Fen Management Handbook - Scottish Natural Heritage

Groundwater discharge to fens is usually a more constant feature, but may be
higher during winter and spring, occurring when the underlying or surrounding soil
and geological layers are saturated with water. If the surface soil and shallow drift
deposits within the fen are permeable, then groundwater is likely to discharge by
diffuse upwards seepage. Alternatively, if the surface and shallow deposits are not
very permeable, groundwater is likely to discharge at the margins of the soil or drift
deposits through discrete springs or seepages. In practice, groundwater usually
discharges to a site through some combination of these two mechanisms.
A pre-requisite for groundwater discharge to a fen, or fen discharge to groundwater,
is an underlying (and/or adjacent) water-bearing formation (aquifer), such as the
chalk of south-east England or the Permo-Triassic sandstones of the Midlands and
north-west England. The high capacity of these aquifers for groundwater storage
means that groundwater discharge is more likely to be maintained over the summer
and autumn periods.
Low permeability rocks or superficial drift such as glacially-derived sands and
gravels can also be important water sources for fens. Groundwater flow in these
formations is usually within the immediate surface water catchment and the
resulting low capacity for groundwater storage often means that groundwater
discharge can be significantly reduced or even interrupted during the summer and
autumn periods.
3.4.1 Fen types and associated water transfer mechanisms
The following diagrams and text provide more detail on the types of fen found in
each landscape setting and their associated water transfer mechanisms.
3.4.1.1 Fens on slopes
Sites occur on gently sloping land and include many valley-head fens that form the
source or headwaters of streams. Water retention is often within low permeability
peat or alluvial deposits. Run-off from surrounding slopes is often an important
water source, but many examples also receive a contribution from groundwater
discharge. Recognised classic examples include Morrone Birkwood (Cairngorm
SAC) and Mynydd Preseli (Pembrokeshire).
Morrone Birkwood SSSI
in Scotland is a good
example of an alkaline
spring-fed fen on a
slope.
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