Spatial variations of arsenic in groundwater from a transect in the Northwestern Hanoi

Vietnam Journal of Earth Sciences, 40(1), 70-77, Doi: 10.15625/0866-7187/40/1/10971 70 (VAST) Vietnam Academy of Science and Technology Vietnam Journal of Earth Sciences Spatial variations of arsenic in groundwater from a transect in the Northwestern Hanoi Dao Viet Nga1, Pham Thi Kim Trang1, Vu Thi Duyen1, Tran Thi Mai1, Vi Thi Mai Lan1, Pham Hung Viet*1, Dieke Postma2, Rasmus Jakobsen2 1Research center for Environmental Technology and Sustainable Development (CETASD), VNU University of Science 2Geological Survey of Denmark and Greenland (GEUS), Denmark Received 5 April 2017; Received in revised form 3 July 2017; Accepted 8 December 2017 ABSTRACT Arsenic contamination of groundwater is a major health problem and has been a growing concern in the last dec- ade in several regions of the world, especially in South and Southeast Asia, including the Red River Delta, Vietnam. Regional groundwater studies have been carried out in the vicinity of Hanoi, on the banks of the Red River and its adjacent floodplains. In this study, the groundwater from a transect in the Northwestern area of Hanoi was examined. The results showed that 28.8% the wells of the B-B’ transect exceed the WHO guideline value for arsenic concentra- tion in drinking water. The arsenic concentrations varied in a wide range from point to point, with the highest concen- tration found at Van Phuc and the lowest one found at Cam Yen. They also varied accordingly to the depth. Keywords: Arsenic; groundwater; Northwestern Hanoi. ©2018 Vietnam Academy of Science and Technology 1. Introduction1 Arsenic is a naturally occurring element in the Earth’s crust. In its natural form in soil, arsenic is harmless. However, if arsenic con- taminates the water and food chain, it will be a threat to human health and other living or- ganisms as it can cause skin, kidney, lung, liver and bladder cancer (Anawar et al., 2004). Drinking water constitutes a major pathway of exposure to arsenic in humans. Drinking wa- ter is derived from a variety of sources such as surface water, groundwater and rainwater *Corresponding author, Email: phamhungviet@hus.edu.vn (Smedley, Kinniburgh, 2002). Among those sources, groundwater is generally more vul- nerable to arsenic contamination than surface water due to the potential interaction between groundwater and the minerals in soil which is favorable for arsenic release (Smedley, 2006). WHO has given the guideline value for arse- nic in groundwater and drinking water is 50 and 10 µg/L, respectively (WHO, 2011). There have been incidents of groundwater with arsenic contamination reported in several regions of the world. Some of the most serve cases of groundwater contaminated with arse- nic occurred in aquifers from South and Southeast Asia (e.g. Bengal delta, Vietnam, Dao Viet Nga, et al./Vietnam Journal of Earth Sciences 40 (2018) 71 Cambodia, etc). Study on the arsenic concen- trations in 6000 tube wells in a 25 km2 area of Bangladesh of van Geen et al. (2003) showed the range of arsenic concentrations from <5 to 900 µg/L, with more than half of the wells did not meet the Bangladesh standard for drinking water (50 µg/L). In West Bengal (India), Chowdhury et al. (2000) studied nine affected districts and found that 55% and 34% of the tube wells contained arsenic above 10 and 50 µg/L, respectively. In Southeast Asia, many researches were also carried out. In Cambodia, the arsenic concentrations ranged from 1-1610 µg/L (av- erage 217 µg/L) and these figures were 1-845 µg/L (average 39 µg/L) in Southern Vietnam (Berg et al., 2006). A similar predicament has been discovered in the Red River Basin, Vietnam. Berg et al. (2001) studied the groundwater of four rural districts around Ha- noi (which are Dong Anh, Tu Liem, Gia Lam and Thanh Tri) and reported that the average arsenic concentrations varied from 1 to 3050 µg/L, with the mean value being 159 µg/L. This is the first report on high arse- nic concentrations in groundwater of the Red River Basin. Since then, more researches have been carried out in the vicinity of Hanoi and along the banks of the Red River. In a more recent study, Winkel et al. (2011) did a research on a large scale in the Red River Delta. The results showed that 27% of the studied wells exceeded the WHO guideline value, which means about three mil- lion people used groundwater contaminated with arsenic concentration >10 µg/L and one million people used groundwater with the ar- senic concentrations >50 µg/L. It is quite a concern because the Red River Delta is one of the most densely populated regions in the world, with a population density of about 1000 people/km2 covering an area of 21,000 km2 (General Statics Office of Vietnam, 2015). However, there still is an unanswered question about arsenic distribution