Free Petroleum Floating on Groundwater — the Apparency
LNAPL means “Light Non-Aqueous Phase Liquid,” meaning a liquid that is not water (non-aqueous), lighter than water, and therefore floats on water. Examples of LNAPLs are:
   
In the case of straight water, LNAPL floats as in this picture:
Therefore, when told there is two feet of oil floating on the water inside a well, many may get a mental image of a two-foot-thick layer of oil resting on the water table, like in the case of the water glass.
NOT NECESSARILY SO.
The picture below depicts a case where LNAPL reached the groundwater table and then spread like a pancake. This picture of a pancake afloat is simplified and true only in a limited number of cases. |
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Let’s discuss CAPILLARITY.
Capillarity is a natural force that makes water rise in thin conduits made of glass or earth materials.
The “capillary fringe” indicates that zone of soil immediately above the water table where water rises into the soil by the forces of capillarity.
Ranges of capillary fringe:
- In Gravel: 1/2 inch
- In Sand: From 5 inches in coarse sand to 16 inches in fine sand
- In Silt: 40 inches
- In Clay: Can reach 40 feet
When an LNAPL travels down to the water table, it will meet the capillary fringe before hitting the water table.
As the capillary fringe is saturated with water, the free LNAPL will reside above and inside this fringe.
The picture below shows a case of gasoline that leaked from an underground storage tank (UST). It shows the “pancake” that forms on and inside the capillary fringe. and it shows the free gasoline that leached from the formation into the monitoring well.
In the illustrated example, the ratio between the thickness of the oil lens in the soil to the thickness of the oil layer in the well is about 1:5
The thickness of LNAPL in a monitoring well typically exceeds the thickness of the mobile LNAPL in the subsurface by a factor estimated to range between 2 and 10.
Due to this difference, the LNAPL thickness measured in a monitoring well is commonly referred to as the "apparent thickness" and is not an accurate measurement of the LNAPL thickness in the subsurface.
The monitoring well acts as a low point into which LNAPL drains. When LNAPL accumulates in the well, its weight depresses the water table in the well resulting in additional LNAPL drainage into the well.
The difference between the actual and apparent LNAPL thickness increases with the increase in the capillary fringe; and the capillary fringe increases as the grain size of a formation get smaller:
It should be recognized that, if LNAPL is detected in a monitoring well, it is unlikely to be as bad as it looks and that estimates of release volumes derived from measurements in wells should take this LNAPL relationship into consideration. |
