A methodology is described for understanding the interaction of karstic aquifers with allogenic rivers, where little information is available. This methodology includes conventional hydrogenology methods tracer tests and measurements of flow into, out of and circulating within the karstic system. The method is designed to understand the hydrogeological behaviour of a river in sufficient detail, given a short study period. The methodology is applied to a karstic system in Spain, obtaining useful, quantitative results for a hydrological year, such as an estimate of the water balance, differentiation between autogenic and allogenic natural recharge, relationship and connection between the river and the aquifer, and measurements of infiltration capacity in watercourses under different hydrological situations. The paper deals with a useful example that could be applied to other rivers and aquifers where few data are available. It can be applied to aquifers under a natural regime and Mediterranean climate.
Interactions between groundwater and surface water basically proceed in three ways: groundwater flows through the streamed into the stream (gaining stream), stream water infiltrates through the sediments into the groundwater (losing streams), and ephemeral stream, where the water table is below the streambed, especially in karstic aquifers and arid river basins. The ephemeral rivers flow due to a response to intense rainfall or allogeneic rivers entries in a karstic system, i.e. Rio Lobos Canyon (Spain). Even in arid climates, the headwaters have ephemeral streams in steeper areas (Barlow and Dickerman, 2001, Granato et al., 2003) [1,2].
For the protection of water resources it is important to understand and quantity exchange processes and pathways between groundwater and surface water. Numerous methods exist for estimating fluxes at the groundwater-surface water interface. Scalon et al. (2002) [
Stream aquifer interactions are complex in space as well as in time. Changes of the rock substratum conductivity, permeability of stream sediments, and seasonal variations in precipitation and evapotranspiration patterns can alter water tables and thereby cause changes in the direction of exchange flows (Lerner et al., 1990) [
The mechanism of the loss of water in a river ephemeral is a transitional process. This is produced by a pulse of infiltration or saturation front which goes down during a significant storm. This infiltration pulse changes very quickly and is very difficult to measure (Abdulrazzak and Morel-Seytoux, 1953; Freyberg, 1983) [6,7]. Intermittent streams may be caused by regional groundwater level fluctuations over time causing the river to be a winner or loser (Winter et al., 1998) [
Methods of assessing losses in ephemeral channels are based on the conceptualization of the flow (Freyberg, 1983; Lerner et al., 1990) [5,7] methods that record changes in soil temperature and water (Stonestrom and Constantz, 2004) [
The dynamics of ephemeral stream-aquifer interaction are still relatively poorly understood. In a few cases have been developed quantitative studies (Goodrich et al., 1997) [
In the Spanish Mediterranean basins there are large areas without permanent rivers, many of them are ephemeral rivers that are usually dry, but periodically have floods.
Karstic aquifers are a source of water resources in large parts of the Mediterranean region. The Mediterranean climate is characterized by scarce, irregular rainfall (600 - 800 mm/year), hot summers and mild winters. Karstic systems typically exhibit variable hydrological conditions through the year, and from year to year. It is frequently the case for rivers in such systems to be losing streams over long periods. It is also frequent that the aquifers drain through large karstic springs under natural regime, which contribute to the base flow and water quality of the river. There are other aquifers, intensively exploited for human water supply, whose vulnerability to pollution needs to be defined and protection zones instated.
A large proportion of karstic systems are scarcely exploited, and under near natural regime, with greater or lesser hydraulic connection with rivers. Some of these large springs, such as La Galiana (Iberian Cordillera, Soria, Spain), represent the only drainage point for aquifers. They contain significant resources of high quality and are responsible for the majority of the base flow of the rivers they give rise to, since the Mediterranean climate is quite arid and precipitation is irregular. These watercourses can also be home to a well-conserved aquatic fauna, whose survival depends to a large degree on the base flow. This type of aquifer also requires significant hydrogeological study.
The study area dealt with in this paper lies in the northwest of the Iberian Cordillera, Central Spain, and extends over approximately 400 km2 (Figures 1 and 2) The relief is determined by the presence of a meseta of limestone layers that have been dissected by the river Lobos, giving rise to a canyon 26 kilometres long. In 1985, the canyon was declared a Natural Park, due to its peculiar landscape and important colonies of Griffin Vulture and Golden Eagle. La Galiana spring, at the downstream end of the canyon, drains a mean flow of some 2000 l/s, giving rise to the river Ucero, a right-bank tributary of the river Duero. Similarly, the river Lobos has a series of left-bank tributaries, whose headwaters lie outside of the calcareous massif in the less permeable rocks surrounding it.
