Hydrogeology of the Chaco Tarijeño (Bolivia)

Hydrogeology of the Chaco Tarijeño (Bolivia)

Hydrogeology of the Chaco Tarijeño (Bolivia)
RONALD PASIG, HERNAN VILLENA & CHRISTIAN NEUMANN-REDLIN
ABSTRACT
RESUMEN
Between 1992 and 1998 the Chaco of the Province
Tarija in Bolivia was investigated to evaluate the
possibility to explore groundwater in this area.
Hydrogeological and geophysical studies as well as
deep borehole drilling revealed in the alluvial plain
and the Tertiary hills of the Chaco the existence of
aquifers beneath 150 m depth. The yield of the
boreholes varies between 0.5 and 2 l/s and the
water quality serves for all kinds of use. Within the
alluvial fan of the Rio Pilcomayo shallow aquifers
occur with fresh water in the paleochanels and
saline water in the paleoterraces. Yields amount to
1 to 3 l/s but the use often is limited through the
existence of saline water.
Entre 1992 y 1998 en el Chaco de la Provincia de
Tarija en Bolivia fueron realizadas investigaciones
para evaluar la posibilidad de explotación de aguas
subterráneas. Estudios hidrogeológicos y geofísicos
como también la perforación de pozos profundos
han revelado en el paisaje de llanura aluvial deposicional y de las colinas bajas terciarias la existencia
de acuíferos en profundidades mayores a 150 m.
Los caudales de los pozos varían entre 0.5 –
2.0 l/seg y la calidad de esta agua subterránea
sirve para todo tipo de uso. En el cono aluvial del
Rio Pilcomayo existen acuíferos someros con agua
fresca en los paleocanales y agua salina en las
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 Cooperation Project named Convenio AlemánBoliviano de Aguas Subterráneas (CABAS), the
Depar tment of Hydrogeology of the Ser vicio
Nacional de Geología y Minería de Bolivia (SERGEOMIN), 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 possibilities 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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Hydrogeology and Environmental Geology
Fig. 2:
Fig. 1:
Location of the Chaco Tarijeño.
the east is only sparsely populated with scattered
cattle farms, military posts (Ibibobo) and Indian
settlements (Crevaux). The Chaco Tarijeño is
crossed by a river called Rio Pilcomayo. It has its
source in the high Andes at Potosí at a height of
about 4,000 m and flows into the Rio Paraguay
near Asunción. It has a permanent water flow, the
rate of which varies considerably between max.
200 m3/sec from Januar y to April and 30 –
60 m3/sec from July to October.
Towards the west the Chaco is limited by the
Subandine Ranges, situated easterly in front of the
proper Andes, and steeply rising from the plain in
the east. They reach a height of 1,500 m and consist of Mesozoic and Tertiary sedimentary rocks.
The Chaco consists of unconsolidated sediments
transported into this plain by rivers and brooks. The
Chaco Tarijeño is divided into three units (Fig 2):
slightly undulating Ter tiar y hills with heights
between 300 and 500 m above sea level, the socalled Chaco plain at about 300 m above sea level
and an alluvial fan, in a way an inland delta, of the
Rio Pilcomayo (NEUMANN-REDLIN et al. 1992).
202
Geomorphological zones of the Chaco Tarijeño
(according to satellite interpretation and
NEUMANN-REDLIN et al. 1992).
Chaco is primarily an ecological expression
describing the typical vegetation of a dense thornbush forest in a semiarid climate. The plants adapted to the extreme evaporation by the reduction of
the transpiring surface, by succulence, by shedding
the leaves in the dry season or the possibility to
store water in the trunk.
In the area of investigation there are four climatological stations, two with records from 1962 –
1995 and two with data from 1992 – 1995.
Additionally, six pluviometrical stations exist with
records since 1977 and 17 stations have been
installed by the project in 1992. The annual precipitation of the proper Chaco Tarijeño is between 300
and 900 mm p. a. and reaches a level of 1,100 mm
in the Subandine Ranges (Fig. 3). This rainfall is limited to the wet season between October and March,
six months from April to September are more or less
dry. The average annual temperature is about 23°C
and at Villamontes the total annual potential evaporation is recorded to be 1.218 mm.
