Spatial chemical variability of surface water in the Nhecolândia, Pantanal (MS), Brazil.
Elisângela Rosemeri Curti Martins1, Sônia Maria Furian Dias2, Laurent Barbiero3, Tatiana Mascari
Parizzoto4, Ary Tavares Rezende Filho5.
Doutoranda do Depto. Geografia Física/FFLCH/USP, Bolsista CNPq, elismartiss@yahoo.com.br
2
Profª Drª Depto Geografia Física/FFLCH/USP, furian@usp.br
3
Prof. Dr. IRD-França/CENA/USP (Lab. de Ecologia Ambiental e Geoprocessamento), barbiero@lmtg.obs-mip.fr
4
Mestranda do Depto. Geografia Física/FFLCH/USP – Bolsista Fapesp - tatiparizotto@hotmail.com
5
Prof. MSc. Depto de Geografia UFMS – CPNA/Campus Nova Andradina/MS, ary.rezende@gmail.com
This research was supported by FAPESP N° 2008/09086-7 and 2009/53524-1
Summary
The upper Paraguay River basin belongs to the second biggest hydrographic system of South
America, i.e. the Platino System. It shows a huge alluvial plain, called “Pantanal of Mato-Grosso”,
which is considered the largest tropical wetland of the planet. The Nhecolândia, a sub region of the
Pantanal located south of the Taquari River, presents an extremely complex sub-surface draining
system. Its landscape consists of thousands of saline or freshwater lakes and ponds, co-existing in close
proximity, and is crossed by draining fields active during the floods of the wet season. The saline lakes
are usually isolated from the draining fields system by forested sandy banks, locally called
‘cordilheiras’, and therefore without apparent connection with the flooding waters. Most authors
considered the presence of salt in these lakes as a heritage of a former accumulation process during the
arid phases of the Pleistocene. However, recent hydrological studies carried out on these lakes suggest
that the salinity results from present day evaporation process of the regional waters. Very few works
were dedicated to the spatial distribution of salinity, whose organization, at regional scale, could help to
understand the origins of salinity and the processes that control it as well.
The objective of this paper is twofold: first it aims at analyzing, at a regional scales, the spatial
distribution of surface waters salinity through the study of two variables the electrical conductivity
(EC) and the pH. Second, at a local scale, it aims to describe the organization of the soil cover through
non destructive electromagnetic inductions methods (Apparent Electrical Conductivity, ECa) in order
to better understand the dynamic of the water.
The regional study of EC and pH of surface waters revealed four fundamental points: 1. the
existence of very wide conductivity and pH range; 2. a correlation between pH and EC of water; 3. a
low range (<200m) for the spatial distribution of these variables; 4. a bimodal distribution of the
variables pH and EC along their range. To this information provided by the study at a regional scale,
we must add a standard distribution of ECa values around the lakes reflecting a standard and singular
morphology in the soil cover and showing that the lakes are surrounded by sub-surface rises of soil
horizons with low permeability, working as a threshold and governing the flow of water.
At the beginning of the wet season, accumulation of rainfall in the Pantanal causes a rise in the
water level. When it exceeds the morphological threshold, it flows down toward the depressions of the
lakes (Fig. X). The average quantity of water Q flowing annually towards the lakes depends on the
relative level of the threshold with respect to the regional fresh watertable. Q is decreasing with
increasing high of the threshold. Two cases may occur: if Q+P<ETP, then the lake salinity increases
every year. Otherwise, if Q+P>ETP, then the lake is flushed every year and the salinity is maintained at
a low level.
The above-described hydrological regime explains how lakes subjected to the same amount of
evaporation and rainfall can have different dissolved ion concentrations. Because it is a cumulative
process, it explains the large range of salinity observed in the region. It also explains the bimodal
distribution of the electrical conductivity observed on a large number of about 80 lakes. Because a
single type of water, evolving in an alkaline pathway, is subjected to concentration and dilution, it
explains the good relationship observed between pH and EC. The very local control of the salinity is in
agreement with the low spatial range for pH and EC (<200m) observed with the geostatistical
treatement.
