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

STUDY SETTING
unconfined aquifer is oxygenated (2 to 10 mg l -1 O2) with Ca and HCO3 the dominant ions
and Eh 300-350 mV. The Wildmoor Formation contains waters that have relatively lower pH
than the other formations and SO4 and/or NO3 and/or Cl are generally the dominant ions. In
the confined aquifer, reducing conditions prevail, Ca and SO4 are the predominant ions,
dissolved oxygen (DO) is generally zero and nitrate is absent.
Three age groups for the waters have been defined (Figure 3.10) on the basis of carbon
isotope analyses (Jackson et al., 1983) and corroborated by the distribution of anthropogenic
contamination. These comprise:
1. modern and contaminated waters within the shallow portions of the unconfined aquifer;
2. intermediate waters (2000 to 4000 years) beneath group one waters in the unconfined
aquifer and in sections of the confined aquifer close to the Birmingham fault; and
3. old waters (>6500 years) within the confined section and at depth in topographic lows in
the base of the unconfined aquifer.
As is the case in all the hydrochemical studies involving abstraction wells within the aquifer,
samples are drawn from over a considerable depth interval resulting in mixing of waters of
different origin. Therefore, waters in the intermediate group may reflect induced mixing of
modern and old waters within the well or natural mixing within the aquifer. Water quality
data from the abstraction wells must be considered in the light of several factors that may
introduce bias into the sampling.
1. The abstraction wells used in the surveys were almost all from sites within the industrial
land-use category which comprises