Environmental geostatistics of heavy metals in fine active sediments

Environmental geostatistics of heavy metals in fine active
sediments of the Western Antioquia subregion
Luis Hernán Sánchez Arredondo a, Jheyson Andres Bedoya Londoño b & Sergio Alejandro
Garavito Higuera b
a
Department of Materials and Minerals, Faculty of Mines, National University of Colombia, Medellín, Colombia, lhsanche@unal.edu.co
b
Curricular Area of Mineral Resources, National University of Colombia, Medellín, Colombia, jabedoyal@unal.edu.co
b
Curricular Area of Mineral Resources, National University of Colombia, Medellín, Colombia, sagaravitoh@unal.edu.co
Corresponding author: Luis Hernán Sánchez Arredondo, lhsanche@unal.edu.co
ABSTRACT
The Western Antioquia subregion is characterized by having a low concentration of rural land ownership
and a high process of ecological destruction and desertification in the municipalities of Santa Fe de
Antioquia, San Jeronimo, Anza, and Sopetran. Tropical dry forest biome, secondary vegetation and growing
forests, sub-Andean forest, Andean forest, wetlands and associated forests, paramos, and basal forest
predominate. One of the most transcendental environmental problems has been the transformation of natural
ecosystems that have become mainly agroecosystems and due to population growth, due to the development
of new projects among other anthropic activities. In the subregion, the main global greenhouse effect (GHG)
emissions are from deforestation, livestock, agriculture, energy consumption, and sanitation. Climate risk
of the subregion is medium to low. Through geostatistical techniques of simulation conditioned by turning
bands (TB), different scenarios were assessed for the analysis of environmental contamination of metals
cobalt (Co), chrome (Cr), nickel (Ni), and lead (Pb) in an area of 7,291 km2, equivalent to the subregion of
west from department of Antioquia. Geostatistical cartography prepared for the pessimistic case of each of
the metals analyzed, report anomalous values above the standards established for the different types of rocks
in the region, which would generate a moderate relative mobility for water contamination, being subjected
to surface weathering processes under oxidation conditions in acidic environments. Under reducing
conditions, the simulated elements would have a behavior of relative immobility and would offer
environmental danger for the soils of the region. Potentially dangerous anomalous values were estimated
for Co ≥ 130 mg/kg, Cr ≥ 2,507 mg/kg, Ni ≥ 138 mg/kg, and Pb 165 ≥ mg/kg, which are consistent with
volcano-sedimentary sequences and acid to intermediate plutonism present in the subregion. Analysis of the
samples were carried out by optical emission spectrography, which although it is a semi-quantitative
method, has made it possible to formulate the state of the art of environmental geochemistry and
identification of possible vulnerable areas in order to focus future studies, with greater precision and detail.
Regarding mining activity, 20% of the area of the subregion is concessioned for extraction of minerals and
34% is requested as mining concession contracts, whose activities of exploration and exploitation of
georesources are concentrated mainly in Au and precious metals. Mine with the highest production is of the
gold type and is in in the municipality of Buritica, which intends to extract 9.1 tons of gold each year;
however, medium and small-scale mining represents 93% of mining titles and applications in the area of
influence. If the high concentrations of Co, Cr, Ni, and Pb in some soils of the subregion of the west of
Antioquia are validated, their use should be technically oriented to economic activities in the mining sector,
instead of agriculture and livestock, due to the negative impacts of heavy metals on human health.
1
KEYWORDS: Colombia, environmental geostatistics, heavy metals.
HIGHLIGHTS
-
An exploratory analysis was made for the heavy metals Co, Cr, Ni, and Pb.
100 conditioned simulations of 400 turning bands were elaborated.
The automatic cartography of the pessimistic case was expounded.
Potentially dangerous anomalous values were estimated.
INTRODUCTION
metamorphic igneous rocks predominate; with
diorites, neises, and amphibolites. The Calima
Terrain is made up of a sequence of volcanic and
sedimentary rocks that make up most of the
outcrops. These are underwater volcanic rocks
with intercalations of claystones, grawacas, and
lidites that would reach an approximate thickness
of 900 m. Several authors have reported fossils
within the cherts of the sedimentary sequence.