The complementary data acquired at different scales, with appropriate statistical and
geostatistical treatments, allow to generalize an hydrochemical functioning described on only one lake
in the region, to the entire Nhecolândia.
Keywords: drains, lakes, space distribution, salinity, electrical conductivity, pH.
Introduction
The common feature of wetlands is the presence of physical, chemical, geochemical and
biological characteristics reflecting the type of flooding or moisture that they are submitted, including
the presence of redoxymorphic soils and hydrophilic vegetation. These landscapes fall into the category
of ‘interface systems’ or ‘intermediate’ as they are distinct from both the terrestrial and aquatic
ecosystems (Fustec & Lefeuvre 2000; Barbiero et al. 2002; 2007; 2008; Reddy & Delaune 2008). The
management of water resources in these areas is complex and requires studies to support geotechnical
appropriate procedures. Therefore, wetlands have attracted a great international scientific interest.
Given these specificities of wetlands, and the number of factors that affect the water
chemistry, it is difficult to assess which factor is predominant in the variability of these environments.
Studies conducted in the Pantanal, in particular in the sub-region of the Nhecolândia showed that the
main axis of variability is the water salinity. The salinity of the many ponds and lakes has always been
attributed to former salt accumulations inherited from dry periods during the Pleistocene (Ab'Saber,
1988; Klammer, 1982; Tricart, 1982), but recent studies have shown that it is the result of current
processes of water concentration by evaporation (Barbiero et al., 2002). Both the saline and freshwater
lakes have different water regimes, since they are largely controlled by the surrounding soil
organization at different stages in a fast dynamic of formation and destruction (Rezende Filho, 2006 ,
Barbiero et al., 2008).
The objective of this study is to analyze both the spatial distribution of salinity of surface
waters at a regional scale and the organization of soil system at a local scale, in order to determine
whether they present a regular distribution patterns in the landscape of the Nhecolândia.
Material and methods
The study area is located in the Upper Paraguay Basin, western-central Brazil. From a
morphological point of view, the basin consists of three macro-physiographic features: plateaus,
depressions, and the plain of the pantanal, with temporary or permanently flooded area (Alvarenga et
al., 1984; ANA, 2004; Ab'Saber, 2006). The Nhecolândia, one of the largest sub-region of the Pantanal,
corresponds to the southern part of the Taquari alluvial fan (Figure 1). It is delimited in the north by the
river Taquari, in the south by the river Negro, in the west by a portion of the Paraguay River, and in the
east by the escarpment of Maracaju plateau. A peculiarity of the Nhecolândia is the presence of about
12000 lakes and ponds. Saline and non-saline waterbodies coexist in the landscape sometimes at a short
distance from each other (200 m). They are usually separated by forested ridges called ‘cordilheiras’.
The lanscapes is crossed by ‘vazantes’ acting as draining fields during the floods. These ponds and
lakes can be permanent or temporary, depending on the frequency and intensity of the floods.
Figure 1. Location of the Nhecolândia region in the Pantanal wetland. The grey circle denotes the
studied area, north of the Negro River.
Two variables, the electrical conductivity (EC) and pH were analyzed in a set of 80 lakes and
ponds, to understand the spatial distribution of salinity at a regional scale. Measurements were
performed directly in the field using of a conductivity meter (HI 9838), and a pH meter (HI 95143).
The points of measurement were georeferenced in UTM coordinates (Universal Transverse Mercator).
The organization of the soil cover has been studied around 4 lakes selected to cover a wide
range of electrical conductivity of the waters. Direct observations (auger holes and excavated pits) were
associated with geophysical surveys by low frequency electromagnetic induction. Two devices were
used EM38-MK2 and EM31-MK2 (Geonics Ltd., Ontario, Canada). Both devices have a different
spacing between the transmitter and receiver coils (1m for the EM38 and 3.66m for the EM31) and a
different induction frequency (14.6kHz for the EM38 and 9.8kHz for the EM31). This gives to each
device a different depth of investigation that is about 2 m for the EM38 and 6 m for the EM31, when
used in vertical position. A maximum spacing of 10 m was maintained between two successive
measurement points.
