Research on quality changes and influencing factors of groundwater in the Guanzhong Basin

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1 Journal Journal of Groundwater of Science Science and and Engineering Vol.5 Vol.5 No.3 No.3 Sept. Jul Research on quality changes and influencing factors of groundwater in the Guanzhong Basin TAO Hong 1,2*, ZHENG Miao-miao 1,2,3, FAN Li-min 1,2, LI Wen-li 1,2, DING Jia 1,2, LI Hui 1,2, HE Xu-bo 1,2, TAO Fu-ping 1,2 1 Shaanxi Institute of Geo-Environment Monitoring, Xi an , China. 2 Key Laboratory of Mine Geological Hazards Mechanism and Control, Xi an , China. 3 School of Geology Engineering and Geomatics, Chang an University, Xi an , China. Abstract: This paper studies the distribution features, the chemical elements beyond standards and the influencing factors of shallow groundwater quality in the Guanzhong Basin through the data monitored in the last twenty years. The final purpose is to comprehensively evaluate the divisions of groundwater quality in the Guanzhong Basin. Results show that, the groundwater quality is in good shape, and suitable for drinking. Drinking accounts for 16.02% of the total, most of which are distributed in the lubotan of Weibei. Some come from loess plateau of Qian-Liquan County and some are generated by the industrial pollution of Xingping City. Materials exceeding standards include chloride, sulfate, three nitrogen, fluoride, manganese, iron, hexavalent chromium and so on. The main factors influencing the quality of shallow groundwater include groundwater exploitation, natural background value of special components and precipitation, among which the groundwater exploitation poses the greatest impact. The depth of water is positively correlated with the concentration of sulfate, nitrate and total hardness. Keywords: Groundwater quality; Comprehensive assessment; Influencing factor; Excessive substance; Guanzhong Basin Introduction The Guanzhong Basin boasts abundant groundwater resources. Exploitation of groundwater has been an important pillar to support the industrial and agricultural production in the basin. As a basic element of ecological environment in the Guanzhong Basin, groundwater quality is key to industrial and agricultural production and people s life. A large amount of data shows that groundwater exploitation posed significant effect on groundwater quality (LI Hui, 2015; ZHANG Mao-sheng, 2005; ZHANG Mao-sheng, 2004; TAO Hong, 2013; TAO Hong, 2014). YANG Xiao-ting et al. (2001) made proposals on the basis of aqueous medium intrinsic factors of vulnerability evaluation of groundwater in the * Corresponding author @qq.com Guanzhong Basin in 2001 analyzed how individual elements such as nitrogen, fluorine respectively impact groundwater quality in the basin in SUN Yi-bo et al. (2013) studied the distribution of groundwater chemical in the Guanzhong Basin. TAO Hong et al. (2014) conducted pollution evaluation division of groundwater in the Guanzhong Basin through DRASTIC evaluation model based on GIS. According to systematic analyses of the water quality distribution and monitoring data of recent twenty years in the Guanzhong Basin, factors influencing the quality of shallow groundwater are human activities, original background value and precipitation, among which groundwater exploitation is the major factor of deteriorating water quality. The paper is thus of great importance to the understanding of regional groundwater quality dynamics, the creation of pollution control measures and the scientific use of water.