Most of the Cretaceous karstic aquifers and their recharge areas fall within the boundaries of the Natural Park. One of the main objectives of declaring the Natural Park was to conserve its waters and achieve compatible use of the aquifer as a groundwater reserve for supply high-quality water to the centres of population in the vicinity. Although the groundwater resources of the Weald Facies around the periphery of the karst would be sufficient in quantity to satisfy current demand, they are mostly degraded by the naturally-poor quality of water, due to their high iron content. For this reason, the karstic aquifer of the Lobos Canyon assumes greater importance as a water resource and as a significant water reserve suitable for human consumption, which need to be protected from contamination.
The object of this study was to provide a sufficientlydetailed understanding of the water balance of the system and the interaction between karstic aquifer and associated rivers over a short period of time, taking into account the scarcity of available data. For this reason, it was necessary to devise a plan of work that could yield quantitative knowledge and include aspects important to the future management and conservation of the aquifer and its rivers, such as.
Detailed evaluation of the water balance over eleven months approximately (341 days, from 12/10/1995 to 07/03/1996), with emphasis on the determination of the natural recharge. One hydrological year is clearly insufficient to evaluate the mean groundwater resources of an aquifer, but this was not the main objective, since the zone clearly has surplus water, and there is plenty to meet water demand. There is a permanent gauging station on the river Ucero 20 km downstream of La Galiana spring, which measures the flow downstream of where other tributaries join it, and so it is not representative of
all of the outflows of the aquifer. Like many other aquifers, there is no permanent, continuous gauging station at the springheads, nor on the rivers that run through it. Thus, to determine its mean resources, a rainfall-run-off model can be applied to reconstruct the phoronomic series. The principal focus at the outset of the study was to quantify the natural allogenic and autogenic recharge separately, and to understand exactly how each is produced, by monitoring several provisional gauging stations of both the karst’s allogenic rivers, and the outflows of the aquifer.
It must be remembered that the area of autogenic recharge of the aquifer is largely forested and, therefore, the risk of pollution is more likely to come from discharges to the allogenic rivers that flow through towns and villages before infiltration into the karstic system.
Another important aim was to gain a detailed knowledge of the hydrogeological relationship between the river Lobos and the karstic aquifer under different hydrological situations, given that this river crosses the aquifer from one side to the other. It was not known if it behaved only as a losing stream along its entire length, nor what the infiltration capacity of its river bed was. Tracing studies were required to elucidate the hydraulic connection between the rivers and the aquifer (Segovia et al., 2011) [
Of the conventional hydrogeological methods, we catalogued 239 water points, including all the points in the karstic aquifer, most of those in the catchments on the northern perimeter that provide allogenous recharge, as well as some on the southern edge of the aquifer that correspond to the border with the Tertiary of the Duero Basin. This inventory also includes resurgences and swallow holes containing water, considering these as “natural wells”. All of the deep potholes have been explored by speleologists in order to identify the regional phreatic levels, as it was already known that there are no significant hanging levels. Dry swallow holes were also included, considering “negative wells” as those whose bases were not wet even when the piezometric level rises under high water conditions.
The inventory of water points was made in both wet and dry seasons, in order to identify temporary springs. The most representative springs were selected from those inventoried for flow gauging, using cup-type meters (or helical flow meters in the larger ones).
We were interested to establish a hydrogeological balance for the aquifer, by monitoring the variables involved, especially the groundwater discharges at the Galiana spring and the recharge from allogenic rivers. For this reason, five gauging stations were installed in the main watercourses that disappear into limestone, located just upstream of the river Lobos karst (Figures 2 and 3).
These gauges were placed to assess the allogenic recharge to the aquifer arising from the less permeable areas surrounding it, the majority of which belong to the Weald. Readings were taken daily to weekly depending on the point and the hydrological situation of the streams (they were sometimes dry) over a almost hydrological year (1995-1996). In addition, four flow gauging surveys were undertaken to calibrate the rating curve.
