TEM and ERT Investigations in Challapampa Aquifer, Bolivia

Abstract

In this study, electrical resistivity tomography (ERT), induced polarization (IP) and transient electromagnetic (TEM) measurements were carried out to investigate two possible bedrock depressions as well as possible thermal water intrusion 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.

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.

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 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.

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 bedrock depression 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 the southern part, there was found no evidence of a bedrock depression.

In order to validate or reject the interpretations presented in this thesis, boreholes should be drilled and logged along one or more of the ERT lines. In order to investigate a larger part of Challapampa aquifer, airborne methods, such as for example SkyTEM, should be used.

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.

Popular Abstract

A possible source of groundwater was found when geological features were mapped near the city of Oruro in Bolivia. With measurements of the electrical resistivity in the ground the authors hope to contribute in solving the water crisis in Bolivia.

Groundwater becomes an important source of water when surficial water bodies are affected by contamination. The groundwater near the city of Oruro in Bolivia is poorly investigated and needs further research to improve the knowledge about the occurrence of groundwater in the region. The groundwater is the main drinking water resource for the city of Oruro and the present extraction rate from the aquifer is not enough to cover the needs of the inhabitants. The climate in Oruro is arid and with climate change it will probably be more arid in the future, which makes groundwater investigations in this region necessary to secure enough drinking water for the inhabitants.

When trying to find suitable locations for ground water wells, the researchers are looking for bedrock depressions containing materials like sand and gravel which are good for storing and leading ground water. In this study, researchers have tried to map the distance to the bedrock from the surface and intrusion of salty thermal water in the bedrock, using instruments to measure the distribution of the electrical resistivity in the ground. The bedrock has much higher resistivity values than the soil above the bedrock, which makes this kind of instruments well suited for geological explorations when trying to map groundwater.

In the northern part of the study area, ca 5 km northeast of Oruro city, a possible bedrock depression was indicated by low values of resistivity. This possible bedrock depression is 150 meters or more in depth. The soil above the bedrock depression contains clay, sand and gravel according to boreholes drilled in the nearby area, which could contain a lot of groundwater. This groundwater could be used to provide drinking water for the city of Oruro in the future.

In the central part of the study area a possible fracture zone with thermal water was indicated by very low resistivity values. In hot springs close to this possible fracture zone salty water is present, which could be an indication that the thermal water is leading salty water from fracture zones within the bedrock to the shallower soil. This salty water is contaminating the groundwater with salt and if the groundwater is contaminated with salty water it can´t be used as a source of drinking water. Therefore it is relevant to study this salt intrusion in more detail in the future.

The instruments used in this study cannot be used to draw conclusions of the underground on their own since one value of resistivity can correspond to many different geological materials. As an example, an electrical resistivity of 10 Ωm could correspond to for example graphite, fresh water or clay and when the geological features are of interest, the three are not interchangeable. In order to work around this uncertainty a knowledge of the local geological conditions is necessary, in order to rule out some of the geological materials which have the correct resistivity but are not present in the area. To further verify the results, boreholes should be drilled at the locations of some of the measurements to test their validity.

Test pumping from drilled wells in this area could also be used to see how much groundwater there is. However, the results concerning the resistivity distribution indicate that there could be enough groundwater in this area to cover the need of drinking water for the inhabitants in the future.

• author: Svensson, Emil and Broman, Viktor
• supervisor: Torleif Dahlin; Etzar Gómez
• organization: Engineering Geology; Civil Engineering (M.Sc.Eng.)
• alternative title: Undersökning med TEM och ERT i Challapampaakviferen, Bolivia
• course: VTG820 20171
• year: 2017
• type: H3 - Professional qualifications (4 Years - )
• subject: Technology and Engineering
• keywords: Geophysics, Resistivity, Chargeability, Sedimentary, Altiplano, Groundwater.
• publication/series: ISRN
• report number: ISRN LUTVDG/(TVTG--5152)/1-50/(2017)
• other publication id: ISRN
• language: English
• id: 8921958
• date added to LUP: 2017-08-15 10:04:55
• date last changed: 2017-08-15 11:01:37