Great tritium ages explain the occurrence of good-quality groundwater in aphreatic aquifer of an urban area, Lublin, Poland

The phreatic aquifer of the Lublin chalk in the watershed of the Bystrzyca River, eastern Poland, is characterized by waters of good quality. Its hydr aulic 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 polluta nts in the aquifer. However, tritium sampling reveals that the mean ages ra nge from about 55 to 92 years in four cases, and about 250 to 350 years in I I cases. One sampling site is free of tritium, which indicates an even gr eater age. Such great ages can be explained only by a retardation of solute transport due to matrix diffusion, that is, an exchange between mobile wat er 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 fiss ure porosity. In consequence, all types of dissolved constituents and, to s ome 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 in fluence of matrix diffusion. Though pollutants are greatly delayed, their a ccumulation in the matrix and slow diffusion into the fissures contribute t o their persistence in groundwater. Regional values of hydraulic conductivi ty, estimated from tritium ages and known matrix porosity, agree reasonably well with the values known from pumping tests, which supports the interpre tation of a dominant influence of matrix diffusion on solute transport.