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Journal of Geoscience and Environment Protection, 2023, 11, 41-61
https://www.scirp.org/journal/gep
ISSN Online: 2327-4344
ISSN Print: 2327-4336
DOI:
10.4236/gep.2023.1111003 Nov. 17, 2023 41
Journal of Geoscience and Environment Protection
Geochemistry of Potentially Toxic Elements in
Soil and Sediments of a Tanzanian Small-Scale
Gold Mining Area
Johnbosco Karungamye1,2*, Mwemezi Rwiza1, Juma Selemani1, Janeth Marwa2
1Minerals Division, Ministry of Minerals, Dodoma, Tanzania
2School of Materials, Energy, Water and Environmental Sciences (MEWES), The Nelson Mandela African Institution of Science
and Technology (NM-AIST), Arusha, Tanzania
3School of Business Studies and Humanities (BuSH), The Nelson Mandela African Institution of Science and Technology
(NM-AIST), Arusha, Tanzania
Abstract
Small-scale gold
mining is linked to significant environmental pollution by
potentially toxic elements (PTEs). However, research on the pollution caused
by such mining activities remains insufficient especially in developing coun-
tries. In the present study, a systematic in
vestigation assessed the pollution
and level of ecological risk of PTEs in soil and stream sediments in an active
small scale gold mining area of Isanga, in Nzega, Tanzania. Samples amount-
ing to 16 soil and 20 sediment were gathered from the study area and ana-
lyzed for five PTEs concentrations (As, Cd, Cr, Hg and Pb) using the AAS
method. The contamination level and ecological risk were assessed using sev-
eral pollution indices. The results suggest that the assessed environmental
systems of the Isanga mining
area and its vicinities are lowly contaminated by
PTEs and have a low potential to pose ecological risks. Hg and Cd with mean
concentrations of 0.09 mg/kg and 0.26 mg/kg respectively were found to be
the most enriched PTEs in soil, compared to their avera
ge continental crust
concentrations (0.056 mg/kg and 0.102 mg/kg respectively). The levels of the
evaluated PTEs in the study area are susceptible to increase over time if
proactive steps are not taken to control mining and waste disposal activities.
Keywords
Environmental Pollution, Pollution Indices, Ecological Risk,
Geo-Accumulation Index, Sediment Quality
How to cite this paper:
Karungamye, J.,
Rwiza
, M., Selemani, J., & Marwa, J. (2023).
Ge
ochemistry of Potentially Toxic Elements
in Soil and Sediments of a Tanz
a
nian
Small
-Scale Gold Mining Area.
Jour
nal of
Geoscience and
Environment Protection
,
11
,
41-61.
https://doi.org/10.4236/gep.2023.1111003
Received:
September 18, 2023
Accepted:
November 14, 2023
Published:
November 17, 2023
Copyright © 20
23 by author(s) and
Scientific
Research Publishing Inc.
This work is licensed under the Creative
Commons Attribution International
License (CC BY
4.0).
http://creativecommons.org/licenses/by/4.0/
Open Access
J. Karungamye et al.
DOI:
10.4236/gep.2023.1111003 42
Journal of Geoscience and Environment Protection
1. Introduction
Environmental contamination by potentially toxic elements (PTEs) from mining
activities is one of the most severe environmental issues in many parts of the
world, due to their possible harm to human and environmental health (Reyes et
al., 2020). Mining signifies the most detrimental environmental impacts as it in-
volves the use of processing chemicals, the production of large amounts of waste,
broad destruction of the landscape and removal of the vegetation (Candeias et
al., 2011). Artisanal and small-scale gold mining (ASGM) in particular is linked to
ideal introduction of PTEs into the environment (Nyanza et al., 2021) as a result
of direct discharge of effluents and waste to surroundings with little to no pollu-
tion controls (Pavilonis et al., 2017; Gyamfi et al., 2019). These materials may
contain various elements that, when discharged into the environment, accumu-
late in soil or get swept into water bodies. According to studies, over 99% of
PTEs that enter water bodies end up in sediments, making them significant PTE
sinks and repositories (Shen et al., 2019; Akoto & Anning, 2021). Elements such
as As, Cd, Cr, Hg and Pb have detrimental effects on flora and fauna even in
small amounts making them toxic at all concentrations (Dybowska et al., 2006).
The mobilization, distribution and concentration of PTEs in gold mining
areas vary depending on geochemical characteristics of soil and tailings, but also
mineralization of the particular environment (ur Rehman et al., 2020). The geo-
chemistry of a mining area can thus be used to examine the type of hazardous
contamination, potential risk to human and ecosystem health, and processes
which dictate the concentrations and behaviors of PTEs in that area (Rivera-Parra
et al., 2021).
