Electrical conductivity and depth of groundwater at the Pergamino zone
(Buenos Aires Province, Argentina) through vertical electrical soundings
and geostatistical analysis
Claudia Sainatoa,*, Griselda Galindob, Cristina Pomposielloc, Horacio Mallevillea,Diego de Abelleyraa, B. Losinnoa
aCatedra de Fısica, Facultad de Agronomıa, Universidad de Buenos Aires, Av. San Martın, 4453 (1417DSQ), Buenos Aires, ArgentinabFacultad de Ciencias Exactas y Naturales, UBA, Depto. De Geologıa, Ciudad Universitaria, Pab. II, Buenos Aires, Argentina
cINGEIS-CONICET, Ciudad Universitaria, Buenos Aires, Argentina
Received 1 August 2001; accepted 1 January 2003
Abstract
In the humid Pampean region of Argentina, a rich agricultural zone, the periodic occurrence of droughts of different intensity is one
of the most important factors in the variability of crop yield. Because complementary irrigation is a highly efficient resource to
increase such yields, an understanding of groundwater resources is important. This knowledge is limited in topographically smooth
zones by the absence of outcroppings and observation boreholes. Water conductivity is another limitation factor if the goal is to avoid
soil degradation by irrigation and negative effects for animal and human consumption. The aquifers of the northeastern zone of the
Buenos Aires province have been studied regionally, but information at the local scale is limited to sparse boreholes.
In this work, a survey using vertical electrical soundings was carried out to determine the depth, thickness, and continuity of
shallower aquifers. Both a mapping of the water table and the electrical conductivity distribution of free aquifers were achieved from
well data and geophysical results using geostatistical techniques. Recharge areas of the aquifer were recognized as those areas with
low conductivity and topographic highs. The discharge areas, mainly at the bed of the Pergamino River, have higher values of
conductivity; two zones north and south of the city of Pergamino have conductivities greater than 2000 mS cm21. Isolines of depth to
the fresh-salty water interface showed different values over the Pergamino River, with some local maxima at the swamp zone and
near Pergamino.
q 2003 Elsevier Science Ltd. All rights reserved.
Keywords: Electrical conductivity; Geostatistics; Groundwater; Vertical electrical soundings
Resumen
En la region de la Pampa Humeda de Argentina, una zona agrıcola muy rica, la ocurrencia de sequıas de diferente intensidad
periodicamente es una de las causas mas importantes de variabilidad en los rendimientos de los cultivos. El riego complementario es un
recurso altamente eficiente para incrementar esos rendimientos. El conocimiento de los recursos de agua subterranea es entonces un punto
importante para desarrollar esta tecnologıa, estando limitado como en otras zonas de llanura por la ausencia de afloramientos y la escasez de
perforaciones de observacion. La conductividad del agua es un factor limitante para evitar degradacion de suelos por riego y efectos
negativos en el consumo animal y humano. Los acuıferos de la zona noreste de la Provincia de Buenos Aires fueron estudiados regionalmente
pero no hay suficiente informacion a escala local, limitada a escasas perforaciones.
En este trabajo, se realizo una exploracion por medio de Sondeos Electricos Verticales (SEV) determinando profundidad, espesor y
continuidad de los acuıferos mas superficiales. Se llevo a cabo un mapeo del nivel freatico ası como de la distribucion de la conductividad
electrica del acuıfero libre a partir de datos de pozos y de los resultados geofısicos usando tecnicas geoestadısticas. Fueron reconocidas las
areas de recarga del acuıfero freatico que tienen baja conductividad electrica coincidentes con altos topograficos. Se encontro que las areas de
descarga, principalmente en el cauce del arroyo Pergamino, tienen valores mas altos de conductividad. Particularmente, dos zonas en el norte
y el sur de la ciudad de Pergamino tienen conductividades mayores que 2000 mS cm21. Las isolıneas de profundidades de la interfase agua
0895-9811/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0895-9811(03)00027-0
Journal of South American Earth Sciences 16 (2003) 177–186
www.elsevier.com/locate/jsames
* Corresponding author. Fax: þ54-11-45248099.
