从地下水赋存运移规律的再认识谈北京平原地面沉降发生的另一原因

通过主要对分层专门监测井进行取样,从水位、水质、水温及氘、氧-18、碳-14等方面进行分析,认为本区在扇缘及以下的冲洪积平原区地下水具有明显的呈层性,层间联系较差;运用氘、氧-18稳定同位素方法及碳-14定年确定出15个古水点,认为扇缘以下冲洪积平原区平均大致250m深度以下多出现古水点,表明深部水交替整体非常缓慢,局部滞留。据此分析,产生分层、深部交替缓慢及局部滞留的主要原因是由于黏土类地层的阻隔及地层的沉积压实,尤其是差异性压实,会形成相对封闭的滞留含水层。并认为地下水交替缓慢及局部滞留而不能及时获得补给是本区地面沉降发生的另一重要原因。

Discussion on Mechanism of Land Subsidence Based on Restudying Storage and Circulation Law of Quaternary Groundwater in Beijing Plain

Through a wide-spread groundwater sampling mainly at nested monitoring wells in the Beijing Plain, this study analyzed the water level, hydrochemistry, temperature, deuterium, oxygen-18, 14 C and other aspects. The result indicates that there exits an obvious stratification phenomenon of the groundwater in the alluvial plain area down in the middle part of alluvial -pluvial fans, and the hydraulic connection between layers is very weak. Based on the analysis of the 15 paleowater samples through the deuterium-oxygen-18 method and 14 C isotopic dating, the deep groundwater circulation in the fine sediments area of Beijing Plain down in the fringe part of alluvial-pluvial fans is relatively slow and partly stagnant. The average depth of the paleowater points is about 250m. The main reasons for the formation of stratification and paleowater points are the water-blocking effect of the clay type strata and the compaction effect during the sedimentary process, especially the differential compaction, which could form closed or relatively closed stagnant aquifers. The deep aquifer could not get recharge timely because of the slow circulation and partly stagnant, which contributes significantly to the land subsidence of the Beijing Plain area.