Research on environmental geochemistry and pollution by PTEs in mining
and mineral processing areas has been globally conducted extensively (Carrillo-
Chávez et al., 2003; Basu et al., 2015; Boonsrang et al., 2018; Akoto & Anning,
2021; Haruna et al., 2021; Tang et al., 2021; Zhao et al., 2021; Olumayowa Olu-
wasola et al., 2023) although in some parts of the world such types of studies are
not many. The studies demonstrate a connection between mining operations
and PTEs-caused contamination, suggesting that mining plays a substantial role
in environmental pollution (Akindele et al., 2023; Olumayowa Oluwasola et al.,
2023). For example, Akindele et al. (2023) revealed that gold mining activities in
the Osun River catchment area, a UNESCO World Heritage Site in Nigeria, re-
sulted in PTEs contamination of benthic sediments, impaired water quality, and
heavy metal bioaccumulation in macroinvertebrates.
For Tanzania where ASGM is characterized by labor-intensiveness with insuf-
ficient technical knowledge of mining and mineral processing; wide scattering of
mine wastes and tailings to the environmental media and poor handling of min-
eral processing chemicals are some of typical outcomes (Dreschler, 2001). How-
ever, published information on environmental geochemistry of PTEs and their
associated risk in small scale gold mines in Tanzania is scant despite the historic
small-scale mining activities in the country. In fact, majority of the few studies
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done in the country have mainly addressed issues of environmental pollution
and degradation by trace elements, heavy metals or metalloids in large scale gold
mines (Kitula, 2006; Bitala et al., 2009; Mganga et al., 2011; Åsgeir & Manoko,
2012; Mganga, 2014; Rwiza et al., 2016; Walwa, 2016) while others have focused
on stratigraphy, rock alteration and petrogenesis (Vos et al., 2009; Kwelwa et al.,
2013). However, studies that have been conducted in small scale mines have fo-
cused mainly on mercury (Hg) with little to no information on contamination
by other PTEs (Taylor et al., 2005; Ikingura et al., 2006; Chibunda & Janssen,
2009; Spiegel, 2009; Bose-O’Reilly et al., 2010; Herman & Kihampa, 2015; Lema
& Mseli, 2017). Nevertheless, the studies have not ascertained comprehensive
information on the extent of pollution, geospatial distribution and environmen-
tal risk associated with PTEs in small-scale gold mining environments. These
significant gaps impede initiatives for a thorough assessment of the environ-
mental impacts and risks associated with artisanal and small-scale gold mining,
not just in Tanzania but throughout sub-Saharan Africa. As such, lack of this
crucial information results in reluctance when handling very toxic wastes and
tailings resulting from mining activities.
Therefore, the present study assessed levels, geochemical distribution and en-
vironmental risk of five PTEs (As, Cd, Cr, Hg, and Pb) in an active small-scale
gold mine in Nzega, Tanzania. Six (6) pollution indices were used for assessment
of the level of contamination and environmental risk namely the enrichment
factor (
Ef
), geo-accumulation index (
Igeo
), Pollution Load Index (PLI), Contami-
nation Factor (
Cf
), degree of contamination (
Cdeg
) and the Potential Ecological
Risk Index (RI).
The results of this study are of utmost importance in determination of envi-
ronmental implication of small-scale mining activities on subsequent environ-
ments for effective and evidence-based environmental management.
2. Materials and Methods
2.1. Study Area Description
Isanga mine is a Small-Scale gold mine situated within the area of the now closed
and rehabilitated Resolute (Golden Pride) gold mine in Nzega district, Tabora
region, Tanzania (Figure 1). It is located southeast of Lake Victoria, approx-
imately 200 km south of Mwanza City and about 24 kilometers from Nzega
town, within the Nzega greenstone belt, part of the Lake Victoria Gold Field
(LVGF). Temperatures typically vary from 22˚C to 27˚C, and there is 700 to 800
mm of annual rainfall on average (Werema et al., 2016).
The Nzega greenstone belt is within the Nyanzian Super-group comprising of
sedimentary rocks, subordinate mafic volcanics, Banded Iron Formations (BIF)
and felsic volcanics (Nkya, 2013). Gold occurrence in the study area is normally
associated with such rock formations with silica and chlorite ore types. Pyrite,
arsenopyrite, pyrrhotite, and accessory minerals such as sulphosalts, galena,
sphalerite, and Ni−Co−Bi sulphides are among the sulphide minerals found in
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Figure 1. Map of Tanzania showing the location of Isanga Mine, in Nzega, Tabora.
both types of ore (Vos et al., 2009). The study area, easily accessible by road,
contains both active and abandoned artisanal and small-scale gold mines.
The area has several rivers and seasonal streams crossing through. Some of
these rivers and streams empty their water into Manonga River which is the
largest river within the area and a distinguished source of water for communities
around (Figure 2(a)). Other streams empty their water into closed Resolute
(Golden Pride) mine pits which are main sources of water used in gold processing
activities at the Isanga mine.
