Desalinated and blended water in Saudi Arabia: human exposure and risk analysis from disinfection byproducts

Saudi Arabia produces the largest amount of desalinated water as a single country. The desalinated water is typically blended with treated groundwater, pH adjusted and chlorinated prior to supply to the communities. The desalinated seawater and/or blended water contains various types of disinfection byproducts (DBPs), some of which may induce cancer risks to human through lifetime exposure. In this study, occurrences of trihalomethanes (THMs) in desalinated and blended water in Saudi Arabia were investigated and their exposure and risks were predicted. The chronic daily intakes of CHCl3, BDCM, DBCM and CHBr3 were estimated to be 8.38×10-5, 7.57×10-5, 2.54×10-5 and 4.32×10-4 mg/kg-day respectively. The overall cancer risk was 1.78×10-5 with the range of 7.40×10-7 – 9.26×105 and the average hazard index was 3.49×10-2 with the range of 1.20×10-3 – 2.34×10-1. The loss of disability adjusted life years (DALY) were estimated to be 25.1 per year and the average cancer risk had 8.48×10-7 DALY per person per year. The financial burden was estimated to be US$2.72 million with the range of US$2.52–2.91 million. The findings may assist in better understanding and reducing cancer risks from DBPs in desalinated and blended water.


Introduction
Desalination of seawater has been in practice for satisfying domestic water demands in many countries in the Middle East, the Mediterranean Basin, as well as in Australia and USA. In the Arabian Gulf, total production of desalinated water was reported to be approximately 11 million m 3 /day while Saudi Arabia produced approximately 4.5 million m 3 /day [1,2], which has made Saudi Arabia as the largest producer of desalinated water as a single country [1,2]. For desalination, two main processes: thermal (MSF: multi-stage flash; MED: multi effect distillation); and membrane (RO: reverse osmosis) are in practice, and the dominant process is MSF, due mainly to low energy cost in this region. In addition, large-scale plants with advanced technologies are being established in these countries indicating the increase in desalinated water supplies in future.
The desalination plants receive feed water mainly from the open seawater intakes while some plants receive feed water via different intakes and coastal locations. The pretreatment is performed using continuous or intermittent chlorination with initial doses of 0.5 -2.0 mg/L to achieve the target residuals of 0.25 -0.5 mg/L. Free chlorine is also applied in different stages of desalination process as well as in final disinfection. Consequently, different types of disinfection byproducts (DBPs) are formed in the desalinated seawater, which can pose public health risks. The average concentration of bromide in seawater from the Arabian Gulf was reported to be 64,000 μg/L [3]. Past studies have reported that seawater could contain bromide and iodide in the ranges of 50,000 -80,000 μg/L and 21 -60 μg/L respectively [3][4][5], which can increase the formation of brominated and iodinated DBPs. The brominated and iodinated DBPs are much more cytotoxic and genotoxic than the chlorinated DBPs [6].
In the past, many studies have focused on human exposure and risk from DBPs in freshwater sourced drinking water. Despite the public health relevance, not much is known about the DBPs in desalinated and blended water, their exposure and risk. In Saudi Arabia, desalinated water supplies more than 60% (~1620 million m 3 /year) of domestic water demand [2], which is likely to be increased in future. The desalinated water is blended with the treated brackish groundwater, pH adjusted and chlorinated prior to supplying to the consumers. The blended water stays in the regional pipelines, storage tanks, blending stations and WDS for several days. In addition, maximum daily temperature during summer often exceeds 40ºC, which can increase the reaction rates by several folds. The water has the environments and constituents conducive to increased formation of DBPs, in particular, the brominated and iodinated DBPs [7]. There is a need to understand DBPs concentrations in desalinated and blended water, and the consequent exposure and risk. In this study, formation and distribution of THMs in desalinated and blended water from different desalination plants in Saudi Arabia were investigated. Exposure to THMs were estimated through ingestion, inhalation and dermal absorption pathways. Cancer and non-cancer risks were predicted which were used to predict the societal burden in terms of disability adjusted life year (DALY) applying the concept of YLL (years of life lost due to premature mortality) and YLD (years of life lost due to disability).

Data Collection
Occurrences of THMs (chloroform: CHCl3; bromodichloromethane: BDCM; dibromochloromethane: DBCM; and bromoform: CHBr3) and water quality parameters were investigated in desalinated seawater and blended water (mixture of desalinated seawater and treated groundwater in the ratio of 40-60%) in the WDS in Al-Khobar and Dhahran, Saudi Arabia for a period of one year (Feb, 2014 -Jan 2015). The samples were collected on biweekly basis and analyzed in duplicates following the standard methods [8,9].

