Industrial Sector

FORMER PETROCHEMICAL PLANT

Lower Mainland, British Columbia

CHALLENGE: Morrow was retained to review the site soil and groundwater characterization work of another consultant.

The site is constructed on uniform alluvial river sand fill dredged and placed over native silt over a sand aquifer. The silts are suspected of being thin or absent at many locations due to site construction activities. A perched groundwater table exists in the sand fill; the regional groundwater flow system located in the alluvial sand is influenced both diurnally by the adjacent river estuary and seasonally by the freshet. As a result vertical gradients between the perched aquifer and the regional aquifer vary daily and seasonally.

Raw products used at the plant included pure phase toluene, xylenes, and inorganic catalysts. Historic releases resulted in significant widespread benzene toluene and xylene contamination of sand fill, overbank silts and the perched groundwater beneath the former plant. Characterization of the site resulted in quantity estimates of BTX in all forms up to 520,000 kg (18% benzene, 69% toluene, 13% xylenes).

SOLUTION: Based on this review, Morrow developed a remediation program consisting of removal of pure phase products and risk management for residuals.

Risk-based remediation criteria were agreed to with the provincial regulators that are intended to be protective of human health (vapour inhalation) and freshwater aquatic life.

A remediation plan for the site was initiated, with a five-year remediation objective. Numerous remedial strategies were considered including air sparging and contaminant excavation and treatment, among others. A conceptual remedial plan was selected, which comprised soil vapour extraction (SVE) in conjunction with dewatering.

Verification of the conceptual remedial plan consisted of the following activities: overbank silt deposit mapping using cone penetrometer testing (CPT); SVE pilot testing; collection of undisturbed (Shelby tube) samples of the overbank silt for triaxial testing to determine vertical permeability (leakage) of the aquitard; and, modelling. Data from the testing was used for modelling dewatering requirements/conditions (ModFlow) and SVE performance (AirFlow SVE). Calibration of the groundwater model indicated the presence of a boundary condition adjacent to the area, which was confirmed by drilling as a buried dyke constructed as part of the site development.

The conceptual remedial design was refined so that remediation will occur in four blocks of approximately equal contaminant mass. The design for dewatering includes perimeter dewatering eductor wells, in conjunction with dual phase dewatering through the SVE wells using a drop tube. Morrow managed the tendering and installation of the system: to date the system has removed over 170,000 kg BTX.