2.2. Sample Collection
A total of 16 soil and 20 sediment samples of about 500 g were gathered from the
study area. Soil samples were collected within 0 - 20 cm depth of the top soil
profile according to the US EPA guidance for environmental data collection
(USEPA, 2002). Sediment samples were collected from areas with low flow in
river channels, streams, processing areas and closed mine pits (pit lakes) within
the study area. The samples were collected using a stainless scoop, placed into
sealed and labelled plastic packages, transported and stored according to stan-
dard protocol (USEPA, 2001). For quality control purposes and avoiding cross-
contamination of samples by digging equipment, digging devices were cleaned
with deionized water after collection of each sample as done by Keith (2017). At
each sampling location, a coordinate was taken for referencing purposes using a
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Figure 2. (a) A map showing soil and sediment sampling locations at the Isanga Mine
and Manonga river. (b) A map showing soil and sediment sampling locations at the Isan-
ga mine.
handheld GPS receiver (Garmin GPS MAP 64s). Sampling locations are indi-
cated in Figure 2(a) and Figure 2(b).
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2.3. Sample Preparation and Analysis
All sediment and soil samples were air dried, ground and sieved through 200
mesh screens. For determination of Cd, Pb, Fe and Cr, the soil and sediment
samples were digested according to the standard digestion protocol (Method
3050B-SW-846) (USEPA, 1996). Methods 245.5 and 7062 (USEPA, 1994) were
used during sample digestion for As and Hg analysis, respectively. As and Hg
determination and their instrumentation protocols were performed as per stan-
dard guidelines (Baird et al., 2017). The atomic absorption spectrophotometer
(AAS) Model WFX 210 was used to determine the concentration of PTEs from
the digested samples. Hydride generation AAS (HG-AAS) was used for As and
Hg analysis. AAS detection limits of As, Cd, Cr, Hg, Pb and Fe were 2 ppb (hy-
dride), 0.1 ppm, 0.5 ppm, 2 ppb (hydride), 1 ppm and 0.5 ppm respectively.
Three replicates assessed the analysis efficiency for each sample.
During soil and sediment digestion, representative dry weight homogeneous
samples were treated in a series of steps. 10 g of each sample were digested by
addition of 10 mg of conc. HNO3 followed by 15 minutes of heating at 90˚C. The
mixture was allowed to cool for sometimes followed by addition of 5 mL conc.
HNO3 and heated until no brown fumes. 2 mL of distilled water and 3 mL of
H2O2 were added until no effervescence was observed. 10 mL of conc. HCl were
added to the mixture and heated again for 15 min until approximately 5 mL re-
main of the sample. The digested samples were filtered using a filter paper
(Whatman, No. 41) after which 5 mL of each sample were diluted to final vo-
lume of 100 mL using distilled water ready for analysis where respective hollow
cathode lamps for the PTEs were applied accordingly. Concentrations of PTEs
have been presented in mg/kg in this study.
2.4. Statistical and Geostatistical Data Analysis
Descriptive statistical analyzes including the Pearson Correlation Matrix (PCM)
and Principal Component Analysis (PCA) were performed using Origin Pro
2022 statistical package (developed by OriginLab Corporation, Northampton,
Massachusetts, USA). Pictographic illustration of level and spatial disparity of
PTEs pollution were performed using QGIS version 3.28.2, an open-source geo-
graphic information system while Microsoft excel Version 2021 was used to
compute geochemical pollution indices.
2.5. Sediment Quality Guidelines
Tanzania doesn’t have national sediment quality guidelines (SQGs) for freshwa-
ter environments. However, the availability of approved SQGs by sediment-based
toxicologists dealing with PTEs is a significant assistance for monitoring aquatic
conditions, protecting aquatic biota, and establishing sound environmental poli-
cies concerning PTEs pollution (Rahman et al., 2022). SQGs were used in this
study to assess the potential biotic impact of PTEs estimated in sediment sam-
ples taken from the study area. These SQGs included the threshold effect level
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(TEL), probable effect level (PEL), effect range low (ERL), severe effect level
(SEL), effect range medium (ERM), and lowest effect level (LEL) as compiled by
Burton (2002).
2.6. Sediment and Soil Pollution Assessment
2.6.1. Geo-Accumulation Index (Igeo)
The
Igeo
is a pollution index that indicates how natural geological processes and
anthropogenic activities have influenced pollution by a given element (Muller,
1969; Abdullah et al., 2020; Napoletano et al., 2023). It is calculated using Equa-
tion (1):
2
log 1.5
geo
Ci
ICb
=
∗
(1)
where
Ci
denotes concentration of element
i
in soil and
Cb
is the geochemical
background value of the element. 1.5 is a factor used to minimize likely varia-
tions in the background value of metal
i
due to lithological processes (Stoffers et
al., 1986). The geochemical background values used in the present study are the
Upper Continental Crust (UCC) averages proposed by Wedepohl (1995) which
are 2, 35, 0.102, 17, and 0.056 for As, Cr, Cd, Pb, and Hg respectively.
