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Previous Hydrochemical and Isotopic studies in the Basin
The Ministry of Water and Environment (M.M.A.y.A) have been working at the implementation of monitoring programs of quantity and quality in the Purapurani zone since 2010 (M.M.A.y.A, 2016). In this context, they have been working in coordination with the IAEA (International Atomic Energy Agency), within the Project ―Using Isotopes for Hydrogeological Assessment of Intensively Exploited Aquifers in Latin America (ARCAL CXXVII), and with the University of San Francisco Xavier de Chuquisaca (Master‘s thesis).
They have conducted several sampling campaigns in this zone (Fig.2.4) and monitored around 50 points of groundwater (wells, springs) and surface water (rivers) sources, since 2010. Thus, major ion facies of water, stable isotopes of oxygen 18 and deuterium, and radioactive isotopes of carbon 14 and tritium information is available at M.M.A.y.A (2016).
Generally, groundwater in the Purapurani zone present and facies. However, there are increasing trends of and in some sectors that could be related to anthropogenic and mining sources of contamination (Limpias et al., 2010; Gómez et al., 2012; M.M.A.y.A, 2016). In addition, isotopic results of deuterium and oxygen 18 in groundwater hosted in the Purapurani zone, showed values isotopically lighter suggesting that recharge of groundwaters were ancient and originated from rains or glacial melting of a different climatic regime (M.M.A.y.A, 2016).
Furthermore, Barroso et al. (2010) reported that groundwater hosted in the glacial-till materials present predominantly facies. Finally, Quino et al. (2019) conducted a local hydrochemical assessment of groundwater hosted in plain area of the Katari and Lago Menor Basin (lower Katari). These authors concluded that , and groundwater facies were not clearly defined.
All the available hydrogeological and hydrochemical data, resultant of the previous local research performed in the study area, are valuable for creating a regional hydrogeological database. Moreover, a basin-scale investigation is compulsory for the correct interpretation of natural flow processes and phenomena, and the comprehension of the final hydrochemical composition of groundwater.
Groundwater source inventory and water level measurements
A regional field campaign, for inventorying groundwater sources and measuring groundwater levels, was carried out during august of 2017. A total of 97 sources were verified and validated in the field: 19 artesian wells, 36 deep wells, 32 dug wells, 8 springs and 2 wetlands (Fig.3.5). The main objectives for performing this regional field campaign were to: i) verify and validate groundwater sources for the establishment of a regional groundwater monitoring network, ii) construct a contour map of groundwater levels for assessing lateral groundwater inflow and outflow and iii) propose a regional groundwater level monitoring network in order to obtain time series of groundwater levels, essential for the construction of regional transient groundwater models.
The groundwater source inventory consisted in the field verification of well conditions or groundwater sources (i.e. springs, wetlands). Each site was georeferenced, with a GPS Garmin Etrex 30 (position accuracy less than 15 meters and +/- 400 ft of height-based on an ellipsoid and not on a vertical datum). Groundwater level measurements were performed using a water level meter Solinst of 100 meters, graduated to millimeter.
The common difficulty experienced when measuring groundwater levels was that most of the wells were provided with submersible electrical pumps. Although it was ensured that the well was not pumped at least 48 hours before measuring groundwater levels, technical issues related to the water well system constrained the progress of this campaign. For this reason, the design, construction and installation of new piezometers were necessary for establishing a regional groundwater level monitoring network in the study site. Figure 3.5. a) Device used for measuring the location of each inventoried groundwater source and example of groundwater level measurements performed in the field, b) deep well, c) well in a pumping house provided with a submersible pump and d) dug well.
Geophysical measurements
A regional field campaign, for performing Time Domain Electromagnetic measurements (TDEM) was conducted during June and July of 2017. The main objectives of conducting this campaign were to: i) investigate electrical resistivity parameters of Quaternary, Tertiary and Devonian rock formations, ii) investigate regional hydraulic continuity in the basin, iii) investigate the geometry (depth and thickness) of potential porous geologic media.
