The vadose zone of a landfill site proposed as an integrated waste management facility was evaluated based on geohydrological, chemical and microbiological characteristics of the groundwater and underlying soil. These data were also used to assess the attenuation capacity of the zone by the use of microbial degradation test of some major constituents including fatty acids, organic nitrogen and chloride of the leachate for a 28-day period. The main soil type in vadose zone consisted of brownish clayey sand of low permeability. The depth to water table which is equal to the thickness of the vadose zone varied from 8 - 13 m. Groundwater flowed with a hydraulic gradient of approximately 4.0 × 10 Dž and a pore velocity of 1.6 × 10 Dž cm/sec. The results of the biodegradation tests showed that the major constituents of the leachate such as ammonia/organic nitrogen, phosphate and organic carbon were completely degraded within 28 days. The population of aerobic bacteria within the 6 m soil depth was sufficient to bring about over 0.05% organic carbon removal. The soil characteristics in the vadose zone are very favourable for the occurrence of natural attenuation. The potential natural attenuation capacity of the vadose zone is therefore classified as moderate to high.
Landfills are covered or uncovered (open dump) waste receiving pits. They fall into different categories depending on the planning, design, construction, operation and closure as follows:
1) Sanitary Landfills (SLF) are engineered sites where environment problems are minimized by a daily programme of spreading the received wastes in layers followed by compacting and then covering them with soils.
2) Secured Landfills are like sanitary SLF but are constructed such that there is no hydraulic connection between the wastes, leachate and natural waters.
3) Modern Landfills are well engineered facilities that are properly located, designed, operated, monitored and closed according to well defined procedures. Monitoring continues even after closure. They are cleaned when necessary and are fenced to ensure compliance with government regulations to protect human health, environment and groundwater resources.
In Nigeria, open dumping of wastes on any available land space is a common method of solid waste disposal by urban dwellers despite the pollution associated with this method compared to the environment friendly waste management techniques of waste avoidance, source reduction, reuse and recycling. The open dumps have their inherent disadvantages but a more serious long term threat is the production of leachate from chemical and biological processes that break down wastes. When water comes in contact with wastes at a dump site, a foul smelling noxious liquid called leachate is produced which may contain toxic materials leached from the wastes. Once spilled in the ground, these contaminants migrate downward through the underlying strata under gravity and capillarity. This movement is enhanced by many processes including molecular diffusion, advection/convection, and mechanical dispersion. Physical, chemical and biological interactions in the subsurface by soil and leachate may ultimately pollute the environment. This implies that potential adverse, long term and irreversible impacts may occur in groundwater systems since the chemical composition of the leachate may exceed permissible limits. In landfill construction, site selection is important because the biophysical conditions of the subsoil (stratification and type) and geohydrologic parameters (aquifer type, depth to water table, flow directions) must be demonstrated to conform to local requirements for environmental protection. Despite advances in landfill construction technology, long term impact on the structural competence of the site by composite geologic, chemical and biological characteristics of both soil and water are always factored into landfill design, thus a thorough investigation of a proposed landfill site is often carried out before construction [
Soils play an important role in groundwater protection and in the transport and fate of the contaminants. The movement and attenuation of contaminants in the subsurface environment depend on several geological, chemical and biological factors including the site geology, thickness of the vadose zone, groundwater flow direction and velocity, aquifer hydraulic properties and quality and abstraction of surface and groundwater. The site in the Niger Delta area of Southern Nigeria where a sanitary landfill is proposed to be constructed is a wetland with potentials for severe environmental impacts. The mean annual rainfall exceeds 2000 mm. Groundwater tables are often very close to the surface. Thus landfills with potentials to cause pollution need to be planned, designed, constructed, operated and maintained according to best practice [
Three kinds of studies are important of proposed landfill sites. First, the geohydrolological integrity of a site must be ascertained to ensure that it is stable and ground water friendly. Geotechnical and hydrogeological data required at landfill sites include, bulk density of the soils, discontinuities, hydrogeological characteristics of vadose and soil and water geochemistry [
The landfill site is located 15 km north of Port Harcourt in southern Nigeria. Vegetation is characteristically rainforest with very tall trees. The area is drained by the Otamiri River which lies about 4 km to the west of the proposed facility site which is also the local groundwater flow direction. Geologically, the site is part of the Tertiary Niger Delta. The local rock cover consists essentially of the Benin Formation which is predominantly sandy with a few shale intercalations. The sand and sandstones are coarse-grained and constitute the main regional aquifer. The location of the site and an outline of the landfill geometry are shown in
The study was undertaken towards the end of the wet season (October-November). Ten environmental boreholes, each 20 m deep, were drilled by the percussion method at strategic locations across the proposed integrated waste management facility as shown in
The attenuation capacity of the vadose zone was evaluated using microbial degradation test of some major constituents of the leachate including fatty acids, organic nitrogen and chloride for a 28-day period. Olive oil served as source of fatty acid. Soil mixed with mercury chloride was used as negative control while surface soil
(0 - 15 cm depth) from the same area was used as control. Two concentrations of olive oil, ammonium sulphate and sodium chloride which served as sources of organic carbon, nitrogen and chloride respectively were used to simulate anticipated leachate from the facility. The objective of the experimental design was to simulate leachate concentrations of fatty acids, chloride and ammonium nitrogen that were similar to the anticipated leachate concentrations during the dry and wet seasons. Ten treatment options in triplicate were employed as follows:
Option 1, 2, 3, 4 had concentration of (NH4)2SO4 (olive oil) that were of dry season value.