E-mail address: [email protected] (C. Sainato).
dulce-agua salada mostraron valores diferentes cruzando el arroyo Pergamino y algunos maximos locales en la zona de los banados y cerca
de la ciudad de Pergamino.
q 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction
The Pampean plain (Humid Pampa) situated north of
Buenos Aires, Argentina (Fig. 1), is a very rich
agriculture zone. The demand for complementary irriga-
tion has grown in this area because of the crop yield
increases produced by the application of irrigation
technology in zones where dry and humid periods alter
climatic conditions. However, exploitation of ground-
water without knowledge of its features and potentialities
risks the sustainable development of this resource.
Furthermore, there is an increasing problem of sodifica-
tion of soils due to the use of poor quality water for
irrigation (Andriulo et al., 1998). The aquifers of the
northeastern zone of the Buenos Aires province have
been studied regionally (e.g. Santa Cruz, 1987; Sala
and Rojo, 1994; Usunoff, 1994), but local-scale studies
of groundwater resources are insufficient because
the hydrogeological and geophysical data are limited
and sparse. The hydrogeology of the Buenos Aires
province is difficult to determine because its smooth
topography limits observation of outcroppings and
requires instead analysis of drilling profiles through
interpolated information. Santa Cruz (1994) and Santa
Cruz and Silva Busso (1995) have characterized a
shallow aquifer, called ‘Pampeano’, through a regional
study with vertical electrical soundings (VES). They
establish a layer of 8–25 Vm of electrical resistivity,
with a thickness of 20–140 m, associated with the
Pampeano aquifer and, at some places, with the
underlying Puelche aquifer. In most sites, a conductive
layer with resistivities between 1–6 Vm was found at
greater depth, associated with the parts of the Pampeano
and Puelche aquifers that contain water of high salinity.
However, Irigoyen (1975) shows that, in geological
sections at the Buenos Aires province, great variations
Fig. 1. Geological map of the location of the study zone in the northern part of Buenos Aires province, Argentina. From Secretarıa de Industria, Comercio y
Minerıa, Gobierno de la Pcia. De Buenos Aires, Argentina.
C. Sainato et al. / Journal of South American Earth Sciences 16 (2003) 177–186178
in the depth of the aquifers may be due to a system of
faults. Different depths to the fresh-salty water interface
were found at Pergamino at both margins of the river
(Pergamino Municipality, pers. comm.).
Therefore, it is necessary to perform local surveys to
determine the depth of the fresh-salty water interface before
drilling projects are started. It is also helpful to understand
the features of the groundwater, which is used for various
agricultural purposes and reflects the hydraulic behavior of
deeper aquifers, as noted by Sala (1975).
Eight VES were performed by Sainato et al. (1997) NW
of Pergamino (NE of Buenos Aires province) to study some
features of the aquifers at the hydrogeological basin of the
Pergamino River. Because of the lack of well data, it is
difficult to determine water properties across the whole
zone. The aim of this work is to obtain the depth and
thicknesses of the aquifers, especially the fresh-salty water
interface, by carrying out nine new VES. The areal
distribution of the water table and the electrical conductivity
of free aquifers were obtained through disposable
information, well data, geophysical results, and geostatis-
tics, taking account the spatial variability of these data.
2. Study area: geological and hydrogeological setting
The study zone is NE of Buenos Aires province,
Argentina. The quaternary sediments, mainly loessic silt,
cover the study zone. There are very few wells in the area,
and no geological cross-sections in the study zone are
available. At the Rosario basin, where the Pergamino zone
is located, the sedimentary sequence is placed over basalts
equivalent to those of Serra Geral, which is found in the
Argentine Mesopotamia. The top of the formation is deeper
at our study zone, reaching approximately 800 m depth
(Fernandez Garrasino and Urba, 1999).
