Other transport models for solutes

SAPREF – Tara Road, Moore Spence Jones (Pty) Ltd – spread of contamination of petrol-based light petroleum hydrocarbons from the pipeline leak at Durban, South Africa.

In June 2001 a huge fuel leak was detected at the SAPREF medium pipeline between the harbour and the refinery in Durban. Immediately after the accident, disaster mitigating measures were adopted and the remediation of the affected site commenced. The major focus was on the removal of LNAPL free phase in the accident epicentre and on the system for control monitoring of the process of natural attenuation in the broader surrounding area.

From the geological point of view the region is made up of unconsolidated, sand to clay-sand deluvial and aluvial sediments of Quaternary age. These sediments rest on unconsolidated sediments of the so-called Harbour Beds, a part of the Berea formation. The sediments are water bearing, in a broader sense they form a continuous unconfined aquifer. At certain points of the aquifer, lenses of clay material may occur. If their spatial range is larger, they split the original continuous aquifer into two. This also applies to the area where the oil product leak occurred. The upper aquifer is separated from the underlying one with a 1-metre-thick clay stratum at a depth of approx 10 to 11 metres below the ground. The hydraulic conductivity of (clay-) sand sediments ranges from x.10-5 to x.10-3 m.s-1.

Mathematical modelling focused on the process of natural attenuation as the second of the selected remediation methods. The subject of modelling was contaminants of the BTEX Group. The reason for choosing these contaminants is their relatively high solubility in water, and thus their great potential to migrate along with the medium. Last but not least, these substances pose a major risk to human health (e.g. benzene is a carcinogen). The mathematical modelling included – apart from advection, hydrodynamic dispersion and sorption – the biodegradation in the sequence: aerobic decomposition->denitrification->iron reduction->sulphate reduction->methanogenesis. The activities also reviewed the monitoring protocol of the processes of natural attenuation. The MODFLOW, BIOSCREEN and RT3D codes were used for the modelling.

Brodek u Přerova, AECOM CZ s.r.o. – spread of contamination with polycyclic aromatic hydrocarbons (PAH), generated by the industrial production of treated wood on the premises of the so-called “Bridge perimeter” at the western edge of the community of Brodek u Přerova. In the course of the work we assembled and calibrated the flow and transport model of the dissolved phase with the subsequent predictive simulation of transported contamination for a period of the future 50 years. A transport of 3 substances was modelled. These substances – in terms of their mobility – represent the group of polycyclic aromatic hydrocarbons, i.e. naphthalene, fluoranthene and benzo(a)pyrene, plus one hypothetical substance called as a sum of PAH, whose physical-chemical parameters correspond to the weighted average of all the represented (analysed) substances, where the weight was the share of the relevant component in the summary concentration of PAH. To determine the amount of contamination that penetrates into the groundwater primarily by the infiltration of rainwater through the contaminated unsaturated zone, the 1D mathematical model VLEACH was used. VLEACH describes the travel of organic contaminants within the framework of and between three different phases: (1) phase dissolved in water, (2) gas phase (volatilisation), and (3) sorbate. The balance between the three phases is determined by the relevant user-defined distribution coefficients. As regards the transport mechanisms, VLEACH simulates the vertical advection of a liquid phase and the diffusion of a gaseous phase. Modelling of groundwater flows and spread of contaminants in groundwater was implemented using the MODFLOW and MT3DMS codes.

Xstrata Horizon Mine, Marsh Environmental Services – deep chrome mine, North West province, South Africa. Evaluation of impact of the deep chrome mining on the groundwater chemistry. Within the evaluation, transport and flow models were designed. The transport model concentrated on the spread of hexavalent chromium and nitrates as the major contaminants generated by the mining operations. The transport model of the hexavalent chromium – apart from advection and hydrodynamic dispersion – also allowed for sorption as per Langmuir Isotherm which was acquired through an experiment using the lab-determined limit reduction capacity of the aquifer rocks. Modelling was implemented using the MODFLOW, Hydrus and MT3DMS codes.

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