paper because of their role as hosts E′ of aquifers or as major aquitards. Shades of green identify units associated with a lower aquifer; yellow indicates a unit that is associated with both a lower and upper aquifer. Shades of orange identify units that are considered in some areas to be aquitards. Distribution of units compiled from Blanco and Tomlinson (2009) and Tomlinson et al. (2010). Abbreviations as in Figure 2. Faults and major folds are shown as thick blue lines (names in Fig. 7), but not all of them deform surface units. Locations of geological and piezometric cross sections are marked as solid black D′ lines, and area of Figure 9 is defined by box. Dashed thin black lines show seismic C′ reflection profiles. B′

folds are important to the groundwater and surface-water systems. A set of aquifer. Only the Chiu Chiu monocline (Table 6; Fig. 7) offsets significantly the faults and associated folds (Mesozoic and Eocene faults of Cerros de Guacate strata that host the upper aquifer. The result of the primary stratigraphic pat- and Sierra de San Lorenzo; the Cere fault; Table 6) controlled the topographic terns and secondary deformation is a complex architecture of the strata that margins of the Calama Basin while sedimentary rocks accumulated. Four are plausible aquifers and aquitards beneath the surface of the Calama Valley major and one minor tectonically controlled fault sets traverse the sedimentary and San Salvador–Loa Valley. basin (Table 6; Fig. 7) and bound hydraulically conducting stratigraphic units. The geological units herein are the lithological units mapped by Marinovic An additional set of nontectonic folds and fractures creates a large-scale and and Lahsen (1984), Marinovic et al. (1995), Blanco and Tomlinson (2009), and laterally continuous zone of likely hydraulic discontinuity within the Calama Tomlinson et al. (2010). These mappable lithological units are not strictly the Valley (Fig. 7; Table 6). Some of the faults (e.g., West fault, Milagro fault; Table same as the aquifers and aquitards defined by hydraulic conductivity. Rather, 6) displace by kilometers the continuity of rocks that are the hosts for the lower this paper documents the positions in space of the geological units whose facies

Figure 5. The major sedimentary units found in three zones of the Calama sedimentary basin. The west of Calama city column applies to the area near Ojos de Opache (O in Fig. 4), based on May (1997) and May et al. (1999, 2005), subsurface data, and new mapping. The east of Calama Hill column applies to the southern margin of the Calama Basin (east of H in Fig. 4), based on Blanco (2008) and Tomlinson et al. (2010). The Eastern Flank basin column applies to the eastern central sector of the basin (general region of the Llalqui lowlands, L in Fig. 4), based on May (1997), May et al. (2005), Jordan et al. (2006), Blanco (2008), and Blanco and Tomlinson (2009). Quat.—Quaternary; Fm.—formation.

are both anticipated, based on textures observed in outcrop, as well as demonstrated in boreholes, to have appropriate porosity and permeability to serve as aquifers or as aquitards. Thus the maps and geological cross sections place bounds on the distributions of the host rocks of the aquifers and aquitards.

MATERIALS AND METHODS

Data for groundwater are from reports that companies file with the Chilean agencies Ministerio del Medio Ambiente (Ministry of the Environment) and/or DGA. Those reports systematically record the static water level and, in some cases, useful aquifer parameters (transmissivity and storativity). The maps of piezometric surfaces (Figs. 2 and 3A) were generated using both a compila- tion of piezometric contours from previous reports (EIA 2005, EIA 2011, Matraz 2012, Mayco 2013, and Minera Leonor 2007, see Table 1) and data from 118 wells (Tables 1 and 7). Time series of water levels are available for more than 50% of these wells (Table 8; Figs. 2 and 3A). Wells reported to be production wells were not used unless a time series was available from which to identify the impacts of pumping, and therefore to select data prior to that impact. Likewise, monthly time series enabled recognition of the impacts of pumping at nearby wells (e.g., Fig. 8), and exclusion of those data. Ideally the maps would represent a single month in a single year, for a time prior to human intervention in the hydrological system. In reality, the data on which the maps are based represent either the oldest reported water levels for each well or data for 2003–2005, which were the earliest years of widespread well monitoring (Table 8). Overall, the oldest measurements used were recorded in 1993 and for a few sectors the earliest monitoring wells reported are as recent as 2011. For the phreatic aquifer, the surface of the water in the Loa and San Salva- dor Rivers was included in the data set. Although the data sources routinely indicate whether each well measures an upper or lower aquifer, the depths of screened intervals are reported for only 39 of the wells, and the year of construction is known for fewer than half of the wells. Well integrity problems may affect the segregation of water in these wells, allowing upper aquifer water to affect the recordings of lower aquifer head. Nevertheless, the clear distinctions between upper and lower aquifer heights of most near-neighbor toring wells successfully isolate the waters of the two aquifers is provided by wells (Table 9) indicates that many of these wells successfully restrict water hydrochemistry studies. For example, Matraz 2012 (see Table 1) examined entry to desired intervals in a single aquifer. In the primary data sets (Table 1) water chemistry for 134 wells that overlap with the set listed in Table 7 (19 there are a few examples of wells whose data suggest the mixing of the two wells in common for upper aquifer; 24 well in common in lower aquifer) and aquifers, and we avoided use of those wells. Further evidence that these moni interpreted from the sulfate concentrations that the upper aquifer and lower

Figure 6. Paleogeographic maps of the principal Cenozoic stages of fill of the Calama Basin, stacked as they occur in the subsurface. Colored and patterned zones show regions in which strata of each time slice occur. The patterns and colors emphasize areas with properties suitable to function as aquifers (shades of yellow) or dominated by facies that are prone to low hydraulic conductivity (shades of orange). Each layer portrays a time slice rather than a geological depth slice. Note that the location of potential aquifer host rocks shifts laterally in successively deeper time slices. (A) Late Miocene–Pliocene basin. Both the limestone-dominated facies (pale yellow, brick pattern) and marginal conglomeratic facies (yellow, gravel pattern) are potential aquifers. R.—river; Ch—Chintoraste hills; O—Ojos de Opache region; H—Calama Hill; L—Llalqui area; T— A Talabre area. (B) Early and middle Miocene basin. Where the pattern is orange mudstone, the Jalquinche Formation dominates and there is little or no potential for facies suited to serve as aquifers. Where the pattern is yellow gravel, conglomerate and well-sorted sandstone of the Lasana Formation and alluvial gravels along the basin margins occur and are plausible aquifers. The Lasana Formation contains thick intervals of mudstone locally that would serve as aquitards. (C) Oligocene–earliest Miocene basin. The Yalqui Formation is shown by conglomerate symbols, but a mud-rich matrix displayed in some of its sparse outcrops indicates that part of this unit is unlikely to serve as an aquifer, and so it is represented by a pale orange color. (D) Eocene basin. Conglomerates of the Calama Formation are represented by yellow gravel symbols. Conglomerates west of the West fault are interbedded with volcanic deposits and mudstones of the Chintoraste complex (in orange). B aquifer waters are distinctive. For those hydrochemistry monitoring wells, the well owners are required to maintain official certification of the well integrity. Subsurface data for rock properties come from a small fraction of the numerous boreholes that have been drilled in the study area to explore for minerals within the rocks underlying the sediments of the Calama Basin. In addition to a very small number of published analyses of exploration boreholes (May, 1997; Blanco, 2008), this study used reports of lithologies from 44 exploration boreholes (Table 7). For 15 of those well reports, the driller or mudlog records include mention of depths at which water or wet rock was encountered. This study utilizes geological information regarding aquifer and aquitard lithologies C from 131 groundwater wells and logged mineral exploration borehole records that appear in reports prepared for the DGA (e.g., Matraz 2012, see Table 1) or to comply with environmental impact and mitigation regulations (e.g., EIA 2005, see Table 1). These were put in the public domain through the website of Chile’s Environmental Evaluation Service (Ministerio del Medio Ambiente, http://sea .gob .cl). In those reports the lithological data appear either in detailed borehole-specific illustrations (e.g., EIA 2011, see Table 1) or embedded within geological cross sections (e.g., EIA 2005, see Table 1). Geophysical profiles collected for minerals exploration, groundwater studies, and petroleum exploration exist in the study area, although only a small D fraction of the results is published (e.g., interpretations of seismic reflection profiles collected for petroleum exploration: Jordan et al., 2006; Blanco, 2008; gravity survey: Matraz 2012 and Mayco 2013, see Table 1). Additional examples consulted for this study that were embedded as supporting documents within environmental impact analyses include NanoTEM™ (http://zonge .com .au/capability /method /nano -tem) profiles (GAC 2012, see Table 1) and TEM (Transient Electromagnetic) profiles (EIA 2011 and Mayco 2013, see Table 1). The lithological information from the boreholes and geophysical profiles was combined with data from geological maps (Marinovic and Lahsen, 1984; Blanco and Tomlinson, 2009; Tomlinson et al., 2010) and stratigraphic studies

TABLE 6. FAULT SETS WITHIN THE LOA SYSTEM Major fault General location Geological time of activity and and/or fold and orientation kinematics in the region Geohydrological impactReferences Mesozoic and North-trending, at western Cretaceous–Paleogene faults and folds created Created limits to the Calama Basin and to Mpodozis et al. (1993); Eocene faults margin (Cerros de highlands against which Calama Basin the most suitable aquifers Tomlinson et al. (2010) west and south Guacate and Sierra Paleogene and Neogene strata abut of basin de San Lorenzo) and In the plain between the San Salvador and Loa southern margins (Sierra Rivers, the positions of paleoridgelines can be Limón Verde) of Calama readily identified on satellite images, through basin the thin Opache Formation cover West fault North-trending, on west side Eocene and early Oligocene: kilometer-scale Sedimentary units hosting lower aquifer are Tomlinson and Blanco (1997); system of Calama Hill (divides right-lateral slip discontinuous across fault Tomlinson et al. (2010); Calama Valley from Loa– Mid-Oligocene–early Miocene: ~37 km left- Zone near fault highly fractured; fault cutting Araya Torres (2010) San Salvador Valley) lateral displacement; locally, to 600m of bedrock is a flow barrier down-to-the-west displacement Impacts on permeability in sedimentary Middle Miocene–Pliocene strata disturbed rocks bounding the fault are not (hundreds of meters) by right-lateral documented displacement Net left-lateral displacement 35 ± 1 km Milagro fault East-trending reverse fault North side displaced up by ~1000 m; Eocene Creates major discontinuity in rocks that Blanco et al., (2003); located between Talabre Incaic deformation host the lower aquifer Blanco(2008); Blanco and and Calama Hill Tomlinson (2009); Tomlinson et al. (2010); Jordan et al. (2006) Loa fault Northeast-trending; in north- Oligocene(?) offset; ~2000 m down-to-east Western boundary of Yalqui Formation Jordan et al. (2006); central Calama Valley to normal slip, possible accompanying strike slip depocenter; little impact on middle Blanco(2008); Blanco north of Talabre Late Miocene small-magnitude folds over buried Miocene and overlying strata so probably and Tomlinson (2009) trace of fault little impact on groundwater Cere fault East-northeast-trending; NW Miocene–Pliocene normal reactivation of None identified; plausible impact on Tomlinson et al. (2012) boundary Calama Valley Paleozoic fault infiltration in bedrock below Loa River San Salvador– Set of east-trending, near Eocene–Oligocene Create physical discontinuities in aquifer This paper Loa Valley vertical, small offset faults Few data, but one strand displays horizontal host rocks and aquitards by juxtaposing fault set North of Chintoraste hills, striations indicative of strike slip and tens of across faults metasedimentary adjacent canyon of meters of displacement rocks, subvolcanic intrusives, lava Loa River and possibly flows, ignimbrites, and volcaniclastic adjacent Canyon of Ojos conglomerates; brecciation may reduce de Opache hydraulic conductivity Chiu Chiu Eastern sector Calama Younger than 3 Ma; vertical displacement Salado and Loa Rivers follow axis of Blanco and Tomlinson (2009) monocline Valley; trace extremely >100 m; origin not tectonic syncline at toe of monocline for 36 km; and adjacent sinuous host rocks for upper aquifer change syncline elevation by >100 m across monocline

(May, 1997; May et al., 1999, 2005; Jordan et al., 2006; Blanco, 2008; Blanco and A subset of both the mineral exploration boreholes and the groundwater Tomlinson, 2009) to map the distributions of sedimentary units that are inde- well reports document the depths to water-bearing rocks or indicate the depth pendently reported to hold the aquifers and form the aquitards. Our geological and lithology at which groundwater wells are screened for water entry (e.g., field work and satellite image analysis using Google Earth led to creation of a EIA 2005, see Table 1). Those reports are of special value in this analysis and new geological map for the study area west of Calama city (Fig. 9). Combin- were used to determine the direct connections between a rock unit and an ing surface information, geophysical data, and borehole data, geological cross aquifer. The few TEM profiles provided data for the depth to aquifers at lo- sections (Figs. 10–14) were created to illuminate major attributes of the three- cations between wells. The positions of the piezometric surfaces (from Figs. dimensional distribution of the mapped units of major importance to aquifer 2 and 3A) and of the corresponding borehole-specific aquifers were super and aquitard architecture. imposed on the geological cross sections (Figs. 10, 11, 13, and 14).

