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a SciTechnol journal
Research Article
Orimoogunje and Ajibola-James, Geoinfor Geostat: An Overview 2013, S1
http://dx.doi.org/10.4172/2327-4581.S1-017
International Publisher of Science,
Technology and Medicine
Geoinformatics &
Geostatistics: An Overview
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Mangrove Ecosystem Recovery
and Restoration from Oil Spill
in the Niger Delta: The GIS
Perspective
Oluwagbenga OI Orimoogunje1* and Opeyemi Ajibola-James2
Abstract
The study evaluated the changes in the land cover of the mangrove
ecosystem in Niger Delta from 1986 to 2008. It also assessed the
time-frame required for mangrove vegetation in the study sites to
recover from oil spill impact with a view to developing a GIS-based
model for mangrove vegetation recovery in the Niger Delta. The
study integrated both primary and secondary data sources. The
data included ground truthing technique with the aid of Global
Positioning System (GPS), satellite imagery, oil spill record and oil
spill map. Two sites were purposively selected for the study due to
the nature of the terrain (non-remediated and remediated site). The
result showed that it took 13 to 14 years for mangrove vegetation
on non-remediated site to recover while it takes only 7 years for
complete vegetation recovery on the remediated site from the
impact of oil spillage. The results further showed a downward trend
of mangrove vegetation recovery from 1986 to 2000 and upward
trend between 2000 and 2007 in the GIS-based model. The study
concluded that oil spillage had serious impact on the health and
growth of mangrove vegetation.
Keywords
Land cover; Mangrove ecosystem; Multi-Date Satellite Imagery;
GIS modelling
*Corresponding author: Oluwagbenga OI Orimoogunje, Department of
Geography, Obafemi Awolowo University, Ile-Ife- 220005, Nigeria, Tel:
+2348035855946; E-mail: orimoogunje2@yahoo.com
Received: June 01, 2013 Accepted: September 16, 2013 Published:
September 24, 2013
has contributed to the reduction in Africa’s mangrove coverage by
55% [6]. Ifeadi and Nwankwo, [7] and Awobajo [8] revealed that
the highest incidence of oil spills occurred in the mangrove swamps
zones and near o shores areas of the Niger Delta which was shown
in an analysis of oil spillage statistics in Nigeria during the period
1976 to 1988. Crude oil spills continue to be a notable threat to the
conservation of the rich mangrove eco-region of the Niger Delta in
Nigeria. According to Ebuehi et al. [9] mangroves areas are the most
productive and sensitive areas in the ecosystem. e predominant
and vulnerable mangrove species in the Niger Delta are Rhizophora
racemosa, Rhizophora mangle (red mangrove), Avicennia germinans
(white mangrove) and Languncularia racemosa (black mangrove).
Mangroves have traditionally provided a variety of plant products,
sh and shellsh for local communities [10]. ey also provide services
such as coastal stabilization, and food chain support for near-shore
sheries. In recent decades there has been increased conversion for
uses which do not sustain the mangrove habitat, such as large-scale
sh culture ponds and industrial salt production, and there is concern
about the resulting loss of mangroves [10]. Nevertheless, all these uses
may be aected following oil spills and need to be considered during
the contingency planning process.
Oil spillage is a major environmental problem in Nigeria.
Between 1976 and 1996 Nigeria recorded a total of 4835 oil spill
incidents, which resulted in a loss of 1,896,960 barrels of oil to the
environment. In 1998, 40,000 barrels of oil from Mobil platform
of the Akwa-Ibom coast were split into the environment causing
severe damage to the coastal environment. Oil spillage has led to
very serious pollution and destruction of ora, fauna and resort
centres, pollution of drinkable water, destruction of properties and
lives along the Nigerian coast. Oil spillage has also caused regional
crisis in the Niger Delta. Factors responsible for oil spillage in the
zone are; corrosion of oil pipes and tanks, sabotage, port operations
and inadequate care in oil production operations and engineering
drills. e consequence is the massive oiling of the environment and
destruction of vulnerable ecological units [9-11]. is is more so in
the mangrove where tidal eects facilitate dispersion of spills. It has
been noted that mangroves are highly susceptible to oil pollution
[12]. e impact of oil spill on mangroves is a function of a number of
factors. According to Lewis [13] there are four factors: rst the type of
spilled oil, second the quantity of spilled oil reaching the mangroves,
third the quantity remaining aer any clean-up eort and fourth the
various physiographic types of mangrove aected. Other factors are
length of mangrove’s exposure to spilled oil [5], site conditions [14]
and the remediation technique employed [12]. Observations from
many spill events around the world have shown that mangrove suers
both lethal and sub-lethal eects from oil exposure [5]. Mangrove
forests are particularly dicult to protect and clean up once a spill has
occurred. is is because they are physically intricate and relatively
hard to access [10]. Each of these considerations contributes to the
overall assessment that mangrove forests are a habitat at risk from
oil spills.