A gauge was installed at Puente de Siete Ojos, which represents an intermediate point on the river Lobos and from where it can be determined whether the river flow exceeds the infiltration capacity of the sinkholes in its upper reaches.
Systematic flow measurements were also made at La Galiana spring, as well as discontinuous water level measurements over a short time period in a borehole located in the central part of the aquifer.
All watercourses were walked at different times of year and under differing hydrological situations, in order to identify sink holes and possible zones of discharge, and so understand the hydrogeological behaviour of the surface water flows in the upper, middle and lower reaches. Differential gauging was also undertaken.
Experimental measurements correspond to a short period of time (eleven months) and cannot cover all hydrological conditions of the basin. For that reason we have added qualitative observations made from 1989 to the present (2012) both for the staff working in the Natural Park, as the authors of this work during numerous visits to the field. The summary of these observations are plotted in
An approximate isoline map was drawn, with elevations, of the base of the carbonate, in order to understand its geometry, and on the north-eastern border and confirm its isolation from the Tertiary.
Sedimentary terrains in the Natural Park date from the Jurassic to the Present Day. The Jurassic rocks are essentially marine carbonate rocks. Overlying these, and concordant with them and in transition towards the Cretaceous, are the detritic sediments of the Purbeck-Weald Facies. Over these are the sands of the Utrillas Facies (Albian), then Coniacian-Santonian-Campanian marls, and the Turonian limestones, which project to form the surface relief, and which form the cliff faces of the canyons in this region, such as the rivers Lobos, Espeja, Abión en Burgo de Osma, Boos, etc. Above these strata comes the Garumnian Facies in transition with the Tertiary and, discordantly, the postorogenic sediments of the Miocene, Plioquaternary and Quaternary (IGME, 1956, 1982) [12-14].
The Coniacian-Santonian-Campanian limestones are 240 m thick and comprise a hydrostratigraphic unit of high permeability that forms the main aquifer. Its impermeable base consists of a marly series with calcareous intercalations towards the top, some 70 - 100 m thick.
Due to the presence of the marly intercalations, the lower part of the aquifer can be locally confined, while the rest is classified as a free aquifer, forming a meseta, with a syncline structure oriented east-west. Its lateral borders are the impermeable Cretaceous marly outcrops that also form the aquifer’s base, except in the south, where there is a discordant contact between the clayey Miocene and the calcareous Upper Cretaceous. The anticlinal axis of Santa María of the Hoyas-Ucero-Aylagas raises the marly base and, whilst it does not always outcrop, it makes the hidden lateral barrier, and the hinge and axis of the anticlinal make the underground watershed between the Lobos aquifer in the north, and the one to the south that feeds the springs at Rejas, Fuencaliente and Ucero. This behaviour was deduced from elevations, as indicated in the block diagram in Figures 3-5.
The eastern border may be open towards Torreblancos, where springs of 200 l/s emerge, and so we opted to fix the limit of the system where the Cretaceous is covered by the Tertiary. In any case, one cannot rule out the possibility of a hydraulic connection with the Tertiary of the Almazán Basin.
The marls mentioned above mark the base of the karst, surrounding the synform on almost all sides. Where this limit occurs at a lower elevation (approximately 900 m, at the entrance to the Lobos Canyon, upstream of Ucero), is where La Galiana spring emerges.
The catchment of the river Lobos can be divided into two completely different zones with distinct hydrogeological characters:
1) The Wealden deposits and Jurassic carbonates, located in the north and northeast comprises mostly detritic deposits (Wealden), and a narrow series of Jurassic carbonates. It forms a watershed with the river Ebrillos to the northeast and the tributaries of the river Arlanza to the north. It is a hilly area of leafy pine forest. It receives similar or slightly higher precipitation, has greater surface runoff and a somewhat similar hydrogeological be-
haviour to the large sedimentary catchments. Its outflows, both surface runoff and groundwater base flow (hypodermic water), flow into the karst, since the headwaters of the rivers Lobos, Navaleno, Chico and others all flow into the Lobos karstic zone.