Figure 7. Zones of faults and associated folds in the study region. Table 6 describes the physical properties, sense of offset, and age of offset of each fault system. The geology of the rock units underlying the river bed alluvium is shown for comparison to Figures 2 and 4. R.—river; S.—sierra; Ch—Chintoraste hills; O—Ojos de Opache region; H—Calama Hill; L—Llalqui area; T— Talabre area.

RESULTS In the central sector of the Calama Valley, most wells encounter an upper phreatic aquifer. In south-central areas the phreatic aquifer occurs in a lime- Aquifers and Hydraulic Parameters stone-dominated rock (EIA 2005 and EIA 2011, see Table 1), the Opache For- mation. In the north-central area, the upper aquifer is variably in limestone In the Llalqui area (Figs. 4 and 13) in the eastern part of the Calama Val- mapped as the Opache Formation or in sandstone of the upper part of the ley, the Opache and Chiquinaputo Formations contain the upper aquifer (Fig. Lasana Formation. A lower set of aquifers also occurs widely; at some lo- 15A–15C) (Houston, 2004, 2006, 2007). A few wells document a lower aquifer, cations the wells are artesian (e.g., northeast of location T in Fig. 3B). In the which is locally artesian. The aquifer corresponds to rocks at >200 m depth that north-central region the lower aquifer occurs in conglomerate below 80–130 m are capped by the Jalquinche Formation mudstone and/or an ignimbrite, with depth (Figs. 10, 13, and 14); current geological mapping places this conglomer- lateral variability in the thicknesses of those low-hydraulic-conductivity units. ate in the Lasana Formation. A mudstone is considered to be the confining unit Houston (2004) presented evidence that the Sifón Ignimbrite is an effective and attributed commonly to the Jalquinche Formation. However, Blanco and confining layer between the two aquifers. Tomlinson (2009) reported that the Jalquinche mudstone facies is only a few

GEOSPHERE | Volume 11 | Number 5 Jordan et al. | Architecture of aquifers, Loa basin Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/5/1438/3335054/1438.pdf 1450 by guest on 01 October 2021 on 01 October 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/5/1438/3335054/1438.pdf Research Paper 59 58 57 56 Upper aquifer water level (with varying amounts of geological data) 55 54 53 51 50 47 45 44 43 41 40 38 37 35 34 32 31 26 25 24 23 21 15 14 13 11 10 9 8 7 6 5 4 3 2 20 52 49 48 46 42 39 36 33 30 29 28 27 22 19 18 17 16 Geological information onl 12 1 Geological information and elevation of wet rocks Identiﬁcatio n UTM* eas t 514436 514220 516520 530838 507100 504032 506787 528837 489150 491600 506700 507166 507786 503307 503600 490528 495700 492640 498368 494989 491215 530660 503900 508391 489060 497150 497987 488612 505170 502856 509530 507339 509970 510836 509610 508981 533772 510836 491650 509278 510608 501981 507662 490696 493620 494012 500922 513943 513408 512745 506200 509775 520319 519953 491500 509261 5241 491 491 y (m) T 172 164 ABLE 7. LOCA 18 TIONS UTM* north 7525613 7524825 7523747 7537100 7530289 7512900 7509705 7529237 7505060 7504190 7513300 7513927 7517186 7516150 7507521 7507285 7502360 7502995 7503986 7502350 7531488 7512500 7513843 7498970 7502000 7505958 7508488 7514873 7516858 7519705 7510902 7509521 7513746 7509517 7507591 7498840 7529867 7513746 7506060 7514215 7516228 7512474 7507517 7507285 7503091 7504999 7525541 7526791 7527235 7507500 7510021 7515057 7520034 7488900 7514004 75 11 75090 11 751 751 (m) 1512 1525 AND INFORMA 20 42 (m above sea level) TION SOURCES FOR WELLS USED IN STUDY Elevatio n 2405 2400 2409 2612 2503 225 1 2273 2315 2246 2207 2213 2034 2264 2161 2569 2286 2258 2179 1938 2226 2212 2290 2256 2324 2286 2278 2308 2013 255 4 206 3 2061 224 7 213 3 2084 2173 2144 2221 2210 2138 2464 2597 2286 2258 2300 2197 2238 2039 2162 2280 2400 2419 2435 2314 2462 2467 2333 2266 2 111 20 3 Blanco (2008) (if no footnote, see Blanco (2008) Blanco (2008) Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 201 2 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 Jordan 2012 EI A 2005 EI A 2005 EIA EI A 2005 EI A 2005 EIA EIA EIA EIA EIA EIA EIA Source 2005 2005 2005 2005 2005 2005 2005 2005 †† ; Matraz 2012 T † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † † †† †† able 1) ( continued )

GEOSPHERE | Volume 11 | Number 5 Jordan et al. | Architecture of aquifers, Loa basin 1451 on 01 October 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/5/1438/3335054/1438.pdf Research Paper 84 89 88 87 86 85 63 62 61 60 Upper aquifer water level (with varying amounts of geological data) Identiﬁcatio n 90 83 82 81 80 79 78 77 76 75 74 73 72 71 70 69 68 67 66 65 64 91 92 93 94 95 96 97 98 99 100 102 101 120 11 11 11 11 11 11 11 11 11 11 109 108 Lower aquifer water level (with varying amounts of geological data) 107 106 105 104 103 1 9 8 7 6 5 4 3 2 0 T UTM* eas t ABLE 7. LOCA 515005 505886 541478 514456 523732 521009 507350 528421 520072 518197 503886 517265 533271 530529 517803 512622 514375 519285 518094 526106 517427 529470 527854 518078 518500 514891 520951 506062 512002 527374 524490 504917 504051 504025 503224 500675 502816 503231 505562 504075 503891 504277 504219 509347 513952 509239 510660 509274 509719 508204 507976 510309 509095 509894 513985 503576 507256 528795 500524 504023 503896 (m) TIONS UTM* north AND INFORMA 7518491 7519610 7529912 7520087 7520244 7522926 7530335 7528715 7524037 7518630 7535028 7526396 7525324 7523369 7525986 7528596 7524377 7522753 7524761 7517137 7525002 7523520 7523153 7522286 7519231 7521816 7520917 7522913 7514605 7515081 7515108 7516369 7514075 7515785 7516371 7516084 7515259 7514882 7514962 7515039 7520294 7519618 7523406 7523728 7525526 7527086 7516058 7521358 7525640 7522483 7518076 7520264 7529222 7513775 7514329 7514520 752031 7521 7521 752581 751 (m) 11 12 100 189 1 1 TION SOURCES FOR WELLS USED IN STUDY ( (m above sea level) ( continued Elevatio n 2391 2253 2568 2485 2474 2389 2250 2481 2494 2459 2428 2432 2621 2487 2438 2382 2409 2467 2443 2485 2432 2478 2470 2438 2440 2391 2471 2230 2326 2475 2488 2220 2214 2215 2205 2172 2198 2206 2230 2215 2216 2216 2284 2365 2336 2342 2397 2442 2266 2278 2301 2399 2403 2368 2225 2249 2539 2175 2213 2212 221 1 ) DGA, 2003; Minera Leonor 2007; Montgomery 2010 DGA, 2003; Minera Leonor 2007; Montgomery 2010 Minera Leonor 2007: Montgomery 2010 Minera Leonor 2007: Montgomery 2010 Minera Leonor 2007: Montgomery 2010 Minera Leonor 2007: Montgomery 2010 Minera Leonor 2007: Montgomery 2010 Minera Leonor 2007: Montgomery 2010 DGA, 2003; Montgomery 2010 DGA, 2003; Montgomery 201 DGA, 2003; Montgomery 2010 DGA, 2003; Montgomery 2010 DGA, 2003; Montgomery 2010 DGA, 2003; Montgomery 2010 (if no footnote, see DGA DGA EI A 2005; EIA Matraz 2012 monitoring well monitoring well EI A 2005 EI A 2005 EIA EIA EIA EI A 2005 EI A 2005 EI A 2005 EI A 2005 EIA EIA EIA EIA EI A 2005 EI A 2005 EI A 2005 EI A 2005 EI A 2005 EIA EIA EIA EI A 2005 EIA EIA EIA EIA EIA EIA EIA EIA EIA EIA EIA EIA EIA EIA EIA EIA EIA EI A 201 EIA EI A 201 EI A 201 Source continued 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 20 11 201 1 1 1 1 20 11 T ) able 1) § § 0 ( continued )

