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286 Chapter Eight

these concerns, a phased approach to ASR implemen- relative ease of shallow groundwater exploitation,
tation is being adopted to evaluate its feasibility and the generally high production capacity and the proxim-
effectiveness as a regional water storage option. ity to demand areas (Doussan et al. 1997). Although
Attempts at ASR in the United Kingdom have proximity to a river can ensure significantly higher
met with mixed success. A full-scale trial of ASR on recharge and pumping rates, water quality problems
a confined Chalk aquifer in Dorset in the south of may be encountered during exploitation of riverbank
England proved that there were no detrimental envir- well-fields (Bertin & Bourg 1994). Even with these
onmental effects; for any injection and recovery scen- problems, groundwater derived from infiltrating
ario, the impacts are absorbed by aquifer storage. river water provides 50% of potable supplies in the
−1
However, a fluoride concentration of 2 mg L , equal Slovak Republic, 45% in Hungary, 16% in Germany
to half of the background groundwater fluoride and 5% in The Netherlands. In Germany, riverbank
concentration, led to disappointingly low recovery filtration supplies 75% of the water supply to the City
efficiencies (<15%) (Eastwood & Stanfield 2001). In of Berlin and is the principal source of drinking water
contrast to the Dorset scheme, ASR has been success- in Düsseldorf, situated on the Rhine (Box 8.3).
fully operated in the confined Chalk and Tertiary In the United States, the water supply industry has
Basal Sands aquifer in North London (Box 8.2). adopted the broadly defined regulatory concept of
‘groundwater under the direct influence’ (GWUDI)
of surface water (variably defined and implemented
8.2.5 Riverbank filtration schemes in response to local conditions by each State, Tribe or
other regulatory agent). Groundwater sources in this
In many countries of the world, alluvial aquifers category are considered at risk of being contamin-
hydraulically connected to a water course are pre- ated with surface water-borne pathogens (specifically
ferred sites for drinking water production given the disinfection-resistant pathogenic protozoa such as

BO X
The North London Artificial Recharge Scheme
8.2

3
−1
3
The North London Artificial Recharge Scheme and its relationship to yield of 90 × 10 m day . The Lea Valley wells and boreholes can
−1
3
3
3
existing surface water resources in the Lea Valley are shown in Fig. supply 60 × 10 m day . Hence, the design yield is 150 m day −1
1. In average rainfall years, flows in the Rivers Lea and Thames, with for a drought period of 200 days and is expected to give only small
the associated pumped-storage reservoirs, are sufficient to meet declines in regional groundwater levels.
current demands. Normally there is surplus water which can be used During the dry years of the 1990s, the North London Artificial
to increase aquifer storage. During a drought, when river flows and Recharge Scheme was used on several occasions in 1995, 1996
associated storage levels in the reservoirs become critical, the stored and 1997 to support low river and reservoir levels in the Lea Valley.
groundwater can be abstracted for supply. The abstracted water Cycles of abstraction and recharge from June to November 1997
from the Enfield–Haringey boreholes is discharged to the New recorded individual daily rates of abstraction averaging about 100
−1
3
3
River, an aqueduct built in 1613, where it is transferred to the × 10 m day . This abstraction rate allowed a decrease in support
Coppermills water treatment works (Fig. 1). All groundwater, includ- for the Lea Valley system from the River Thames which, in turn,
ing water abstracted from the Lea Valley wells and boreholes and decreased the rate of decline in the Thames stored-water system,
discharged directly into the surface reservoirs, is blended with raw while conserving aquifer storage. In total during this period, 10.7 ×
3
6
surface water and treated at Coppermills, thus minimizing capital 10 m were withdrawn from groundwater storage in North
and operational costs. This is an important consideration in that the London, equivalent to 25% of the useable capacity of the Lea valley
scheme has been designed for infrequent use with long periods of reservoirs (O’Shea & Sage 1999).
relatively small-scale recharge, followed by shorter periods of large- This innovative artificial storage and recovery scheme is there-
scale abstraction (O’Shea & Sage 1999). fore considered successful in providing good quality water with the
The North London scheme utilizes fully treated drinking water as environmental benefit of balancing groundwater abstraction with
the source of the gravity-fed artificial recharge water, via the normal natural and artificial recharge with no net effect on long-term
distribution system. The recharge water quality is similar to the groundwater levels. In addition, the confined nature of the Chalk
background groundwater in the aquifer. The Enfield–Haringey and Basal Sands aquifer ensures that abstraction has no impact
boreholes vary in depth from 80 to 130 m and can provide a total upon the overlying river system.