that is how two wells only 50 m away from each other can have a different arsenic concentration. Study of Eiche et al. (2008) in Van Phuc vil- lage (about 10 km Southeast of Hanoi), where groundwater is exploited as the main source of drinking water, showed the same phenom- ena: very different arsenic concentrations were obtained at two sites 700 m apart (one had the concentration <10 µg/L and one ranged from 170-600 µg/L). In their study in 2013, van Geen et al. found out that due to the groundwater pumping, arsenic can move from the Holocene aquifer into a previously uncon- taminated Pleistocene aquifer. This might be the explanation for the difference in arsenic distribution in groundwater; however, this can only be applied to a specific area. In this study, groundwater from a transect in the Northeast-Southwest direction in the Northwestern Hanoi was examined. Different from Van Phuc village in the aforementioned studies of Eiche and Van Geen, this is a less dense area where groundwater chemical com- position is less affected by human activities. Therefore, this is an appropriate area to study the natural occurrence and distribution of ar- senic in groundwater. The objectives of this study are (i) to investigate the spatial varia- tions of arsenic concentration in groundwater of the area in the Northwestern Hanoi to see whether or not the groundwater here contains arsenic; (ii) to investigate the relationship be- tween arsenic and some chemical compounds and (iii) to partially predict the tendency of arsenic concentration in groundwater. 2. Methodology 2.1. Research area The research area is located in the North West of Hanoi and about 30 km away from the city center. This location of this area is be- tween Ba Vi Mountain and the Red River, in- cluding a part of the Day River. This is the ar- ea with lower population density, where there is neither groundwater exploit for irrigation nor any water supply plants using groundwa- ter. Therefore, the impact of human activities Vietnam Journal of Earth Sciences, 40(1), 70-77  72 on the chemical composition and distribution in groundwater is limited. For this study, observation wells were constructed alongside the transect: Cam Yen - Phuc Hoa - Xuan Phu - Van Phuc (B-B’ tran- sect) (Figure 1). Figure 1. Research transect Among the four points of the transect, Cam Yen is the closest to the Ba Vi Mountain (ap- proximately 16 km) and Van Phuc is the clos- est to the Red River (approximately 1 km). The cross-section of the research transect is shown in Figure 2. The uppermost layer of this area is clay layer. Beneath that are the silty sand and sandy layer. And at the bottom is the gravel layer. The depth of the construct- ed wells depends on the depth of the sand lev- el in the aquifer. From Figure 2, one can see that the depth of the wells at Xuan Phuc varies in the smallest range, while the wells at Phuc Hoa have the widest variation in depth. Figure 2. Cross-section of the research transect To be more precise, the depth of the obser- vation wells which are constructed for this study at Cam Yen are 13.27-28.79 m (16 wells), at Phuc Hoa are 6.5-26.7 m (18 wells), Dao Viet Nga, et al./Vietnam Journal of Earth Sciences 40 (2018) 73 at Xuan Phu are 10.02-15.47 m (7 wells) and at Van Phuc are 11.78-26.88 m (16 wells). The total number of groundwater samples is 57. 2.2. Sampling of groundwater After the wells were constructed, they were rested for approximately 3 months be- fore sampling to stabilize the geological con- ditions and chemical composition of water. Groundwater water was sampled from the borehole using a down hole pump. Before tak- ing the water samples from the wells, 3 to 5 borehole volumes of the well were flushed. During flushing, a flow cell equipped with pH, EC and DO electrodes was mounted di- rectly on the sampling tube. The water sam- ples will be taken only until the values of pH, EC and DO were stable. CH4 samples were injected directly from the sampling tube into a preweighed evacuat- ed glass vial, leaving a headspace of one-half to two-thirds of the total volume. After sam- pling, the samples were frozen immediately in an upside down position to avoid sample leakage. Samples for other parameters were collected in the syringes and filtered through 0.45 µm cellulose acetate filters. As (V) and As (III) were separated using an anion ex- change cartridge, contained 0.8 g alumino- silicate adsorbent to adsorb As (V). Cation determination samples were kept in 50 mL PE bottles, using HNO3 1:1 to pre- serve. DOC determination samples were kept in 24 mL baked glass vials and pre- served by HCl 1:1. 