Yong-jun Jiang¹, Cheng Zhang^1,3, Dao-xian Yuan^1,3, Shi-you Xie¹, Gui Zhang², Rao-sheng He²

1 School of geographical Sciences, Southwest University， Chongqing  400715, China; 

2 Institute of Geology Investigation in Yunnan province, Kunming 650041, China.

3 Institute of Karst Geology, CAGS, Karst Dynamics Laboratory, M L R Guilin 541004,China;

Abstract: Supported by RS and GIS, land use change from 1982 to 2004 was analyzed, and the impact of land use change on groundwater quality of Xiaojiang watershed, a typical karst agricultural region of YunnanProvince, Southwest China was assessed. The results indicate that: (1)The total land use converted during the past 22 years in the Xiaojiang watershed covers an area of 610.12 km², of which 134.29 km² of forestland was converted into cultivated land, and 210 km² of unused land was converted into cultivated land. (2) There was a dynamic relation between the groundwater quality change and the land use change.The groundwater quality change is related to the non-point pollution of using fertilizer caused by the increasing of cultivated land and construction land, and the decreas of forestland and unused land. As forestland and unused land transformed into cultivated land, the pH value, and the concentration of NH₄⁺,SO₄^2-, NO₃^-, NO₂^-, and Cl^- in groundwater increased, but the concentration of Ca²⁺ and HCO₃^- in groundwater declined.

Key words: Land use change; typical karst watershed; groundwater quality; Xiaojiang watershed; Yunnan Province

Introduction

Karst has been regarded as a fragile environment by environmental scientists. Because of karst system with a low capacity, it is difficult to restore once disturbed (Yuan 1988,1993). Changes in hydrological balances are not unique to karst regions, but karst regions are more sensitive than others (LeGrand 1984). Because there is often little or no soil or cover which means poor filtration, poor pre-purification, rapid infiltration, and that flow is turbulent and conduit dominated, high flow velocities allow transit times that may be too short for microorganisms to die off, particularly in shallow groundwater systems. Because large numbers of interconnected fissures mean that pollution inputs are possible from the surface almost anywhere, karst groundwater’s particularly liable to contamination (Prohic 1989). Recent study has shown that land use change may give rise to a significant impact on karst groundwater quality. With forest clearance, the concentration of nitrate and chloride in the groundwater increase significantly (Kastrinos and White 1986; Mark 1998),and the expanding of cultivated land and developing intensive agriculture result in the increasing in the concentration of the nitrate, sulphate, phosphate and pesticides (Libra, et al. 1986,1987; Robert, et al, 1998; Molerio, et al 1998; Ramón et, al. 1998; Pertti et, al. 1999; Alice et, al. 1999; Andrzej 1999; Milde, et, al 1988; Zhang and Yuan 2004). Industrialisation and urbanization result in increasing concentrations of nitrate, sulphate, phosphate, and even causing bacteriological pollution (Howard 1989), organic contaminants (Barbee 1994; Fetter 1993; Barker 1996), and heavy metal contamination (André 1999; Jia and Yuan 2003).

In the present study we analyzed the land use change from 1982 to 2004, and assessed the effects of land use change on groundwater quality of the Xiaojiang watershed, a typical karst agricultural region ofYunnan Province, Southwest China.

The study area

The Xiaojiang watershed of Yunnan Province, Southwest China(Figure1), covers an area of about 1034 km². It extends 24^°12^′-24^°45^′N and 103^°32^′-104^°00^′E with an average elevation of about 1900 m. It has a subtropical plateau monsoon climate with mean annual precipitation of 1000 mm and mean air temperature of 15.2 ^oC. The watershed is mainly underlain by Triassic system with carbonate rocks of 617 km² or about 60%. The soil mainly consists of red soil in the Xiaojiang watershed, which covers 730 km² or 71%. The population was 22.7×10⁴, of which 20×10⁴ was rural in 2004. The gross domestic product (GDP) was 0.4×10⁸＄, of which agriculture gross domestic product was 0.3×10⁸＄ in 2004. Xiaojiang watershed is a typical karst agricultural region for this pant of China.

Figure1 Location of the Xiaojiang watershed

Materials and Methods

Land use data and analysis

To facilitate the operation between series of spatial data, the land use category in Xiaojiang watershed is limited to 6 classes, i.e., forestland, cultivated land, unused land, orchard land, water bodies and construction land.

Land use data in 1982 (scale 1:50,000) was obtained from aerial photos from 1982 by interpreting, and verified and rectified through field survey.

Land use data in 2004 (scale 1:50,000) was obtained from Land-sat TM digital image in 2004 by supervised classifications, and checked by field survey.

The analysis of land use change was completed by Geographical Information System (GIS) software (ArcGIS and Professional MapInfo).

Water sampling and analysis

Thirty groundwaterwater points were monitored, of which 12 were cultivated land converted from forestland, and 11 were cultivated land converted from unused land, 4 forestland converted from cultivated land, and 3 construction land converted from cultivated land (Figure 2). Water samples were collected in the dry season (April) and rainy season (July) in 1982 and 2004, and measured in the field for pH, Ca²⁺, Mg²⁺, HCO₃^-by Multiline P3 pH/LF and Aquamerck, and subsequently analyzed in the laboratory for SO₄^2-, NH₄⁺, NO₃^-, NO₂^-, and Cl^-.

