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Environmental Geochemistry of Mine Tailings Soils Paper

Words: 3120, Paragraphs: 184, Pages: 11

Paper type: Essay , Subject: Chemistry

Changes in trace element concentrations in mine tailings (dry and wet) were investigated in the gold mining area

of B?tar?-Oya, Eastern Cameroon. Forty-one surface sediment samples were analyzed using ICP-MS for heavy

metals and pollution was assessed using Enrichment Ratio and Geo Accumulation Index (Igeo); using a sample

from a remote area as control.Trace elements in mine tailings show significant increase compared to the

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background (control) values, with the exception of Sr and Nb. It is suggested that allochtonous deposition may

account for Sr distribution. A strong correlation is observed between the lithophile elements; Y, Nb, Ce, La, and

Pb. Their association with each other is also strong, i.e. Nb/Ce (.73) and La/Ce (.63). The close association of

these elements in the samples may be an indication of the effect of resistant heavy minerals from felsic rocks

(mainly granites and gneisses). Cu, Zn, Sr and Ba distribution is associated with mineralisation of

sulphide-bearing minerals and clay formation from weathering of mica schist. Cu, Zn, As, W, Mo and Ag have

been identified as potential pollutants. Compared to the Dutch soil quality guidelines, these elements are above

the target values and below the intervention levels. Arsenic is considered to be the most threatening element,

with regards to its potency. The distribution of As in this area appears to be controlled by mining activities.

Keywords: gold mining, trace elements, tailings, Cameroon

1. Introduction

1.1 Background

Environmental impacts due to mining are commonly associated with large-scale mechanised activities. However,

several studies are reporting that artisanal, informal mining also gives rise to environmental problems (e.g.

Tarras-Wahlberg et al., 2000; Hilson, 2002; Babut et. al., 2003; Waziri, 2014) and that the consequences could be

more serious due to higher levels of exposure, in addition to the fact because these non-formal operations are

hardly regulated by government agencies, their impact on the environment may go completely unnoticed.

During mining, a fine grind of the ore is often necessary to release metals and minerals, so the mining industry

produces enormous quantities of fine rock particles, in sizes ranging from sand-size down to as low as a few

microns (USEPA, 1994). These fine-grained wastes are known as “”tailings””. By far, the larger proportion of ore

mined in most industry sectors ultimately becomes tailings that must be disposed of. In the gold industry, only a

few hundredths of an ounce of gold may be produced for every ton of dry tailings generated (USEPA, 1994).

These substances can be easily dispersed into the surrounding areas and can be carried off by processes of

weathering into waterbodies. Tailings constitute a major source of pollution due to its potentially high chemical

reactivity resulting from its large surface area. The potential for elements present in soils and sediments to be

mobilised/immobilised and redistributed depends on several soil factors such as organic matter, type and amount

of clay, pH, redox conditions and cation exchange capacity (CEC). Under the right geochemical conditions such

as the pH and the prevailing redox conditions; and pathways, these elements can easily be mobilized and

transmitted through for example, water and the food chain to humans. Previous researchers have highlighted the

potentially negative environmental impacts of these processes on biodiversity, ecosystem function and ecosystem

services (Malaviya, Munsi, Oinam & Joshi, 2010; Simmons et al. 2008). Amonoo-Neizer, Nyamah, & Bakiamoh

(1995) reported the significant distribution of As and Hg in the top soils, plantain, water fern, elephant grass,

ep.ccsenet. o

cassava a n

element d i

1.2 Locati o

B?tar?-O y

Republic (

south and

comprisin

g

influence o

October) w

Humidity

the extre

m

24.7°C.

The B?tar

?

Lom rive r

constitute

and Fofo,

uniform r

e

The vege t

progressi v

still intact .

1.3 Geolo g

The regi o

metamorp h

Pan-Afric a

and shale s

foliation r e

feldspars [

veins whi c

org

nd mud fish a

istribution an d

on, Climatic S

ya (Figure 1) i s

(CAR) at 5°35

d the savanna h

g largely of m

of a tropical, h

with abunda n

and cloud co v

me east and n o

?-Oya gold di

rs.These rive r

a significant p

which drain i n

esistance to er o

tation is dom i

vely thicker to w

.

gic Setting an d

onal geology i

hosed under l o

an granitoids.