Igeo
is clas-
sified as
Igeo
< 0 unpoluted; 0 ≤
Igeo
< 1 unpolluted to moderately polluted; 1 ≤
Igeo
< 2 moderately polluted; 2 ≤
Igeo
< 3 moderately to strongly polluted; 3 ≤
Igeo
< 4
strongly polluted; 4 ≤
Igeo
< 5 strongly to extremely polluted; and
Igeo
≥ 5 ex-
tremely polluted (Rabin et al., 2023).
2.6.2. Enrichment Factor (Ef)
Ef
is used to determine possible anthropogenic source and degree of accumula-
tion of heavy metals in sediments and soil in comparison with the typical occur-
rence of a given metal (Loska et al., 2005). It is calculated by making a metal with
low occurrence variability and high chemical stability a reference element and
using its concentration to estimate the
Ef
of the metal of interest (Barbieri, 2016).
Fe and Al are mostly used as reference elements because their geochemical na-
tures resemble many PTEs of environmental concern in both oxic and anoxic
conditions (Rubio et al., 2000; Nowrouzi & Pourkhabbaz, 2014; Manna & Maiti,
2018). In the present study, Fe was used. Equation (2) is used for calculation of
Ef
:
( )
( )
sample
background
f
Ci Cref
EBi Bref
=
(2)
where
Ci
denotes measured concentration of PTE
i
;
Cref
denotes concentration
of a reference element;
Bi
is the background concentration of PTE
i
in soil (con-
trol); and
Bref
is the background concentration of the reference element. The
Ef
values are categorized as follows:
Ef
< 2 shows deficiency to minimal enrichment;
2 ≤
Ef
< 5 purports moderate enrichment; 5 ≤
Ef
< 20 indicates significant
enrichment; 20 ≤
Ef
< 40 indicates very high enrichment, and
Ef
> 40 indicates
extremely high enrichment (Barbieri et al., 2015).
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2.6.3. Contamination Factor (Cf) and Degree of Contamination (Cd)
Cf
is an index that allows the assessment of soil contamination, by considering
the present concentration of a given element in relation to the background value
of the same element in soil or sediments. The degree of contamination (
Cd
) is
then computed by summing up all factors of contamination of PTEs examined
implying an inclusive pollution index of multi contaminants (Rutkowski et al.,
2020).
Cf
is is calculated using Equation (3):
f
Ci
CBi
=
(3)
where
Ci
is the mean concentration of a given PTE in soil from at least five sam-
ples and
Bi
is the background concentration of that PTE in soil prior to mining
activities. According to Justus Reymond & Sudalaimuthu (2023), contamination
factors are classified as
Cf
< 1 (low factor of contamination); 1 ≤
Cf
< 3 (mod-
erate factor of contamination); 3 ≤
Cf
< 6 (considerable factor of contamination);
and
Cf
> 6 (very high factor of contamination) while degrees of contamination
are categorized as
Cd
< 6 (low contamination degree); 6 ≤
Cd
< 12 (moderate
contamination degree); 12 ≤
Cd
< 24 (considerable contamination degree) and
Cd
> 24 (high contamination degree)
2.6.4. Pollution Load Index (PLI)
PLI is used for assessment of the presence of soil pollution by a given PTE and
determines the overall soil contamination degree (Sezgina et al., 2019). This in-
dex makes available a means to demonstrate the alteration of soil conditions due
to increase in the amount of PTEs within that soil (Kowalska et al., 2018). The
PLI is calculated based on Equation (4):
1 23
PLI
nn
CF CF CF CF= × ××
(4)
where
n
is the number of analyzed PTEs in soil,
CFn
is a computed factor of
contamination of PTE
n
. PLI values were classified into two classes where a val-
ue of PLI < 1 implies unpolluted while PLI > 1 implies pollution. The values of
PLI > 1 are further classified by Sezgina et al. (2019) as follows: (1 < PLI ≤ 2)
represents low to moderate pollution; (2 < PLI ≤ 3) represents moderate pollu-
tion; (3 < PLI ≤ 4) represents moderate to severe pollution; (4 < PLI ≤ 5)
represents severe pollution; and (PLI > 5) represents very severe pollution (Sez-
gina et al., 2019).
2.6.5. Potential Environmental Risk Index (PERI)
The PERI of PTEs is a useful index for assessment of the extent of ecological risk
caused by concentrations of PTEs in environmental media (Romzaykina et al.,
2021). It is estimated by computing the overall potential ecological risk index
(PERI) (Romzaykina et al., 2021) using Equation (5):
1
PERI
n
i
i
E
=
=
∑
(5)
Ei
=
Ti
×
CFi
, where, n is the number of PTEs being accessed;
Ei
is a risk factor of
PTE
i
function of the contamination factor;
Ti
is a factor for toxic response on
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the assessed PTE.