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HYDROGEOLOGY OF THE CHACO TARIJEÑO (BOLIVIA)
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 confined 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 eastern slopes of the Subandine Ranges, has an easterly 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 precipitation 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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Hydrogeology and Environmental Geology
Fig. 6:
Sample points for isotopical studies.
Fig. 5:
Map of groundwater flow.
areas, due to denser vegetation and better infiltration capacity of the top soil layer, rainfall does not
lead to surface run-off and thus direct infiltration
takes place. But the annual precipitation does not
exceed the effective field capacity of the fine
grained near surface layers. Hence, the infiltrating
rain water is almost completely used by the vegetation, and nearly no recharge towards the saturated
zone of the groundwater occurs. Due to this very
small amount of recharge the groundwater is brackish to saline. Figure 5 demonstrates the situation of
an unconfined shallow aquifer in the alluvial fan
which partly is fed by the Rio Pilcomayo.
For better understanding of the overall recharge
and hydraulic situation in the Chaco Tarijeño, water
samples of 15 boreholes were taken to analyse the
content of the environmental isotopes. Figure 6
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 topographical height the groundwater recharge
occurred and, furthermore, which climatic conditions were dominating during the time of rainfall
and infiltration.
204
As mentioned above, the groundwater at the
Tertiary hill zone and the Chaco plain is recharged
on and at the heights of the Subandine Ranges,
flowing in eastern direction (Fig. 5). Concerning the
conventional age values there can be observed an
increase in ages from west to east, for example
between sample of borehole 4 with 10,090 B.P. and
the water of borehole 11 with 25,730 B.P. At wells
1 and 13 locally direct recharge occurs with values
less than 8,000 B.P. and the water of well 2
revealed a very old water age of 38,660 B.P. The
analyses of 18O values in the Chaco Tarijeño outside the alluvial fan of the Rio Pilcomayo yielded
values between – 7.20 ‰ and – 8.09 ‰ which indicates recharge areas at approximately 1,500 m
above sea level (GEYH et al. 1996) corresponding to
the above-mentioned high sectors of the Subandine
Ranges.
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HYDROGEOLOGY OF THE CHACO TARIJEÑO (BOLIVIA)
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, diminish 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 paleochannels (cañadas) of the inland, delta groundwater is classified good to bad depending on the
purpose of use, whereas in the terraces of the alluvial 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):
Sondeos eléctricos y electromagnéticos para la
exploración de aguas subterráneas en el Chaco
Tarijeño. – Informe Técnico CABAS N° 24;
Cochabamba, Bolivia.
GEYH, M., PASIG, R., FELDHAUS, L. & ZAEPKE, M.
(1996): Estudio isotópico para la determinación
del origen y edad del agua subterránea en el
Chaco Tarijeño de Bolivia. – XII Congreso
Geológico Boliviano, 3, P. 713-724; Tarija, Bolivia.
NEUMANN-REDLIN, CHR. & VILLENA, H. (1992): Primeros
aspectos sobre la Hidrogeología del Chaco
Tarijeño. – Informe Técnico CABAS N° 1;
Cochabamba, Bolivia.
Ing. RONALD PASIG,
Proyecto SARO,
Calle Ciencias Veterinarias 215,
San Lorenzo (Paraguay)
Ing. HERNAN VILLENA,
Proyecto PRONAR,
Calle Badilla, Esc. La Madrid,
Tarija (Bolivia)
Dr. CHRISTIAN NEUMANN-REDLIN,
Diplom-Geologe,
Federal Institute for Geosciences
and Natural Resources (BGR),
Stilleweg 2,
30655 Hannover (Germany)
PASIG, R. (1998): Estudio Hidrogeológico del Chaco
Tarijeño de Bolivia. – Boletín del Ser vicio
Nacional de Geología y Minería, 15; La Paz,
Bolivia.
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