Three main hydrogeological units are recognized at the
Pergamino zone: the Hipoparaniana, the Paraniana, and the
Epiparaniana. Sala (1969) and Sala et al. (1983) describe
them as follows: the tertiary sediments of the Hipoparaniana
section are above the impermeable basement. The lower
part of this section is called the ‘Red Miocene’ and is
formed by sandstones and red clays with an intercalation of
ash and gypsum of continental origin. The thickness reaches
up to 250 m, and its top is 400–500 m depth, deepening to
the southwest. The water is generally salty.
The Paraniana section is formed by marine sediments
called ‘Green Miocene’ (upper Miocene). It is formed by
gray–blue and green clays with some intercalation of sand
(Parana formation). Its thickness varies between 75–135 m.
This section contains very saline water.
The Epiparaniana section is located above the Green
Miocene and contains horizontal and vertical flows that
represent the recharge or discharge path of the deeper
aquifers. It is formed by the Puelches Formation (upper
tertiary–quaternary) and the sediments of the Pampeano
(quaternary) and post-Pampeano. The Puelches sands
constitute a semiconfined aquifer and appear as quartzi-
ferous yellowish sands of medium grain, with intercala-
tions of gravel at greater depths and silt contents at lesser
depths. Santa Cruz and Silva Busso (1995) report a
variable thickness from 10 to 25 m and a top between 50
and 100 m depth, approximately. Water quality of the
Puelches aquifer is worse to the west of the northern
Buenos Aires province (Santa Cruz and Silva Busso,
1995), where saline residual values are greater than
2 g l21. To the east, better quality conditions are indicated
by values ,500 mg l21. Salinity varies with the zone
(recharge and discharge areas). In general, it is considered
a bicarbonate sodium water type.
The Pampeano aquifier, whose thickness may vary
between 20 and 120 m (Santa Cruz and Silva Busso,
1995), contains a phreatic aquifer and some deeper,
semiconfined aquifers. It has a sequence of permeable
(with greater contents of sand) and impermeable (more
clayey) horizontally layered levels, which constitute a
multiple or multiunitarian aquifer. There are also nodules
and continuous layers of permeable calcareous, formed by
agglomerates with spherical shapes. In general, the
direction of the regional phreatic runoff at the northern
part of the Buenos Aires province is W–NW to E–SE,
with local variations at the different hydrogeological
basins. There is an autochthonous recharge (by means of
precipitation) and indirect recharge by upward flow added
to the regional horizontal component (Santa Cruz and
Silva Busso, 1995). The Pampeano aquifer shows an
increase in water salinity to the west; the dry residue is
800 mg l21 at Arrecifes (approximately 50 km SE of the
study zone) and 1000 mg l21 at Pergamino. The water
type is bicarbonate sodium. The salinity of this section
increases at the flooding plains and toward the beds
(discharge zones) with values of conductivity s
.1000 mS cm21 N and S of Pergamino.
3. Geophysical prospecting
Nine VES, which together with previous soundings of
Sainato et al. (1997) constitute a set of 17 VES (Fig. 2), were
carried out using the Schlumberger array. For these
soundings, distances (AB) of up to 1000 m at some sites
and 2000 m at others were reached. The use of the whole set
of soundings enabled a better description of the features of
the aquifers.
Models of electrical resistivity of the earth varying with
depth (horizontally layered, medium, one-dimensional
(1D)) were proposed for each site. The responses of
apparent resistivity fit the experimental data in a least
square sense. An inversion 1D modeling code (Cooper,
1992) was used. Curves of experimental apparent resistivity
as a function of the semidistance between current electrodes
C. Sainato et al. / Journal of South American Earth Sciences 16 (2003) 177–186 179
(AB/2), together with the model fit, are shown in Fig. 3 for
the nine new sites.
