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

7.2 The pH and redox environment
WATER QUALITY INTERACTIONS
Table 7.3 indicates a mean pH for the deep, shallow and riverbed urban groundwater system
of 7.0 (median 7.0). A minimum value for pH of 5.4 detected in the riverbed at Profile 8 is
believed to be associated with a discharging contaminant plume. The pH of the groundwater
system is dominated by the dissolution of calcite cement from the sandstone that buffers the
pH by the formation of HCO3 - ions. This has maintained near neutral levels of pH despite
acidification of the groundwater (Ford et al., 1992) associated with the leakage of industrial
acids, N-species transformations, and hydrocarbon oxidation which have either released
hydrogen ions or increased the content of carbon dioxide.
The surface water pH ranges between 7.4 and 8.1 with a mean of 7.6 and displays significant
daily fluctuations. Agency data from monthly water quality sampling at Water Orton between
1989 and 1996 show a range in pH of between 7.1 and 8.7. Data from the continuous
monitoring of pH were available for Water Orton during July 1998 and the maximum pH
range observed in a single day was from 7.3 to 8.1 over a nine-hour period. The rural
catchment displays a mean pH of 6 in the shallow groundwater and 6.6 in the riverbed
associated with acidic peat bogs in the valley. These conditions are perhaps representative of
the original environment in the Tame Valley where peat deposits have been observed at
several locations (Powell et al., 2000). The acidic conditions prevalent within the bogs may
have contributed, along with recent anthropogenic acidification, to the formation of a shallow
(