s (Freyssinet, L

esulting from

[(K,Ca,Na)(A l

ch are confin e

at Obuasi, (G h

d contaminatio n

Setting and Dr a

s located on t h

?59? N, and 1 4

h to the nort h

monotonous, ri d

humid type A

nt rainfall an d

ver are relativ e

orth regions w h

strict has sev e

rs flow all y e

part of the dra i

nto the Lom. T

osion, which f

inantly of pr i

wards the sout h

d Gold Miner a

is dominated

ow- to mediu m

The Lom Se r

Lecomte, & E

a regional rig h

l,Si) 3O8], biot i

ed in brecciate d

Environ m

hana) and its e

n in mine taili n

ainage

he south easter n

4°04?59? E. It l

h. It is found

dges, deep riv e

wet Equatori a

d a short dry

ely high and p r

here it is slig h

Figure 1. M

eral river syst e

ear round. Ot h

inage system e

The prevalenc e

favours dendri t

imary nature c

h. The land ar

alziation

by the Neo

p

m-grade condi t

ries is compri s

Edimo, 1989).

ht-lateral tran s

ite and remna n

d and locally

ment and Pollut i

53

environs. The

ng from artisa n

n edge of the A

lies in the tra n

at an elevat i

er valleys and

al climate wit h

season (Oct o

recipitation ra n

htly less. Aver a

Map of the stu d

ems that trave r

her secondar y

e.g. the Mari R

e of metasedi m

tic configurati o

comprising o f

ea is sparsely

proterozoic v o

tions (Suh, L e

sed of biotite

These rocks

a

spressive defo r

nts of amphi b

silicified met a

ion

aim of this w

nal gold mini n

Adamawa pla t

nsition zone b e

ion of 779m

gently undul

a

h two seasons ;

ober to mid- M

nges from 15 0

age annual te m

dy area

rse the plains:

y streams tog e

River, Ngueng u

mentary rocks i

on of the indi v

f low land tr o

cultivated an d

olcano-sedime n

ehmann, & M a

schists, serici t

are characteri z

rmation. The g

boles and pyr o

avolcanic and

work is there f

ng in Betare O y

teau, close to t

etween the eq u

asl with a r e

ating plains.T h

; the wet rain y

March) with a

00 to 2000m m

mperature ran g

the Nyong, K

ether with th e

ue River, Mbi g

in the study a r

vidual stream c

opical rain f o

d most of the n

ntary rocks o

afan y, 2006) a n

te and chlorit e

zed by a NE- S

granites are ri c

oxenes. Gold i

metasediment

Vol. 6, No. 1;

fore to assess

ya, Cameroon .

the Central A f

uatorial forest t

egional topog r

his area is und e

y season (Ma r

abundant sun s

m per year exc e

ge from 22.8 °

Kadei, Boumb e

eir tributaries

gala, Lisso,

M

rea offers rela t

courses in the

orest, which g

natural vegeta ti

of the Lom g r

nd c ross-cut b

e schists, quar t

SW steeply di p

ch in quartz ( S

is hosted in q

ary rocks, int r

2017

trace

frican

to the

raphy

er the

ch to

shine.

ept in

°C to

e and

also

Mbaya

tively

area.

grows

ion is

roup,

y the

tzites

pping

SiO2),

uartz

ruded

ep.ccsenet.org Environment and Pollution Vol. 6, No. 1; 2017

54

by post-tectonic mafic and felsic magmas of unknown age (Suh et al., 2006). Gold occurs as small particles

disseminated within the quartz matrix and along microfractures within the veins. These veins have as much as

54ppm gold. The quartz matrix is associated with sphalerite (ZnS), pyrite (FeS), chalcopyrite (CuFeS

2),

arsenopyrite [(As,Fe)S], galena (PbS) and barite (BaSO

4) and rare pyrrhotite. Freyssinet et al. (1989) identified

Au-Mo-W-Pb-Bi element association in the host rock and overburden as pathfinder elements for gold in the Lom

area.Other ore minerals produced by weathering in the quartz veins are hematite (Fe

2O3), covellite and chalcocite

(complex hydroxides of Cu and Fe), and psilomelane (Mn hydroxides) (Suh et al., 2006). Although Vairon (1986)

attributed a Proterozoic age (2100–2200 Ma, Birrimian) to the Lom series, recent data (Soba et al. 1991) provide

evidence of inherited Archaean ages (c. 2500 Ma) and Neoproterozoic ages (700–1100 Ma) on zircons from

detrital material and Neoproterozoic ages (c. 700 Ma) from metavolcanic rocks in this district. The soil in the

area is predominantly acrisols, alisols, plinthosols, acid soil with clay-enriched lower horizon and low saturation

of bases.

2. Method

2.1 Sampling Analysis

Field work was done in the month of February. Sampling was random but purposive (only tailings) over the

entire region of Mari artisanal gold mining site. Forty (40) samples were collected comprising of thirty (30) dry

tailing samples from the tailing pile and ten (10) wet tailing samples from both active and inactive mining sites.

