Apparent vs. Actual Free-floating Petroleum on Groundwater
Environmental Enlightenment #1
LNAPL is short for Light Non-Aqueous Phase Liquid: a liquid substance that is not water (non-aqueous), lighter than water, and therefore will float on water.
Here are some LNAPLs that we meet in contaminated soil and groundwater:
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.
When an LNAPL (like gasoline, diesel or oil) is discharged into the soil and travels down, it will meet the capillary fringe before hitting the water table. The free LNAPL will reside above and inside this fringe.
The diagram above depicts a groundwater situation where there is no LNAPL present. The water level in the open well on the right marks the level of the water table in the formation.
The next diagram demonstrates the problem we face with the presence of LNAPL. The legend on the left explains the shades of grey and other info.
See the monitoring well. The screen segment is perforated to allow entry of water. The horizontal, straight, thick black line says, “WATER TABLE:” If there were no LNAPL in the area, the water face inside the well would coincide with this line.
However, the actual water face inside the well is DEPRESSED by the weight of the LNAPL that entered the well.
The next solid black line up is crooked and says, “CAPILLARY FRINGE.” If not for the LNAPL, this is the level in the soil to which the clear water would have risen.
The LNAPL presses down on the water in the capillary fringe. The dark-grey zone is where enough LNAPL accumulated to have mobility in the soil. The light-grey above is where LNAPL resides in and tied to the soil by capillarity. In this zone the LNAPL is not mobile.
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 LNAPL drains into the well, accumulates, and its weight depresses the water in the well resulting in additional LNAPL drainage into the well.
The capillary fringe increases as the grain size of a formation get smaller: the capillary fringe in silt being 1000 millimeter while in coarse grain sand it is only 125 millimeters.
The difference between the actual and apparent LNAPL thickness increases with the increase in the capillary fringe.
The result is that in a silty formation, for example, a 20-inch thick layer of petroleum inside a well may represent a mere 2-inch layer of mobile petroleum in the formation; while in a coarse sand formation these same 20 inches may represent 10 inches in the formation.
Many studies have been performed to correlate LNAPL thickness in a monitoring well to actual LNAPL thickness. These correlations typically produce only order-of-magnitude estimates.
It should be recognized that, if free LNAPL is detected in a monitoring well, it is unlikely to be as bad in the formation, and estimates of its presence in the formation should consider the “Actual vs. Apparent” described above.
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Environmental Enlightenment #219—Here’s a data-packed site that provides handy information on environmental data at sites of interest.