Fen Management Handbook - Scottish Natural Heritage

238
Case Study 10.1
Monitoring to Inform Fen Management
– Foulden and Gooderstone Commons
Foulden and Gooderstone Commons are a series of groundwater-fed fens overlying
the middle, upper and lower chalk aquifers between Downham Market and Thetford
in north Norfolk (OSGRTF 762002). The chalk is capped with drift deposits that
range from clay to sand and gravel. The fens occur where water can escape from
the aquifer through ‘windows’ in the overlying aquitard, giving rise to springs and
seepages, and to areas of more permanent water in pingos. It is important to
maintain adequate groundwater pressure, and a groundwater table to sustain fen
within the pingoes.
The fens support rich-fen vegetation of the NVC M13 plant community type,
and are designated SSSI and SACs within the Norfolk Valley Fens. The shallowrooting
higher plants such as black-bog rush and bryophytes such as Scorpidium
scorpioides rely on a positive water pressure to maintain their calcium-rich
soil water at close to ground level. The degree of water-level variation found in
monitored examples of these fens, and its relationship with species diversity, is
described by Wheeler & Shaw (2001). Provided such measured values can provide
a reliable index of the required status quo the soil water level, as sustained by
aquifer pressure, should not fall more than about 15 cm below ground level.
Gooderstone Common:
spring and seepage area
North Norfolk is an important area for arable agriculture. Survival of crops and the
meeting of quality criteria means irrigation is often necessary. Water is abstracted
under licence from boreholes sunk into the chalk aquifers. The Environment Agency
has to demonstrate that the licensed abstraction would have no adverse effect on
the SAC.
Hydrological modelling techniques
The only basis on which predictions of drawdown can be made is through
hydrological modelling. Many models are available, each making a set of
assumptions, and each dependent on the quality of the data available to feed into it.
Important factors include the slope of the land relative to the positions of boreholes
and fens, as this determines the direction of flow within the aquifer. For example
applying the THEIS equation to data for Foulden Common predicted a drawdown
of 10-20 cm. Compared with the water fluctuation information provided by Wheeler
& Shaw for NVC community M13, this could cause significant damage.