Therefore, a total of 187 TDEM (Time Domain ElectroMagnetic) soundings were performed in the field with a TEMfast 48 equipment (AEMR technology, The Netherlands) (Fig. 3.6). TDEM measurements were performed using a coincident loop configuration, a transmitter/receiver (Tx/Rx) loop of 100 x 100 m² and a 25 x 25 m² (or 50 x 50 m² loop when urban sites were surveyed). Detailed explanation about the TDEM method, survey strategy and data processing is presented in Chapter 4 (Article I).
Piezometer construction and installation
A total of five new piezometers were installed in the Katari and Lago Menor Basin during the first semester of 2018 (Table 3.1). These piezometers were installed in order to: i) investigate temporal variations of groundwater-lake interactions, ii) study the connection between the piedmont and plain areas through a buried channel, iii) investigate the behavior of temporal groundwater levels in a natural wetland area, iv) incorporate observation wells in a regional groundwater level monitoring network and for v) monitoring groundwater quality. Table 3. 1. Location and position of the new piezometers using the Universal Transverse Mercator (UTM) coordinate system.
Hydrogeological setting of the study area
Previous local-scale research has investigated and monitored groundwater resources primarily in the two largest cities of the Katari and Lago Menor Basin (El Alto and Viacha, Fig.1) (M.M.A.yA, 2016). M.M.A.yA (2014) delimited the Purapurani zone ( ) (Fig.2) according to the lithology and general stratigraphy of the Quaternary deposits: fluvioglacial and alluvial deposits surrounded by glacial deposits to the north, and the Paleozoic rocks towards the west and south.
The lithological information of several boreholes drilled in the Purapurani zone to a depth of 60 to 110 meters (GEOSIRH, 2017) indicated that the Quaternary geologic media is saturated. However, there are no studies that reported the total thickness of these deposits (depth to bedrock) or their spatial continuity (hydraulic continuity).
Field data from two pumping tests performed by M.M.A.yA established that fluvioglacial materials in the Purapurani zone behave as potential permeable layers with a hydraulic conductivity ranging from . Moreover, long-term groundwater level monitoring data (50 observation wells) have shown that groundwater flow direction, towards NE to SW in this zone, is consistent with flow from the high surface elevations to the low elevations. Close to the east boundary, groundwater flow diverts in the SE direction, towards the valleys of the City of La Paz (M.M.A.yA, 2016). Barroso (2010) conducted a detailed hydrogeological assessment in the location of El Alto Sanitary landfill (Fig.2), towards the NE of the Purapurani zone. This author concluded that the landfill had been placed over clayey glacial-till materials of granite blocks, quartzite, quartzes and intermingled sand, with horizontal hydraulic conductivities ranging from .
Time Domain Electromagnetic (TDEM) soundings
Time Domain Electromagnetic (TDEM) is a geophysical sounding method based on the investigation of the electrical resistivity of rocks. Resistivity is a well-known parameter in hydrogeophysics. For our study, electrical resistivity is a well adapted parameter because our objective is to discriminate permeable formations from more clayey formations, possibly identify conductive brackish groundwater close to the lake (if any) or any lateral variations of groundwater electrical conductivity. Previous studies have demonstrated the usefulness of TDEM for groundwater investigations in arid and semi-arid areas, as highlighted by Descloitres et al. (2013) in the semiarid Lake Chad Basin (Niger), Viguier et al. (2018) in the hyperarid Atacama Desert (Northern Chile), or by Guérin et al. (2001) and Gonzales Amaya et al. (2018) in the semiarid Altiplano (Bolivia).