e Remediation by Enhanced Natural Attenuation (RENA)
technique established in the oil spill response plan of Shell Petroleum
Introduction
Mangrove forests are the dening feature of the coastal
environment in many tropical regions. ey are the dominant
ecosystem along the sheltered shoreline of the Nigerian coast.
Mangroves are vital resource that serves the inhabitants of the Niger
Delta and are areas of active oil exploration in Nigeria. Mangroves
provide logs, fuel wood, charcoal, wood-chips, paper pulp, scaold
poles, piling and construction materials, stakes for sh traps, and
shing platforms, railway sleepers, wood for furniture making and
carvings, materials for roof thatching, bark for tannin, medicinal
products, sugar, alcohol, acetic and dyes [1-3]. According to
Robertson et al. [4] mangroves serve as habitat and breeding areas for
many commercially important sh and crustaceans while it provide
detritus for oshore sheries, controls coastal erosion as well as
maintaining water quality. ey are principal places that are impacted
by oil exploration and associated activities [5], an occurrence that
Citation: Orimoogunje OOI , Ajibola-James O (2013) Mangrove Ecosystem Recovery and Restoration from Oil Spill in the Niger Delta: The GIS Perspective.
Geoinfor Geostat: An Overview S1.
• Page 2 of 5 •
Special Issue 1 • 017
doi:http://dx.doi.org/10.4172/2327-4581.S1-017
Development Company of Nigeria (SPDC) has limited scientic
proof of its eectiveness in mangrove recovery. Remediation by
Enhance Natural Attenuation (RENA) is a land farming treatment
technology for intervention in petroleum hydrocarbon contaminated
soils in the Niger Delta regions [7,15,16]. But the earlier study on
RENA’s eectiveness adopted the rate of hydrocarbon degradation in
soil and swamp; and suitability of contaminants for biodegradation
as major criteria, but lacked considerations of mangrove vegetation
recovery time-frame [17]. erefore, this study attempts to assess the
eectiveness of RENA approach adopted and the time frame required
for mangrove vegetation in the study sites to recover from oil spill
impact.
Materials and Methods
Study area
e study area consists of two sites impacted by single major oil
spill in the mangrove of the Niger Delta (Figure 1). ese sites have
spillage impact of more than 20 km2 and spilled oil quantity more
than 25 barrels. One of the sites was impacted on the 15th June, 1994
and subsequently remediated between 2000 and 2002. e other
was impacted on the 22nd November, 1995 but was not remediated
as at December, 2008 [18]. e remediated site covers about 27.93
km2 lying approximately within Top Le Coordinates: 473414.012E;
78304.938N and Bottom Right Coordinates: 478383.632E;
71709.313N. e Non-remediated site covers about 23.99 km2
lying approximately within Top Le Coordinates: 478797.298E;
87122.644N and Bottom Right Coordinates: 484975.276; 77901.781.