There are no wells with geologic logs in the study
area, except at the city or towns, where two (M. Ocampo
and M. Alfonzo) are located a few km from sites VES17
and VES3, respectively. Their lithological descriptions
(Fig. 4) were compared with VES models. The
relationship between electrical layers and lithology is
mainly based on clay sediments being less resistive than
sands. Well data from Fig. 4 do not provide great
contrast in the type of sediments. The presence of a
calcareous crust may increase the resistivity of the layer
that includes it, but it is likely that contrasts in
resistivities are mainly due to water resistivity.
Electrical sections were constructed with 1D models at
each site (Fig. 2) in two profiles, AA0 and BB0, across
the river bed. The electrical sections with resistivities and
associated lithology are shown in Fig. 4. Well data at
M. Alfonzo and M. Ocampo were used for the
interpretation of profiles AA0 and BB0, respectively.
4. VES results
The first layer in the profiles, with electrical resistivities
between 7 and 33 Vm, corresponds to the unsaturated zone
with loessic silt, and the first change in resistivity indicates
the top of the Pampeano aquifer, where, at its upper part, the
water table aquifer is located.
In profile AA0, the groundwater level is at depths ,5 m.
Beneath the water table, changes in electrical resistivity are
influenced by lithology and water quality. From the
soundings, two layers may be recognized with resistivities
between 4 and 38 Vm, with the lower more resistive. The
upper layer is considered clayey silt with calcareous (as
indicated by different layers of the M. Alfonzo well), and
the lower has greater values of resistivities (15–38 Vm)
with more sand content. The presence of calcareous
intercalations (Sala and Rojo, 1994) as nodules or
continuous plates may increase the resistivity of the layers.
A lens of higher resistivity appears beneath SEV12, perhaps
with a great content of calcareous crust, but the poor fitting
Fig. 2. Study zone showing sites of VES.
C. Sainato et al. / Journal of South American Earth Sciences 16 (2003) 177–186180
of this site may indicate lateral inhomogeneities not taken
into account in a 1D model. Below, a more conductive layer
appears (r , 5 Vm). The sounding at the left margin of the
Pergamino River (VES6) shows a greater depth of the
conductive layer than at VES12 and VES3 at the right
margin. The very conductive layer is at 28 m depth to the
SW and 60 m depth at VES6. Very low resistivity values
(,5 Vm) may be attributed to the deterioration of water
quality because the lower Puelche aquifier is composed of
sand. However, because there are no well data for this depth,
we cannot confirm that this layer corresponds to the Puelche
aquifer—which at areas in the western part of Buenos Aires
province was known to have high salinity water values—or
if it also includes the underlying Green Miocene Formation.
The top of this layer deepens toward VES6.
In profile BB0, the groundwater level is at ,5 m depth.
Below, there is a lower resistivity layer
(3 Vm , r , 41 Vm; Pampeano aquifer with silty–clayey
or clayey–silty sediments from M. Ocampo well) with
intercalations of layers of higher resistivity beneath the bed
of the river. This irregular sequence may be influenced by the
river’s dragging material. At VES13, the greater data misfit
may indicate the presence of lateral inhomogeneities. Below,
a silty layer with intercalations of sands and calcareous (M.
Ocampo well) is more resistive. Beneath that, the conductive
layer, which corresponds to a deterioration of water quality,
may be observed to correspond to salty strata of the lower
Puelche or Green Miocene Formation. Its top lies between 30
and 60 m depth, deepening across the river in the NE.
Different depths to the top of the conductive layer were
found. Sites VES9 and VES6 are separated by the Botija
River, a tributary of the Pergamino Stream. At 25–30 m
depth, electrical resistivity is higher at VES9 (113 Vm) than
at VES6 (15 Vm). The top of the conductive layer is at 60 m
(VES6) or 40 m (VES9) depth. In profile BB0, the top of the
salty aquifers deepen to the left margin of the Pergamino
Fig. 3. Fit of 1D model response to experimental apparent resistivity curves from VES.
