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

STUDY SETTING
The importance of fracture flow has yet to be fully investigated but is believed to be
significant in some cases. The calcite cementation of the formations varies from strong to
weak. A value of 0.15 for specific yield and 0.0005 for the confined storage coefficient are
thought to be generally representative of the aquifer (Knipe et al.,1993). The increasing
occurrence of mudstone beds adjacent to the Mercia Mudstone contact is thought to produce
multi-aquifer conditions (Ramingwong, 1974). The impact of these mudstone beds throughout
the aquifer is unknown, but may significantly reduce vertical conductivity, with fractures
acting as the dominant control on vertical flow. The extent of the active groundwater system
is highly dependent on the degree of vertical connectivity, with the likelihood that the older
groundwater at depth will be relatively stagnant with long residence times in the aquifer.
Recharge is complex and has a high spatial variability in the urban catchment. The
distribution of precipitation across the region is not uniform with average annual rainfall of
800 mm in the south-west and only 650 mm in the north-east (Powell et al., 2000). Run-off is
high due to the extensive urban coverage and recharge may vary considerably over relatively
short distances of tens to hundreds of metres dependent on land use and coverage. The
reduced recharge from precipitation may be supplemented by seepage from the extensive
Birmingham canal network and leakage from sewers and water mains. Recharge from mains
leakage into the aquifer has been estimated as 600 mmyr -1 equivalent to 25% of
Birmingham’s public supply (Lerner et al., 1996) the majority of which is derived from
Wales via the Elan Aqueduct. A major control on recharge to the underlying bedrock is the
conductivity, thickness and distribution of the superficial drift deposits. Large thicknesses of
boulder clay are present in some areas (e.g. more than 40 m in the vicinity of Smethwick ) and
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