**2.2 Geological and hydro-geological settings**

436 Pesticides in the Modern World - Risks and Benefits

water supply and their vulnerability to contamination by chlorinated solvents are reasons to seek improved understanding of how chlorinated solvents behave in karst aquifers (Wolfe et al., 1997). This chapter discusses CCl4 transport and fate in a karst aquifer in a northern

Karst aquifer investigated is located in the Northern China Plain. Karst aquifers provided averagely 25.8×104 m3/d to urban public water supplies from 1981 to 2008 (Liu, 2010). As illustrated in the Fig.1, the karst water is in a relatively confined groundwater system unit and its east and west boundaries are coal seam water-resisting layers and the south and north are groundwater watershed. The karst groundwater system is composed several *relatively independent aquifers. The CCl4 pollution occurs in the* southern Qiligou water-bearing basin.

city of China based on *years of continuous monitoring* of CCl4 concentrations*.*

Fig. 1. *Hydro-geological zonation of the karst groundwater system in the city*

According to monitoring data obtained in November, 2000, it has been contaminated with CCl4 in Qiligou water-bearing basin. The pollution source is a pesticide plant which produced a pesticide that used CCl4 as a solvent and it has used more than 42 tons of CCl4 in the past ten years. This plant is located at hill slope in southwestern recharge area of the karst aquifer (Fig. 2). However, emergency measures were taken in 2001, including closing the pesticide plant and intensive pumping from heavily polluted wells. Untill May 2001, carbon tetrachloride was found in 53 wells (contaminated area is about 17.3 km2). The highest CCl4 concentration in karst water was over 3900 μg/L in a water supply well approximately 465m away from the pesticide plant. The concentration in Chinese standards for drinking water quality is lower than 2μg/L (GB5749-2006) (China's Ministry of Heatlth, 2006). Since then, the contaminated wells have not been used for dinking water. While, some lightly contaminated wells have been pumping for *agricultural and industrial production.*

**2. Site characterization** 

**2.1 Site location and pollution source** 

As shown in the Fig.3, the contaminated site is a NE synclinal basin with area of approximately 200 km2. Its southeastern and northwestern boundaries are two NE mountain chains composed of Cambrian and Ordovician limestone with elevations from 100 m to 248 m above the sea level. Quaternary deposits in the central lowlying area of the basin are composed of alluvium, proluvium, sand, sandyclay and subclay. The thickness of Quaternary is from 5 to 30 m, and the elevation varies from 30m to 40m above the sea level.

Fig. 3. Bedrock geologic map of Qiligou water-bearing basin

Transport of Carbon Tetrachloride in a Karst Aquifer in a Northern City, China 439

According to borehole data, there are four well developed underground karst or paleokarst zones and they are regarded as the horizontal runoff layers in the karst aquifer (Fig. 6). Of them, the third karst zone with a depth from 90 m to 150 m is the most important runoff layer of karst grounwater. Karst groundwater is recharged mainly from precipitation (about 835mm per year). Rain water seeps into karst aquifer from sinkholes, fissures in outcrop areas or in covered karst area. It first infiltrates into Quaternary phreatic aquifer in lowlying basin area, then infiltrates into underlying karst aquifer from recharge skylight. In middle sub-area, there is a layer of igneous rock and the karst aquifer can be divided into upper

Fig. 6. Vertical zonation of karst development along the groundwater flow path in the site (1. limestone; 2. dolomite; 3. igneous rock; 4. cave; 5. karst fissure;6. fault fracture zone; 7 water

The variation in groundwater level from 2000 to 2008 is presented in Fig.7. Although the range of groundwater fluctuation in different wells is different, the trend is similar. It suggests there is a good hydraulic connection within the aquifer system, which makes the aquifer more vulnerable to contamination. Many years of groundwater level observations

In conclusion, karst water-bearing medium in the site is distinguished as extreme anisotropic and heterogeneous. Hence it can be classified as the combination of fissure network and runoff zones type, which has unified hydraulic field. Tracer results indicates that the karst conduits are well developed along the syncline basin axis and the velocity of groundwater is fast in the runoff zone, which can attain 3027.8m/h when the water source in the mining conditions (Pei, 2007). Therefore the convection is predominant in mass transport of the pollutants in the aquifer. Hence the pollutant in the subsurface can move

indicate that there is very little change in the karst groundwater flow field.

faster and further with the groundwater flow.

table; 8 karst zone)

water-bearing zone and lower zone because of igneous rock watertight.

karren and *grooves* in exposed karst *area*

Corrosion on the surface of limestone

Fig. 4. Surface karst formation in the site

Limestone cropped out along hills contains abundant karst landforms such as caves, blind valleys and sinkholes, which provide pathways for the rapid transport of contaminants into the aquifer. Fig.4 presents the surface karst landform in the studied region.

Fig.5 demonstrates the *degree* of *development* of *karst* in the *subsurface*. Karst caves and fissures are the major structure of water storage. Especially, the honeycomb-like dissolved solution pores are quite well-developed.

