Peng ZHOU，Meiling REN，Wenwu ZHOU，Danzeng,3*，Qiongdazhuoma，Yang LI

1. College of Science，Tibet University，Lhasa 850000，China; 2. Lhasa Ali Senior High School，Lhasa 850000，China; 3. College of Environmental Science and Engineering，Tianjin University，Tianjin 300072，China

Abstract [Objectives] The research aimed to grasp environmental quality of groundwater in a landfill site，predict health risk of pollution factors to regional residents，provide scientific basis for landfill management，pollution prevention and control，and ensure safety of residents’ drinking water supply. [Methods] Through on-site sampling of three groundwater monitoring wells at Shigatse landfill，and testing and analysis of conventional indexes and heavy metal elements，standard index method and comprehensive index method were used to evaluate environmental quality of groundwater in the landfill area. On this basis，a health risk evaluation model was applied to assess the health risk of residents in Shigatse landfill area. [Results] Comprehensive water quality was good in individual well of Shigatse landfill. Because NH3-N and Fe in the monitoring well exceeded class III standard of groundwater environmental quality in 2014，pollution of monitoring well was relatively heavy. The total carcinogenic risk of landfill groundwater for male and female was 1.855×10-6 and 2.238×10-6，respectively，exceeding the maximum acceptable level of carcinogenic risk for contaminants (1×10-6). Cr6+ should be paid attention to. Moreover，risk value of health hazard for females was generally higher than that of males. The total non-carcinogenic risk values for males and females were 0.057 7 and 0.069 6，respectively，far lower than the non-carcinogenic risk threshold of "1". [Conclusions] The groundwater environmental quality of Shigatse landfill site was good，and it posed a little health risk to regional residents.

Key words Landfill site，Groundwater，Health risk assessment，Shigatse

1 Introduction

Sanitary landfill treatment of domestic waste is widely used at home and abroad because of its low construction cost，low requirements for incoming waste，simple operation and other characteristics[1]. According to the trend of economic and social development in China，sanitary landfill will still be the main method of urban domestic waste treatment in China for a long time in the future[2-5]. Landfill treatment plays an important role in economic construction and waste treatment，but it also brings serious pollution to the surrounding environment，such as heavy metals pollution of soil，water and air pollution，and ecological safety[6-7]. In recent years，reports on groundwater pollution from landfills are common at home and abroad[8-10]. For example，Yao Jiangangetal.[11-13]investigated and evaluated the groundwater quality and pollution risk in the domestic landfill area. It was found that the quality of groundwater in the landfill area has decreased significantly compared with the background well water，and the pollution of permanganate index，NH3-N，NO2-N，F(xiàn)e and so on was the most serious; Cd，Cr and NO3-N had a high risk of health hazards to exposed population. As an important national security barrier and ecological security barrier in China，the construction and protection of ecological environment in Tibet has always attracted much attention[14-15]. According to the statistics of the Department of Housing and Urban-rural Development of Tibet Autonomous Region: 104 sanitary landfills of domestic waste have been built and operated in Tibet，covering each county of seven regions (cities) in the region[16]. The output of urban and rural household garbage in Tibet is increasing day by day，and the composition is becoming increasingly complex. Garbage landfill has become one of main sources of pollution that may pollute the regional environment. In this paper，Shigatse domestic waste landfill of Tibet was taken as the research object. Based on the sample collection，detection and analysis of regional groundwater，the health risk analysis and evaluation of groundwater quality in the landfill area were carried out. The research aimed to provide scientific basis for guaranteeing urban water supply safety，human health，pollution control，and prevention and control of ecological environment health risks.

2 Materials and methods

2.1 Sample collection and detectionBackground well (50 m upstream of groundwater flow in Shigatse landfill site)，diffusion well (50 m downstream of vertical to groundwater flow of landfill) and monitoring well (150 m downstream of groundwater flow of landfill) were taken as groundwater quality monitoring points of landfill site. In August 2014，October 2015，September 2016，October 2017，and August 2018，three parallel samples were collected respectively from background well，diffusion well，and monitoring well. According to the requirements of theStandardforGroundwaterQuality(GB/T 14848-2017)，there were 17 conventional detection indexes: pH，total hardness，sulfate，NO3-N，NO2-N，NH3-N，Cr6+，chloride，F(xiàn)-，Cd，Pb，As，Zn，F(xiàn)e，cyanide，Hg，and total dissolved solids. Water sample analysis was implemented according to the standard method stipulated by theStandardExaminationMethodsforDrinkingWater(GB/T 5750-2006)，and the specific testing work was entrusted to the Ecological Environment Testing Center of the Tibet Autonomous Region.

