湖沼と地下水 琵琶湖における調査・研究を中心として

Abstract

The interrelation of groundwater and lake is perhaps the least understood aspect of lake hydrology. The net groundwater input to Lake Biwa had been estimated from hydrological budget and showed the figures of 1 billion tons per year or 15-20 % of total input (4.5 billion tons per year). As a result, the nutrient loading level due to groundwater component becomes a problem one should not ignore.The development of a simple device which permit direct measurement of groundwater seepage has made it possible to get an independent estimate of groundwater input, and to collect seepage water sample. The usefulness of this meter has been documented at many lakes especially in USA and Canada.The seepage meters were installed at 34 stations around the littral zone of Lake Biwa. Groundwater seepage rate at every station ranged from 3.5 to 460l/m2 · day, and total discharge was estimated to be 0.71 to 1.14 billion tons per year, which accounted for a substantial amount (16-26 %) of total input.The water qualilty of seepage meter sample was varied with sampling localities and times. At places, water samples very similar in composition to the lake water were found, but the almost samples were similar to the well water around the lake, except for nutrient elements. The nutrient concentrations of seepage water were considerably lower in nitrate nitrogen but higher in ammoniun nitrogen than the well water. The concentration of phosphate phosphorus was practically lower than the well water but slightly higher concentration accompanied by high alkalinity was observed at places where muddy sediments were prevailing.The seepage flux and seepage chemistry data can be combined to obtain an estimation of nutrient loading due to groundwater. The annual nitrogen and phosphorus loadings were estimated to be 270-580 tons and 41-77 tons respectively.