Statistical and geostatistical treatments were performed on the raw data (EC, pH and ECa)
including, an analysis of the distribution of the variables and of the experimental variogram, and an
adjustment of a theoretical model by the least squares method, then kriging and mapping.
Results
The EC values range from 42 to 24350 µS/cm and pH from 5.9 to 9.95. These large ranges of
pH and EC reflect highly contrasted environments. The distributions of the EC and pH are shown in the
histograms in Figure 2b and 2c.
Figure 2. Histograms showing the distribution of: a – EC; b - Log10(EC) and c - pH.
The histogram of the variable EC (Figure 2a) shows a distribution shifted towards the lowest
class of conductivity (0-2500 µS/cm), which included about 70% of the samples. This kind of
distribution is far from a normal distribution and cannot be used directly to study the spatial
distribution. A logarithmic transformation (Figure 2b) allows to better apportion the data along the EC
range. Therefore, the variable log10(EC) will be studied in the following. This variable log10(EC) shows
a bimodal distribution, with a first mode focused on the value 2.6 (~ EC 400 µS/cm) and a second
lower mode on the value 4 (EC ~ 10000 µS/cm). There is therefore a deficit of surface water with EC
values close to 1250 µS/cm.
The pH values are relatively well distributed along the range. Therefore, the raw pH data were
directly used in spatial analysis. This histogram has also bimodal feature, with the first mode from 6 to
8, involving 26 measurement points, and the second one from 8 to 10, obtained on 24 waterbodies.
Descriptive statistics performed from the data set, Log10(EC) and pH are presented in Table 1. The
coefficient of variation of these logarithmic variables, reflecting the great heterogeneity.
Table 1. Descriptve statistics obtained from Log10(CE) e pH values.
The experimental semivariograms for log10(EC) and pH values are presented on Figure 3.
Both are characterized by a very short range, less than 200m, indicating that the maximum of
variability is reached for small distances in the landscape.
pH
Electrical Condutivity (EC)
30000000
1.8
1.6
25000000
1.4
1.2
Variogram
Variogram
20000000
15000000
10000000
1
0.8
0.6
0.4
5000000
0.2
0
0
500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500
Distance
0
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
5500
Distance
Figure 3. Semivariograms of Log10(EC) and pH showing the low spatial range below 200 m.
Indeed, in the field we observed that the lakes and ponds have very contrasting salinity,
although they are located very close to each other. The results emphasizes that there is no dependence
between the waterbodies for the studied variables in the Nhecolândia.
Figure 4 shows an example of distribution of ECa values obtained with the EM38 around the
‘Salina Verde’ lake. The ECa values range from 50 to 220 mS/s. The distribution is unimodal, centered
in a mode of about 120 mS/m. This kind of distribution approximates a normal distribution, and can be
directly used to study the spatial distribution.
Figure 4. Frequency histogram of ECa values obtained around ‘Salina Verde’ lake.
The experimental variogram has a range of about 60 m without nugget effect (Figure 5). The
sill is approximately 1000. This variogram shows that the minimal distance of 10 meters between
successive measurement points is sufficient to evaluate the spatial variability of the variable ECa. The
experimental variogram was better fitted with an exponential model.
Experimental Variogram
1000
900
800
Variogram
700
600
500
400
300
200
100
0
0
20
40
60
80
100
120
140
160
180
200
220
240
Distance
Figure 5. Experimental variograma of ECa data (EM38 in vertical position) and adjusted model for the
soil cover around ‘Salina Verde’.
7847000
220
a
200
7846900
180
b
d
160
140
c
7846800
120
mS/m
100
7846700
80
60
7846600
40
20
7846500
7846400
598800
598900
599000
599100
599200
599300
Figure 6. Distribution map for ECa values around ‘Salina Verde’.
