Description
TABLE OF CONTENTS
Title Page
Certification i
Dedication ii
Acknowledgements
iii
Table of Contents iv
List of Tables
vi
List of Figures vii
List of Plates viii
Abstract ix
CHAPTER 1: INTRODUCTION
1.1 Preamble
1.2 Statement of the Problem
1.3 Area of Study
1.4 Location and Accessibility
1.5 Geomorphology and Soil Type
1.6 Surface and Groundwater Resources
1.7 Objectives of the Study
1.8 Literature Review
CHAPTER 2: GEOLOGIC SETTING
2.1 Tectonic and Stratigraphic Setting
2.2. The Asu River Group
2.3 The Abakaliki Shale
2.4 Mineralization of the Study Area
CHAPTER 3: METHOD OF INVESTIGATION
3.1 Field Mapping and Sample Collection
3.2 Laboratory Analyses of Samples
CHAPTER 4: RESULTS OF THE INVESTIGATION
4.1 Hydro-Chemical Results
4.1.1 Microbial Contaminants
4.1.2 Heavy Metal Contaminants
4.2 Geo-Chemical Results
4.2.1 Soil Samples
4.2.2 Mine-pit Samples
4.2.3 Mine-Waste Rocks
CHAPTER 5: IMPLICATIONS OF RESULTS AND
ENVIRONMENTAL/HEALTH CONCERNS
5.1 Implications of the Hydro-Chemical Results
5.2 Implications of the Soil Geo-Chemical Results
5.3 Environmental/Health Concerns
5.3.1 Extraction Wastes
5.3.2 Mine Pits
5.3.3 Human Exposure
CHAPTER 6: CONCLUSIONS AND RECOMMENDATIONS
6.1 Conclusions
6.2 Recommendations
References
LIST OF TABLES
1. WHO Regulatory limits of heavy metals on soils
2. WHO Guidelines for drinking water quality
3. Cretaceous sequence in southern Nigeria
4. List of microbial contaminants isolated from water samples
5. Heavy metal constituents in water samples
6. Chemical composition of soil samples
7. Chemical composition of mine pit sediments
8. Chemical composition of Ameri waste rock composite
9. Chemical composition of Enyigba waste rock composite
10. Chemical characteristics of mine water sample
11. Chemical constituents of soil samples from farmlands
12. Chemical characteristics of water samples from abandoned mine-pits
LIST OF FIGURES
Figure 1: Study area in relation to the Benue Trough
Inset (Nigeria showing the location of Benue Trough)
Figure 2: Geological map of the study area
Figure 3: Accessibility map of the study area showing lead-zinc mines
Figure 4: Map showing locations where soil samples were collected
Figure 5: Map showing locations where water samples were collected
vii.
LIST OF PLATES
Plate 1: Highly weathered clay soil showing yellow colour
Plate 2: The largest pond within the study area
Plate 3: Shale outcrop exposed at „Okpuru Nkume‟
Plate 4: Overburden soil layers and underlying shale units exposed in a mine pit
Plate 5: Collection of sample at Akpara River
Plate 6: Mine waste rock piles at mine sites
Plate 7: Acid Mine Drainage on mine waste rocks
Plate 8: Mine water being pumped out of mine-pit into farmlands
Plate 9: Accumulation salt crystals within the mine area
Plate 10: Iron-deficient sweet potato plants
Plate 11: Unaffected sweet potato plants
Plate 12: Heaps of withered sweet potato plants
Plate 13: Mine sites as erosion sites during rainy season
Plate 14: A 30ft mine-pit filled with water after mine operation
Plate 15: Children engaged in mining activities at the mine site
Abstract
The extraction and processing of lead-zinc ore deposits involve large amounts of
waste materials. These mining wastes contain heavy metals which are potentially
toxic to human life and the environment. Heavy metals such as Cd, Cu, Ni, and
Zn are all potential soil contaminants and excess accumulation of these metals in
soil is toxic to human and other animals. Potentially contaminated soil may occur
at areas in or around mining waste rock piles.
This study assessed the environmental effects of lead-zinc mining at Enyigba to
document the present state of the environment in areas affected by the mining
operations. The study examined the extent to which heavy metals from
abandoned mine pits and mine waste rocks have polluted the soil as well as the
ground water and surface water. The occurrence of heavy metals in the soils,
streams, lakes, rivers, hand dug wells and boreholes within Enyigba and the
environs was investigated to determine the level of contamination in the area.
