Assessment of geospatial capability index for siting waste dump/landfill to control groundwater geopollution using geographic information system (GIS) approach: case study of Abakaliki area and environs, Southeastern Nigeria
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Understanding spatial variation of rock and soil is important for capability precision as well as groundwater contamination or geopollution management. In view of waste dump/landfills, geoenvironmental hazards ravaging our land including surface and groundwater contamination, site suitability indexing, spatial distribution and characteristics of underlying rock and soil matter are studied. Rock and soil samples were collected in a systematic grid pattern using simple drill core rig and hand auger. Geostatistical and soil property analyses were conducted on each grid. Groundwater aquifer vulnerability to leachate was studied using depth to water table, recharge (precipitation), aquifer material, topographic slope, impact on vadose zone/vadose zone material and hydraulic conductivity (DRASTIC) and geophysical methods. Human population growth analysis indicates tremendous waste generation. Flash points of waste generation and dumping were noted. Geographical positioning system was used to take record of sampling point coordinates. Using a sufficient dataset, each grid indicative factor is objectively scaled, weighed and assigned a numerical rating according to their relative importance employing site suitability (S) analysis approach in an empirical equation. Results were imported into a geographic information system (GIS) platform to generate thematic maps. The results showed that subsurface rock and soil characteristics are neither spatially nor vertically homogeneously distributed. Thematic maps were generated and further interpolated in the GIS domain to produce a composite waste dump/landfill suitability index map.
KeywordsContamination Geoenvironmental hazards GIS Spatial Suitability Variability Waste dump
Geology and soils are naturally never homogeneous; hence, there is growing interest in the study of geospatial variation properties of rock and soil using geostatistics since 1970s. This technique is yet well developed, but successful in characterizing the spatial variations of subsurface and soil (Liu et al. 2006). Many studies have been carried out on land capability index. Their results indicate diverse and dynamic system (Kavianpoor et al. 2012) changing their properties with time and space continuously (Maniyunda et al. 2013; Onwe et al. 2018). This heterogeneity in rock and soil properties is anisotropic across landscape. This inconsistency can be as a result of several interacting factors, their different intensities with different scales acting simultaneously (Serrano et al. 2014). Consequently, application of conventional land planning and allocation cannot overcome the issue of the inconsistency, resulting in mismatching when trying to assign capability to each of the many interests competing for an area of land. Severe environmental damage, deterioration and geohazards such as earthquake, structural failures, landslide and soil creep, including flooding, erosion, groundwater contamination/pollution arise due to land capability index mismatching (Onwe et al. 2018). Estimation of rock and soil properties’ spatial variance is important for evaluating geoenvironmental resource vulnerability (Iňigo et al. 2012, Onwe et al. 2017). It provides the influential processes and factors controlling potential contamination and or pollution of groundwater resource (Akbas 2014). It is also an important determinant to water well as well as waste dump or landfill sitting, their design, yield efficiency and management.
The capability of an area for waste dump or landfill is a function of different parameters, including soil pH, soil organic matter, texture and biological activities as well as rock type and lineament including aquifer media characteristics (Karaca 2004; Onwe et al. 2017). Hence, determination of such parameters is important for evaluating geospatial capability index.
To facilitate development in line with sustainable development goals (SDGs), it is important to know the limitations of a land and the uses it is capable of supporting and how susceptible a land is to such factors as erosion and contamination influences. Nwankwor et al. (2004) and Onunkwo-A et al. (2011) determined geological parameters and used analytical manual overlay and GIS approach, respectively, to index land use. One important way to gather this knowledge is to undertake geostatistical and soil survey and prepare each factors’ common georeferenced thematic map and through spatial integration by interpolation and simulation in a GIS platform. While some are small scale (Wilcke 2000), relatively few have been done at large scale (Qu et al. 2014). This study is undertaken in the research interest for analyzing the spatial variation and properties and identifying waste dump/landfill site probable or possible geohazard vulnerable zones for groundwater specific management using GIS platform.
