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

WATER QUALITY INTERACTIONS
Oxygen levels in the urban groundwater drop from a mean of 8.4 mgl -1 (median 9 mgl -1 ) in
the deep to a mean of 6.1mgl -1 (median 7 mgl -1 ) in the shallow. This, perhaps, reflects oxygen
consumption related to microbial oxidation of shallow hydrocarbon and ammonium
contamination as the recent recharge water would be expected to contain more dissolved
oxygen than observed. The main aquifer comprises red bed sandstones with hematite and iron
oxyhydroxide grain coatings that has been extensively oxidised under previous geologic
conditions. There are unlikely to be large amounts of original organic material or sulphides
that would consume significant amounts of oxygen and it is reasonable, therefore, to expect
high levels of oxygen at depth. However, oxygen levels from the abstraction wells are higher
than the mean value of 5.6 mgl -1 recorded from previous sampling (Ford, 1990), perhaps
indicating exposure to the atmosphere between abstraction and sampling. The riverbed
sediments have a low mean oxygen content of 3 mgl -1 (median 3.5 mgl -1 ) with anoxic
conditions observed in several locations. The drop in oxygen levels is associated with
microbial activity in the riverbed sediments where a supply of organic compounds from the
surface water is readily available for degradation. Oxygen levels observed in Sutton Park
follow a similar trend, with the shallow groundwater having a mean of 5.3 mgl -1 , dropping to
a mean of 3.2 mgl -1 in the riverbed. Data on oxygen content from the 2001 surface water
survey is limited (n=3, mean 6 mgl -1 ) but Agency data from monthly spot sampling between
1993 and 1996 indicate a fall in dissolved oxygen across the aquifer from mean 10.1 mgl -1 at
Profile 5 (upstream) to 9.1 mgl -1 at Profile 18 (downstream). The Agency data show
fluctuation in oxygen levels between the 5 th percentile and the 95 th percentile of 6.8 to 12.7
mgl -1 , with a minimum of 1.7 mgl -1 reported at the downstream end reflecting high biological
oxygen demand.
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