Also, the climate data from 1980 to 2004 was collected in the Xiaojiang watershed.

Figure 2 Distribution of the monitored groundwater points

Results and Discussion

Land use change

Through spatial overlay analyzing two-period land use maps, which was supported by Geographical Information System, the land use change matrix was obtained (Table 1).

As shown in Table 1, land use changed significantly in the Xiaojiang watershed in the past 22 years. The percentages of land use for each type were 19.63 cultivated land, 27.84 forestland, 0.02 orchard land, 1.89 construction land, 0.72 water bodies and 49.9

unused land in 1982, respectively, but they were converted into 46.5, 27.47, 0.15, 3.24, 0.77 and 22.77 in 2004, respectively. The total land converted covers an area of 610.12 km² in the Xiaojiang watershed during the past 22 years. The main land use type changes were from unused land into cultivated land and forestland, and from forestland into cultivated land. So the cultivated land increased by 268.5 km² or 132.7%, but unused land decreased by 280.5 km² or 54.36% during the last 22 years.

Table 1 The land use change matrix in Xiaojiang watershed from 1982 to 2004 (km²)

Groundwater Quality Change

Figure 3 reveals that the indices of groundwater quality have been modified in the past 22 years. As shown in Figure 3, the pH value, and the concentration of Mg²⁺, NH₄⁺, SO₄^2-, NO₃^-, NO₂^-,Cl^- in groundwaterincreased by 0.74, 10.26 mg/L, 0.40 mg/L, 38.13 mg/L, 25.75 mg/L, 0.24 mg/L, 14.19 mg/L, respectively, but the concentration of Ca²⁺ and HCO₃^- in groundwater decreased by 6.60 mg/L, 24.20 mg/L,respectively.

As shown in Figure 4, the fluctuation of climate was very little during the past 22 years in the Xiaojiang watershed. So the modification in groundwater quality was primarily caused by human activities, especially land use change.

The groundwater quality modified following forestland conversion into cultivated land

As show in Table 2, after forestland conversion into cultivated land, the pH value, and the concentration of Mg²⁺, NH₄⁺, SO₄^2-, NO₃^-, NO₂^-,Cl^- in groundwater showed an increasing trend, but the concentration of Ca²⁺ and HCO₃^- in groundwater showed a decreasing trend. The pH value, and the concentration of Mg²⁺, NH₄⁺, SO₄^2-, NO₃^-, NO₂^-,Cl^- in groundwater increased by 0.79, 16.34 mg/L, 0.58 mg/L, 48.16 mg/L, 23.28 mg/L, 0.25 mg/L, 6.96 mg/L, respectively, but the concentration of Ca²⁺ and HCO₃^- in groundwater decreased by 14.98 mg/L, 52.74 mg/L, respectively, after forestland conversion into cultivated land in the past 22 years. Furthermore, the concentration of NH₄⁺, NO₃^- and NO₂^- in groundwater exceeding the drinking standards in 2004. The groundwter pH value increased due to forest clearance, which brings about a decline in the content of CO₂ and the concentration of H⁺ result from the decreasing in biological activities. Also, with a decline in content of CO₂, the concentration of Ca²⁺ and HCO₃^- in groundwater decreased.

With the forestland conversion into cultivated land, the increase in the use of fertilizer should be primarily responsible for the increase in the concentration of NH₄⁺, SO₄^2-, NO₃^-, NO₂^-, and Cl^- of the groundwater. Because these ions, especially the nitrate ion and chloride ion, are highly soluble, they rapidly flush into groundwater in karst region. Furthermore, with the forest clearance, which leads to soil erosion, soil cover overlying the aquifer becomes thin which leads to an increased risk of leaching. The increase the concentration of NH₄⁺, SO₄^2-, NO₃^-, NO₂^-, and Cl^- in groundwater are believed to be caused by leaching of stored sulphate , nitrate, and chloride in the soil profile.

Figure 3. The modification of the indces of groundwater quality from 1982 to 2004 in Xiaojiang watershed

Figure 4. The fluctuation curve of precipitation and air temperature from 1980 to 2004 in Xiaojiang watershed

Tab2 The modification of the indces of groundwater quality following land use change

There is a selecting dissolution for Ca²⁺ and Mg²⁺ in groundwater. So, with decreasing in Ca²⁺ concentration, the Mg²⁺ concentration in the groundwater showed an increase trend.

