Page 50 - Materials Chemistry, Second Edition

Site Assessment and Remedial Investigation 33

Discussion:
1. Specific gravity is the ratio of the density of a substance to the
density of a reference substance (commonly, water at 4°C).
2. As shown in this example, the actual thickness of the free prod-
uct is only 3 in., while the apparent thickness within the moni-
toring well is much larger at 75 in. (a 25-fold difference).

Example 2.18: Estimate the Mass and Volume of
the Free-Floating Product
Recent groundwater-monitoring results at an impacted site indicate that the
areal extent of the free-floating product has an approximately rectangular
shape of 50 ft × 40 ft. From the apparent thicknesses of free product in four
monitoring wells inside the plume, the true thicknesses of free product in
the vicinities of these four wells were estimated to be 2, 2.6, 2.8, and 3 ft,
respectively. The effective porosity of the subsurface is 0.35. Estimate the
mass and volume of the free-floating product present at the site. Assume the
specific gravity of the free-floating product is equal to 0.8.

Solution:
(a) The areal extent of the free-floating product
= (50′)(40′) = 2,000 ft 2
(b) The average thickness of the free-floating product
= (2 + 2.6 + 2.8 + 3)/4 = 2.6 ft
(c) The volume of the free-floating product
= (volume of the free-floating product zone) × (effective
porosity of the formation)
= [(area)(thickness)] × (effective porosity of the formation)
= [(2,000 ft )(2.6 ft)](0.35)
2
= (5,200)(0.35) = 1,820 ft = 13,610 gal
3
(d) Mass of the free-floating product
= (volume of the free-floating product)(density of the free-
floating product)
= (1,820 ft ){0.8 g/cm ) × [62.4 lb/ft /(1 g/cm )]}
3
3
3
3
= 90,854 lb = 41,300 kg
Discussion:
Effective porosity should be used instead of porosity for these types of
estimates. The effective porosity represents the portion of pore space
that contributes to flow of the fluid (i.e., free product here) through
the porous medium.