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Great tritium ages explain the occurrence of good-quality groundwater in a phreatic aquifer of an urban area, Lublin, Poland |
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| Authors: | A Zuber Z Michalczyk P Maloszewski |
| Institution: | Institute of Nuclear Physics, Radzikowskiego 152, 31342 Cracow, Poland, Institute of Earth Sciences, UMCS, Akademicka 19, 20033 Lublin, Poland, GSF-Institut für Hydrologie, Ingolst?dter Landstr. 1, 85764 Oberschlei?heim, Germany, |
| Abstract: | The phreatic aquifer of the Lublin chalk in the watershed of the Bystrzyca River, eastern Poland, is characterized by waters of good quality. Its hydraulic conductivity is related to a fissure network of low porosity, as much as 0.006 at the outcrops. The good quality of water in this urban setting is surprising because, according to estimates based on Darcy's law, a high seepage velocity would be expected, resulting in rapid transport of pollutants in the aquifer. However, tritium sampling reveals that the mean ages range from about 55 to 92 years in four cases, and about 250 to 350 years in 11 cases. One sampling site is free of tritium, which indicates an even greater age. Such great ages can be explained only by a retardation of solute transport due to matrix diffusion, that is, an exchange between mobile water in the fissures of low porosity and stagnant water in the matrix of high porosity. The delay factor is given as the ratio of total porosity to fissure porosity. In consequence, all types of dissolved constituents and, to some degree, fine suspended matter are delayed in their transport to springs and wells. In addition, non-conservative constituents have longer times to sorb, react, and decay than in the case of transport without a dominant influence of matrix diffusion. Though pollutants are greatly delayed, their accumulation in the matrix and slow diffusion into the fissures contribute to their persistence in groundwater. Regional values of hydraulic conductivity, estimated from tritium ages and known matrix porosity, agree reasonably well with the values known from pumping tests, which supports the interpretation of a dominant influence of matrix diffusion on solute transport. Electronic Publication |
| Keywords: | phreatic aquifer fissured marls porous matrix tritium ages groundwater quality |
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