Climate and weather
The area is a tropical temperate and characterized by seasonal conditions caused by the North–South fluctuation of a discontinuity zone between the dry continental (Saharan) air and the humid maritime (Atlantic) air. This culminates in two main seasons dominating the area—rainy and dry seasons. Other minor climatic conditions in the area are the short dry season—August break and the harmattan patches of November to February. Mean annual rainfall is 2000 to 2500 mm and mean monthly rainfall varies from 50 to 300 mm, while mean annual temperature of 31.2 °C ranges from 33 °C in dry season to 28 °C in wet season. The area falls within the tropical rainforest to savannah belts with lush vegetation characterized by variety of tree shrubs, grasses and abundant palms (Onwe et al. 2015, 2016). Dominant vegetation is cereals (rice, maize), vegetables and fruits. The area consists predominantly of shale rock type. This weathers to lateritic clays, silts and clay soil deposits.
Materials and methods
Soil sampling and analysis
Compressive strength test of residual soils determining the stress required to break a loaded sample that is unconfined at the sides was also examined. The compressive strength of soil depends on their porosity. Shear strength, the resistance of soil/rock mass to shearing stress depending on the angle of internal friction, (ϕ), and cohesion, (c), between the particles of the materials were measured using direct shear test arrangement. They were studied with a view to understand the engineering and attenuation characteristics. Information on soil class/type, depth and texture (porosity) was obtained.
Core drilling, vertical electrical sounding (VES) and profiling were used to undertake surface and subsurface spatial geostatistical studies. Measurements on lineaments (fracture) orientation, aperture, trace length and spacing were taken with the aid of Garmin 78sc geographical positioning system (GPS), callipers, tape and Brunton compass. Their geospatial analyses were carried out employing geostatistical/geospatial mapping software (ArcGIS, Global mapper, Rockworks 16). The spatial variability parameters were calculated in numerical value, and interpolation was performed in ArcView GIS 10.2.
Hydrology and drainage
Water table approximate static level elevation and DRASTIC analysis were employed to understand aquifer vulnerability to contamination and possible pollution of the groundwater. Drainage network, patterns and regime were assessed to enable the determination of channel geometry (depth, width and mean flow velocity) and flood incursion risk area and scenario.
The population data of the study area (population in thousands).
Source: National Population Commission of Nigeria and National Bureau of Statistics web
Using the population data, the growth rate from 2006 to 2015 is 15.31%/year. Based on this growth rate, a projection to the year 2020 is forecasted.
Waste disposal siting criteria
Wastes are worthless, unwanted, defective or of no other value substance which constitutes a scrap, (U.S. EPA 1990; Oyediran 1997). It could be toxic and has the potential to defect other substance. Site evaluation for waste disposal involves a basic understanding of soil and subsurface properties and the treatment and limitations of the waste mechanisms polluting groundwater or loading rate. Sites are selected after a reconnaissance survey of an area. The criteria checklist includes population, climate, geology, lineament, soil class, soil depth, topographic/elevation, slope, water table, hydrology, land use, drainage characteristics and flood incursion scenario.
In considering waste dump site, shallow spelt groundwater table, recharge and discharge areas should be avoided. Proximity to streams or lakes causes runoff contamination and pollution including groundwater, wells and aquifers. Sites closer than 100 m to a high-yield well may be excluded.
Prolonged wet period impairs waste renovation, affects aeration and causes the likelihood of increased surface runoff. Topography informs the nature of slope. Slopes > 15% are considered too steep and not good for waste disposal land use. Medium-textured soils ranging from sandy loams to silty clay loams are generally suitable. The actual thickness of material above a permanent water table, bedrock or some other restricting layer constitutes the effective depth of soil useful in the renovation of waste; at least 3.05 m of permeable, unconsolidated material above a water table is generally recommended (Raymond 1979). Poorly drained terrains are not very suitable for waste disposal. Well-drained soils offer the greatest potential for waste renovation and application. An insufficient volume of well-aerated material reduces the soil’s ability to purify effluent. Carbonate rocks, fractured rocks and permeable rocks form solution channels and are susceptible to solution and facilitate leachate fluid migration. These affect stability and potential release of pollutants. Higher-cation-exchange-capability (CEC) areas are preferable waste disposal sites.
Exploratory statistical analysis
Thematic map layers dataset for waste disposal land use option using geometry calculator to determine percentage influence
0–9 (gentle slope)
Down-migration of leachate
Used in the derivation of DEM for slope
Fractured. Enhances migration of leachate
Poorly drained can lead to reducing condition
May be polluted when shallow. Difficult to exploit if deep
Reducing condition, distribution of waste and leachate. Buffer ≥ 1 km
Can create pathway for leachate migration to groundwater
Buffer ≥ 1 km
Attenuation of pollutants
Based on percentage of fines
≥ 1 km
Dumpsite descriptive statistics
Site should be in an area of mild slope, not water logged.
