Methane Emissions from Riparian Zones Formed by Sediment Deposition in Hydropower Reservoirs

This study was conducted in a riparian zone on a sediment deposited island inside Manwan Reservoir on the upper Mekong River, which is not directly disturbed by riverbank runoff. Spatial patterns of methane emissions across the island were analyzed by measuring methane fluxes at seventeen sites on the island and eight reference sites in the adjacent reservoir. All the measurements were conducted in triplicate at each site.Methane fluxes were measured using the static chamber method (Duchemin et al., 1999). The plexiglass chamber consisted of a 6.28-L cylinder (20 cm in diameter, 20 cm in height), an outlet for gas collection, a pressure-equilibrium tube (3 mm in diameter, 20 cm in length) and a removable Styrofoam. The outlet of the chamber was open during the chamber deployment, and was left to stand for 20 min to completely equilibrate with ambient pressure outside before sample collection. During gas collection in the reservoir, the chamber was fitted with the Styrofoam, which maintained the upper closed portion of the chamber about 10 cm above the water surface (Figure S3), and the measurements were conducted in an anchored boat. During gas collection on the island, a collar was installed in each site a few days before the measurements. Gas samples (20 mL) were collected every 10 min over a 40-min period using a 25-mL polypropylene syringe and were injected into a 12-mL pre-evacuated Exetainer® vial (839 W, Labco, UK) for storage until analysis. Exetainer® vials have been evaluated for storage of gas samples and they can limit sample loss to an insignificant amount for at least three months. Gas samples were analyzed by a gas chromatograph (7890B, Agilent Technologies, USA; the detection limit is 0.1ppmv at 25°C). Gas fluxes were calculated using linear regression based on the concentration changes of five samples over time. Linear regression correlation coefficients of less than 0.95 were not accepted for further calculations (Duchemin et al., 1999).The triple measurements were conducted by placing three individual chambers at each site, and nine chambers were employed by three work groups to finish measurements as quickly as possible. Methane fluxes at these sampling sites were used to feature spatial patterns of methane emission across the island and the adjacent reservoir using simple spline tension interpolator (Arcgis v10.2, ESRI) based on the assumed smoothness of variation (Immerzeel et al., (2009).