Ti
values used for Hg, Cd, As, Cu, Ni, Pb, Cr and Zn are 40, 30,
10, 5, 6, 5, 2, and 1 respectively (Hakanson, 1980; Kumar et al., 2021). Romzay-
kina et al. (2021) classified
Ei
values as
Ei
< 40—low ecological risk, 40 ≤
Ei
< 80—
moderate ecological risk, 80 ≤
Ei
< 160—considerable ecological risk, 160 ≤
Ei
<
320—high ecological risk,
Ei
≥ 320—very high ecological risk. The PERI was
classified as follows: PERI < 150—low ecological risk, 150 ≤ PERI < 300—
moderate ecological risk, 300 ≤ PERI < 600—considerable ecological risk, PERI
≥ 600—very high ecological risk (Hakanson, 1980; Romzaykina et al., 2021).
3. Results and Discussion
3.1. PTEs Concentration in Sediments and Soil
The minimum, maximum, mean and standard deviation concentrations of five
PTEs (As, Hg, Pb, Cd and Cr) in 20 sediment and 16 soil samples collected
within the study area (Isanga gold mine and vicinities) are summarized in Table
1. The concentrations of PTEs in both sediment and soil matrices were in the
following order: Cr > Pb > Cd > As > Hg, and the ranges of their concentrations
(mg/kg) in both sediment and soil samples are presented in Table 1.
Hg and Cd mean concentrations in soil were above the average concentrations
in the upper continental crust (0.056 mg/kg and 0.102 mg/kg respectively) as es-
timated by Wedepohl (1995). The results were also compared to PTE concentra-
tions in control samples collected about 6 kilometers from the mining area in
order to better understand the relationship between the lithology of the sur-
rounding environment and the level of contamination in the study area. The
Table 1. Concentrations of PTEs in samples of soil and sediments collected from the
Isanga mine and its vicinities.
PARAMETERS (mg/kg), dry weight basis
As Pb Cd Hg Cr Fe
Soil Samples
(n = 16)
Mean 0.40 3.96 0.26 0.09 27.45 574.03
Standard Deviation
0.20 1.66 0.07 0.07 0.18 141.07
Median 0.365 4.015 0.24 0.085 27.445 592.2
Minimum 0.11 0.99 0.15 0 27.11 336.91
Maximum 0.87 6.67 0.38 0.22 27.79 803.58
UCCa 2 17 0.102 0.056 35 483.17
Sediment
samples
(n = 20)
Mean 0.38 4.58 0.27 0.14 27.51 522.434
SD 0.231 1.48 0.12 0.11 0.20 103.559
Median 0.315 4.585 0.27 0.125 27.55 510.9
Minimum 0.02 1.64 0.03 0 27.18 373.08
Maximum 0.85 7.6 0.48 0.41 27.83 736.85
Backgroundb 0.31 4.87 0.09 0 27.41 417.86
aUCC = Upper continental crust background concentrations as per Wedepohl (1995);
bBackground values from the control sample.
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comparison revealed that, with the exception of Cd, which had a mean concen-
tration of 27.45 mg/kg as compared to the control (27.585 mg/kg), all the ana-
lyzed PTEs had average concentrations that were slightly above background le-
vels in control samples. Such results can be linked to the impact of mining oper-
ations in the study area as proposed in a similar study by Zhao et al. (2021). The
higher concentrations of Hg as compared to its average concentration in the
continental crust and in the control samples than other heavy metals, can be
linked to Hg use in gold amalgamation processes in the study area.
Similarly, the mean concentrations of all the assessed PTEs in sediments were
above concentrations of the same in the control sample collected from upstream
of Manonga river about 9 km from the Isanga mine. These findings suggest that
there was a slight degree of pollution in the mining area in both soil and sedi-
ment matrices. The slight difference of concentration of other PTEs in relation
to their concentration in control samples is most likely related to less discharge
to the environment.
Hg and Cd mean concentrations in soil were above the average concentrations
in the upper continental crust (0.056 mg/kg and 0.102 mg/kg respectively) as es-
timated by Wedepohl (1995). The results were also compared to PTE concentra-
tions in control samples collected about 6 kilometers from the mining area in
order to better understand the relationship between the lithology of the sur-
rounding environment and the level of contamination in the study area. The
comparison revealed that, with the exception of Cd, which had a mean concen-
tration of 27.45 mg/kg as compared to the control (27.585 mg/kg), all the ana-
lyzed PTEs had average concentrations that were slightly above background le-
vels in control samples. Such results can be linked to the impact of mining oper-
ations in the study area as proposed in a similar study by Zhao et al. (2021). The
higher concentrations of Hg as compared to its average concentration in the
continental crust and in the control samples than other heavy metals, can be
linked to Hg use in gold amalgamation processes in the study area.
Similarly, the mean concentrations of all the assessed PTEs in sediments were
above concentrations of the same in the control sample collected from upstream
of Manonga river about 9 km from the Isanga mine. These findings suggest that
there was a slight degree of pollution in the mining area in both soil and sedi-
ment matrices. The slight difference of concentration of other PTEs in relation
to their concentration in control samples is most likely related to less discharge
to the environment.
