Environmental Engineering Reference
In-Depth Information
16.2.4
Spring fens
In north-western Poland, however, many spring
systems are still present and their infi ltration areas
have not been hydrologically disturbed. Yet most of
them have been damaged by drainage in downstream
lake areas (Plate 16.3c; Wolejko
et al
. 1994 ). Decreas-
ing lake levels in the nineteenth century have triggered
erosional processes that have washed away most of
the peat that has accumulated around these spring
systems.
Spring fens are almost exclusively fed by groundwater
discharge (Plate 16.2c). They generally occur in land-
scapes with rather complex geological and hydrological
conditions. Usually they occur in places where ground-
water fl owing through an artesian aquifer bursts
through a rupture in the overlying confi ning layer and
discharges at the land surface. Such a rupture is often
the result of changes in the original stratifi cation of
sediments (e.g. where tectonic movements have
occurred) (Wilcox
et al
. 1986 ; Grootjans
et al
. 2005 ).
Spring systems can be found as elevated spots, in low-
lands in the middle of a sloping landscape or even close
to a hilltop. In all these settings, spring fens occur at
the infl ection point where local groundwater fl ow
systems intersect the land surface (Almendinger &
Leete 1998 ).
In calcareous substrate areas, the regular discharge
of groundwater may accumulate large amounts of
CaCO
3
(
travertine
) up to a height of 30 m (Grootjans
et al
. 2005 ; Pentecost 2005 ). When groundwater,
supersaturated with CaCO
3
, comes into contact with
the atmosphere, CO
2
escapes and CaCO
3
precipitates.
Spring mire plants, and mosses in particular, stimulate
this process by using the CO
2
in the discharging
groundwater as a source of carbon. The result is that
CaCO
3
precipitates on plant leaves, and when they die
off and decompose, CaCO
3
is deposited. Under favour-
able climatic conditions, peat formation may occur.
Peat formation in spring mires is sensitive to changes
in landscape hydrology and climatic conditions that
trigger changes in
groundwater discharge
leading to the
decomposition
of the peat layers. Strongly decomposed
peat has a high resistance to water fl ow, thus blocking
water transport in the spring mound. The discharging
groundwater will then force its way out somewhere
else in the spring system, thereby creating new oppor-
tunities for peat or travertine formation (Wolejko
et al
.
1994 ). Some of the best - preserved spring - mire systems
anywhere can be found at the base of the Alps and the
Slovakian Tatra Mountains, and in Latvia (Pakalne &
Kalnina 2005). Outside Europe, spring mires occur in
the United States (Komor 1994; Glaser
et al
. 2004 ;
Middleton
et al
. 2006), South Africa (Scott & Vogel
1983), Siberia (Schipper
et al
. 2007 ) and Australia
(Whinam & Hope 2005 ).
Most spring mires in western Europe have disap-
peared due to decreased infl ow of groundwater from
surrounding areas as a result of changes in land use.
16.2.5
Floodplain fens
In hydrological terms, fl oodplain fens (Plate 16.2d) are
very dynamic systems. In winter and spring, intensive
fl ooding may deposit large amounts of sand, silt or clay.
In summer, water tables may drop to 1 meter or more
below the soil surface. Under such conditions, nutrient
availability can be very high (Olde Venterink
et al
.
2003) resulting in high productivity of fl oodplain veg-
etation. On a geological time scale, rivers frequently
change their course, leading in turn to modifi ed fl ood-
ing frequencies and sedimentation rates. Eutrophic
fl oodplains may change into mesotrophic fens and
existing fens, or even bogs, may become fl ooded with
surface water. Examples of large fl oodplain mires in
Europe include the Danube fl oodplain in Romania
(Vadineanu 2009), the Oder fl oodplains on the border
of Poland and Germany, the Narev fl oodplain in Poland
and the Rhône River delta in southern France (see
also Chapter 17). Large fl oodplain mires outside Europe
are the Okavanko Delta in Botswana (McCarthy &
Ellery 1998) and the Mesopotamian marshes in Iraq
(Maltby 2009 ).
16.3 CAUSES OF DISTURBANCE OF
NATURAL MIRES
The main causes of destruction of and disturbance in
natural mire systems vary from direct peat extraction
for fuel to indirect changes in landscape hydrology and
atmospheric nitrogen deposition due to changes in
land
use
by agriculture, forestry or urbanization. The indi-
rect hydrological changes and their effect on nutrient
availability in the peatland may be subtle, but can lead
to natural mires changing into systems that no longer
accumulate peat, but instead actually release carbon
into the atmosphere. The change from natural mires to
disturbed ecosystems is illustrated in Plate 16.3, which