2 Journal of Groundwater Journal of Science Groundwater and Engineering Science and Vol.5 Engineering No.3 Jul. Vol No.3 Sept Study site 1.1 Topography and geomorphology Located in the northwest of China, the Guanzhong Basin is between the plateau of Northern Shaanxi and the Qinling Mountains. The basin is crescent-shaped, with an area of km 2. It is about 360 km from east to west, and about km from north to south. The elevation is about m. The terrain is higher in the east and lower in the west. River Wei runs through the basin in the middle from west to east, and into the Yellow River at Tongguan. The main landform includes alluvial-proluvial Plain of the Wei River, loess platform, the Qinling Mountains and alluvialproluvial fan of North Mountain. The terrain is relatively flat. 1.2 Meteorology and hydrology The Guanzhong Basin belongs to the semi-arid and semi-humid climate zone, where temperature differs greatly in different seasons, with cold winters and hot summers. The boundary of dry season and wet season is clear; precipitation is low in the late of autumn, winter and spring, but high in summer and early autumn. The annual average evaporation is mm, temperature , with the highest temperature above 40 and the lowest below -10, precipitation mm, most of which falls in July, August and September. Generally speaking, precipitation in the southern basin is higher than that in the north while the west sees more rainfall than the east. The Guanzhong basin is situated in the Yellow River basin. The Yellow River flows through the eastern boundary of the basin from north to south to Tongguan, and converges with the Wei River before running to the east. River Wei is the major river in this area. The riverbed undergoes a sudden widening after running across the Baoji gorge, with twists, turns, and large sediment. Once the Wei River passes the entry, it accepts hundreds of tributaries from its north and south. The south bank of Wei River originates from the Qinling Mountains, parallel and overcast, short runoffs and rapid water flow. The water level and flow vary greatly due to the precipitation. The number of tributaries is less in the north part of Wei River, most of which features long standing, with sudden rises and falls and large sediment. 1.3 Hydrogeological condition The Guanzhong Basin formed in the late Cretaceous and early Tertiary. It is a typical Cenozoic faulted-basin, accumulated more than km of Quaternary loose sediments, which provide sufficient space for groundwater storage and flow. Groundwater in the basin is divided into three types according to different geomorphic units, including unconsolidated formation pore water of alluvial plain, fracture pore water of loess tableland, and alluvial-pluvial fan pore water of piedmont. The exploitation of River Wei and its tributaries mainly focus on the exploitation of unconsolidated formation pore water of alluvial plain. The water containing medium is composed of Quaternary alluvial facies sand and sand gravel. The yield of each well is m 3 /d on average. Water within the depth of m is divided into phreatic water and lower confined water according to cohesive soil. Phreatic water is mainly distributed in the Wei River, the floodplain of its tributaries and level terrace areas, with a buried depth of about 1-20 m, and aquifer thickness of m. Confined water is distributed in the lower parts of the geomorphic units at different levels, with total thickness of m, and becomes increasingly thinner along the Wei River from both northern and southern sides. On the south bank of the Wei River, pore water runs from southwest to northeast in general, while on the north, it runs from northwest to southeast on the whole. Confined aquifer provides most of the centralized water supply. Alluvial-pluvial fan pore water of piedmont of the Beishan Mountain and the Qinling Mountains also contribute to the water supply, though with no stable water-resisting layer. Fracture pore water of loess tableland has not been exploited. 2 Regional hydro-chemical characteristics The chemical and geomorphic characteristics of groundwater in the Guanzhong basin are closely 297

3 Journal of Groundwater Journal Science of Groundwater and Engineering Science Vol.5 and Engineering No.3 Sept Vol.5 No.3 Jul linked. The salinity increases gradually and the hydro-chemical types become more complex from the piedmont to the center of the basin and from the upstream to downstream of Wei River. In piedmont pluvial fan zone and the west region of the Jing River and Bai River, where groundwater hydraulic gradient is larger, there are mostly alluvial facies sand and sand gravel of aquifer, and the groundwater smoothly runs off, mainly in the leaching and more formed bicarbonate water whose salinity is less than 1 g/l. The eastern region of River Jing and the northern region of River Ba see a relatively flat terrain, and poor permeability. Runoff lagging of groundwater goes through vertical evaporation discharge. The strong inspissation and high salinity forms chloride, water of sulfur acid, and more complicated hydrochemical types. The mineralization degree is more than 10 g/l from north of Dali and Gushi to the large area of salt pond depression and lubotan. The luorine ion content of the Loess Plateau area is generally greater than 1 g/l, which is the major reason behind the worsening of water quality in Loess Plateau. 3 Evaluation of water quality This part aims to comprehensively evaluate the division of the phreatic water and confined water in the Guanzhong Basin on the basis of the data generated from monitoring in It adopts a composite index evaluation method by categorizing the results into four levels: Excellent, good, weak, and very poor. 3.1 Evaluation model The evaluation model is used to calculate the comprehensive evaluation value F by using the composite index evaluation method, and to determine groundwater quality at the monitoring point according to the F value. The scores of every unit group according to Quality Standard for Ground Water (GB/T ) are as follows. 2 2 F + Fmax F = (3-1) 2 1 F = n n F i i= 1 (3-2) F - The average value of F i, the score of every monomial group; F max - The maximum value of F i, the score of monomial group; n- number of terms. Table 1 The score of monomial group Classification Ⅰ Ⅱ Ⅲ Ⅳ Ⅴ F i Table 2 The comprehensive evaluation value Category Excellent Good Weak Poor F F< F< F< Division evaluation Generally speaking, the quality of groundwater in Guanzhong basin is in good shape. Taking River Wei as the boundary, water quality of the south bank is better than that of the north bank. According to Quality Standard for Ground Water (GB/T ), the evaluation grade of water quality that is above the weak or poor is suitable for drinking; the evaluation grade that falls on very poor is not suitable for drinking. The area where the phreatic water is suitable for drinking accounts for 83.23% of the total evaluated area. The area where the phreatic water is not suitable for drinking accounts for 16.02% of the total evaluated area, most of which are distributed in the lubotan of Weibei, some coming from loess plateau of Qian-Liquan County and some from Xingping City. Materials exceeding standards include total hardness, sulfate, nitrate, nitrite, ammonia nitrogen, COD, fluorine ion and so on. (1) Excellent area (F<0.8): Distributed sporadically in recharge area of diluvial fan Qinling piedmont, big slope of terrain, with good condition of runoffs. Human activity in this region is relatively rare. It covers an area of km 2, accounting for 0.36% of evaluated areas. (2) Good area (0.8 F<2.50): Mainly distributed in the south of River Wei. The total size of this area is km 2, accounting for 21.28% of evaluated areas. (3) Weak area (2.50 F<7.20): Widely distributed in the whole area, mainly along the major

