Oil migration from sediment is driven by the dynamics of surface tension relationships among oil, water and gas, and makes this everyday phenomenon difficult to observe and measure.
Detecting oil, or non-aqueous phase liquids (NAPLs), migrating from contaminated sediment can be tricky business. The right conditions must be present for NAPLs to migrate from contaminated sediment to the water surface, and migration is commonly driven by forces other than groundwater flow. Oil migration from sediment is driven by the dynamics of surface tension relationships among oil, water and gas, and makes this everyday phenomenon difficult to observe and measure.
Recently a client called on Burns & McDonnell to evaluate sheens developing in a surface water body. During a short daytime period, hydrocarbon sheens were visible on the water surface, and at other times throughout the day no sheens were visible.
The difference was in the timing. Specific conditions involving the water level, which varied throughout the day, were needed for the hydrocarbons to migrate and be detectable. Because the interfacial tension of the oil was less than that of water, the oil spread across the water surface. If the water level was low, only water and air were in contact at the water surface and migration did not occur. If the water level was just right and intersected the area of oily sediment, then oil, water and air were in contact at the water surface and sheens formed. If the water level was too high, only air and water were in contact and the oily sediment was submerged, meaning no migration. The difference in water levels that drove this migration was less than one foot.
"The oil sheen forms only when oil, water and air are present at the same location," says Gene McLinn, senior associate environmental specialist at Burns & McDonnell. "You have to understand why the sheens form in order to measure them effectively."
With that understanding, creating a sampling process is much simpler because the question of when to test is answered. Where testing should be completed is typically determined by where the hydrocarbons can be seen on the surface of the water.
"Understanding the mechanism of oil migration in this setting allows us to proceed with confidence to flesh out a solution for mitigation and allows our client to manage its liability," McLinn says.
For more information, contact Gene McLinn, 608-630-4961.