An Introduction to Environmental Chemistry

atively charged species and become much more mobile than in the neutral
conditions generally typical of groundwater.

Petroleum contamination—Bowling Green, Kentucky, USA

Although groundwater flow rates are generally low when compared to surface
waters, large cracks and conduits in some contaminated aquifers cause specific
problems. The US city of Bowling Green, Kentucky, is built on limestone
bedrock (Ste Geneviève limestone), with underground drainage through the Lost
River Cave (Fig. 5.18). Limestone bedrock is often heavily fissured and joints in
the rock are enlarged by dissolution, resulting in interconnected caves. Sinkholes
may divert surface streams into these fissures and caves, resulting in a subsurface
drainage system. Accidental spillage of toxic chemicals or any other contaminant
is rapidly dispersed in these conduits, making remediation particularly difficult.
In the 1970s and 1980s up to 22 000 litres of petroleum leaked from storage
tanks at auto service stations in Bowling Green into the subsurface water. Petro-
leum, being a non-aqueous phase liquid (NAPL), floats on the surface of ground-
water and its volatile components (see Box 4.14), for example benzene, rapidly
fill air spaces with explosive fumes, particularly at sumps or traps in the cave
system (Fig. 5.18). The trapped fumes then escape into the basements of build-
ings, water wells and storm drains.
In addition, leaking tanks at a chemical company are believed to have
delivered benzene, methylene chloride (CH 2 Cl 2 ), toluene (C 6 H 5 CH 3 ), xylene
(C 6 H 4 (CH 3 ) 2 ) and aliphatic hydrocarbons (see Section 2.7) to the subsurface
water. These toxic (some carcinogenic) chemicals vaporize in the cave atmos-
phere, collect at traps and then rise into homes in a similar way to petroleum
fumes.
The potential explosive/toxicity risk in Bowling Green has resulted in a
number of evacuations of homes in the last 20 years. Remediation measures have

The Chemistry of Continental Waters 177

Pond

200

300 200 200 150 100

N 1000

0 0.5 1

km

50

50

Line of equal chloride concentration,

1974—dashed where approximately located.

Interval 50 and 100 mg l–1

Landfill deposits

Fig. 5.17Map of Cl-plume at 9–12 m depth below the water table, Babylon landfill site,
1974, showing the extent of groundwater contamination. After Kimmel and Braids (1980).