Exposure and Risk Assessment
Exposure to THMs occurs through multiple pathways, including ingestion of drinking water, and inhalation and dermal contact during showering, bathing, house cleaning and swimming [10]. To incorporate uncertainty, 5000 random data are generated for each parameter following the statistical distributions. Exposure to THMs through ingestion is predicted using the THMs in cold water. THMs exposure through inhalation pathway is predicted using THMs in the shower air, which is estimated using the partition coefficients and mass-balance equations. In assessing dermal exposure during showering, showering events are divided into the unsteady and steady states and exposure. Moreover, to incorporate the higher level of exposure during early life adjustment factor was used (i.e. adjustment factor of 10 fold and 3 fold for the age group <2 years and 2 -16 years, respectively; no adjustment factor for the age group > 16 years) [11]. Further details are summarized below.

Dermal contact pathway
THMs can be absorbed through human skin during showering. Human skin is a complex organ, which acts as a barrier to chemical intrusion through dermal pathway. Depending on shower duration, dermal exposure often needs unsteady and steady state exposure assessments while unsteady state exposure can be different from the steady state assessments [7]. However, diffusion of chemicals before reaching steady state can be significantly different from the steady state values [15]. To achieve steady state between the chemicals in water attached to the skin surface (i.e. upper layer of stratum corneum) and lower layer of stratum corneum, there is a need of lag time, which was reported to be in the range of 9.8 -391.2 minutes for different DBPs [7]. The lag time prior to achieving steady state condition, diffusion through skin, octanol-air partition coefficient, CDI during unsteady and steady states are estimated following past studies [7,16]. Upon estimation of CDI through the unsteady and steady states of exposure, the total CDI through dermal route is calculated as: Where, CDIGHUPíXVW = CDI of THMs during unsteady-state period (mg/kg-day); CDIGHUPíVV = CDI of THMs through dermal route (mg/kg-day) during steady-state condition; CDIGHUPíVV = total CDI through dermal route. Upon estimation of route specific CDI, lifetime cancer and non-cancer risks can be estimated as: Where, i = 1,2,3…m represent different THMs (i.e. CHCl3, BDCM, DBCM and CHBr3); j = 1,2,3…n representing different routes of exposure (i.e. ingestion, inhalation and dermal contact); CR = cancer risk; SF = slope factor ([mg/kg/day] -1 ) for specific route; RfD = reference dose (mg/kg/day). The oral SF for BDCM, DBCM and CHBr3 are 0.062, 0.084 and 0.0079 per mg/kg-day respectively [17], while for the other routes, SF data are not available. In this study, oral SF from USEPA were used to estimate human health risk. The RfD for CHCl3, BDCM, DBCM and CHBr3 are 0.01, 0.02, 0.02 and 0.02 mg/kg-day respectively [17].

DALY analysis
The disability adjusted life year (DALY) estimates the number of years lost due to premature death and disability because of a disease or injury [18]. To asses DALY, three possible scenarios are considered: (i) a proportion of affected people will die from cancer; (ii) a proportion will be cured from cancer; and (iii) rest of them will live with cancer sequelaes. The healthy years of life lost in a group of populations is calculated following Soerjomataram et al. (2012) [19] as: Where, DALY = total disability adjusted years of life lost; YLL = total years of life lost due to premature death from cancer; YLD = total years of life lost due to disability. YLL is calculated by multiplying the number of death with the years of life expected to live at the time of death. YLD is calculated by multiplying the incidence rate with the disable years caused by the disease and severity of the disease (e.g., disability weight). YLL and YLD were estimated following Pan et al., (2014) [20] as: Where, nx = number of population for each age group; ex = standard life expectancy, as reported in the Global Burden of Disease Studies (GBDS) [18]; TD = median time to die (e.g., 2.20 years for bladder cancer) [21]. DW = adjusted disability weight (range: 0.0 -1.0) obtained from GBDS [18]; L = duration of disability, which is divided into three stages: time

