By Christopher Nielsen, B.Sc., T. Harris Environmental Management Inc.
Identifying the source of groundwater contamination can not only be extremely difficult, but the results can sometimes astound property owners. In one case, a client noticed heavy oil in the sump pit of his building. It was winter and there was no obvious source of the contamination. Earlier, there had been a spill of diesel fuel due to over-filling during the filling of boiler tanks in winter. But a clean-up of the diesel fuel and soil had been completed.
During the following spring thaw, the client identified heavy oil seeping from the ground to the west of the building. He deduced that oil flowed into the catchbasin and into a weeping tile system and into a sump pit. The client surmised that a neighbour's recently removed Underground Storage Tank (UST) must have been leaking. The neighbour had removed the tank quickly with no evidence of leaking, odour, or a clean-up noted. The client was aware of only two USTs on his own site: one 10,000 gallon diesel tank for the furnace and one 500 gallon tank for the back-up generator.
The client contacted us to arrange for a Phase I ESA (Historical) and a Phase II sub-surface investigation to gather evidence against the neighbour. During the course of the sub-surface investigation, it was discovered that contamination was not present between the neighbour's property and the visual oil seepage.
A geomagnetic investigation revealed an additional UST that was historically used for Bunker C oil for the old boilers before they were converted to diesel. After the discovery, rumours surfaced that the old tank had been emptied and filled with sand. Investigators found a tank cover in a landscaped area - covered with soil and stones, approximately 0.75 metres below grade in a round access tube.
A remediation program was arranged. When three tanks had been removed, only the oil bunker C tank had leaked and the contamination followed the weeping tile bed.
Confirmatory sampling showed evidence of bunker oil and diesel from previous spills and fill overflows. The contamination had migrated along weeping tile beds surrounding the building footprint. Contamination was made worse by the fact that the weeping tiles had failed and standing water built up in the area of the USTs and up-gradient along the weeping tile beds.
Remediation was completed successfully; however, many difficulties were encountered. Challenges included the proximity of a major roadway (access and permits), steep grade and the building footings situated very close to the tanks, and the only equipment access was through the neighbour's property. But ultimately, it was confirmed that it was the client's own tank that had leaked and that it did not contaminate adjacent properties.
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