Water resources are essential for the development of cities, industrial, mining and
agriculture activities in arid Northern Chile. Groundwater is in general the main
component of water resources in this region. An optimum use of groundwater resources
requires a comprehensive evaluation of the water balance of the aquifer in order to
extract water at a rate that will not diminish the groundwater resources. One of the
key components of the water balance is the evaluation of recharge rate in the recharge
areas. The other component is the residence time of the groundwater that has implications
for groundwater extraction and groundwater protection. Environmental isotopes have been
an integral part of the approach used in water resources studies in northern Chile.
One good case study that can be use to illustrate the application of environmental isotopes
in northern Chile is the work done at the Pampa del Tamarugal basin. This region is the
northern extension of the Atacama Desert, one of the most arid regions of the world.
Annual precipitation is nil at lower elevation and reaches about 200 mm/yr above an altitude
of 3500 m.a.s.l. The Pampa del Tamarugal is a north-south plain at 1000-12000 m.a.s.l, located
between the Cordillera de la Costa an the foot of the Cordillera de los Andes. This plain is a
closed terminal basin formerly of a Tertiary-aged drainage system that had the headwater in the
Cordillera de los Andes. The aquifers are within a sedimentary sequence that was deposited during
the Tertiary and Quaternary and has a thickness of up to 3000 m. The Quaternary sediments have a
maximum thickness of 500 m and include a variety of alluvial, fluvial and lacustrine deposits. The
upper 250 m are being taped for groundwater exploitation.
One key aspect that was evaluated using environmental isotopes was the origin of the groundwater.
This is related to the location of the recharge areas. It was important to understand if the aquifer
was a multiaquifer system associated to different recharge areas. The environmental isotopes used in
these studies were oxygen-18 and deuterium, stable isotopes of the oxygen and hydrogen, respectively.
These isotopes are part of the water molecule then there are ideal tracers to study water behavior
within the water cycle. The isotope data is reported in delta units that is defined as (Rsm -Rst)/Rst * 1000,
where Rsm and Rst are the isotope ratio (18O/16O, 2H/1H) of the sample and standard, respectively. The use
of oxygen-18 and deuterium relies in that the isotopic composition of the groundwater is a reflection of
the average weighted-mean isotopic composition of the precipitation in recharge areas. It is well known that
the precipitation get depleted in the heavy isotope (O-18, H-2) as the air masses move from the coast into
the interior of the continent (continental effect). The same relationship is observed when the air masses
move toward high altitude (altitude effect). Then, coastal precipitation are much more enriched isotopically
than rains inside the continent and in the mountains. This isotopic fingerprint is transmitted to the groundwater
in recharge areas. It is also important to highlight that the isotopic composition of the groundwater is
conservative along the groundwater flow system.
In case of northern Chile, as part of the studies, it was established for the precipitation the existence
of an isotopic gradient with altitude. It means that rains at 2,700 m.a.s.l are more enriched isotopically
than rains above 4,000 m.a.s.l. This relationship was confirmed in springs collected at different altitudes.
Then, the next part of the study was to test if this relationship was preserved in the groundwater. Four
main areas in the Pampa del Tamarugal basin, northern, central, south and the Pica region were selected
for groundwater sampling. The Pica
area is characterized by the discharge of hot springs, that provide water for the community and for
agricultural activities. The isotope data for the groundwater show a wide range in delta values that
vary between -5 and -13 0 for O-18 and -35 and -105 0 for H-2. The more enriched delta values
were observed in the north and the most depleted delta values were observed in the Pica area. The high
temperature of the Pica groundwater is associated to the existence of a deep groundwater flow system linked
to the high part of the basin. This wide isotopic range has to be associated to precipitation in the recharge
areas. The isotopic pattern of the groundwater, precipitation and springs at different altitudes clearly
indicates the existence of a multiaquifer system in the Pampa del Tamarugal. The aquifers have recharge
areas located at different altitudes.
These findings provide information to understand the relationship between modern precipitation and
groundwater recharge. For example, the existence of groundwater in the range of -5 to -70 for delta 18O,
indicates that these waters were recharged at altitudes no higher than 2700 m.a.s.l. The precipitation today
at these altitudes is irrelevant, implying that these groundwater were recharged during wetter periods in
the past characterized by higher precipitation at lower altitudes. Evaluation of the groundwater residence
time using C-14, a radioactive isotope of the carbon, and hydrological data indicated that a large portion
of the groundwater in the Pampa aquifers should be considered as a non-renewable water resources. These
findings provide by the use of environmental isotopes have significant implications for water resources
management in Northern Chile.
The use of environmental isotopes is widespread in studies of water resources in Latin America.
The environmental isotope laboratory (EIL) of the University of Waterloo, Canada, has provided
the analytical support for several of these studies.
This laboratory has the state of art facilities for analysis of environmental isotopes in
water, and inorganic and organic compounds.
