**4.1 Plane distribution of the CCl4 plume**

442 Pesticides in the Modern World - Risks and Benefits

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

K2 9.1 0.8-8.7 1.3-2.9 4.6-4.8 NT NT NT K3 6.0 0.8-5.3 1.1-7.7 6.5-7.0 NT NT NT

04-3 10.5 4.0-6.7 0.7-1.1 5.5-5.7 2.5-10.2 5.3-11.7 9.0-9.2 04-4 9.2 5.0-8.7 0.7-1.1 8.5-8.7 7.8-8.0 15.8 7.8-8.0 04-6 5.0 0.5-4.7 0.7-34.0 4.0-4.2 ND ND ND 05-1 8.1 ND ND ND 0.5-8.1 5.3-9.2 5.0-5.2 05-2 4.7 2.5-4.7 3.3-42.2 4.0-4.2 0.2-4.7 2.7-26.5 4.0-4.2 05-3 5.7 ND ND ND 1.2-5.7 2.7-7.3 1.0-1.4 05-4 6.7 5.5-6.2 1.0-1.8 5.5-5.7 1.0-5.7 2.6-8.0 3.5-3.7 05-6 6.2 ND ND ND 0.2-6.2 6.9-11.3 2.5-2.7 05-7 5.7 2.7-4.9 7.6-33.5 4.2-4.4 ND ND ND 05-8 5.4 3.2-5.4 1.8-47.1 5.2-5.4 ND ND ND

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;

Depth of maximum content (m)

Content (μg/kg)

CCl4 Chloroform

6.5-7.0 0.9-2.8 6.5-6.7 5.0-6.7 9.5-19.8 6.5-6.7

Detected depth (m) Content (μg/kg)

Depth of maximum content (m)

fissured clay layer (depth 7.8-8.0m) respectively.

depth (m) Detected

depth (m)

Borehole

04-2 7.0 1.5-1.7,

ND- Not detected, NT-Not test.

**3.3 Pollution pathways** 

Table 3. CCl4 and chloroform contents in the soils

6. sand; 7. Movement direction of CCl4)

Borehole number

> 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 shown in Fig. 12.

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

Transport of Carbon Tetrachloride in a Karst Aquifer in a Northern City, China 445

According to geologic, hydro-geologic setting and monitoring data of CCl4 concentration in the past years, transport of CCl4 in the complex karst aquifer can be generalized as shown in

The changes in CCl4 concentration in typical wells are presented in Table 4 and Fig.15. However, there is a general downward trend in concentration, CCl4 concentration in most of the wells increased in 2010. This may be due to the decrease in groundwater exploitation. Well 2001 2004 2005 2006 2007 2008 2009 2010

quantile 1279.4 369.6 90.1 500.0 201.3 146.6 70.1 222.3

quantile 181.5 248.0 61.8 17.7 9.1 13.0 20.3 65.1

quantile 59.6 8.9 21.6 18.5 25.5 25.0 8.8 11.8

quantile 6.3 17.8 14.5 16.2 10.4 15.5

quantile 14.6 13.2 8.8 10.9 7.5 4.9 20.8

1313.6

168.1

136.6

115.8

39.2

81.2

67.8

quantile 92.0 24.5 13.6 11.9 10.9 8.7 33.6

quantile 30.0 22.4 16.0 14.4 11.9 8.2 29.8

n=27

n=27

n=27

n=25

n=27

n=25

n=27

620.5

163.7

107.2

78.8

39.5

31.1

27.8

n=19

n=25

n=23

25

n=25

n=25

n=25

587.9

104.0

116.9

75.7

16.9

28.2

30.0

n=12

n=12

n=11

n=12

n=12

n=12

n=12

193.8

122.4

27.7

21.4

19.2

17.4

16.9

n=49

n=55

n=49

n=53

n=53

n=24

n=48

627.6

152.0

39.0

29.2

27.7

37.2

45.2

quantile X-49 1722.6 1662.6 104.6 815.0 264.5 218.3 119.6 272.5

quantile X-61 907.7 325.2 96.0 52.2 19.0 21.6 30.4 108.5

quantile X-47 69.0 29.3 68.5 53.4 74.5 48.5 18.4 15.3

quantile X-83 25.1 27.3 45.4 24.6 11.3 18.9

quantile X-59 24.2 23.1 14.0 14.2 11.3 6.5 23.6

quantile X-64 40.2 31.0 23.6 16.2 14.4 8.7 33.1

quantile X-56 115.8 43.2 23.8 18.1 13.5 9.9 37.8

The Fig. 15 shows that: (1) the CCl4 concentration in karst auifer has obvious seasonal variation. In general, CCl4 concentration of groundwater during the drought period from February to June is relative lower and during the rainy period from August to October is much higher. (2) CCl4 changes rapidly with time, which is notably different from the

n=32

n=29

n=30

n=32

n=28

n=32

n=29

**5. Temporal change in CCl4 plume in the karst aquifer 5.1 Temporal change in CCl4 concentration in the aquifer** 

Fig. 14.

