GROUNDWATER - Water Quality Engineering in Natural Systems - page 199

5.30. (a) What is the maximum concentration of
TCE that can be found in a soil gas sample
at 20°C?
(b) If a liquid consisting of 35% TCE is spilled,
what is the maximum concentration of TCE
expected in the soil gas?

calculated value of c w exceeds the saturation con-
centration of the compound.

5.34. Consider the generic case of a shallow unconfined
aquifer where the water table is located a distance
d below the ground surface. The soil between the
ground surface and water table has a density ρ b , a
porosity n , and a fraction of organic carbon equal
to f oc . You are concerned about siting a commer-
cial facility that will be handling a hazardous
chemical whose maximum allowable concentra-
tion in the groundwater is c *, density is ρ , and
organic carbon distribution coefficient is K oc . The
spilled contaminant would potentially enter the
groundwater when rainfall percolates through
the soil in which the contaminant is spilled.

5.31. What is the maximum concentration of o -xylene
in a plume of contaminated groundwater? Con-
trast this with the maximum concentration of
o -xylene in groundwater resulting from a spill
of gasoline containing a mole fraction of 9%
o -xylene.

5.32. An contaminated volume in an aquifer has a
plan area of 5 × 5 m and a depth of 2 m. The
contaminated volume is located just below the
water table and the seepage velocity in the
aquifer is estimated as 0.45 m/d in a direction that
is aligned with the sides of the contaminated
area. The contaminant in the aquifer is a mixture
of several chemical compounds and contains PCE
at a mole fraction (and approximate mass frac-
tion) of 20%. Laboratory tests indicate that the
residual saturation of the mixture in the aquifer
matrix is 10%, and that the aquifer matrix has
material density of 2.65 g/cm 3 , bulk density of
1.67 g/cm 3 , porosity of 0.37, and organic carbon
fraction of 1%. A literature review indicates that
PCE has the following properties: saturation con-
centration in water = 160 mg/L, half life of PCE
dissolved in water = 500 days, octanol-water par-
tition coefficient = 200, organic carbon partition
coefficient = 100 cm 3 /g, and density = 1620 kg/m 3 .
(a) Determine the maximum concentration of
PCE in the groundwater that is expected at a loca-
tion 200 m downstream from the contaminated
volume, and (b) estimate how long it will take for
the contaminated volume to be cleaned up by
natural processes.

(a) Derive a general expression for the maximum
spill volume per unit ground area that would
be acceptable without the concentration in
percolating rainwater exceeding the maximum
allowable concentration in the groundwater.
(b) Derive a general expression for the limiting
soil concentration (in mg/kg) that would be
consistent with the percolation rainwater
meeting the groundwater quality standard.
(c) You are concerned with the siting of a com-
mercial pesticide facility that will be handling
moderate amounts of chlordane on-site. The
water table is 1 m below the ground surface,
the soil has a bulk density of 1700 kg/m 3 , a
porosity of 0.32, and a 2% fraction of organic
carbon. The average annual precipitation on
the site is 1500 mm, of which approximately
50% percolates through the soil and enters
the saturated zone on the aquifer. The maxi-
mum allowable concentration of chlordane in
groundwater is 2 µ g/L, the density of chlor-
dane is 1600 kg/m 3 , and the organic carbon
distribution coefficient is 3090 cm 3 /g. Estimate
the maximum amount of chlordane that can
be spilled at the site (in mL/m 2 ) and not
violate the groundwater quality standards.
Estimate the maximum amount of chlordane
in the soil (in mg/kg) that would be consistent
with meeting groundwater quality standards.
(d) A soil investigation of the pesticide facility
described above indicates that chlordane
exists in the soil at a level of 4 mg/kg over an
area of 5 m 2 . If 50% of the annual rainfall
percolates through the vadose zone and into
the groundwater, estimate how long it will
take for the leachate concentration to be less
that the water-quality standard of 2 µ g/L.

5.33. Analyses of a soil sample show that the mass of
an organic compound per unit mass of the solid
matrix is c s (m/m), the bulk density of the soil is
ρ b , the water-filled porosity is θ w , the fraction of
organic carbon in the soil is f oc , and the organic
carbon sorption coefficient is K oc . Show that the
equilibrium concentration, c w , of the organic com-
pound in the interstitial groundwater is given by

c
f K

ρ

s b

c =

w

ρ + θ

b oc oc w

Explain the implications if c w is calculated from
measurements of c s , ρ b , θ w , f oc , and K oc , and the