C. Sainato et al. / Journal of South American Earth Sciences 16 (2003) 177–186 181
Fig. 4. Profiles AA0 and BB0 with geoelectrical sections and the associated lithology of two wells, M. Alfonzo and M. Ocampo.
C. Sainato et al. / Journal of South American Earth Sciences 16 (2003) 177–186182
River (NE). These features may be evidence of structural
controls. Cross-sections at different places in the province
show fractures that caused stratigraphic throw in the Red
and Green Miocene Formations (Irigoyen, 1975). Reactiva-
tion of some faults affected the shallower and more recent
formations. Wells at Pergamino, located at both margins of
the river, also show different depths of the fresh-salty water
interface (Pergamino Municipality, pers. comm.).
4.1. Geostatistical analysis of depth and electrical
conductivity of aquifers
To study the spatial distribution of the groundwater
properties at the Pergamino basin, a geostatistical analysis
was carried out to map such properties in the study area. The
geostatistical interpolation, called kriging (Trangmar et al.,
1985; Webster, 1985), is a common tool to achieve this
purpose and estimates the unknown values inside a grid by
taking into account the spatial correlation of known values. It
thereby provides information when no well data are available.
The variogram (Trangmar et al., 1985) measures the
average dissimilarity between data values of the studied
property as a function of the distance between them.
Through modeling the variograms, the range, the maximum
distance of influence of any data point, is obtained.
In the process of kriging, the value at each point of the
grid is estimated as a linear combination of neighboring
points, where the weights depend on the values of
variogram, thus providing unbiased estimates of minimum
variance (Webster, 1985).
4.2. Mapping of water table and depth to the fresh-salty
water interface
The depth of the phreatic level obtained from the VES
and some well data (INTA Pergamino, pers. comm.) was
Fig. 5. Water table contours showing direction and sense of groundwater flow and water parting, together with VES and well sites.
C. Sainato et al. / Journal of South American Earth Sciences 16 (2003) 177–186 183
used to carry out a geostatistical kriging interpolation of the
water table. The isolines of depth to the water table are
shown in Fig. 5, together with well and VES sites.
Groundwater parting and the direction and sense of
groundwater flow are shown and mainly orient toward the
bed of the Pergamino River, with some SE components
coincident with regional flow.
Fig. 6 shows the isolines of depth to the fresh-salty water
interface as provided by kriging. Depths increase below
60 m at the SE zone near Pergamino, surrounding B0
(extreme profile BB0), and the swamp zone on the right
margin of the rivulet. In general, east of 608500W, the top of
the conductive layer deepens toward the NE and raises again
near the Botija River. To the west, the situation reverses,
and the depth of the interface increases to the SW.
4.3. Mapping of the groundwater electrical conductivity
To map water electrical conductivity of the phreatic
aquifer, a cokriging interpolation was carried out that made
use of all the disposable information (VES and wells) and
thus provided a map of higher precision. Cokriging
interpolation allows mapping of one property (primary)
and uses a secondary property spatially correlated with the
first. The value at each point in the grid is a linear
combination of neighboring primary and secondary data. It
makes use of the cross-correlation between the two
variables (Webster, 1985).
In the case of water conductivity, this correlation is based
on the relationship between the bulk electrical conductivity
of the aquifer, sbulk; and the conductivity of water filling the
pores, sw: Using Archie’s law (McNeill, 1990) in the
presence of clay, as is the case for this lithology, this
relationship is as follows:
sbulk ¼ fmsw þ sclay; ð1Þ
where f is the porosity, and m is a factor that depends on
grain shape.
A geostatistical cross-correlation was evaluated between
these two variables because the bulk conductivity obtained
from VES models and water conductivity from samples are
located at different sites. The cross-plot of sbulk versus sw is
estimated for within a search distance; that is, the range for
the bulk conductivity is used as the maximum distance at
which a water sample is associated (Geostat, 1996).
The lineal fitting of cross-plot data (Fig. 7(a)) is
evaluated through the normalized correlation between the
two variables:
s ¼Covsbsw
½Covsw·Covsb�; ð2Þ
as estimated with Geostat (1996) program.
