Risk-Based Decommissioning of a Sulphur Storage Facility
This project illustrates how multiple Millennium EMS Solutions Ltd. (MEMS) disciplines work in partnership to develop optimum solutions for our clients at complex contaminated sites. In this case, the site-assessment, contaminant hydrogeology and risk assessment groups worked closely together to dramatically decrease the volumes of soil requiring remediation in the decommissioning of a sulphur storage facility in northern Alberta. Examples of this close collaboration included early risk assessment involvement in the site assessment and hydrogeology data collection programs to ensure that the right data were collected for eventual risk assessment use, and contaminant hydrogeology input into the groundwater modelling stages of the risk assessment.
This facility had been used since the mid-1960s for the storage of elemental sulphur recovered from the treatment of sour natural gas. Over the operational life of the facility, sulphur was stored in block, prill, and liquid form. Run-off water from the sulphur blocks was routed to on-site storage ponds where it was treated before release. At the end of life of the facility, some residual sulphur remained in soil after all marketable sulphur was removed. Elemental sulphur in the subsurface can oxidise to generate dissolved sulphate ions and acidic conditions. Acidic conditions, in turn, can mobilize trace metals from naturally occurring soil minerals.
Conceptual Site Model
Building a conceptual site model involves compiling and organizing relevant information on site stratigraphy, hydrogeology and contaminant distribution and is a critical step in developing optimized contaminated site solutions for MEMS clients. In this case, four distinct stratigraphic units were identified, each corresponding to a different hydrogeological unit. The primary contaminant issue identified at the site was dissolved sulphate currently present in porewater and groundwater together with dissolved sulphate predicted to be generated by future oxidation of residual sulphur. Secondary contaminant issues identified included dissolved trace metals mobilized by acidic subsurface conditions.
Risk Assessment Approach
Careful assessment of the various exposure pathways at this site demonstrated that the migration of dissolved sulphate downwards towards a domestic use aquifer was a limiting issue that would drive remedial efforts. Information gathered in the conceptual site model was used to develop finite-difference groundwater flow and transport models for several vertical 2-D cross sections at the site. The results of these models were used to develop a remedial plan through iterative removal of sulphate hotspots from the model until predicted future concentrations of sulphate in the domestic use aquifer no longer exceeded drinking water guidelines. Geochemical modelling was used to investigate future movement of mobilized trace metals and demonstrate that additional remediation was not required in relation to this issue. Overall, the multi-disciplinary approach which MEMS applied to this project enabled a true optimized remedial plan to be developed, and a dramatic reduction in remedial volumes to be realized.