Back to Contents
The average inflows into the Guadalquivir River basin (63,822 km2) are 8,940 Hm3 per year. Current regulation amounts to 34% (3,034 Hm3/year) and is expected to reach 40% in the near future.
The topographical configuration of the basin, coupled with irregular seasonal rainfall patterns, make an increase in regulation by traditional means (dams and reservoirs) economically and technically impracticable.
The favorable hydrogeological conditions of the Guadalquivir basin and the availability of water surpluses during the winter make artificial recharge an appropriate means of regulation. The Lower Guadalquivir is the area best suited for this purpose, in view of the favorable hydrogeological conditions for storage, the availability of water from the Lower Guadalquivir Canal and the area’s high water demand.
It is here that the CHG (Hydrographic Confederation of the Guadalquivir) and the ITGE (Geomining Technological Institute of Spain) are carrying out important joint experiments in artificial recharging, in order to define the technical and economic parameters that will permit an additional regulation of 100 Hm3 a year in industrial applications.
The Guadalquivir River Basin, which covers an area of about 60,000 km2, is the fourth largest in Spain. Its main feature is the extreme irregularity of its regime, which means that its normal flows can vary in a proportion of 1 to 1,000 throughout the year and its overall resources in a proportion of 1 to 5, between the driest and the rainiest years, the average annual volume being in the order of 8,900 Hm3.
Since the forties, a series of works have been carried out to regulate the surface waters. The current total capacity of those works is approximately 5,000 Hm3 and they regulate a flow of 3,034 Hm3/year, produced mainly by the headwaters of the Guadalquivir River and its main tributaries (Fig. 1).
Net total demand, which at present is balanced with the available resources, is about 3000 Hm3. Approximately 80% of this total is accounted for by irrigation, about 82% of which is provided by surface resources and the remainder by extraction from the aquifers.
The above figures clearly explain the need to increase the volume of regulated water.
A very interesting option is the recharge of aquifers, particularly if they are located in areas where dams cannot be built. Such is the case of the Alluvial of the Guadalquivir or the Calcarenitas de Carmona area. This paper makes reference to the latter.
The first step is to define the basic parameters of the aquifer with sufficient accuracy to determine the volumes involved, the time and area of infiltration in accordance with the available flows, and in short, to obtain all the necessary data for a study of the project’s technical and economic feasibility. The experiment carried out is described below.
The experiment was carried out to the south of Sevilla, within the Sevilla-Carmona aquifer system. This system covers 1,150 km2 along the Lower Guadalquivir’s left bank. (Fig. 2)
The aquiferous materials are calcareous sandstones of the Upper Miocene and different alluvials from the Quaternary terraces of the Guadalquivir. From east to west there are successive layers of sandstone, in the aquifer’s catchment area, and Early, Middle and Late Quaternary terraces descending to the Guadalquivir River. The limits and the substratum of these aquiferous materials are the Blue Marls also from the Upper Miocene (Tortoniensis).
The calcareous sandstones outcrop in the form of fringes running from SW to NE. Towards the base, they turn into loamy sands. In some places, their thickness can exceed 50 m.
The Quaternary terraces are formed by alluvials from the Guadalquivir (silt, gravel, cobbles and sand); and their thickness varies from 10 m for the Early Quaternary to over 20 m for the Late Quaternary.
The aquifers formed by these materials are free, with piezometric levels between 0 and 30 m, most being less than 10 m. Seasonal variations in level are smaller than in the Late Quaternary terrace (0 to 2 m) and greater in the sandstones (2 to 8 m) where progressively lower levels can be observed as a result of intensive exploitation.
The region’s average hydraulic parameters, show permeability values of between 100 and 500 m2/day for the sandstone and between 50 and 1,000 m2/day for the Quaternary deposits. The storage coefficient is 1 to 10% in sandstone and 1 to 20% in the Quaternary terraces.
With regard to groundwater quality, the dry residues in the sandstone are less than 0.5 r/l, while in the different Quaternary terraces they reach values of between 0.5 and 2 gr/l. Organic pollution is significant: maximum concentrations of 140 mg/l of nitrates have been measured in the Late Quaternary deposits.
The aquifers are charged by direct percolation of rainwater, which occurs mostly in the sandstone outcrops in the catchment area of the groundwater table. It should be pointed out that the Late Quaternary terrace is hydraulically connected to the Guadalquivir River, which means there is a very close relationship between the river and the aquifer.
The Guadalquivir River and the Guadaira River (Fig. 2) are the aquifer’s two most important drainage axes. The movement of the water is conditioned by this circumstance, flowing NW, W and SW from the sandstone directly into the Guadaira River, or indirectly into the Guadalquivir, after passing through the different Quaternary terraces and descending from an elevation of 200 m down to 5 m.
