The interactions between surface water (SW) and groundwater (GW) and the resulting landscape connectivity can happen at different time scales with implications for biogeochemical cycling. In forested headwater wetlands, this hydrologic connectivity and its influence on functions such as greenhouse gas dynamics are still not well understood. To begin to understand how groundwater-surface water connectivity impact carbon emissions in headwater wetlands, our study employs a combination of synoptic sampling, dissolved gas sondes, and water level loggers. Over the course of two years, we sampled 18 wetlands and adjacent wells in the Delmarva Peninsula, Maryland every 2-3 months for dissolved greenhouse gases (CO2 and CH4) and other water chemistry parameters. We also instrumented a subset of sites with dissolved CO2 and O2 high-frequency sensors.
We found that dissolved CH4 in wetland surface water (15uM) is, on average, higher than in the adjacent groundwater (6uM). By contrast, groundwater (1,041uM) has higher CO2 concentrations than wetland surface water (315uM). From our discrete sampling, we found that dissolved CO2 in wetlands increases on average from 250uM to 450M after a long-term dry down period. However, on a shorter time scale of hours to days, we saw an increase in CO2 concentrations during a wet-up after a dry period. On longer time scales, changing redox conditions could be controlling greenhouse gas patterns, while short-lived increases could result from the accumulation of dissolved CO2 in soil pores that is transported as the surface water connects to the adjacent terrestrial environment. Integrating comprehensive discrete sampling with high-frequency sensor data allows us to evaluate how hydrology is changing on a seasonal and event basis and how these changing conditions influence the greenhouse gases along a landscape gradient. This understanding will be crucial as we move into a future of increased climatic variability.