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

GROUNDWATER FLUX
aquifer is abstracted (13.3 Mld -1 ). The mean values for total dissolved mass in the riverbed
piezometer and abstraction well data from the 2001 sampling (Section 7.1, Tables 7.1 and 7.3)
were 1101 mgl -1 and 504 mgl -1 respectively. Therefore, in terms of the total groundwater
chemical mass flux, 76-91% goes to the river and only 9-24% to the abstraction wells. Of the
total groundwater flux, some comprises relatively recent recharge affected by anthropogenic
pollution and the remainder consists of older groundwater with a composition reflecting
previous recharge history and mineral dissolution. The relative contribution to the surface
water flux from the different water types is unknown. The calculation of mass flux through
the aquifer is highly complex but some estimates of current mass loading via recharge can be
made for comparison with the estimates of groundwater flux to surface water. The current
recharge to the aquifer is estimated to comprise 52% rainfall run-off and 48% from other
sources such as mains and sewer leakage (Section 6.4.2.2). A recent survey of roof run-off in
a residential area of Birmingham (Harris,2002) and from a study on pollutant flux to the
aquifer (Thomas, 2001) provide data on the likely composition of recharge water (Table 8.2).
The residential roof run-off is generally representative of atmospheric loading, while the
general urban and industrial categories reflect the higher concentrations observed in storm
drains associated with different land uses. Many of the concentrations in rainfall from the roof
run-off survey are one to two orders of magnitude lower than the groundwater discharging to
the riverbed, showing the extensive input from natural and anthropogenic sources. However,
the run-off concentrations for the metals Cu, Pb, Zn are similar to, or greater than, the
concentrations in the riverbed. Given the sources in addition to the run-off, this implies
attenuation is occurring within the aquifer.
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