2.2 Water quality evaluation methods

2.2.1Standard index method. Standard index method is a kind of evaluation method recommended by theTechnicalGuidelinesforEnvironmentalImpactAssessment:GroundwaterEnvironment(HJ 610-2016)，to reflect the exceedance and pollution of a single component in groundwater. The calculation formula is as below:

(1)

wherePiis pollution index of a pollutant in groundwater，dimensionless;Siis measured mass concentration of a pollutant in groundwater，mg/L;Ssiis background value or reference value of a pollutant in groundwater，mg/L.

According toPi，the pollution of a single pollutant in water can be divided into five levels:Pi≤0.4 is level 1，no pollution; 0.45.0 is level 5，serious pollution.

2.2.2Comprehensive pollution index method. Comprehensive pollution index method is used for assessment of groundwater pollution status[17]，and calculation formula is as below:

wherePIis comprehensive pollution index of groundwater，dimensionless;Piis pollution index of pollutanti，dimensionless;nis number of pollutant types. The comprehensive pollution degree of water body is divided into 6 levels according toPI:PI≤0.20，clean water (most items are not detected，and some items are detected but within the standard); 0.202.0，serious pollution (a considerable number of detected values exceed the standard by several times or dozens of times).

2.3 Health risk assessmentIn this paper，health risk assessment model stipulated by "Guidelinesforhealthriskassessmentofgroundwaterpollution(2019)" is used to analyze and evaluate the management risks of carcinogenicity and non carcinogenicity suffered by the health of residents in the study area. Exposure route mainly considers drinking water and skin contact (taking water randomly during bathing，etc.).

(i) The calculation formulas of carcinogenic and non-carcinogenic risks in drinking water are as follows:

(3)

(4)

whereCRcgwis carcinogenic risk of exposure to a single pollutant by oral intake of groundwater，dimensionless;HQcgwis non-carcinogenic hazard quotient of exposure to a single pollutant by oral intake of groundwater，dimensionless;WAFis reference dose distribution coefficient of exposure to groundwater，dimensionless;SFois carcinogenic slope factor by oral intake of groundwater，mg (pollutant)/(kg (body weight)·d);RfDois reference dose by oral intake of groundwater，mg (pollutant)/(kg (body weight)·d);Cgwis pollutant concentration in groundwater，mg/L. The meanings，units and values of other parameters are shown in Table 1 and Table 2.

(ii) The calculation formulas of carcinogenic and non-carcinogenic risks through skin contact are as follows:

(5)

(6)

whereCRdgwis carcinogenic risk of exposure to single polluted groundwater through skin contact，dimensionless;HQdgwis non-carcinogenic hazard quotient of exposure to a single pollutant through skin contact with groundwater，dimensionless;DAeais absorbed dose of adult skin contact，mg/cm2;RfDois reference dose by oral intake，mg (pollutant)/(kg (body weight)·d);RfDdis reference dose by skin contact，mg (pollutant)/(kg (body weight)·d);ABSgiis absorption efficiency factor of digestive tract，dimensionless. The meanings，units and values of other parameters are shown in Table 1 and Table 2.

(iii) Formulas for calculating carcinogenic risk and non-carcinogenic hazard quotient of a single pollutant in groundwater through all exposure routes are as below:

CRca=CRcgw+CRdgw

(7)

HQnc=HQcgw+HQdgw

(8)

whereCRcais carcinogenic risk of exposure to a single pollutant through all exposure routes，dimensionless;HQncis non-carcinogenic hazard quotient of a single pollutant in groundwater through all exposure routes，dimensionless.

Table 1 Average daily exposure dose of contaminants via the ingestion route[18-20]

Table 2 Meanings of human exposure parameters and their recommended values[21-23]

3 Results and analysis

3.1 Groundwater quality evaluation of landfill siteSeen from Table 3，the groundwater quality of Shigatse landfill site was generally good. In 17 groundwater monitoring indexes，the concentrations of Cd，Pb，As，Zn，cyanide and Hg were below the detection limit，and they were respectively 0.001，0.001，0.001，0.01，0.004 and 0.000 1 L. The groundwater quality was weakly alkaline. Each detection factor of background well all met class III standard of theStandardforGroundwaterQuality(GB/T 14848-2017) and standard for drinking water quality of residents. Fe content in diffusion well exceeded class III standard of theStandardforGroundwaterQuality(GB/T 14848-2017) by more than 1.0 times，and it was moderate pollution. Other evaluation factors all met the drinking water quality standards of residents. Water quality of monitoring well was relatively worse. Among them，when NH3-N and Fe contents exceed class III standard of theStandardforGroundwaterQuality(GB/T 14848-2017) by about 2.0 times，which was heavy pollution. Other evaluation factors all met the drinking water quality standards of residents. The main pollutants in the groundwater environment of Shigatse landfill were NH3-N and Fe，and the main carcinogenic pollutant was Cr6+.

