某市岩溶地下水四氯化碳污染特征研究

Groundwater contamination by carbon tetrachloride in Karstic area in China

赋存于寒武系和奥陶系灰岩中的岩溶水是某市的主要供水水源,每天供水量约为35×104t.2000年11月地下水监测资料表明,南郊水源地的岩溶水已受到了四氯化碳污染.污染源为一农药厂,该厂位于七里沟水源地补给区的山坡上,生产农药时用四氯化碳做溶剂.该厂生产废水中四氯化碳的浓度从281 0μg·L-1到2584 3μg·L-1.到2001年5月,水源地中53口水井中发现了四氯化碳,污染面积达17 5km2,井水中四氯化碳浓度最高达3909 2μg·L-1.根据四氯化碳浓度,污染区可以被划分为3个亚区:污染源附近亚区(初始最大浓度为3909 2μg·L-1,2003年4月最大浓度1891 5μg·L-1),集中抽水排泄区污染亚区(初始最高浓度195 5μg·L-1,2003年4月最大浓度504 5μg·L-1)和中间污染相对较轻的过渡区(一般20～200μg·L-1).过去3a间64口水井水质资料监测表明,岩溶含水层中四氯化碳含量受降雨、承压水位和含水层深度等多种因素的综合控制.由于四氯化碳为一种微溶于水的比水重的非水相有机物(DNAPL),它主要沿含水层底部运移,因此比较浅的水井(深度<150m)中四氯化碳含量比较低,而在深度较大的水井(深度>150m)中则浓度较高.

Groudwater formed in Ordovician and Cambrian Limestone aquifers in Karstic area is the major source of drinking water supply in a city in northern China with a yield up to 35,000?m^3 per day. According to monitoring data in November 2000, the water in Qiligou water-bearing basin has been contaminated by carbon tetrachloride and the contamination is located in southern suburban of the city. The contaminant comes from a pesticide manufacturer that used carbon tetrachloride as solvent in its process. The manufacturer is located at hill slope and in the recharge area of the Karstic aquifer. Monitoring data showed that concentration of carbon tetrachloride in the effluent ranged from 281.0?(μg·L^(-1) )to 2584.3?μg·L^(-1). In the recent survey in May 2001, carbon tetrachloride could be detected in 53 wells with contaminated area of 17.5?km^2. The highest concentration in groundwater was up to 3909.2?μg·L^(-1). According to spatial distribution of carbon tetrachloride, the contaminated area can be divided into three sub-areas: source sub-area where the highest carbon tetrachloride concentration was 3909.2?μg·L^(-1) in the past and 1891.5?μg·L^(-1) at present respectively; sub-area in drainage area where the highest carbon tetrachloride concentration was 504.5?μg·L^(-1) at present; and the transitional sub-area with moderate contamination. Monitoring data from 64 wells in past three years has shown that carbon tetrachloride concentration in the Karstic aquifer was regulated synthetically by precipitation, piezometric level and the depth of aquifer. Carbon tetrachloride concentration in most wells near the pollution source increases with the precipitation due to the eluviation, while in transition sub-area and drainage sub-area, it most decreases because of the dilution. The diffusion of the chemical was confined by higher water level around the contaminated plume and the shape of the plume is similar with the groundwater flow field. Carbon tetrachloride as a dense nonaqueous phase liquid (DNAPL) with slight dissolubility mainly transports along the bottom of the Karstic aquifer, therefore, carbon tetrachloride concentration in well depends on the depth of the well, usually relative lower concentration occurs in the wells with the depth less than 150?m and higher in those with the depth more than 150?m.