The impact of urban groundwater upon surface water - eTheses ...

WATER QUALITY INTERACTIONS
The only other VOC that occurred at a high level was the PCE/TCE biodegradation product
cis 1,2-DCE, with a maximum concentration of 110 μgl -1 detected in the same abstraction
well as the maximum PCE content. The other VOCs occurred infrequently and at low levels.
There is little data available on the toxicity of VOCs to benthic organisms living in the
hyproheic zone but if the environmental quality standards of 10 μgl -1 are applied then the
discharge of VOCs will have limited effect on the hyporheic zone and even less after dilution
in the river. However, if lower limits (~1 μgl -1 ) are imposed then the impact is considerably
more widespread. A compound that may have considerable toxicity even at low levels is vinyl
chloride, which forms as the end member of the breakdown of TCE and PCE. Analysis for
this compound is difficult and expensive and was undertaken for only 16 samples in areas of
TCE and PCE contamination. Vinyl chloride was detected in 10 samples (maximum value of
1.9μgl -1 ) and its occurrence at low levels is widespread, associated with the biodegradation of
TCE and PCE, as indicated by the common occurrence of cis-1,2 DCE.
Chlorinated solvents are frequently detected in the groundwater due to their widespread use,
relatively high solubility and resistance to natural attenuation (NA). Other groups of organic
compounds such as BTEX and PAHs have similar or higher levels of usage (e.g. from
petroleum fuels, gas works hydrocarbons, road run off) but were either infrequent or not
detected, showing a greater susceptibility to NA. Napthalene is a common PAH but was not
detected in any sample. This is likely due to it being a large molecule with a low solubility
that fairly readily sorbs to the aquifer material. The BTEX compounds have multiple sources
associated with petrol stations and industrial usage but benzene and toluene were detected
only once and ethylbenzene and xylene not at all. The most likely reason for the limited
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