Guo Fang^1,2, Jiang Guanghui¹, Yuan Daoxian¹

1 College of Resources and Environmental Science，Southwest University, Chongqing 400715

2 Karst Dynamics Laboratory, Institute of Karst Geology, MLR, Guilin 541004

Abstract: Subterranean river is an important reservation and floating location for karst water resources in Southwest China, and important water source for the local people. The water quality of subterranean river not only affected the people’s health, but also an important factor affected the water sustainable development of karst areas in Southwest China. Long-term auto-monitored for subterranean river, measuring the index which sensitivity to environmental change such as pH, EC, water temperature, HCO₃^-and Ca²⁺ in the scene, observation once a month and in flood process, especially in fertilization season adding observation density are approaches to explore the water quality change discipline in southwest karst area. Bases on the previous study, we find NO₃^-,SO₄^2-,Cl^-,K⁺ are the most important observation objects. According to study for about a year in Guangcun subterranean river, it shows that except for the precipitation, the agriculture activity is also a major factor affected water quality. K⁺,NO₃^-,SO₄^2-,Cl^- are affected by fertilization obviously. In the past two decades, NO₃^- concentration increase heavily; the variety of SO₄^2- is very complex, it need longer time observation to explore it’s discipline; Cl^- rises a little; the contents of K⁺, Na⁺ have no obviously change; Mg²⁺ is likely to rise; HCO₃^- and Ca²⁺ have no change. We suggest that strengthen the protection of karst aquifer, using the fertilizer reasonable, at the same time we should enhance the mechanism study of water quality of subterranean rive affected by human activity. 

Key words: subterranean river ,land use ,ion concentration ,auto-monitor

1 The Study Status and Significance in National and International

Groundwater in karst area is an important water resource, about 25 percent of population mostly or entirely use karst water in the global. The karst ground water is mainly distributed in southwest karst mountain areas include Guizhou, Guangxi, Yunnan, Hunan, Guangdong, Hubei, Sicuan Province and Chongqing, it covers a karst area of 7.8 million square kilometer here, and has a population of eight hundred million. It has been found out primary that only in Guangxi there are 3066 subterranean rivers with an annual natural resource of karst groundwater of 180.8 thousand million cube meter. There are 435 subterranean rivers and 736 karst springs having 100l/s flow quantity in dry season, with the total runoff of 191m³/s in dry season[1]. We can see the abundance degree of groundwater here. But in the past two decades, due to unreasonable disposition of domestic waste from city and “three waste” of industry, mineral quarrying, agriculture activity, over used of pesticides and fertilizers, groundwater pollution aggravate gradually, which has endangered the safety of drinking water.

A lot of studies for karst water quality have been done in China, but most of them are centralized on the city, especially the city which suffers greatly from industry activity, waste water from city, mineral quarrying heavily, such as Shuicheng basin[2], Liupanshui, Guizou Province, Anshun city, Xuzhou, Jiangshu Province[3] and so on. Some people discuss the variety of water quality from the change of land use, they figure out there are some relativity between space-time variety of water quality and land-use. But in rural area water quality change due to agriculture activity is rarely involved in[4,5,6,7]. In foreign country, they have done a lot of studies about water quality affected by environment change, considering that the major ions of underground water or chemistry components are influenced by human activity directly or indirectly[8,9,10]. In recent years people attach more importance to the relationship between agriculture and karst, but they study the relation between agriculture and karst form, environment, water resources and water quality mostly from macroscopical point mainly.  

The main population is rural population in South karst area, groundwater quality is well as a whole here, most of them belong to I, II grade, but when they are compared with those in twenty years ago, the major ions concentration of ground water have observably increased, showing that ground water quality has a tendency of deterioration. Agriculture activity is a main factor responsible for ground water deterioration. Thus in order to find out the change disciplinarian of ions concentration, we need to take a long-term monitor for subterranean rivers, according to this we can have a scientific evaluation of ground water variation tendency. We had carried out the hydrogeological general investigation in late 70’s to early 80’s in twenty century, from there we have the analysis data of water quality in every important ground water outcrop. In the new round land and resources investigation, in order to understand the variety trend of water quality in the past two decades, by the way of comparing with the former period water quality, it has a very important signification for us to understand water resources status in our country. Due to there is only one or two samples for a subterranean river before, but the water quality is in ceaseless variation all the year round, it is necessary to have a long-term monitor to study the ground water variation discipline.

