Drainage and siphoning of a karstic spring: a case study

Abstract

The Fuentetoba Spring, with a mean flow of 210 L s À1 , releases the discharge from an aquifer that comprises three hydraulically-connected synclines. The spring has a very irregular flow of between 8 L s À1 and 3,400 L s À1 due to the predominant water circulation that is non-Darcian turbulent flow as an underground torrent in the vadose zone, as well as through well-developed karstic conduits in the phreatic and epiphreatic zones, as attested to by speleological explorations. The long response times to recharge by the Fuentetoba karstic system, seen in the spring's hydrograph, are controlled by regional factors. Nevertheless, certain responses that have very long time lags under high water conditions might be governed in the final stretch of the flowpath by mechanisms of pressure or siphoning of the floodwave, and perhaps by constrictions or plugs of sediment in the conduits.

Modeling and Prefeasibility Management, and Conservation Strategies for Fuentetoba Springs, (Spain)

“…With the support of three speleological groups (Club Deportes Espeleo, Club Deportivo Terrasub, and Sociedad Espeleológica Alto Duero), it was possible to discover a well-organized karst network of caves that channels the flow towards a single spring. There are 3 km of development in the unsaturated zone [10,11] and about 400 m of conduit connected to the spring in the saturated zone ( Figure 3). Thanks to these explorations and the geological structure of the area, it was possible to better understand the hydrogeological conceptual model of the aquifer.…”

“…Thanks to these explorations and the geological structure of the area, it was possible to better understand the hydrogeological conceptual model of the aquifer. Two important aspects stood out regarding the regulation studies of the Fuentetoba spring, as follows: (1) The hydraulic communication between the synclinal furthest from the spring (Villaciervos syncline) and the Fuentetoba syncline is through an underground river that circulates through the unsaturated zone ( Figure 2) [10]. This conditions the way of organizing the approach to the regulation calculations and the mathematical model.…”

“…This aquifer has very active hydrodynamics, with high filling and emptying speeds. The groundwater velocity is very high, with hydrodynamic reserves emptying within one month [10]. This is reflected in Figure 7 for the example of using 2 hm 3 of the underground reserves (Option 1)-inputs to the system (natural recharge) can fill the aquifer in a single month, going from a situation of deficit in supply to surplus water, and diverting it to the river.…”

“…As this spring is the natural discharge of a syncline hanging from extremely karstified aquifer limestones that lean on impermeable marls, the project consisted of regulating the spring by drilling a gallery with a "tap" under the discharge point in order to drain this entire bucket, thus controlling the groundwater outlet as appropriate, but the project was not executed. Thanks to recent hydrogeological and speleological studies [9,10], it was possible to know its detailed operation and the network of submerged galleries associated with the source, so regulation possibilities could be proposed. Apart from its speleological and scientific potential [11], mapping submerged galleries may be essential in designing catchment structures for the future regulation of the spring, as was done in other springs with siphons [12,13].…”

“…At this stage, the most influential and significant risk group is geological and hydrogeological knowledge. Although new prospections and specific studies could give us more details, the main conceptual behavior was fully studied by the authors [9][10][11].…”