The Carbon Tetrachloride Pollution Mechanism Of A Karst Groundwater Supply Source In One City

Based on the geological and hydro-geological conditions of a Karst water source, as well as long-term water pollution information and means of simulation experiment, we studied the mechanism of carbon tetrachloride contamination of the water sources systematically, and the conclusions are as follows:(1) A pesticide plant is the pollution source, so the ways of carbon tetrachloride into the Karst aquifer are mainly: waste water from the pesticide factory flowing directly into the Karst aquifer through the exposed limestone Karst or through the leakage into the soil at the bottom of drainage.The field tracer tests show that the main transport channel of carbon tetrachloride is in the axis of a syncline basin. In the main migration channel, the contaminant transport speed can be up to 3027.8 m/h.(2) The migration and pollution characteristics of carbon tetrachloride of the Karst groundwater source are as follows:Carbon tetrachloride contamination plume in the plane is a "dumbbell" shape, and the focus of the distribution of pollution are in southern pollution source sub-region and northern artificial centeralized excretion sub-region, and the concentration of CT in the south is significantly higher than that in the north.The developments of the carbon tetrachloride contamination plume are affected by pollution inputs, groundwater extraction, artificial flow field and other factors. Before 2004, the scope of carbon tetrachloride contaminated plume in wet period was greater than that in the normal river flow period and dry period, and the pollution became serious in wet period. Affected by a significant reduction in the amount of northern artificial centeralized extraction and reduction of hydraulic head difference (less than 10 m) between north and south area at the beginning of 2005, for the two heavily polluted poles, changes both the scope of carbon tetrachloride contaminated plume, and pollution levels, between wet and dry seasons, were little in the same year, and the pollution concentration showed a declining trend in the different years.(3) The simulation study on the vertical penetration of CT in soils shows:The content of organic matter in soil is an important factor that affects the permeability of CT in soil. The soil interlayer has a great influence on the vertical penetration of CT. The clay interlayer has resistance and arrearage influence on the transposition of CT and it can retard the vertical transposition of CT, which can cause the accumulation and increase of concentration of CT.The eluviation results show that the content of soil organic matter is the main factor affecting the leaching of CT. The leaching of CT is poor in soils with high organic matter content, so it is more difficult to remove CT in this condition. Another important factor affecting the leaching of CT is soil particle size. The leaching of CT in sandy loam soil is better than that in sticky nature soil.(4) The study on the volatilization and transfer of CT through the bedrock aquifers and soil shows:Soil moisture content has great influence on the volatilization and transportation of CT in soil. When the soil organic matter content is lower (<3.02% in this research), moisture content plays a leading role in affecting the adsorption of CT. The greater the moisture content, the smaller the absorption capacity. Similarly, the moisture has a great blocking effect on the volatilization and transfer of CT in soil-gaseous phase. On the contrary, the volatilization and migration of CT in the soil-gaseous phase is affected little by the content of soil organic matter, and the soil moisture content still plays a leading role in this situation. The relation between concentration of CT in soil-gaseous phase and that in liquid storage bottle conform to the Henry's Law.The migration of CT in the soil accorded with the first order reaction kinetics. The kinetic analysis shows that the dominant facor which affected the migration of CT in soil was also the moisture. The moisture had a blocking effect on the migration of CT.The ventilation experiment shows that the soil organic matter content plays an important role in the residues of CT in the soil, and the higher the soil organic matter content, the more residual amount. Temperature significantly affects the volatilization of CT in the soil, when the temperature increases from 10℃to 20℃, the latter evaporation rate of CT is nearly three times of the former. The ventilation experiment can simulate the dynamic change of volatilization and residues of CT in unsaturated soil.(5) We have studied the mechanism of contamination of CT in the soil through the indoor simulation experiment:The waste water containing carbon tetrachloride in the ditch of the outfall is leaked; it is why the soil is contaminated near the ditch.The overlying soil of contaminated groundwater is polluted by the volatilization and migration of CT from the groundwater. As the concentration of CT in groundwater near pesticide plant is on the high level (average 1336.5μg/L), the soil in this region is polluted by CT severely, and the concentration is up to 52.1μg/kg. Generally, the most polluted layer is below 2.5 meters deep.(6) The main input source of CT in the basin is the leakage of pollutant in sewage ditch near the pesticide plant and the leakage or secret emissions of pesticide plant and so on. The main output pathway of CT in groundwater is volatilization transfer and artificial pumping excretion and so on. Actually, the input carbon tetrachloride by means of the leakage in the sewage ditch is little. The output item of CT is mainly by artificial pumping of water.Input-output gap is large every year, it's estimated that it was more than 5.0 t/a in 2001, 1.30 t/a in 2004 and 0.27 t/a in 2005. This gap is owing to the unorganized pesticide plant emissions or conscious secretly emissions.The natural and artificial excretion and excretion rate of carbon tetrachloride are large in the Karst groundwater of the basin. So long as a large number of secret emissions do not occur, the contamination of carbon tetrachloride pollution will rapidly reduce in the Karst groundwater.