Hydrogeology of the Chaco Tarijeño (Bolivia)

RONALD PASIG, HERNAN VILLENA & CHRISTIAN NEUMANN-REDLIN

ABSTRACT RESUMEN

Between 1992 and 1998 the Chaco of the Province Entre 1992 y 1998 en el Chaco de la Provincia de Tarija in Bolivia was investigated to evaluate the Tarija en Bolivia fueron realizadas investigaciones possibility to explore groundwater in this area. para evaluar la posibilidad de explotación de aguas Hydrogeological and geophysical studies as well as subterráneas. Estudios hidrogeológicos y geofísicos deep borehole drilling revealed in the alluvial plain como también la perforación de pozos profundos and the Tertiary hills of the Chaco the existence of han revelado en el paisaje de llanura aluvial deposi- aquifers beneath 150 m depth. The yield of the cional y de las colinas bajas terciarias la existencia boreholes varies between 0.5 and 2 l/s and the de acuíferos en profundidades mayores a 150 m. water quality serves for all kinds of use. Within the Los caudales de los pozos varían entre 0.5 – alluvial fan of the Rio Pilcomayo shallow aquifers 2.0 l/seg y la calidad de esta agua subterránea occur with fresh water in the paleochanels and sirve para todo tipo de uso. En el cono aluvial del saline water in the paleoterraces. Yields amount to Rio Pilcomayo existen acuíferos someros con agua 1 to 3 l/s but the use often is limited through the fresca en los paleocanales y agua salina en las existence of saline water. paleoterrazas. Los caudales suman a 1 a 3 l/seg, pero el uso es limitado por la existencia de aguas salinas.

During 1992 –1998 within a Technical Co- operation Project named Convenio Alemán- Boliviano de Aguas Subterráneas (CABAS), the Department of Hydrogeology of the Servicio Nacional de Geología y Minería de Bolivia (SER- GEOMIN), the Prefecture of the Bolivian Tarija Department and the German Federal Institute for Geosciences and Natural Resources (BGR) carried out studies with the purpose to evaluate the possi- bilities of exploitation and use of groundwater in the Chaco Tarijeño. In 1998 the project finalised with the publication of a Hydrogeological Map of the Chaco Tarijeño at scale 1 : 250,000 together with a comprehensive explanatory note (PASIG 1998).

The geographical location of the Chaco Tarijeño is shown in Figure 1. The extension of the project area is approximately 120 120 km and limited in the north by the province of Chuquisaca, in the east and south by the state borders of Paraguay and Argentina. About 30,000 people are living in this region settling mostly in the western part along the railway line and road which pass the places of Villamontes and Yacuiba. The neighboring Chaco in

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Fig. 2: Geomorphological zones of the Chaco Tarijeño Fig. 1: (according to satellite interpretation and Location of the Chaco Tarijeño. NEUMANN-REDLIN et al. 1992).

the east is only sparsely populated with scattered Chaco is primarily an ecological expression cattle farms, military posts (Ibibobo) and Indian describing the typical vegetation of a dense thorn- settlements (Crevaux). The Chaco Tarijeño is bush forest in a semiarid climate. The plants adapt- crossed by a river called Rio Pilcomayo. It has its ed to the extreme evaporation by the reduction of source in the high Andes at Potosí at a height of the transpiring surface, by succulence, by shedding about 4,000 m and flows into the Rio Paraguay the leaves in the dry season or the possibility to near Asunción. It has a permanent water flow, the store water in the trunk. rate of which varies considerably between max. 200 m3/sec from January to April and 30 – In the area of investigation there are four clima- 60 m3/sec from July to October. tological stations, two with records from 1962 – 1995 and two with data from 1992 – 1995. Towards the west the Chaco is limited by the Additionally, six pluviometrical stations exist with Subandine Ranges, situated easterly in front of the records since 1977 and 17 stations have been proper Andes, and steeply rising from the plain in installed by the project in 1992. The annual precip- the east. They reach a height of 1,500 m and con- itation of the proper Chaco Tarijeño is between 300 sist of Mesozoic and Tertiary sedimentary rocks. and 900 mm p. a. and reaches a level of 1,100 mm The Chaco consists of unconsolidated sediments in the Subandine Ranges (Fig. 3). This rainfall is lim- transported into this plain by rivers and brooks. The ited to the wet season between October and March, Chaco Tarijeño is divided into three units (Fig 2): six months from April to September are more or less slightly undulating Tertiary hills with heights dry. The average annual temperature is about 23°C between 300 and 500 m above sea level, the so- and at Villamontes the total annual potential evap- called Chaco plain at about 300 m above sea level oration is recorded to be 1.218 mm. and an alluvial fan, in a way an inland delta, of the Rio Pilcomayo (NEUMANN-REDLIN et al. 1992).