A control sample was collected about 50km from B?tar?-Oya town at Ndokayo; a location remote from the

mining area in the outskirts of B?tar?-Oya and far removed from the influence of mining activities. Forty one

soil samples were shipped to ACME Analytical Laboratory, Canada for chemical analysis. The samples (0.5g)

were leached in hot (95°C) Aqua Regia (HNO

3/HCl) for 2 hours. The digested sampleswere then cooled and

diluted with deionised water and analyzed for their trace metal content by Inductively Coupled Plasma Mass

Spectrometry (ICP-MS). Spiked duplicate samples were used to determine precision and bias. In order to allow

for data analysis, concentrations below LOQ (limits of quantification) were replaced with half the LOQ for some

of the trace elements (USEPA, 2000; Waziri, 2014). While those data points may not be very reliable, this is

probably a better alternative to assigning zero concentration or removing the affected samples from the database

(Waziri, 2014).

2.2 Data Treatment Proceduress

Two pollution indices were used to assess the levels of contamination of the sediments: Enrichment Ratio (ER)

and Geoaccumulation index, Igeo:

Enrichment Ratio (ER): Enrichment ratios, adopted from Albright, (2004) were calculated in order to assess the

extent of enrichment and /or depletion of trace elements in the soils of the two study areas relative to their crustal

concentrations. In this case, the control value was used as the background concentration (1).

ER= C

m/Bm (1)

Where:

C

m is the concentration of an element measured in a sample and B m is the background or baseline concentration,

in this case the control value.

Geoaccumulation Index: The index of geoaccumulation (Igeo) actually enables the assessment of contamination

by comparing the current and pre-industrial concentrations. Originally used with bottom sediments by Muller

(1969), Igeo has widely been applied in the assessment of soil contamination (Likuku, Mmolawa & Gaboutloeloe,

2013). The geoaccumulation index (Igeo) is expressed as follows (Singh, 2001) (2):

+

? ? ? = H K C 6 @ ?

? 5. 9 ? ?

? A (2)

Where:

Cn = measured metal concentration at sampling point; Bn = background concentration value for element; 1.5 = the

background matrix correction factor. The factor 1.5 is introduced to minimize the effect of possible variations in

the background values which may be attributed to lithologic variations in soils.

The geoaccumulation index assess contamination based on seven grades (0 – 6) ranging from unpolluted to very

highly polluted. These seven descriptive classes are as follows: 5 = very

ep.ccsenet.org Environment and Pollution Vol. 6, No. 1; 2017

55

highly/strongly contaminated.

3. Results and Discussion

3.1

Trace Element Concentrations

Table 1 is a summary statistics of chemical analysis of tailings samples (dry and sediment) and comparative

concentrations of global crustal averages and trigger values. Gold has the lowest mean concentration followed by

Cd and W, whereas Ba and Zr have the highest mean concentrations (579ppm and 651ppm respectively), similar

observations were recorded in the Birim-Gwari (Nigeria) gold mine (Waziri, 2014). Ce, Zn, Sr and Nb have

average values of about 50ppm, while the least abundant is Au; for the later, as the element of interest, it is most

depleted in the tailings relative to the control soil. Low content can be as a result of low concentration in the

original parent rock, and removal during mineral extraction and weathering processes. W, Ce and Nb are

generally known to occur in trace quantities in rocks as reflected in the average continental crustal abundance.

The elements with the highest concentration, Ba and Zr display different patterns that may have a bearing on the

processes accounting for their distribution. The abundance of barite, micaschists and granatoids (Suh et al., 2006)

of the parent rock which are host to Ba possibly accounts for the high Ba content. In addition, the substitution of

Ba and Rb for K in K-feldspar and mica also serves as a source of this element. Mineralisation of feldspar will

liberate Ba which adsorbs onto solid (secondary clay mineral)Fe-(oxy)hydroxides or phosphates particularly

during the early stages of weathering (Price, Gray, Wilson, Fray, & Taylor, 1991).However, considering that

K-bearing minerals are relatively more resistant to chemical weathering (Berner and Berner, 1996), release of Ba

from them is likely to be limited. In addition, due to its lower solubility (relative to the other alkaline earth

metals) Ba in aqueous systems is likely to be retained in neo-formed clays; thereby increasing its concentration

in sediments. Mining activities seem to increase its concentration, such that it is a-par with the values of crustal

abundances. Zr content in the control sample is higher relative to crustal abundance; the former is negatively

affected in the mining area (as indicated by the test statistic), considering the mean and standard deviation. Zr is

primarily present in zircon and other heavy minerals in the parent rock.