Samples were collected from several points to capture the variability of heavy
metals in various locations. The analysis focused on twelve (12) metals (Pb, Zn,
Cu, Cd, Ag, As, Ni, Mo, Sb, Hg, Se, and Te).
The heavy metals concentrations in soil were measured at surface and near
surface depth and were found to be highest in the soil around the mine areas.
The soil sample collected from a farmland close to the waste rock dumps at
Ameri had concentrations of lead and arsenic up to the values of 1.9% and 0.8%
respectively. The probable source of these metals is dust and small particles of
mine waste rock spread locally at the mines. Cadmium, zinc, copper, and nickel
were detected in some other soil samples collected close to the mines, the
highest concentration of each of the metals being present in the clay–rich soils.
The concentrations of these metals were found to be below dangerous
thresholds for plants and animals although contamination of soil with heavy
metals such as lead, arsenic or cadmium is considered a serious environmental
concern as these elements can persist in soil for a long time with chances of
increased concentration.
Statistically, insignificant concentrations (below WHO regulatory standard for
drinking water) of zinc, silver and selenium were also noted in few water samples
tested in the study area. ix.
CHAPTER ONE
1.0 INTRODUCTION
Heavy metals occur naturally in the environment, but may also be introduced as a
result of land use activities. Mining industry throughout history has seriously
caused immense environmental pollution in industrialized and developing
countries. According to the Environmental Data Report of the United Nations
Environmental Programme (UNEP, 1993), there is an increasing awareness that
the health of populations is at risk from pollution hazards. Mining generates 2.7
billion tons of waste, in part hazardous, an amount which far exceeds the world‟s
total accumulated municipal garbage. Mining activities are responsible for an
increase in certain heavy metals in soils, sediments and groundwater reserves
occurring within the influence sphere of the mine (Aucamp, 2003).
According to Brandy, et.al., (1999), the soil is a multi-component system
consisting of solid, liquid and gaseous phases as well as living organisms. Soil is
dynamic and sensitive to almost every aspect of its surroundings and represents
a chemical system where mineral transformation occurs continuously. Thus,
when environmental changes occur, such as mining activity, the soil responds.
Any such change produces a gradual alteration of soil characteristics until a new
balance is reached. Naturally occurring as well as anthropogenically introduced
concentrations of metals in near-surface soil can vary significantly due to different
physical and chemical processes operating within soils geographic regions.
Amongst the range of contaminants that may be found in soils, potentially toxic
elements or heavy metals are of particular interest for a number of reasons.
Firstly, they show a tendency, under normal circumstances, to accumulate in
soils and have a long persistence time because of the interactions with particular
soil components. It is thus possible that environmental change may lead to
conditions, which could result in dramatic releases of stored metals, the so-called
„chemical time-bomb‟ effect. Secondly, they are ubiquitous in soils and arise
from both natural and anthropogenic sources, with pathways including
inheritance from the parent rocks, application of wastes, as well as local and
long-range atmospheric and fluvial deposition of emissions from industry and
mining. Also, many of the heavy metals are essential at low concentrations for
plant, animal and human health but at higher concentrations, they can be toxic.
Toxic metals are being released to the environment in increasing amounts and
they are daily ingested by humans, either through the air or through food, water
and soil. Unfortunately heavy metals are not subject to chemical degradation.
Once the elements become part of this cycle, they may accumulate in animal and
human body tissue to toxic levels. Exposure to heavy metals is normally chronic
(exposure over a long period of time), due to food chain transfer. Acute
(immediate) poisoning from heavy metals is rare through ingestion or dermal
contact, but possible. This situation is especially critical for fish and other wildlife
and humans at the top of the food chain. The most common problem causing
metals are mercury, cadmium, lead, nickel, copper, zinc, chromium, manganese,
arsenic, molybdenum, selenium, and boron. There are screening levels or
regulatory limits for metal concentration in soils set to protect wildlife. Also,
standards exist for protection of aquatic life from metal toxicity in surface waters.
Table 1, shows the World Health Organization (WHO) regulatory limits of heavy
metals on soils while Table 2 shows the water quality criteria for heavy metals in
fresh water. Exceeding these concentrations (ug/l) is unacceptable for protection
of natural resources and human health.