GEOSPHERE | Volume 11 | Number 5 Jordan et al. | Architecture of aquifers, Loa basin 1452 on 01 October 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/5/1438/3335054/1438.pdf Research Paper 121 162 161 160 159 158 157 156 155 154 153 152 151 150 149 148 147 146 145 144 143 142 141 140 139 138 137 136 135 134 133 132 131 130 129 128 127 126 125 124 123 122 Lower aquifer water level (with varying amounts of geological data) Identiﬁcatio n 163 164 166 165 167 168 169 170 171 172 but in some sources the datum is not stated and might be WGS84. The elevation was estimated from Google Earth elevations. is 400–450 173 †† § † **http://www *Locations given in Universal Note: Dirección General de T. See text for reference. Jordan 2012 (own interpretation), extracted information from original CODELCO exploration well logs and driller reports. Italics indicate wells for which the elevation is approximate. For these, original report provided location data, but not m. .codelco T .com/prontus_codelco UTM* eas t ABLE 7. LOCA Aguas (DGA) monitoring reports available monthl y, 502825 503583 504548 506221 503812 505920 505241 505236 503804 512210 533248 519999 512815 514180 526730 529427 517231 523766 528125 518400 522584 526186 528837 525895 508082 513586 533261 513943 530659 513408 527340 512804 516297 515437 514388 523720 531210 503313 505564 503645 500670 503585 504072 503081 502513 501801 498849 500545 51 5056 11 51 5259 11 5091 (m) 1672 1697 11 T ransverse Mercator (UTM Zone 19S). Most locations are reported by primary sources relative to da TIONS /site/artic/20120926/asocﬁle/20120926174425 UTM* north AND INFORMA 7516939 7518029 7521318 7520263 7522754 7519620 7520516 7520524 7522760 7518395 7535083 7533648 7526556 7524759 7531584 7525469 7520467 7518135 7523545 7521327 7522345 7522584 7529237 7528443 7531310 7523226 7534989 7525541 7531487 7526791 7520902 7526587 7526719 7527593 7518671 7520309 7527219 7522272 7526499 7524158 7524708 7517179 7516093 7514581 7514080 7514673 7515269 7515095 7514927 7514492 7513604 7513794 7522 11 (m) 3 TION SOURCES FOR WELLS USED IN STUDY ( (m above sea level) The location uncertainty caused by inconsistent use of these da ( continued Elevatio n 2329 2212 2224 2305 2259 2373 2253 2269 2269 2374 2339 2621 2547 2416 2401 2540 2475 2437 2485 2470 2453 2478 2486 2540 2505 2630 2375 2619 2400 2568 2419 2475 2417 2423 2428 2380 2483 2524 2322 2418 2452 2372 2216 2231 2209 2172 2209 2216 2204 2196 2188 2036 2176 http:// ) snia.dga.cl/BNAConsultas/. DG A 2003; Minera Leonor 2007; Montgomery 2010 DG A 2003; Minera Leonor 2007; Montgomery 2010 /monitoreo Minera Leonor 2007; Montgomery 2010 Minera Leonor 2007; Montgomery 2010 Minera Leonor 2007; Montgomery 2010 Minera Leonor 2007; Montgomery 2010 Minera Leonor 2007; Montgomery 2010 Minera Leonor 2007; Montgomery 2010 Minera Leonor 2007; Montgomery 2010 Minera Leonor 2007; Montgomery 2010 Minera Leonor 2007: Montgomery 2010 _ambiental_0912.pdf. (if no footnote, see CODELCO publicity** EIA EIA EI A 2005 EI A 2005 EI A 2005 EI A 2005 EIA EIA EI A 2005 EI A 2005 EI A 2005 EI A 2005 EIA EIA EIA EIA EI A 2005 EI A 2005 EI A 2005 EI A 2005 EI A 2005 EIA EIA EIA EI A 2005 EI A 2005 EI A 2005 EI A 2005 EI A 2005 EIA EI A 2005 EIA EIA EIA EIA EIA EIA EIA EI A 201 EI A 201 EIA Source continued 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 2005 201 201 201 201 201 201 201 201 1 1 1 1 1 1 1 1 1 1 T ) able 1)

elevation data. tum systems tum PSAD56,

GEOSPHERE | Volume 11 | Number 5 Jordan et al. | Architecture of aquifers, Loa basin 1453 on 01 October 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/5/1438/3335054/1438.pdf Research Paper 107 106 105 104 103 102 101 100 99 98 97 96 95 94 93 92 91 87 90 89 88 86 85 84 83 82 81 80 79 78 77 76 75 74 73 72 71 70 69 68 67 66 65 64 63 62 61 60 59 58 57 56 Upper aquife r Identiﬁcation (seeT able 7) Bottom hole depth (m below surface ) 210 N.A. N.A. 102 100 45 79 60 60 60 60 60 60 61 51 65 86 60 55 60 60 53 62 48 54 66 70.5 54 60 59 42 17 15 42 18 48 75 60 60 24 41 84 24 60 57 66 42 66 48 24 36 24 screened depths Reported 66–204 21–42 12–56 10–56 10–57 25–61 25–50 30–48 20–80 31–55 10–56 14–52 26–50 m2 11 9–57 7–56 8–57 9–53 N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. (m) –29 m T ABLE 8. WA TER WELL constructe d Y ear well 1993 1993 1993 2007 1999 1993 1999 1999 201 N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. 01 MONIT 1 1 ORING INFORMA 1998–2001 monitored monthly; neighboring well PPR-3 began 1994–2000 monitored before 1993–2000 monitored before 1994–2000 monitored before 1995–2000 monitored before then made a production well 1994–2000 monitored; then neighbor well PPR-2 began neighboring PPR-5 began made a production wel l ﬁeld pumping began pre–April 2005 pre–April 2005 pre–April 2005 pre–April 2005 pre–April 2005 pre–April 2005 pre–April 2005 January 2005 level reporte d Y 2007, 2009 1993–2007 1994–2007 1990–2006 2004–2005 2004–2005 2004–2005 2004–2005 2004–2005 2003–2005 2003–2005 2004–2005 2004–2005 2004–2005 2004–2005 2004–2005 2004–2005 2004–2005 2004–2005 2004–2005 2004–2005 2004–2005 2004–2005 2004–2005 2004–2005 2007–20 11 2007–20 11 production production production ears water 2004 20 11 201 N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. TION 1 months missing from complete series) which water level reported (number of Number of sequential months over 180 (28) 162 (5) 190 (45) 19 (0 ) 19 (0 ) N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. 3 (1 ) 5 (0 ) 5 (0 ) 5 (0 ) 5 (0 ) 5 (0 ) 5 (0 ) 5 (0 ) 7 (0 ) 5 (0 ) 5 (0 ) 4 (1 ) 5 (0 ) 5 (0 ) 5 (0 ) 5 (0 ) 5 (0 ) 5 (0 ) 8 (0 ) 5 (0 ) ( continued

)

GEOSPHERE | Volume 11 | Number 5 Jordan et al. | Architecture of aquifers, Loa basin 1454 on 01 October 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/5/1438/3335054/1438.pdf Research Paper 170 169 168 167 166 165 164 163 162 161 160 159 158 157 156 154 152 155 153 151 150 149 148 135 134 133 132 131 130 129 128 127 126 125 124 123 122 121 120 11 11 11 11 11 11 147 146 145 144 143 142 14 11 140 139 138 137 136 11 11 11 11 109 174 173 172 108 Lower aquife r 171 Note: 1 9 8 7 6 58 4 3 2 0 Identiﬁcation (seeT N.A. indicates data not available. able 7) Bottom hole depth (m below surface ) N.A. N.A. N.A. 350 350 N.A. 267 285 303 N.A. N.A. 263 244 120 108 185 236 159 148 148 196 231 128 200 121 173 190 270 180 237 200 150 300 300 100 250 250 250 300 250 300 204 252 222 300 204 220 192 191 230 276 171 300 300 185 176 320 11 11 21 60 60 50 50 60 36 0 4 0 1 181–216, 222–233, 187–234; 260–273, 236–247, 249–342 275–297, 335–340 screened depths T ABLE 8. WA 196–264 219–279 207–289 157–257 191–238 100–108 167–185 142–225 135–147 Reported 86– 11 46–103 95–154 41–141 40–141 60–190 66–165 24–35 m 23–35 1 1–29 N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. (m) 0 TER WELL well for pump test well for pump test 2001; pumping 2001; pumping 2009 or earlier 2009 or earlier MONIT constructe d Y ear well 2007 2007 2007 2007 2007 1993 1993 2009 2008 20 11 201 201 201 201 201 201 N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. 1 1 1 1 1 1 ORING INFORMA production from lower aquifer 2008–2010 monitored pre- pre–April 2005 pre–April 2005 pre–April 2005 pre–April 2005 pre–April 2005 level reporte d TION ( Y 1995–2007 1994–2007 2003–2004 2003–2004 2002–2004 2003–2004 2003–2004 2003–2004 2003–2004 2003–2004 2003–2004 2003–2004 2003–2004 2003–2004 2003–2004 2003–2004 2003–2004 2003–2004 2003–2004 2003–2004 2003–2004 2004–2005 2004–2005 2004–2005 2003–2004 2003–2004 2003–2004 2003–2004 2003–2004 2000–2004 2007–20 11 2007–20 11 2007–20 11 2007–20 11 2007–20 11 2010–20 11 2008–20 11 2007–20 11 ears water 2007 2004 2004 2004 2004 2005 2005 2004 2004 2004 2004 2004 N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. continued ) months missing from complete series) which water level reported (number of Number of sequential months over artesian at all times 150 (4) 162 (8) 59 (0) 18 (1 ) 19 (1 ) 14 (12) 24 (4 ) 16 (0 ) 16 (0 ) 21 (1 ) 17 (3 ) 21 (1 ) 16 (1 ) 10 (0 ) 12 (0 ) 12 (0 ) 17 (1 ) 13 (3 ) 18 (2 ) 20 (2 ) 21 (1 ) 14 (0 ) 20 (1 ) 21 (1 ) 19 (2 ) 16 (0 ) 11 N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. 6 (0 ) 6 (0) 3 (0) 4 (0) 4 (0) 3 (0 ) 5 (0) 3 (0 ) 4 (0 ) 4 (0 ) (1)

GEOSPHERE | Volume 11 | Number 5 Jordan et al. | Architecture of aquifers, Loa basin 1455 Research Paper

Figure 8. (A) Variability during 2003–2005 in discharge of water in the Calama Valley. Loa River discharge monitored by summing reported releases by two paths from the Conchi Reservoir. Letter abbreviations at top are months of the year. (B–F) Variability during 2003–2005 in head of water in the Calama Valley. Monitored heads of lower and upper aquifer wells, organized by general location in the Calama Valley and San Salvador–Loa Valley. Black lines indicate water levels in the Loa River and in the upper aquifer. Blue lines indicate water levels in the lower aquifer. Well locations are in Table 7, and data are described in Tables 8 and 9. Locations of well pairs selected for direct comparisons of upper and lower aquifer variability are shown in Figure 3B. B is a shallow well in eastern Calama Valley near Salado River that is monitored by the Direccíon General de Aquas. It is reported as depth in meters below the surface (m.b.s.) because contradictory elevation data for the well are published. C and D are in the northeastern part of the Calama Valley (m.a.s.l.—meters above sea level). E–G are monitoring wells in the central part of the basin. H is located near the southern margin of the basin. I and J are located west of Calama city, in the San Salvador–Loa Valley.

meters thick in the north-central area, and therefore is probably not an effective confining layer. Instead, mudstone intervals within the Lasana Formation (Blanco and Tomlinson, 2009) are more likely candidates for the low-transmissivity layers in that area. South of Talabre (T in Fig. 2) but north of the Loa River, there is a region where there is no phreatic aquifer even though there appears to be lateral continuity of the limestone-rich unit. In that area, several wells prove a confined lower aquifer in a conglomerate unit (EIA 2005, see Table 1). Both upper and lower aquifers are found near and west of Calama Hill (sector between H and O in Figs. 2 and 3) (Mayco 2013, see Table 1). An upper phreatic aquifer exists in unconsolidated alluvium, karstic limestone, calcareous sandstone, sandstone, and conglomerate. The calcareous upper part is referred to as the Opache Formation (Fig. 15D) and the lower detrital strata has informal unit names (e.g., black sandstone). A lower aquifer occurs in conglomerate referred to in most reports as the Calama Formation (Fig. 16C), although evidence presented in the following indicates that the aquifer is in multiple sedimentary and volcanic units. Above the lower aquifer is a thick clay-rich siltstone, commonly attributed to the Jalquinche Formation. West of the central Calama Valley and north of the San Salvador River and Calama city, where the surface elevations rise toward the northwestern mountain range, the confining layer is an ignimbrite (EIA 2011, see Table 1). However, that ignimbrite pinches out northward so that the two aquifers become one phreatic aquifer (EIA 2011, see Table 1). The sparse data for the hydraulic properties of the rocks are summarized in Table 10. Typical permeability for the upper aquifer in the central Calama Valley (L and T in Fig. 4) is 0.7–1.2 m/day. In the San Salvador–Loa Valley area west of Calama Hill (sector between H and O, Fig. 4), productive well fields in the upper aquifer report higher permeability (3.9 m/day average) (Fuentes Carrasco, 2009). Houston (2004) reported that some upper aquifer horizons in the eastern part of the study area have much higher permeability (120 m/day). Heterogeneity was also emphasized by Fuentes Carrasco (2009), who considered the upper aquifer west of Calama Hill to contain elongate channels of exceptionally high permeability.

