Apparent vs. Actual Free Floating Petroleum
Recognition: Parts in this letter have been copied from a GeoInsight newsletter at http://www.geoinsightinc.com/newsletter.htm.
LNAPL is short for Light Non-Aqueous Phase Liquid; indicating 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 the contaminated soil and groundwater cases:
   
When discharged to the soil and reach groundwater, LNAPLs stay at and above the water table.
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 (for example, gasoline, diesel or oil) is discharged into the soil and 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.
When we investigate the magnitude of a release from, say, leaking underground petroleum storage tanks; estimating the thickness of LNAPLs is critical to proper evaluation of the volumes released and appropriate remediation programs.
The diagram below 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. Explanations follow.
The diagram shows a monitoring well. The screen segment is perforated (or slotted) to allow entry to the groundwater. The horizontal 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 resides on and presses down on the water in the capillary fringe. The dark-grey zone is where enough LNAPL accumulated so as 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 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: the capillary fringe in silt being 1000 millimeter while in coarse grain sand it is only 125 millimeter.
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 inside the well 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 studies have produced correlations that can be used to estimate the actual LNAPL thickness from apparent LNAPL thickness measured in a well. However, these correlations may not be accurate under a variety of field conditions and typically produce only order-of-magnitude estimates.
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.
|