The resulting fit shows a slope of 0.207 and a sclay value
of 550 mS cm21. The m value, depending on the shape of
particles, was taken as an average of 1.5 between possible
values (McNeill, 1990). This results in a porosity f of 35%,
which is in agreement with general values known for this
type of lithology (silty/clayey) (McNeill, 1990).
Water conductivity obtained from wells ðswÞ (primary
property), together with the bulk conductivity values ðsbÞ
Fig. 6. Isolines of depth, in meters, to the fresh-salty water interface.
C. Sainato et al. / Journal of South American Earth Sciences 16 (2003) 177–186184
of the phreatic layer from VES (secondary property), was
used to interpolate sw by cokriging, taking into account
the spatial correlation estimated between the two
variables (fit of Eq. (1)). The mapping of the conduc-
tivity of the phreatic aquifer is shown in Fig. 7(b) These
results show good quality salinity zones ðs , 1000 mS �
cm21Þ to the north near M. Ocampo and to the south at
M. Alfonzo. They coincide with recharge areas or
transition zones with greater slopes of the hydraulic
gradient (Fig. 5). Good quality zones are characterized to
the south by a soil texture that enables the quick
circulation of infiltrated water (Argiudoll soils) and a
moderate hydraulic gradient. Higher conductivity zones
appear near the bed of the Pergamino River, a discharge
area. There are also two zones of highest conductivity.
At the northwestern topographic depression of the swamp
zone, conductivity is s . 1800 mS cm21, and the
hydraulic gradient is rather gentle. To the southeast,
conductivity is s . 2000 mS cm21 close to the north and
south of Pergamino, which coincides with transition areas
that contain a component of groundwater flow to the SE.
Surrounding VES6 and VES7, there is a slight slope of
the water table.
Less permeable soils (Natracuolls) with deficient drai-
nage found along the river (discharge zone) and the low
hydraulic gradients at different places in the study area
Fig. 7. (a) Cross-correlation of bulk electrical resistivity ðsbÞ and water conductivity ðswÞ for the phreatic aquifer. Regression line (correlation coefficient 0.8).
(b) Mapping of water conductivity ðswÞ for phreatic aquifer using cokriging between ðsbÞ obtained from VES and sw from well data.
C. Sainato et al. / Journal of South American Earth Sciences 16 (2003) 177–186 185
cause a greater time of residence, and water moves more
slowly with a larger time of contact with the subsoil
material, thereby dissolving salts.
5. Conclusions
VES carried out at the study zone have improved the
understanding of groundwater resources. The phreatic
aquifer flow has a recharge that coincides with topographic
high zones, and the discharge is mainly toward the
Pergamino River. The analysis of water electrical conduc-
tivity distribution over the area was performed by
geostatistical cokriging, which enabled the use of not only
well data but also geophysical results. This methodology is
suitable when well data are scarce, as is the case in several
areas of Argentina. Groundwater electrical conductivity is
greater than 1800 mS cm21 at discharge zones where the
soils have a fine texture and at places where the hydraulic
gradient is very small, which increases the water time of
residence. Water of better quality may be found at the
recharge areas or transition zones with a moderate hydraulic
gradient.
The fresh-salty water interface represents a limitation of
the available water for irrigation. Different depths of this
interface were found at two profiles across the river. Water
conductivity greatly increases below 28 m on the right
margin of the Pergamino River and at 60 m depth at the left
margin, which means the top of this conductive layer
deepens to the NE. However, the kriging interpolation of
this interface, using all VES data, confirmed this behavior at
the central part of the stream, and two maximum depths
were also found near the swamp zone and at Pergamino.
Acknowledgements
This work was financially supported by the University of
Buenos Aires. The authors thank Amalia Gonzalez for
helping with the drawings and Daniel de Oto and Martın
Nothardt for their collaboration in the field.
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