2Description of Sampled Subterranean River System

Guancun subterranean river lies in Daliang, Rong’an county, with a distance of 60km from Liuzhou city. The elevation of subterranean rive outcrop is 160m. It covers an area of 30.5km². The topography is high in northeast and northwest, with an average elevation of 420m and the highest of 770m and the lowest of 160m. The annual average temperature is 20 ^oC, and it has an annual average rainfall of 1750m. The relief belongs to fengcong depression, with a characteristic of developed dolines. The underground river is developed in Rongxian carbonate, it has a characteristics of surface and underground interphase, seasonal surface river interspersing in the depression (Fig.1). The bury depth of water level is shallow. Flood water level is higher than ground surface. The water level of berry depth in low water season is 8-12m. Flow quantity of subterranean river is affected by rainfall strongly. Usually a bout of flood takes a period of about a weeks. The bottom of depression in upriver of subterranean river is flatness with abundance land resources there. The styles of land use are paddy field, dry land and resident. There is a total population of 653 in the drainage area, but actually one third of population working out, they come back to inhabitance rarely. The quantity of population decrease compared with that of 1980. The infield area is 76 ha, taking account of 2 percent in the drainage area only. Paddy field is 58.13ha, taking account of 76.5 percent in the infield. The other is dry land, most of them are planting sugar cane. 

Fig.1 Hydrogeological map of Guancun subterranean river drainage basin

3 Materials and Methods

Guancun subterranean river is selected for the study area. We use multi-auto memorized instrument(CDTP300) produced by Greenspan Co. Australia to auto-monitor water level, water temperature, pH, electrical conductivity, precipitation at the exit(15min/once) from Nov.2004. From Jun 2004, with WTW and portable Ca²⁺ and HCO₃^- testing box we test pH, EC, water temperature, Ca²⁺、HCO₃^- in the scene once a month, the concentration of K⁺,Na⁺,Ca²⁺,Mg²⁺,Cl^-,SO₄^2-,NO₃^-,NO₂^-,NH₄⁺ were analyzed in the laboratory. After rainstorm in fertilization season, we increase sampling density when the subterranean river swelling. Water chemistry data of 1978 and 1980 were colleted; land use and human activity in the drainage area were surveyed.

4 Results and Discussion

The water type of Guancun is Ca-HCO₃, in this Ca²⁺、HCO₃^- are the dominant component, Mg²⁺、Na⁺、K⁺ 、Cl^-、SO₄^2- 、NO₃^- are secondly, the other ions and microelement are very few. Ca²⁺、HCO₃^-、Mg²⁺ are mostly derived from D_3r carbonate and dolomite, so the contents of them  have a compact correlation with karst process. All factors affected karst dynamics system would have an impact on the concentration of three ions. The concentration of K⁺、Na⁺ in the rock are less than those in water and soil, while they are equally in soil and water, showing that K⁺、Na⁺ are mostly derived from soil, some studies also show that Cl^-,SO₄^2- ,NO₃^- mostly come from soil. 

We take a water sample as behave of the month water quality from Jun 2004 to Jul. 2005. We observe several floods process and take water samples after an interval of 30min in May and Jul. 2005. The comparative results are showed in the table below. Although only the concentration of Ca²⁺ and Cl^- in May 1980 and NO₃^- in April 1978 are less than those variational range now, most of the concentrations of ions sampled in 1978 and 1980 are lower than variational range of 2004 and 2005, but the average of K⁺,Na⁺,Mg²⁺,Cl^-,SO₄^2-,NO₃^- are higher than those of 1978 and 1980, we have do more study to answer these questions.

Table.1 Comparison of ion concentrations in two periods

4.1 Stabilization and variety of water chemistry

The results of auto-monitored show that precipitation is a major factor induced the variety of ions concentration. In dry season(Sept.1004-Fre.2005), with small rainfall, water level keeping stability, water in the conduit and gravity water in soil having released completely, drainage water from the export of subterranean river is the groundwater storage in karst fracture, so the ions concentration is relative with water-rock reaction at this time. The fractures in carbonate are under half opened state, in this condition matter and energy communicating with atmosphere little; the exterior conditions of karst dynamics system keep stability, thus pH, EC(Electric Conductivity), water temperature of groundwater retain steady state(Fig.2).      

