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Groundwater resources and environmental management 293

8.2.6 Horizontal well schemes particular location. Hence, making decisions about
the impacts of groundwater abstraction on rivers is
It has long been recognized that well yields can technically challenging.
be increased by driving horizontal tunnels (adits) An important determining factor of the flux of
below the water table which radiate away from a well water between a river and an aquifer is the degree
or borehole shaft. Systems of adits, typically 1.2 m of connection between the river and aquifer as con-
wide and 1.8 m high, are associated with many trolled by the material properties of the river bed and
large groundwater sources in the Chalk of south-east river bank sediments, and the extent to which the
England, Belgium and the Netherlands. The water channel of the river intersects the saturated part of
supply to Brighton on the south coast of England the aquifer. In general, and as shown in Fig. 8.8, there
includes 13.6 km of adits and in east London there are three types of hydrogeological situation that lead
are 18 km. Generally, the adits were driven to inter- to flow between an aquifer and a river.
sect the principal fissure or fracture directions in the The rate of change in river flow and the attenu-
Chalk aquifer (Downing et al. 1993). Groundwater ation of short-term fluctuations in river flow, for
flow in an adit may be pipe or open channel flow. example a flood event, can be strongly influenced by
Adits in the Chalk of south-east England are norm- storage of water in the floodplain deposits surround-
ally full of water contained under pressure. In this ing a river. As the river stage and groundwater level
situation, Darcy’s law (see eq. 2.5) is not applicable increase, the extra water saturates the alluvial sedi-
and alternative methods are required for model- ments and fills the available bank storage. When
ling flow in aquifer–adit systems (Zhang & Lerner water levels recede this bank storage is released and
2000). can have a short-term beneficial effect in alleviating
More recently, and with advances in drilling tech- the immediate impact of adjacent abstractions on
nology, horizontal and slanted wells have been invest- the river flow.
igated for various hydrogeological situations (Chen In the long term, and usually within one or two
et al. 2003; Park & Zhan 2003) and also for environ- years for boreholes a few hundred metres from a
mental applications such as vapour extraction in con- river, groundwater abstraction will deplete the river
taminated aquifers (Plummer et al. 1997; Zhan & flow at a rate equal to the pumping rate. As shown
Park 2002). Horizontal wells have screened sections in Fig. 8.9a, the river flow depletion consists of two
that can be positioned parallel to the horizontal components:
flow direction. These wells have several advantages, 1 interception of flow that would otherwise reach
including: interception of vertical components of the river;
groundwater flow; greater control over the dynamics 2 induced recharge from the river.
of the water table; better contact between well In general, the depletion of river flow caused by
screens and horizontal aquifer units; easier drilling op- pumping increases with time and will increase more
erations close to ground surfaces that are obstructed rapidly the closer the abstraction point is to the
by infrastructure (airport runways, roads, buildings, river. The degree of depletion is also dependent on
etc.); and the possibility of installing long screen sec- the aquifer properties of transmissivity and storage
tions in aquifers of limited thickness. coefficient.
A dramatic illustration of the impacts of ground-
water abstraction in depleting river flows is the case
8.3 Groundwater abstraction and river flows of the River Colne valley, north of London (Fig. 8.10).
In the first half of the twentieth century, the substan-
The link between groundwater and river flows is tial growth of residential areas here and elsewhere
fundamental to conserving the riparian environment on the outskirts of London was supplied by direct
yet is one of the more difficult hydrogeological situ- groundwater abstractions from the underlying Chalk
ations to predict. This difficulty is due to the complex aquifer. This rapid development had marked effects
nature of river–groundwater interactions and uncer- on certain rivers, particularly the Rivers Ver and
tainties in the nature of the hydraulic connection at a Misbourne, tributaries of the River Colne, in the