Health risk analysis of benzene, toluene, ethylbenzene, and xylene (BTEX) in groundwater in Yogyakarta City, Indonesia

Benzene, Toluene, Ethylbenzene, and Xylene (BTEX) are persistent and bio-accumulative compounds that occur in petroleum-based products. Benzene is categorized as a carcinogenic substance while ethylbenzene is potentially carcinogenic. One of the sources of BTEX contamination in the environment is gas stations. The leakage of underground storage tanks (UST) of gas stations is a potential source of groundwater contamination, while in many areas, it is the main source of daily water consumption. The long-term consumption of this BTEX-contaminated groundwater may generate health risks. This paper will present the health risk analysis of the consumption of BTEX-contaminated groundwater taken from dug wells near a gas station in Yogyakarta City which has a history of major leakage. Analysis of BTEX in groundwater samples has been already presented in the previous study. Data for intake calculation was acquired through questionnaires and interviews on water consumption patterns. Risk analysis was conducted for both the carcinogenic effect of benzene and the non-carcinogenic effect for all BTEX compounds. Health risk analysis on BTEX exposure in groundwater resulted in the Hazard Quotient (HQ) value for all compounds which were still less than one, meanwhile, the Excess Cancer Risk (ECR) for benzene was less than 10-4. The result indicates the low health risk associated with the consumption of BTEX-contaminated groundwater in the study area.


Introduction
Benzene, Toluene, Ethylbenzene, and Xylene (BTEX) are compounds that naturally occur in crude oil and become the major constituents in many petroleum-based products such as gasoline [1].This group of compounds posed a major threat to human health.International Agency for Research on Cancer (IARC) of the World Health Organization (WHO) stated that benzene belongs to group 1 which means carcinogenic to humans, and ethylbenzene belongs to group 2B which means possible carcinogenic to humans.BTEX has been included as a priority pollutant in water by the U.S. Environmental Protection Agency (EPA) [2].It is of major concern because of its characteristic of good solubility in water and mobility into soils [3].
One of the potential sources of BTEX contamination in the environment is gas stations, especially from leakage from its underground storage tanks (UST) [3][4][5][6].Some factors that cause the leakage to occur are construction error, overfilling, or fuel spillage.One case of leakage of gas stations was reported 1263 (2023) 012003 IOP Publishing doi:10.1088/1755-1315/1263/1/012003 2 in 1999 in Yogyakarta City [7].As it is in the vicinity of housing area, the leakage directly affected the inhabitants living in the area.
Despite the high possibility of occurrence and potential adverse health effects it can pose, health risk analysis of exposure of BTEX from gas stations to humans is still rarely conducted.Most studies focused on the monitoring of groundwater quality surrounding the source of BTEX contamination or modeling or simulating its transport in the environment [8][9][10].Health risk analysis can be used to estimate the potential health impact that occurred due to BTEX exposure from the environment.The result can be the basis to plan or design the most appropriate environmental management to minimize the risk.Environmental management may include remediation by using for example bioremediation technology, establishment and enforcement of a better regulation, or environmental education to raise people's awareness.This paper present the health risk analysis of the consumption of BTEX-contaminated groundwater based on the previous study [10] due to a leakage incident in one gas station in Yogyakarta City [7].

BTEX concentration in groundwater
To conduct a health risk analysis for this study, the concentration of BTEX in drinking water is based on the concentration of groundwater based on the previous study [10].Samples were taken from dug wells surrounding the gas station which has a history of leakage.This gas station is located in Yogyakarta City, Indonesia.

Data on exposed population and water consumption pattern
Data on the exposed population and water consumption pattern were acquired through questionnaire distribution and interviews.In total, 26 respondents for this data are people living within a radius of 100-400 m from the gas station in the west, south, north, and east directions.Respondents are those using groundwater as their source of daily water consumption.A smaller number of respondents were taken from the eastern part as people in this area mostly use water from municipal water companies instead of groundwater.
Data acquired including respondent profile (age, weight, and duration of stay in the area) and data on activity and water consumption pattern: quantity of water for consumption, frequency, and duration of exposure.Respondent profiles represents the exposed population whose data is the input for the intake calculation along with the data on activity and water consumption pattern.

Health risk analysis
Health risk analysis was conducted for both the carcinogenic effect for benzene and the noncarcinogenic effect for all BTEX compounds.The basis for health risk analysis is the calculation of intake from every possible route of exposure.The route of exposure considered in this study is oral exposure.Intake for both oral and dermal exposure was calculated using equation (1).
I = intake (mg/kg-day) CW = concentration of BTEX in water (mg/L) IR = ingestion rate (L/day) EF = exposure frequency (day/year) ED = exposure duration (year) BW = body weight during exposure duration (kg) AT = averaging time, a period of average exposure to occur (day) Intake is the concentration of one substance taken into the human body per body weight per year, CW is the concentration of pollutant (BTEX) in groundwater, IR is the ingestion rate or daily drinking water consumption represents the oral route of exposure of BTEX into the human body, EF is the exposure frequency or how many days the exposure occurred within a year, ED is exposure duration or how many years the exposure occurred, BW is the average body weight of the exposed population, AT is averaging time or a period of average exposure to occur, AT is days/year times 30 years for non-carcinogenic effect while it is 70 years for carcinogenic effect [11].
The result of the intake calculation was used to determine carcinogenic and non-carcinogenic effects.Non-carcinogenic risk is determined by comparing the intake calculation result with the reference dose (RfD).RfD is an estimate of the daily exposure of one substance to the human population that is likely to be without an appreciable risk of deleterious effects over the course of a lifetime [12].The population of concern includes sensitive subgroups whose more susceptible to environmental pollution due to various affecting factors such as age, sex, nutrition, or pregnancy status [13].RfD values used in this study (Table 1) for each compound were those stated in the Integrated Risk Information System (IRIS) of United States Environmental Protection Agency (US EPA).Non-carcinogenic risk is represented by Hazard Quotient (HQ).HQ value of more than 1 means that non-carcinogenic risk is not acceptable, and some risk management effort should be conducted.