The area has been subjected to various tectonic
phases with compression, extensional horizontal
movements and especially large displacements
due to dextral-inverse transpression. The Cuna
Terrain is made up of three important structural
elements: the Mandé magmatic arc, the basin of
the Atrato and San Juan rivers, and the Serranía
de Baudó. The Santa Cecilia Unit outcrops to the
east of the Mandé Batholith, between Antioquia
and Chocó, as a continuous belt with a northwest
orientation, variable amplitude between 2 and 7
km, in faulted contact with sedimentites and
vulcanites of the Cañasgordas Group and to the
south is limited by the Batholith of Mandé.
Lithologically it is a heterogeneous unit made up
of basic lava flows, breccias, agglomerates, tuffs
with local levels of pillow lavas and intercalations
of siltstones, calcareous mudstones, chert, and
limestone. The flows are basaltic in composition,
solid, vesicular or porphyritic, with a dense paste,
greenish-black in color when altered. The
breccias and agglomerates are solids made up of
basalt fragments, tuffs and abundant cavities
filled with chlorite. The tuffs are solid, greenish
gray in color and generally show good
Current active sediments (CAS) are the main
source of polluting heavy metals in the world and
their excess is harmful to ecosystems due to their
high toxicity and susceptibility to microbial
degradation, when they enter water bodies and the
atmosphere. Heavy metals studied from the point
of view of environmental pollution are important
because they are good indicators of regional
pollution (Wang, Lin, & Liu, 2022).
The Western Antioquia subregion drains its
waters into two large basins: the Cauca River
basin and the Atrato River basin; in the first, the
rivers of the western flank of the central mountain
range and the drainages of the eastern flank of the
western mountain range converge, in both cases
the ravines form deep and narrow "V" valleys. In
the first we have the municipalities of Peque,
Sabanalarga, Liborina, Buriticá, Olaya, Santafé
de Antioquia, Sopetrán, San Jerónimo, Ebéjico,
Anzá, Heliconia, and Armenia; while Dabeiba,
Giraldo, Cañasgordas, Uramita, Abriaquí, and
Frontino drain their waters to the Atrato River
basin.
In the study area (Figure 1), the three types of rock
with different ages outcrop: sedimentary,
igneous, both volcanic and intrusive, and
metamorphic (Servicio Geológico Colombiano,
2015). The subregion is crossed by three
geological terranes: the Cauca Romeral Terrain
(Servicio Geológico Colombiano, 2019a), the
Calima Terrain, and the Cuna Terrain (Servicio
Geológico Colombiano, 2019b). In the former,
2
stratification; they are intercalated in the basaltic
flows and the crystallolites predominate and
occasionally, tuffs of vitrocrystalline crystals.
Both in the tuffs and in the basaltic flows with
which they are associated, there are vesicles filled
with zeolites, chlorite and calcite, pumpellyite
and chlorite, prehnite, and epidote. The
pyroclastic sequence is interbedded with lenses of
sedimentary rocks such as chert, siltstones,
arenites, and epiclastites; locally there are
calcareous strata of biosparites with abundant
microfauna and terrigenous contribution
represented by quartz clasts, feldspars, and lithic
basalts. The name of La Equis is used to designate
the unit of this complex located to the west of the
Batholith of Mandé, which constitutes an
elongated north-south strip with an average width
of 6 km.
Figure 1. Geological map of the subregion Western Antioquia (Servicio Geológico Colombiano, 2015).
3
2. MATERIALS AND METHODS
turning bands method that allows the construction
of simulations in space from simulations in lines.
Therefore, one-line simulation methods are of
great interest even if the ultimate goal is to
simulate in 2D or 3D. The general principle of the
method applied to regionalized variables is due to
Matheron (Chilés & Delfiner, 1999). With the
turning bands method, 100 conditioned
simulations in 400 bands were elaborated.
The field data of CAS are part of the regional
geochemical sampling scale 1:100,000 of plates
114, 115, 129, 130 and 146 provided to the
Geochemical Map of Antioquia project of the
Faculty of Mines of the National University of
Colombia, by Ingeominas (current Colombian
Geological Service). In total, 2,652 Co, 2,670 Cr,
2,389 Ni, and 1,045 Pb specimens were
processed. The analysis of samples was carried
out by optical emission spectrography (semiqualitative) which is recommended in the regional
sampling stage (Sánchez Arredondo & Molina
Escobar, 2009).
2.1 Mathematical Formulation
The simulation by turning bands is based on the
simultaneous generation of a series of lines in
space. If P is a 2D or 3D region where it is
required to generate simulations at a series of
points along a grid in an x-y coordinate system, a
series of lines i whose azimuth θi is a random
variable uniformly distributed between 0 and 2π.