Site should be in an area moderately shallow or deep water table.
NOT near water catchments area, surface water and other sources of fresh water.
Sandy clay environment is preferably better.
Site should not be in an active fault, landslide or flood incursion, seismic zones and unstable areas like swamps.
Site should be at a threshold distance of 1 km from major streams and not nearer than 200 m to populated areas.
The site shall not be 3–5 km nearer to the airport area.
Site shall not be in a highly permeable soil areas.
May be close to an agricultural land use area, mainly a small-scale farming.
Difficult long hauling distances (≥ 5 km) access from main roads to the surroundings of the site and to the site itself should not be a site.
Site should not be too little available volume.
Results and discussion
Results of the soil properties of Abakaliki area and environs
Specific gravity, Gs
Plastic limit, PL (%)
Liquid limit, LL (%)
Plasticity index, PI
Natural moisture content, %
Maximum dry density, MDD (mg/m3)
Optimum moisture content, OMC (%)
California bearing capacity, aCBRu (%)
California bearing capacity, bCBRs (%)
Shearing angle resistance, ϕ (o)
Cohesion, c (kN/m2)
CEC (milli equiv.)
Particle size distribution at different areas
Little plasticity. Cohesive. Porous but little permeability. Limited ability to retain organic nutrient. Offers the good potential for waste renovation and application
Porous, permeable, low cohesion and plasticity. Poor retention of water. Well-drained soils offer the greatest potential for waste renovation and application
Most plastic and cohesive of all. Good attenuation medium. Ability to retain more plant nutrients. Poor engineering property
The flood incursion map was then imported into ArcGIS software for spatial analyses. Slope thematic map was generated from digital elevation model (DEM). The DEM was converted to raster data format before being used for spatial analysis. The presence of factors can significantly increase the vulnerability of the groundwater aquifer. GIS techniques have been used to present the spatial range of the areas with different levels of risk for increased vulnerability, together with data on the spatial range of its groundwater vulnerability.
The predominant shale in the area has favoured the low erodability of the lithology, resulting in the absence or near absence of deep and steep cut slopes, valleys and erosion channels but thick clayey top horizon. The areas’ shale formation and considerable clayey soil thickness with high percentage of fines form groundwater protector and ensure waste dump/landfill suitability having attenuation and renovation potential for waste Leachate especially to the Northeastern region of the study area. Its suitability for waste disposal sites is not without concern due to the geostructural or lineament implication which serves as a leachate conduit and needs consideration. The entire area is highly fractured, and composite Rose diagram indicates a NE–SW orientation. The presence of fractures and faulted formations contributes to low or unsuitability. Its conduit permits possible infiltration of leachates into groundwater.
Results indicate soil cover is thicker toward the northern part of the area, while shallower toward the southern part. This suggests groundwater may be more vulnerable to contamination toward the southern region. This may be further worsened with lineament and drainage flow direction, generally NE–SW and N–S, respectively. The unsuitable areas for waste dump correspond to the area around the Juju hill which has a fairly steep slope. Unsuitable areas further fall mainly around major drainage channels and flood-vulnerable areas measuring about 0.5–1 km from major rivers, streams and tributaries.
Waste dump/landfill suitable area falls around the uppermost northeast and a marginal portion in the southwest of the study area corresponding to area of thick soil cover. The northeastern region suitable for waste dump option falls within the sandy clay with reasonable soil and water table depth thickness ranging between 3.9–5.4 m and 41.7–46.5 m, respectively. Some other parts with higher soil or water table thickness that would have been suitable for waste dump option are knocked off or nullified by other characteristics such as soil texture, geostructural effects, drainage and flood scenario. The area not suitable for waste dump/landfill purposes may be suitable for other options not considered in this research.
Summary and conclusion
In view of increasing depletion in groundwater resources mainly because of excessive mismatch in waste dump/landfill sitting, a GIS-based approach to estimate spatial distribution of soil and subsurface rock properties to delineate potential areas is pertinent. Result show that human population density, drainage pattern and flow direction, flood-vulnerable areas, depth to groundwater, soil texture, rock type, geostructural features (faults, fractures) and slope nature are consideration factors in sitting waste dump/landfill especially to checkmate groundwater contamination and possible pollution.
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