The Quantification of Biogas Emissions from a Korean Groundwater-Dependent Ecosystem through Novel Field and Laboratory Methods

Currently, there is a lack of quantification to the hydrogeologic sensitivities (groundwater level, soil textures, waters chemical condition, precipitation quantities, soil, and air temperature) that contribute to the fluctuating emissions from wetlands. Focusing on nitrous oxide (N2O) production rates within a palustrine groundwater-dependent ecosystem in Gunsan South Korea named Baeksukjae, a novel methodology has been applied to measure the emission rates of N2O and N2 gases from the field site. Five sampling sites were utilized where a total of 246 gas, 59 water, 38 soil, and 52 kinetic cell samples were collected. During sample collection, field parameters of pH, conductivity, dissolved oxygen, redox potential, soil, air temperature, and groundwater levels were measured daily over a 4-week field survey conducted in summer, 2021. Field data were then used within a denitrification and decomposition (DNDC) model for calibration. Water ion data at all sites reinforced several expected relationships such as the diminishment of nitrification during moments of high silicon concentration, increased delivery of alkaline earth metals during moments of higher acidity, and strong positive correlations between water electrical conductivity and alkalinity. Although the chemical diversity of water samples taken during the sampling stint are high, investigation revealed only a few significant parameters that vary NO2 emissions. Dramatic changes in water chemistry can be attributed to high precipitation events. Precipitation events diluted chemical constituents directly, as well as delivered nutrients to the wetland through runoff. Site mA consistently showed increases in nitrate (NO3-) concentrations (maximum values of 19.1 mg/L) in the groundwater and increased N2O gas production rates that mirror the frequency of precipitation and temperature variations. Analyzing kinetic cell data confirmed initial hypotheses that the production of N2O gas is occurring 20-32 cm beneath the surface and can be explained by the denitrification of dissolved NO3- in groundwater.DNDC model calibration was performed on the mA site using soil parameters measured by Dr. Eung Seok Lee, groundwater level data and maximum and minimum air temperature, in addition to vegetation type (vascular plants) for two years (September 2019- August 2021) and for July 2021. Modeling over large periods of time suggests that small changes in the groundwater and air temperature (+/- 2 oC) have little impact on annual N2O gas production when precipitation is held constant. When precipitation was modeled to be at 150%, annual N2O production rose by 17.2%. This relationship was consistent with reiterations of the model and nearly all N2O production occurred in the summers with very little to zero N2O gas formation in the winter.