25%

50%

n=16

n=12

n=14

2584.3

2241.5

134.8

n=20

n=20

n=20

n=21

n=21

n=20

2911.9

609.0

162.8

44.6

115.0

196.4

n=52

n=54

n=52

n=53

n=56

n=40

n=56

681.4

212.7

216.6

204.0

58.8

67.5

89.1

Table 4. Summarizes of changes in the average CCl4 concentrations with time (μg/L)

90% quantile

25%

50%

90% quantile

25%

50%

90% quantile

25%

50%

90% quantile

25%

50%

90% quantile

25%

50%

90% quantile

25%

50%

90% quantile

"n" is the number of the samples

Because of the obstruction of higher-level water in the southern and western parts of the pollution source, polluted water could transport to northern sub-area along well-developed karst conduits. Transition sub-area has formed an obvious depression cone by artificial withdrawal and the water level was about 5.00 m lower than of the southern sub-area. Karst fissures and caves are most well developed in both horizontal and vertical direction in the northern sub-area. Water development experience in past fifty years has revealed this zone is the most water-yielding section and also the most intensive pumping area in the water-bearing basin. Consequently, northern sub-area is the centre of the depression cone and the CCl4 is accumulated in this area. In the middle sub-area, there is a layer of diabase igneous rock aquifuge at a depth from 100 m to 150 m, which separates the aquifer into two individual layers without hydraulic connection. Because the depth of most wells in the middle sub-area is less than 150 m, CCl4 concentration of the wells in this sub-area is relative lower.

#### **4.2 Vertical distribution of CCl4 in the karst aquifer**

The high density and low viscosity of CCl4 cause it to migrate downward until they encounter openings too small to enter. The influence of well depth on the CCl4 concentration in wells was studied. CCl4 concentration in Qiligou wells and Sanguanmiao wells increased with the increase of the well depth (Fig. 13). The transport of CCl4 in the groundwater is controlled primarily with gravity under similar hydro-geological conditions. Therefore, with deeper the wells, there is higher CCl4 concentration of groundwater is.

Fig. 13. Relationship between CCl4 concentration and the well depth (A: Sanguanmiao wells; B: Qiligou wells)

Fig. 14. Conceptual model for CCl4 transport in the karst aquifer (Han et al., 2004)

Because of the obstruction of higher-level water in the southern and western parts of the pollution source, polluted water could transport to northern sub-area along well-developed karst conduits. Transition sub-area has formed an obvious depression cone by artificial withdrawal and the water level was about 5.00 m lower than of the southern sub-area. Karst fissures and caves are most well developed in both horizontal and vertical direction in the northern sub-area. Water development experience in past fifty years has revealed this zone is the most water-yielding section and also the most intensive pumping area in the water-bearing basin. Consequently, northern sub-area is the centre of the depression cone and the CCl4 is accumulated in this area. In the middle sub-area, there is a layer of diabase igneous rock aquifuge at a depth from 100 m to 150 m, which separates the aquifer into two individual layers without hydraulic connection. Because the depth of most wells in the middle sub-area is

The high density and low viscosity of CCl4 cause it to migrate downward until they encounter openings too small to enter. The influence of well depth on the CCl4 concentration in wells was studied. CCl4 concentration in Qiligou wells and Sanguanmiao wells increased with the increase of the well depth (Fig. 13). The transport of CCl4 in the groundwater is controlled primarily with gravity under similar hydro-geological conditions. Therefore, with

y = 0.0224x2

B Well depth(m)

Discharge sub-area

Abandoned Yellow River Fault

R2 = 0.9615

90 110 130 150 170 190


Fig. 13. Relationship between CCl4 concentration and the well depth (A: Sanguanmiao wells;

Quaternary loose Ordovician limestone Igneous rock Aquifuge Flow direction

Fig. 14. Conceptual model for CCl4 transport in the karst aquifer (Han et al., 2004)

0

Middle sub-area

60

120

CCl4 Concentration (μg/L)

180

240

less than 150 m, CCl4 concentration of the wells in this sub-area is relative lower.

deeper the wells, there is higher CCl4 concentration of groundwater is.

**4.2 Vertical distribution of CCl4 in the karst aquifer** 


90 100 110 120 130 140 150 160

Pesticide plant

Pollution source sub-area

y = 0.0044x2

A Well depth (m)

0 -50 -100 -150 -200 -250

Recharge area

15

B: Qiligou wells)

20

CCl4 concentration(μg/L)

25

30

R2 = 0.9025 According to geologic, hydro-geologic setting and monitoring data of CCl4 concentration in the past years, transport of CCl4 in the complex karst aquifer can be generalized as shown in Fig. 14.
