2019 CSCE Annual Conference - Laval (Greater Montreal)

2019 CSCE Annual Conference - Laval (Greater Montreal) Conference


Title
Assessment of Groundwater below a Contaminated Soil Landfill Including Modeling of Vadose Zone

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Author(s)
Dr. Mahad Baawain, Sultan Qaboos University (Presenter)
Dr. Ahmed Sana, Sultan Qaboos University
Dr. Yahia Mohamedzein, Sultan Qaboos University
Abstract

This study aims at assessing the groundwater contamination underneath a waste yard that receives oily contaminated soil (CS). So, comprehensive analysis was conducted for the groundwater samples from upstream and downstream of the CS site. Furthermore, the study modeled the solute (hydrocarbons) transport in the unsaturated zone (i.e. the zone between ground surface and groundwater table).  The geotechnical investigation consisted of drilling and installation of two 50 m deep groundwater monitoring wells (one well located 500 m upstream and another well located 300 m downstream of the dumping site). The groundwater was encountered at a depth of 42.5 m in borehole 1 and 43 m in borehole 2.  The wells were developed using a pumping rate of 2.8 m3/hr. After pumping, the water level in the wells was 24.5 to 24.8 m below the top of the casing.

According to the collected samples, the groundwater underneath CS site is brackish water (salinity > 40 mS/cm and TDS > 24,000 mg/L) with relatively neutral pH (7 to 7.4). Hence, it cannot be used for potable uses without significant desalination process. Both samples taken from upstream and downstream wells show high hardness (~6,000 mg/L as CaCO3) and relatively low alkalinity (50 mg/L - 80 mg/L as CaCO3).  The sulphate content is very high (>2,700 mg/L). The results did not show any heavy metal content. Furthermore, the amount of the different types of hydrocarbons in the groundwater samples were below detection limits of 0.01 mg/L).

A well-known model for flow and solute transport in variably saturated porous media has been applied to study the infiltration of water and solute (dissolved hydrocarbons) transport. The computations were done using in-situ values of percolation rates and laboratory measured permeability values of compacted soils. The concentration of solute (dissolved hydrocarbons) was computed for a period of two years over a depth of 2 m below ground. For the highest percolation rate observed at dumping site, the concentration of solute 2 m below the ground after two years was computed to be 0.0007 mg/cm3 (i.e. 0.00007%) approximately.  Therefore, the chances of the oil contamination reaching the water table are remote. Moreover, the migration of contaminants can be reduced further by compacting the soil underneath the CS sites as evident from the modeling results by using lab permeability of compacted soil.