Doctoral Dissertation by Andres Gonzales Amaya

Andres is having his doctoral dissertation: “A multidisciplinary approach: geophysics, hydrochemistry and hydrometeorology for addressing water security in alluvial fans”. Tuesday, October 23, 2018.

Click here for the dissertation’s invitation and link for the thesis.

Abstract

Water security offers to the local population an adequate quality and quantity of water for health, economy, livelihoods and ecosystems. Hence, achieving basic levels of water security should be a priority in areas where access to fresh water is scarce. The Punata alluvial fan (Bolivia) is a semiarid zone and an important agricultural zone, where the main source of water is groundwater. Therefore, it is important to retrieve and broaden the knowledge about the main hydrogeological mechanisms, and the physical and chemical properties of the groundwater in the Punata fan. This research aims to apply and combine different disciplines such as hydrometeorology, geophysics and hydrochemistry to retrieve important hydrogeological information in the Punata fan. This information is of particular interest for achieving water security, so that freshwater (i.e. groundwater) can be provided to the local population, industry and environment.

The hydrometeorological results suggests that there is a cycling of El Niño events which are related to regional drought periods, hence the recharge of groundwater can be affected. Records from wells suggest that there is a lowering of the water table, which is more evident when an El Niño event take place in the same or in the previous years. Therefore stakeholders should take into consideration the occurrence of El Niño events in order to plan a more efficient usage of water. The geophysical results and interpretation are important for proposing and refining hydrogeological conceptual models, where a more accurate description of the layering, depth and thicknesses of the different aquifers units are displayed. One main finding is the delimitation of a saline layer at the bottom of the aquifer system. The delimitation of this saline layer is important in order to prevent the pollution of fresh water when drilling and pumping take place. Another application of the geophysical results is for elaboration of groundwater vulnerability maps, which are important for preventing groundwater contamination. The hydrochemical results highlight that groundwater in the Punata fan is mainly recharged by the Pucara River, while recharge from precipitation and irrigation are of less importance. The analysis of groundwater salinization suggests that saline deposits lying in the lacustrine layer are the main cause of salinization, which is also supported with the geophysical results where a saline layer was detected.

A multidisciplinary approach can produce more complete, accurate and reliable hydrogeological information in alluvial fans, where groundwater is the main source of water supply. The results and findings of this research are important for the local actors, due to the fact that a better understanding of the major hydrogeological processes is provided. Therefore, with this knowledge, further plans for sustainable groundwater management and protection can be proposed. Thereafter, if access to fresh water (i.e. groundwater) is guaranteed in the study area, basic levels of water security can be reached and with this an improvement of the local wellness.

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Presentation of partial results in international congresses

Several successful presentations have been performed through the year:

  • Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP 2017). Colorado, United States of America. Presented by Torleif Dahlin and Andres Gonzales Amaya
  • Novel Methods for Subsurface Characterization and Monitoring (NOVCARE 2017): From Theory to Practice. Dresden, Germany. Presented by Andres Gonzales Amaya
  • Meeting on Hydrogeology (Flowpath 2017). Cagliari, Italy. Presented by Andres Gonzales Amaya
  • Society of Exploration Geophysicists – Annual Meeting (SEG 2017). Houston, United States of America. Presented by Etzar Gomez
  • 4th Annual Congress of the International Association of Hydrogeologists (IAH 2017). Dubrovnik, Croatia. Presented by Andres Gonzales Amaya

The abstracts are available in the Publication/Conference abstracts section.

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Fieldwork data acquisition at Pucarani aquifer La Paz

A new field campaign with the Ministry of Environment and Water (MMAyA) has started this week aiming to acquire geophysical data from the Pucarani aquifer. ERT and TEM measurements are going to be conducted during the coming five weeks. The research team for this campaign is formed by Nicolai Friis Mortensen (DK), Giulia De Pasquale (IT), Michelle Pedrazas (BO), Micaela Pedrazas (BO), Waldo Medinaceli (BO), personel from MMAyA and local municipalities.

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Presentation of MSc thesis by Viktor Broman and Emil Svensson

Viktor Broman and Emil Svensson presented their MSc thesis “TEM and ERT Investigations in Challapampa Aquifer, Bolivia” Friday 16 June 13:15 at Lund University.

 

Abstract

In this study, electrical resistivity tomography (ERT), induced polarization (IP) and transient electromagnetic (TEM) measurements were carried out to investigate features in the Challapampa aquifer, located on the Bolivian Altiplano north of the city of Oruro. Oruro presently cannot satisfy the demand for water and Challapampa aquifer acts as the main source of drinking water for the city. The objective of this study was to investigate fault systems and thermal water intrusion in a study area located in the aquifers south eastern part.

The ERT method uses an array of electrodes, set up along the surface or in boreholes, to generate resistivity-depth cross sections of the subsurface. The time-domain IP method uses the same electrode configurations as the ERT method but instead measures potentials during charge up or directly after the current is shut down. The IP method can thus be used to determine the chargeability (or IP-effect) of a geological material.  For the TEM method, a direct current is sent through an ungrounded loop. When the current is shut off, the magnetic response of the ground is measured as induced current in receiver(s) on the surface.

