Seasonal variations in chemistry and 13C/12C ratios of total inorganic carbon of carbonate dissolving waters are considered in the frame-work of a simplified approach dealing explicitly with major ionic species. The unidirectional evolution models of Deines et al. (1974) are supplemented by taking mixing effects into consideration (Wigley and Plummer 1976). These effects, caused mainly by the redistribution of carbon-bearing species in the mixture, are to a great extent nonlinear and unsymmetric with respect to the fraction of the second solution in a binary mixture. The theoretical considerations are supported by observations of many samples taken during a period from January to May 1977 in SW Lublin Upland.

The parameters PCO2 and δ13C of a reservoir lose their primary meaning in the case of mixing of waters but investigation of their changes may help in studying some conditions in the soil. During the period of observation (except March) the estimated values of PCO2 of a reservoir tended to he in a narrow range of values from 10−1.3 to 10−1.4 atm. In March those values were higher due to the large inflow of CO2-rich waters into the aquifer. The estimated δ13C values of a reservoir CO2 spread from –17 to –27 permil during January to March. In April and May those values focused in a narrow range from –22 to –26 permil. This tendency indicates that waters inflowing in March and April transported isotopically lighter CO2.

Both HCO3 content and δ13C of total carbon versus pH allowed to search inflows of infiltrating water into particular areas of the aquifer. It has been shown that δ13C investigations are necessary to notice the admixing of waters in certain cases.