Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Sustainable Risk Based Analysis towards Remediation of a Fractured Aquifer impacted by Crude Oil Spill

Auteur : Zahran Al-Busaidi

Université de soutenance : Sultan Qaboos University

Grade : Master Civil and Architecture Engineering 2017

Résumé
Often groundwater remediation is complex due to the difficulty to understand the behaviour of liquid in the sub-surface. To have an aquifer that is fractured, it makes the understanding of it extremely difficult. The complexity was increased by the fact that this aquifer has very poor primary porosity and permeability which means that the fluid movement will happen mainly in the uncemented fractures/joints. The Site of interest is dominated by three main rock structures ; namely in sequence from the top to the bottom, the unconsolidated alluvium (0.4 – 4.26 m thick), the fractured cemented conglomerate (4.65 – 15.7 m thick) and the Peridotite as bed rock. The approximate area impacted by contamination is around 0.24km2. There are 21 domestic wells completed in conglomerate or peridotite on the Site and the surrounding nearby areas owned by the Citizens and are used for irrigation of the farms in the study area. Fifteen (15) monitoring wells were drilled in the area in the year 2000 helped in delineating the contamination plume. The monitoring wells indicated that the thickness of free floating product in these wells varied with time from as high as 4.7m to 2.0mm. Four (4) monitoring wells drilled in 2016 to help in defining if the plume has migrated longer distance along the downstream side. This thesis is aiming to analyse data available about the Site of interest in order to help in proposing a sustainable risk-based approach for remediation. In order to this, it is necessary to characterise the spilled crude oil and assess the aquifer impacted. Geological and hydrogeological data, such as hydraulic conductivity, soil properties, lithology and other parameters were analysed in order to develop a simplified conceptual site model. Various remediation techniques were analysed to help choosing the most suitable one for the remediation. Unsteady pumping tests were conducted in the Site using 3 pumping wells for 8 hours in each well. The tests indicated that transmissivity in the area is variable and the highest calculated transmissivity value was 183.12 m2/day. These tests did not result in the recovery of any LNAPL from any of the 3 wells. These results indicate that although the LNAPL plume is slightly mobile but it is not practically recoverable. Based on these results, it was not feasible to conduct pump-and-treat in the Site. Another result that was made in this thesis and that is after more than 16 years from drilling the monitoring wells, the plume did not migrate significantly downstream. In addition, some of the impacted wells in the year 2000 are no longer contaminated. The thesis was concluded with various recommendations to remediate the area in sustainable risk-based manner.

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