Options 5, 6, 7, and 8 had concentration of olive oil that were of wet season value
Options 2, 4, 5, and 8 had concentration of (NH4)2SO4 that were of dry season value
Options 1, 3, 6, and 7 had concentrations of (NH4)2SO4 that were of wet season value
Options 3, 4, 7 and 8 had concentration of NaCl that were of dry season value while
Options 1, 2, 5, and 6 had concentration of NaCl that were of wet season value
Options 4 and 6 were representative of leachate during dry and wet season respectively.
Options 1, 2, 3, 5, 7 and 8 were mixed.
Option 9 was the negative control which contained olive oil and mercury chloride poison to kill bacteria.
Option 10 was the control sample which contained dry season levels of olive oil (NH4)2SO4 and NaCl in top soil (0 - 15 cm depth) which is expected to contain more bacteria than the deeper soil. The total heterotrophic bacteria (THB) count of soil samples was performed by inoculating 0.1ml of appropriate dilution of soil suspension in deionized water on nutrient agar plates using the spread plate technique [
Higher concentrations of constituents in the 10 options represented dry season levels, while the lower concentrations represented the wet season levels. Each option was placed in a dark plastic container of 1.5 L volume biodegradation test vessel and then kept in a thermos cooler which contained a lit candle to eliminate most of the air for the 28-day duration of the study. The candle light was extinguished after 3 hours as a result of depletion of oxygen level with the exception of option 10. The objective of this low oxygen incubation at room temperature was to simulate microaerophilic to anaerobic condition in the soil (at depth 6.5 m) where low levels of oxygen are characteristic. The options were watered every 7 days with 25 ml of distilled water to maintain a fairly constant moisture content of soil in all the options. Analysis of soil physicochemical properties and total heterotrophic bacteria count was performed on surface and 6.5 m deep soils before and after treatment with simulated leachate on day 0, 7, 14, 21, and 28. The physical and chemical characteristics of the soils before treatment are shown in
Parameters | Soil samples | |
---|---|---|
(0 - 15 cm depth) | (6.5 m depth) | |
Description | Darkish top soil | Brownish clayey soil |
Porosity | 0.38 | 0.38 |
Bulk density (kg/m3) | 1750 | 1890 |
Permeability (cm/sec) | 3.6 × 10−5 | 0.0144 |
Sand | 75% | 75% |
Silt | 20% | 20% |
Clay | 5% | 5% |
pH | 6.65 ± 0.03 | 6.2 ± 0.2 |
% Moisture | 14.4 ± 0.5 | 10.7 ± 0.03 |
% Total Organic Carbon | 0.58 ± 0.04 | 0.18 ± 0.02 |
% Organic Nitrogen | 0.105 ± 0.02 | 0.0035 ± 0.0001 |
% Ammonium Nitrogen | 0.02 ± 0.01 | 0.0014 ± 0.0001 |
Chloride (mg/g) | 780 ± 10.5 | 750 ± 0.5 |
Sulphate (mg/g) | 120 ± 5.0 | 71.4 ± 2.5 |
Phosphate (mg/g) | 12.0 ± 0.8 | 1.6 ± 0.2 |
conditions using hydrogeological, chemical and biological characteristics which are considered as important site parameters that influence the occurrence of natural attenuation.
