AbstractThere are six classes of water and five geologic environments in the subarctic Nahanni karst. During the summers of 1972 and 1973, 214 water samples were collected from 15 of the 30 hydrogeologic categories. Linear discriminant function analysis, using five measured and two derived chemical variables, indicates that there are statistically significant (0.005 level) differences in water chemistry between similar waters in different geologic environments, between waters in the same geologic environment, and between waters in different hydrogeological categories. Geological environment labels a natural water because it determines the availability of soil CO2and of soluble minerals. Measurements indicate that mean soil log PCO2is greatest in areas of shale mantled by till (−2.39), and least in areas of sandy fluvioglacial drift (−3.27). Low values on the sandy drift are due to the sparse shrub vegetation, and to the high degree of soil aeration; soils in areas of shale are clay‐rich and support a dense boreal forest. Hydrology influences water chemistry because it determines how much CO2natural waters pick up from the environment and how much they subsequently lose to the atmosphere, and as a result, whether they dissolve or deposit soluble materials. The similarity between mean calculated log PCO2in natural waters (−2.92) and mean measured soil log PCO2(−2.80) suggests that natural waters in Nahanni are dose to equilibrium with mean soil