23.1 Distribution
The distribution of endorheic salt lakes (>3 gl ’ total dissolved solids) is shown in Figure 2.4. A catalogue compiled by Pardo (1948) listed 240 salt lakes, a significant number of which have now disappeared through drainage or been altered by input of fresh water or urban waste (Montes & Martino, 1987). The majority are located in the Ebro Basin of Aragôn, the Guadiana Basin of La Mancha and the Guadalquivir Basin of Andalucfa. Smaller clusters occur in the Segura and Jucar basins of Albacete, La Mancha and between the Duero and Tajo basins in the northern Meseta.
Other saline water bodies comprise artificial salt pans and coastal lagoons which are located intermittently along all but the northwestern Spanish coastline.
2 3 2 Lake origins
Endorheic lakes occur within semi-arid or semi-humid climate zones where annual évapotranspiration exceeds precipitation. Their distribution in Spain is further restricted to regions of flat topography where endorheism is favoured by a poorly developed drainage network and by the proximity of groundwater levels to the surface (Alonso, 1985). Spanish salt lakes are unusual in that the origins of most are related to processes of limestone and e v ^ r ite dissolution and subsidence, in regions where Tertiary or earlier continental and marine deposits are preserved at or beneath the surface.
Most salt lakes of the Ebro Basin, Aragôn, are ephemeral dolines formed either by gradual dissolution of gypsum after infiltration of rainwater through dissolution cracks in the upper limestone crust (Sânchez Navarro et al., 1989), or by less structured dissolution and subsidence of limestone or gypsum- rich evaporites. Aeolian erosion is particularly significant here; many basins are elongated due to deflation and this is the main mechanism by which salts are removed when lakes are dry (Pueyo Mur, 1978). Wind deflation is the most important variable in the formation of lakes to the south of the region, in lake basins whose geomorphology is constrained by the structure of sandstone palaeocanals (Sânchez Navarro et at., 1991).
The southern Meseta lakes of La Mancha are in a karstic zone where, in addition to both limestone and evaporite dissolution (carbonate-rich marls or gypsum), origins are often also linked to geological factors such as structural faulting, as is the case with the lakes of Alcâzar de San Juan, Ciudad Real
Figure 2.4 Map showing the distribution of Spanish salt lakes (closed circles). (Modified from Conun & Alonso [1988].)
Aragon Duero Basin p. Duej Tajo p.Jucan La Mancha * Salt Lake Endorheic zone Andaiucia 100 km 46
(Tello Ripa & Lôpez Bermudez, 1988, Florin et ai, 1993).
The solution lakes of Côrdoba and some others in Andaiucia were formed after limestone dissolution by meteoric water and groundwater (Torres Esquivias et al., 1990). The origins of others such as the Puente de Piedra, Mâlaga and the lakes of Câdiz are related to the dissolution of either Triassic or Tertiary gypsum- or halite-rich evapaites (Bemuds Sanz, 1990, Femândez-Palacios, 1990).
Deep freshwater karstic lakes occur outside the main salt lake regions in Albacete (the Ruidera System; Gonzâlez Martin et al., 1987), Cuenca (e.g. Vicente & Miracle, 1988) and to the north of Barcelona, northeast Spain (e.g. the Laguna de Banyoles; Pdrez-Obiol & Julia, 1994).
Some salt lakes originated independently of dissolution. Small volcanic crater lakes southwest of Ciudad Real in La Mancha were formed in the Pliocene (Hemândez-Pacheco, 1932); many have now been drained for agriculture (Montes & Martino, 1987). The main origin of others such as the Laguna de Gallocanta, Temel (Comin et at., 1983) and the Laguna de Pétrola, Albacete (Ordofiez et al, 1973) is tectonic, related to either faults or fractures. The lakes of the northern Meseta are of ancient fluvial origin and were isolated within Tertiary detritic sediments in endorheic areas at the centre of large interfluves (Gonzâlez Bemâldez, 1992); some lakes in La Mancha such as El Hito, Cuenca are also of fluvial origin (Florhi et ai, 1993).
2 3 3 Hydrology
Most Spanish salt lakes are ephemeral and dry out every sununer. The majority of permanent or semi permanent lakes, which dry out every few years, are located in Andalucfa. With the probable exception of the few crater lakes, which often have small, well-defined drainage areas, pattems of groundwater flow in Spanish salt lakes are complex and vary considerably between lakes, from minor flow through sediments of low permeability, shallow aquifers or springs, to major aquifer flow in lakes connected to the regional groundwater table (Montes & Martino, 1987). Groundwater outflow can occur through internal seepage or aquifer flow; a lake-bed can exhibit groundwater inflow and seepage simultaneously in different areas and net input/output alternates over time in a complex relationship with lake level (Gonzâlez Bemâldez, 1992).