The maps produced by electromagnetic induction show similar structures around all the lakes,
which can be illustrated on the example of ‘Salina Verde’ (Figure 6) and ‘Salina 60mil’ (Figure 7).
Immediately around the lakes, there are high ECa values (140-190 mS/m) that reflect to the presence of
a clay horizon at topsoil, with a maximum thickness of 0.3m (a). Further outside, there is a continuous
ring-shaped structure (b) with values in the range of 100-140 mS/m. Auger surveys show that this ringshaped structure corresponds to a rise, in sub-surface, of a sandy clay green horizon, very consistent
and locally cemented. Just behind, there are discontinuous areas (c) with higher ECa values (in the
range of 180-220 mS/m) corresponding to the presence of highly saline water (15000 to 80000 µS/cm)
at approximately 1 m deep. Further away from the saline lake, ECa values decrease (d) due to the
topographic elevation associated with the thickening of the sandy surface horizon, which becomes
more than 2 m thick. The ECa variations result from deep changes in the soil, which are not anymore
detectable with the EM38 device. In contrast, the EM31 detects a decrease in ECa values, then a slight
increase at about 60-100m from the lake shore (arrow on Figure 7), before a further decline. Direct
auger observations show that the increased in ECa again corresponds to a rise in the sandy clay
consistent green material, generally approaching 2 m from the soil surface.
Figure 7: Oblique view of ECa distribution around ‘Salina 60 mil’ using EM31 in vertical position.
Discussion
The regional study of the electrical conductivity of surface waters revealed four fundamental
points: 1. the existence of a very high conductivity range, while the climatic conditions (ETP-P ± 300
mm) do not favor a salinization by evaporation, neither to preserve the salinity in the landscape; 2. a
correlation between pH and EC of water; 3. a low range (<200m) for the spatial distribution of
log10(EC); 4. a bimodal distribution of log10(EC) and pH values. To this information provided by the
study at a regional scale, we must add a standard distribution of ECa values around the lakes reflecting
a standard morphology in the soil cover. These points will be discussed below.
The identification of high EC and pH ranges in surface waters of the Nhecolândia have already
been mentioned in previous studies, and it can be regarded as a peculiarity of this region (Barbiero et
al. 2002; Fernandez, 2007, Mariot et al., 2007, Barbiero et al., 2008, Furquim et al., 2008, 2010 b). The
current climate conditions, with mean annual precipitation of about 1100 mm and mean evaporation of
about 1300 mm, are not favorable to the accumulation or maintenance of salinity. In this context, the
high range in the surface water EC and pH may reflect two characteristics of the process responsible
for this range: 1. this can be a very powerful process, such as the presence of a stock of salt
accumulated during earlier arid phases, and that would still mark locally the water chemistry; 2. It can
also be a system of accumulation of dissolved elements, cumulative from year to year, leading to saline
waters isolated from the current transfers of fresh water across the Pantanal, and thus protected from a
desalination by the circulating water.
The correlation between pH and EC is also consistent with previous work (Barbiero et al., 2002,
2007, 2008) carried out at a local scale or on a reduced number of samples. In the region, the waters
may result from each other by simple process of concentration and dilution. They evolve in a sodic
alkaline pathway, and the concentration is then accompanied by an increase in the pH that can reach
quite rapidly values close to 10. This good correlation between pH and EC indicates that the chemistry
is framed in a unique chemical regional pattern that is relatively well known.
The short range in the distribution of conductivity values of surface waters shows that there is
no possible extrapolation of EC beyond the same lake, the value of 200 m corresponding approximately
to the minimum distance between two neighbouring lakes. We can therefore conclude that the process
that controls the chemical characteristics of surface waters acts in the soil cover between two
neighbouring lakes.