Table 3 Evaluation results of standard index method

In order to analyze the current situation of groundwater pollution in Shigatse landfill，a comprehensive pollution index method was used for analysis and evaluation，and the results were shown in Table 4. The results showed that comprehensive pollution index evaluation results (Pvalue) of background well and diffusion well in the landfill site were less than 0.4，which was still clean; evaluation result of monitoring well (Pvalue) was less than 0.7，which was mild pollution. Pollution degree sequence of each monitoring well was monitoring well>diffusion well>background well.

Table 4 Evaluation results of groundwater environmental quality

3.2 Health risk assessment resultsThe non-carcinogenic exposure and non-carcinogenic risk of a single pollutant for adults under oral intake and skin contact were calculated by the calculation model of health risk value. The evaluation results were shown in Table 5 and Table 6.

Table 5 Non-carcinogenic hazard index evaluation results of groundwater evaluation factors

Table 5 showed that the non-carcinogenic risk of a single pollution factor by oral intake was generally higher than that caused by skin contact. Under the same pollution route，the non-carcinogenic risk of females was generally higher than that of males. Non-carcinogenic risk of males caused by oral intake of a single pollution factor was between (1.50E-02±3.04E-03) and (3.37E-04±4.65E-04)，while non-carcinogenic risk caused by skin contact route was between (2.10E-08±7.52E-09) and (3.08E-11±4.25E-11). Non-carcinogenic risk of females caused by oral intake of a single pollution factor was between (1.81E-02±3.69E-03) and (4.07E-04±5.62E-04)，while non-carcinogenic risk caused by skin contact route was between (2.25E-08±8.08E-09) and (3.31E-11±4.57E-11). The non-carcinogenic risk of each monitoring factor in a single way was lower than risk characterization threshold "1".

Table 6 Results of carcinogenic risk evaluation of groundwater evaluation factors

Table 6 showed that the carcinogenic risk of a single pollution factor by oral intake was generally higher than that caused by skin contact. Under the same pollution route，the carcinogenic risk for females was generally higher than that for males. Cr6+had the greatest carcinogenic risk to males through oral intake and skin contact: 9.78E-07 and 3.58E-12，and its maximum carcinogenic risk for females was 1.18E-06 and 3.84E-12. The carcinogenic risk caused by oral intake in females was slightly higher than the carcinogenic risk characterization threshold "1.0×10-6". Due to the lower content of Cd and As，the risk of carcinogenesis to human body was small through oral intake and skin contact. Carcinogenic risk characterization values were shown in Table 7.

As shown in Fig.1，the comprehensive non-carcinogenic risk of each pollution factor was higher in females than in male. Non-carcinogenic risk of NO3-N and NO2-N showed an upward trend. Non-carcinogenic risk of NH3-N changed little except higher in 2014. Except higher in 2017，non-carcinogenic risk of Cr6+changed little. Except lower in 2015，non-carcinogenic risk of F-changed little. Non-carcinogenic risk assessment results of Cd and As remain unchanged. Its main cause was that the concentration was lower than detection limit，showing not detected status. Total hazard quotient of Fe showed a declining trend. The non-carcinogenic risk of each pollution factor was shown as F->As>Cr6+>Cd>Fe>NO3-N>NH3-N>NO2-N (Fig.1)，and comprehensive non-carcinogenic risk of each monitoring factor was lower than the risk characterization threshold "1".

Table 7 Carcinogenic risk characterization values[24]

The overall performance of total carcinogenic risk of each pollution factor was that men were greater than women. Except total carcinogenic risk of Cr6+was higher in 2017 and slightly higher than threshold of carcinogenic risk characterization "1.0×10-6"，the change was little in other years. The total hazard quotient evaluation results of Cd and As remained unchanged because their concentrations were lower than the detection limit. The total carcinogenic risk of each pollution factor was Cd>Cr6+>As (Fig.2).

