HYDROLOGICAL PROCESSESHydrol. Process. 18, 2303–2312 (2004)Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.5531

Assessment of groundwater contamination by nitratesassociated with wastewater irrigation: A case study in

Shijiazhuang region, China

C. Tang,1* J. Chen,1 S. Shindo,2 Y. Sakura,2 W. Zhang3 and Y. Shen1

1 Graduate School of Science and Technology, Chiba University, Chiba 263-8522, Japan2 Faculty of Science, Chiba University, Chiba 263-8522, Japan

3 Shijiazhuang Institute of Agriculture Modernization, Chinese Academy of Sciences, Shijiazhuang, People’s Republic of China

Abstract:

In arid and semi-arid regions there is usually a shortage of irrigation water; thus, wastewater water, as well as other low-quality water resources, may become an important source of water and nutrients. However, (pre)treated wastewatermay contain elements and compounds that can damage the environment. It also has the potential to affect waterquality adversely in an aquifer that may be the source of drinking water in the area. In order to assess the impacts ofwastewater on the environment, groundwater samples were taken and analysed in typical croplands in the North ChinaPlain, where urban wastewater or groundwater have been used for irrigation for several decades. Concentrations ofnitrate �NO3

�� in groundwater in the study area varied from 50 to 130 mg l�1 in the croplands irrigated by wastewater,but in the croplands irrigated by pumping wells, away from the Dongming Canal, NO3

� concentrations are less than35 mg l�1. It was found that values of υ15N ranged from C5 to C13‰, and dominantly from C7 to C11‰, and theNO3

� concentration in most wells with depths of less than 40 m was higher than the drinking water standard setby the WHO. Cluster analysis was used to classify the spatial distribution of nitrates resulting from the wastewater.Identification of chemical patterns is found to be effective for the comprehensive assessment of the spatial distributionof groundwater quality. It is also emphasized that the wastewater in this area controls the NO3

� distribution in thegroundwater, and should be used carefully to protect both soil and groundwater from NO3

� pollution. Copyright 2004 John Wiley & Sons, Ltd.

KEY WORDS groundwater quality; wastewater irrigation; nitrate pollution; North China Plain

INTRODUCTION

There is always the difficult problem of water shortage in arid and semi-arid regions, where water scarcity onthe national level is seemingly contradicted by the local occurrence of sometimes large amounts of domesticwastewater. Consequently, in many countries this wastewater is considered as an additional source of water,particularly for irrigation purposes.

Irrigation with wastewater effluent has been practised successfully over a long period. The earliestdocumented wastewater farms were those of Bunzlom, Germany, and Edinburgh, Scotland, which were inoperation in 1531 and around 1650 respectively. Many other farms, such as in London, Paris (1868), Berlin(1876) and Melbourne (1892), were established later.

In the 1800s, the gradually increasing pollution of streams, first in Britain and then in other manufacturingcountries, led a demand for wastewater purification, and the probable value of wastewater in agriculture. Thegrowth in Britain of great manufacturing industries and the consequent pollution of the streams of that countryat the beginning of the 1800s, forced city authorities to consider the necessity of wastewater purification at

* Correspondence to: C. Tang, Graduate School of Science and Technology, Chiba University, Chiba 263-8522, Japan.E-mail: cytang@earth.s.chiba-u.ac.jp

Received 20 December 2002Copyright 2004 John Wiley & Sons, Ltd. Accepted 30 July 2003

2304 C. TANG ET AL.

an earlier date than elsewhere. Wastewater irrigation as a branch of applied science may, therefore, be saidto have originated in Britain (Rafter, 1897, 1899).

Land application of wastewater has been used in the USA since the late 1800s to irrigate crops anddispose of wastewater. In the 1970s, Federal regulations were enacted that required that land applicationbe considered as a disposal method before Federal grants were approved for wastewater treatment plantconstruction. Land application of secondary treated wastewater is considered a viable management practice,because land application provides further filtering and biological treatment of the treated wastewater throughthe soils, recharges local water resources, and, if crops are grown, provides revenue.