Fe, which was analyzed to serve as a reference element in the enrichment
factor computation (Barbieri et al., 2015), was found to be slightly higher in
both soil and sediment samples. Fe was found to have an average concentration
of 574.03 mg/kg in soil, higher than the background value of 483.17 mg/kg for
the upper continental crust, and 522.434 mg/kg in sediments, higher than the
background concentration of 417.86 mg/kg. Such elevated Fe concentrations can
be linked to the existence of Banded Iron Formations (BIF) as well as pyrite, ar-
senopyrite, and pyrrhotite mineralization in gold-bearing rocks in the studied
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area (Vos et al., 2009; Nkya, 2013).
3.2. Sediment Quality
For the purpose of assessing the potential biotic impact of PTEs estimated in se-
diment samples, the comparison between average concentrations and different
SQGs is shown in Table 2. The SQGs in Table 2 indicated that the sediments
contained acceptable concentrations of As, Pb, Cd, and Hg below threshold bio-
tic impact. However, with regard to the LEL and SEL guidelines, Cr concentra-
tion (27.51 mg/kg) was above threshold biotic effect (26 mg/kg) and above ex-
treme biotic effect (10 mg/kg). These findings suggest that PTEs, particularly Cr,
have contaminated the sediments that were collected from various riverine sys-
tems, ore washing bays, and water swamps within the study area and have a po-
tential to cause biotic impacts.
It should however be noted that, in determining whether or not sediments are
harmful or have been polluted by PTEs, SQGs exhibit different percentages of
false positive and false negative outcomes. When a PTE concentration in a sedi-
ment surpasses a SQG that indicates it is dangerous when it is not, this is known
as a false positive. The reverse is a false negative, where the concentration of a
PTE in the sediment is below the SQG and purports to be nontoxic but is harm-
ful to aquatic biota under normal circumstances (Burton, 2002).
3.3. Pollution Source Apportionment and Elemental Composition
Relationships
3.3.1. Pearson’s Correlation Analysis of PTEs
The Pearson Correlation Matrix (PCM) is a multivariate analysis method used to
determine the potential sources of PTEs and determine the extent of their quan-
titative relationships (Zhao et al., 2021). PTEs having a shared geochemical his-
tory or origin may exhibit strong correlations with one another. Table 3 shows
PCM results for PTEs in soil and sediments collected from the Isanga mine and
its vicinities. A moderate positive correlation was observed for As – Hg in soil (r
Table 2. Concentrations of PTEs in sediments compared to different sediment quality
guidelines for metals (in mg/kg).
As
Pb
Cd
Hg
Cr
Effect Level
PTEs’ Mean
Concentration
0.38 4.58 0.27 0.14 27.51
TELa 5.9 35 0.6 0.17 37.3
Threshold
ERLa 33 35 5 0.15 80
LELa 6 31 6 0.2 26
PELa 17 91.3 3.53 0.486 90 Midrange
ERMa 85 110 9 1.3 145
SELa 33 250 10 2 10 Extreme
aValues for SQGs adopted from Burton (2002); aAbbreviations defined in Section 2.5.
J. Karungamye et al.
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Table 3. Pearson correlation matrix for PTEs concentrations in soils and sediments.
Pearson Correlation
Elements
As
Pb
Cd
Hg
Cr
Fe
Soil Samples
As
1
Pb
0.471 1
Cd
0.460 0.370 1
Hg
0.686* 0.488 0.173 1
Cr
−0.019 0.435 0.229 −0.233 1
Fe
−0.234 0.079 −0.004 −0.077 −0.222 1
Sediment
Samples
As
1
Pb
0.084 1
Cd
0.177 −0.181 1
Hg
−0.045 −0.400 0.552** 1
Cr
−0.176 0.083 0.223 0.051 1
Fe
−0.294 −0.013 0.194 0.361 0.246 1
*Correlation is significant at
p
< 0.01 (2-tailed); **correlation is significant at
p
< 0.05
(2-tailed).
= 0.686,
p
< 0.01) and Cd – Hg in sediments (r = 0.552,
p
< 0.05) indicating that
Hg may have similar sources, similar geochemical processes or similar pathways
with As in soils and Cd in sediments within the Isanga mine and its nearby en-
vironment. The Hg – Cd correlation in sediments may also indicate that Hg and
Cd behaved similarly during deposition into sediments (Nkinda et al., 2021;
Tang et al., 2021). There was also a notable but not significant positive correla-
tion for As – Pb (r = 0.471,
p
> 0.05), As – Cd (r = 0.460,
p
> 0.05), and Pb – Hg
(r = 0.488,
p
> 0.05) suggesting a similarity in input sources for As, Pb, Cd and
Hg in soil.
3.3.2. Principal Component Analysis (PCA)
Figure 3(a) and Figure 3(b) show PCA biplots for soil and sediment samples
respectively. The PCA was used to determine the relationship and variations of
soil and sediment datasets and explain possible sources of PTEs by computing
the eigenvectors and determining the major components. The eigenvalues and
extracted eigenvectors (PC1 and PC2) for both soils and sediments are presented
in Table 4.
