Page 300 - Hydrogeology Principles and Practice

HYDC08 12/5/05 5:31 PM Page 283

Groundwater resources and environmental management 283

Fig. 8.3 Hydrograph records for 1970–1971 showing the net gain in flow of the River Thet from groundwater abstracted from Chalk
boreholes in the pilot area of the Great Ouse Groundwater Scheme (see Fig. 8.2 for location). As shown, the net gain is the difference
between the actual discharge with groundwater abstraction and the estimated natural discharge without abstraction. As a result of
measurements taken for identical 24-hour periods without allowance for travel times (or temporary storage effects), occasional zero flows
(or even apparent losses) can arise. After Backshall et al. (1972).

groundwater pumped. Concerns were expressed that levels steadily recovered towards natural levels dur-
pumping the Chalk aquifer could affect the Breckland ing the following winter. Natural river flows were
Meres situated within the pilot area. Theses meres re-established in time for the beginning on the follow-
are groundwater-fed lakes of notable scientific and ing baseflow recession period (Fig. 8.3). The delay in
ecological importance. However, drawdown of the recovery is a feature of river regulation schemes and
groundwater level by more than 1 m due to extensive is due to winter groundwater recharge first replenish-
pumping in 1970 was relatively limited within the ing the groundwater storage depleted by pumping
pilot area (Backshall et al. 1972). during the summer regulation period. By definition,
The success of a river regulation scheme can be the net gain to the river over a long period of years
expressed by the net gain to the river. The actual net must be zero if environmental impacts are not to be
gain at any given time depends on the aquifer proper- permanent.
ties and the distance of boreholes from natural spring Full development of the Great Ouse Groundwater
discharges. During 1970 and 1971, and as shown in Scheme was envisaged to include almost 350 new
2
Fig. 8.3, the net gain in the Great Ouse Groundwater boreholes distributed over an area of 2500 km . The
Scheme varied, but during summer periods values possibility of developing the Chalk groundwater
were about 70% of the quantity pumped. The resource in conjunction with surface runoff was estim-
3
3
scheme was therefore successful in maintaining river ated to yield 360 × 10 m day −1 of water for export
flows at a high proportion of the mean flow. After to demand areas outside of the basin. The drawdown
pumping stopped, river flows and groundwater of the Chalk aquifer for the fully developed scheme