By Christopher Nielsen, T. Harris Environmental Management Inc.
When it comes to investigations of Underground Storage Tanks (USTs), there are all sorts of surprises. Several years ago, one of our clients called to complain about heavy, black oil that showed up in one of their building's sump pits. The sumps were connected to exterior weeping tiles that surrounded the building. The client was aware of only two USTs on their site. One was a 10,000 gallon diesel fuel tank for the boilers, and the other a 500 gallon diesel tank for the back-up generator. No heavy, Bunker C-type oil had been used at the building since the old Bunker C tank had been converted to diesel fuel many years ago.
Shortly thereafter, during an early spring thaw, what appeared to be the same oil was noted to be seeping out of the ground near the northwest corner of the building. It was believed that this was the same oil that had appeared in the sump pit several weeks earlier, having leached into the weeping tile bed and/or catch basin and flowed into the sump.
The client was perplexed as to the source of the oil. It was then suspected that the neighbour, having several months before removed a UST approximately 150 feet away from the oil seeping from the soil, must have caused the problem. The tank had been removed quickly and quietly with no indication of leaking, testing, or clean-up at the time.
The client commissioned the services of T. Harris Environmental Management Inc. (THEM) to conduct a Phase I ESA (Historical) and a Phase II sub-surface investigation in order to gather evidence against the neighbour. The Phase I ESA identified the two USTs currently present on site but no other items of concern. During the course of the Phase II sub-surface investigation, it was discovered that the contamination was not present between the neighbour and the visual oil seepage on site. A subsequent geomagnetic investigation revealed an additional UST that was not identified in historical documentation and which the current owner of the property did not know existed.
The access port for the tank was covered by rock landscaping and was approximately 0.5 metres below grade. It appears that the newly discovered tank had been used for Bunker C oil for the old boilers before they were converted to diesel. Apparently, the tank was not converted to diesel fuel but simply abandoned, with a new diesel tank installed nearby. After the discovery, rumours surfaced that the old tank had been emptied and filled with sand.
A full site remediation was performed, with all three USTs removed and one new tank installed. It became apparent during the remediation that the old Bunker C tank had leaked and that it had not even been emptied before it was decommissioned. The process was also complicated by the fact that the oil had followed the weeping tiles partially around the building. In addition, the weeping tile system had failed and a significant amount of water in the system buoyed the fuel to the top. This caused the oil to travel substantially up-gradient in the tile bed system. Confirmatory sampling showed evidence of bunker oil as well as diesel fuel from previous spills and fill overflows associated with the newer tanks.
Despite the surprises, complications, and difficulties of the job, full remediation of the site was eventually completed. This case is a perfect example of opening a "tank" of worms when investigating USTs and their potential impact on sub-surface environments.
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