Most of the ephemeral lakes are groundwater windows', or areas of local or regional groundwater discharge where inflow is enhanced by capillary action (Sânchez Navarro et al, 1991) such that water levels are maintained into the season of water deficit (Gonzâlez Bemâldez, 1992). Water retention is enhanced by an impermeable substrate (marl or deflocculated clay) which prevents loss of surface waters by infiltration (Recio Espejo & Tirado Coello, 1982a).
Other ephemeral lakes, such as the Laguna de la Ballestera, Sevilla, are in recharge areas where shallow water levels are maintained by the proximity of groundwater levels to the surface. This impedes infiltration of meteoric waters and water is retained in the basins for some part of the year even though groundwater makes a negligible contribution to the net water balance. Small (<20m diameter) lakes in recharge areas of the northern Meseta are thought to be unconnected to groundwaters and water retention is a product of substrate impermeability alone (Gonzâlez Bemâldez, 1992).
In addition to freshwater karstic lakes, the relatively high water depth (>2m) of a small number of Spanish salt lakes is mainly due to their connection to major aquifers. Unlike other lakes of the Ebro Basin, the Laguna Salada de Chiprana is 5m deep and permanent due to its connection to steady regional groundwater flows (Gonzâlez Bemâldez, 1992), although its current permanence is mainly a function of recent human impact (Davis, 1994). Estimates for aquifer input exceeded total surface input (direct precipitation + mnoff) for three permanent lakes in southem Côrdoba, Andalucfa (Torres Esquivias et al, 1989).
Few detailed studies have been made of groundwater outflow. The models of lake function proposed by Sânchez Navarro et al (1989 and 1991) assume no outflow for both solution and deflation basins in Aragôn. This is not necessarily the case, however. Significant groundwater outflow to the nearby river has been demonstrated for the Laguna de Gallocanta, a semi permanent tectonic basin in Temel, northeast Spain, for example (Gonzâlez Montermbio et al, 1982 in Montes & Martino 1987). Major aquifer outflow is an important characteristic of the freshwater karstic system of Ruidera, where a series of lakes located along a gentle gradient are linked by subterranean flow (Gonzâlez Martin et al, 1987).
23.4 Physical lake characteristics
Compared to salt lakes globally, most Spanish salt lakes are small (mean lake area <0.5km^) and shallow (mean water depth <lm). Notable exceptions are the Puente de Piedra, Mâlaga (12.2km^, the Laguna de Medina (12.1km^) and the Laguna de Gallocanta, Temel (13.3km^. The Laguna Salada de Chiprana, Aragôn (Balsa et al, 1991) and some of the lakes of Andalucfa are 5-10m deep.
Many of the shallow, ephemeral lakes which predominate in Spain are typical playa lakes with characteristically flat lake-beds and, where wind deflation is significant, small lunettes on the lake shore (Sânchez Navarro et al, 1989). The morphometry of both small recharge lakes and ephemeral solution lakes is often circular or subcircular; deflation basins are more elongated. Lakes with an origin related to stmctural faulting are more irregular in shape. Deeper, karstic lakes in relatively pure limestone or carbonate-rich marl areas such as the Laguna Chica, Villafranca de los Caballeros, tend to be circular or subcircular with conical lake basins (Florfn et al, 1993).
2 3 S Hydrochemistry
Carbonate-rich salt lakes are rare in Spain, and are restricted largely to the northern Meseta. In this region, the salinity of lakes such as the Lagunas de Coca is due to the input of alkaline groundwaters, whose mineralisation is a consequence of long groundwater flow time in siliceous aquifers (Gonzâlez Bemâldez, 1992).
In most other lakes groundwater input has an overriding significance in determining lake salinity and brine composition since it is charged with dissolved evaporite salts, and lake waters are saline even with short groundwater flow and residence time (Gonzâlez Bemâldez, 1992). Runoff tends to make a relatively small contribution to total inflow but may also be charged with salts from evaporite outcrops (Torres Esquivias et al., 1989). As a result the ionic composition of lake waters varies with local or regional geology. Almost without exception the dominant ions are Na*, Mg^"^, Cl and . Na-Cl or mixed Na-Mg-Cl-SO^ lakes are associated with marine evaporites in Andalucfa and, with lower Mg* content, with continental evaporites in the Ebro Basin; lakes of La Mancha are within continental evaporites and are mainly Mg-SO^ type. Apart from freshwater karstic lakes, carbonates are significant (>25% of the anion sum) only in the lakes of the northem Meseta (Comfn & Alonso, 1988).