Geology Reference
In-Depth Information
occurrence and possible sources of MTBE in groundwater in the United
States in 1993/1994, as part of the US Geological Survey's National
Water-Quality Assessment programme, Squillace and co-workers
191
found that, out of 60 volatile organic chemicals determined, MTBE
was the second most frequently detected chemical in samples of shallow
ambient groundwater from urban areas. MTBE was detected above 0.2
mgL
1
in 17% of shallow groundwater samples from urban areas, 1.3%
of shallow groundwater from agricultural areas, and 1.0% of deeper
groundwater samples from major aquifers. Only 3% of the shallow wells
sampled in urban areas had MTBE concentrations 420 mgL
1
, the
estimated lower limit of the USEPA draft drinking water health
advisory. The 1999 Report of the Blue Ribbons Panel on oxygenates
in gasoline found that 5-10% of community drinking water supplies in
high-MTBE-use areas showed at least detectable concentrations of
MTBE, with 1% 420 mgL
1
.
192
As MTBE generally was not found,
in shallow urban groundwater, along with benzene, toluene, ethylben-
zene, or xylene, which are commonly associated with petrol spills, it was
concluded that possible sources of MTBE in groundwater include
leaking storage tanks and non-point sources such as recharge of
precipitation and storm water runoff. In the summer of 1999, 13
municipal wells on the south shore of Lake Tahoe, located close to
commercial areas with gasoline stations, were shut down as a result of
actual or threatened MTBE contamination.
193
In Meyers, an unknown
amount of gasoline leaked from a pipe into the soil for about two weeks
before the release was repaired and five months later a groundwater
sample collected at the gasoline station was 28,000 mgL
1
in MTBE.
193
As MTBE is thought to be potentially carcinogenic to humans, its use
poses an interesting dilemma for regulators, given that it helps to reduce
carbon monoxide emissions from cars. Regardless of what happens to
MTBE use in the future, it is clear that significant amounts of this
compound are already present in the sub-surface.
192
In Europe, how-
ever, the problem seems much less severe.
194,195
QUESTIONS AND PROBLEMS
(i) Calculate the ionic strength of a solution which is 0.01 mol L
1
with respect to potassium sulfate and 0.002 mol L
1
with respect
to magnesium chloride. Use this value of ionic strength and the
Gu
¨
ntelberg approximation to determine the mixed acidity con-
stant, K
0
, for methanoic acid (K
¼
10
3.75
).
(ii) Construct a plot to show how
log{activity} varies with pH for
all species in one litre of groundwater containing 3
10
4
moles