Along each line i, a realization of a onedimensional random function with zero mean and
covariance C1 (hi) is generated, where hi is the
coordinate along line i. If N is the point in the
region P where it is desired to simulate Z(x), N is
projected onto each of the lines and assigned the
value Zi (hNi) corresponding to the contribution of
line i, where hNi is the projection of N on line i
(Figure 2). Let be ui the unit vector along direction
i whereby hNi = XN.ui where (.) denotes dot
product. Finally, if L is the total number of lines,
the simulated value at point N is given by:
Geostatistical methods are in strong demand
across all fields of earth sciences, as powerful
tools to predict spatial attributes and simulate
forecast uncertainty at unsampled locations,
important as automated mapping to identify
potentially hazardous areas of greatest attention to
control and monitoring.
According to Chilés & Delfiner (1999),
conditional simulations are useful to obtain
realistic images of spatial variability.
Quantitatively, they are the tool of choice for
evaluating the impact of spatial uncertainty on the
results of complex procedures, such as numerical
modeling of a dynamical system or economic
optimization of natural resource development.
Conditional simulations fall into the scope of socalled Monte Carlo methods; in fact, they are
nothing more than Monte Carlo simulations. The
objective pursued here is not to reproduce the
genetic mechanisms that generated the observed
phenomenon. It is simply to mimic its spatial
variations as realistically as possible. By
modeling a stationary field over an area much
larger than the range of influence in the
variogram, a single simulation can give insight
into a variety of possible local situations. This is
usually sufficient, for example, to evaluate the
performance of a mining scenario, which depends
mainly on the local variability of the ore grades or
thicknesses.
1
ZS (XN) = 𝐿 ∑𝐿1 𝑍𝑖 (𝑥𝑁 . 𝑢𝑖 )
For the two-dimensional case, the form that C1
must have so that Zs has the covariance C(h) is:
ℎ 𝐶1 (𝑡)
(π /2) C (h) =∫0
√ℎ 2 −𝑡 2
The elaborated procedure was the following: 1)
transformation of original data to Gaussians
(Anamorphosis), 2) variographic analysis of the
transformed data, 3) simulation conditioned by
turning bands, 4) post-process simulation (reverse
transformation to the process carried out in 1),
and 5) automatic mapping and interpretation of
the results (see Figure 3).
Some covariance models can be simulated
directly on Rn. But it is often easier to use the
4
3. RESULTS
3.1 Variography
The elements cobalt, chromium, nickel, and lead
were selected for the elaboration of experimental
and theoretical semivariograms (dotted in black
and solid line in red respectively, Figure 4), which
showed a marked nugget effect; between 32% and
42% of the statistical variance, which may be due
to the lithological heterogeneity of the samples
and the different genetic environments of the
metals analyzed. Exponential type models
predominate for Co, Cr and Pb, while Ni was
better coupled to spherical type models. The
continuity expressed through the influence range
is in the 20,000-34,000 m interval, with two
nested structures standing out; spherical type for
nickel and exponential type for lead (Table 1).
The
statistical
variance
underestimates
(horizontal dashed line in the semivariograms) the
dispersion of the data, real variance (sill) for
cobalt since it is above, indicating that this is the
most dispersed element in the sediments of the
West Antioquia subregion, while for the other
elements it is overestimated.
Figure 2. The principle of turning bands in 2D
(Chilés & Delfiner, 1999).
3.2 Cross-validation
Because one of the applications of the
semivariogram is the spatial estimation of a
regionalized variable, cross-validation is used to
evaluate estimation errors and has been widely
used to evaluate the degree of goodness of a
semivariogram model. For Zi real points, Zi*
estimated data are obtained by ordinary point
kriging. The results obtained are shown
graphically in Figure 4, a diagram whose
correlation coefficient (rho) between Zi against
Zi*, which in our case for nickel is 82%. A
histogram of the errors relative to the estimated
standard deviation and a scatter plot of the
estimated Zi* against the relative errors, with rho
very close to zero.
Figure 3. Flowchart of the various stages of
construction of a simulation matching the
histogram, the variograma and the data (Chilés
& Delfiner, 1999).
5
Figure 4. Experimental (dashed black lines) and theoretical (red lines) semivariograms of the elements Co,
Cr, Ni, and Pb.
Table 1. Geostatistical parameters of the theoretical variograms.
