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

GROUNDWATER FLUX
An estimate of mass loading from rainfall run-off may be calculated for the unconfined
aquifer using a surface area of 1.063 x 10 8 m 2 and an average recharge from rainfall of
0.00036 md -1 . This does not include the recharge of 0.00033 md -1 estimated to come from
other sources such as leaking water mains (Section 6.4.2.2). The estimate of pollutant mass
flux from industry is based on a land use of 20% in the industrial/commercial category
(Thomas, 2001). The estimated groundwater mass flux to the river for Cl was 1,271 tonnes
per year -1 (Table 8.1) which may be compared with an atmospheric loading (residential roof
top run-off) to the aquifer of 71 ty -1 . A combined estimate of Cl mass flux for urban and
industrial recharge to the aquifer was ~ 1,700 ty -1 . In addition to this an average amount of
8,781 ty -1 of NaCl ( 5,326 ty -1 as Cl) is applied in the Birmingham area during winter road
salting (Jenkins, 1995). The estimated groundwater mass flux to the river for SO4 was 1,904
ty -1 and for NO3 515 ty -1 compared with an atmospheric loading to the aquifer of 85 and 87 ty -
1 respectively. The combined estimate for urban and industrial recharge to the aquifer for NO3
was 219 ty -1 . This compares with estimates by Ford (1994) for the atmospheric fallout on the
Birmingham area for SO4 of 275 ty -1 and for NO3 160 ty -1 .The fluxes to the river are
considerably higher than the contributions from run-off suggesting considerable input from
other sources such as leaking sewers and water mains, contaminated land and mineral
dissolution.
The mass flux of the chlorinated solvents to the river is limited despite the large mass
estimated to be resident in the aquifer (Rivett, 1989). For TCE a total mass flux of 95 kgy -1
equates to 65 litres, ~1/3 of a drum, and for TCA 82 kgy -1 equates to 61 litres, ~1/3 drum, and
PCE 2 kgy -1 equates to 1.2 litres, ~ 1 lemonade bottle. Calculations from the sampling of
abstraction wells indicated loss of TCE from the aquifer of 1,500 kgy -1 (1987) and 160 kgy -
1 (1990) (Taylor et al.,1999). The lower value in 1990 was the result of the more restricted
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