At first, many studies were on the methods of applying wastewater, such as the best crops for wastewaterfarming, the proper quantity of wastewater to be applied, limiting temperatures, cost of labour on wastewaterfarms, and sanitary conditions on such farms. The first attempt at wastewater irrigation in the USA was madein Maine in 1872. About 7000 gallons disposed from the State Insane Asylum at Angusta, Maine, every daywas used for irrigation after the solids were settled in large tanks.

It is true that irrigation was primarily a convenient and relatively inexpensive method of disposing ofwastewater, but times and standards have changed since the 1930s. The US Government considers that waternot safe enough to discharge into stream channels is not safe for general irrigation use, where it may beapplied promiscuously to crops and may become a health hazard. Therefore, wastewater for general irrigationuse requires pretreatment, such as commonly given before returning it to streams, in which case use is beingmade of wastewater that a city, to all intents and purposes, has already disposed of.

In China, wastewater from industries and cities was used for irrigation as early as the 1940s. Wastewater-irrigated land increased sharply from 11 700 ha in 1957 to 1 390 000 ha in 1982. The largest area irrigated bywastewater in China can be found in Tianjin, with its farm area of 149 000 ha. In 1988, wastewater disposalin China was as large as 368 ð 108 m3.

Nitrate contamination of groundwater is typically a result of nitrogen movement into the unsaturated(vadose) zone, the occurrence of transformation processes in either the unsaturated or saturated zones, andnitrate movement with groundwater. From the viewpoint of the water cycle, groundwater can be consideredas a renewable source, but from the viewpoint of groundwater resource or ecology the groundwater may notbe seen as a renewable source if it is polluted. In particular, large amounts of nitrogenous-fertilizers and poorutilization efficiency may lead to nitrate leaching, and hence pollution of groundwater.

Untreated wastewater from Mexico City has been used for decades to irrigate the Mezquital Valley, Hidalgo,Mexico. Data suggest that the irrigation region is acting as a huge open-system slow filter, the main reservoiras a large waste-stabilization lagoon, and the canals as extremely long, narrow stabilization channels. Elevatednitrate concentrations were found in all of the Mezquital Valley groundwaters. As all groundwater nitrate levelsexceeded drinking water standards and were greater than those in surface water, wastewater irrigation appearsto have a negative impact on groundwater quality (Gallegos et al., 1999; Downs et al., 2000)

Also, in the North China Plain (NCP), where average annual precipitation is less than 600 mm, 80% ofirrigation water is urban wastewater in the farmland near big cities. However, much of the wastewater thathas been used for irrigation has caused groundwater pollution problems (Ye et al., 1996). Chen and Qu (1996)have discussed the use of wastewater in the NCP. About 80% of the wastewater discharged from cities isindustrial wastewater, 80% of which is effluent to the environment without treatment. As a result, surfacewater and groundwater quality problems have been very serious.

Owing to increasingly stringent regulations on the discharge of wastewaters, as well as the decreasingavailability of freshwater resources, there is a need to consider the expanded use and reuse of marginal-quality waters for irrigation and other purposes. Nitrate pollution of groundwater is an increasing problemworldwide. In a global context, five main causes of groundwater nitrate accumulation are known: (1) naturallyoccurring nitrogen fixation in desert areas, such as the arid regions of Australia (Barnes et al., 1992); (2) thedecomposition of soil organic matter in the absence of a nitrate sink, e.g. in the case of deforestation (Faillatand Rambaud, 1991); (3) the use of nitrogen fertilizers in intensive agriculture (Dudley, 1990); (4) theconcentration and decomposition of human and animal waste (Smith et al., 1999); and (5) land treatment

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ASSESSMENT OF GROUNDWATER CONTAMINATION 2305

of urban wastewater, the nature of which, in the context of crop production regions near urban centres in aridand semi-arid countries, is the subject of this paper. This paper addresses broad issues related to the potentialreuse of municipal wastewaters on cropland systems. The long-term efficacy of irrigation with marginal-quality waters is dependent on several factors, such as unique limiting water quality characteristics andsite-specific crop, soil and climatic conditions. Opportunities exist to use marginal-quality waters beneficially.However, their use requires more intensive management and monitoring than use of higher quality waters.Site-specific management options are used to overcome many of the potentially adverse impacts partially, butsome impacts are unavoidable. In many instances, the upper bounds of wastewater usage are not fully known,and the potential long-term cumulative impacts on the environment require further research. In this study,attention has also been put upon the nitrate distribution in farmland groundwater where wastewater has beenand is used.