The bimodal distribution of salinity provides additional information. It shows that the process
that controls the concentration of lake water either guides the water towards high salinity, or conversely
towards dilute water, more in line with the waters that flow regionally in the landscape.
The soil mantle as evidenced by the ECa survey shows that the lakes are surrounded by subsurface rises of soil horizons with low permeability. Such morphology of the soil cover is in every
respect similar to what has been described in other parts of the Nhecolândia by Sakamoto (1997). This
soil system is therefore representative of the region. The concordance between these ring-shaped
structures and the presence of lakes, and the mineralogical composition of their clay fraction,
emphasize a chemical sedimentation, probably induced by the concentration of the solutions. The
genesis of these concentric structures was discussed by Barbiero et al. (2008). The information
collected at both the regional and the local scale is in agreement with the singular limnological
functioning described below.
At the beginning of the wet season, accumulation of rainfall in the Pantanal causes a rise in the
water level. When it exceeds the morphological threshold, it flows down toward the depressions of the
lakes (Figure 8). Conversely, during the dry season, saline and fresh watertables are disconnected and
the threshold acts as a barrier against the backward propagation of saline water toward the fresh sandy
aquifer.
Figura 8. Model of hydrochemical functioning of a saline lake during the flood (1) and during the dry
season (2), modified from Barbiero et al. (2008).
In summary, the salinity in the lakes depends on the average quantity of water Q flowing
annually towards the lakes, which in its turn depends on the relative height of the threshold with respect
to the regional fresh watertable (Figure 9). Q is decreasing with increasing height of the threshold. Two
cases may occur: if Q<ETP-P, then the lake salinity increases every year leading to a saline
environment. Otherwise, if the threshold is low, it is easily exceeded by the watertable and then we
have Q>ETP-P. The lake is flushed every year and the salinity is maintained at a low level.
Figure 9. Influence of the height of the morphological threshold on the hydrological regime of lakes in
the Nhecolândia: 1: the high level of the threshold leads to Q<ETP-P; 2: the low level of the threshold
leads to Q>ETP-P.
The above-described limnological regime explains how lakes subjected to the same amount of
evaporation and rainfall can have different dissolved ion concentrations. Because it is a cumulative
process, it explains the large range of salinity observed in the region. It also explains the bimodal
distribution of the electrical conductivity observed on a total number of about 80 lakes. Because a
single type of water, evolving in an alkaline pathway, is subjected to concentration and dilution, it
explains the good relationship observed between pH and EC. The very local control of the salinity by
the presence of threshold between the lakes is in agreement with the low spatial range for EC (<200m)
observed on the variogram (Figure 3).
Conclusion
The objective of this study was to understand the distribution of the salinity, a major aspect
in the variability of surface waters in the Nhecolândia. Through an appropriate statistical and
geostatistical treatment, the data collected show that there is no organization of the salinity at the
regional scale. In contrast, the low spatial range measured indicates that the salinity is controlled
locally by the presence of thresholds governing the hydrological regime of each lake. These thresholds
were here identified by geophysical surveys using electromagnetic induction methods. The study
confirms the existence of close relationships between the soil system and the limnological functioning
of the lakes of the Nhecolândia.
References
Ab’Sáber, A. N. 1988. O Pantanal Mato-Grossense e a teoria dos refúgios. Revista Brasileira de
Geografia, 50 (número especial 1-2): 9-57p.
Ab’Saber, A. N. 2006. Brasil: Paisagens de Exceção: o litoral e o Pantanal Mato-Grossense patrimônios
básicos. Cotia, SP: Ateliê Editorial, 182p.
Alvarenga, S. M.; Brasil, A. E.; Pinheiro, R.; Kux, H. J. H. 1984. Estudo geomorfológico à Bacia do
Alto Rio Paraguai e Pantanais Mato-Grossenses. In: Brasil. Ministério das Minas e Energia. Secretaria
Geral. Projeto RadamBrasil. Boletim Técnico.
ANA; CEF; PNUMA; OEA. 2004. Modelo de simulação hidrológica da bacia do Alto Paraguai.