Using the Kaiser criterion, PTEs with eigenvalues greater than 1 are selected
as principal components in this study. For soil samples, values in Table 4 with
eigenvalues greater than one accounted for 80.91% of the cumulative variance.
PC1 had a strong positive correlation with As (0.558), Pb (0.505), Hg (0.482) and
Cd (0.404) whereas PC2 had strong positive correlation with Cr (0.784) and a
negative correlation with Hg (−0.457) (Table 4 and Figure 3(a)). PC1 was, to a
larger extent, loaded with As that explained 39.92% of the total variance, while
PC2 was highly loaded with Pb that explained 22.97% of the total variance.
J. Karungamye et al.
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Protection
Figure 3. PCA biplots of PTEs in soils from Isanga gold mine for soil (a) and for sedi-
ments (b).
Similarly for sediment samples, PC1 was highly loaded with As that explained
33.10% of the total variance while PC2 was loaded with Pb that explained 22.69%
of the overall variations. PC1 and PC2 with eigenvalues greater than 1 for sedi-
ment samples accounted for 73.88% of the cumulative variance. PC1 of sediment
samples had a strong positive correlation with Hg (0.600) and Cd (0.514), whereas
J. Karungamye et al.
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Journal of Geoscience and Environment Protection
Table 4. Eigenvalues and PCA analysis results of PTEs in soil and sediments.
Element Eigenvalue
Percentage of
Variance (%)
Cumulative
(%) PC1 PC2
Soil
As 2.395 39.92 39.92 0.558 −0.216
Pb 1.378 22.97 62.89 0.505 0.217
Cd 1.081 18.02 80.91 0.404 0.217
Hg 0.745 12.41 93.32 0.482 −0.457
Cr 0.237 3.94 97.27 0.154 0.784
Fe 0.164 2.73 100.00 −0.117 −0.190
Sediment
As 1.986 33.10 33.10 −0.137 −0.607
Pb 1.362 22.69 55.79 −0.334 0.331
Cd 1.086 18.09 73.88 0.514 −0.278
Hg 0.754 12.56 86.45 0.600 −0.218
Cr 0.488 8.13 94.57 0.251 0.462
Fe 0.326 5.43 100.00 0.429 0.430
PC2 had strong negative correlation with As (−0.607) and a positive correlation
with Cr (0.462) (Table 4 and Figure 3(b)).
From these results, it is possible to conclude that Cr, As, Pb and Hg concen-
trations in soil as well as Hg, Cd and Cr concentrations in sediments could be
associated with gold processing activities (Tang et al., 2021) especially irres-
ponsible storage and management of tailings, process effluent waters and waste
rock in the study area. The negative correlation of Hg in soil can also be related
to a source different from other PTEs which may be its use in gold amalgama-
tion activities. As in sediments may have a different loading mechanism com-
pared to other PTEs.
3.4. Pollution Assessment of PTEs in Soil and Sediments
Based on the average concentrations of PTEs in the continental crust as esti-
mated by Wedepohl (1995) and the background concentration of PTEs in sedi-
ments derived from the control sample collected upstream of the Manonga river,
several pollution indices for soil and sediments respectively were computed. The
indices are the
Igeo
,
Ef
,
Cf
, Cd, PLI and PERI as indicated in Table 5.
The
Igeo
for the assessed PTEs was in order Cd > Hg > Fe > Cr > Pb > As for
soil and Cd > Fe > As > Cr > Pb > Hg for sediments. The
Igeo
was less than 0 for
As, Pb, and Cr in both surface soils and sediments indicating no pollution by
these PTEs. Although the
Igeo
was less than 0 for Hg in sediments indicating no
pollution, it was 0.019 for soil indicating slight pollution of soil by Hg. The
Igeo
for Cd was greater than zero (0.749 for soil and 0.968 for sediments), indicating
no to moderate Cd pollution in both soil and sediments. All of the examined
PTEs showed an
Ef
less than 2, suggesting depletion to little anthropogenic
enrichment, with the exception of Cd, whose
Ef
was slightly above 2 in both soil
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10.4236/gep.2023.1111003 55 Journal of Geoscience and Environment
Protection
Table 5. Computed pollution indices in soil and sediments.
Geochemical index
Compartment
As
Pb
Cd
Hg
Cr
Fe
Geo Accumulation
Index
Soil −2.891 −2.687 0.749 0.019 −0.936 −0.336
Sediment −0.291 −0.674 0.968 −2.856 −0.579 −0.263
Enrichment Factor Soil 0.170 0.196 2.121 1.279 0.660 1.000
Sediment 0.980 0.752 2.347 0.138 0.803 1.000
Contamination
Factor
Soil 0.202 0.233 2.520 1.520 0.784 1.188
Sediment 1.226 0.940 2.935 0.138 1.004 1.250
Risk Factor Soil 2.022 1.165 75.607 60.804 1.569 1.188
Sediment 12.258 4.702 88.038 5.524 2.008 1.250
Degree of
contamination
Soil 6.448
Sediment 7.493
PLI Soil 0.743
Sediment 0.915
PERI Soil 142.353
Sediment 113.781
and sediments (2.121 for soil and 2.347 for sediment), indicating moderate
anthropogenic enrichment of the PTE. In contrast to As and Cr, which both had
contamination factors in soil below one (0.202 and 0.784, respectively), Hg had a
contamination factor below one (0.138) in sediments, indicating a low contami-
nation factor, and above one (1.520) in soil, indicating a moderate contamina-
tion factor. Cd had soil and sediments contamination factors above 1 that were
moderate (2.520 and 2.935, respectively), while Pb showed low contamination
factors of 0.233 and 0.940 in soil and sediments respectively. The contamination
degree of the study area was 6.448 for soil and 7.493 for sediments, indicating a
low soil contamination degree and a moderate sediment contamination degree.