Fig. 5. Corrosion of rock core samples

 karren and *grooves* in exposed karst *area*

 Corrosion on the surface of limestone

Limestone cropped out along hills contains abundant karst landforms such as caves, blind valleys and sinkholes, which provide pathways for the rapid transport of contaminants into

Fig.5 demonstrates the *degree* of *development* of *karst* in the *subsurface*. Karst caves and fissures are the major structure of water storage. Especially, the honeycomb-like dissolved

the aquifer. Fig.4 presents the surface karst landform in the studied region.

Fig. 4. Surface karst formation in the site

solution pores are quite well-developed.

Fig. 5. Corrosion of rock core samples

According to borehole data, there are four well developed underground karst or paleokarst zones and they are regarded as the horizontal runoff layers in the karst aquifer (Fig. 6). Of them, the third karst zone with a depth from 90 m to 150 m is the most important runoff layer of karst grounwater. Karst groundwater is recharged mainly from precipitation (about 835mm per year). Rain water seeps into karst aquifer from sinkholes, fissures in outcrop areas or in covered karst area. It first infiltrates into Quaternary phreatic aquifer in lowlying basin area, then infiltrates into underlying karst aquifer from recharge skylight. In middle sub-area, there is a layer of igneous rock and the karst aquifer can be divided into upper water-bearing zone and lower zone because of igneous rock watertight.

Fig. 6. Vertical zonation of karst development along the groundwater flow path in the site (1. limestone; 2. dolomite; 3. igneous rock; 4. cave; 5. karst fissure;6. fault fracture zone; 7 water table; 8 karst zone)

The variation in groundwater level from 2000 to 2008 is presented in Fig.7. Although the range of groundwater fluctuation in different wells is different, the trend is similar. It suggests there is a good hydraulic connection within the aquifer system, which makes the aquifer more vulnerable to contamination. Many years of groundwater level observations indicate that there is very little change in the karst groundwater flow field.

In conclusion, karst water-bearing medium in the site is distinguished as extreme anisotropic and heterogeneous. Hence it can be classified as the combination of fissure network and runoff zones type, which has unified hydraulic field. Tracer results indicates that the karst conduits are well developed along the syncline basin axis and the velocity of groundwater is fast in the runoff zone, which can attain 3027.8m/h when the water source in the mining conditions (Pei, 2007). Therefore the convection is predominant in mass transport of the pollutants in the aquifer. Hence the pollutant in the subsurface can move faster and further with the groundwater flow.

Transport of Carbon Tetrachloride in a Karst Aquifer in a Northern City, China 441

**05-9 05-8 05-7 05-2**

**farmland**

**pesticide plant well**(**X-49**) **04-6**

**farmland**

**05-3 05-4**

**abandoned chloroactic acid plant**

**04-3**

**K2**

S4

**road**

**05-1**

**04-1**

road

**large calibre well (X-63)**

**04-4**

**K3**

**05-5**

**04-5**

**ditch**

**phreatic water well (X-70)**

**village**

X-47

**large calibre well**

**village**

**0 50 100m**

**thruway**

**S6 5**

**05-6**

S1 45.82 45.34

water well; 6, borehole; 7, village; 8, leakage observation spot.)

**effluent drain**

**04-2**

Fig. 9. Boreholes and leakage observation spots location in pollution source sub-area

(1, lined ditch; 2, bared limestone; 3, karst groundwater well; 4, large calibre well; 5, phreatic

Discharge (m3/h) Leakage rate (%) *Observation spot* Distance (m) 2001.7.22 2001.8.31 2001.7.22 2001.8.31

In order to investigate the feature of effluent leakage into karst aquifer, 17 soil sampling boreholes numbered K2 and K3(2001), 04-1 to 04-6(2004) and 05-1 to 05-9(2005) were drilled with auto-driller (Model: DPP100-3B) and a total 206 soil samples were collected(for locations see Fig. 9). The quaternary deposits are 2.4-10.2m thick. CCl4 and chloroform were detected in the soil (soil samples of K2 and K3 were analyzed only for CCl4). CCl4 was found in the drilling soil along the drainage ditch and nearby the west part of well X-49, and the highest concentration reached 47.1μg/kg (Table 3). The CCl4 content of soil in the boreholes nearby well X-49, (e.g. 04-6, 05-2, 05-7 and 05-8) are much higher and their highest content is 34.0μg/kg, 42.2μg/kg, 33.5μg/kg and 47.1μg/kg respectively. In general, CCl4 was found in the soil at depths than 3 meter, and the content increased with the increase of the soil depth. CCl4 was not detected or was relatively low in the topsoil. Chloroform, the daughter product of CCl4, was also detected, of which was in the range of 2.6 to 26.5μg/kg. For borehole 04-2, 04-3 and 04-3, the chloroform distribution was larger than that for CCl4.

S6 1148.0 35.66 35.66 22.10 21.34

**S2**

N

**pesticide plant**

**S3**

**2**

**1 Legend**

**3**

**4**

**8 7**

Table 2. Results of the leak off tests

**3.2 Soil pollution investigation** 

**6**

S1 S5

**farmland**

**pit**

**abandoned reservoir**

Fig. 7. Variation in piezometric levels in the research region
