Reactive transport modeling helps explain the origin of the microbiological zoning observed in pristine freshwater aquifers. Zoned aquifers have been described previously as either thermodynamic or kinetic phenomena, but neither interpretation has proved fully satisfactory. Drawing on concepts of population dynamics, the modeling reported here offers an alternative explanation of how certain microbes exclude others from zones: one functional group maintains conditions under which cells in another group die more rapidly than they can reproduce. The modeling also lends support to the idea that a group of microbes that appears to dominate a particular zone in an aquifer may in fact coexist with, or even be subordinate to, another group.

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