4 Journal of Groundwater Journal of Science Groundwater and Engineering Science and Vol.5 Engineering No.3 Jul. Vol No.3 Sept cities and the surrounding areas of Wei River. It covers an area of km2, accounting for 62.33% of evaluated areas. (4) Very poor area (F 7.20): Mainly distributed in the lubotan of Weibei, some from loess plateau of Qian-Liquan County and some from Xingping City. The topography of lubotan of Weibei is relatively flat and the runoffs are slow. The ground water is shallow, and tends to evaporate while holding salt, leading to hardness, higher mineralization, and poor water quality. The loess plateau of Qian - Liquan County is one of the reasons that cause native fluoride pollution to exceed bid. Meanwhile, since the groundwater has run through the loess plateau for a long time, the total hardness, TDS and nitrate in it are higher than standards. On the other hand, Xinping has developed chemical industry. The total hardness, TDS, nitrate and ammonia nitrogen are higher than standards as well. The size of this area totals km2, accounting for 16.02% of the evaluated areas. Fig. 1 Division map of comprehensive evaluation of the phreatic water quality in the Guanzhong Basin in Analysis on dynamic characteristics and influencing factors of groundwater quality 4.1 Determination of excess value According to the Quality Standard for Ground Water (GB/T ), class III water is appropriate to serve as supplies of centralized drinking water, industry and agriculture in line with the reference value of human health. Therefore, this part is to determine whether the groundwater chemical component stays in the range of class III water and utilizes it as the standard of pollution. Through the analysis of monitoring data in the past years, materials exceeding standards in the groundwater are mainly total hardness, sulfate, nitrate, nitrite, fluoride, ammonia nitrogen, manganese ions, iron ions (Table 3). Table 3 Groundwater chemical composition exceeding the range (Unit: mg/l) Total hardness sulfate Fe Mn Nitrate Nitrite Ammonia (In terms N) (In terms N) nitrogen Fluoride

5 Journal Journal of Groundwater of Science Science and and Engineering Vol.5 Vol.5 No.3 No.3 Sept. Jul Dynamic characteristics of major excessive materials The Guanzhong Basin is the economic center of Shaanxi Province, and even the northwest China. When its population increases with time, the discharges of industrial and domestic sewage are also on the increase. The exploitation of groundwater differs greatly in different periods. The spatial distribution of shallow groundwater quality is affected by human pollution and the amount of special components in nature. The main substances in groundwater also differ in different regions. In loess plateau, the value of fluorine ion is fairly high (over 1 mg/l in general), making it the major excessive material in this area. While in Weibei region, where the land is flat and the groundwater runoff is slow, the major reason of poor water quality is the high value of mineralization. In the place where many old chemical enterprises gathered, it is the human activities that lead to TDS, and the serious high value of three nitrogen. Hence according to the comprehensive evaluation, water quality in this area is very poor. Fig. 2 Dynamic curve of sulfate, nitrate graph, total hardness with typical monitoring wells of phreatic water for years The data monitored in the past thirty years show that groundwater in the basin is heavily polluted from the 1970s and 1980s to the mid-1990s, when the items exceeding the standards include total hardness, sulfate, nitrate, nitrite, ammonia, COD. The amount of nitrite in some areas once hit 1.85 mg/l (ZHAI Zhai 2005), 7.08 times higher than the standard. After the mid-1990s, the amount of excessive substances is on the down track year. The average value of total hardness measured is less than 450 mg/l in the region in 2014, 3-4 times lower than that of twenty years ago. The amounts of sulfate, nitrate and chloride also decrease significantly, and have basically returned to normal range (Fig. 2). The amount of ammonia nitrogen and COD varies in different regions, which reflects the influence of human activities on shallow groundwater. On the other hand, the amount of fluoride sees a trend of increase in some areas year on year base. It increases by 8-9 times in the past 10 years (Fig. 3). The average value is higher than the standard, up to mg /L. Fig. 3 Dynamic curve of ammonia, fluoride of typical monitoring wells of phreatic water for years 4.3 Analysis of influencing factors Results show that groundwater exploitation, natural background value and precipitation are the main factors affecting shallow groundwater quality in the Guanzhong Basin. The impact of exploitation on water quality is particularly significant. The heaviest water exploitation in the basin happened from the 1970s and 1980s to the mid 1990s, reaching m 3 /a in the main City of Xi an, Xianyang Baoji and Weinan in It is one of the periods when groundwater exploitation is heaviest in the Guanzhong Basin. Meanwhile, it is also a period of serious pollution, the amount of sulfate, nitrate, chloride and other items all reach their highest values, 4-5 times higher than standards in general. After the mid-1990s, groundwater exploitation in the basin is restricted, and the amount of groundwater exploitation increases to m 3 /a in Xi an, Xianyang, Baoji in 2011, which equals to 31.8% of that in The water level rises sharply (Fig. 4), growing by nearly 100 m, the largest increase in the past twenty years. At the

