Page 48 - Materials Chemistry, Second Edition

Site Assessment and Remedial Investigation 31

2.2.6 Estimating the Mass and Volume of the Free-Floating Product
The LNAPL product leaked from a UST may accumulate on the top of the
capillary fringe of a water-table (unconfined) aquifer or on the top of the
upper confining layer of a confined aquifer to form a free-product layer. For
site remediation, it is often necessary to estimate the volume or mass of this
free-floating product. The thickness of the free product found in the moni-
toring wells had been directly used to calculate the volume of free product
outside the wells. However, these calculated values are seldom representa-
tive of the actual free-product volume existing in the formation.
It is now well known that the thickness of free product found in the forma-
tion (the actual thickness) is much smaller than that floating on top of the
water in the monitoring well (the apparent thickness). Using the apparent
thickness, without any adjustment, to estimate the volume of free product
may lead to an overestimate of the free-product volume and overdesign of
the remediation system. The overestimate of free product in the RI phase may
cause difficulties in obtaining an approval for final site closure, because the
remedial action can never recover the full amount of free product reported
in the site assessment report.
Factors affecting the difference between the actual thickness and the
apparent thickness include the densities (or specific gravity) of the free
product and the groundwater as well as the characteristics of the formation
(especially the pore sizes). Several approaches have been presented in the
literature to correlate these two thicknesses. Ballestero, Fiedler, and Kinner
[7] developed an equation using heterogeneous fluid flow mechanics and
hydrostatics to determine the actual free-product thickness in an unconfined
aquifer. The equation is

g t = (1 SG)t − − h (2.12)
a

where
t = actual (formation) free-product thickness
g
t = apparent (wellbore) free-product thickness
SG = specific gravity of the free product
h = distance from the bottom of the free product to the water table.
a
If no further data for h are available, average wetting capillary rise can be
a
used as h . Information on capillary rise can be found in Section 2.2.5.
a
To estimate the actual thickness of free product, the following procedure
can be used:
Step 1: Determine the specific gravity of free product. (The specific
gravity of gasoline can be reasonably assumed as 0.75 to 0.80, if
no additional information is available.)