Aside from Cd and Hg, all other PTEs tested had an
Ei
< 40, indicating a low
ecological risk in both soil and sediments. Cd had an
Ei
of 75.607 for soil, sug-
gesting a moderate ecological risk, and an
Ei
of 88.038 for sediments, indicating a
significant ecological risk. Hg posed a moderate ecological danger in soil (
Ei
=
60.804) but a low ecological risk in sediments (
Ei
= 5.524). Although Hg had a
low ecological risk in sediments, it had high concentrations of up to 0.41 mg/kg
in samples obtained from amalgamation bays, which is greater than threshold
concentrations set in the TEL, ERL, and LEL sediment quality guidelines (0.17,
0.15, and 0.2 mg/kg, respectively) implying that it can induce biotic impacts. The
PLI values for both soil and sediments were less than 1, indicating that the site is
not considerably polluted by the assessed PTEs. The PERI was less than 150
(142.353 for soil and 113.78 for sediments), indicating that the study area poses
little ecological risk.
J. Karungamye et al.
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Journal of Geoscience and Environment Protection
4. Conclusions and Recommendations
The present study investigated the extent of PTEs pollution for Cr, Cd, Pb, Hg,
and As in soils and stream sediments of the Isanga small scale gold mine situated
in the area of the now closed and remediated Resolute gold mine, in Nzega,
Tanzania. Hg concentrations in the study area were higher than average concen-
trations in the continental crust and control samples, implying that such pollu-
tion may be caused by Hg use and disposal practices during gold amalgamation
processes.
The soil contamination in the study area was low, and the sediment contami-
nation was moderate. Cd was identified as a significant contaminant in both the
soil and sediment compartments, with contamination factors of 2.520 and 2.935,
respectively. Except for Cd, which showed moderate enrichment and was most
likely derived from mining activities, the other PTEs in soil and sediments had
no significant anthropogenic enrichment. Despite the moderate enrichment of
Cd in both soil and sediments, as well as the moderate degree of sediment con-
tamination in the study area, the PTEs in the study area were found to have a
low potential to cause ecological risks. However, if proactive measures are not
taken to control mining and waste disposal operations, the levels of the assessed
PTEs in the study area will certainly continue to rise over time and finally result
in a high ecological risk.
The current study suggests that proper run-of-mine and mine water and waste
management controls should be implemented to reduce environmental pollu-
tion caused by PTEs. Mine waste management plans that allow for the manage-
ment of wastes and tailings in a linear fashion from production to disposal, the
construction of lined tailing storage facilities suitable for small scale mining op-
erations, and eventually in situ closure of mines or waste storage facilities, could
be the most effective controls as per the observations made in this study. Addi-
tionally, a routine environmental audit of the mine environment is necessary to
make sure that conducts that cause pollution are avoided and under control.
Small-scale gold miners in Tanzania are characterized by inadequate technical
knowledge of mining and mineral processing operations and improper waste
management controls. Therefore, the government should step up routine sensi-
tization and awareness raising initiatives on mine water and mine waste man-
agement in small scale mining operations.
This study successfully demonstrated that multivariate statistical approaches
such as Pearson’s Correlation and Principal Component analyses of geochemical
data are useful and effective techniques for determining elemental composition
relationships in both soil and sediment compartments. Such statistical tools ena-
ble the identification and prediction of likely causes and sources of pollution in a
specific area. However, the study didn’t apportion the exact origins of PTEs
identified to have the potential to cause ecological risks in the study area. Such
apportionment was out of scope of this study, but future research may take it
into account and broaden the study’s coverage of the Lake Victoria Greenstone
J. Karungamye et al.
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10.4236/gep.2023.1111003 57 Journal of Geoscience and Environment
Protection
Belt, which is known to be home to hundreds of small-scale gold mining opera-
tions.
Acknowledgements
The authors gratefully acknowledge the scholarship granted to Johnbosco Ka-
rungamye by the Ministry of Minerals of Tanzania that funded this study. Au-
thors also thank the Arusha Technical College (ATC) laboratory technicians for
their help and suggestions during the analytical procedure.
Conflicts of Interest
The authors declare no conflicts of interest regarding the publication of this pa-
per.
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