Procedia Earth and Planetary Science 7 ( 2013 ) 525 – 528

1878-5220 © 2013 The Authors. Published by Elsevier B.V.Selection and/or peer-review under responsibility of the Organizing and Scientific Committee of WRI 14 – 2013doi: 10.1016/j.proeps.2013.03.146

Water Rock Interaction [WRI 14]

Characteristics and reasons for groundwater pollution of the Qingshuiquan underground river system

LU Haipinga*, ZHAO Chunhonga, LIU Qinqina, KONG Xiangshenga, MIAO Yinga,b,YIN Yafanga,b, ZOU Shengzhanga

a Institute of Karst Geology, CAGS, Karst Dynamics Laboratory, MLR Guangxi Autonomous Region, Guilin, 541004, China b College of Environment Science and Engineering, Guilin University of Technology, Guilin, 541004, China

Abstract

In this study, we established 7 sampling points to collect water samples from the Qingshuiquan underground river system in December 2011, which was during the dry season. By analyzing those samples, we determined the present quality of underground water of the Qingshuiquan underground river system, the characteristics of its contamination situation, the source of pollutants, and the reasons for the pollution. Despite some light-compound pollution caused by PAHs and HCHs, the overall quality of the underground water of the Qingshuiquan underground river system is good. PAHs are due to coal burning; HCHs are from the use of Lindane. ‘Skylights’ (roof collapse features) and sinkholes can lead pollutants to the underground water intermittently and to pollute the underground water. © 2012 The Authors.Published by Elsevier B.V. Selection and/or peer-review under responsibility of Organizing and Scientific Committee of WRI 14 – 2013.

Keywords: Qingshuiquan; underground river system; pollution characteristics; interpretation of results

1. Introduction

The area of the karst regions in southwest China is about 78×104 km2, which is the largest bare karst area in the world, and is one of the most economically backward areas of China. At present, except for some environmental geological problems such as, drought, flood, water loss and soil erosion, stony desertification and Karst engineering geological disasters [1], the groundwater pollution also exists in this area [2-4]. With the development of economy and urbanization, the diversity of pollution accidents has occurred. The special "surface-underground double hydrological structure" in southwest karst region contributes to a weak antifouling system of karst water, which results in a bad quality of water environment, and the majority of underground water has been polluted [4-6]. In addition, since karst water is secluded and not-reversible, we can hardly completely recover it after pollution which threats the safety of water supply in karst regions in southwest China [7]. In this paper, we investigated the Qingshuiquan underground river system, collected groundwater samples of dry season in 2011 and analyzed them. In combination with the survey of hydrological geology and pollution source, we eventually figured out the current situation of groundwater water quality of Qingshuiquan underground river system, and then discussed the pollution characteristics and pollution reasons.

Available online at www.sciencedirect.com

© 2013 The Authors. Published by Elsevier B.V.Selection and/or peer-review under responsibility of the Organizing and Scientific Committee of WRI 14 – 2013

* Corresponding author. Tel.: +86 773 5809810; fax: +86 773 5831303. E-mail address: karstuhp@163.com ( LU Haiping).

526 LU Haiping et al. / Procedia Earth and Planetary Science 7 ( 2013 ) 525 – 528

2. General situation of the study area

Qingshuiquan underground water system, which located in the 4 km south of Pumiaotown, Nanning city, is the most important source of groundwater of Nanning, which supplies about 50 km3 water per day. It originates from the clastic rock area in east Naba village of Yongning district. The underground river runs from east Naba village to the west along the rock stratum through skylight ND004, ND007and ND001 (Fig.1). The length of the flow path is about 10 km and the total area is about 40 km2, including 22 km2 of the total area is bare karst area or shallow covered karst area and the Tertiary and Cretaceous conglomerate, sandstone and mudstone. Geomorphology of the study area is hill depression type karst, and the lithology mainly contains Carboniferous thick massive pure limestone and marl. There are small amount of siliceous rocks and local residual old Tertiary conglomerate. The direction of dissolved mound spreading keeps the same with that of the fold. The relative relief is about 20~60m and the residual butte ratio is less than 100m. The river system is developing and many small gullies have formed.

Fig. 1. Hydrogeological sketch of Qingshuiquan underground river.

In the limestone-exposed area, the underground skylights, karst depressions and caverns have developed, and the rock near the ND004 skylight is best developed. Near the outlet of underground river (Qingshuiquan), water outflows from east –west direction lane- style cave. Due to the dam, the water level is more than one meter and the cave is flooded.

3. Sample collection and test

3.1. The principle of sampling point layout

Groundwater sampling points layout is decided by the shape and size of underground river system tributaries formed by main pipe and branched pipe line, the number of the sampling points is at least two in branches, and at least three in the main channel.

The sampling points in the recharge-runoff area, located between the main and branch stream, are laid with the density of 5 to 10 per 100 km2, and in the significant potential pollution sources, the number of the sampling points should appropriately increase.