In spite of these regulations, there is still uncertainty as to the nature and extent
of heavy metal pollution in mine areas, particularly the soils on which crops are
grown. Thus, there is little information available on the level of heavy metals in
the surface or near-surface soil in areas associated with mining activities in
Nigeria. A detailed study of lead-zinc mine areas could uncover the inherent
danger posed to the communities where these mining activities occur. Heavy
metal pollution could constitute serious environmental and health hazards in such
communities within our country. This study focuses on the lead-zinc mining
activities in Enyigba, southeastern Nigeria and their effect on the environment
with emphasis on heavy metal concentration. The importance of the study lies on
the fact that although the degradation of soil quality by pollution may be localized,
the environmental impacts are usually quite large. It is believed that
contamination of the soil can pose a significant threat to human health and
aquatic life.
1.2 Statement of the Problem
The environmental consequences of open-pit mining are several. As ore deposits
consist of relatively small percentages of economically valuable minerals, their
extraction and processing involves large amounts of waste materials. The
extraction process entails the removal of large amounts of materials which
become waste products. These materials are both the overburden that covers
the deposits and wastes from the ore that are produced in the extraction and
separation of the ore. The lead/zinc mining at Enyigba was by the old open-pit
mining technique which meant that once the mine became uneconomic, it was
abandoned with little or no attempt to rehabilitate the land. Thus, there was total
destruction of the vegetation of mined areas, and considerable reshaping of the
natural topography. These activities left several heaps of abandoned lead-zinc
mine waste rocks at various mine sites in Enyigba. Also, mining activity brought
to the surface large quantities of minerals that are unstable in the weathering
environment. The mine itself disrupts the landscape, and the increased surface
area of the broken and crushed rocks from mining and extraction set the stage
for erosion and leaching of metals to the environment.
Recently, the Ebonyi State Government in collaboration with UNIDO carried out
analyses of saline water in ponds, lakes and springs within the state. It was
discovered that most of the brine occurrences in the area are contaminated with
heavy metals particularly, lead (Pb). The Ishiagu brine was noted to have the
highest concentration of Pb.
Akwaokuko and Lefin to the north-east of the town. The Enyigba, Ameri-Ameka
deposits which are the best known deposits were first mined in 1925. These
deposits have constituted an important source of lead-zinc since the 1940‟s.
During the 1920‟s, a British Mining Company mined the Ameri and Enyigba
deposits by open cast and underground mining respectively, although it is known
that long before the coming of the British to Nigeria, the natives worked the
Abakaliki lodes near the surface. Further down in depth, galena became an
object of search for use as a cosmetic. The natives of the area used hand-picked,
finely ground cubic galena as a cosmetic (Bogue, 1952). They also used lead
smelted from galena as fishing net weights. The deposits were also exploited by
a German Mining Company before the Nigerian civil war. The company
employed open cast mining and the galena and sphalerite were beneficiated at
the site by differential floatation using xanthate collector. However, mining
ceased at the onset of the civil war and the natives shifted their interest on salt
production from the brine pools associated with the lead-zinc deposits. In recent
years, however, mining has continued intermittently and many firms have
renewed interests in operations within the area. This has resulted to abandoned
open pits and several heaps of leadzinc mine waste rocks, which have
considerably reshaped the natural topography of the area.
The study area in relation to the Benue Trough is indicated in fig.1, while fig. 2
shows the geological map of the study area. The area is bounded by Latitude 6 o
10 / N – 6 o 13 / N and Longitude 8 o 07 / E – 8 o 10 / E and covers a surface area of
33.06km 2 . It falls within the lower region of the Benue Trough. Deposits of galena
(PbS) and sphalerite (ZnS) occur as veins and lodes in the oldest exposed
sequence (the Asu River Group) of the Abakaliki Basin within which are the
villages of Enyigba, Amagu, Alibaluhu and Ameri in Ikwo LGA are located. The
villages are about 14 kilometers south of Abakaliki , the capital city of Ebonyi
State, in the southeastern part of Nigeria. The routes in the area consist mainly of
footpaths and narrow road networks. Fig. 3 is the accessibility map of the study
area showing lead-zinc mines.
The most prominent topographical features in the study area are ridges and flat
lands which are underlain by the shales of Asu River Group. These ridges and
flat lands host the lead–zinc veins and form watersheds from which many springs
and streams are generated. The streams generally flow along narrow valleys
beneath the escarpment. Some of the saline springs are found along stream
valleys and are consequently flooded during the rainy season.