TABLE 9. VARIABILITY THROUGH TIME OF RIVER DISCHARGE AND OF HEADS IN UPPER AND LOWER AQUIFERS, CALAMA VALLEY REGION Difference in Maximum Location Months overlap Height on map piezometric variability in Would relative Location (numbered star, Wells upper and lower at star center height from map nearby wells Maximum Minimum vertical positions description Fig. 3B) compared Years data aquifer data (m) (m) (m) height height change seasonally? Northeast 1 central basin 30 June 2004– –1 lower aquifer SI-5D 2555 13 2561.5 2548.5 31 October 2004 June–October possible 11 June 2004– 2004 upper aquiferTT-4E 25540.062 2554.031 2553.969 13 January 2005 Southeast 2 central basin 20 October 2004– lower aquifer TL-14 2460–51.652460.8252459.175 13 January 2005 October 2004– not with sampled variability 28 September 2004– January 2005 upper aquifer TL-06C 24550.072455.0352454.965 13 January 2005 3 20 October 2004– lower aquifer TL-15 2454–60.595 2454.298 2453.703 13 January 2005 28 September 2004– 23 September 2004– upper aquifer TL-02C 20.033 2448.017 2447.984 not with sampled variability 13 January 2005 13 January 2005 23 September 2004– upper aquifer TL-03C* 0.0412448.0212447.980 13 January 2005 Southwest 4 central basin 24 July 2003– lower aquifer PBMM-7 232300.1572323.0792322.922 4 October 2004 September– by <1 m; too short a time 8 September 2004– October 2004 to gain farther insight upper aquifer SI-18C 23230.362 2323.181 2322.819 14 January 2005 Central basin 5 16 July 2003– lower aquifer SI-12E 242460.9932424.4972423.504 6 October 2004 July 2003– not with sampled variability 16 July 2003– October 2004 upper aquifer SI-14E 24301.284 2430.642 2429.358 11 January 2005 6 12 March 2003– lower aquifer SI-6B† –4 1.3312397.6662396.335 5 October 2004 2397 12 March 2003– March 2003– lower aquifer SI-1B 9.8882401.9442392.056 possible 7 October 2004 October 2004 2 July 2002– upper aquifer SI-17C 23930.706 2393.353 2392.647 5 October 2004 Western basin 7 20 January 2003– lower aquifer CHUCA-7B§ 4 October 2004 2210164.849 2212.425 2207.576 8 July 2004– September– upper aquifer SI-8C 0.14 2226.070 2225.930 not with sampled variability 6 January 2005 October 2004 7 September 2004 upper aquifer SI-23C 22260.689 2226.345 2225.656 –14 January 2005 (continued)

TABLE 9. VARIABILITY THROUGH TIME OF RIVER DISCHARGE AND OF HEADS IN UPPER AND LOWER AQUIFERS, CALAMA VALLEY REGION (continued) Difference in Maximum Location Months overlap Height on map piezometric variability in Would relative Location (numbered star, Wells upper and lower at star center height from map nearby wells Maximum Minimum vertical positions description Fig. 3B) compared Years data aquifer data (m) (m) (m) height height change seasonally? 8 September 1994– lower aquifer LE-1** 2175 30 2.1 2176.050 2173.950 January 2008 July 1995– lower aquifer LE-2†† 4.6 2177.300 2172.700 January 2008 September 2004– not with sampled variability 8 July 2004– January 2005 upper aquifer SI-8C 2205 0.14 2205.070 2204.930 6 January 2005 7 September 2004– upper aquifer SI-23C 0.689 2205.345 2204.656 14 January 2005 9 July 2007– lower aquifer OBS-7C§§ 2110 44 2.32111.150 2108.850 June 2011 July 2007– not with sampled variability July 2007– June 2011 upper aquifer OBS-7L*** 2154 0.35 2154.175 2153.825 June 2011 *Aquifer tested 18 December 2004. †Range reported here for only 2003–January 2004 because of aquifer tests on nearby wells later in 2004. §Aquifer test 14 September 1993. **Variation reported 1994–2005; range during 2003–2005 is ~1.5 m. ††Variation reported 1994–2006; range during 2003–2005 is ~1 m. §§Water level for 2008; variation 2007–2008. ***Water level for 2008; variation 2007–2008.

The permeability of the conglomeratic lower aquifer in the eastern region West of Calama city, the flow direction is west-southwest (Fig. 2) where the ranges from 1 to 4 m/day (Table 10), with localized horizons of much higher water table declines 90 m to the springs of Ojos de Opache. fissure permeability (40–100 m/day) (Houston, 2004). In the central Calama Val- The aquifers discharge to surface water bodies in several areas. In the cen- ley as well as in the area west of Calama Hill, well tests for the lower aquifer tral Calama Valley, short-term stream gauging campaigns document signifi- display generally higher permeability, 2–21 m/day (Table 10). cant transfer of water (e.g., several hundred liters/second at multiple locations) Data for the basement rocks upon which the Eocene–Quaternary sedimen- from the aquifers into the south-trending reach of the Loa River (between blue tary rocks accumulated and for units reported to serve as confining layers con- diamonds 2 and 3 in Fig. 2) (EIA 2005 and Matraz 2012, see Table 1). West of firm that they are significantly less permeable. The few well tests conducted Calama city (Fig. 2), examples of discharge into the Loa River occur near the for the mudstone or ignimbrite that overlies the lower aquifer reveal perme- location of the La Cascada waterfalls (LC in Fig. 2) and through diffuse zones of ability of ~10–3 m/day (Table 10). Araya Torres (2010) documented that most springs that produce extensive wetlands in two tributary canyons on the Loa’s permeability values for basement rocks in the adjacent mountain range on north bank (blue diamond 5 in Fig. 2). Although no gauging stations document the west side of the basin, where hundreds of measurements exist because of the flow of the Loa above and below those springs, at times of low flow there mining-related geotechnical studies, are 10–3 to 10–6 m/day. is a visually pronounced downstream increase in the flow of the Loa River. Similarly, the San Salvador River is entirely spring fed, primarily from a set of springs at Ojos de Opache (O in Fig. 2; CORFO 1973, see Table 1). Klohn (1972) Groundwater Flow reported that the water chemistry of surface streams, springs, and wetlands in the area between Calama city and Ojos de Opache reveals a connection Across the eastern, central and northern sectors of the Calama Valley, the between the upper aquifer and the surface waters. piezometric surface of a phreatic aquifer (Fig. 2) declines from 2660 m in the For a set of wells with roughly monthly measurements during the inter- northeast to ~2300 m at the West fault, driving flow toward the southwest. val of time corresponding to most of the hydrological data, 2004–2005, the

to B

Figure 9. Geological map of the Calama San Salvador Basin west of Calama city, in the eastern sector of the San Salvador–Loa Valley (location is shown by box in Fig. 4). Mapped ON relations are based on field work and our satellite image analysis. Dashed-line faults F5? (thick black lines) are inferred based on satellite image interpretation and sparse data. Solid-line faults were mapped in the Ojo de Opache field. Gray lines identify zones in which 2000 the structural grain of basement rocks can be discerned through the small-scale F4? relief even though the basement unit is draped by the surface unit, the Opache LC Formation (Fm.). Shades of green iden- 0 m 100 0 tify units associated with a lower aquifer. m Blue patches are areas covered by tufa F3 m a carbonate deposits that are many meters E Lo thick. The brown line labeled 0 m (solid F2 where mapped; dashed where inferred) traces the position where the Jalquinche Formation pinches out below the base of a thin sandstone unit that underlies the Opache Formation (Fig. 5). The green line labeled 0 m (solid where mapped; C C′ dashed where inferred) traces where the F1 Chintoraste unit pinches out. Between Uncertain where confining layer terminates the brown and green lines, there is no regionally extensive aquitard, and therefore 0 2100 200 0 1 2 3 4 km water-bearing horizons in the Chintoraste ? unit will have direct contact with the base ? 20 of the rocks that regionally host the upper Ch N 00 altitude (m) above sea level aquifer. Ch—Chintoraste hills; LC—La Cas- ? cada waterfalls; ON—Ojos de Opache at Pleistocene(?) terrace or Nacimiento spring. Jalquinche Fm. Intrusive igneous rock modern channel sediments conglomerate overlying tufa carbonate platform Mesozoic strata Chintoraste complex Opache Fm. and underlying Chintoraste pyroclastic and undifferentiated basement sand and gravel volcaniclastic complex inferred basement ridge

time-variable groundwater levels (Fig. 8) reveal a mixture of natural variabil- river that is a conveyance from the high-elevation parts of the catchment, as ity and human management. An annual cycle of increased river flow during it is located <500 m from the Salado River near its juncture with the Loa. This austral summer (December–March) is perceptible in the Loa River flow (Fig. well displayed a small-magnitude increase in the phreatic water table late in 8A) after it exits the Conchi Reservoir to pass into the Calama Valley, although 2003, and a subsequent slow decline. That September increase predated by the seasonal variations are managed and spikes in discharge are likely related three months the anticipated annual precipitation cycle in the catchment high- to managed flow through dam drains. The Chiu Chiu well (Fig. 8B) offers lands (Chaffaut, 1998). For wells located farther downflow and at considerable a history of an upper aquifer well that is tightly coupled to the other major distance from the rivers (Fig. 3B), neither the upper nor lower aquifer heads

A′ D′ A B′

Figure 10. (A) Geological cross section A-A’ through the main part of Calama Valley. Thick black lines are faults. Thin vertical black lines are boreholes whose data constrain the interpretations. Dashed unit contacts indicate a high degree of uncertainty about the position. Mudstone of the Jalquinche Formation and conglomerate and coarse sandstone of the Lasana Formation interfinger extensively. (B) Cross section illustrating the elevation of the piezometric surfaces for the two stacked aquifers, overlaid on the same geology shown in A, with greater vertical exaggeration. The rocks associated with the two aquifers are also identified where construction (screened intervals) and associated geology are reported for wells near the line of section.