e two sites for the study are in the Rivers State and the SPDC
Eastern division of the Niger Delta in Nigeria. e Niger Delta,
with an area of about 30,000 km2 is rich in biodiversity [19]. e
mangrove plants in the Niger Delta cover approximately 6000 km2
between the inland fresh water zone and the belt of beach-ridges,
which form the seaward boundary [20-22]. e Niger Delta region
has ve species of true mangroves: Rhizohpora racemosa, Rhizophora
harrisonii, Rhizophora mangle, Avicennia africana and Langucularia
racemosa [23], Acrostichum aureum, Conocarpus erectus, and Nypa
fruticans have also been identied as species of plants associated with
mangroves in the Niger Delta [24]. In term of climate, the study area
lies within the Wet Equatorial climate zone where cloud cover is very
high and the coastal parts are under more or less permanent cloud
cover. Sunshine hours are very low with very high relative humidity
most of the year, ranges between 80% in March and greater 90% in
July. Temperature means values range between 24°C and 32°C while
rain falls every month of the year but with a short dry spell between
January and March. Mean annual rainfall ranges from 4500 mm in
Bonny to 2000 mm in Ndoni [25].
Methods
is study utilized data acquired by ground truthing technique
(eld observation) with the aid of Global Positioning System
(GPS) coupled with data sourced from the archive of an oil and
gas exploration company. Two sites were purposively selected for
the study in the Niger Delta (one remediated and the other non-
remediated). e sites selected were based on the history of oil
spillage which impacted more than 20 km2of the study area. Landsat 5
TM of 1986 and Landsat ETM+ of 2000, 2003 and 2007 multi-spectral
sensor of 30 m spatial (pixel) resolution were used to monitor, assess
and contrast the trend of mangrove vegetation recovery from the oil
spill’s impact. e imagery was pre-processed using geo-rectication,
region creation and co-registration techniques embedded in the ER
Mapper (version 7.0.) soware. e image processing techniques
of Normalised Dierence Vegetation Index (NDVI) and Density
Slicing of NDVI Values were used for analyzing the imagery data sets.
Vegetation monitoring by remotely sensed data was carried out using
vegetation indices, which are mathematical transformations designed
to assess the spectral contribution of green plants to multi-spectral
observation [26]. us, the analysis of Mangrove vegetation health
status was attempted using Normalized Dierence Vegetation Index
(NDVI) function, which is given by:
( ) ( ) ( )
DN output NIR R / NIR R= − +
(1)
where,
( )
NIR Near Infrared Band 4=
,
and
( )
R Red Band 3=
of LANDSAT TM and ETM+.
is function exists as an algorithm in the ER Mapper and was
applied to the imagery data set over the study area. e output
NDVI imagery data sets were brought into ArcGIS 9.2 soware
environment by using the “add data” command tab in the window
of the soware. Subsequently, the density slicing analysis was carried
out by employing the capabilities of the GIS soware to partition and
colour code the observed feature on the imagery data set. e density
slicing approach was based on the principle of Electro Magnetic
Radiation. e adopted principle for the land cover classication
is in consonance with the ndings of Budde and Nijmeijer [27] on
spectral signatures of land cover. Based on this theory, a classication
scale was evolved for the study which represents the NDVI values
classication range detailing the land cover classes, their respective
range of NDVI values, colour and RGB (Red, Green, Blue) codes. It
should be noted that the classication range was determined with
the aid of Pseudo-Natural Colour Composite (PNCC) of the data
sets. In eect, the NDVI imagery data set for each acquisition was
classied into 7 distinct clusters of land cover (Creeks & Rivers, Deep
Swamp, Shallow Swamp, Swamp Soil, Stunted Mangrove, Medium
Mangrove, Tall Mangrove, and Very Tall Mangrove) by using the
density slicing approach. Additionally, through the process of re-
classication, generic statistics of Water, Swamp Soil, and Vegetation
were generated from the initial 7 clusters’ statistics.
Figure 1: The Study Area in the Nigeria Niger Delta.
Citation: Orimoogunje OOI , Ajibola-James O (2013) Mangrove Ecosystem Recovery and Restoration from Oil Spill in the Niger Delta: The GIS Perspective.
Geoinfor Geostat: An Overview S1.
