Current World Environment

Introduction

Increasing population and anthropogenic activities have brought about many environmental problems globally. One of such problems is land subsidence described as  the gradual  differential settling or sudden sinking of the ground surface due to the movement of ground materials.^1,2 Land subsidence is generally caused by human activities, alterations to the earth’s surface and underground geologic processes. Specific causes include: Underground mining of solid minerals, and the collapse of such mines; withdrawal of groundwater and petroleum; dewatering or drainage of organic soils; sink holes, wetting of dry low density soil; and, natural  sediment compaction.

The growth of world population and the increasing   need for water  has led to this geological hazard that has affected  all continents of the world except Antarctica.³ Land subsidence has received the attention of researchers in different parts of the world. Land subsidence problems that has resulted from the overuse of ground water in Bangkok in Thailand has been investigated.⁴ According to the study, subsidence occurred at rates up to  10cm/year  in critical areas. The crises worsened flood conditions and caused damages to buildings and infrastructure. Hence since 1983, government agencies  have monitored  subsidence and ground water levels  and implemented  general measures to mitigate the problem. Also, the effect of ground water on rural housing in USA was equally studied leading to the conclusion that land subsidence occurred where large amounts  of ground water was withdrawn from  a thick layer of saturated fine gravel sediment  that is susceptible  to  compaction. Remote sensing and Geographic Information System (GIS) techniques were used to detect and quantify  land subsidence caused by aquifer compaction in Antelope Valley, California.⁶ The resultant maps with high spatial detail and resolution allowed  a comprehensive comparison between recent subsidence  patterns and those detected  historically by traditional methods.

A 24 year  data was used to  analyze establish trends and causes of land deformations in Yangtze River delta, China.⁷ This area is characterized by the occurrence of land subsidence due to the exploitation of ground water resources  that serve as a major water  supply for industrial and municipal purposes. An assessment of land subsidence in the Karst region of Rongkop, Indonesia was carried out with the purpose of delineating the areas that are at risk. A land subsidence risk map was developed  based on 5 parameters – slope degree, lithology, relative relief, distance to  lineaments and land use. The result revealed  that the highest risk zones coincided with sink hole locations.

As part of mitigation strategies, the government of New South Wales, Ausrtalia established  the Mine Subsidence Board (MSB) as a service organization operating  for the community  responsible for the administering the Mine Subsidence Compensation Act. This provided the basis for compensation or repair services where subsidence induced damages occur in this coal mining area.⁸ The Board is responsible  for  reducing the risk of more subsidence damage  to properties by assessing and controlling the type of buildings that can be erected in subsidence prone districts. It is worthy of note that  although subsidence damages were reported in New Castle  from late 19^th century, comprehensive  subsidence monitoring programmes  were not initiated until 1970s.

The study area, Akwa Ibom State of Nigeria is susceptible to subsidence as a result of complex interaction between anthropogenic activities and natural processes. It is located between latitude 4 ⁰ 32 ^I and 5⁰ 33 ^I North and Longitude 7⁰ 25 ^I and 8⁰ 25 ^I  East. To the East of the state is Cross River State, to the West  Rivers and Abia and to the South Atlantic ocean. The geology of the study area consists of four recognizable distinct stratigraphical units.⁹ The shale – limestone unit is the oldest geological fancy in the state belongs to the late Cretaceous (Nsukka formation) and the early Eocene ( Imo clay shale group) periods. They are found in the north and north east parts of the state. The coastal plain sand also found in the north and north east part of the state belong to the Oligocene – Pleistocene  period, the younger Benin formation Coastal plains sand and the beach ridge complex and alluvial deposits cover the remaining parts of the state.

As an area of subterranean fluid extraction, there is the need for subsidence monitoring so as to minimize the resultant risk. Considering the sensitivity of the Niger Delta ecosystem of which the State is a part, the need  for utilization of GIS so as to quantify spatial changes in this  dynamic environment cannot be overemphasised.¹⁰

Methodology

Data on Subsidence Susceptibility Index of the study was used to produce subsidence surface  map which was divided into 4 subsidence zones using a Reclass tool of  Arcmap’s Spatial Anayst extension. Figures 1 & 2   show the spatial distribution of the subsidence while Table 2 shows the areal distribution of the zones, From the table it can be seen that Very High zone covers 0.59% while the Marginal zone covers 75.71%.  The analysis using water extraction is a good  estimate of land subsidence hence serves as a good tool for policy makers to monitor environmental hazards. Areas of special interests could also be monitored. For example, the map shows that areas of High and Very High zones are found in established urban areas of Uyo, Ikot Ekpene and Ikot Abasi. This emphasizes the need for the population growth and the water supply needs of  our urban centers to be monitored.

The  study utilized  water extraction data obtained from Akwa Ibom Water Campany Ltd, and Ministry of Rural Development, to model the subsidence susceptibility surface of the study area. Total quantity of water extracted in the study area was calculated by combining extraction figures that include the annual discharge for all the hand pump boreholes per LGA; annual discharge for all the mini water schemes in the LGA; and, the annual discharge of all the Akwa Ibom Water Company Ltd head works in each of our urban LGAs. Although Akwa Ibom State  is a petroleum oil producing state in the Niger Delta of Nigeria, most of the extraction is done offshore. For this reason, petroleum oil could not be used as the basis to estimate the areas’ susceptibility to subsidence. Since the data  required for  the study have spatial components that can change over time. GIS  proved useful in the storage, integration and display of the data.

