Abstract
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 × 10Dž and a pore velocity of 1.6 × 10Dž 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.
Highlights
Landfills are covered or uncovered waste receiving pits
At the time of the investigation in October which is about the end of the rainy season, the depth to water table which is equal to the thickness of the vadose zone varied from 8 - 13 m
When these depths (1.5 m + 1.5 m) are added to the proposed 5 m thickness of the landfill, the minimum thickness of the vadose zone that is required is 8m which is adequate at this location
Summary
The wells were completed with 5 cm diameter PVC casing and screens. The annulus of the boreholes was backfilled with uniformly graded coarse sand up to 0.5 m above screen level. Well cuttings were used to backfill the remaining part of the annulus up to 0.75 m below ground surface and cement grouted to the surface, ensuring that the cement grout completely sealed the casing in order to prevent the introduction of surface contaminants into the aquifer. To avoid contamination by drilling chemicals, soil samples for moisture content, physico-chemical properties and microbiological studies were collected at 0.5 m intervals from 3 m auger holes located beside each of the 10 boreholes. The geotechnical index properties of the soils including particle size distribution, bulk density, permeability, Atterberg limits, and porosity were determined as specified in [15].
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