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Fig. 3: Distribution of precipitation.

After a profound reconnaissance of the dug wells and boreholes (194 in total) which exist in the Chaco Tarijeño geoelectrical resistivity soundings were carried out to identify subsurface hydraulic parameters and the distribution of fresh and saline water in the underground (FIELITZ et al. 1994). In the following, the project team drilled deep wells to explore the groundwater situation. They indicate that the groundwater level in the Tertiary hill zone and in the Chaco plain is deep and the usable aquifers only can be reached at a depth of more than 150 m (Fig. 4). The deep groundwater is con- fined and the content of total dissolved solids (TDS) of the water is normally less than 1,500 mg/l. The map of groundwater flow (Fig. 5) indicates that the groundwater beyond the alluvial fan of the Rio Pilcomayo, recharged at the heights and the east- ern slopes of the Subandine Ranges, has an east- erly flow direction with at first a high and, later on, a diminished hydraulic gradient. In this part of the Chaco the Rio Pilcomayo serves as collector for the groundwater.

There is a completely different situation in the alluvial fan of the Rio Pilcomayo which is divided into the so-called paleochannels (cañadas), former river arms of the Rio Pilcomayo, and higher terraces lying between (Fig. 2). The cañadas are formed by argillaceous soils and near-surface sediments. Therefore, vegetation is scarce and after heavy pre- cipitation rain water flows superfically towards small intermittent water bearing grooves. These brooklets cut into the clayey superficial layers until fine sands are deposited below rainwater is able to infiltrate. As a result, indirect groundwater recharge takes place leading to shallow aquifers of fresh groundwater. The cañadas are surrounded by the paleoterraces where the soil and the near surface sediments consist of silt and fine sands. In these

Fig. 4: Lithologic profile of a deep well (No. 139 on Fig. 5) drilled in the Tertiary hill zone.

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Fig. 6: Sample points for isotopical studies. Fig. 5: Map of groundwater flow.

areas, due to denser vegetation and better infiltra- As mentioned above, the groundwater at the tion capacity of the top soil layer, rainfall does not Ter tiary hill zone and the Chaco plain is recharged lead to surface run-off and thus direct infiltration on and at the heights of the Subandine Ranges, takes place. But the annual precipitation does not flowing in eastern direction (Fig. 5). Concerning the exceed the effective field capacity of the fine conventional age values there can be observed an grained near surface layers. Hence, the infiltrating increase in ages from west to east, for example rain water is almost completely used by the vegeta- between sample of borehole 4 with 10,090 B.P. and tion, and nearly no recharge towards the saturated the water of borehole 11 with 25,730 B.P. At wells zone of the groundwater occurs. Due to this very 1 and 13 locally direct recharge occurs with values small amount of recharge the groundwater is brack- less than 8,000 B.P. and the water of well 2 ish to saline. Figure 5 demonstrates the situation of revealed a very old water age of 38,660 B.P. The an unconfined shallow aquifer in the alluvial fan analyses of 18O values in the Chaco Tarijeño out- which partly is fed by the Rio Pilcomayo. side the alluvial fan of the Rio Pilcomayo yielded values between – 7.20 ‰ and – 8.09 ‰ which indi- For better understanding of the overall recharge cates recharge areas at approximately 1,500 m and hydraulic situation in the Chaco Tarijeño, water above sea level (GEYH et al. 1996) corresponding to samples of 15 boreholes were taken to analyse the the above-mentioned high sectors of the Subandine content of the environmental isotopes. Figure 6 Ranges. shows the conventional 14C ages of the analysed water in years before present (B.P.) as well as the 18O values in ‰. The latter indicates in which top- ographical height the groundwater recharge occurred and, furthermore, which climatic condi- tions were dominating during the time of rainfall and infiltration.