The stratigraphy of the area delineated from soil samples obtained in the 20 m deep boreholes is characterized by three major soil types: a 1-m thick top darkish clayey fine sand underlain by a 10-m thick brownish clayey sand which is in turn succeeded by a 9-m thick fine to coarse sand horizon. The characteristics of the various horizons are presented in the
The groundwater chemistry is shown in
Layer | Depth (m) | Description | Texture/percentage composition | Uniformity coefficient | Permeability (cm/sec) | Liquid Limit (%) | Plastic Limit (%) | Plasticity index | Natural Moisture content (%) |
---|---|---|---|---|---|---|---|---|---|
I | 0 - 0.5 | clayey fine sand) | Sand (65% - 76%) Silt & clay (24% - 35%) | - | 10−5 | 34 - 45 | 17 - 21 | 17 - 24 | 14 - 35 |
II | 0.5 - 11 | Brownish clayey sand | Sand (75% - 96%) Silt & clay (4% - 25%) | 4 | 10−3 - 10−4 | 40 - 44 | 21 - 22 | 19 - 22 | 15 - 37 |
III | 11 - 20 | fine-coarse Sand | Sand (99% - 100%) Silt & clay (0% - 1%) | 4 - 6 | 10−2 - 10−3 | 33 - 40 | 15 - 18 | 16 - 22 | 12 - 22 |
BH No | pH | Cond. (ms/cm) | TDS (mg/1) | DO mg/1 | Temp ˚C | TSS mg/1 | BOD (mg/1) | Turbidity (NTU) | Hardn (mg/1) | Cl− mg/1 | |||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
BH1 | 7.11 | 287 | 142 | 4.8 | 28 | 650 | 15 | 1.60 | 65 | 4.5 | 0.02 | 0.09 | 0.06 |
BH3 | 7.11 | 80.2 | 42.4 | 5.6 | 28 | 7.30 | 50 | 2.30 | 95 | 3.0 | 0.04 | 0.04 | 0.03 |
BH4 | 7.17 | 119.7 | 55.8 | 5.5 | 27.5 | 15.60 | 70 | 3.5 | 75 | 4.0 | 0.62 | 0.07 | 0.05 |
BH5 | 6.99 | 282 | 140 | 4.8 | 28 | 460 | 180 | 2.8 | 80 | 5.0 | 0.06 | 0.10 | 0.07 |
BH6 | 7.25 | 125.9 | 62.9 | 5.3 | 28.5 | 10.40 | 65 | 3.2 | 85 | 4.5 | 0.06 | 0.08 | 0.05 |
BH8 | 6.97 | 185.8 | 93.0 | 5.2 | 27.5 | 20.80 | 190 | 2.6 | 255 | 3.5 | 0.04 | 0.06 | 0.0.07 |
BH9 | 7.13 | 84.6 | 42.1 | 5.8 | 28.0 | 12.20 | 150 | 2.6 | 55 | 23.8 | 0.04 | 0.05 | 0.04 |
BH10 | 7.04 | 115.6 | 57.9 | 5.4 | 27.5 | 5.63 | 75 | 3.0 | 75 | 3.5 | 0.04 | 0.08 | 0.06 |
The mean microbial count in the soil is summarized in
The percentage reduction in the various properties with time during the degradation period is presented in
Ammonium nitrogen and organic nitrogen decreased steadily in all the options except option 9) due to microbial utilization of nitrogen [
S/No | Sample Description | Mean Microbial count (cfu/g. or ml. of sample) | |||
---|---|---|---|---|---|
Heterotrophic | Fungal | Coliform | Hydrocarbon | ||
count | count | count | degraders | ||
1. | Surface (Soil, 0 - 15 cm) | 8.30 × 105 | 1.44 × 104 | <×103 | 3.7 × 104 |
2. | Subsurface (Soil, 0.5 m) | 3.86 × 104 | 2.13 × 103 | <×103 | 1.0 × 103 |
3. | Subsurface (Soil, 1.0 m) | 5.87 × 103 | <×103 | <×103 | 1 × 103 |
4. | Subsurface (Soil, 1.5 - 3 m) | 1 × 103 | <×103 | <×103 | 1 × 103 |
5. | Subsurface (aquifer mud) | 4.80 × 105 | 8.53 × 103 | <×103 | 1.08 × 104 |
6. | Subsurface (aquifer water) | 4.78 × 103 | ×103 | <×103 | 1 × 103 |
Dry season samples | Wet season samples | |||||||
---|---|---|---|---|---|---|---|---|
% reduction from weeks 1 to 4 | % reduction from weeks 1 to 4 | |||||||
Week | 1 | 2 | 3 | 4 | 1 | 2 | 3 | 4 |
PH | 3 | 8 | 8 | 8 | 2 | 2 | 3 | 4 |
Soil moisture | 10 | 10 | 10 | 10 | 10 | |||
Ammonia Nitrogen | 29 | 63 | 82 | 100 | 26 | 66 | 84 | 100 |
Organic nitrogen | 26 | 48 | 75 | 97 | 17 | 26 | 30 | 41 |
Chloride | 5 | 5 | 5 | 52 | 0 | 0 | 0 | 20 |
Phosphate | 57 | 87 | 92 | 95 | 30 | 55 | 80 | 100 |
Sulphate | 16 | 35 | 44 | 50 | 3 | 7 | 6 | 7 |
Organic carbon | 49 | 94 | 95 | 95 | 50 | 99 | 100 | 100 |
THB (increase) | 900 | 2100 | 20,900 | 79,900 | 1500 | 3900 | 15,900 | 74,900 |
