Modeling methane transport and transformation in a first-order tidal river floodplain

The recent restoration of tidal exchange to Beaver Creek (BC), a first-order tidal stream in western Washington state, provides a unique observational platform for studying the hydrobiogeochemical effects of seawater encroachment in established freshwater ecosystems. We posit that understanding the systematic behavior of transitional coastal terrestrial-aquatic ecosystems is fundamental to the development of models capable of assessing the impact of regional conditions on the biogeochemical functioning of coastal landscapes.

Here we present insights gained from a 3D reactive transport model focused on methane (CH₄) transport and transformation across the BC floodplain. Stream water levels are tidally-regulated resulting in monthly floodplain inundation. Infiltration during floods accounts for most of the water mass input to the floodplain. Low permeability floodplain soils limit lateral groundwater flow and transport, increasing residence times. With limited lateral groundwater exchange across the floodplain, inundation events lead to moderate, topographically controlled salinization of floodplain soils. Sulfate loading from seawater disrupts CH₄ cycling in the floodplain. However, significant soil CH₄ emissions were detected after inundation events, highlighting the complicated link between seawater exposure and CH₄ cycling in coastal floodplains.

The model was therefore designed to probe the interplay of processes/properties linking CH₄ cycling to inundation events. We furthermore simulate the transition of the floodplain from fresh to seasonally saline and the impact this has on the CH₄ budget of BC. The pre-salinization floodplain conditions were reconstructed from upland and upstream freshwater wetland data, suggesting that the floodplain was likely an actively emitting acetoclastic CH₄ source before seawater infiltration began. The model indicates that BC is still in a state of transition and will not achieve a dynamic steady state for several more years.