4 Conclusions and discussion

4.1 DiscussionThe concentrations of Cd，Pb，As，Zn，cyanide and Hg in the groundwater monitoring indexes of Shigatse landfill were all below the detection limit. The groundwater pollution in the monitoring well was relatively serious. NH3-N and Fe exceeded class III standard of theStandardforGroundwaterQualityand were the main pollutants. The pollution of background well and diffusion well was relatively mild，which was still clean，and the monitoring well was slightly polluted. Through consulting data and sampling，monitoring and analysis of three monitoring wells in Shigatse waste landfill，it was found that NH3-N and Fe were polluted to varying degrees，and the content of other elements was lower. Since the groundwater in Shigatse landfill area is mainly bedrock weathered fissure water，the excessive iron is greatly affected by the stratum lithology. Additionally，groundwater monitoring well also has a certain impact on groundwater environment. NH3-N is an important domestic pollution source. Due to the lack of strict management of the monitoring well in the landfill for a long time，the monitoring well has been open for a long time，which is greatly affected by regional environmental pollution. Before the Shigatse landfill was put into operation，there were natural pits (between the diffusion well and the monitoring well) in the north of the landfill，which was the early dumping and stacking site of domestic waste in Shigatse. After the phase I project of Shigatse landfill was put into operation，the original landfill area was only covered with soil，compacted and sealed，without any anti-seepage and pollution prevention and control measures. Through field investigation，it is found that the original landfill area is still a low-lying area. It is the rainy season during April-October in Tibet，and rainwater is mostly deposited in the original landfill area，which is one of the reasons for the relatively poor water quality of the monitoring well. Since only NH3-N in the wet season of 2014 exceeded the standard，and the detection results in other years met class III standard of theStandardforGroundwaterQuality，the comprehensive evaluation result of NH3-N was greatly affected by the monitoring results in 2014.

4.2 Uncertainty analysis(i) Due to the concealment and complexity of groundwater environment，the impact of regional hydrogeology on water environment was not considered in the evaluation process，and the health analysis was not comprehensive to a certain extent. (ii) The five-year monitoring data of groundwater in the wet season were used in this paper. As the monitoring data in 2014 were obtained by entrusting Shigatse Environmental Monitoring Station to detect and analyze，among which the monitoring concentrations of NH3-N，Cr6+and NO3-N were higher than those in other years，resulting in the results of health risk assessment in 2014 being higher than those in other years. It could make comprehensive carcinogenic risk and non-carcinogenic hazard quotient higher. (iii) The parameters in the health risk assessment model，such as drinking water volume，per capita weight，average height，skin contact frequency，contact time，skin permeability，etc.，were estimated values，which were not accurate statistical values. These parameters could affect the evaluation results and produce a certain degree of uncertainty. In addition，exposure frequency，exposure cycle，an average time of carcinogenic effect，average time of non-carcinogenic effect，etc.were all based on relevant foreign standard values and domestic average values，which also had certain uncertainty on the evaluation results. (iv)Due to the lack of relevant evaluation reference dose standards in China，standards such as reference measurement of various non-carcinogens and carcinogenic intensity coefficient published by the U.S. Environmental Protection Agency were mostly adopted. Due to the existence of human body differentiation，a direct reference to these standards will also produce a certain degree of uncertainty. (v) In this study，the health risk assessment only calculated the carcinogenic risk or non-carcinogenic hazard quotient caused by a single pollutant in various ways，and then calculated the total carcinogenic risk or non-carcinogenic total hazard quotient by the cumulative method，ignoring the joint synergy or antagonism between pollutants and the superposition effect of pollution risks. It had certain uncertainty on the current assessment results.

Fig.1 Comprehensive non-carcinogenic risk of a single pollutant

Fig.2 Comprehensive carcinogenic risk of a single pollutant

4.3 ConclusionsAs an important source of groundwater pollution，the landfill has a pollution risk that can not be ignored to the regional groundwater environment. Relevant studies also show that the total carcinogenic risk (Cr，As) of groundwater in the landfill area will exceed the maximum acceptable risk level of 10-6，and the total non-carcinogenic hazard quotient (Fe，Mn，Cu，NH3-N，F(xiàn)，NO2-N，NO3-N) will exceed the maximum acceptable level[25]，or the order of magnitude of the total carcinogenic risk is between 10-6-10-4，and the total non-carcinogenic risk is less than 1[26]. The total carcinogenic risk of groundwater in Shigatse landfill site for males and females was 1.855 ×10-6and 2.238×10-6，respectively，exceeding the maximum acceptable level of carcinogenic risk of pollutants in Sweden，the Netherlands and other countries (1×10-6). Moreover，the risk value of health hazards for womales was higher than that of men. This showed that long-term exposure to groundwater in monitoring well could cause certain health hazards to human body. The total non-carcinogenic risk values of males and females in the assessed area were 0.057 7 and 0.069 6，respectively，meeting the non-carcinogenic risk threshold "1". The non-carcinogenic risk contribution of each pollutant was cyanide>Cr6+>fluoride>NH3-N>Fe> NO3-N>Zn>NO2-N.

There are many residential areas，substations，building materials production bases，commercial concrete companies，etc.in the downstream (north) of Shigatse landfill. The evaluation results can provide a scientific basis for the management of Shigatse landfill，the prevention and control of pollutants，and the safety of residential water use.