The first aquifer‐wide calculations of the clay‐cation compositions in equilibrium with the present‐day groundwater in a clay‐rich coastal aquifer are provided, and downgradient changes in Ca on the clay‐exchange site identified three sequential‐downgradient zones, Updip (95–80% Ca), Transition (80–60% Ca), and Downdip (60–40% Ca). These zones form a cation‐exchange front which exteI nds from a meteoric‐water plume, with clays near equilibrium with the fresh‐recharge water (Updip Zone), to clays that are increasingly equilibrated with seawater downgradient (Transition and Downdip zones). The sediments progress from predominantly fluvial clays in the Updip Zone to shallow‐marine clays downdip. Updip‐Zone clays are Na poor, with cation exchange downdip progressively dominated by more Ca in the water exchanging for Mg on the clays. In the Transitional and Downdip zones which have Na on the clays from the original seawater equilibrium, Ca, and further downdip Mg, in the water exchange for Na on the clays. Perturbations in these trends were compared to elements in the groundwater not involved in ion exchange to identify areas of upwelling and halite dissolution. Mass‐balance calculations show that the effect of cation exchange on each pore volume of fluid that flows in the aquifer is large, whereas the effect on the clay composition is small, so the changes in the clay exchange composition can be used to identify the aquifer's past chemical evolution and historical‐flow paths. This understanding coupled with tracers of modern flow (such as Cl) can be used to recognize areas of modern perturbations, such as contamination or salt dissolution.

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Differentiation of a fluvial from a marine depositional environment is essential to understanding the potential hydrocarbon reservoir and source rock characteristics of a sedimentary unit (e.g., Chen et al., 2015;Posamentier & Walker, 2006). Fluvial reservoirs in particular, with their typical meander-belt characteristics, are much less continuous, less laterally extensive, and more compartmentalized than marine reservoirs with their typical sheetlike characteristics (e.g.,