Data statistics
Concentrations of THMs and water quality parameters for the desalinated and blended water are shown in Table 1. Concentrations of THMs in blended water were significantly higher than that in the desalinated water (Table 1). In this study, average concentrations of THMs in the desalinated and blended water were 10.08 and 19.2 μg/L respectively with the ranges of 0.1 -33.6 and 2.1 -52.4 μg/L respectively ( Table 1). Concentrations of THMs in desalinated and blended water from few other plants in Saudi Arabia, and some Arabian Gulf countries are summarized in Table 2.
In the major cities of Saudi Arabia, THMs in desalinated water were in the range of 0.1 -41.7 μg/L [22]. In blended water, THMs were in the ranges of 0.1 -66.7 μg/L [5,23]. THMs in the desalinated and blended water from Bahrain, Kuwait, Qatar and UAE were in the ranges of 0.27 -6.4, 7.11 -104.89, 4.02 -83.31 and 7.0 -15.0 μg/L respectively (Table  2). Figure 1 shows the concentration variability of THMs in Saudi Arabia. On average, CHCl3, BDCM, DBCM and CHBr3 were approximately 5.2, 6.2, 9.5 and 79.1% respectively. The THMs database obtained in this study was expanded using additional data from the desalinated and blended waters in different desalination plants and WDS in the major cities in Saudi Arabia [5,7,16,[22][23][24][25]. Using the combined data, the best-fit statistical distributions were developed for THMs. In this study, CHCl3 and BDCM followed triangular distribution while DBCM and CHBr3 followed the Gamma distribution (Table 3).  The proportion of various sequelaes and disability weight of various sequelaes are shown in Table 4. The other relevant parameters for estimating exposure and risk assessment are obtained from previous studies [7, 13-17, 20, 26].

Risks of THMs
The lifetime excess cancer risks and hazard indices are shown in Table 5. The average cancer risk considering all age groups was predicted to be 1.78×10 -5 with range of 7.40×10 -7 -9.26×10 -5 . The ingestion, inhalation and dermal routes contributed approximately 65.4, 23. of the overall risk, indicating that appropriate protection during this period (birth to < 2 years) might reduce the overall risk significantly. The overall hazard index was estimated to be 3.49×10 -2 with the range of 1.20×10 -3 -2.34×10 -1 . Hazard indices through ingestion, inhalation and dermal routes were estimated to be 2.23×10 -2 (range: 1.05×10 -3 -1.45×10 -1 ), 7.89×10 -3 (range: 1.21×10 -4 -8.18×10 -2 ) and 4.77×10 -3 (range: 3.84×10 -5 -4.53×10 -2 ) respectively (Table 5). The overall DALY was estimated to be 25.1 with the range of 23.2 -26.8, meaning that on average 25.1 years are likely to be lost due to cancer risks from exposure to THMs. Among the DALY, the YLL and YLD were estimated to be 18.6 (17.1 -19.9) and 6.5 (6.0 -6.9) respectively. The YLL and YLD contributed approximately 74 and 26% of total DALY. DALY for different age groups are shown in Figure 3. The largest DALY (5.6) were estimated for 15-39 age group (Figure 3). The cancer risk in terms of DALY was obtained as 8.48×10 -7 per person per year (PPPY), which was lower than the USEPA guideline value of 1.0×10 -6 [27]. One DALY was associated with a loss of US$ 108,600 [28], which means one year of healthy human life costs US$ 108,600, indicating that the total financial burden for Saudi Arabia was US$ 2.72 million with the range of US$ 2.52 -2.91 million per year.

Conclusion
This study presented and applied the methodologies of estimating human health risk and DALY from exposure to DBPs in desalinated and blended water in Saudi Arabia. The cancer risks of DBPs in desalinated and blended water was predicted to be 1.78×10 -5 , which was higher than the minimum risk recommended by the USEPA. Cancer risks through ingestion route was highest (65.4%) followed by dermal (23.5%) and inhalation (11.1%) routes. With respect to age groups, the highest contributor group was >16 years (45.3%), which had the largest lifespan (16+ to death). In contrast, the exposure during the early life (birth to < 2 years) contributed approximately 25.4% of the overall risks. Control of early life exposure can reduce the risk significantly. The cancer risks in terms of DALY was estimated to be 8.48×10 -7 per person per year, which is below the reference risk level as recommended by the USEPA. The DALY can be used for analyzing financial burden from cancer risks. It can also be used for cost effectiveness analysis, which may help to improve quality of desalinated water and may help to prioritize the hazardous material present in desalinated water. In this study, THMs were used for analyses, which were a fraction of various DBPs present in desalinated water. Few other DBPs with possible cancer risks are HAAs, iodo-THMs, bromate, NDMA while sufficient information on these DBPs are not available to date to conduct risk analysis for desalinated and blended water. Future study may further look into these DBPs and their risks.