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

WATER QUALITY INTERACTIONS
riverbed, implying conditions of decreasing pH and Eh. Iron shows mean concentrations of
0.31 mgl -1 (median 0.16 mgl -1 ) in the deep, 0.18 mgl -1 (median 0.13 mgl -1 ) in the shallow
and 1.41 mgl -1 (median 0.24 mgl -1 ) in the riverbed, also implying more reducing and lower
pH conditions higher in the system. Manganese has a greater solubility (as Mn 2+ ) than iron
(Fe 2+ ) at higher levels of Eh which may explain the higher concentrations. Both are likely to
precipitate (Fe(OH)3, MnCO3/MnO2) with the increase in pH. This is likely to occur during
mixing with the surface water and evidence of this can be seen in the multilevel profiles
(Figures 7.5 a and b). Other workers (Harvey et al., 1998) have indicated the importance of
manganese and iron in the co-precipitation and removal of heavy metals from groundwater.
The abrupt change in redox conditions to a more oxidising and higher pH environment across
the upper section of the riverbed, combined with elevated levels of microbial activity, may
result in significant changes to the dissolved mass flux from the groundwater to the river.
7.3 Anion Hydrochemistry
The total anion content increases upwards through the groundwater system (Table 7.3)
showing higher levels of calcite dissolution and chloride, sulphate and nitrate contamination
in the shallow and riverbed piezometers. Elevated concentrations of anions and other
contaminants provide evidence of modern recharge in all 14 of the abstraction wells with the
exception of a deep (154 m ) well beneath the city centre which has chloride, nitrate and
sulphate levels of