Abstract
ABSTRACTThe alluvial deposits that occupy the Madrid Basin in central Spain form an aquifer system covering an area of 5,000 km2 (2,000 mi2) and with thicknesses of 2,000 m (6,600 ft) or more. Average annual precipitation is 500 mm (15 in.) and average annual temperature is 15°C (59°F). The precipitation is sufficient to provide a net surplus to ground‐water recharge which, in turn, supports dry weather flow of major streams in the basin. A distribution of surface recharge and hydraulic conductivities were obtained from a previous study utilizing a two‐dimensional finite‐difference model of the same vertical cross section as this study. In this study a flow net and a discrete‐state compartment (or “mixing‐cell”) model were employed to calculate the age distribution of ground water circulating through the aquifer. Carbon‐14 decay ages were determined for nine ground‐water samples taken from eight locations. The ages obtained with the flow net and with the mixing‐cell models are mutually consistent and generally agree with the carbon‐14 decay ages. The calculated ages range from zero at the recharge boundaries to over 100,000 years at discharge boundaries in stream channels. The results obtained are to be regarded as preliminary. Their principal value will be to guide future C‐14 field sampling programs.
Published Version
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