Abstract
The present study was conducted at Hirosato area in Kushiro Mire, where the aim of the nature restoration project by the Ministry of Environment is to restore pasture areas back to original mire. In order to understand the effects of pasture development and river improvement, the groundwater environment was investigated in pasture and surrounding areas. The results revealed that the mire area exhibited a transitional groundwater system from fen to bog. In comparison to the mire area, the average groundwater table in the pasture was lower with higher fluctuations. The pasture was developed along the Kyu-Setsuri River where the groundwater conditions are regulated by the river water level. This was greatly reduced by the diversion of the original course through artificial dikes. Thus the groundwater in the pasture was rapidly drained into the Kyu-Setsuri River. Moreover, Na+, Cl-, Mg2+ concentrations of groundwater were significantly lower in the pasture than in the mire area, suggesting a drainage of these nutrients from the pasture through the groundwater. In addition, the Ca2+ concentration of the groundwater in the pasture area was twice as high as in the mire area. This can be attributed to remains of calcium carbonate from pasture soil amendment, although about 30 years have passed since the pasture was developed. Furthermore, the river diversion by artificial dikes conducted almost 70 years ago has been influencing the groundwater system and its water quality. In order to restore the original groundwater environment, an increase of the water level of the Kyu-Setsuri River and the treatment of soil amendments are necessary.
