Characteristics and cause of monocyclic aromatic hydrocarbon contamination in shallow groundwater in an area of the Yangtze River delta

Abstract：The paper deals with the study of monocyclic aromatic hydrocarbon (MAH) in shallow groundwater in three cities (C, W and S) in an area of the Yangtze River delta. Based on the data of water samples, the authors summarize the contamination features and distribution characteristics of MAH and then discuss the cause of MAH contamination in shallow groundwater in terms of the distribution of contamination sources, MAH volatility, precipitation and anti-pollution performance of the aeration zone in the study area. Study indicates that the MAH contamination in shallow groundwater is of point distribution within a small scope and not so serious, while the MAH contamination in surface water is serious. The characteristics of MAH contamination in shallow groundwater of the study area are closely related to the distribution of factories, volatile features of MAH, precipitation and anti-pollution performance of the aeration zone in the study area. The contamination shows good consistency with the distribution of factories. The detection points of organic pollutants with a high concentration are all distributed near polluted factories, while no organic pollutants have been detected from shallow groundwater samples in places without marked contamination sources. Henry's constants of various MAH components detected are all >1.01×102 Pa·m3·mol-1, so volatilization is its main migration mechanism. The column test of benzene indicates that it only takes 26 days for the concentration of the benzene solution to decrease from 1079.0μg/L to 6.9μg/L. The aeration zone in the study area is composed of sludge clay of fluviolacustrine deltaic facies. It contains plenty of clay granules and clay minerals and is rich in organic matter (>1.0%). Therefore the soils have a high adsorptivity to stop the pollutants from migrating to shallow groundwater. When it rains, both the detected MAH rate and concentration are high. The combination of the above-mentioned four factors makes the shallow groundwater in the study area have the following features: the pollution degree is low, MAH is of sporadic distribution and the pollution scope is small.