The volumes from percolation, calculated by the amount of useful rainfall, have been estimated at 174 Hm3/year, while exploitation of the system takes place mainly in the sandstone areas, where the inflows are approximately 28 Hm3/year and the outflows slightly higher. The Late and Middle Quaternary terraces are largely irrigated by surface waters from the Lower Guadalquivir canal, and they therefore receive a supplementary supply from the repercolation of irrigation water. Overall extraction in the system as a whole is approximately 40 Hm3/year.
The following infrastructure and control works needed to be built so that the artificial recharge experiment could be carried out:
Pumping Station. A 25 h.p. electric engine capable of providing a flow of 90 l/sec. See fig. 3.
Conveyance. An asbestos fiber cement underground pipeline, 1,050 m long and with a 300 mm diameter, was installed from the pumping station to the recharge area.
Decantation Pond. With a capacity of 650 m.
Infiltration Pond. Excavated mechanically to a depth of 6.5 m, which is the thickness of the less permeable upper stratum. The basin has a capacity of 6,000 m3, with a useful percolation surface of 480 m2. The bottom of the basin has a siliceous sand filter bed, 90 cm3. thick.
The sedimentation and percolation basins are connected by a spillway with a movable metal gate.
Meteorological Station. Located within the recharge area, it is equipped with a Hellman-type rain gage, minimum and maximum temperature thermometer, a Piche evaporimeter and an evaporation tank.
Gage. To control the inflow, a Thompson-type gage was built with a 90° V- shaped discharge outlet.
Control Piezometers. Eleven piezometric boreholes were made in the area of the recharge basin, at depths varying from 20 to 56 meters to monitor changes in levels in the area. Continuous monitoring equipment was installed in four of those piezometers.
In July 1990 and April 1991, two preliminary short-term studies lasting 8 and 10 days, respectively, were carried out to determine percolation rates, filling coefficients and permeability during and after the recharge.
These studies showed the presence of two significant fracture zones at the bottom of the basin, through which the entire volume of water percolated almost instantly.
At the conclusion of both these experiments, an examination was made to determine the amount of silting in the filtering layer, and reaming and restitution in the fractured zones.
From May 23 to August 6, 1991, a third recharge trial was launched. The recharge volume injected the 76 days of the experimental period was 326,000 m3 at an average flow rate of 38 liters/second. During this long-term experiment, 280,000 m3 were pumped from a well 300 meters to the northeast of the basin.
The water table in the percolation basin varied between 1.20 meters and 5 centimeters as a function of the output volume pumped from the Lower Guadalquiver Canal, with an average infiltration rate of 9 meters/day.
The maximum rise in the piezometers was between 5.12 and 0.92 meters, and directly related to the distance from the recharge site. The first twenty-four hours of the injections (Fig. 4a), a predominantly vertrical flow phase, was associated with a rapid elevation in the levels nearest the recharge site. During the following 15 days (Fig. 4b) there was a slow horizontal expansion in the dome and little change in the pressure in the infiltration basin. Pressure in these zones stabilized in 25 to 30 days after the initiation of the recharge process. (Fig. 4c)
There was a clear tendency towards the stabilization of pressure in the areas farthest from the injection site in the last phase. (Fig 4d) The hydrodinamic parameters forty-five days after the trial were similar to initial values.
Piezometric data for the period from October 1990 to October 1991 (Fig. 5) showed a 1 meter increase in the area adjacent to the recharge site, although the rainfall received in hydrological year 1990-1991 (608 mm) was less that received in hydrological year 1989-1990 (873 mm) and the volume of groundwater extracted from October 1990 to October 1991 (280,000 m3) exceeded that for the period from October 1989 to October 1990 (190,000 m3).
Calculations made from point observation data taken at the control piezometers (Fig. 6) provided transmissibility coefficients varying from 300-400 m2/day and fill coefficients of 10-1 to 3 x 10-1. The area of influence (320,000 m2), extrapolated from the observed hydraulic gradients, varied from 250 to 450 meters depending on the direction considered.
From the results obtained, it may be seen that artifical recharge is technically feasible and highly efficient to improve the control of hydraulic resources in an aquifer in the Calcarenitas de Carmona area. Under a conservative hypotesis, with an average infiltration coefficient of 5 m/day, an infiltration surface of 1.5 hectares would be required to recharge the 9.5 hm3/year deficit in the aquifer during an operating period of 4 months/year.
For the hydrogeographical Guadalquivir basin, in which there is an extraction volume of 3,034 hm3/year, through large-scale artificial recharge in areas where there is an excess of surface water and where an increase in extraction by traditional methods (reservoirs) is not feasible, the extracted volume could be increased to 100 hm3/year.
Back to Contents