2) The karstic zone delimits the catchment in the south and southeast by permeable layers of low permeability. It receives slightly less rainfall. It discharges mainly towards La Galiana spring, but also to the springs at Rejas de Ucero, Santervás and Fuencaliente in the south; one cannot rule out a small southward groundwater transfer towards the Tertiary of the Almazán Basin on the lowlands of the river Ucero.
This zone does not produce surface runoff, except after very intense rainfall. In addition, recharge from precipitation falling over its outcrops goes entirely to the Wealden/Jurassic zone. When the infiltration capacity of the watercourses of the river Lobo and river Chico in the other zone is exceeded, excess surface water leaves the karstic system along the river Ucero.
The surface areas of the two zones are:
Karstic zone: 164 km2
Wealden zone and Jurassic carbonate: 197 km2
TOTAL: 361 km2.
Given that the peripheral Wealden zone is in part of the catchment that is under natural regime, flow gauging of its streams was done to provide basic information for a hydrological study of both this and the karstic zone.
The gauging survey lasted eleven months approximately (341 days, from 12/10/1995 to 07/03/1996), undertaken along the main rivers and streams that descend from the north and north-eastern periphery. Gauges were installed at the following points.
Lobos River, at Hontoria del Pinar (Burgos), just before it enters into the karstic system of the Lobos Canyon. The catchment covers 80 km2 corresponding to the headwaters of the Lobos. Gauging was done to characterize and quantify the inflow from the detritic Wealden and Jurassic carbonate terrains. It also allowed for differential gauging with the station downstream at Puente de Siete Ojos, which lies within the karstic zone, in order to quantify losses or discharges in this stretch of the river.
Navaleno River, at Arganza (Soria), before this river enters the karst. Two gauges had to be installed, one in the natural course of the river Navaleno, and the other in the take-off to a mill that frequently draws water from the river. The two were summed to give the total flow for the river. The catchment covers 85 km2, consisting of detritic Wealden terrains and occasional Jurassic limestone and dolomite.
Valderrueda stream, at La Cantera bridge from Ucero to San Leonardo, before the stream is lost to the limestone of the karstic aquifer. Its catchment is small, some 12 km2 and comprises mostly detritic Wealden deposits. It serves as a representative catchment to indicate what the other small, non-gauged, periphery catchments could contribute.
Chico River at Talveila, before the river suffers its first losses as it enters the karstic aquifer. It drains 15 km2 and comprises solely detritic Wealden deposits.
In addition to the above stations monitoring allogenic inflows to the karst, two further stations were set up within the karstic aquifer itself.
Lobos River at Puente de Siete Ojos, upstream of the confluence with the river Navaleno. This station was designed to characterize the hydrogeological regime of the Lobos as it enters the Canyon, and to quantify the losses or inflows, calculated by difference between the headwater stations at Hontoria and the end of the canyon at Ucero. The catchment between Hontoria del Pinar and this point is 55 km2.
Ucero River at Ucero, downstream of the confluence with the river Chico. Effectively, it monitors the outflows of the system, consisting of the Galiana flow (princepally), the discharge of the river Lobos at La Galiana, and the surface discharge of the rivers Lobos and Chico. The surface area of the catchment as far as this station is 344 km2, which includes both the karstic zone (164 km2) and peripheral detritic zone (180 km2), commented above.
A gauge board was installed at each station and direct gauging was done under different hydrological conditions, including fierce floods, with the object of constructing the rating curve.
The total allogenic recharge of the gauged rivers amounted to an equivalent flow of 1049.01 l/s. However, we must consider the other small peripheral catchments (5 km2) that were not gauged. These were assigned a value equivalent to the mean unitary flow of the gauged catchments (6.1 l/s/m2). This amounts to a further 30.5 l/s, which brings the total allogenic recharge to 1080 l/s, which is equivalent to 31.8 hm3 for the period considered, 341 days (
The following section gives a detailed description of the hydrological regime of these losing rivers in the karst, and of their relationship with the gaining rivers of the detritic Wealden aquifer.
Recharge in the karstic zone is autogenic and diffuse over the 164 km2 of the outcrop. This is not to say that, in intense rainfall, there is no running water in the streams, but that this rapidly infiltrates into the streambeds.
The area of the outcrop apparently has a low degree of karstification, since there is no great profusion of dolines and uvalas. Nevertheless, the ground has a high absorp-