Element
Co
Cr
Ni
Pb
Nugget
effect
0.39
0.42
0.32
0.36
Range
Sill
Range
Sill
Model 2
(m) (mg/kg)2
(m) (mg/kg)2
Exponential 24,258
0.72
Exponential 20,186
0.51
Spherical 4,655
0.17
Spherical 22,591
0.46
Exponential 33,595
0.08
Exponential 20,588
0.28
Model 1
6
Figure 5. Cross-validation analysis for the Ni case study in the West Antioquia subregion.
With 2,377 estimated data for nickel, a rejection
of 94 points was obtained, which is equivalent to
4% of the estimated Zi*. Table 2 reports the
results obtained from the cross-validation for the
elements cobalt, chromium, nickel, and lead.
Table 2. Cross-validation parameters.
Cross-Validation
(Z*-Z)/S*
Element
rho
Mean Std. Dev. Rejection
2%
Co
0.722 -0.004
0.987
3%
Cr
0.745 0.007
1.082
4%
Ni
0.817 0.128
1.073
3%
Pb
0.786 -0.007
0.981
3.3 Simulation conditioned by turning bands
100 conditioned simulations of 400 turning bands
were elaborated, with the objective of estimating
blocks of 500 m * 500 m. On Table 3, the average
of 100 simulations, the pessimistic case (largest
realization) and the optimistic case (smallest
realization) are reported for each element.
The maps in Figure 6 show the automatic
mapping for the pessimistic case of each of the
elements under study. These have been elaborated
7
on a scale of 7 colors corresponding to the
minimum value, quantiles 5, 25, 50, 75, 90, 98
and the maximum value. Potentially dangerous
anomalous concentrations were considered for
values ≥ the 90th quantile, corresponding to Co ≥
130 mg/kg, Cr ≥ 2,507 mg/kg, Ni ≥ 138 mg/kg,
and Pb ≥ 165 mg/kg, which could be sources of
contribution of the volcano-sedimentary
sequences and the acidic to intermediate
plutonism genetically related to the subregion.
These results would generate a moderate relative
mobility for water contamination, when subjected
to surface weathering processes under oxidation
conditions in acidic environments. Under
reducing conditions, the simulated elements
would have a behavior of relative immobility and
would offer environmental danger to the soils of
the region.
Table 3. Results obtained by 100 turning band simulations for the elements Co, Cr, Ni, and Pb.
Variable
Co average 100 Simu TB
Co optimistic 100 Simu TB
Co pessimistic 100 Simu TB
Cr average 100 Simu TB
Cr optimistic 100 Simu TB
Cr pessimistic 100 Simu TB
Ni average 100 Simu TB
Ni optimistic 100 Simu TB
Ni pessimistic 100 Simu TB
Pb average 100 Simu TB
Pb optimistic 100 Simu TB
Pb pessimistic 100 Simu TB
Count
Minimum Maximum
30,390
5.39
140.60
30,390
3.00
76.55
30,390
8.49
477.99
30,390
7.94
3,732.40
30,390
5.00
1,776.62
30,390
13.45
6,782.57
29,693
4.32
107.57
29,693
3.00
58.55
29,693
5.88
500.00
30,328
5.01
204.55
30,328
5.00
21.86
30,328
5.12
1,000.00
8
Median
30.22
13.26
75.25
278.82
65.69
1,121.03
32.56
13.04
78.92
17.70
17.70
84.49
Figure 6. Automatic mapping of the pessimistic case for the elements Co, Cr, Ni, and Pb.
9
CONCLUSIONS
normal to find 150 mg/kg of Ni (diabases and
basalts very common in the subregion) and in
ultrabasic rocks 2,000 mg/kg. The median of 100
simulations showed a value of 32 mg/kg of Ni,
which is somewhat reassuring when compared to
the normal content of average crustal rocks.
According to the pessimistic cartography of this
element in the Western Antioquia subregion, the
highest values are located in the municipalities of
Frontino, Dabeiba, Caicedo, and Anzá, with some
small foci in Peque, Sabanalarga, and Liborina,
where there are areas that exceed 138 mg/kg of
Ni. The similarity in the spatial distribution of
nickel and cobalt is clear (Figure 6), that is, the
areas with high cobalt values are the same ones
where we find nickel anomalies. Essential for
some organisms. Ni (2+) compounds are relatively
non-toxic. Some other compounds are extremely
toxic and/or carcinogenic. The environmental
trajectory of Ni is medium under oxidizing
conditions, high under acidic conditions, very low
under neutral to alkaline and reducing conditions.
Ni deficiencies can retard growth in animals. At
high levels in the soil, they can result in plant
growth disorders (chlorosis and plant death).