• Page 3 of 5 •
Special Issue 1 • 017
doi:http://dx.doi.org/10.4172/2327-4581.S1-017
Results and Discussion
Satellite imagery analyses
Table 1 shows the NDVI values for each site for the 4 epochs in
the study area between 1986 and 2007. Table 1 also shows that in 1986
before the oil spills the two sites had equal NDVI average value (0.32),
which indicates equilibrium mangrove vegetation health which is
essential basis for the assessment study. us, the 1986 imagery
represents the base line data for the study. However, the equal NDVI
values obtained do not necessarily represent the optimal vegetation
health status [28,29].
e Table 1 also shows that in 2000 (6/7 years aer the oil spill)
the NDVI value of the non-remediated site (0.27) was less than that
of remediated site (0.30). is shows a decline in the health condition
of the vegetation on both sites and thus suggests the spilled oil as an
inducing factor. It also indicates that the vegetation on the remediated
site was healthier than the one on the non-remediated site. It should
be noted that an information gap exists between 1986 and 2000
due to data inaccessibility. In spite of the information gap, the 0.03
dierence in the NDVI values for the two sites in 2000 coupled
with the fact that remediation was done between 2000 and 2002
may indicate the eectiveness of RENA. is was in support with
Shekwolo [17] argument that if RENA is applied to an oil impacted
site, remediation may be observable within 8 to 12 weeks if moisture
content and nutrient level are appropriate. ey also suggest that a
signicant reduction of the hydrocarbon compounds in crude oil spill
contaminated soil could be achieved within 40 days of application of
RENA. In 2003 the NDVI value of non-remediated site (0.31) was less
than that of remediated site (0.34). is shows that the vegetation on
the remediated site continued to be healthier than that on the non-
remediated site. e value also implies that there was an increase in
the health status of the vegetation on both sites and thus suggests
progressive recovery from the spilled oil impact. e observed 4 units
of improvement in the NDVI value of non-remediated site from 0.27
in 2000 to 0.31 in 2003 seems to be induced by normal rate natural
attenuation. is attenuation would be partly due to the weathering
of spilled oil overtime on the non-remediated site. e weathering
processes might be one or combination of factors like oxidation,
evaporation, dissolution and biological degradation [30]. Freegarde
and Hatchett [31] estimated that an oil-slick of 2.5mm thickness
could be degraded in 100hours of continuous sunlight. Floodgate [30]
opines that up to 50% of spilled oil can be lost through evaporation in
a few hours depending on the original composition of the oil. Bacteria,
yeasts and other microbes attack petroleum and biodegrade it [30].
In 2007 the NDVI value of the vegetation on the non-remediated
site (0.34) was less than that of remediated site (0.36). is indicates
further increase in the health status of the vegetation on both sites.
However, the vegetation on the remediated site was healthier than
that of the non-remediated site. is shows the eectiveness of RENA
in mangrove restoration. Moreover, the NDVI values of remediated
site (0.36) and non-remediated site (0.34) in 2007, which rose above
the baseline values (0.32) in 1986 show that vegetation on the two
sites may have completely recovered from the oil spill impact and
proceeded towards attainment of full physiognomy. is collaborated
Lewis ndings, that from the 10th year upward, complete recovery of
oil impacted mangrove vegetation is attainable [13].
Tables 2 and 3 reveal the land cover area for both sites respectively.
e tall mangrove vegetation class on non-remediated and remediated
sites yielded the most unusual trend pattern between 2003 and 2007
(up-trend). Figures 1 (a) and (b) represent the trend of land cover for
the former and later sites respectively over the 4 epochs, the general
pattern of the trend is quite inconsistent. For instance, on the non-
remediated site, the tall mangrove accounted for 12% of the land
cover in 1986, 15% in 2000, 13% in 2003 and 64% in 2007. Similar
inconsistent pattern was observed in other land cover classes like
medium and stunted mangroves. e ideal pattern for a recovering
vegetal cover area should at least be consistent over time. at is, an
initial downtrend followed by an up-ward trend is expected on a site
impacted with crude oil. is could be inferred from the observation
Lewis on generalized responses of mangrove forests to crude oil [13].
He suggests initial defoliation and death of some sizes of mangroves,
which correspond with the expected downtrend in the area of
vegetal cover. e subsequent recolonization/ reduction in litter fall
correspond with expected uptrend in the area of vegetal cover over
time. e challenge of inconsistency might be due to the thin line
(1 unit dierence) between lower and upper class boundaries of the
NDVI Values Classication.
