Question

What occurs when used engine oil is poured into the pit in the ground?

The oil remains sealed in the small disposal pit and does not affect the groundwater.


The oil seeps through the soil until it leaks into, and mixes with, the groundwater.


The oil seeps through the soil but stops before it reaches the groundwater due to the difference in densities.


The oil creates a membrane barrier between the water table and the groundwater.

Answer 1

Best answer: The oil will seep through the soil until it reaches the groundwater (closest to option 2), but with an important clarification: motor oil is hydrophobic and typically less dense than water, so when it reaches the water table it usually does not fully “mix” — it accumulates as a separate floating layer (an LNAPL) and can spread laterally. Small amounts of soluble/toxic hydrocarbons can dissolve into the groundwater and create a contaminant plume, and oil sorbs to soils and persists, causing long‑term contamination.

Key points:
- Oil percolates through the unsaturated zone and can reach the water table.
- It tends to float on groundwater rather than mix uniformly, but still contaminates via dissolved constituents and soil contamination.
- Proper disposal is required (recycle or take to a hazardous waste facility).