Due to continually rainfall in rainy season(June-August 2004, March-July 2005), water level and flow are in a state of continually variety, which results in the other ion concentrations changed constantly too. The total rainfall is 54mm in May 27,2005. During raining water level has a great range rise, the reflection is very sensitive, because of it rains all along in prophase. The decline of water level is a slowly process, 9 hours(at 12:00)after the rain stop water temperature become rise, at 28(0:00) it reached highest, the ascend extension was 0.84℃, after that it declined slowly. During the rainfall pH also has a decline process, pH resume the value of pre-rain when water temperature rise. EC starts to decline when it begin raining, after 16 hours(at 19:00) it reaches the lowest, after that it raise gradually. For the big intension of rainfall, supply of subterranean river give priority to inpouring form, surface section flowing into subterranean conduit firstly, which induces water level in the conduit increase drastically. Surface section flow has a higher temperature, but lower pH and EC than underground water in summer. When two types of water mixed together, water temperature will increase but pH and EC will decrease, yet these three parameters don’t change synchronization. When the mixing process end, due to karst and other process in karst aquifer environment, water temperature, pH and EC rise gradually. The change of water temperature is consistent with pH, which shows pH is greatly affected by temperature. The lower EC is lagged by the higher water level, with no corresponding relation with temperature and pH(Fig.3).

Fig.2 Dynamics variation of hydrochemical in exit of subterranean river in dry season

4.2 Ion concentrations in flood process

Precipitation supply to subterranean river, following that ion concentration will change. There is osculation relationship between ions variety characteristic and the sources of them. The ions which derived from soil will enter into subterranean river while rains, they will not react with other ions, due to the quantity released from base rock is small, they have a possibly of adsorbing or exchanging with clay in subterranean river motherboard. The relationship between ion concentration and recharge quantity of new coming subterranean river and the original of them determines the variety of ion concentration. The rainfall in late May 2005 didn’t result in descend of K⁺ concentrations, even it rains frequently, with water level fluctuating acutely, K⁺ concentrations don’t have a continuous decrease, instead they will ascend sometime. High water level doesn’t correspond with low concentrations(Fig.4). Continuance rainfall since Mid-June results in K⁺ concentrations drop to the rainwater’s concentration. Following the decline of water level K⁺ concentrations decrease, finally they return to rise(Fig.5). The lower concentration of K⁺ is 0.14mg/l, while K⁺ concentrations of rainwater in June is 0.19mg/l, indicating that K⁺ concentration decrease continually in soil due to rainfall leaching, till to the lowest value in a hydro-year. It also shows K⁺ is an activity cation, it will not resort in soil for a long time. Na⁺ is sensitive to rain, the rainfall beginning at May didn’t result in observably change of Na⁺, but rainfall from late May to June make Na⁺ concentration decreased rapidly. After decline they will decrease immediately, it is normal to Ca²⁺ and HCO₃^- those derived from rock but Na⁺ won’t do that. The possibly reason may be water from fracture has sufficiency contact with soil which result in high concentration of Na⁺, this water lag to water recharge to subterranean via dolines, after it entering into river Na⁺ concentration will rise.

Fig.3 Dynamics variation of hydrochemical in exit of subterranean river in rainy season

Ca²⁺ is derived from rock, with the concentration variety discipline of decline after water level rising before long, and then it will ascend. These phenomenon appear repeated in two rainstorms observation, the reason is low concentration of water recharge to subterranean river when rains, which results in Ca²⁺ concentration decrease, and then Ca²⁺ increase due to CaCO₃ dissolution in karst process. The variety of Mg²⁺ is similar with Ca²⁺, because both of them come from karst process, but the concentration of Mg²⁺ is lower than that of Ca²⁺, so Mg²⁺ derived from soil is more important to the variety of it’s concentration. Mg²⁺ rise gradually in June, while Ca²⁺ will decrease during increase process, the different reason between them may be this. The variety of Cl^- in May is instability, but rainstorm observation in June it became steady(Fig.7,8). From unsteadiness to steady showing that they affected by anthropogenic activities. Because late April is fertilization and transplant rice seedlings season. In paddy field collects large quantity of fertilizer, when rains fertilizer transfer to subterranean river, which makes Cl^- concentration fluctuated. Cl^- concentrations reach normal level in cropland in June, so the concentrations don’t fluctuate any longer.

