Binghua LI¹， Honghan CHEN¹，Xiaojuan CAO²

1 Beijing Key Laboratory of Water Resources and Environment Engineering, China University of Geosciences   (Beijing), 100083, China

2 National Research Center for Geoanalysis，Beijing,100037, China

 

Abstract: This study aimed to survey the contamination of chlorinated hydrocarbons and BTEX in Yangtze River delta region shallow groundwater, and then explored the factors of contaminated characteristic. A total of 129 shallow groundwater samples were collected from 218 different wells from early 2002 to late 2003. Tetrachloroethylene (PCE) was detected in about half of all samples (42.6%), followed by carbon tetrachloride (CT); trichloromethane (TCM); and trichloroethylene(TCE). Benzene, toluene, ethylbenzene, xylene isomers were detected in approximate 20% of the samples analyzed. The detected contamination concentrations varied from 105.7µg/l for toluene to 0.05µg/l for carbon tetrachloride; all of detected the concentrations were lower than those corresponding standards set by the U.S. Environmental Protection Agency (U.S.EPA) for drinking water except for benzene in a few wells. The maximum observed benzene concentration was 8.05µg/l, which was over an order of magnitude higher than current drinking water standards (5.0µg/l). It indicated that almost all the concentrations of BTEX were over an order of magnitude higher than those of chlorinated hydrocarbon. Thus BETX were the major contaminants in the study area. All of the wells with significantly higher contamination concentrations were associated with the channels where industries directly discharge sewage. Further study showed that the contaminated characteristics of the study area were relative to these factors, such as the land use, the physical and chemical features of those contaminants, especially the high anti-pollution capability of the study area unsaturated zone.

Keywords: Shallow groundwater，BTEX，Chlorinated hydrocarbons，Unsaturated zone

1 Introduction

The growing rural population in the Yangtze River delta region demand more fresh water resources. A great deal of shallow groundwater has been found in the unsaturated zone underlying the region. Protection of this valuable resource from contamination is essential to the health and safety of the citizens inhabiting there. The organic contaminants in groundwater mainly involved chlorinated hydrocarbons, BTEX, pesticides, and polycyclic aromatic hydrocarbons (PAHs). The presence of chlorinated hydrocarbons and BETX in water sources is a significant concern because they are known for animal carcinogen and suspected human carcinogen (U.S.EPA 822-R-02-038, 2002).In addition, biodegradation of them appears to be limited in most environments.Like other petroleum products, chlorinated hydrocarbons and BTEX are produced in large quantities and usually used in solvents. As a result of their widespread use, the release of them to the environment has by no means been associated with pollution, but they are generally found in water supplies in dissolved form at µg/l (ppb) to mg/l (ppm) concentrations.Traditionally, the electronic, instrument manufacturing, and aerospace industries are regarded the sources of hydrocarbons. Also, a range of small businesses in every community using hydrocarbons have been responsible for problematic spills. The common chlorinated hydrocarbons are the most ubiquitous. In a survey in New Jersey of 1,070 wells, TCE is found in 58% of the wells, carbon tetrachloride in 65%, and PCE in 43% (.Page,G.W.1981). The main source of BTEX contamination is the leakage of gasoline from faulty and poorly maintained underground storage tanks. Other sources of BTEX contamination are releases from large bulk facilities, surface spills, and pipeline leaks. When gasoline comes into contact with water, BTEX will readily partition into water because of their comparatively high aqueous solubility. Once in the environment, they also rapidly partition from water to air because of their high Henry’s Law coefficient. Since BTEX are mobile, they are also particularly common in groundwater. (Kao and Borden, 1997; Swoboda-Colberg, 1995; Chapelle, 1993; Barbaro, J.R., Barker, J.F., Lemon, L.A., et al., 1992).

2 Hydrogeology

The study area is located in Yangtze River delta, Jiangsu Province, China. There are lots of rivers like a net to connect one another. The medium of unconfined aquifer is made up of low permeability alluvial clay and silt, and the buried depth of it is less than 15 meters. The topography gradient is low and easy for unconfined aquifer to receive recharge. However, due to low exploitation of shallow groundwater in the region, the recycle of it is very slow. The recharge relation between surface water and unconfined aquifer is difficult to define. Generally, surface water receives recharge from unconfined aquifer during flood season, and this condition is opposite during dry season. The main discharge styles of unconfined aquifer are evaporation, drainage to rivers during dry season, and exploitation by rural residents. Recently, the hydraulic gradient of this unconfined aquifer increased because the first confined aquifer had been exploited largely, which activated the first confined aquifer received recharge from that unconfined aquifer.

