某市X供水井群四氯化碳污染特征研究

Characteristics of Pollutant Carbon Tetrachloride in X Water Supply Wells in a City

X供水井群位于某市南郊水源地的北区,日供水量约5×104m3,是该市的主要供水水源,2000年11月发现该供水井群受到四氯化碳污染。3年多的监测资料表明,污染初期井水中四氯化碳含量迅速上升,强排抽水治理期间除个别时段井水中四氯化碳浓度显著升高外,井水中四氯化碳含量明显下降,目前四氯化碳含量相对较低,在30～130μg·L-1内波动,但仍大大高于国家饮用水标准。地下水流场控制着四氯化碳污染的扩散,X井群井水中四氯化碳含量随地下水位的上升而下降。自由相的四氯化碳是一种DNAPL,沿着深部或含水层底板运移,在含水层底部的部分自由相四氯化碳不断转化为溶解相并在浓度梯度下向上扩散,因此井孔越深井水中四氯化碳含量越高。

X wells, located in Northern part of the Southern suburban water source, are a major source of water supply in a city. Water development experience in past years has revealed X well district is the most water- yielding area of karst water and also the center of exhaustion cone. Its recharge mainly comes from the southwestern runoff zone. According to borehole data, there are four well- developed karst zones, of which the third with the depth of 110～ 140 m and the fourth with the depth of 170～ 210 m are the main runoff layers in the aquifer. Monitoring data in November 2000 indicated that the X wells have been polluted by carbon tetrachloride (CT), the pollution source is located at hill slope in southern recharge area. During the initial period of pollution, CT concentration in the wells increased significantly. From May 2001, X wells were intensively pumped for three months and as results the CT concentration decreased. At present, the CT concentration is relatively lower and fluctuates from 30 to 130 μ g.L- 1. Groundwater flow field controls the extending of CT contamination plume and the concentration decreases with the rising of the groundwater. As a DNAPL, CT will migrate along the deeper aquifer or bedrock of the aquifer. Due to dissolution of free phase and residual phase of CT at bottom of the aquifer and dissolved CT diffusion by the concentration gradient, the deeper the well is, the higher the CT concentration of groundwater is.