Range used for the land cover features on the Study Area, coupled
with the close spectral values of similar features like tall, medium
and stunted mangroves. e analysis shows further that the non-
remediated site over the 4 epochs, area covered with water had a
Time-Lapse 1986 Pre-oil
spill 2000 2003 2007
Non-remediated site
(Nov 1995) 0.32 0.27 0.31 0.34
Remediated site (June
1994) 0.32 0.30 0.34 0.36
Table 1: NDVI values for the four Epochs (1986 to 2007).
Land Cover 1986
Km2 %
2000
Km2 %
2003
Km2 %
2007
Km2 %
Creeks and rivers 1.36 6 3.40 14 1.42 6 1.03 4
Deep swamp 3.67 15 4.82 20 2.53 11 0.79 3
Shallow swamp 4.53 19 0.00 00 3.95 16 2.09 9
Swamp soil 4.89 20 6.21 26 5.12 21 4.61 19
Stunted mangrove 3.74 16 2.62 11 4.46 19 0.00 0
Medium mangrove 2.57 12 3.35 14 3.33 14 0.00 0
Tall mangrove 3.23 12 3.59 15 3.18 13 15.47 64
Total 23.99 100 23.99 100 23.99 100 23.99 100
Table 2: Land Cover Area for non-remediated Site for the 4 Epochs.
Land Cover 1986
Km2 %
2000
Km2 %
2003
Km2 %
2007
Km2 %
Creeks and rivers 1.54 6 3.59 13 2.63 9 0.94 3
Deep swamp 2.94 11 3.96 14 0.00 0 2.14 8
Shallow swamp 3.68 13 0.00 0 3.78 14 3.39 12
Swamp soil 5.10 18 8.23 29 9.53 34 6.53 23
Stunted mangrove 5.54 20 4.26 15 0.00 0 0.00 0
Medium mangrove 5.07 18 4.64 17 4.76 17 0.00 0
Tall mangrove 4.06 15 3.25 12 7.23 26 14.93 53
Total 27.93 100 27.93 100 27.93 100 27.93 100
Table 3: Land Cover Area for remediated Site for the 4 Epochs.
Citation: Orimoogunje OOI , Ajibola-James O (2013) Mangrove Ecosystem Recovery and Restoration from Oil Spill in the Niger Delta: The GIS Perspective.
Geoinfor Geostat: An Overview S1.
• Page 4 of 5 •
Special Issue 1 • 017
doi:http://dx.doi.org/10.4172/2327-4581.S1-017
consistent downward trend from 1986 till 2007 from 9.56 km2 (∼40%)
to 3.91 km2 (∼40%). e Swamp Soil seems to have an upward trend
between 1986 and 2000. e trend reversed consistently between
2000 and 2007 for three consecutive epochs. e vegetation cover had
upward trend between 1986 and 2007 for four consecutive epochs.
is follows a normal distribution and thus suggests a gradual
recovery of vegetation coverage from the oil spill impact on the non-
remediated site.
Figure 2a and b for the remediated site over the 4 epochs
revealed that the area covered with water i.e. creeks and Rivers, Deep
and Shallow Swamps, seems to have an insignicant downtrend
between 1986 and 2000 from 8.16 km2 (∼29%) to 7.91 km2 (∼27%).
Between 2000 and 2003 the downtrend continued but it became
more signicant: 7.91 km2 (∼27%) to 6.41 km2 (∼23%). ere was an
upward but insignicant movement of the area trend between 2003
and 2007 from 6.41 km2 (∼23%) to 6.47 km2 (∼23%).
From 1986 through 2000 to 2003, the trend of area covered by
Swamp Soil progressively followed an upward course: 5.10 km2 (∼18%)
to 8.23 km2 (∼29%) to 9.53 km2 (∼34%) respectively. However, there
was downtrend between 2003 and 2007 up till 2008: 9.53 km2 (∼34%)
to 6.53 km2 (∼23%) to 4.72 km2 (∼17%). Concerning the vegetation
cover, the amalgamation of Stunted, Medium, and Tall mangroves
coverage had signicant downtrend between 1986 and 2000 from
14.67 km2 (∼53%) to 12.15 km2 (∼44%). e downtrend continued,
though insignicant between 2000 and 2003: 12.15 km2 (∼44%) 11.99
km2 (∼43%). Between 2003 and 2007, there was a signicant trend
reversal from 11.99 km2 (∼43%) to 14.93 km2(∼53%).