GEOGRAPHICAL SETTING

The NCP has a very high population density of about 70 000 000 in an area of about 260 000 km2, and it isa very important region for agriculture in China. Over its long history, agricultural production in China hasalways been recognized as a closed system for material and energy cycles. At the same time, groundwaterin the NCP is widely used as a source of primary or supplementary irrigation. Radical changes in farmingpractice, aimed at increasing productivity and reducing reliance on imported food, have led to the widespreadintroduction of agricultural monocultures, sustained by major increases in the use of agrochemicals andirrigation. As a result, nitrogenous-fertilizers application is usually over 500 kg ha�1 year�1 (as nitrogen)with an accompanying decrease in utilization efficiency in the NCP (Zhang et al., 1996).

Located in the piedmont plain of Taihang Mountain, the study area (Figure 1) is south of Shijiazhuang, oneof the big cities in the NPC. Geologically, thick Quaternary loose accumulation deposits on the top of thestudy region, which abounds with good-quality groundwater, make up important water resources for domestic,industrial and agricultural uses. The major aquifer in the study area is a layer of gravel with coarse sand. Theannual rainfall ranges from 400 to 600 mm in the study area, with 70 to 80% of precipitation available fromJuly to September. There are only 40 to 60 mm of rainfall, or even no rainfall at all, in more than 100 days inspring. The variation of seasonal precipitation is so large that it is common to have a dry spring and floodingsummer.

There are two rivers in the Shijiazhuang region, the Hutuo River and the Jiao River. Both rivers supplyirrigation water for croplands. Dongming Canal, which runs thought the study area, was built to transportwastewater from the city to the Jiao River. Untreated wastewater from the Shijiazhuang City basin has beenused for decades to irrigate croplands in the study area, mainly for wheat and maize. Excess irrigation waterrecharges the aquifer that is used as a domestic water supply source. Geologically, there are sandy clay, mid-fine sand, gravel with mid-coarse sand, clayed sand and gravel with mid-coarse sand depositing in sequencesfrom the Earth’s surface. In Figure 1, zone A contains croplands irrigated by wastewater and zones B and Ccontain farmland irrigated by pumped groundwater.

Agricultural reuse of wastewater has to be integrated into comprehensive land and water management plans,taking into account the water supply, wastewater collection, reclamation, and reuse. It may be incorporated,as a land treatment system, to the treatment cycle and considered as the nutrient recycling part of the loop.However, the water used for irrigation purposes has to meet public health and agronomy quality requirements.Since the 1960s, wastewater has been used for irrigation in the regions around Shijiazhung, like many otherbig cities in the NCP (Bi et al., 2000). The amounts of wastewater used for irrigation depended fundamentallyon rainfall during the crop growing seasons. The average amounts of wastewater used for irrigating wheatand maize are estimated as 265 mm year�1 and 155 mm year�1 respectively.

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2306 C. TANG ET AL.

Figure 1. Sampling points in study area

METHODS

Chemical analysis of groundwater in the study area

For simplicity, we set the origin at the bottom left-hand corner in the study area, which is as large as 96 km2

(Figure 1). In the study area, groundwater samples were taken from 27 wells with well depths ranged from40 to 60 m, which are believed to be the same aquifer. Figure 1 also shows the sampling points in the studyarea. In addition to variation in groundwater use for economic development and urbanization, groundwaterquality has also become a serious problem in the NCP. From 2000 to 2001, we carried out field surveys fourtimes and took groundwater samples for the study. During the field surveys, the pH, electrical conductivity(EC) and temperature of the groundwater were measured in situ. The water samples were analysed in thelaboratory at Chiba University. Anions such as NO3

�, Cl� and SO42� were measured by ion chromatography

and cations such as NaC, KC, Ca2C and Mg2C were measured by inductively coupled plasma spectrometer.HCO�

3 was measured by titration. 15N was analysed by using a Delta S mass spectrometer. Information suchas groundwater usage, well depth, etc. were obtained by interviewing the well managers or farmers.