B′ E′ A A′

Figure 11. (A) Geological cross section B-B’ across the eastern B part of the San Salvador–Loa Valley and southwestern part of the Calama Valley. Thick black lines are faults. Thin vertical black lines are boreholes whose data constrain the interpretations. Dashed unit contacts indicate a high degree of uncertainty about the position. (B) Cross section illustrating the elevation of the piezometric surfaces for the two stacked aquifers, overlaid on the same geology shown in A, with greater vertical exaggeration. The rocks associated with the two aquifers are also identified where construction (screened intervals) and associated geology are reported for wells near the line of section.

display a seasonal variability that is tightly related to the upper Loa and Salado 2003–2005 the natural range is interpreted to be <2 m for both the upper and inflow to the Calama Valley, even with a phase shift. For those wells, rapid fluc- lower aquifers (Table 9). tuations in water levels are more likely the result of pumping of nearby wells The elevations of the aquifer surface in the north-central Calama Valley and (e.g., Figs. 8C, 8I) than natural causes. For the wells that are distant from the of the Loa River bed constrain the plausible geology of the aquifer in the area rivers, the range of variability of the head during 2003–2005 is 1–13 m, inclu- north of the well data. The elevation of the phreatic aquifer in the two north- sive of large values interpreted to reflect human management (Table 9). With ernmost control wells (Fig. 2) is ~2610 m asl in the valley center and ~2660 m the available data, identification of natural variability is subjective, but during asl near the western margin of the valley. Following the piezometric gradient

C′ deposits occur along some sectors of the river bed north of Chintoraste hills (tr in Fig. 3A), and spring-like tufa carbonate deposits occur along the traces of some east-trending faults on the walls of the Loa canyon. These deposits suggest spring drainage that is not tied to the upper aquifer. The relative heights of the piezometric surfaces of the upper and lower aquifers vary across broad regions (Fig. 3B). In most of the northern Calama Valley west of the Loa River and in a zone along the west-central margin of the valley, the lower aquifer head is above that of the upper aquifer, produc- ing localized artesian or near-artesian conditions (Fig. 3B). For well pairs 1 and 6 (Fig. 3B; Table 9), the variability of head over the reported months is sufficiently large to plausibly reverse these relative piezometric heights in some months. Nevertheless, persistently flowing artesian wells near well pair 1 demonstrate the robustness of the relative pressures within some parts of the northern region. In other broad areas, especially one in the south-central region and another near and west of Calama city, the head in the upper aquifer is higher than that in the lower aquifer (Fig. 3B). There are two smaller regions in which the lower aquifer head is similar to that of the upper aquifer (Fig. 3B). In most of the subareas these relative heights are robust over the 2003–2005 data years (Table 9).

Figure 12. Geological cross section C-C’ in the San Salvador–Loa Valley south of Loa River. There Geology are no known boreholes in close proximity to this cross section. Geohydrological Consequences of Faults and Folds to the northeast, the landscape surface elevation rises to 2720–2800 m asl near the Loa River, before dropping 40–60 m to the river bed. Although the Opache The long-lived, north-trending, oblique-slip West fault system occurs near Formation is exposed widely in the northern part of the Calama Valley, it was the boundary between the Calama Valley and the San Salvador–Loa Val- removed by erosion at the Loa canyon. Instead, the bedrock exposed at the ley (Reutter et al., 1996). Due to lateral offset during the mid-Oligocene–early depth of the river bed is within the Lasana Formation (Figs. 2 and 4). If a phre- Miocene, the Eocene and lower Oligocene strata in the southwestern Calama atic aquifer is recharged by the Loa River south of the Conchi Dam (Fig. 2), the Valley would not have formed in continuity with similar-aged deposits in the most likely aquifer host is the Lasana Formation. Groundwater must then flow San Salvador–Loa Valley (Figs. 7, 10, 11, and 14). Within the Calama Basin, ver- southwestward into what become both the lower and the upper aquifers. tical offset across the West fault apparently displaces the contact of the crystal- Through the central and northern sectors of the Calama Valley, the head line basement with strata by <200 m (Figs. 11 and 14). of the lower aquifer declines ~400 m and the form of its piezometric surface Where the West fault zone cuts crystalline basement rock in the >800-m-deep is similar to that of the upper aquifer. The flow direction is from northeast to Chuquicamata open pit mine, the zone of fault gouge and breccia is at least southwest (Fig. 3A). For the lower aquifer in the western part of the Calama Val- 3 m wide (Tomlinson and Blanco, 1997). Araya Torres (2010) documented that ley and San Salvador–Loa Valley, the pattern of the piezometric surface is more the fault within the mine region is a barrier to groundwater flow; however, irregular. Immediately west of Calama Hill, the piezometric gradient is low (~10 that data set examined the hydraulic conductivity of fracture zones related to m/km) over a 5-km-wide zone before transitioning westward to a steep slope basement rock and did not evaluate hydraulic properties where the faults cut (~20 m/km) in the region of the Ojos de Opache springs (Fig. 3A). Although the moderately lithified sedimentary units that are the aquifers. sparse data south of Calama Hill (H in Fig. 3) suggest that lower aquifer water The east-trending Milagro fault system (between locations H and T in Fig. 7) may flow toward the Loa canyon south of Calama city, there are no control formed contemporaneously with accumulation of the Eocene Calama Forma- wells near the Loa River southwest of Calama city or river gauges west of tion, and is buried by post-Eocene strata along most of its trace (Blanco, 2008; station 5 (Fig. 3) with which to verify possible lower aquifer conditions. Station Tomlinson et al., 2010). Available outcrop and subsurface data imply that it is a 5 (Fig. 3A) marks the western point at which the Opache Formation, the typical north-dipping reverse fault in its central sector (Fig. 10) and that it declines in upper aquifer host rock throughout the western part of the basin, crops out at offset in the western and eastern sectors, where folds dominate the structure the base of the Loa canyon. West of station 5, extensive volumes of carbonate (Fig. 14). In general, lower aquifer rocks north of the Milagro deformation zone

NW SE D cross-section cross-section east end D′ A E-E′ A-A′ Chiu Chiu seismic line 3000 monocline

2000 vertical exaggeration 5:1 Loa Fault

1000

B piezo- water- metric bearing surface rocks upper aquifer

vertical lower aquifer exaggeration 15:1 2000 m 01020 30 km

alluvial or wetland deposits, basement rocks0 (compacted Ignimbrite El Yeso Formation (gypsum) unconsolidated strata; volcanic and intrusive) kilometers Lasana Formation Opache Formation Yalqui Formation (conglomerate) folded strata (schematic) conglomerate or sandstone Upper Miocene-Pliocene Yalqui Formation fault (arrow indicates Jalquinche Formation (mudstone) Chiquinaputo Formation (muddy conglomerate) sense of displacement)

Figure 13. (A) Geological cross section D-D’ through the central Calama Valley. The line of profile matches seismic line 99–06 interpreted by Jordan et al. (2006) and the cross section of Blanco and Tomlinson (2009). Thick black line is a fault. Thin vertical black lines are boreholes whose data constrain the interpretations. Dashed unit contacts indicate a high degree of uncertainty about the position. (B) Cross section illustrating the elevation of the piezometric surfaces for the two stacked aquifers, overlaid on the same geology shown in A, with greater vertical exaggeration. The rocks associated with the two aquifers are also identified where construction (screened intervals) and associated geology are reported for wells near the line of section.

cross-section st Faul cross-section E E′

B-B″ We D-D″

t 3000 A st Faul east branch We ?? 2000

Milagro ?? vertical exaggeration 4.9:1 ?? ?? Fault ??

1000 m B t st Faul We

vertical exaggeration 15:1 ??? ? water- piezometric bearing surface rocks 2000 upper aquifer lower aquifer

010203040 km

alluvial or wetland deposits, Lasana Formation distance (km) Chintoraste complex unconsolidated conglomerate or sandstone (distal volcanic and conglomerate) basement rocks (compacted Opache Formation Jalquinche Formation (mudstone) 10 strata; volcanic30 and intrusive) Upper Miocene-Pliocene Calama Formation folded strata (schematic in Calama Fm.) sandstone or conglomerate (conglomerate) fault (arrow indicates sense of displacement)

Figure 14. (A) Geological cross section E-E’ through the western sector of the Calama Valley. Thick black lines are faults. Thin vertical black lines are boreholes whose data constrain the interpretations. Dashed unit contacts and faults indicate a high degree of uncertainty about the position. Mudstone of the Jalquinche Formation and conglomerate and coarse sandstone of the Lasana Formation interfinger extensively. (B) Cross section illustrating the elevation of the piezometric surfaces for the two stacked aquifers, overlaid on the same geology shown in A, with greater vertical exaggeration. The rocks associated with the two aquifers are also identified where construction (screened intervals) and associated geology are reported for wells near the line of section.

A B

Figure 15. Photographs of rocks that host the upper aquifer illustrate the lithological diversity as well as some of the primary porosity and fracture porosity. (A) Opache Formation limestone in a quarry wall in the southeastern part of the Calama Valley (near 22.45745°S, 68.731°W). The limestone has little visi- ble porosity except in the upper 1 m, but fractures that are both parallel to bedding and approximately perpendicular to bedding (arrows) display centimeter-scale open space. (B) Chiquinaputo Formation conglomerate bed (upper half) and cross-bedded sandstone bed (lower half) in the northeastern part of the Calama Valley, south of the Salado River (near 22.3°S, 68.5°W). (C) Opache Formation limestone near northern limit of study area (near 22.031°S, 68.620°W) where the Opache is only ~3 m C thick. Note the abundant centimeter-scale vugs. (D) Opache For- D mation limestone in the southwestern part of the study area, in the San Salvador–Loa Valley (near 22.4924°S, 69.0081°W). Note that the upper left half of outcrop is well bedded, whereas the rock of the right half is entirely broken into meter-scale breccia. The brown coloration and white vertical streaks in the lower left area are indicative of alteration and mineralization during water seepage. An active spring exists at the same horizon 25 m to the left. Note person for scale in white oval.

occur in the Lasana Formation, whereas lower aquifer rocks near and south of these faults, to form extensive calcium carbonate mineral deposits along the this fault occur in the Calama Formation (Figs. 10 and 14). Loa River bed (tr in Fig. 3) and at paleosprings located on the canyon walls. The fault within the basin with the largest vertical displacement, the north- There are no stream gauge data for the Loa River at suitable locations to test east-trending Loa fault (Fig. 7; Table 6), displaces the pre–middle Miocene units this discharge hypothesis. With many fewer constraints, it is inferred that and is a major discontinuity in the deeply buried Yalqui Formation (Figs. 10 and parallel faults of similar magnitude occur 3–4 km to the north. These inferred 13). Well data in the northern part of the Calama Valley are insufficient to test faults would control the east-trending walls of the Quebrada de Opache can- whether the fault impacts the aquifers (Figs. 10 and 13). yon, and might control vertical displacement by tens of meters of some of In the western sector, in the San Salvador–Loa Valley, near-vertical, the lithologic units described in the water monitoring wells (Fig. 14, southern small-displacement (tens to hundreds of meters), east-trending faults that are extreme of cross section E-E’). To date, the potential affects by these faults on exposed in the canyon of the Loa River (Figs. 7 and 9) have an important local the hydrology of the Ojos de Opache springs area have not been considered, impact on the groundwater system. Because these faults juxtapose rocks of and no wells monitor the region down-gradient (southwest) of this set of in- markedly different physical properties, such as coarse Eocene conglomerate ferred faults. against Jurassic metasediments, hydraulic conductivity changes abruptly. Pre- The final pair of structures known to cause major displacement of the liminary data suggest that groundwater is forced to the surface along one of strata that serve as aquifers are the Chiu Chiu monocline and accompanying

Figure 16. Photographs of rocks that are reported to host the lower aquifer. (A) The Yalqui For- A mation of the eastern extreme of the Calama sedimentary basin (near 22.4°S, 68.3°W) consists of pebble conglomerate interbedded with coarse sandstone. There is a strong vertical heterogeneity at the decimeter scale, yet individual layers are well sorted and locally cross-bedded (upper 30 cm). (B) The Yalqui Formation at a position ~10 km more centrally located within the Calama sedimentary basin compared to A (near 22.401°S, 68.364°W). The angular clasts, poor sorting, and fine-grained matrix together produce a texture that is unlikely to support high values of hydraulic conductivity. (C) Calama Formation near the western margin of Calama Valley, exposed on Calama Hill (near 22.45°S, 68.88°W). The gravel is clast supported, with moderately well sorted, rounded to subrounded cobbles and pebbles.