Implication of the study
e study shows that there was relatively fast recovery rate and
short recovery period of the vegetation on crude oil impacted site
where RENA was applied. It also shows that complete recovery of
mangrove vegetation was possible between 13 and 14 years, that is
7 years post RENA application, aer the oil spill impact. is was
in agreement with Lewis argument on mangrove complete recovery
period of 10 – 50 years aer oil spill impact. In spite of the armed
possibility of mangrove recovery via RENA, the initial negative
impacts on land, sheries and the ecosystem with consequences on
community livelihood are better imagined than experienced. is has
been of greater concern to the aected communities over the years.
us, zero oil spillage in the mangroves is preferred as the ideal for a
sustainable and productive human-environment relationship. Lastly,
this study also observes that Remote Sensing approach to vegetation
recovery from oil spill impact has much to do with chlorophyll content
of the vegetation than vegetation area coverage. Consequently, the
assessment and modelling of vegetation recovery from oil spill impact
appear to yield a more reliable result when approached from NDVI
values perspective than from vegetal cover area. However, time-lapse
analysis of vegetation area on oil-impacted site could be a supportive
technique depending on the degree of impact.
Conclusion
In conclusion, the study has showed that oil spillage has serious
impact on the health and growth of mangrove vegetation and it
established that GIS technique is eective for mangrove vegetation
recovery from oil spillage impact. Finally, it should be established
that man cannot run away from reality but face the reality if he don’t
want to cut his life short on this planet, therefore, it is of paramount
importance to concentrate on preventing spills but despite the best
eorts of individual organizations, spills will continue to occur and
will aect the local environment. Response to spills should seek to
minimize the severity of the environmental damage and to hasten the
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
1986 1991 1996 2001 2006 2011
Land Cover Area (Km2)
Time Lapse
Trend of Land Cover for Unremediated Site (Km2)
Creeks & Rivers
Deep Swamp
Shallow Swamp
Swamp Soil
Stunted Mangrove
Medium Mangrove
Tall Mangrove
0
10
20
30
40
50
60
70
1986 1991 1996 2001 2006 2011
Land Cover Area (%)
Time Lapse
Trend of Land Cover for Unremediated Site (%)
Creeks & Rivers
Deep Swamp
Shallow Swamp
Swamp Soil
Stunted Mangrove
Medium Mangrove
Tall Mangrove
(a)
(b)
Figure 1a,b: Scatter Plots Showing the Trend of Land Cover Area for Non-
remediated Site over 4 Epochs (1986 - 2007).
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
1986 1991 1996 2001 2006 2011
Land Cover Area (Km2)
Time Lapse
Trend of Land Cover for Remediated Site (Km2)
Creeks &
Rivers
Deep Swamp
Shallow
Swamp
Swamp Soil
Stunted
Mangrove
Medium
Mangrove
0
10
20
30
40
50
60
1986 1991 1996 2001 2006 2011
Land Cover Area (%)
Time Lapse
Trend of Land Cover for Remediated Site (%)
Creeks & Rivers
Deep Swamp
Shallow Swamp
Swamp Soil
Stunted
Mangrove
Medium
Mangrove
(a)
(b)
Figure 2a,b: Scatter plots showing the trend of landcover area for remediated
over 4 epochs (1986 – 2007).
Citation: Orimoogunje OOI , Ajibola-James O (2013) Mangrove Ecosystem Recovery and Restoration from Oil Spill in the Niger Delta: The GIS Perspective.
Geoinfor Geostat: An Overview S1.
• Page 5 of 5 •
Special Issue 1 • 017
doi:http://dx.doi.org/10.4172/2327-4581.S1-017
recovery of any damaged ecosystem; and the response should always
seek to complement and make use of natural forces to the fullest
extent practicable.
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Author Afliations Top
1Department of Geography, Obafemi Awolowo University, Ile-Ife, Nigeria
2Geo Inheritance Limited, Port Harcourt, Nigeria