Fig.4 Variation of cation concentrations in floodwater in May, 2005

Fig.5 Variation of cation concentrations in floodwater in June-July, 2005

From May to June Cl^- in subterranean river system decrease due to rainfall leaching, representing in notable decrease. Though it suffers from dilution from rainfall replenishment, Cl^- keeps continuance stabilization in flood events. The likely reason is Cl^- concentration in underground is equaled to that in surface space, when they mix together, they will not affect the total concentration, or maybe Cl^- in underground system have a strong balance ability(they can reach balance via adsorbing and exchanging). More studies should be done for these.

SO₄^2- concentrations greatly increase after fertilization shows fertilizer in cropland also has a great impact on SO₄^2-. When rains, water level swells in a short time inducing SO₄^2- decrease, but they rise to a high level soon. We consider it may be enriched highly in surface, which have some similar with Cl^- in late May, while the difference is SO₄^2- keep this phenomenon until June.

The variety discipline of HCO₃^- is similar with Ca²⁺, because both of them come from rock, they have an osculation relation with karst process.

NO₃^- has a tendency of degression from May to June, this is because of two big rainfalls. NO₃^- appears an up fluctuated in flood process in late May, but they disappear in late June, instead they keep steady, this is very similar with Cl^-, because both of them greatly affect by fertilization in late April. We can’t draw out why NO₃^- doesn’t have a low value, but this doesn’t affect the total concentration of NO₃^-.

Fig.6 The relationship between anion concentrations variety and water table in May, 2005

Fig.7 The relationship between anion concentrations variety and water table in Jun-Jul, 2005

The main reasons for ion concentrations variety are rainfall and human activity. In dry season(from Sept.-Fre.), rainfall account for 11% of the total in a year. Rainfall centralized on May, June and July gradually increase from March and from August it begin small. Agriculture activity is connected with the climate. In the study area, Farmer often fertilize dry land in February to plant sugar cane, harvesting in November. For cropland fertilization quantity is 373-597kg/ha. of compound fertilizers(the component includes K₂SO₄) before transplant rice seedlings in early May. After one week farmers superadd 5-10kg carbamide, and after two weeks add 5kg carbamide and 7.5kg potassium fertilizers(KCl). Fertilizer rarely used after that which determined by the growth status of the ripe. Harvest begins at August, after that till to March in next year most of cropland leave nature, some of them are planted corn and vegetable. Of course except for field and terra, the nature soil also has nutrient loss, which is relation to the intensity of rainfall and the condition of vegetation. Sloping field in drainage basin gives priority to arbor in 70’s. From 80’s people cut firewood for charcoal gradually spring up in the aim of business resulted in arbor gradually reduced. From 90’s a large scale of grazing and calcite mineral quarrying intensify the degradation of vegetable, which result in water and soil loss. Extra materials are inpoured into subterranean by human activities in February to May, which include N(they are mostly exit in a behavior of NO₃^- due to nitrification process), P(PO₄^3-), K(K⁺), Cl(Cl^-) and S(SO₄^2-). In the function of rain these material enter subterranean river, after transferring for some distance they are discharged in the vent. Four main cations have different characteristic of variety disciplinarian in the vent. In flood event observation showing K⁺ increase instead of decrease in May, in June they continuous decline, this likely be explained by fertilization activity in May. The behavior of Cl^-, SO₄^2-, NO₃^- in four anions show groundwater are influenced by fertilization activity. Correlation results for eight ions shows that the high relative degree is Mg²⁺-HCO₃^-(R²=0.5214), secondly is K⁺-SO₄^2-(R²=0.2654), K⁺-Cl^-(R²=0.1358) is also high compared with other ions. All of these can be interpreted by K⁺,SO₄^2-,Cl^- in subterranean river are likely derived from fertilizer. The absolute correlation coeffcient of K⁺ and SO₄^2-, K⁺ and Cl^- are not very high, the reasons are K⁺ can be adsorbed by crop, while SO₄^2- and Cl^- can’t.