Relatório final. IPH – UFRGS, Porto Alegre.
Barbiero, L., Queiroz Neto, J. P., Ciornei, G., Sakamoto, A., Capellari, B., Fernandes, E., Valles, V.
2002. Geochemistry of water and groundwater in the Nhecolândia, Pantanal of Mato Grosso, Brazil:
variability and associated processes. Wetlands, .22, 528-540p.
Barbiero L., Furquim S.C., Valles V., Furian S., Sakamoto A., Rezende Filho A.T., Graham R.C., Fort
M., 2007 – Natural arsenic in Groundwater and alkaline lakes at the upper Paraguay basin, Pantanal,
Brazil. In Battacharya P., Mukherjee A.B., Bundschuh J., Zevenhoven R., Loeppert R.H. (Ed.) Arsenic
in Soil and Groundwater Environment: Biogeochemical interactions. Elsevier Book Series “Trace
metals and other contaminants in the environment” (J.O. Nriagu, Serie Ed.), Vol 9, Chapter 4, 101-126.
Barbiero, L.; Rezende Filho, A. T.; Furquim, S. A. C.; Furian, S.; Sakamoto, A. Y.; Valles, V.; Graham,
R. C.; Fort, M.; Fereira, R. P. D.; Queiroz Neto, J.P. 2008. Soil morphological control on saline and
freshwater lake hydrogeochemistry in the Pantanal of Nhecolândia, Brazil. Geoderma, 148: 91–106p.
Fernandes, E. 2007. Organização espacial dos componentes da paisagem da Baixa Nhecolândia –
Pantanal de Mato Grosso do Sul. Tese de Doutorado (Universidade de São Paulo), São Paulo, SP, 176p.
Furquim, S. A. C.; Graham, R.; Barbiero, L.; Queiroz Neto, J. P. de; Valles, V. 2008. Mineralogy and
genesis of smectites in an alkaline-saline environment of Pantanal wetland, Brazil. Clays and Clay
Minerals, v. 56, 580-596p.
Furquim, S. A. C.; Graham, R. C.; Barbiero, L.; Queiroz Neto, J. P.; Vidal-Torrado, P. 2010. Soil
mineral Genesis and distribution in a saline lake landscape of the Pantanal Wetland, Brazil. Geoderma,
154, 518-528.
Furquim, S. A. C., Barbiero, L., Graham, R. C., Queiroz Neto, J. P., Ferreira, R. P. D., Furian, S. 2010.
Neoformation of micas in soils surrounding and alkaline-saline lake of Pantanal Wetland, Brazil.
Geoderma, 158, 331-342.
Fustec, E.; Lefeubre, J. C. 2000. Fonstions et valeurs des zones humides. Ed. Dunod, 426p.
Klammer, G. 1982. Die Palaeowuste des Pantanal von Mato Grosso und die Pleistozane Klimageschichte
des brasilianischen Randtropen. Zeitschrift für Geomorphologie, 26: 393-416.
Mariot, M., Dubal, Y., Furian, S., Sakamoto, A., Vallès, V., Fort, M., Barbiero, L. 2007. Dissolved
organic matter fluorescence as a water-flow tracer in the tropical wetland of Pantanal of Nhecolândia,
Brazil. Science of the Total Environment, 338, 184-193p.
Reddy K. R., Delaune, R. D. 2008. Biogeochemistry of wetlands. CRC Press, London, 774p.
Rezende Filho, A. T. 2006. Estudo da variabilidade e espacialização das unidades da paisagem:
Banhado (baía/vazante), Lagoa Salina e Lagoa Salitrada no Pantanal da Nhecolândia, MS. Dissertação
de Mestrado (Universidade Federal do Mato Grosso do Sul), Aquidauana, MS, 122p.
Tricart, J. 1982. Paisagem e Ecologia. Inter-Facies – Escritos e documentos. UNESP, São José do Rio
Preto, n. 76, 43p.