千葉県鋸南町の地すべり地小流域における渓流水の水質形成機構

Abstract

222Rn is an inert, naturally occurring, radioactive gas. The source of 222Rn in groundwater is 226Ra which is ubiquitous in sediments and sedimentary rocks, incorporated in aquifer material. As the 226Ra decays, its daughter product, 222Rn, diffuses into the pore waters of rock formations and becomes concentrated in the aqueous phase. Because 222Rn is present in much higher concentrations in groundwater than surface flow, anomalously high concentrations detected in a stream can be indicative of the influx of groundwater to surface flow.Volumes of stream flow were measured and water samples of stream were systematically collected for analysis of chemical compositions and 222Rn concentrations at nineteen locations along the stream in a small watershed of landslide area in Kyonan City, Chiba prefecture. Sevevteen samples of branches and springs were also collected.Two types of groundwater, the shallow groundwater and the deep groundwater, have been identified, clearly defined on the (Ca2+ + Mg2+) -HCO3 daiagram. The chemical compositions of streamwater and branch water are the intermediate compositions between two types on the diagram.Based on the fact of the change of chemical compositions, discharge volume and concentration of 222Rn in stream water, we found several possible groundwater seepage points along the river.The geochemical evolution of the streamwater along the river course presents a gradual increase in Mg2+, Ca2+, HCO3-, SO42-, K+ and PO4-P and a relative decrease in NO3-N, and Si. However, there are step changes at mixing points of branch water and groundwater with streamwater.From the present results, it is concluded that the compositions of streamwater are initially originate from shallow groundwater and are modified along the river course, mainly due to the mixing of branche water and deep groundwater, then yielding the range of deep groundwater compositions.222Rn data can also be used in a simple mass-balance equation in conjunction with stream discharge data to quantify groundwater inputs to surface discharge in stream.