6 Journal of of Groundwater Science and and Engineering Vol.5 No.3 Sept. Jul same time, the centralized water supply source also increases greatly. The amount of major excessive materials are also on the decrease year by year. Through comprehensive analyses of exploitation, groundwater level and water quality monitoring data in , it can be concluded that the concentration of sulfate, nitrate and total hardness is positively correlated with the depth of water. The correlation coefficient between the depth of water level and total hardness is pollution, native background value, precipitation. Groundwater exploitation is the major factor influencing the groundwater quality in the Guanzhong Basin, the amount of excessive materials increases with the increase of the exploitation. The depth of water is positively correlated with the concentration of major excessive items such as sulfate, nitrate and total hardness. And the correlation coefficient is Acknowledgements The research was sponsored by the construction of urban geological environment monitoring network in Guanzhong urban agglomeration, China geological survey, Project No. Water [2012] and the research of groundwater dynamic monitoring in Shaanxi province, Shaanxi public welfare geological project, Project No Fig. 4 Curve of precipitation and shallow groundwater depth in Guanzhong Basin in different periods 5 Conclusions (1) The groundwater quality in the Guanzhong Basin is good generally. Water in most of the area is suitable for drinking except some parts where pollution exists. Groundwater unsuitable for drinking is mainly distributed in the Weibei lubotan of east Weibei, partly coming from the arid region in the loess plateau of Qian-Liquan County and industrial pollution area of Xingping City. The area is about km 2, accounting for 16.02% of the whole evaluated area. (2) The major excessive substances in the shallow groundwater in the Guanzhong Basin are total hardness, sulfate, nitrate, nitrite, ammonia. Data in recent twenty years show that groundwater exploitation in the Guanzhong Basin is now under control and thus the amount of major excessive items such as sulfate, nitrate, chloride, total hardness, has been significantly decreased in recent years, with the total hardness close to normal standard. (3) The main factors affecting the quality of groundwater in the Guanzhong Basin is human References DUAN Lei, WANG Wen-ke, et al. 2011a. Temporal and spatial variation of Three Nitrogen in shallow groundwater in the Guanzhong Basin and its control measures. Journal of Arid Land Resources of Enviroment, 25(8): DUAN Lei, WANG Wen-ke, et al. 2011b. Health risk assessment of Three Nitrogen in shallow groundwater in the Guanzhong Basin. Hydrogology & Engineering Geology, 38(3): LI Hui, LI Wen-li, DING Jia Groundwater dynamic monitoring report in Shaanxi Province. SUN Yi-bo, WANG Wen-ke, et al Evolution mechanism of shallow high fluorine groundwater in the Guanzhong Basin. Hydrogeology & Engineering Geology, 40(6): SUN Yi-bo, WANG Wen-ke, et al Geochemistry evolution mechanism of shallow groundwater in the Guanzhong Basin, China. Hydrogeology & Engineering Geology, 41(3): TAO Hong, TAO Fu-ping, LIU Wen-bo Characteristics and influencing factors of groundwater dynamic in Guanzhong urban agglomeration during the last 50 years. Hydrogeology & Engineering Geology, 40(6):

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