Cluster analysis for chemical patterns

In order to classify the spatial distribution of nitrates resulting from wastewater, the identification of featuresin chemical patterns is an effective means of obtaining a comprehensive assessment of the spatial distributionof groundwater quality. Placing the chemical data into sets of multivariable classes is carried out using thetechniques of mathematical pattern recognition or pattern classification. The patterns are the sampling pointsthemselves, or strictly the mathematical point vectors that contain the sets of the chemical component valuesfor the points arranged in the form

P D [p1, p2, . . . , pn

]where p1 to pn are the various chemical components’ concentrations at point P. The use of the chemicaldata can only specify the information classes. Occasionally, it might be possible to guess the number of

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ASSESSMENT OF GROUNDWATER CONTAMINATION 2307

chemical classes in a particular information class; but, in general, the one will have little idea of the numberof distinct unimodal groups that the data fall into in the multivariable space. Clustering procedures can beused for this purpose.

In this paper, points belonging to a particular cluster are, therefore, chemically similar. In order to quantifythis relationship, it is necessary to devise a similarity measure. The method most frequently used is theEuclidean distance. Suppose that P1 and P2 are two points whose similarity is to be checked, then theEuclidean distance between them is

d�P1, P2� D[

N∑iD1

�P1i � P2i�

]1/2

where N is the number of chemical components. By using a distance measure it should be possible todetermine clusters in the data. Often, however, there could be several acceptable cluster assignments in thedata, so that once the candidate clustering has been found it is desirable to have a means by which the ‘quality’of clustering can be measured. The availability of such a measure should allow one cluster assignment of thedata to be chosen over all others.

A common clustering criterion or quality indicator is the sum of squared error (SSE) measure, defined as

SSE D∑Ci

∑P2Ci

�P � mi�T�P � mi�

where mi is the ith cluster and P 2 Ci is a pattern assigned to that cluster. The outer sum is over all theclusters. This measure computes the cumulative distance of each pattern from its cluster centre for each clusterindividually, and then sums these measures over all the clusters. If it is small, then the distances from thepatterns to the cluster means are all small and the clustering would by regarded favourably.

RESULTS AND DISCUSSION

Groundwater flow system

Figure 2 shows the water-table distribution in the study area measured on 2 July 2001. It was found thatthe water table was highest along the wastewater canal that became the local divide line for groundwater.Since the wastewater in the canal flows from north to south, the water table in the northern part of the studyregion is higher than that in the southern part. Wastewater recharges to groundwater by direct seepage at thecanal bottom and by irrigation activities. In the study region, wastewater percolated into groundwater, andthen moved along the groundwater flow direction. The water table changed sharply near the wastewater canaland became gently in the area away from the canal. As a result, groundwater moved to the southeast. Thegroundwater flow directions were also supported by palaeogeomorphology data.

Characteristics of EC, nitrate and υ15N in groundwater of study area

Figure 3 shows the EC distribution in groundwater. It was found that the EC value of 130 mS m�1 matchedwell with the boundary of wastewater irrigation. The EC decreased with distance from the wastewater canal;this trend is also supported by the groundwater flow direction described above.

Figure 4 shows the distribution of nitrate concentration in groundwater. Clearly, nitrate concentrations ingroundwater are highest in the area near the canal and decrease sharply away from it. In the croplands irrigatedby wastewater, nitrate concentration ranged from 50 to 130 mg l�1, which is higher than the drinking waterstandard set by the WHO. However, in the croplands irrigated by pumping wells away from the canal, nitrateconcentrations in the area are less than 35 mg l�1.

The nitrogen isotopic component in groundwater nitrate was used to identify or implicate the source. Ingeneral, the υ15N values from soil organic matter sources varied from about 0 to C4‰, and the mean was

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2308 C. TANG ET AL.

