Fig. 1.

Schematic representation of the stratified reactivity framework. Potential vertical profiles of reactivity in an aquifer (Left) and resulting apparent reaction times (Right) are shown. The framework assumes similar stratum reaction times, τ_stratum, (within a given layer) for O₂ and NO₃⁻. A late start or an early stop of reactions along the flow paths results in differences in apparent O₂ and NO₃⁻ reaction times. (A) Late start of reactivity creates the late start pattern, where the subsequent O₂ and NO₃⁻ reduction only starts after a time, τ_enter, when the water reaches the reactive layer. The late start increases the apparent O₂ reaction time compared with the stratum O₂ reaction time. The subsequent apparent NO₃⁻ reduction is only marginally affected, because the time for NO₃⁻ reduction starts only after O₂ is depleted, resulting in longer observed apparent O₂ reaction times compared with NO₃⁻. (B) Early stop of reactivity results in NO₃⁻ degradation first being limited by O₂ and then by the absence of electron donors. In the early stop scenario, the apparent reaction time for O₂ is smaller than for NO₃⁻, and the difference between the two informs the characteristic time, τ_leave, when reactive elements leave the reactive stratum. Evenly distributed electron donors throughout the aquifer correspond to a small τ_enter and a large τ_leave and result in a uniform reactive stratum with a sequential reduction of O₂ and NO₃⁻ starting at the water table. The relation of apparent O₂ and NO₃⁻ reaction times in the case of a uniform reactive stratum is represented by the dashed line in the plot of apparent reaction times (also SI Appendix, section S2). The hot spot pattern is compatible with both stratified reactivity patterns (a*/b*).