4.3 Ion concentrations vary monthly

Ion concentration in subterranean river is affected by climate change in a year and human activities discipline, in which rainfall is the most important climate factor. Rainfall change and human activities are contributed to the discipline change of ion concentrations. The high value of NO₃^- is in February to May in a year, the concentrations in other months are similarity; they are belonged to low value. One reason for high value is NO₃^-enriched in surface are leached into subterranean river when the rain begin; the other reason is a large quantity of fertilizer contain nitrogen are used in this period. SO₄^2- appear high value in May to July too, we have educed the reason before, tit is affected by fertilization too. Cl^- concentrations don’t vary greatly in a year, only in February to May, 2005 they present a little decrease, but they truly affected by rainfall and human activities, we can make sure by high density observation in rainfall process. The influence don’t present in the monthly variety curve, indicating the influence only keep a short time. All of HCO₃^-,Mg²⁺,Ca²⁺,Na⁺,K⁺concentrations have the discipline of high value in dry seasons and low value in wet seasons, they are closely connect with rainfall. Because HCO₃^-,Mg²⁺,Ca²⁺ come from dissolution of carbonate, the influence of human activities keep short time, so while in low density sampling they reflect the phenomenon of  dilution by rainfall and enrich in dry seasons. Cl^-,SO₄^2-,NO₃^- in anions are correlation with human activities closely, the total proportions variety of them in anions are opposition with those of K+ and Na⁺, they representing low in winter and high in summer. Values in rainy seasons are higher than those in dry seasons, which reveals the function of human activites.

Fig.8 Cation concentrations variety affected by rainfall after fertilization

Fig.9 Anion concentrations variety affected by rainfall after fertilization

4.4 The trend of ion concentrations variety in the past two decades

Because only one or two groundwater samples analyzed data were done in the last hydrogeological survey of the whole country, it is difficult in comparing with them. We have compared the analyzed data with the pre-data, and find some of them decrease. The low value of NO₃^- is in the month except for February-May, the average value is 4mg/l, the high value is ranged in 9-16mg/l, individual one is less than 1mg/l, value in 1978 and 1980 are greatly lower than the lowest value, which shows NO₃^- decrease certainly. Low value of SO₄^2- doesn’t have any discipline, for example, the lowest value was in August and Sept. 2004, but they didn’t appear that in August 2005. Low value is smaller than 5mg/l, middle value is about 9mg/l, high value is about 15-19mg/l in May-July due to fertilize, the appearance of low value doesn’t have some rule, so we can’t say SO₄^2- has decreased generally. Low value of Cl^- appears after rainfall in Summer due to dilution of rainfall, low value is ranged in 2-4mg/l. High value is 7-10mg/l in February to June, the normal value is about 5mg/l. Although the value of 1978 is in the normal range, value of 1980 is greatly lower than lowest value, showing that Cl^- increased a little. Low values of K⁺ appear in the flood process of Spring and Summer, they are less than 0.3mg/l, the normal is about 0.4mg/l, high value is seldom. Results in 1978 and 1980 are between the lowest and the highest value, which shows that K⁺ has no distinctively change. High value of Na⁺ appear in winter, which is about 1mg/l, the normal value is 0.6mg/l, low value is less than 0.55mg/l, they have no distinctly change comparison with those of them before. High value of Mg²⁺ is in winter, which is about 9-11mg/l, in the initial stages of flood even it has low values transitorily, which is about 6mg/l. The two results of prophase are in the range of low value, so Mg²⁺ has a possibility of increase. Ca²⁺ and HCO₃^- don’t have a possibility tendency of increase.

Fig.10 Relationship between eight ions concentration variety in four periods and rainfall

Fig.11 Variety of calcite saturation index in 2004-2005

Fig.12 Monthly variety trend of K⁺ and Na⁺ proportion in cations

Fig.13 Monthly variety trend of SO₄^2-, Cl^- and NO₃^- proportion in anions

5 Conclusion

Long-term auto-monitor for subterranean river, measuring the index which sensitive to environmental change such as pH, EC, water tempreture, HCO₃- and Ca²⁺ in the scene, observation monthly and in flood process, especially in fertilization seasons increasing observation density are approaches of exploring water quality variety discipline of subterranean river in South karst areas. NO₃^-,SO₄^2-,Cl^-,K⁺ are the major objects of observation.

Observation and Sampling for Guancun subterranean river for about a year shows, except for rainfall, agriculture activities are main factors having greatly impact on groundwater quality. K⁺,NO₃^-,SO₄^2-,Cl^- are affected by fertilization distinctively.

NO₃^- increase in recent twenty years distinctly; the variety discipline of SO₄^2- is very complex, it need observation for longer time; Cl^- increase a little; K⁺ and Na⁺ don’t have a obvious change; Mg²⁺ has a possibility of increase; HCO₃^- and Ca²⁺ don’t change.

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