Figure 2. Distribution of water table (m) in July 2001

Figure 3. Distribution of EC �mS m�1�

not significantly different from that of fertilizer sources. The υ15N values of animal sources varied from aboutC8 to C20‰ and were dependent on site and animal source. In the study area, it was found that value ofυ15N ranged from C5 to C13‰, and dominantly from C7 to C11‰. High υ15N values also suggest that thesources of nitrate in groundwater can be considered as wastewater, since no fertilizers have been used foragriculture in the wastewater irrigated area after the wastewater canal was built. At the same time, the υ15Nof on-site wastewater was significantly different from that of animal sources.

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ASSESSMENT OF GROUNDWATER CONTAMINATION 2309

Figure 4. Distribution of nitrate concentration �mg l�1�

Identification of nitrate sources of wastewater irrigation in the study area

Table I show the relative coefficients among positions from wastewater canal as well as chemicals ingroundwater. In the study region, wastewater canal runs from north to south, all items in Table I show verylow relativity in NS direction and have fairly high ones in EW. It hints that wastewater percolated intogroundwater, then moved east along the groundwater flow direction. Nitrate has its highest relation coefficientof 0Ð738 with the chloride ion. As the chloride concentration in wastewater is very high (about 561 mg l�1),the sources of nitrate in groundwater can be considered as wastewater, since no fertilizers have been used foragriculture in the study region since the wastewater canal was built. In general, the nitrate concentration washigher in shallower wells, and the significant correlation between nitrate and chloride concentrations clearlyindicated the same source, i.e. wastewater, for these two anions, since both of them are not easily absorbedby the soil and/or assimilated directly by crops, but subject to leaching from the unsaturated to the saturatedzone.

Table I. Relative coefficients of chemical components in groundwatera

X Y HCO3� Cl� NO3

� SO42� pH EC Eh

Y �0Ð102HCO3

� �0Ð724 �0Ð304Cl� �0Ð780 �0Ð009 0Ð706NO3

� �0Ð540 �0Ð002 0Ð449 0Ð738SO4

2� �0Ð715 �0Ð169 0Ð849 0Ð843 0Ð713pH 0Ð636 �0Ð088 �0Ð613 �0Ð753 �0Ð514 �0Ð667EC �0Ð569 0Ð033 0Ð601 0Ð626 0Ð529 0Ð632 �0Ð452Eh �0Ð601 �0Ð015 0Ð765 0Ð459 0Ð346 0Ð682 �0Ð557 0Ð341DO 0Ð419 0Ð291 �0Ð622 �0Ð447 �0Ð001 �0Ð487 0Ð685 �0Ð121 �0Ð575

a X, Y are distances from the origin in the east and north directions respectively (refer to Figure 2). DO: dissolved oxygen.

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2310 C. TANG ET AL.

Since the wastewater irrigation activity has been carried out for 40 years, we have to identify whether andhow extending wastewater use has made some pollution problems in the study area. According to chemicalanalysis data for the study area, it is found that the nitrate concentration can be detected not only in thecroplands irrigated by wastewater but also in farmland croplands irrigated by groundwater with pumpingwells.

In order to classify the spatial distribution of nitrates resulting from the wastewater, we use cluster analysisto identify features in the chemical patterns of the spatial distribution of groundwater quality. Figure 5 showsthe classification map of groundwater quality obtained by the clustering method based on concentrations ofnitrate, chloride, sulphate and distance from the canal. Evidently, there are three groups of water quality in thestudy area. Group X, located near the canal, shows strong relations with wastewater, and Group Z belongs toan area affected only by irrigation from local groundwater. Group Y can be considered as due to the effectsof irrigation by wastewater.

Though, As mentioned above, the source of nitrate comes from the wastewater, direct irrigation withwastewater and seepage from the canal both affect the distribution of nitrate, with high nitrate levels shownnear the canal (Figure 4). The results of groundwater measurement in the immediate vicinity of the wastewatercanal strongly indicate that the major source of nitrate is the canal that runs through the study area from northto south. High nitrate concentrations in the groundwater could be the result of an input by a source at the soilsurface, such as wastewater seepage from the canal and irrigation activities. In the first of these two cases,wastewater discharge occurs all year around from the city. In the latter case, infiltration occurs just in theperiod of rainfall or irrigation. The results given in Figure 4 strongly suggest that the nitrate is generatedclose to the water table, indicating that the canal is the only nitrate source, since no fertilizers have been usedin the croplands irrigated by wastewater. Any water well located immediately down gradient from the canalis, therefore, likely to be highly contaminated with nitrate in the wastewater-irrigated croplands.

