Geological and environmental implications of the evaporite karst in Spain

960 Environ Geol (2008) 53:951–965
The Beuda Gypsum karst (Eastern Pyrenees)
Although, outcrops of this formation are very limited, its
interstratal and suballuvial karstification has produced a
considerable amount of endo and exokarstic features. The
most significant manifestation corresponds to the Bañolas
Lake, 1.12 km 2 in area (Fig. 1). This lacustrine system has
been developed in a group of coalescent collapse sinkholes
generated by upward flows derived from a deep carbonate
aquifer confined by the Beuda Gypsum (Sanz and Trilla
1982; Canals et al. 1990). According to Bischoff et al.
(1994), dissolution of the gypsum in this karstic aquifer is
favored by dedolomitization reactions that reduce the calcium
concentration in the groundwater. During rainy
periods, the upward discharge through a trop plein located
40 m above the Bañolas Lake gives rise to an ephemeral
lake, 4 ha in area, called the Clot d’Espolla (Vila et al.
1989). Some sudden changes in the water level of Bañolas
Lake have been attributed to collapse events in its bottom
(Brusí et al. 1992). The occurrence of sinkholes is relatively
common in the surroundings of the lake, mainly
during low discharge periods. On the other hand, the
numerous gravitational deformations and paleosinkholes
that display the Plio-Pleistocene lacustrine sediments in the
Bañoles-Besalú sedimentary basin reveal the significant
role played by karstic subsidence phenomena on its morpho-sedimentary
evolution (Fleta et al. 1996; Ros et al.
1996).
In Sant Miquel de Campmajor valley, Pallí and Trilla
(1979) have identified 88 active and relict sinkholes, some
of them hosting ponds. In Borró River valley, a group of
funnel-shaped sinkholes up to several tens of meters across
connect to the 1,315 m long Bores de Borró Cave system
(Miret and García 1999). In Beuda village area, the 962 m
long La Mosquera Cave, with excellent examples of scallops
in its walls, has been severely spoiled by the disposal
of excrement from a pig factory (Cardona 1989–1990;
Miret and García 1999). The 920 m long Rotgers Cave, in
Borredá village, is fed by a swallow hole located in the
bottom of a stream. Some buildings in Besalú village have
been irreversibly damaged by the formation of sinkholes.
In this area, terrace deposits of the Fluviá River show
numerous deformations and thickenings higher than 60 m
that record subsidence phenomena caused by karstification
of the Beuda Gypsum (Solá et al. 1996).
Evaporite karst in the Ebro Tertiary Basin
The Cardona salt karst
Piercing of the overburden by salt in the Cardona Diapir
has given rise to the largest salt outcrop in Western
Europe, covering around 0.9 km 2 . This salt stock, with a
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NE-SW trending ellipsoidal geometry in plan view, is
crossed by a meander of the Cardener River in its
northeastern edge. Underground mining of potassium salts
and halite has caused dramatic geomorphological and
hydrological changes in the karst system (Cardona 1989–
1990; Gutiérrez et al. 2001; Cardona and Viver 2002;
Lucha et al. 2008a). The diapir has a well-developed
endokarstic system, including historical caves whose
genesis has been induced by the mining operations. The
680 m long Forat Mico Cave, discovered in 1967, has
been for 15 years the longest known salt cave in the
world. It is composed of two levels. The upper passage
displays unusual scallops of aeolian origin. Generation of
the 280 m long Del Riu Cave was due to inflow of fresh
water from the surrounding sandstone aquifer; this inflow
was caused by the excavation of a ventilation gallery
(Fig. 3f). The 335 m long Riera Salada Cave is carved in
the halite debris of a slag heap built between 1925 and
1972. This cave was primarily generated by sewage waters
infiltrated in an artificial depression generated by the
slag heap. In March 1998, the interception of a phreatic
conduit by a shallow mine led to the inflow of fresh water
from the Cardener River into the mine galleries, resulting
in generation of the 4,300 m long Salt Meanders Cave,
which is the third longest explored salt cave in the world.
This flooding event caused a sudden decline in the piezometric
level of the karstic aquifer and massive dissolution
of salt with the consequent generation of a large
number of sinkholes which caused severe damage in
roads, buildings, and the mine infrastructure. Lucha et al.
(2008a), based on an inventory of 178 sinkholes, have
estimated minimum probability of occurrence values of
4.7 and 8 sinkholes/km 2 year, respectively for the time
intervals previous and subsequent to the 1998 mine flood
event.
The main exokarstic landforms found in the salt outcrops
and the halite slag heaps are dolines and different
types of karren, mainly rillenkarren, spitzkarren, and salt
pedestals. Measurements carried out in solution flutes
(Mottershead et al. 2006) and pedestals (hoodoos) have
yielded lowering rates of several centimeters per year. The
Bofia Gran (meaning big sinkhole), located in the southwestern
edge of the diapir, is a 300 m long and 220 m wide
polygenetic karstic depression with nested bedrock collapse
sinkholes and swallow holes. In 1986, collapse of the
Riera Salada Cave roof generated a subcircular collapse
sinkhole 50 m across in the slag heap.
The flow of saline water into the Cardener River has
caused a significant hydrochemical degradation of the river
waters. To partially overcome this problem, a 100 km long
pipe was constructed in 1989 to divert the brine to the sea.
Subsequent to the 1998 mine flood event, the Cardener
River meander, affected by numerous sinkholes, was