Geographical map of the administrative districts in Seoul: monitoring data collected from 14 out of all 25 districts in the city (for each number assigned to each district in Seoul, refer to Table 1) 

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... quality is often considered one of the most sensitive issues worldwide as potable water is needed to ensure health and safety among the populace. Ac- cess to safe potable water is of prior importance in the development of any country, as it is a key element to ensure economic growth by sustaining a healthy population (WHO 2004). It contributes to the quality of life of households through the provision of basic needs for water quality and sanitation. Potable water should be free from compounds that can cause color, taste, odor, and pathogens while low in concentrations of compounds with acutely toxic or serious long-term effects on health (Pritchard et al. 2007). In this era of heavy industrialization and numerous human activities, there is an ever increasing demand for clean water. The availability of surface water sources is decreasing with the fast deterioration of their quality which in turn can cause damages on human health with increases in medical treatment costs (Pritchard et al. 2007). It is in fact reported that 80 % of all illness in developing countries are related to water and sanitation (Tebutt 1998). This finding ultimately reflects an increasing demand for pure water sources with an added pressure on the clean environment. The demand for extra clean water resources has now placed much emphasis on groundwater exploita- tion for economical water supply (Gelinas et al. 1996). A way of extracting water from the ground is by drilling boreholes and shallow wells through the exist- ing water table. Unfortunately, certain hydrogeologi- cal conditions, such as permeable soil and shallow aquifer, may lead to an enhanced groundwater pollution threat (Drew and Hötzl 1999). As the water per- colates through the soil, it carries considerable quantities of harmful physical, biological, and chemical constituents (e.g., fine suspended matter and fluoride). Thereby such processes can cause contamination of the well water (Pritchard et al. 2007). The existence of urban sewage or unsewered sanitation systems nearby a water source should be the dominant factors contributing to such groundwater contamination (Kaown et al. 2009a, b). These water sources, if not adequately monitored and managed, may thus become a source of chronic health problems with the limited suitability for water resources (Bissen and Frimmel 2003; Reimann and Banks 2004). In Korea, hundreds of shallow wells are located in clean mountainous areas. Most wells are covered and built using carved rocks or concrete enclosures. These wells are shallow at the collection sites so that any- body can access the water. However, the origin of these well waters may all be different in nature. These waters are commonly used for drinking by mountain- eers and picnic goers. As people consider them as the source of fresh pure mineral or spring waters, they are commonly collected in water containers and brought back home. Although locations for most wells are clean and far from known sources of pollution, there is an eminent need for water quality monitoring for various usage. Hence, in an effort to maintain the quality of well waters located in the capital city of Seoul, South Korea, evaluation of their water quality was made through the measurements of various chemical and physical parameters in vast areas within the city boundary. The data obtained from these comprehensive monitoring efforts will thus be used to assess the present pollution status of well waters and their corresponding suitability for drinking water and to help derive most reasonable management strategies of those resources. The locations of 14 districts investigated in this study are depicted in Fig. 1. Although the testing sites in each district changed throughout the year, they recorded as many as 289 in 2011. In terms of sampling density, these 14 districts can be distinguished between ten major and four minor ones. The number of monitoring at the former was made from 321 to 808 times during the whole study period, while its counterpart at the latter only ranged from 7 to 22 (Table 1). This type of distinc- tion between different districts is mainly made by the abundance of well sites for routine monitoring. The water quality parameters of wells in the model districts of Seoul city have been tested routinely such as six times (once per each quarter (1, 2, and 4th) and once per each month between July and September) on an annual basis by the joint effort of the Seoul Metropolitan Research Institute of Public Health and Environment (SIPHE) and the Local Health Department (LHD). SIPHE initially analyzes up to 48 water quality parameters in the 2nd quarter of the year. Those wells which failed the water quality standards (from the initial analysis made in the 2nd quarter) are re- evaluated in the 4th quarter of the same year. In addition, LHD analyzes up to seven water quality parameters (e.g., NH 3 – N, etc.) for five times per year basis (two quarters (1st and 4th) and 3 months (July to September)). LHD collects samples from sites that failed during the 2nd-quarter inspection and submits them to SIPHE for analysis. All acid-washed clean polyethylene bottles were rinsed with sample aliquots prior to actual sampling. The collection of well water samples was made with the least disturbance to the well waters. The various water quality parameters such as elemental components, volatile organic compounds (VOC), and ionic species were measured, and results of 14 (ten major plus four minor) districts from the whole study period are summarized in Table 2. In addition, the data for the ten major districts are described in detail for each annual basis in Table 7 in the Appendix. However, only water quality parameters which exceeded various drinking water guidelines (e.g., As, F, NO 3 – N, B, 1,2- dichloroethene (1,2-DCE), dichloromethane, Cu, and Pb) or those critical for drinking water (e.g., Cl, hardness, Fe, Mn, SO 42 − , NH 3 – N, Al, Zn, TDS, and turbidity) were assessed in detail in this study. Water samples contained in 4 L clean bottles were brought in for the analysis of other chemical parameters, while those collected in 5 L containers were subject to VOC analysis. The details of instrumentation and analysis methods for quantitative analysis of water quality parameters used are listed in Table 2. The basic information covering sample volume, detection limit, precision, and accuracy is also summarized in Table 2. A rigor- ous quality control program was routinely imple- mented such as reagent blanks, replicate samples, and in-house reference materials. (Refer to the standard operation procedure of each method (e.g., in the case of fluoride, Bulletin of the Ministry of Environment (BME) ES 05351.1 or standard method 3500) provided in Table 2). In general, consumers determine the acceptability of the water by relying on their senses (e.g., appearance, taste, and odor). Microbial, chemical, and physical constituents of water may affect aesthetic water quality. Water with high turbidity, color, objectionable taste, or odor may be regarded as unsafe and rejected by consumers, although some of them may have no direct health effects. In extreme cases, consumers may avoid aesthet- ically unacceptable but otherwise safe drinking water in favor of more pleasant but potentially unsafe sources. As a key component to water quality management, the reliability and chemical quality of drinking water can be assessed primarily against regulation guideline values. Hence, in this study, we evaluated our water quality data (Table 3) with a main emphasis on the regulation guideline set by the World Health Organization (Table 4; WHO 2011a). From 14 out of all 25 districts in Seoul, we investigated the well-water data exceeding guideline values for key water quality parameters set by international (or national) agencies (Fig. 2). Chemicals with health significance in drinking water A few key chemicals have been identified to cause clearly defined human health effects with excessive intake via drinking water. These include naturally occurring chemicals such as arsenic and fluoride. In- organic arsenic compounds are classified by IARC (2004) and (American Conference of Industrial Hygienists (ACGIH), 2004a) as carcinogenic to humans (Group 1) with the WHO ’ s provisional guideline value set at 0.01 mgL − 1 (WHO 2004). In this study, we found As concentration from 18 well waters (0.01 – 0.62 mgL − 1 ) to exceed the WHO ’ s guideline value (0.01 mgL − 1 ), mainly coming from the districts of DJ, EP, GB, JR, MP, and NW (Table 5). These levels are considerably high compared with its common values in natural waters, which is ...