浅析广西北海市偏酸性地下水的形成原因

A Preliminary Analysis of the Formation of the Weak Acidic Groundwater in Beihai, Guangxi

广西北海市滨海平原松散沉积物中分布有潜水含水层和承压含水层,地下水的pH值普遍偏低,一般4.0～6.0者居多。第四系松散沉积物的矿物成分石英占50%～80%,含少量粘土矿物,化学成分中SiO2占绝大多数。以难溶成分为主的沉积物和长期的淋滤作用使地下水具有低矿化度。天然状态下偏酸性地下水的H+来源于碳酸的离解、粘土层中的H2O+以及雨水中的酸度。碳酸则是由CO2溶解于水而形成。其中CO2主要来源于生物成因。地下含水系统中缺少可以中和酸的碱性物质,有利于H+集聚,使地下水pH值偏低。

Unconfined and confined aquifers occur in the unconsolidated sediments in the coastal plain near Beihai, Guangxi. Groundwater in the aquifers is of low pH ranging commonly from 4.0 to 6.0. Minerals in unconsolidated Quaternary sediments are predominated by quartz with small amount of clay minerals. SiO2 accounts for most of the chemical compositions of the sediments. The sediments containing mainly insoluble constitutes and dissolution of a long time lead to low TDS of the groundwater. Under the natural conditions, H＋ in the groundwater is derived from dissociation of H2CO3, release of the absorbed H2O^＋ in clay layers and the acidity of rainwater. The H2 CO3 in the groundwater is formed by dissolution of CO2. The CO2 comes mainly from biological contribution. Lack of alkaline substances in the groundwater system is also helpful in the accumulation of acidity, resulting in a decrease in pH of the groundwater.