HQ = I/RfD
(2) HQ = Hazard Quotient I = Intake (mg/kg-day) RfD = Reference Dose (mg/kg-day) Carcinogenic risk represented by Excess Cancer Risk (ECR) was estimated by using slope factors (SLF).It was only conducted for benzene because no SLF value available for toluene, ethylbenzene, and xylene.The ECR value higher than 10 -4 means unacceptable level of cancer risk and some management effort should be conducted.

ECR = I x SLF
(3) Source: [10] Toluene and ethylbenzene were both detected below LoD.Therefore, these two compounds would not be considered for the following health risk analysis.

Results of questionnaire distribution and interview
The distribution of respondents according to their age is presented in Figure 1.As it represents the exposed population, the data will be divided into three

Risk Calculation
As suggested by USEPA, a conservative approach was used for risk calculation, hence the maximum potential risk was considered.Applying this approach, the maximum concentration of benzene and total xylene were used to calculate the risk.Intake calculation following equation (1) used input value acquired from questionnaire data as presented in Table 3.The ingestion rate value was taken from the data on the amount of BTEX-contaminated groundwater as the source of daily drinking water consumption.As it is consumed every day, the exposure frequency is the whole day in a year or 365 days.For exposure duration, three conditions are calculated separately.Realtime duration is based on the average duration of stay in the study area for each age group, while for non-carcinogenic and cancer duration, 30 and 70 years are used respectively following recommended value for residential cases [11].Another input value is body weight which uses the average body weight data for each age group.Finally, 30 years and 70 years are used for non-cancer and cancer effect respectively to get averaging time as recommended by the guidance [11].Risk calculation for those two compounds is presented in Table 4 and Table 5.The calculation results presented in Table 4 show that the HQ value for real-time and non-cancer for all age classification is less than one.This shows that non-cancer risk is still acceptable.Comparing the value among all age groups, the real-time hazard quotient for the older population is the highest.This is due to the much longer exposure duration and lower body weight compared to the adult group.This group is included as the sensitive subgroup in a population who are more susceptible to pollutant [13].
A similar result was given for cancer risk calculation.ECR value for benzene exposure through consumption of contaminated groundwater for all age groups was still acceptable as those did not exceed 10 -4 .Comparing the three age groups, the children group gives the highest ECR value.This result differs from a similar study of BTEX groundwater contamination in India that resulted in a higher ECR in adults than children [14].This is due to different input values, especially for body weight.
The result of this study may show an acceptable level of risk related to BTEX contamination in groundwater in the study area.However, it should be interpreted cautiously, as some uncertainty is involved in the study.This includes the lack of groundwater monitoring data on BTEX compounds [15].It is not known whether the concentration will remain constant during exposure duration.The reference value may also be the source of uncertainty.For example, the RfD value for the calculation of the noncancer risk is the subject of uncertainty.This value was derived by dividing the NOAEL (No Observed Adverse Effect Level) by UF (Uncertainty Factor).The source of uncertainty is for example from the use of animal toxicological data on human risk calculation [12].The use of the deterministic method to calculate the risk may also contribute to uncertainty.It can be reduced by using probabilistic methods such as Monte Carlo simulation for exposure input value in risk calculation [16].On the other hand, this study was only conducted for the oral route of exposure, while dermal is also a possible route for BTEX exposure to humans [1,17].Calculation of every possible route of exposure will give a better estimation of the risk posed by BTEX contamination in groundwater or any case of environmental pollution.

Conclusion
Health risk analysis towards the exposure of BTEX compounds in the study area resulted in a Hazard Quotient (HQ) value of less than 1 and an Excess Cancer Risk (ECR) value for benzene exposure of less than 10 -4 .The result indicates that the health risk associated with the consumption of BTEXcontaminated groundwater in the study area was still acceptable.
age groups: children, adults, and the older population.Children are those under the age of 15, adults are those within the age of 15 -65, and the older population are those above 65 years of age following age grouping according to the Decree of Ministry of Health No. HK.01.07/MENKES/5675/2021 on Population Data for the Target of Health Development Program 2021-2025.

Figure 1 .
Figure 1.Age distribution of respondents

Table 1 .
RfD and SLF value for BTEX according to IRIS.

Table 2 .
BTEX concentration in groundwater samples.
*Limit of Detection

Table 3 .
Exposure data from questionnaire distribution and interview.

Table 4 .
Calculation of oral intake for real-time, non-cancer, cancer, and HQ value for benzene exposure.

Table 5 .
Calculation of oral intake for real-time, non-cancer, cancer, and HQ value for xylene (total) exposure.