Analysis of dissolved gas and water chemistry in mountain region of Goaping river watershed in southern Taiwan

Due to high topographic relief, groundwater is critical for water supply in Taiwan. The mountain region of the Goaping river in southern Taiwan is a large catchment with few tributaries, that makes it a suitable area to understand the potential of groundwater recharge in the mountain area. The aim of this study is to interpret the characteristic of groundwater in the study area using dissolved gas and water chemistry. Nine groundwater, 19 surface water and three hot spring samples were collected from the mountain region﹔6 groundwater and 2 surface water samples were collected from the plain area. All samples were analyzed for stable hydrogen and oxygen isotopes, dissolved gases (including N2, O2, Ar, CH4 and CO2), noble gases (helium and radon) and major ions dissolved in the water samples. For surface water samples, results of stable hydrogen and oxygen isotopic ratios are plotted on the local meteoric water line. Dissolved gases are dominated by N2 (>90%) and O2 (>5%). Helium isotopic ratio is approximately equal to 1 RA (RA is 3He/4He ratio of air). Radon-222 concentration is below the detection limit (<0.2 Bq/L); and concentrations of major anions and cations are low (Na+ <20 mg/L, Ca2+ < 60 mg/L, Cl- <2 mg/L). All these features indicate that surface water is predominately recharged by local precipitation. For groundwater samples, results of stable hydrogen and oxygen isotopic ratios are also aligned along the local meteoric water line. Helium isotopic ratios (0.90-0.23 RA) are lower, and radon-222 concentrations (0.30-19.6 Bq/L) are much higher than the surface water samples. Some samples show that CH4 (>20%) or CO2 (>10%) are dominated in dissolved gases other than N2, most likely contributed by biogenic or terrigenic sources. On the other hand, samples that have temperature 5℃ higher than the average of other samples, show ion concentration similar to hot springs. In the mountain region of the Goaping river catchment, shallow bedrocks have well developed fractures or thick layer of regolith, which make groundwater discharge more easily to the Pingtung plain. This study suggests that groundwater in the Pingtung plain are recharged not only by local rainfall and river waters, but also by significant amount of shallow groundwater in the mountain area.