The remediation of an arsenic-contaminated site has successfully paved the way for new commercial uses.
When the Department of Environmental Affairs decided not to approve off-site disposal as a default remediation strategy, an alternative in situ methodology needed to be adopted in remediating a section of industrial land in Durban.
The chosen approach was to use ferrous sulfate to stabilise the contaminated soil.
“Our conceptual site model had identified potential risks to human receptors, as well as groundwater and surface water resources,” explains Richard O’Brien, principal environmental geochemist, SRK Consulting.
“The human health risk meant that any action plan had to ensure that impacted soil would be capped with hardstanding paving to mitigate the exposure of workers to arsenic impacted soil.”
The remedial strategy focused on protecting the groundwater resource, occurring at a depth of 7 m to 13 m below surface. The risk assessment involved site-specific geochemical analysis of arsenic mobility and partitioning within the soil – an uncommon approach in this part of the world, says O’Brien, but one that proved effective in assessing groundwater risks.
Eastern and western zones
Results from the initial soil assessment delineated the site into two in terms of soil arsenic concentrations. In the western portion, the concentration was usually over the soil screening value for industrial land use of 150 mg/kg; the eastern portion was primarily below this level.
The eastern portion was subjected to an expedited approach using a field-portable X-ray fluorescence analyser to delineate hotspots for excavation and stockpiling: their concentrations did not indicate pervasive contamination.
In turn, the western portion progressed to a detailed site geochemical characterisation and risk assessment using geohydrological modelling due to the high arsenic concentrations.
“Geochemical and hydrogeological evaluation enabled remediation target levels specific to the site to be calculated, and the stabilisation of the residual arsenic – by adding ferrous sulfate – was assessed in laboratory-scale bench experiments,” says O’Brien.
This testing was followed by a small-scale field trial, which confirmed the efficacy of mixing the ferrous sulfate with soil. Full-scale remediation began in November 2017, with about 3 200 tonnes of arsenic-impacted soil being stabilised in batches with ferrous sulfate heptahydrate.
The treatment of the soil entailed excavating to a depth of 1.5 m and placing the soil into stockpile layers of about 0.5 m. The ferrous sulfate was then thoroughly mixed into the soil using an excavator fitted with a skeleton bucket.
“We also recognised that dust generation was an important safety issue, so our human health monitoring strategy included three different levels of dust monitoring,” he continues.
“There were dust buckets, fence-line monitoring by an external air quality consultant, and an in-field dust monitor. It was also vital to protect the remediation crew through disposable overalls, and checking their health with medical entry and exit tests.”
The sheer scale of the project is reflected in the logistics of packaging and transporting the ferrous sulfate. This comprised some 50 tonnes of the chemical delivered in 25 kg bags for distribution around the site before mixing. Subsequently, the site has been signed off for other industrial uses.