抄録
Water levels of 39 wells and water qualities of samples from the wells have been measured to determine the groundwater movement in the Wani river delta, on the western shore of Lake Biwa. In the littoral zone around the delta, direct seepage measurements and sampling of seepage water and porewater have been carried out to clarify the seepage process of groundwater flow into Lake Biwa.
Shallow groundwater is strongly recharged by effluent water from the Wani river an d moves into the lake in the direction perpendicular to the shore line. Slopes of water table near the shore range from 0.005∼0.01 at Wani beach and about 0.005 at Imajyuku beach throughout the year.
Both seepage flux and electrical- conductivity of seepage water change spatially along the shore line. Areas along the transect perpendicular to the shore line show high values of seepage flux and electrical conductivity while near the shore, these values decrease abruptly with increasing distance from the shore. These patterns along the transect perpendicular to the shore line, however, do not show any significant spatial and seasonal changes throughout the study period. There are similar correlation between the quality of seepage waterwithin about 20m from the shore and that of well water near the shore. The distribution of chloride concentrations of groundwater in the vicinity of the lake indicates that the shallow groundwater (3∼5m depth below water table) in the aquifer discharges almost vertically into the lake within 20∼30m from the shore; the deeper water discharges into the lake at a distance of over 30m offshore.
These results indicate that it is important to measure groundwater seepage at many sites near and along the shore line in order to estimate seepage amount into a lake and also suggest that the direct method employed is available to investigate shallow groundwater flow system near shore.