Challapampa aquifer consists of Quaternary sediments of fluvial and alluvial deposits from lakes and rivers that have occupied the Altiplano for thousands of years. The bedrock that lays beneath the Quaternary sediments consists of sandstone and shale with intrusion of magmatic rock in some parts. The fault direction of the bedrock on the Altiplano shows a Southeast-Northwest trend. In the southeastern part of the aquifer within the study area there are some hot springs where the thermal water is believed to origin from fractures in the bedrock where the magmatic rock has intruded into the sandstone. In the part where the hot springs are the water is salty, which indicates that the thermal water from the hot springs leads to a salt intrusion in the southeastern part of the aquifer.

Two especially interesting areas of low resistivity were found. One in the northern part of the study area, where the low resistivity area was interpreted as a fault beneath the Quaternary sediments and one in the central part, where the low resistivity area was interpreted as a fissure system beneath the hot springs.

In some parts of the study area, measurements were planned but could not be executed due to villagers not allowing measurements to take place on their land.

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Presentation of project results in a seminar in Cochabamba

Project results were presented by Andrés Gonzales and Torleif Dahlin in a seminar at organised by Centro Agua, UMSS, in Cochabamba on Friday 2017-03-31. The presentations included an overview of the project, preliminary results from the Challapampa and Punata aquifers plus a short introduction to the methods used.

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Field data acquisition campaign at Challapampa

A field data acquisition campaign is in progress at Challapampa during March-April 2017. Both ERT (electrical resistivity tomography) and TEM (transient electromagnetic) measurements are made. The unusually late rainy season with frequent thunderstorms have slowed down the progress, but that is expected to come to an end soon.

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Presentation of project results in a seminar in Oruro

Project results were presented by Etzar Gomez and Torleif Dahlin in a seminar at Geology Dept., Oruro Technical University (UTO,) in Oruro on Tuesday 2017-03-28. The presentations included an overview of the project, preliminary results from the Challapampa and Punata aquifers plus a short introduction to the methods used.

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Presentation of project at SAGEEP 2017

The project is presented in the “Applications of Humanitarian Geophysics” at SAGEEP 2017 in Denver, Colorado:
http://www.eegs.org/sageep-2017-program
Date and time: Wednesday 22 March 2:40 PM – 3:00 PM
Presenters: Andrés Gonzales and Torleif Dahlin, Lund University
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MSc thesis presentation by Joakim Mårdh

A TEM study, with complementary ERT, of Punata alluvial fan, Bolivia

Abstract:

The Punata alluvial fan, situated in central Bolivia (Valle Alto) between the Altiplano and the lowlands, is an important aquifer for the local rural population. Due to rapid development and population growth in the area, the demand for fresh water has increased in recent year. In combination with decreasing annual rainfall, in the already semi-arid climate, the groundwater level is steadily sinking and thus depleting shallow wells. The current solution is to drill new, deeper, wells and continue to overexploit the reservoir in an unsustainable manner. In order to map the aquifer geometry this paper presents a TEM (Transient Electromagnetic Method) survey with the aim to find the sediment – bedrock boundary, which is thought be at >300 m depth. The survey acts as a compliment to the previously conducted ERT (Electrical Resistivity Tomography) surveys in the area. Valle Alto is a tectonic basin, in the department of Cochabamba, with predominantly Palaeozoic sedimentary bedrock (Ordovician and Silurian) and minor Mesozoic formations from the late Cretaceous. The lithology varies between shales, siltstones and sandstones, deposited in a marine environment during the Palaeozoic and in a continental rift basin during the Mesozoic. Valle Alto is the result of tectonic activity during the Pliocene, which yielded an enclosed lake in the area. An unquantified amount of lacustrine clay was deposited in the basin before the lake was drained due to renewed tectonic activity. On top of the clay there is intermingling alluvial fans and colluvial deposits of different generations, with the Punata alluvial fan being one of them. In an attempt to achieve adequate depth of penetration, TEM was used in this survey. The method utilizes the fact that an electrical field always yields a proportional magnetic field and vice versa. In short, an electrical pulse is sent through transmitter loop and then abruptly turned off. This induces an electromotive force which propagate into the ground and in turn induces currents proportional to the resistivity. The currents yield a secondary magnetic field which can be measured by receiver coils, and then used to create resistivity models of the subsurface. Unfortunately, the lacustrine clay restricted the depth penetration to 90 – 200 m and the models did not reach the bedrock boundary. The survey did however present a few other interesting features such as a distinct thin layer with very low resistivity, interpreted to be brine, on top of the alluvium – clay boundary. There might also be a fault line beneath the fan, but the results of the survey are inconclusive and further studies of the tectonic regime necessary in order to verify or disregard the hypothesis.

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Article in Bolivian newspaper: Crisis de suministro de agua en Bolivia

Click the link below to read:

Crisis de suministro de agua en Bolivia

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