Trend in nitrate distribution in groundwater in the study area

Wastewater has been used for irrigation in the study area since the 1960s, and the depth that the nitratehas shifted to is closely related to the component of water balance and cycling, such as rainfall, percolation,

Figure 5. Map of classification of groundwater quality by the clustering method

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ASSESSMENT OF GROUNDWATER CONTAMINATION 2311

etc. It is appropriate on an average basis to consider that the depth is affected only by annual rainfall, andthe annual nitrate leachate rate can then be assessed by dividing the depth by the years elapsed. Because ofthe possible effects of other factors in the leaching process, the filtered (by polynomial) rainfall and averagenitrate-depth series were use to trace the depth by assuming the nitrate depth as a temporal infiltration series.As a result, nitrate is leached downward at a rate of around 3–4 m year�1.

The aquifer is an important source of groundwater in the study region. Figure 6 shows the relation betweennitrate concentration and well depth in the study region. In a survey of 27 wells located within or near theaquifer’s recharge zone, nitrate concentrations ranged from less than 24 mg l�1 to 125 mg l�1 . In general,nitrate concentrations were higher in shallower wells, and only one observation exceeded 125 mg l�1. It wasfound that almost all wells with depths less than 40 m have nitrate concentrations exceeding 50 mg l�1, thedrinking water standard of the WHO. Even on well with a depth of about 100 m has a nitrate concentrationof 37 mg l�1. Since the drinking water is from a well over 60 m in depth, the nitrate trends show evidence ofa nitrate pollution threat in the near future. Potential sources of nitrate in the study area include animal wastefrom feedlots, and urban wastewater used on crops. The results of this study suggest that nitrate concentrationsare locally elevated above background levels, groundwater is more susceptible to nitrate contamination, andwastewater is a probable source of groundwater pollution. The use of deep wells and careful land-use planningmay alleviate future contamination of groundwater supplies.

Therefore, there is an urgent need for rapid surveys of groundwater utilization, aquifer pollution vulnerabilityand subsurface contaminant load to be undertaken. The groundwater pollution risk and susceptibility tooverexploitation effects can then be assessed and protection measures prioritized and initiated.

CONCLUSIONS

Groundwater is one of the most valuable natural resources that the NCP possesses. As one of the mostimportant agricultural production regions, the NCP has problems with groundwater in terms of both itsquality and quantity due to over exploitation. Consequently, information about the evolution of groundwaterquality in this region is very important for economic development.

The nitrate distribution is controlled by the local groundwater flow system, which was affected greatlyby the wastewater canal. As a result, concentrations of nitrate in the study area were high in most of the

Figure 6. Relation between nitrate concentration and well depth in study region

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2312 C. TANG ET AL.

wells in the shallow aquifer with a depth of less than 40 m, and υ15N values ranged from C5 to C13‰, anddominantly from C7 to C11‰.

The results given by cluster analysis strongly suggest that the nitrate is generated close to the water table,indicating that the canal is the only nitrate source, since no fertilizers have been used in the croplands irrigatedby wastewater. Any water well located immediately down gradient from the canal is, therefore, likely to behighly contaminated with nitrate in the wastewater-irrigated croplands.

Nitrates in groundwater represent a widely distributed pollution concern; nitrates are perhaps the mostubiquitous of all groundwater contaminants in the study area. Man-induced sources of nitrates in groundwaterare a result of water usage for irrigation, excessive applications of waste disposal practices associated withland application of wastewater effluents in the study area. In order to mitigate this, treatment of wastewaterprior to irrigation and careful irrigation management are recommended.

ACKNOWLEDGEMENTS

We are grateful to Professor Kondo, Center for Environmental Remote Sensing, Chiba University, for valuablesuggestions. We also thank Mr Cao Jianshen, Mr Li Fadong and Mrs Ma Zhongqiu of Shijiazhuang Instituteof Agriculture Modernization, CAS, for the water sampling.

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