The site for this project - an active petroleum refinery in southern Wyoming - had previously been used for residuum storage, including the storage of spent sulfuric acid from refinery processes. Subsurface investigations previously conducted by Trihydro identified widespread residuum and low pH conditions due to the co-mingling of spent sulfuric acid.
Trihydro was tasked with evaluating total excavation and disposal in an on-site corrective action management unit (CAMU) versus in-situ soil treatment utilizing industrial by-products of other Wyoming industries. Based on the volume of the area and the potential costs associated with excavation (approximately $31 million for excavation and disposal costs) alternatives to excavation were explored.
An evaluation of in-situ neutralization and stabilization was conducted and initial pilot testing showed that the soil pH could be increased from existing levels to a pH of approximately 5.0 by mixing approximately 20% by volume of a neutralizing agent. Through research and discussion, precipitated calcium carbonate (PCC), a by-product of the sugar refining industry, was identified as a potential effective neutralizing agent. PCC, which is a non-reusable resource for sugar facilities, was tested at the pilot scale and selected as an effective and extremely cost-effective neutralizing agent for the acidic soils. Its use provided the added benefit of productive re-use of material that would otherwise be disposed as a waste.
In addition to neutralization of the acidic soils, the asphaltic wastes/free residuum were mechanically stabilized in place to reduce residuum mobility in the soils. Stabilization was conducted by mixing impacted site soils with material from the surface (e.g. PCC, Portland cement, fly ash, or onsite soils) using an extended-reach track-hoe.