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

WATER QUALITY INTERACTIONS
There is very limited evidence that biodegradation is occurring within the riverbed. The total
VOC content (Figure 7.26e) does show a general decrease in the shallower samples from 80
μgl -1 to 70 μgl -1 in the autumn and 137 μgl -1 to 121 μgl -1 in the summer (between 92 cm and
18 cm below the riverbed). However, there is no rise in the levels of daughter products such
as VC, DCE and DCA, and the ratio of TCE to DCE shows no obvious fall. The riverbed
environment does not appear to provide the necessary reducing conditions to initiate the
microbial breakdown processes. Oxygen levels are high (3 mgl -1 ) and neither sulphate nor
nitrate appear to be reduced below a depth of 20 cm (Figure 7.19). However, both sulphate
and nitrate concentrations appear to decrease by a greater amount than predicted by simple
dilution with surface water based on chloride concentrations. The increase in chloride
concentrations at 15 cm indicates a surface water content of 12.5% and a surface water
content of 19% at 5 cm. At these mixing ratios, if the sulphate is conservative, the
concentration should be 8.6 meql -1 at 15cm (actual 8.15 meql -1 ) and 8.24 meql -1 at 5 cm
(actual 4.24 meql -1 ). This implies that the sulphate is not conservative (i.e. is being reduced)
from 0 to 15 cm depth below the riverbed. This calculation is not wholly reliable as there may
have been some analytical error and because surface water concentrations are known to vary
over time. If biodegradation does occur in this profile it lies within the surface water-
groundwater mixing profile within the top 15 cm of the riverbed. There is insufficient VOC
sample coverage in this area to provide any further information.
7.9 Comparison of results with general toxicity standards
The results of the water quality sampling were compared against the U.K. Environmental
Quality Standards (EQS) for freshwater. These comprise a set of general toxicity standards
which indicate levels harmful to aquatic life (Table 7.7). The U.K. Drinking Water
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