Salado syncline (Figs. 7 and 17). At gross scale, the monocline is a gentle fold with down-to-the-east sense of stratigraphic offset of 100–200 m (Blanco and Tomlinson, 2009). The synclinal axis is a broad, shallow, and complexly folded zone, within which the Loa and Salado Rivers flow. The Chiu Chiu monocline is younger than the Opache Formation and older than the Quaternary Chiu Chiu B Formation. The serpentine form of the monocline and evident subsidence of the area encircled by the monocline (Fig. 17) led Blanco and Tomlinson (2009) to interpret it to be the result of the subsurface dissolution and removal of an evaporite-rich unit located at many hundred meters depth (Fig. 13A). The 2–3-km-wide syncline would thus represent the topographic low formed above the area of maximum subsurface material loss and subsidence. The vertical position of the strata that contain the upper aquifer rises >100 m from east to west across the monocline (Fig. 13). South of the Loa River the monocline diminishes progressively in relief. Fractures related to the original dissolution and subsidence along the axis of the syncline, as well as fractures related to strain within overlying units (Blanco and Tomlinson, 2009), may have en- hanced permeability through some rock units.

Distribution of Aquifer Host Rocks East and North of Calama Hill

Three cross sections (Figs. 10, 13, and 14) illustrate the geometry of the strata that contain the aquifers in the Calama Valley. Cross sections A-A’ (Fig. C 10) and E-E’ (Fig. 14) are approximately parallel to the groundwater flow direction; cross section D-D’ (Fig. 13) is essentially perpendicular to groundwater flow. The regionally extensive limestone of the upper Miocene and Pliocene Opache Formation, the host for the upper aquifer in many areas, displays a va- riety of facies (Figs. 15A, 15C), some with centimeter-scale vugs (Fig. 15C) and microkarst (May et al., 1999; Houston, 2004). Both bedding-parallel and nearly vertical fractures are common (Fig. 15A) and likely contribute to the hydraulic conductivity of the Opache. Laterally toward all the basin boundaries, the Opache grades to conglomerates (Figs. 6A and 17). In the northeastern Calama Valley, the Chiquinaputo Formation, which is also a host to the phreatic aquifer (Houston, 2004), interfingers with the Opache limestone (Figs. 5 and 13) as well as locally underlying the limestone (Blanco, 2008). The Chiquinaputo consists of well-sorted fluvial gravels (Fig. 15B). Zones of siltstone within the Chiquina-

TABLE 10. VALUES OF PERMEABILITY REPORTED IN PUBLISHED AND UNPUBLISHED REPORTS, ORGANIZED BY GEOGRAPHICAL SECTOR AND HYDROGEOLOGICAL UNIT Geological unit Local Matrix Matrix Sector in central Loa (Formation name hydrogeological permeability range permeability average Source groundwater basin used by source) role (m/day) (m/day) of dataComments Opache and Houston Aquifer test; matrix Upper aquifer 1–2 Chiquinaputo (2004) permeability Opache and Houston Aquifer test; fissure Upper aquifer ≤120 Chiquinaputo (2004) permeability Eastern (Llalqui) Lasana? and Houston Aquifer test; matrix (Fig. 4, broad area Lower aquifer1–4 Yalqui (Calama) (2004) permeability surrounding location L) Lasana? and Houston Aquifer test; fissure Lower aquifer 40–100 Yalqui (Calama) (2004) permeability Houston Sifón Confining 0.00003 Outcrop data and theory (2004) Average of 8 tests Opache Upper aquifer0.04–2.25 1.2 EIA 2005 in 3 wells

Lasana Average of 3 tests conglomerate Upper aquifer0.53–0.79 0.7 EIA 2005 Central (Fig. 4, broad in 1 well area surrounding member location T) Aquitard or Average of 11 tests Jalquinche 0.000321–0.0062 0.0017 EIA 2005 conﬁning in 4 wells mostly Lasana Average of 19 tests conglomerate; Lower aquifer0.71–23.15.5 EIA 2005 in 9 wells (Calama) Northwestern margin Average of 11 tests (Fig. 4, near West fault Calama Lower aquifer2.0 0.02–9.96 EIA 2005 in 6 wells northwest of location H) Fuentes Calama city–West fault Opache Upper aquifer3.9 Carrasco Pump tests region (Fig. 4, between (2009) locations H and O) Average of 6 tests Calama Lower aquifer 20.60.24–106.22 EIA 2005 in 3 wells Note: Matraz 2012 (see Table 1) summarized the span of permeability values from 205 wells for an upper aquifer (1 x 10–3–3 x 102 m/day), lower aquifer (1 x 10–5–1 x 102 m/day), aquitard (1 x 10–6 to <5 x 10–3 m/day), and basement (1 x 10–9–1 x 10–5m/day), ranges similar to those compiled here. The report does not permit examination of spatial variations of those values.

puto Formation, reported by Blanco (2008), likely have poor hydraulic conduc- and fold system (Figs. 7, 10, and 14) (Blanco, 2008; Tomlinson et al., 2010). In tivity and diminish the continuity of flow within the upper aquifer. Elsewhere, the eastern sector of the Calama Valley, depths to a local lower aquifer coincide the basin-margin conglomerates are alluvial fan gravels (May, 1997; Blanco, with the interpreted depth to the Yalqui Formation (Figs. 6C and 13). Although 2008) that are not well sorted and likely have low hydraulic conductivity. facies of some exposures of the Oligocene–lowest Miocene Yalqui Formation The Eocene Calama conglomerate (Figs. 6D and 16C) is the host to the are suitable to serve as a lower aquifer (Fig. 16A), much of the Yalqui Forma- lower aquifer in most of the southern Calama Valley. Blanco et al. (2003) and tion is a matrix-supported conglomerate (Fig. 16B) that is not likely to have Blanco (2008) documented that, in outcrop, the lower 100 m of the Calama adequate permeability. The kilometer-scale lateral variations from clast-sup- Formation contains several interbeds of andesite lava overlain by ~450 m of ported to matrix-supported conglomerate texture (Figs. 16A, 16B) seen in out- alluvial and fluvial conglomerate cemented by either gypsum or clays. The crop suggest that aquifers in the Yalqui Formation in the eastern sector of the Calama Formation pinches out northward near the Eocene-age Milagro fault basin must be laterally complex. There is inadequate information in the zone

Widespread ignimbrites in the eastern and northern extremes of the Calama Valley (Ramírez and Gardeweg, 1982; Marinovic and Lahsen, 1984; de Silva, 1989) may be aquitards. The distribution of the Sifón Ignimbrite is well established in the east-central and southern parts of the Loa catchment basin. In the northern and easternmost sector, published maps and reports do not clarify the locations of the boundaries between the Sifón Ignimbrite and other very thick ignimbrites (e.g., Cupo, Divísico, Rio Salado, and lower San Pedro Ignimbrites; e.g., de Silva, 1989) in similar stratigraphic positions. Herein, these five ignimbrites are treated as a single map unit (Figs. 4, 10, and 13). Although locally at least part of the 1–100-m-thick ignimbrites is welded tuff, more widely the ignimbrites are not welded. The effectiveness of these ignimbrites to impede water flow is unresolved: common vertical fractures may serve as flow paths (Montgomery et al., 2003), yet water-pressure data in the Llalqui area demonstrate that the Sifón Ignimbrite locally confines an artesian aquifer (Houston, 2004, 2007). In the southern part of the Calama Valley a major aquitard or confining Figure 17. Map of the upper Miocene–Pliocene Opache For- mation (brick pattern), the Chiquinaputo Formation, and lat- unit is created by the fine sandstone and mudstone of the middle Miocene erally equivalent alluvial conglomerates (both the Chiquina- Jalquinche Formation, which is laterally extensive (Figs. 5 and 9–14) and as puto and marginal conglomerates are represented by gravel much as 200 m thick (May, 1997; May et al., 2005; Blanco, 2008; Blanco and pattern), showing the positions of interpreted subsurface Tomlinson, 2009; Tomlinson et al., 2010). Red mudstone is an important com- dissolution pathways. The positions are constrained by seismic reflection data where the line is solid. Where dashed, ponent of the Jalquinche Formation (Blanco, 2008), and the reddish color is the positions are constrained only by subtle variations in suggestive of a diagenetic Fe-rich clay. Fine-grained reddish sandstone is both landforms and are speculative. The path of the syncline con- common and rich in gypsum (May, 1997; Blanco, 2008). Its color and gyp- trols the position of the lower Salado River (R.) and a reach of the Loa River. Hypothetically, the southern dissolution sum content together suggest that even in the sandstone facies the primary path may also influence groundwater flow south of Calama porosity may have been occluded. North of the Talabre area (T in Fig. 4) the Hill (H) in an area where there are no monitoring wells. Ch— Jalquinche Formation thins to only 10 m, and is replaced by coarse-grained Chintoraste hills; O—Ojos de Opache region; S.—San. facies of the Lasana Formation (Fig. 6B) (Blanco and Tomlinson, 2009). Not only are the geological units that serve as aquitards laterally variable between the Talabre, Llalqui, and Calama Hill areas (T, L, and H in Fig. 3A) to from mudstone to ignimbrite, in addition their effectiveness as aquitards is deduce how the gravel-dominated lower aquifer host rocks change across the heterogeneous within a single geological unit. Both the Jalquinche Formation Chiu Chiu monocline. and the ignimbrites change markedly in thickness as well as pinch out entirely. In the northern and eastern Calama Valley the middle Miocene Lasana For- In some parts of the central Calama Valley the coarse facies of the lower mem- mation (Fig. 5) is the most likely candidate to be the gravel reported to host ber of the Lasana Formation either underlies the Sifón Ignimbrite or underlies both the lower and upper aquifers (Figs. 10, 13, and 14). The Lasana Formation a thick mudstone interpreted as the Jalquinche Formation or as a facies vari- is at least 100 m thick where its lower member crops out along the Loa canyon ation within the Lasana Formation (Figs. 10, 13, and 14); however, elsewhere north of Chiu Chiu (Fig. 4) (Blanco, 2008). Blanco (2008) and Blanco and Tom- available well data are not conclusive that any of these units is the aquitard linson (2009) described fluvial conglomerate, sandstone, and siltstone in the (see especially Fig. 10, where boundaries are dashed lines). As a consequence lower member, in repeated complex fining-upward series several meters thick, of the heterogeneity of the overlying set of aquitards, down-gradient flow in that become progressively dominated by siltstone to the south and west. The deep aquifer horizons is likely to pass laterally from phreatic zones to confined upper member has a higher percentage of mudstone (Blanco, 2008). Aquifers zones (Fig. 3B). composed of the Lasana Formation are likely internally complex and laterally limited by facies changes. Blanco (2008) described preferred orientations of the sedimentological features of the lower member that may cause a favored Distribution of Units West of Calama Hill south-southwest alignment of conductive aquifer properties. The Lasana Formation is age equivalent to and interfingers laterally with the Jalquinche Three cross sections, B-B’ (highly oblique to groundwater flow), C-C’, and Formation clay-rich siltstone unit (Figs. 5, 13, and 14), which functions as an the southern 15 km of E-E’ (the latter two subparallel to groundwater flow; aquitard (Table 10). Figs. 12, 13, and 14), illustrate major changes in the distribution of Calama