Table 1. Descriptive statistics for heavy metals and one-sample statistic t-test with test value as Control and

comparative trigger concentrations of Continental crust and soils

This study Continental

crust Trigger &

intervention

c

Min Max Mean SD Control P-value A b

Au .05 4.2 1.10 0.95 .05 1.12E-08

Cu 6 58 27.60 12.83 1 3.5E-16 25 25 140- 720

Zn 26 111 59.00 20.59 4 7.68E-20 71 65 50-100

As 5 43 19.78 12.55 .05 1.5E12 1.5 29- 59

Sr 20 152 53.33 24.20 50

0.195 350 333

Y 16 82 39.35 12.09 20 9.07E-13

Nb 23 99 48.28 14.37 52

0.055 25 29

Mo 1 5 2.08 0.92 .05 4.13E-17 1-5

Ag 2.1 18 8.21 3.90 .05 2.75E-16

Cd 1 2 1.20 0.41 1 9.38E-21 0.8- 12

Sb 5 11 6.68 2.22 1 1.31E-05

Ba 220 990 578.95 183.66 112 4.19E-19 550 584

La 10 54 20.25 10.71 8 5.18E-09 30 30

Ce 36 164 80.85 26.07 14 3.15E-19 64 60

W 1 13 3.88 3.41 .05 7.74E-09

Pb 18 54 31.33 8.23 5 1.4E-22 20 15 85- 530

Zr 369 1143 650.85 198.43 919 8.96E-11 190 203

a: Upper continental crust, Taylor and McLennan, (1995); b: Wedepohl, (1995); c: Dutch values Bird et al., (2003)

ep.ccsenet.org Environment and Pollution Vol. 6, No. 1; 2017

56

The difference in concentration between the tailings sediment and the control is evaluated by the one sample

t-test (with the control as the test value). The derived p-value (?=0.05) of the test statistics, indicates that the null

hypothesis can be rejected (i.e. the difference between the means is less than 0) for all elements with the

exception of Sr and Nb.The statistically significant (compared to control) concentration of all elements

introduced into the surface soil of the mining area, indicates increases in elemental concentrations in the mining

area (i.e. Au, Cu, Zn, As, Y, Mo, Ag, Cd, Sb, Ba, La, Ce, W and Pb). Similar situations have been observed

worldwide, such as in the Migori Gold Belt, Kenya, wherein the concentrations of heavy metals in the surface

soil of this region, are above acceptable and background levels (Ogola, Mitullah, & Omula, 2001). Some of these

elements can even exceed critical levels.Compared to the crustal abundances, Sr content in the control sample is

lower (about three times), while Nb is almost twice as high relative to the crustal abundance. Mining has not

affected the concentrations of these two elements (following the results of the test statistic). The behavior of Nb

may be due to intense chemical weathering in both the mining and control site. Sr on the other hand may have its

concentrations affected by allochtonous depositions in the control area, or leaching from the mining area; thereby

maintaining concentrations between the two areas close/unchanged.

A comparison of the content of the tailings and published soil concentration guidelines is undertaken to assess

the toxicity of certain elements. As is an element of particular concern in gold mining areas (Carvalho, Neiva, &

Silva, 2009; Inam et al, 2011) where high releases are associated with pollution. Arsenic concentrations in the

sediments are significantly higher compared to the control sample as evident in the results of the t-test; inferring

a very strong possibility of potential environmental pollution. Gold mine tailings at Obuasi, Ghana for instance,

contain very high amounts of As, averagely 8305 mg/kg (Ahmad & Carboo, 2000). This has been linked to the

considerable level of naturally occurring arsenic at Obuasi, as well as liberations from arsenic-bearing gold ores

during gold extraction (Amonoo-Neizer et al., 1995; Ahmad & Carboo, 2000; Kumi-Boateng, 2007). The As

content in the study area is not comparable to that of Obuasi (Ghana).However compared to the soil quality

guidelines (Dutch), the values (including standard deviation) have exceeded the target value. The Dutch target

values indicate that if concentrations do not exceed this value, the site is considered clean with no

eco-toxicological risk. Zn content also falls above the target value.

3.2 Pollution Indices

The ER provides an estimate of the changes in concentrations in the examined tailings soils, relative to the

background or undisturbed soils (Table 2). Soil from the control area was used as baseline and is referred here as

control sample soil. ER of 1 indicates that the soil or sediment is neither enriched nor depleted in a particular

element relative to the soil in the control area. Enrichment and depletion are represented by ER values of >1 and

About the author

This academic paper is composed by Samuel. He studies Biological Sciences at Ohio State University. All the content of this work reflects his personal knowledge about Environmental Geochemistry of Mine Tailings Soils and can be used only as a source for writing a similar paper.

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