As show in table 2, the groundwater quality modified following unused land conversion into cultivated land, the pH value, and the concentration of Mg²⁺, NH₄⁺, SO₄^2-, NO₃^-, NO₂^-, and Cl^- in groundwater showed an increasing trend, but the concentration of Ca²⁺ and HCO₃^- in groundwater showed a decreasing trend. The pH value, and the concentration of Mg²⁺, NH₄⁺, SO₄^2-, NO₃^-, NO₂^-, and Cl^- in groundwater increased by 0.82, 8.95mg/L, 0.31mg/L, 34.79mg/L, 21.92mg/L, 0.12mg/L, 12.89mg/L, respectively, but the concentration of Ca²⁺ and HCO₃^- in the groundwater decreased by 9.99mg/L, 42.84mg/L, respectively, after unused land conversion into cultivated land in the past 22 years. Furthermore, the concentration of NH₄⁺, NO₃^- and NO₂^- of the groundwater exceeded the drinking water standards in 2004.

Variation of groundwater quality following unused land conversion into cultivated land follows the same pattern as forestland converted into cultivated land.

The groundwater quality modified following unused land conversion into forestland.

As show in Table 2, after unused land conversion into forestland, the concentration of Ca²⁺, HCO₃^-, SO₄^2-, NO₃^-, NO₂^-, and Cl^- in groundwater showed an increasing trend, but the pH value and the Mg²⁺concentration in groundwater showed a decreasing trend. The concentration of Ca²⁺ , HCO₃^-, SO₄^2-, NO₃^-, NO₂^-, and Cl^- in the groundwater increased by 10.26 mg/L, 30.78 mg/L, 11.32 mg/L, 18.01 mg/L,0.03 mg/L, 11.80 mg/L, respectively, but the pH value and the Mg²⁺ concentration in groundwater decreased by 0.20 and 1.83 mg/L, respectively, after unused land conversion into forestland in the past 22 years.

The groundwter pH value decreased due to artificial afforestation, which brings about a increase in the content of CO₂ and the concentration of H⁺ result from the increasing in biological activities. Also, with increasing in the content of CO₂, the concentration of Ca²⁺ and HCO₃^- in groundwater increased obviously.

With the unused land conversion into forestland, the nitrate and organic matter in the soil profile will increase, which leads to a increasing in the risk of leaching.

Because there is a selecting dissolution for Ca²⁺ and Mg²⁺ in groundwater, with the increase in Ca²⁺ concentration, the Mg²⁺ concentration in groundwater showed a decline trend.

The groundwater quality modified following cultivated land conversion into construction land

As show in Table 2, after cultivated land conversion into constrauction land, the pH vaule, and the concentration of Ca²⁺, Mg²⁺ , HCO₃^-, SO₄^2-, NO₃^-, NO₂^-, and Cl^- in groundwater showed an increasing trend. The pH value, and the concentration of Ca²⁺ , Mg²⁺, HCO₃^-, SO₄^2-, NO₃^-, NO₂^-, and Cl^- in the groundwater increased by 1.47, 16.90 mg/L, 6.90 mg/L, 84.99 mg/L, 46.04 mg/L,59.94 mg/L, 0.91 mg/L, 50.00 mg/L,respectively, after cultivated land conversion into constrauction land in the past 22 years. Furthermore, the concentration of NH₄⁺, NO₃^- and NO₂^- in groundwater exceeded the drinking water standards in 2004.

The HCO₃^- concentration increased significantly due to the dense population, which brings about an increasing in the content of CO₂. Also, with increasing in content of CO₂ and HCO₃^- concentration, the dissolution capacity of the groundwater will increase, which leads to an increas in the concentration of Ca²⁺ and Mg²⁺ in groundwater.

Untreated domestic waste water, coming from toilets, bath and showers, dishwashers and washing machines, should be responsible for the increasing in the pH value. Also, because domestic waste water is generally characterised by increased content of nitrate, chloride,sulphate and ammonium, the concentration of SO₄^2-, NO₃^-, NO₂^- and Cl^- in groundwater increased significantly after cultivated land conversion intoconstruction land.

Summary and Conclusions

This study reveals that the groundwater quality has been extensively modified by human activities in karst agricultural region, Southwest China. Because of modifying land-cover, land use change can cause significant modifications in groundwater quality. After forestland and unused land were converted into cultivated land, groundwater quality modified significantly which showed an increasing trend in the pH value and the concentration of Mg²⁺, NH₄⁺, SO₄^2-, NO₃^-, NO₂^-, and Cl^- in groundwater, but a declining trend in the concentration of Ca²⁺ and HCO₃^- in groundwater. While unused land conversion into forestland, the concentration of Ca²⁺, HCO₃^-, SO₄^2-, NO₃^-, NO₂^-, and Cl^- in groundwater showed an increasing trend, but the pH value and the Mg²⁺concentration in groundwater showed a decreasing trend. After cultivated land conversion intoconstrauction land, the pH vaule and the concentration of Ca²⁺, Mg²⁺, HCO₃^-, SO₄^2-, NO₃^-, NO₂^-, and Cl^- in groundwater increased significantly.

Because the groundwater quality has not been continued to be monitored during a day or a month, this paper does not reveal the short term variation of the groundwater quality.

Acknowledgements

This research was funded by the physical geography doctor program open foundation of southwest university China, project code 250-411109, the doctor foundation of Southwest University of China, project code SWNUB2005035, and the project of Ministry of Land and Resources, China, project code 2003104000.

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