Environmental Isotope-based Estimation of Groundwater Discharge Area and Classification of Hydrochemical Aspects of Distributed Ponds in the Kushiro Wetland

Abstract

  The Kushiro Wetland, which is designated for inclusion in the Ramsar List of Wetlands of International Importance, is located in the northeastern part of Hokkaido Prefecture, Japan. This study is associated with the environmental isotope-based investigations of hydrochemical aspects of distributed riparian ponds close to the Chiruwatsunai River, which is one of major natural rivers in the Kushiro Wetland. The observation results of dissolved radioisotope radon-222 in the ponds water showed that the discharge of deep groundwater below the underlying clay layer in the wetland might occur in the upstream area of the Chiruwatsunai River and the border area of low-lying wetland and hilly plateau areas. It is found from environmental isotope-based investigations that the distributed ponds in the wetland can be classified into two different types; a peatland water-fed type that is mainly formed by rainwater and peatland water, and a deep groundwater-fed type that receives the supply of groundwater from deeper aquifer in addition to rainwater and peatland water. It is also shown that the deep groundwater-fed type ponds have a small evaporative loss of ponds water compared with the peatland water-fed type ponds, and therefore maintain relatively steady hydrochemical environments by the contribution of the groundwater discharge.