The principal soil forming process in the area is by rock weathering hence
climatic factors influence the rate and depth of weathering and soil formation. The
soil is derived mainly from the old sedimentary rocks. The organic matter content
of the soil, which is important to their productivity and structural stability, varies
with the natural and density of the vegetation cover. There are different types of
soil in the area, namely: lateritic clay soil, sandy soil with mudstone and clayey
shaly soil. The lateritic clay soil supports most of the important cash and food
crops. The soil becomes increasingly clayey especially down the profile because
of intense leaching and eluviations from above. The productivity of the sandy soil
depends on the supply of litter by the vegetation. The clayey soil has a higher
fertility than the sandy soil, which suffers from rapid internal drainage and high
susceptibility to erosion once the vegetation cover is removed. The clay
enhances runoff, thus making the area dominated by sheet erosion. The
tributaries which develop into rectangular drainage patterns are closely spaced
because local soils and bedrocks have poor resistance to erosion. Runoff
response is so quick that much of the precipitation is lost by direct runoff, surface
retention and overland flow.
The clayey shaly soil is seasonally or permanently waterlogged and occurs in
pockets within the valleys and flood plains. The clay soils are more highly
weathered and their subsurface horizons are commonly red or yellow in colour
(Plate 1). This material is soft and can be dug easily from the profile so long as it
remains moist. When dried, it hardens into a kind of ironstone that is virtually
useless for cultivation.
The main potential sources of surface water supply to the area are the Ebonyi
River and its tributaries. The Ebonyi River is large and drains the entire land area
including the Akpara River. These surface water bodies lie within the lowland
dominated by shale. Only the Ebonyi River in the area is perennial. This might be
attributed to the perennial nature of the streams that flow into the river. For
example, the Ameri stream which upfluxes from a spring source is a major
tributary of the Ebonyi River. Most of the streams overflow their banks during
rainy season whereas the discharge through the Ebonyi River and its tributaries
are drastically reduced during the dry season resulting from limited water supply,
evaporation and infiltration. During extreme dry periods, some of the rivers also
dry up completely as witnessed in the month of April when the field investigation
was conducted. Almost all the streams as well as the Akpara River are
ephemeral. The Akpara River had completely dried up except for the point where
the river took a U-turn along its course and an oxbow lake was formed. Most of
its tributaries are seasonal and they also dry up during the dry season.
There are many springs in the area, which usually issue out from lithologic
boundaries or from fractures. Some of the springs also issue out due to fracture
intersections at aquiferous zones. They occur in areas underlain by the thin fine
to medium grained sandstone units within the formation, especially at the
intersection of the sandstone and the less permeable underlying shales. Some of
these springs are saline while others are not. Many of the springs within the area
are seasonal while others are perennial and in the mine areas the water is
continuously drained out to avoid flooding of the mines. The occurrence of the
springs might be due to juxtaposition of a permeable rock mass with an
impermeable one, a situation that makes the movement of water through the rock
deflective as the impermeable materials cause it to upflux at the ground surface
as springs.
There are several ponds in the study area. shows the one at Enyim-Agalagu
village in Ameri. This pond is the largest of them all and overflows its “banks”
during the rainy season. Most of these ponds have point sources and their water
level fluctuates as the seasons change from rainy to dry or vice versa. In many of
the ponds, tadpoles and toads were seen swimming freely but no fishes were
seen. Report, however, has it that fishes abound in these ponds during rainy
season but as soon the dry season is approaching they disappear into the mud
or migrate to other places.
The general geology of the area controls the occurrence and flow of groundwater.
Evaluation of the groundwater resources within the area was not carried out due
to lack of drilling data. During the field investigation, most of the boreholes had
been completed and hydro-geologic data could not be obtained. A total of eight
(8) boreholes were visited but water samples were collected from only five (5)
because some of the boreholes had completely dried up. Three hand dug wells
owned by individuals were encountered within the area and their water level
elevations were estimated. Water samples were collected from two hand dug
wells for hydro-chemical analyses and the depths of the wells were given as 35ft
and 40ft respectively. These wells form additional source of water for the rural
communities beside the ponds available in the area since the boreholes are
situated several kilometers away from some of the inhabitants of the area.
1.7: Objectives of the Study
The primary objective of the study was to determine if there are statistically
significant differences between the metal concentrations in surface and near-
surface soils as well as surface water and groundwater in areas affected by
mining and in areas not affected by mining. To achieve this aim, it was
necessary to investigate the distribution of the heavy metal concentrations in the
lead-zinc mine waste rocks as well as the soils and waters affected by
abandoned waste rocks and minepits. Using the data, it was possible to:
• Assess the degree of heavy metal pollution of soils, surface water and
groundwater;
• Determine the potential environmental effects associated with mining other
lead-zinc deposits in similar physiographic and geologic settings.
• Provide information about the natural background concentrations in control
sampled
areas.