Basin strata near the West fault and in the region where the Calama Valley mudstones and very fine to fine sandstones. Heterogeneities in the Jalquinche meets the San Salvador–Loa Valley. Cross section C-C’, drawn south of the Formation encompass local horizons of coarser sandstone and conglomerate. Loa River, is isolated by the deep Loa canyon from the regional groundwater Overall, the thickness of the Jalquinche Formation varies markedly, from 0 to flow. Nevertheless, the exposures in this southern zone permit detailed under- 200 m (May, 1997), while it thins westward, toward a basement paleoridgeline standing of the architecture of the sedimentary and volcanic units (Figs. 2 and near which it pinches out (brown line, Fig. 9). 9), unlike the area between the Loa and San Salvador Rivers where only the Rocks with properties suitable to act as aquifers are heterogeneous in this Opache Formation is exposed. It is assumed that cross section C-C’ (Fig. 12) western area. A persistence throughout the Neogene of topography that fun- approximates the geometries and depths of formations that are appropriate neled rivers through the narrow San Salvador–Loa Valley (Figs. 6A–6C) would for the aquifer-hosting rocks between the two rivers. likely have created preferred elongations of sedimentary facies in an east-west Rocks related to a local Eocene volcanic center (Mpodozis et al., 1993; direction. Near what is today the canyon of the Loa River exists evidence of Trumbull et al., 2006) at the Chintoraste hills are an important part of the geo- paleo–Loa River positions, expressed both in cross sections of channel forms hydrological setting west of the West fault (Figs. 4 and 9). Pyroclastic deposits and in landforms. One stratigraphically deep example is a paleochannel with are abundant, as well as epiclastic conglomerates, minor lava flows, and sub- an apparent width of 250 m that cuts the Chintoraste unit and has well-bedded volcanic intrusives. The Chintoraste hills are a circular feature ~4 km in diam- siliciclastic fill that underlies the Jalquinche Formation. Shallow examples may eter with an outer ring of outward-tilting (~40°–50°) ignimbrites and a center either control zones of karst in limestone of the Opache limestone or control that includes contact-metamorphosed Mesozoic strata. Reports from mineral the distribution of Quaternary gravels. exploration boreholes in the 10-km-wide valley between the West fault and To the west of Calama Hill, bedrock of the Precordillera constricts the sedi- Chintoraste hills indicate widespread pyroclastic deposits, primarily tuffs, mentary units that form the aquifers. This is especially true of the upper aqui- and interbedded coarse fluvial siliciclastics. The lithologic distribution sug- fer, where the horizontal distribution and vertical relationships of rocks with gests that pyroclastic deposits near the Chintoraste center interfinger north- relatively high permeability are reduced from an eastern wide area (~25 km ward and eastward with conglomerates. In some of the mineral boreholes, north-south distance) to a narrow western area (~5 km wide, measured north- water was reported within these deposits. south) (Fig. 18). Although the width of the lower aquifer also narrows west- Capping the localized Chintoraste volcanic center is an Oligocene(?) or ward near Calama Hill, the Chintoraste volcanic and volcaniclastic unit may Miocene conglomerate that constitutes a more widespread sheet, ~10 m thick continue at a similar elevation for more than 10 km southward, underlying a (Fig. 6). From a depositional history perspective, the Eocene Chintoraste strata broad valley for which there are few data (Fig. 4). Given that the Jalquinche and the overlying conglomerate sheet are very different. However, few subsur- Formation coarsens southward from the Loa River, and given that the scant face data exist that enable differentiation of this capping conglomerate from borehole reports in the southern valley reveal no evidence of a Jalquinche-like Chintoraste lithologies. mudstone, an aquifer within that southern valley is likely to be phreatic. Borehole reports and paleospring locations suggest that the lower aquifer In addition to the narrow passage between the north and south bedrock west of the West fault is dominated by pyroclastic deposits and interbedded boundaries of the San Salvador–Loa Valley, water that passes north of Chinto conglomerates. This study attributes most of these water-bearing horizons to raste hills in the lower aquifer encounters a second major bedrock constric- the Chintoraste complex (Figs. 11 and 14) and the overlying thin conglomerate tion. A north-trending bedrock ridge separates the eastern sedimentary basin sheet. A consequence of this interpretation of the lower aquifer host rocks is domain from a western domain of high-standing folded Mesozoic metasedi- that, at the West fault, down-gradient flow in the lower aquifer (Fig. 3) transfers mentary rocks and Cretaceous–Eocene intrusive bodies (Figs. 9 and 12), with from an eastern host rock dominated by conglomerate (Calama Formation) to a north-trending fault at the boundary. South of the Loa River the western a western host rock dominated by pyroclastic facies and volcaniclastic facies bedrock domain and faulted boundary have a thin cover of Miocene–Pliocene (Chintoraste unit). gravel; between the San Salvador and Loa Rivers there is a thicker cover of In the area west of Calama Hill (H in Fig. 4), the Opache Formation lime- Opache Formation, which obscures the details of the bedrock ridge (Fig. 9). On stone and immediately underlying sands and gravels serve as the regionally the east side of that bedrock ridge (west end of section C-C’, Fig. 12), both the extensive phreatic aquifer (Figs. 11, 14 and 15D; Table 10). The sub-Opache Jalquinche mudstones and the Eocene Chintoraste unit with its permeable in- medium-grained sandstone to well-sorted cobble conglomerate ranges in terbeds thin westward. In the intercanyon plain the host rocks for the regional thickness between 0 and 4 m (May et al., 1999) to 20–30 m (our mapping), too lower aquifer shallow westward until they are in direct contact with the base thin to distinguish in Figure 9 as a separate map unit. The outcrop belt of the of the thin sandstone and gravel that underlies the Opache Formation. In Fig- sandstone and conglomerate at the base of the Opache Formation is associ- ure 9, the brown contour marked 0 m traces where the Jalquinche Formation ated with important springs. pinches out, and the green contour labeled 0 m traces where the Chintoraste The uppermost major unit with poor capacity to transmit water is a part of unit pinches out. Between the brown and green lines and for some distance the Jalquinche Formation. It is composed mostly of gypsum-rich and clay-rich to the east of the brown line, where the Jalquinche is thin, there is no effective

Figure 18. (A) River locations, major faults, and the positions of two schematic geohydrology cross sections (black lines). The reach of the Loa River that probably recharges in part the Calama Valley aquifers is highlighted blue, and river elevations of 2660 m, 2760 m, and 2865 m are noted in x′ that reach. The brown line marks the border of the Calama z′ sedimentary basin and its potential aquifer host rocks. The pale green area encompasses both the Jalquinche Forma- tion and the ignimbrites that effectively confine a lower artesian aquifer in some area. The tan zones within the green region mark areas where the two aquifers are in pressure balance or the upper aquifer head exceeds that of the lower aquifer, from Figure 3B. The dotted pattern indicates regions where no data test the effectiveness of potential confining units. Contours for the head in the lower aquifer are simplified from Figure 3A. There are no well data in the northern sector of the basin and no known suitable confining layer there, so hypothetical head contours are shown (blue) to tie to the elevations of the Loa River. The blue area in the northeast map corner is a zone above 4000 m elevation where primary recharge likely occurs. The star marks the location of Calama city. Ch—Chintoraste hills; O—Ojos de Opache region; H—Calama Hill; L—Llalqui area; T—Talabre area. (B) Schematic cross sections of the geo hydrology of the Calama Valley. The vertical scale is exag- gerated relative to the horizontal scale, and neither is exact. z′ The moderate- to high-permeability zones (white units) are a set of formations in which a mosaic of sedimentary facies exist and, by inference, variable permeability exists B that is not illustrated. The black arrows indicate parts of those units that contain groundwater. The arrows indicate the part of the vector of groundwater flow that resolves to the plane of the cross section; additional flow may occur into or out of the plane of section. The blue arrows indicate flow across the low-permeability geological units that separate the upper and lower aquifers, inferred based on relative heads of the lower and upper aquifers (Fig. 3B; Table 9). Open arrows indicate exchanges of the rivers with the aquifers.

aquitard and therefore there is a single aquifer that is phreatic. Thus within a Groundwater in the east-central Calama Valley, east of the Loa River, is span of 10–15 km west of the West fault, it is inferred that the lower aquifer sourced by water that precipitated in the highlands to the east and northeast, connects to the upper aquifer (intercanyon region) and can discharge directly by recharge that may be both direct (Houston, 2007) as well as indirect, e.g., by to both rivers. infiltration of Salado River water into its alluvial bed. The extent of or locations At a smaller scale near the Loa River, two intersecting sets of faults control where the groundwater from the eastern aquifers (L, Llalqui area, Fig. 18A) the position of the western boundary of the rock unit that hosts the lower aqui- mixes with the water in the central Calama Basin aquifers are not yet docu- fer. The first fault set is north trending (F1 and F3, Fig. 9) and forms the western mented (Fig. 18B; note question marks where the eastern and northern cross boundary of the Eocene Chintoraste unit. The second set, east trending and sections should connect). of small displacement, juxtaposes the Chintoraste unit against the Mesozoic From the perspective of the broad Loa water system, within the Calama impermeable basement (F2 in Fig. 9). That set also juxtaposes multiple lith- Valley there is no net increase in water because there is essentially no direct ologies within the Eocene volcano-sedimentary package against one another, precipitation. However, within the study area the rivers exchange with the some likely permeable and some of low permeability. Locally, the result is that aquifers at various locations. A key river-groundwater exchange may occur the lower aquifer host rocks terminate at a corner. near the northern limit of the Calama sedimentary basin, where the Loa River likely loses water into the Lasana Formation, which down-gradient hosts both a lower confined aquifer and the upper phreatic aquifer (Figs. 10 and 14). Else- DISCUSSION where in the basin, east of the West fault, the available data suggest that only the upper aquifer exchanges with the rivers. However, at the West fault and Overview within 20 km distance to its west, most of the water in the lower aquifer must discharge to the upper aquifer or to the rivers directly. For the Loa system, the existing subsurface and surface flow data are inadequate to quantify numerous parts of the hydrological system. It would be convenient to assume that aquifer properties are laterally homogeneous, so Calama Valley and West Fault System Hydrogeology that sparse data can be widely applied. However, the geological properties of the sedimentary basin that hosts the major aquifers of the Loa system point to- Across the western and central Calama Valley the piezometric surfaces of ward considerable heterogeneity (Fig. 18). This paper contributes an improved the two aquifers are of similar elevations in some sectors, but elsewhere the understanding of the architecture of the rocks of suitable hydraulic conduc- lower aquifer piezometric surface is higher than that of the upper aquifer (Fig. tivities to serve as aquifers or as aquitards within and adjacent to the Calama 3B). Within those broad areas (Fig. 3B) the aquitard likely permits a small de- Valley, and their influences on the preferential flow pathways and possible gree of slow flux upward and the lower aquifer may recharge the upper aqui- discharge regions. fer. The sedimentary rocks that serve as the aquifer are highly heterogeneous Within the Calama Valley the groundwater supplied from the eastern across this region. Near the north-trending reach of the upper Loa River and mountains and from the northern mountains join at multiple levels. At the sur- the Chiu Chiu monocline, southward and westward thinning of the Sifón face, the baseflow-fed Salado River and baseflow-fed Loa River merge. In the (Blanco, 2008; Blanco and Tomlinson, 2009; Tomlinson et al., 2010) and Cupo subsurface, groundwater enters from the highlands and encounters sedimen- Ignimbrites (Figs. 10 and 13) lead to their loss of effectiveness as confining tary rocks of the Calama Basin. Figure 18 illustrates in a simple geohydrolog- units. Similarly, lateral variations in the thickness of the Jalquinche Formation ical sketch the vertical changes in hydraulic conductivity but displays only the and mudstones in the Lasana Formation lead to transitions in their capacity to most rudimentary aspects of the horizontal variability. The steep regional topo- be effective confining layers (Figs. 10, 14, and 18B). graphic gradient imposes a strong piezometric gradient that directs ground In areas in the western and central Calama Valley where the head in the water to the southwest and then west across the study area. upper aquifer exceeds the piezometric surface of the lower aquifer (red in Fig. The aquifers of the north sector of the Calama Valley are filled at least in part 3B), we expect that slow downward flow from the upper aquifer may recharge by infiltration of water from the Loa River into the conglomerate and sandstone the lower aquifer. The narrow strip north and east of the Talabre region (Fig. of the Lasana Formation along the sector of the river south of the Conchi Dam 3B) where the pressure in the two aquifers is similar corresponds to a transition (Figs. 2 and 18). Additional groundwater may enter the Calama Valley from the zone from excess head in the lower aquifer to excess head in the upper aquifer. peaks above 4000 m elevation beyond the northeast end of Figure 18 section The piezometric gradient of the lower aquifer in the 5 km east of the West X-X’, passing beneath the bed of the Loa River in the lower parts of the Lasana fault is markedly steeper than in the central sector of the Calama Valley (broad Formation. Once in the Lasana Formation, the groundwater migrates southwest region around T in Figs. 3A and 14). A less pronounced increase in gradient and encounters lenses and formations of variable permeability, with the out- occurs also within the upper aquifer east of the West fault (Fig. 2). Changes in come that the groundwater is progressively split into multiple aquifers (Fig. 18B). the piezometric gradient of the lower aquifer may reflect the Milagro deforma-

