A forensic study to differentiate two dissolved gasoline plumes at adjoining service stations


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Three gasoline service stations are present at a highway intersection in central New Jersey. In the mid- to late- 1980s, service station “B” began to notice large discrepancies in their gasoline inventory. The USTs were removed in 1988 and a significant release was discovered. Separate-phase gasoline, as much as 1.5 feet (0.5 m) thick was present on the water table for about an area of one acre. Monitoring wells were installed and a plume extending downgradient by as much as 300 feet (100 m) was found. Downgradient potable wells were also impacted and had to be replaced. An upgradient well was installed and this well also contained separate-phase gasoline. Accordingly, station “B” began to petition the state regulatory agency to include station “A” in the cleanup.

Based on a calculation using the product thickness, porosity of the soil and the area affected (among other parameters), the quantity of gasoline released was in the hundreds of thousands of gallons (>one million litres).

Monitoring wells were installed at station “A” and a release from underground piping was discovered. However, except for the one well installed between the two sites, separate-phase gasoline was not detected at station “A”.

In the mean time, station “B” undertook a ground-water cleanup. Several recovery wells were installed and a treatment system was built. The treated effluent was then directed to an infiltration gallery installed into the former UST excavation. Unfortunately, this area was still underlain by separate-phase petroleum and the infiltration of effluent caused the gasoline to be flushed radially in all directions, including upgradient.

The problem to be addressed at this site was the contribution of each party to the total problem. Because station “A” is hydraulically upgradient (ground water flows from station “A” to station “B”), it might appear that station “A” is the source of the problem. To differentiate the two plumes (and possibly a third from station “C” located just to the south), an isotopic study was conducted.

An isotope is one of two or more atoms of the same element that have the same number of protons in their nucleus, but a different number of neutrons. For example, carbon has three isotopes: 12C, 13C and 14C. 12C has 6 protons and 6 neutrons in its nucleus, while 13C has 6 protons and 7 neutrons. 14C has 6 protons, 8 neutrons and is also radioactive.

The isotopic content of specific contaminants in ground water can be assessed through a technology known as gas chromatography with isotope ratio mass spectrometry (GCIRMS). In environmental investigations where hydrocarbons are involved, the ratio of 13C to 12C is commonly used to fingerprint contaminants. Through this technique, values of d13C (the 13C/12C ratio) for a specific compound, such as methyl-tert-butyl ether (MTBE) or benzene, can be determined.

Values of d13C can be used as a proxy for weathering. Bacteria tend to favor the 12C and as biodegradation proceeds, the compound can become enriched in 13C. Accordingly, as weathering becomes more severe, the d13C values increase.

The map to the right shows the d13C values for MTBE in samples collected from 20 monitoring wells at the two sites (A and B). The d13C values reveal that, beneath station “B”, the MTBE is lighter or contains less 13C and, therefore, could be considered fresh (the d13C values for MTBE are within the range for fresh product). On the other hand, the d13C values for MTBE beneath station “A” are heavier and enriched in 13C.

The MTBE beneath Station “A” at this location is weathered. Accordingly, the two plumes can be distinguished by their weathering characteristics and they are outlined in the map provided above. The quantity of gasoline associated with each release can be calculated with: the areas delineated above; the contaminant concentrations, and the quantity of separate-phase gasoline known to have existed. Based on these data, greater than 95% of the gasoline originated from station “B”.

Why were the d13C values so different? Because the weathering conditions were different at the two sites. Gasoline at station “A” originated from a piping leak and had to travel through the unsaturated zone to reach the ground water. Within the unsaturated zone, the MTBE was subjected to increased weathering processes. At station “B”, the gasoline originated from USTs which where were buried to greater depths. Therefore, this release began deeper and were not subjected to the same weathering processes. The differences between the two sites are depicted on the figure below.

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