The data on water extraction  was linked to an existing data base of the base map of the study area. To obtain the subsidence susceptibility index, the total water extraction data for each LGA was normalized using the formulae:

Dimension index = Actual value – max. value
                               Max. value – min. value

The data was then converted into grid using ArcMap 9.1 GIS software with Spatial Analyst Extension. The grid was interpolated with Inverse Distance Weighted (IDW) tool to create an isoline map. IDW is used to create a continuous surface from sampled point values. The method is a deterministic interpolation that assigns values to locations based on  the surrounding and on a specified mathematical formulae that determines the smoothness of the resulting surface. Isoline map that resulted, based on the assumption that the phenomena represented has a continuous distribution and smoothly changes in value in all direction of the plain; was used to show the spatial distribution of water extraction in the study area urban areas of Uyo, Ikot Ekpene and Ikot Abasi. This emphasizes the need for the population growth and the water supply needs of  our urban centers to be monitored.  

Result and Discussion

Data on Subsidence Susceptibility Index of the study was used to produce subsidence surface  map which was divided into 4 subsidence zones using a Reclass tool of  Arcmap’s Spatial Analyst extension. Figures 1 & 2   show the spatial distribution of the subsidence while Table 2 shows the areal distribution of the zones, From the table it can be seen that Very High zone covers 0.59% while the Marginal zone covers 75.71%.  The analysis using water extraction is a good  estimate of land subsidence hence serves as a good tool for policy makers to monitor environmental hazards. Areas of special interests could also be monitored. For example, the map shows that areas of High and Very High zones are found in established urban areas of Uyo, Ikot Ekpene and Ikot Abasi. This emphasizes the need for the population growth and the water supply needs of  our urban centers to be monitored.

An analysis of the pattern of population growth, age distribution of the population and trend of urbanization will  enable one  to  understand the implications of the study. The spatial distribution of the population density of the state shows that the state is located in  the high population zone of the country. It has an average density of 541 persons/km²  based on the 2006 population census. Oron has the most  highest density of 2128 persons per km² followed by Uyo (1986), Ikot Ekpene (1116), Eket (983), Nsit Ibom (998) and Etinan (928).  All other LGAs have densities that are  lower than 300 except Uruan (280). The age characteristics of the population to the state show that 60.75% of the population are below 20 years.¹¹

Table 1:  Akwa Ibom State Showing Annual Water Discharge Rates

Source Compiled from data Supplied by AK-RUWATSAN, Akwa Ibom Water Campany Ltd, Ministry of Rural Development;  and,  Cross River Basin Development Authority (CRBDA).

This shows that the area has a youthful,  dynamic and  growing population. It has been observed that urban areas in the state have grown significantly in the last 100 years with Uyo the State capital being the most populous with 118,250 people.¹² The generally high population  growth result in the spilling over of urban residents into unplanned urban suburbs. The pressure on urban services and facilities has exceeded the coping capacity resulting in acute shortages of essential services like water, electricity, health, transportation and education.

Table 2:  Akwa Ibom State Showing coverage of subsidence zones.  

The analysis of population characteristics, pattern of growth and urbanization show a long term pressure in the underground water reserves, hence land subsidence is a real threat in the study area. The study has demonstrated the use of GIS to visualize the subsidence of the study area. This environment allows for ease of data editing, integration, analysis, and  storage. The resultant product will help  to identify, and locate sensitive areas that can be impacted by subsidence so that emergency managers can customize disaster  relief efforts. Using a GIS for the modeling has enabled a data base to be created hence could be updated as new facts emerge. It is recommended that a subsidence unit should be established within the Akwa Ibom State Water Corporation so that the rate land subsidence could be monitored especially in the vulnerable areas identified in the study.

Conclusion

The study has used water discharge figures to model the susceptibility of the study to land subsidence IN A GIS environment. The map produced shows that the Very High and High Subsidence Susceptibility zones are found in the Uyo, Ikot Ekpene Itu and Ikot Abasi and the surrounding areas. Using a GIS for the modeling has enabled a data base to be created which could be updated as new facts emerged.

References
 

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2. Devin, G. Jones, D and  Ingebritsen, S. E., Land Subsidence in the United States , U.S. Geological Survey Circular 1182, 1999.
3. Prokopovich, N. P. Detection of Aquifers Susceptibility to Land Subsidence, Land Subsidence (Proceedings of the Fourth International Symposium on Land Subsidence). IAHS Publ. no. 200,  3333 Braeburn Street, Sacramento, CA 95821, U.S.A, 1991.
4. UNEP , The Bangkok City State of the Environment,  Thailand, 2001.
5. Waller, R. M., Ground Water and the Rural Homeowner, U.S. Department of the Interior / U.S. Geological Survey,1994.
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8. Mine Subsidence Board,  Graduated Guidelines for Residential Construction (New South Wales), Vol. 1:  Historical and  Technical Background, NSW, Australia, 2000.
9. Usoro, E. J., Geology, In: P Akpan and  Usoro E Ed., Akwa Ibom State: Geographical Perspective, Department of Geography Publications, Uyo, pp 23 -34, 2010.
10. Ogba, C. O. and B. P. Utang,  Geospatial Evaluation of Niger Delta Coastal Susceptibility to Climate Change,  FIG Congress Sydney, Australia, 11-16 April 2010. Mid:20453883 PMCid:PMC2892028
11. Inyang, I. B., Population and Settlement,  In: P. Akpan and E. Usoro, Ed., Akwa Ibom State: Geographical Perspective, Department of Geography Publications, Uyo, pp. 179 – 190, 2010.
12. Akpan, P. A., Urban Settlement,  In: P. Akpan and  E. Usoro, Ed., Akwa Ibom State: Geographical Perspective, Department of Geography Publications, Uyo, pp 191 – 202, 2010.