tion zone and related changes in the thickness of suitable aquifer host rocks height of the upper aquifer is 20–40 m higher than that of the lower aquifer. (Fig. 14), but lithological data for the lower aquifer are not sufficient to test this This relative loss of head in the lower aquifer might occur for either of two hypothesis. reasons. First, the rocks with high permeability below the aquitard layer might The heads of the two aquifers come into equilibrium near the West fault thicken westward, allowing more vertical dispersion in the lower aquifer of zone, which comprises the West fault and a subsidiary parallel fault, located water that infiltrated across the West fault. Second, part of the water from the 1 km to the east (Fig. 3B) (Tomlinson et al., 2010). The Loa River turns sharply lower aquifer east of the fault might have transferred upward within the fault to the south near the trace of the eastern branch of the fault, continues ~6 km zone into the upper aquifer. Data are not available to test which explanation is in the zone between the two faults, and then resumes its westward direction more viable. Whatever the cause, the result is a zone with relative pressures (Fig. 7). In the subsurface, rocks that host the lower aquifer, the Eocene Calama that create a hydraulic gradient favoring downward seepage of upper aquifer Formation conglomerate to the east and Eocene Chintoraste pyroclastic and water into the lower aquifer, across the intermediate Jalquinche Formation sedimentary rocks to the west, meet at the West fault (Figs. 10, 11, and 14). (Fig. 18). Lower aquifer water flow from east of the West fault to west of the West fault Unlike prior studies, this study concludes that within 20 km west of the West system navigates through permeability pathways that are not stratigraphically fault the lower aquifer discharges significantly, if not completely, to the surface continuous. Although those deeper units are discontinuous, the Jalquinche water system, because the host rocks for the lower aquifer unit thin between Formation and the upper aquifer-bearing Opache Formation accumulated after impermeable basement and a thinning confining unit. This lower aquifer dis- most of the displacement across the West fault zone, and likely underwent charge is in part direct, because the host rocks crop out in the San Salvador, much less disruption (Tomlinson et al., 2010). Nevertheless, the nearly equal Ojos de Opache, and Loa canyon walls (Fig. 9). Although hydrologi cal evidence heads (blue zone north of H in Fig. 3B) likely indicate an active flow between of groundwater discharges into the Loa River canyon where the Chintoraste the lower and upper aquifers through a less effective aquitard. The faults may unit crops out is lacking, extensive carbonate deposits in the bed of the Loa increase the heterogeneity of rock units that would otherwise act as aqui- River north of Chintoraste hills (Fig. 9) and paleospring carbonates located tards, and they may place low-conductivity rocks adjacent to high-conductivity above the modern water level on the canyon walls are hypothesized to be by- rocks. Recent research shows that large faults can effectively produce a greater products of lower aquifer springs. The lower aquifer discharge is also indirect hydraulic connection between shallow and deep aquifers (Bense et al., 2013). in part, through the upper aquifer between the San Salvador and Loa canyons. This indirect discharge is inferred from the westward termination against a basement ridge of a major aquitard, the Jalquinche Formation (brown dashed San Salvador–Loa Valley Hydrogeology line in Fig. 9), and of the subjacent units with lithologies that are suitable for moderately high permeability (green line in Fig. 9). Borehole data imply that The interpretation that the lower aquifer in the western region is hosted little mudstone separates the Chintoraste unit from the sub-Opache sandstone by the Chintoraste complex and a thin overlying conglomerate is a departure of the upper aquifer (westernmost 4 km E-E’, Fig. 14), and hence water could from prior interpretations, which ascribed the aquifer to the Calama Forma- migrate easily from the lower aquifer to the upper aquifer. tion. The borehole geology and the outcrops at Chintoraste hills reveal that the Important improvements in knowledge of the hydrogeology of the criti- host rocks west of the West fault are much more pyroclastic and volcaniclastic cal region west of Calama city will require geological mapping at high spatial than is the Calama Formation (Blanco et al., 2003; Blanco, 2008). Furthermore, resolu tion. In addition to providing greater precision on positions of formation considering that tens of kilometers of left-lateral displacement along the West boundaries, mapping is needed to specify facies variations in the water-bear- fault are interpreted to have postdated accumulation of the Eocene Calama ing rocks, to establish the positions of channelized strata of high hydraulic Formation (Tomlinson and Blanco, 1997), it is unlikely that the Calama Forma- conductivity, and to relate paleospring deposits to the modern groundwater tion continues to the west of that major fault. hydrology. Given the widespread cover by the Opache Formation of underly- Although the Loa River turns abruptly southward where it intersects the east ing complex lateral changes in aquifer-host units and aquitard units, and the branch of the West fault and parallels the fault set for ~2 km (Fig. 7), potential vertical canyon walls along the Loa River, novel observation techniques and exchanges between the river and aquifers cannot be quantified with the sparse high-resolution geophysical surveys may be needed. stream gauge data (Table 2), especially because several irrigation channels tap the river in this reach. Only a few kilometers farther west, widespread springs discharge from the upper aquifer to the Loa River channel and to the main tribu- Intersection of the Aquifers of the Eastern and Central Sectors tary to the San Salvador River. Much of the discharge occurs because the upper aquifer is intersected by the canyon walls (e.g., springs near LC, Fig. 9). A lack of publicly available piezometric data for the eastern sector of the Whereas at the West fault the piezometric height of both aquifers is ~2240 m basin (from Llalqui to the Salado River; Figs. 2 and 3A) results in very little (Figs. 2 and 3A), throughout the area with data west of the fault the piezometric documentation of what happens to either aquifer near the Chiu Chiu mono-

cline (Figs. 13B and 18). In that sector, where the Loa River flows parallel to ness and in hydraulic properties of the aquitards, which are both ignimbrites and immediately east of the monocline, a short-term stream gauge campaign and mudstones. Much of the lateral variability in aquifer properties results from conducted late in a summer season (March) indicated that the upper aquifer facies changes in the middle Miocene Lasana and Jalquinche Formations, as discharges ~700 L/s to the river (Matraz 2012, see Table 1), accounting for about well as in the upper Miocene to Pliocene Opache and Chiquinaputo Formations. a quarter of the surface water flow. Another discharge from the upper aquifer Folds and faults add to the architectural complexity of the aquifers. A princi- to a spring on the north bank of the Loa River, ~100 L/s, occurs at the southern pal example occurs west of Calama city, where a fault-controlled, north-trend- crossing of the Loa River canyon over the monocline (Fig. 4) (Matraz 2012, see ing basement ridge against which most of the Calama Basin sedimentary rocks Table 1). This spring location suggests a likely structural control on upper aqui- terminate controls discharge from a lower, semiconfined aquifer to springs fer groundwater flow, which is combined with insight into aquifer host rocks and rivers. A second important example is the West fault, across which pie- developed elsewhere in the Calama Valley to put forth a hypothesis for ground- zometric gradients change and groundwater flow navigates major changes in water flow near the Chiu Chiu monocline. The lower aquifer occurs at a depth the host rocks. exceeding 200 m (hosted in the Yalqui Formation and a conglomerate that Two general conclusions of this analysis should be useful in the design may be age equivalent with the Lasana Formation; Fig. 13; Blanco and Tomlin- of studies that will improve understanding of the coupled surface water and son, 2009), with a considerable thicknesses of two overlying low-permeability groundwater system. First, the spatial variations of the aquitards exert a key units (Jalquinche Formation and Sifón Ignimbrite). Given the westward dip control on the exchanges of water between the upper and lower aquifers. Addi- of the aquifer host strata and aquitards east of the Loa River and their higher tional research focused on the sedimentary architecture of the middle and late elevations across the monocline west of the river (Fig. 13B), it seems likely Miocene sedimentary basin, with a focus on the aquitard facies, would likely that the Salado-Llalqui region groundwater does not cross the monocline to lead to better understanding both of preferential flow pathways and the loca- mix with the northern source region groundwater of the Talabre area. Blocked tions where there are exchanges between an upper and lower aquifer. Second, by the rise in elevation of the aquifer hosts at the monocline, lower aquifer faults and folds near which specific sedimentary units terminate, change in water may flow south into the region where the thick (hundreds of meters) thickness, or change in elevation likely have groundwater flow consequences. Eocene Calama Formation is expected to occur (Figs. 4 and 6D). Within the The hypothetical consequences can be tested by monitoring the groundwater Calama conglomerates, water may continue southward until the monocline and surface water at locations near those faults and folds, in both upflow and tip is reached, south of which it then flows westward between, and paralleling, downflow directions, or with new geophysical and geochemical studies. the Loa River and the southern basin boundary. Some lower aquifer water would thus flow south of Calama Hill (H in Fig. 4). An absence of data near and ACKNOWLEDGMENTS south of the west-flowing reach of the Loa River (Fig. 3A) precludes any further We thank Nicolás Blanco Pavez and Andrew J. Tomlinson of the Servicio Nacional de Geología y evaluation of this hypothesis. Minería (Chile), Luís Baeza Assis, Jorge Jemio Figueroa, and Manuel Bucci Ramirez of CODELCO (Corporación Nacional del Cobre de Chile), and Luís Rojas B. and Arturo Beltrán Schwartz of the Dirección General de Aguas (Chile) for initial access to locations, reports, and data, as well as for in-depth discussions of the Loa system. We appreciate the early encouragement by and discus- CONCLUSIONS sions with John Houston. Alex Covarrubias Aranda and Rodrigo Riquelme Salazar of Universidad Católica del Norte, Oscar Cristi of the Universidad del Desarrollo, Gary Libecap and Eric Edwards The integration of data describing spatial variations in sedimentary facies of University of California Santa Barbara, and Lovell Jarvis of University of California Davis con- and their thicknesses has the potential to improve understanding of any com- tributed greatly to our understanding of the management of the Loa water. U.S. National Science Foundation grant OISE-1037929 enabled Jordan, Godfrey, and Kirk-Lawlor to gain an understand- plex groundwater system located within the fill of a sedimentary basin. This ing of the multifaceted challenges for water management in the study area. A Fulbright Fellowship Calama Basin study integrated information about lateral variations in the po- for Jordan in 2012 provided partial support of this project. We also thank the makers of Google tential to store and transmit water with an assessment of the primary aquifers, Earth and the agencies and companies who acquire satellite images displayed on Google Earth for the free availability of this information and tool, which were fundamental for this study. A and thereby clarified the spatial distribution of the units with which the con- critique of an earlier manuscript by Maria-Theresia Schafmeister was very helpful. We are grateful fined or semiconfined aquifer system is associated. 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