A very striking and unique feature of the groundwater system within the Upper Kettle Creek watershed is the marked concentration decrease of nearly all ionic species in the direction of flow. The conceptual model proposed here attributes this decrease to selective ion filtration by the Port Stanley till. Ionic constituents generated in the shallow groundwater zone by dissolution of the porous medium either pass through the till or accumulate depending upon their relative mobilities. Other important processes that influence the composition of the groundwaters are cation exchange within the Port Stanley till, CO2 gas generation within the soil zones of recharge areas, and the upward leakage of H2S gas from deeper in the Paleozoic section.