Phosphate fertilizers can increase Ni availability.
Limestone and potash fertilizers reduce Ni
availability. Most Ni compounds are relatively
soluble at pH < 6.5, but insoluble at pH > 6.7.
More than 70 Ni-accumulating plants are known.
Other plants do not absorb Ni such as corn,
potatoes, carrots, and spinach. Drinking water
should not contain more than 0.1 mg/l Ni. A soil
with more than 210 mg/kg of Ni must be
remediated.
Co is a central atom in vitamin B12, toxic to
humans at doses ≥ 25 mg/day and in water at
doses > 2 mg/l. The median of 100 simulations
developed in this study is 30 mg/kg, a value that
is considered higher than the average content of
the rocks of the earth's crust estimated at 25
mg/kg. Basic and ultrabasic rocks predominate in
the Western Antioquia subregion, in which it is
normal to find values between 50-150 mg/kg of
Co respectively. From the environmental point of
view (pessimistic case, Figure 6), 10% of the
estimated area contains cobalt values ≥ 130
mg/kg and are essentially located in the
municipalities of Frontino, Dabeiba, Buriticá,
Peque, and Sabanalarga. In Frontino it has been
referenced as a source of Co la Cabaltina, which
was observed in association with gold and
molybdenite in the Cerro Plateado (Molina,
Molina, & Ortiz, 1990).
It is known that some Cr (6+) compounds are
carcinogenic and their average abundance in the
earth's crust is 100 mg/kg, but in basic rocks it is
normal to find Cr contents of up to 200 mg/kg,
while in ultrabasic of 2,000 mg/kg. The median of
100 simulations gives us a value of 279 mg/kg,
while in a pessimistic case 10% of the simulation
values reported values ≥ 2,507 mg/kg (Figure 6).
Although the mobility of chromium is very low
under all conditions, its environmental trajectory
due to geogenic dust resulting from weathering
can be very dangerous. The US EPA suggests that
drinking water in excess of action levels can lead
to short-term skin irritation or ulceration, and
long-term damage to circulatory and nerve tissue
and skin irritation. Drinking water should not
contain more than 0.05 mg/l of Cr and the
maximum tolerance of a soil intended for
agriculture should not be higher than 100 mg/kg.
The normal contents of lead in the rocks of the
earth's crust are estimated at 12 mg/kg, its highest
value is found in granitic rocks, where it is normal
to observe 20 mg/kg. The median for 100
simulations in the Western Antioquia subregion
was estimated at 18 mg/kg and is surely related to
materials associated with the granitic facies of the
Sabanalarga Batholith in the municipalities of
The average abundance of Ni in the rocks of the
earth's crust is 75 mg/kg, but in basic rocks it is
10
DECLARATION
INTEREST
Peque and Sabanalarga. The map in Figure 6
shows the pessimistic case, where 10% of the
estimated values exceed 165 mg/kg of Pb, but 5%
are above 767 mg/kg. Lead is a particularly
dangerous chemical element, and it can
accumulate in individual organisms, but also
enter food chains. Its mobility in the environment
is low, in oxidizing, acid and neutral to alkaline
conditions, and very low, under reducing
conditions. The main mineral in the area that may
contain Pb is Galena (PbS).
OF
COMPETING
The authors declare that they have no known
competing financial interests or personal
relationships that could have appeared to
influence the work reported in this paper.
ACKNOWLEDGEMENTS
To the Faculty of Mines of the National
University of Colombia, for their support in the
preparation of this manuscript. Through this
article, the authors wish to pay a posthumous
tribute to Professor of Mineral Deposits Franklin
Ortíz Bejarano, who unfortunately died on June 7,
2021, a victim of COVID-19 and who was for
more than 30 years professor and guide of many
generations of graduates of the Faculty of Mining.
If the high concentrations of Co, Cr, Ni, and Pb
are validated in some soils of the Western
Antioquia subregion, their use should be
technically oriented to economic activities in the
mining sector, instead of agriculture, due to the
negative impacts of heavy metals on human
health (Londoño Franco, Londoño Muñoz, &
Muñoz Garcia, 2016).
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20% of the area of the subregion is under
concession for the extraction of minerals and 34%
is requested as mining concession contracts,
whose activities of exploration and exploitation of
georesources are mainly concentrated in gold and
precious metals. The mine with the highest
production is of the gold type and is located in the
municipality of Buriticá, which intends to extract
9.1 tons/year of gold; however, medium and small
mining represents 93% of mining titles and
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