Recharge systems for protecting and enhancing groundwate

104
TOPIC 1
Recharge systems / Injection well issues, aquifer storage and recovery
River sources). Water then gravitates
to the major Coppermills Water Treatment
Works at Walthamstow or the
smaller Water Treatment Works at
Hornsey, negating the need for a raw
water network. A number of the
sources are equipped for artificial
recharge with connections to the water
mains network. During periods of low
demand, excess water can be recharged
to the aquifer at a rate appropriate to
maintain network pressures.
Phase 1 Wells –
Lea Valley
Phase 2 Wells & Boreholes
– Lea Valley
Phase 2 Borehole – New River
Phase 3 Boreholes
- Lea Valley
Phase 3 Wells & Boreholes
- New River
Phase 4 Boreholes -
Lea Valley
Phase 4 Borehole
- New River
Recharge connection
0 1 2
Kilometres
Northern New
River Wells
N
Northern
Transfer Tunnel
(backfilled)
GEOLOGY AND
HYDROGEOLOGY
NEW
RIVER
LEA
VALLEY
Treated Water
to Supply and
Surplus to Aquifer
Hornsey
Water Treatment
Works
Amhurst
Main
Stoke Newington
Reservoir
(backfilled)
Coppermills
Water Treatment
Works
Figure 2. Borehole locations within the NLARS Scheme
Treated Water
to Supply and
Surplus to Aquifer
NLARS is located within the London
Basin (Fig. 1). The geological succession
comprises alluvium resting upon
a Tertiary sequence of London Clay,
Lambeth Group and Thanet Sands, in
turn resting on Cretaceous Chalk. The
strata have an overall dip to the south.
The Chalk and sandy sequence (Basal
Sands) above comprise a major aquifer
system, with a general hydraulic gradient
to the south. Natural recharge in to the
area is very limited as a result of the
distribution of low flow boundaries
produced by low permeability Chalk
and faulting. This limits flow into the
confined aquifer. It is also reflected
in source transmissivity values ranging
from 80 m 2 /day to > 2,000 m 2 /day.
Historical over-abstraction has dewatered
the Basal Sands producing a potential zone for storage by artificial recharge. The abstraction and recharge
points comprise an amalgamation of old hand-dug wells with adits and drilled boreholes. These vary in diameter
between three metres and 760 mm, extending to a depth of up to 130 m, and produce yields of between 20 Ml/d
and 1.3 Ml/d.
LICENSING
The Scheme is currently regulated by an abstraction licence and an Operating Agreement. The licence restricts
daily output to 275 Ml/d, but this is not a constraint as the total peak output is just over 200 Ml/d. There is also an
annual abstraction limit of 55,000 Ml, equivalent to 150 Ml/d. The Operating Agreement describes the trigger for
Scheme use in a drought, as defined by flows in the River Thames and levels of reservoir storage. The Agreement
also specifies a water level monitoring network to assess the degree of aquifer storage. It also stipulates a
ISMAR 2005 ■ AQUIFER RECHARGE ■ 5th International Symposium ■ 10 –16 June 2005, Berlin