[en] A comprehensive study of the hydrogeological framework and groundwater resources using geochemical and isotope methods was recently carried out in the Los Arenales hydrogeologic unit. The aquifer extends over ∼7.600 km² on the central part of the Iberian Peninsula, about 200 km north of Madrid. The stratigraphic sequence is composed of Upper Tertiary sediments (mostly Miocene) of continental origin that conform the Duero Basin. The sediments are mostly of terrigenous character, generally of low permeability, due to the presence of clay minerals. Lenses of sandy and silty materials, linked to paleoriver-beds, constitute the main aquifers, extending to a maximum depth of 500 metres. However, most of these lenses are poorly interconnected due to the characteristics of the clayey matrix of the rock formation. This lithologic control is strongly limiting the existence of a regional groundwater flow from the mountainous regions towards the Duero river, as has been generally accepted till now in most of the previous hydrogeological studies. The aquifer is phreatic in certain areas, partly confined or confined in others. Despite the abundant geological information available and the large number of boreholes and wells drilled and monitored in the area, the existing conceptual models were unable to explain the observed changes in the piezometric levels, the development of depressions as well as the variations in water quality. The lack of lateral groundwater flow, which is controlled by the existence of more permeable lenses, led to the development during the past 20 years of some serious piezometric depressions. The situation forced the local Water Authority to establish serious limitations to the exploitation of the aquifer. The main characteristics of the most representative chemical components of groundwater, as well as its spatial distribution has been studied by sampling 141 pumping wells. Similarly, the same sampling points have been considered for stable isotopes (141 wells), while 50 wells were sampled for carbon-14 and carbon-13. The large spatial variability of all these parameters confirmed previous indications on the high heterogeneity of the flow pattern in the aquifer. In fact, the different hydrochemical maps produced suggest the existence of numerous aquifers poorly or not interconnected. Both carbon-14 and stable isotopes results have provided evidence of the presence of paleowaters, mainly, in the deepest wells. Similarly, the existence of sodium-bicarbonate facies, as result of cation exchange processes in the aquifer, pointed to a large residence time of most groundwaters in the system. An estimation of the average recharge-rate R_a for the period 1953-1994 has been made on the basis of the tritium balance using the following equation: (R_a)_53-94 = (P_a)_53-94 (I_T)_aquifer / (I_T)_{precipitation} where P_a is the average precipitation and I_T the tritium inventory. The value of (I_T)_{precipitation} has been obtained from the tritium contents in precipitation measured in the Madrid station since 1970, and a general distribution model of tritium in precipitation valid for the Northern Hemisphere. A value of 45.000 Bq/m² was obtained. The tritium inventory in the aquifer (saturated zone) has been calculated individually for the 141 wells from the measured concentration values and the thickness of the water column deduced from the lithological columns. The contribution to the unsaturated zone has been considered null for those areas (Thyessen polygonal areas) where no or negligible tritium concentration was found and the carbon-14 'age' was higher than 1.000 years. The tritium balance has been corrected for tritium losses due to the water extraction made during the period 1953-1994. An average recharge-rate for the indicated period of time equal to 32.2 mm/year was obtained, which represents a total annual recharge of 244.7x10⁶ m³/y. In spite of the uncertainty of some of the parameters used for the calculation, this recharge-rate is very similar to the estimations made by some authors using conventional hydrometeorological techniques. However, the most interesting result obtained by this investigation is the spatial distribution of the recharge. It was concluded that about 96% of the recharge is concentrated in a surface of 3.530 km², that is to say, in ∼46% of the aquifer surface. No modern recharge was obtained for the areas affected by the above-mentioned piezometric depressions