**3.2 Soil pollution investigation**

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.

Transport of Carbon Tetrachloride in a Karst Aquifer in a Northern City, China 443

The size and shape of the CCl4 plume in the aquifer was confirmed by multiple samples from multiple water supply wells. In porous media aquifer or unconsolidated aquifer, the plume concentration decreases with the distance from the pollution source. But the plume distribution in the studied site was quite different. Karst conduits develop along preferential pathways between areas of groundwater recharge and discharge. CCl4 in groundwater was recharged from the southern pollution source and transported into northern supply wells forming a long belt-like plume. Based on CCl4 concentration data, the contaminated area can be divided into three sub-areas: southern pollution source sub-area, northern sub-area of artificial discharge center and transition sub-area or middle sub-area. The CCl4 plume in the karst aquifer was "dumbbell" shaped, with high contamination located in the southern and northern sub-area and relatively light concentrations in the middle transitional sub-area, as

Fig. 11. Wastewater with CCl4 seeping *through* the soil *into the karst aquifer*

**4. Spatial distribution of CCl4 in the karst aquifer** 

**4.1 Plane distribution of the CCl4 plume** 

shown in Fig. 12.

X-63 X-53 X-54 X-51 X-48

X-66

**Pesticide Plant**

X-67

X-47 X-83

X-50

X-61 X-46

X-68 X-75

3

10

50

50

X-43 X-42 X-44 X-60

3

10

X-59

X-58 X-38 X-37

> X-80 X-81 X-82

X-36 X-57

50

10

3

X-32 X-28

X-88

X-84

X-25 X-26

X-23

X-90

X-33 X-34 X-65

X-39

X-83

10 3

**Kuihe River**

<sup>a</sup>

X-35 X-30 X-31

X-91 X-68

50

X-18 X-17 X-21

X-22

X-89

**Abandoned Yellow River**

X-27

X-29 X-64

150

X-56 X-24 3

10

X-85 X-87

10

Fig. 12. CCl4 plume distribution in the karst aquifer (a. 2001-4-18; b. 2008-4-30)

Groundwater sampling point Groundwater flow direction Concentration contour Residential area Railway, highway River 0 1 Km 2Km

X-63 X-53 X-54

**Yun Long Lake** 

**150**

**50**

**3**

X-47 X-49

**50**

X-66

b

Pesticide Plant

X-51

X-50

X-43 X-42 X-44

**33**

**10**

X-60 X-59

**3**

**3**

**10**

X-84

X-48 X-83 X-61 X-46

1.0

X-58 X-39 X-38 X-37

**10**

X-80 X-81 X-82

X-57

X-34 X-65

X-67

X-90

**10 10**

X-25 X-26 X-29 X-64

X-23 X-20 X-19

X-83

X-33

X-32 X-28

**3**

X-36 **3**

X-88

X-35 X-30 X-31

**10**

X-24

X-56

X-85 X-87

X-91 X-68

**3**

X-22 X-18 X-17 X-21

X-89

Abandoned Yellow River

X-27

X-40

X-45

10

Groundwater sampling point Groundwater flow direction Concentration contour Residential area Railway, highway River 0 1 Km 2Km

X-69

Kuihe River

X-20 X-19

X-40

X-45

X-69

X-49

300

150

X-41

**Yun Long Lake** 


Samples with the highest content chloroform were colleted from clay-limestone interlayer (depth at 6.5 to 6.7m), intensive weathered igneous rock layer (depth at 9.0 to 9.2m) and fissured clay layer (depth 7.8-8.0m) respectively.

ND- Not detected, NT-Not test.

Table 3. CCl4 and chloroform contents in the soils

#### **3.3 Pollution pathways**

There are three pollution pathways of karst groundwater. Specifically: Wastewater directly entering the karst aquifer in the pesticide plant area (Fig.10); Flowing into the aquifer through bare section of limestone in drainage ditch; and Leaking under ditch by soil (Fig11).

Fig. 10. Generalized pollution pathway of direct seepage into the karst aquifer within the pesticide plant (1. Intrusive igneous rock; 2. limestone; 3. clay; 4. silty clay; 5. cultivated soil; 6. sand; 7. Movement direction of CCl4)

Fig. 11. Wastewater with CCl4 seeping *through* the soil *into the karst aquifer*
