Controls over hydrogen and oxygen isotopes of surface water and groundwater in the Mun River catchment, northeast Thailand: implications for the water cycle

Stable isotopic composition (delta H-2, delta O-18) of river water, groundwater, and paddy water in the Mun River catchment, northeast Thailand, were determined to investigate the hydrological processes and the impacts of natural and anthropogenic activities on the water cycle. Quantities of delta H-2 (-93.9 to -25.4 parts per thousand) and delta O-18 (-12.24 to -2.22 parts per thousand) in river water in the wet season follow the trend: upper reaches > middle reaches approximate to lower reaches. Trends for delta H-2 (-52.3 to -22.0 parts per thousand) and delta O-18 (-6.37 to -1.36 parts per thousand) in the dry season are: upper reaches approximate to middle reaches > lower reaches. In the dry season, groundwater (delta H-2: -57.5 to -34.6 parts per thousand, delta O-18: -8.24 to -4.40 parts per thousand) shows a lighter isotopic composition, and paddy water (delta H-2: -18.2 parts per thousand, delta O-18: -0.72 parts per thousand) shows the highest isotopic composition. Spatial variation of delta O-18 and deuterium excess suggests that groundwater exchanges with surface water frequently. Rainfall and river water recharge groundwater in the wet season, and groundwater flows back to the river in the dry season, especially in the middle reaches. This process is most likely related to impoundment of the rivers by large dams. On the other hand, the lowest values of stable isotopes of river water are coincident with the extreme flooding that was produced by Tropical Storm Sonca in July 2017. This study contributes to a better understanding of hydrological processes in the Mun River catchment and provides a perspective on the application of stable isotopes to other large tropical monsoon catchments around the world.