Sampling point layout. According to the “1:50,000 Nanning groundwater pollution survey” made by the Institute of Karst Geology, CAGS, sampling points are laid along the main development direction of the underground river, and the type of sampling points include: spring, well, skylight, outlet and so on. Finally, NS203 NS202 NS223 ND007 ND006 NS093 NS224 are selected as the sampling points.

3.2. Sample collection and test analysis

Dry season samples in December 2011 were collected. Four bottle of inorganic samples was gathered using 600 ml high density polyethylene plastic bottle, and 4 ml 1:1 HNO3 was added into one

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bottle immediately to make the pH of the sample less than 2; 2g solid NaOH was added into another bottle to make the pH of the sample above 12. Use 40 ml brown VOA special bottle which have already contained 4 drops of 1:1 HCl to collect 2 bottles of volatile organic samples; Use 1000 ml brown VOA special bottle to collect 2 bottles of semi volatile organic compounds samples. All the samples should be preserved in constant temperature box and set the 4 temperature, and take to the lab for analysis test immediately.

4. Test result analysis

The results of the groundwater environmental quality assessment show that groundwater water quality is good. The "First Class water", the "II class water" and "III and IV Class water" respectively accounts for 74%, 10%, 8%. The main impact factor are Benzo(a)pyrene, total BHC.

The majority of organic chlorine pesticide detected in the sample is HCHs, DDTs are not be found in the detection (Figure 2(a)). HCHs detection value range from 1.65 to 9.61 ng/L. The concentration of HCHs increases in the beginning and then decreases from upstream to downstream. The average of it is 5.43 ng/L. The lowest concentration (1.65 ng/L) appears in the skylight ND006, which illustrates the weakness contract between skylight and underground river system. There are few farmlands nearby, so the detection concentration is low.

Generally, if α-HCH/γ-HCH is between 4 and 7 in the sample, the sample must originate from industrial products after a long atmospheric transmission. If the ratio is less than 4, it shows that Lindane is playing the role of HCHs [8]. In 1983, China has banned the production of industrial HCHs, but Lindane is still widely used as pesticide currently [9]. α-HCH cannot be detected among HCHs isomers, (detection limit 0.91 ng/L), indicating that Lindane is still used in Qingshuiquan drainage area.

About 11 kinds of OCPs identified as priority control polycyclic aromatic hydrocarbon by EPA were detected, the detection rate is 68.7%, and the detection concentration increases gradually from upstream to downstream, indicating that the degree of pollution is gradually increasing (Figure 2(b)). From the value of BaP/BPE, we can judge the source of polycyclic aromatic hydrocarbon. If the value is between 0.3 ~ 0.44, it illustrates that the main source is traffic pollution, and if the value is between 0.9~6.6, it illustrates that PAHs is from coal burning [10, 11]. Values are 3.85, 4.01 and 5.20 from upstream to downstream indicating that PAHs is from coal burning.

Fig. 2. (a) Distribution of organic chloride pesticides in Qingshuiquan groundwater river (dry season); (b) Distribution of polycyclic aromatic hydrocarbons in Qingshuiquan underground water river (dry season).

At present, water source of Qingshuiquan is suffering from mild compound pollution. Besides of the pollution situation of underground water analyzed in this study, two large-scale "water bloom” phenomena have taken place in mid April and late May, 2008. Since then, serious pollution of underground water nearly happens once or twice every summer.

5. Analysis on the causes of pollution in Qingshuiquan underground river system

The primary cause of pollution in Qingshuiquan underground river system is the vulnerability of its karst aquifer. Along the distribution of the underground river, there are lots of skylights with different scales. Pollutants run with rainwater to get into the underground river intermittently. The dominating entrance of pollutants is the skylight ND007 located in Ganhuai Slope of Xinxin village. The physiognomy of Ganhuai Slope is a ring sector shape slope, and dozens of paddy field and plant slope lie around the skylight. The entrance of skylight ND007 is exactly the collection point of the slope.

(a) (b)

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Rainwater will wash dirt into the underground river through skylights, which contributes to a intermittently muddy of underground water, and turbidity index will increase. Economic activities around underground river system is agricultural cultivation, including rice, corn etc. And these skylights give the convenience for the entrance of pesticides, fertilizer, large numbers of hard degradation organic pollutants (such as pesticides) leading to underground water pollution (Figure 3).In addition, due to vandalism, the underground river system is becoming more fragile. Explosion of rock uncovered skylight ND004, skylight ND005 even become a poultry farms.

When it comes to the source of pollution, Yongning paper mill, cement plant, and waste landfill are conventional sources of pollution. In addition, extensive use of pesticides and fertilizers during agricultural planting in bare karst area is also the main source of underground river pollution.

1.Conglomerate 2. Limestone 3. Outlet of underground river 4. Water and pollution migration way

Fig. 3. Pollution model map of Qingshuiquan underground river.

Acknowledgements

This research was supported by China Geological Survey Geological Survey Projects (1212011121166) and the national basic research program of China (973 Program) project (2011CB201001).

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