GROUNDWATER - Water Quality Engineering in Natural Systems - page 182

Using the relation proposed by Charbeneau (2000),
Equation (5.87) gives

above-ground oil-water separator and minimize the
load on aboveground treatment systems. In dual-pump
systems, both the water pump and the product recovery
pump are installed in a single well, which must have a
large enough diameter to accommodate both pumps
and also accomodate product detection probes that are
used to turn the pumps on and off. Dual-pump systems
have greater costs and maintenance requirements than
single-pump systems.
Free-product recovery systems frequently include
interceptor trenches, which are used in cases where the
water table is shallow, or some shallow, confining layer
limits the maximum depth of liquid penetration to less
than 6 m (20 ft) below the ground surface. Trenches are
excavated to a depth below the liquid product, trench
widths are typically 1-3 m (3-10 ft) depending on the
stability of the soil, and the length of the trench extends
beyond the limits of the free product lens. Trench depths
typically extend 1-2 m (3-6 ft) below the water table.
One or more vertical wells are placed in the trench,
which is then backfilled with very permeable gravel, and
a surface seal is typically placed on top of the trench to
minimize vapor losses. Drawdown in the trench must be
sufficient to reverse the groundwater gradient on the
downgradient side of the trench so that the floating
product cannot flow out. Liquid product recovery rates
rarely exceed 20 L/min (5 gpm). Vertical recovery wells
are typically used where interceptor trenches are not
feasible.

H = β ( H − α ) = 0 393 0 30 0 167 . ( . − . ) = 0 052 . m = 5 2 . cm

a w

The volume of gasoline, V , floating on the water table
is given by

V nA H = = ( . 0 23 2500 )( ) H = 575 H

spill a a a

Therefore, the Kemblowski and Chiang equation
gives V = 575(0.06) = 35 m 3 , the Hampton and miller
equation gives V = 575(0.075) = 43 m 3 , and the Charbe-
neau equation gives V = 575(0.052) = 30 m 3 . A conser-
vative estimate of the volume of gasoline is 43 m 3 .

Pumping well recovery schemes include (1) skim-
ming systems for nAPLs only; (2) single, total fluids
pumps that pump water and free product mixtures;
and (3) dual-recovery pump systems in which a ground-
water pump is used to depress the water table, while a
second pump skims the LnAPL free product that flows
into the well. Skimming systems are used when there is
not a significant quantity of nAPL trapped below the
water table, water table depression is not needed to
accelerate flow to the well, the aquifer is composed of
very low permeability materials, and it is cost-prohibitive
or not possible to treat produced groundwater above
ground. Single- and dual-pump recovery systems are
used when a significant quantity of nAPL is trapped
below the water table, a water table depression is needed
to accelerate the flow to the well, and the formation is
composed of relatively permeable material. A typical
dual-recovery pump system is illustrated in Figure 5.18.
Single-pump systems tend to emulsify the water and oil,
requiring the use of an oil-water separation system,
while dual-pump systems eliminate the need for an

5.8.2.2 Excavation and Disposal. In cases where a
limited amount of soil is contaminated with hazardous
materials, excavation and disposal of the contaminated
soil is an option to be considered. The excavated soil is
transported to a hazardous waste incinerator for com-
plete thermal destruction of organic contaminants. This
approach is typically used for highly refractory organic

treatment or disposal

product recovery

ground surface

leaking tank

water table

product recovery pump

groundwater depression pump

recovery well

Figure 5.18. Dual-recovery pump system.