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Fig. 7: Recommended yields of production wells.

In the alluvial fan of the Rio Pilcomayo exists another situation. The fresh shallow groundwater in the cañadas is relatively young and vary in ages between 700 in borehole sample Nr. 12 and 3870 years B.P. in the water of well 15. The mineralized water in the terraces, however, show ages between 14,465 (sample 10) and 19,940 years B.P. (No. 14). 18O values of the groundwater in the alluvial fan are low and with one exception at borehole 12 (– 7.57 ‰) less than – 8.22 ‰. This indicates that a certain part of the groundwater within the inland delta originates from the Rio Pilcomayo of which the head water are descending from areas with heights of more than 3,000 m (see above).

What is the practical use of these investigations for the farmer living in the Chaco, the new settlers or the planning authorities, respectively? Figure 7 shows the recommended yields of boreholes. They amount to 3 – 5 l/sec in a valley filled with coarse sediments situated in the north of Yacuiba, dimin- ish in the Chaco plains and reach only 0,5 – 1 l/sec in the sediments of the Tertiary hill zone where silt and clay sediments prevail.

Figure 8 gives a general view of the quality of the water and its aptitude for various kinds of use. The map shows that in the Chaco Tarijeño beyond the alluvial fan, the groundwater can generally be used for all kinds of purposes as the total salt content exceeds only exceptionally 1,500 mg/l. In the pa- leochannels (cañadas) of the inland, delta ground- water is classified good to bad depending on the purpose of use, whereas in the terraces of the allu- vial fan and in an area in the south of Crevaux saline groundwater has been identified.

Fig. 8: Aptitude of the groundwater quality for various purposes.

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References

FIELITZ, K., SORUCO, W. & INSINGER, J. (1994): Ing. RONALD PASIG, Sondeos eléctricos y electromagnéticos para la Proyecto SARO, exploración de aguas subterráneas en el Chaco Calle Ciencias Veterinarias 215, Tarijeño. – Informe Técnico CABAS N° 24; San Lorenzo (Paraguay) Cochabamba, Bolivia. Ing. HERNAN VILLENA, GEYH, M., PASIG, R., FELDHAUS, L. & ZAEPKE, M. Proyecto PRONAR, (1996): Estudio isotópico para la determinación Calle Badilla, Esc. La Madrid, del origen y edad del agua subterránea en el Tarija (Bolivia) Chaco Tarijeño de Bolivia. – XII Congreso Geológico Boliviano, 3, P. 713-724; Tarija, Bolivia. Dr. CHRISTIAN NEUMANN-REDLIN, Diplom-Geologe, NEUMANN-REDLIN, CHR. & VILLENA, H. (1992): Primeros Federal Institute for Geosciences aspectos sobre la Hidrogeología del Chaco and Natural Resources (BGR), Tarijeño. – Informe Técnico CABAS N° 1; Stilleweg 2, Cochabamba, Bolivia. 30655 Hannover (Germany)

PASIG, R. (1998): Estudio Hidrogeológico del Chaco Tarijeño de Bolivia. – Boletín del Servicio Nacional de Geología y Minería, 15; La Paz, Bolivia.

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