3 Methods

Shallow groundwater samples were collected from wells in different land uses around the three cities in Yangtze River delta and analyzed for chlorinated hydrocarbons (Tetrachloroethylene, carbon tetrachloride, trichloromethane, trichloroethylene) and BTEX (benzene, toluene, ethylbenzene, xylene isomers). All samples were collected manually in 22ml vials with no headspace and then transported to the laboratory for analysis. During sampling period, shallow groundwater were poured into a common container in the field. This common container was then used to fill the 22ml specified time intervals and put them into a plastic box. Organic compounds were analyzed using a Hewlett Packard 6890 Gas Chromatograph equipped with ECD, FID, and 7694E headspace autosampler. The quantitation limits for chlorinated hydrocarbons were 0.05µg/l more or less, while for BTEX were about 0.5µg/l. Over the course of this project, 13 sample blanks were analyzed. All the target compounds were not observed in any blank samples above the quantitation limits. The detailed parameters for chlorinated hydrocarbons and BTEX were presented in Tab.1. It is noted that the conditions for chlorinated hydrocarbons are a little different from those of BTEX.

4 Results and Discussions

4.1. Contaminant concentration and detection frequency

The locations of sampling wells were selected throughout the study area to cover the range of urban land uses where chlorinated hydrocarbons and BTEX might be expected to occur. These included open space as a control, low density residential, high-density residential, commercial, industrial, and service stations. In addition, there were a number of locations where there was no single dominant land use.

A total of 129 shallow groundwater samples were collected from 218 different sampling wells over 2-year period. Concentrations of all contaminants were low, with mean detected concentrations varying from 0.51µg/l for trichloroethylene to 10.32µg/l for ethylbenzene (Tab.2).

The highest observed concentration was 105.7µg/l for ethylbenzene, and the lowest was 0.05µg/l for carbon tetrachloride. All of the concentrations were lower than the corresponding standards set by the U.S. Environmental Protection Agency (U.S.EPA) for drinking water except for benzene in a few wells. The maximum observed benzene concentration is 8.05µg/l, which was over an order of magnitude higher than current drinking water standards (5.0µg/l).It was noted that almost all the concentrations of BTEX were over an order of magnitude higher than those of chlorinated hydrocarbon. Thus BETX were major contaminants in the study area.

Fig.1 and Fig2 showed the distribution of those observed contaminants’ frequencies in all samples analyzed. The observed frequencies of chlorinated hydrocarbon mainly distributed between 0.05µg/l and 1µg/l; Tetrachloroethylene was detected in about half of all samples (42.64.0%), followed by carbon tetrachloride (20.93%); trichloromethane (17.14%); and TCE (15.34%). The observed frequencies of BTEX mainly distributed between 0.5µg/l and 5.0µg/l;Benzene, toluene, ethylbenzene, and xylene isomers were detected in approximate 20% of all the samples analyzed. The highest observed frequency was xylene isomers (20.16%). Fig1 and Fig2 also presented that the most concentrations of detected chlorinated hydrocarbons ranged from 0.05µg/l to 1µg/l, and those of BTEX located between 0.5µg/l and 10.0µg/l.

4.2 Land use and shallow groundwater contamination

The different monitoring wells were grouped to identify land use types where chlorinated hydrocarbons and BTEX were more likely to be presented. As expected, open space and low-density residential had the lowest detection frequency and the lowest detected concentration for most contaminants (2.3% for TCM, 0.05µg/l for CT). Also as expected, industrial used land had high detection frequencies and maximum observed concentrations for most contaminants (42.6% for Tetrachloroethylene, 105.7µg/l for ethylbenzene). Average percent impervious surface of a land use was a good predictor of contaminant detection frequency for the contaminants. Runoff from industrial and high-intensity urban land uses had relatively high contaminant concentrations (Honghan Chen and Jiangtao He, 2005), indicating that most of the contaminants might come from them.

4.3 The physical and chemical futures of these contaminants

The analyzed chlorinated hydrocarbons and BTEX have high volatilities, high relative solubilities, and low degradabilities (McCarthy and Johnson, 1992). The Henry’s Law coefficients of them ranged from 0.32*10^-3 (atm.m³.mol^-1) for trichloromethane to 2.4*10^-2 (atm.m³.mol^-1) for carbon tetrachloride, which indicated they were readily to volatilize. Once they released to the unsaturated zone they would easily volatilize to the atmosphere. However, some still would be transported into the subsurface by gaseous diffusion, by infiltration of contaminated water, and as a moving soluble phase. The high relative solubility of them denoted that they could cause groundwater contamination at levels which were high relative to those concentrations which appeared harmful to human health. The low degradation of them indicated that they were relatively persistent in the environment and could exist as pollution source for a long time. The reason for this persistence was unclear but might be related to the relatively low cell yield for organisms growing on them, the lack of adapted microorganisms or other environmental factors. Compared with the solubilites of the analyzed chlorinated hydrocarbons, those of BTEX are a little higher, thus the concentrations of BTEX were higher than those of chlorinated hydrocarbons. Based on those characteristics of them, it could be reasoned why they were detected at so high frequencies while the concentrations of them were so low in the study area shallow groundwater samples.

