This data product includes measurements of dissolved CO2 and CH4 in surface water and groundwater, along with hydrological and biogeochemical variables, across 20 headwater wetlands in the Delmarva Peninsula in the Mid-Atlantic region of the United States. Greenhouse gas samples and water chemistry samples were collected every 1-3 months over a period of 2 years. We also monitored water level at each of the wetlands with a pressure transducer.