Recharge systems for protecting and enhancing groundwate

778
TOPIC 7
Sustainability of managing recharge systems / MAR strategies
METHOD
The mathematical function of a GIS is a very powerful and generally underused tool and can be used to calculate
spatial distributions of more useful parameters that cannot be directly measured. As each vector (numerical value)
map essentially defines the distribution of one term of an equation, spatial distributions of useful parameters can be
calculated using an appropriate equation and separate maps defining the spatial distribution of each term in the
equation. Here we show how this approach can produce useful spatial distributions of the three hydraulic and
hydrological factors critical to identifying both feasible and optimum artificial recharge locations.
Deptford Window
180000
River Thames
Retention Time
(days)
175000
Ladywell Fields
2000
1000
Streatham
500
170000
200
165000
Ravensbourne
Window
100
50
10
0
160000
520000 530000 540000
Thanet Sands
Chalk
SLARS TEST ABH's
Wimbledon and
Deptford Fault Zone
0 5 10 15 20
Km
Outside Chalk and Thanet Sands subcrop, Chalk is
confined by Lambeth Group Clays and London Clay
Figure 1. Map of retention time in the SLARS study area
Hydraulic constraints on artificial recharge potential are essentially limited to just three fundamental factors;
1) borehole recharge capacity; 2) borehole abstraction capacity; and 3) recharge retention time. Optimum artificial
recharge conditions occur at locations where all three factors are maximised. Borehole recharge and abstraction
capacity are important factors because the greater the capacity of each borehole, the greater the economies of scale
that can be achieved. Target design recharge and abstraction rates of 5 to 10 Ml/day were selected on the basis of
Thames Water’s previous and current abstraction operations in the SLARS area.
Retention time is a new concept developed by MWH to define the significance to artificial recharge operations of
aquifer-surface water connections, where stream flow depletion can occur during aquifer abstraction and loss
of injected water can occur during artificial recharge. However, losses and gains from the aquifer to the connected
surface watercourse do not occur instantaneously once recharge or abstraction begins. If there is a significant time
delay before unacceptable losses occur then artificial recharge is feasible. Where the purpose of artificial recharge is
to allow increased abstraction to support seasonal peak demand, retention times of six months or more are required.
ISMAR 2005 ■ AQUIFER RECHARGE ■ 5th International Symposium ■ 10 –16 June 2005, Berlin