4.4 The anti-pollution capability of the study area unsaturated zone

The anti-pollution capability of unsaturated zone was related closely to some factors, such as soil style, thickness, permeability, and topography gradient. In the study area, the soil styles were alluvial silt and clay, and its thickness reached to 15 meters. The content of particles whose diameters were less than 0.005mm ranged from 45.2 % to 84.6 % by weight. In addition, the content of montmorillonite and illite was approximate 15.0 %. Such soil had very low permeability (lower than 10^-7cm/s), high specific surface area, good expansion character, and high adsorption capability. Although the topography gradient was low and easy for unconfined aquifer to receive recharge, the recycle of groundwater was very slow because of the low exploitation of shallow groundwater in the study area. All those feature just described above could retard the movement of organic contaminants in unsaturated. So it was very difficult for the organic contaminants penetrating the unsaturated zone. As a result, it appeared the light contamination of shallow groundwater.

Aside from those factors affecting the behavior of organic contamination in subsurface, the organic matter content of soil played an important role. It was known that organic compounds could be easily absorbed by organic matter in soil. Almost all the organic matter contents of the soil samples were above 1.5 %. Turfs also were found in different locations, and their organic matter content reached to 18.7 %. Since the chlorinated hydrocarbon and BTEX could readily partition to the soil with high content of organic matter, the concentrations of those contaminants in shallow groundwater were very low.

In order to quantify the retardation capability of the unsaturated zone, a parameter R called retardation factor was introduced. It could be described by this equation: 

Where ρ_b is the soil bulk density, n is the soil porosity, K_oc is the organic carbon-water partition coefficient, f_oc is the content of organic matter ( Fetter,C.W., 1993). The higher the value of R was, the higher retardation capability the soil had. The average values of R in Tab.3 indicated that the soil with high content of organic matter had a high retardation capability for the organic contaminants.

Furthermore, a soil methanol-soaked test was conducted in laboratory to verify that conclusion. The result was presented in Tab.4. It could be seen that the absorbed BTEX content of soil samples were hundreds of milligram per kilogram. The highest concentration of xylene isomers was 1069.53 mg/kg in the 32.5mm depth of unsaturated zone. Due to absorption and volatilization functioning together, the absorbed BTEX contents of soil samples fluctuate regularly in different depth of unsaturated zone. Near surface, toluene, ethylbenzene, and xylene isomers could easily volatilize to atmosphere，so their concentrations were a little lower.

5 Conclusions

All shallow groundwater samples were analyzed by gas chromatography to achieve a quantitation limit of 0.05µg/l for chlorinated hydrocarbons, and 0.5µg/l for BTEX. Tetrachloroethylene were detected in about half of all samples, followed by carbon tetrachloride; trichloromethane; and trichloroethylene. Benzene, toluene, ethylbenzene, xylene isomers were detected in approximate 20% of the samples analyzed. While chlorinated hydrocarbons and BTEX were commonly detected, their concentrations were typically very low in most samples. Mean contaminants concentrations (when detected) varied from 0.51µg/l for trichloroethylene to 10.32µg/l for ethylbenzene. All of the detected concentrations were lower than those corresponding standards set by the U.S. Environmental Protection Agency (U.S.EPA) for drinking water except for benzene in a few wells. The maximum observed benzene concentration was 8.05µg/l, which was over an order of magnitude higher than current drinking water standards (5.0µg/l). For all of the BTEX, the observed contaminant concentrations were over an order of magnitude higher than those of observed chlorinated hydrocarbons.

       Thus the BTEX were the major contaminants in the study area. The presence of detected contaminants appeared to be reasonably well correlated with land use; contaminant detection frequencies were higher in land uses with higher impervious surfaces. The maximum concentrations appeared in a few wells near channels where many industries discharge organic contaminants. The analyzed organic compounds with high Henry’s Law coefficients were easy to volatilize to atmosphere, which, to some degree, caused the low concentrations in shallow groundwater. The unsaturated zone was made up of clay and silt with high content of organic matter (above 1.5%). The content of particles whose diameters were less than 0.005mm ranged from 45.2 % to 84.6 % by weight. In addition, the content of montmorillonite and illite was approximate 15.0 %. So the unsaturated zone had a high retardation capability for organic contaminants. A soil methanol-soaked test had verified that the absorbed BTEX content could reach hundreds of milligram / kilogram. Thus it might retard contaminants moving to shallow groundwater. As a result, the contamination of shallow groundwater was a little light.

Acknowledgements

The paper was supported by the China Geological Survey (NO.200310400046). Collection of the samples would not have been possible without the help of Licai Liu, Hongwei Chen, and Shi Liu. Special thanks go to the staff of the Key Laboratory of Water Resources and Environment Engineering, China University of Geosciences (Beijing) for providing all the samples analytical data. Thanks also go to Professor Zhaoli Shen, Associate Professor Jiangtao He and Fei LIU for giving valuable suggestion to focus the discussion on the causes of contaminated characteristics of chlorinated hydrocarbons and BTEX in Yangtze River delta region shallow groundwater.

 

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