Basically TDEM uses a large transmitter (Tx) square loop (typically 50x50m² or 100x100m²) and a receiver loop (Rx) both laid out on the surface. Rx can be either a small coil at the centre of Tx (central loop system), or be the same cable as Tx (coincident loop system, as used in our study). A direct current is injected in Tx creating a primary static magnetic field that diffuses into the ground. Once the primary magnetic field is established, the current is abruptly turned-off creating a decreasing flux with time. Due to the induction principle, eddy currents are then created in the soil immediately below Tx cable. As the time elapse, and because the primary magnetic field no longer exists, the eddy current are decreasing with time and further current loops are created deeper and expand laterally in the sub-surface, as smoke rings. Each eddy current loop creates a secondary magnetic field (with the same polarity that the primary magnetic field) also decreasing with time, creating an induced current in Rx loop. The result (the decreasing voltage with time) is then analysed to derive the basic information given by TDEM: the sounding curve, i.e the variation of apparent resistivity with time. Field data are then fitted to simulated data curves, using a layered resistivity model of the ground by means of an automatic adjustment (inversion) of a starting 1-D model with a minimum of layers (usually 3). If the RMS calculated after the inversion remains high (i.e. above 2-3 %) one or two additional layers are introduced to smooth the model and decrease the RMS. The final results are interpreted as a 1-D (layered) model of the resistivity with depth, which can be related to the geology and related aquifers. Further information can be found in the comprehensive review of TDEM for groundwater exploration published by Fitterman and Stewart (1986).
TDEM survey strategy and interpretation
The locations of the TDEM profiles were chosen in order to investigate the different outcropping formations (Tertiary/Devonian substratum) and the different Quaternary deposits (alluvial fan, fluvioglacial, glacial, paleo-lacustrine and fluvio-lacustrine deposits). The locations of these profiles are presented in Figure 2.
Profile 1 is 44 km long (75 soundings, Table 2), and has a NE-SW direction. It stretched from the Eastern Cordillera to the Piedmont subsystem and the Lacustrine plain. This profile investigated also Devonian and Tertiary formations perpendicular to their main structural direction. It crosses Quaternary glacial and fluvioglacial deposits, alluvial fan deposits, and paleo-lacustrine deposits. Profile 2 is 33 km long (32 soundings, Table 2), and has a SE-NW direction. It is perpendicular to profile 1 is used to explore Quaternary fluvioglacial, alluvial fan and paleo-lacustrine deposits.
Table of contents :
Preface
Abstract
Résumé
Aknowledgments
Chapter 1 Introduction
1.1 Research Objectives
1.2 Limitations
1.3 Organization of the thesis
Chapter 2 Regional Environmental Setting of the Study Site
2.1 Location and description
2.2 Climate
2.2.1 Precipitation
2.2.2 Temperature and Evapotranspiration
2.3 Surface topography, drainage, wetland and alpine features
2.4 Geology and Paleohydrology
2.5 Population and Groundwater dependent sectors in the Basin
2.6 Potential environmental threats to groundwater resources in the Basin
2.7 Previous hydrogeological studies in the Basin
2.8 Previous Hydrochemical and Isotopic studies in the Basin
Chapter 3 Methods and Techniques
3.1 Introduction
3.2 Field work
3.2.1. Sampling campaign
3.2.2. Groundwater source inventory and water level measurements
3.2.3. Geophysical measurements
3.2.4. Piezometer construction and installation
3.3 Laboratory work
3.4 Data analysis methods
Chapter 4 Regional geophysical characterization
4.1 Introduction
4.2 Article I: Insight into the Bolivian Lake Titicaca aquifer system, inferred from geophysical (TDEM), hydrogeological and hydrogeochemical data
4.3 Conclusions
Chapter 5 Basin-scale conceptual groundwater flow model
5.1 Introduction
5.2 Article II: Basin-scale conceptual groundwater flow model under natural conditions of the semi-arid, Bolivian Lake Titicaca Aquifer System
5.3 Conclusions
Chapter 6 Groundwater basin-scale hydrochemical and dual isotopic characterization
6.1 Introduction
6.2 Article III: Tracing nitrate contamination in a gravity-driven groundwater flow system of Lake Titicaca Basin, Bolivian Altiplano: influence on water chemistry and nitrogen i
6.3 Conclusions
Chapter 7 Conclusions and perspectives
References