this phenomenon. Two concentrations of NaCl (0.5 g/kg and 1.5 g/kg of soil) were employed in this study. Results revealed that the higher concentration of NaCl promoted disappearance of chloride. Other than option 9, all other options recorded significant percentage organic carbon removal at day 14. However, the removal stabilized by day 28. The inference from these results is that olive oil, the source of organic carbon was easily biodegraded in the vadose zone. The total heterotrophic bacterial count of all the options except 9 shows an increase with time. The results suggest that the population of aerobic bacteria at soil depth 6 m is sufficient to bring about over 90% organic carbon removal. Biodegradation will however be faster in surface soils where aerobic bacteria predominate and oxygen is more readily available to the resident bacterial flora. The general trend of other results indicates that the values of most components analysed declined steadily except moisture content and Total Hetrerotrophic Bacteria count which showed increased values. The decrease in chloride and sulphate are much smaller than the other components. A summary of the qualitative assessment of the various site specific characteristics that favour natural attenuation is shown in
The results of this study may be summarized as follows:
1) The soils of the vadose zone (0.5 - 6.5 m) at the landfill site are mainly brownish clayey sands of low permeability. pH of the groundwater implies that it is basic in nature.
2) The population of aerobic bacteria within the zone is sufficient to bring about over 90% organic carbon removal.
3) The major constituents of the leachate such as ammonia/organic nitrogen, phosphate and organic carbon were completely degraded within 28 days while less than 50% of sulphate and chloride were degraded within the same period.
4) Although the decrease in the components of the leachate is generally higher in the dry season than in the wet season, the seasonal variations are quite insignificant.
Parameter | site conditions | Values/conditions favourable for natural attenuation | Ranking (natural attenuation) |
---|---|---|---|
Soil Permeability | Sands; K = 10−2 - 10−4 cm/sec | Moderate to high (i.e. sands, gravels,) K > 10−4 cm/sec | Favourable |
Groundwater gradient | 0.4% | Low to moderate | Favourable |
Depth to groundwater (thickness of vadose zone) | >8 m | Moderate to deep | Favourable |
Recharge | Mean annual rainfall = 2500 m; recharge rate = 0.83 m/yr | High | Favourable |
Moisture content | 12% - 35% | Moderate (avoid excessively wet or dry soil) | Favourable |
Dissolved oxygen (DO) | 4.8 - 5.8 mg/l | Minimum is 1 - 2 mg/l | Favourable |
Soil/water PH | 6.97 - 7.13 | 6 - 8 | Favourable |
Total Heterotrophic (TH) | 1 × 104 - 8.3 × 105 | - | Favourable |
Hydrocarbon Degraders (HD) | 3 × 104 - 1 × 193 | >1 × 105 | Favourable |
Ratio of TH/HD | 20% - 60% | HD is significant percentage of TH | Slightly Favourable |
Results of the assessment of the various site specific characteristics reveal that both hydrogeological and chemical conditions are very favorable for natural attenuation while the in-situ biological conditions appear to be less favourable. The overall site conditions strongly indicate that the potential natural attenuation capacity of the vadose zone is moderate to high.
Akaha C. Tse,Lucky O. Odokuma, (2016) Biophysical Evaluation of the Vadose Zone at a Landfill Site in the Niger Delta, Nigeria. Journal of Geoscience and Environment Protection,04,33-42. doi: 10.4236/gep.2016.44006