2.3. Laboratory analysis Cations were determined using Shimadzu atomic absorption spectrophotometer instru- ment (AA 6800). Arsenic was measured using the same instrument with an HVG hydride generator. Anions were determined by ion chromatography using a Shimadzu LC20AD/HIC-20ASuper. CH4 head space concentrations were determined by gas chro- matography using a Shimadzu GC-14A. NH4 concentrations were determined by Shimadzu spectrophotometer UV-Vis 1800. DOC con- centrations were determined by Shimadzu TOC V-CSH. %RSD of all analysis was under 2% with the recovery from 95-105%. 3. Results and Discussions The total arsenic concentrations from groundwater of the B-B’ transect ranged from <5 to 115 µg/L. Among them, only 5 wells at Van Phuc and 1 well at Xuan Phu contained more than 50 µg/L of arsenic (10.5% of the observation wells). The average concentration at Cam Yen was the lowest (<5 µg/L), where- as the average concentration at Van Phuc was the highest (50 µg/L) (Figure 3). However, if compared with the WHO guidelines for arse- nic concentrations for drinking water, only the wells at Cam Yen, 1 well at Phuc Hoa and 1 well at Xuan Phu met the limitation value (28.8% of the well). Therefore, most of these wells water cannot be used for drinking. Figure 3. Total Arsenic concentrations in groundwater from the B-B’ transect Vietnam Journal of Earth Sciences, 40(1), 70-77  74 It can be seen also that the As concentra- tions increased from Cam Yen to Van Phuc. As mentioned before, Van Phuc is the closest to the Red River, which carries heavy sedi- ment loads, together with organic and inor- ganic matters. When this sediment deposits, these organic compounds stay in the soil. With that being said, the sediment at Van Phuc is expected to be the richest in organic compounds among the four points. This also means that the intensity of reduction activities in Van Phuc will be higher than other loca- tion, thus intensify the dissolution process of iron oxides minerals, which controls the re- lease of As. On the other hand, the sediment at Cam Yen is the farthest from the Red River, which suggests that there are not many organic com- pounds in the sediment here. This will lead to the less of reduction activities, and as a result, less in Arsenic concentrations. Another indication of organic compounds contents in sediment is the Ec value. The high concentrations of organic compounds will lead to the degradation process of organic matters and release CO2 into groundwater. This CO2 will take part in the dissolution pro-cess of minerals and thus, increases the Ec value (Appelo & Postma, 2004). Figure 4 shows the EC results of the transect. The Ec values at Van Phuc were approximately 1100 µS/cm. This is much higher than the Ec found in the Red River, which is 290 µS/cm (Postma et al., 2007). Whilst, the Ec values at Cam Yen were around 200 µS/cm. This is corresponding with Postma’s re- sults in his study in 2012. In this study, he found that the highest As concentrations were detected at the H-transect, which is closest to the Red River. While the lowest ones were found at Phung Thuong, which is farther from the River. He also concluded that the As con- centrations in groundwater were proved to be in correlation with sediment burial age. The younger the sediment is, the higher arsenic was found in groundwater. Therefore, the sed- iment in Cam Yen is predicted to be the oldest among the four points. The sediment then is younger at Phuc Hoa, Xuan Phu and Van Phuc are believed to be the youngest because it is the closest to the Red River. Figure 4. Ec values of the four sites from the B-B’ transect Not only varied from point to point, the As concentrations also varied with depth. At Cam Yen, the As concentrations were practically similar for all the depths. But at Phuc Hoa, Xuan Phu and Van Phuc, the As concentra- tions generally increased with depth. The As concentrations at Van Phuc varied the most with depth, with the lowest concentration (15 µg/L) was found at the depth 17.8 m, while the highest one (115 µg/L) was found at the depth 24.6 m. It was the same for Xuan Phu. The variation range of As concentrations at Phuc Hoa was less wildly. The variations of DOC and NH4 concentra-tions in groundwater of this transect were quite similar. At Cam Yen, Phuc Hoa and Van Phuc, the maximum values of DOC and NH4 were found at the depth of 20-25 m, and at the depth of 15 m, the DOC and NH4 concentra-tions at Xuan Phu reached the highest values. As for CH4, the concentrations at Xuan Phuc and Van Phuc had the same trends with DOC and NH4, while in Cam Yen and Phuc Hoa, the CH4 concentrations did not show huge dif-ferences between different depths. According to Smedley & Kinniburgh (2002) and Fendorf et al. (2010), one of the principal causes of high As concentrations in water is the reductive dissolution of hydrous Fe Dao Viet Nga, et al./Vietnam Journal of Earth Sciences 40 (2018) 75 oxides. As Fe oxides dissolve under strongly reducing conditions, one can predict that the stronger the reducing conditions are, the higher As concentrations occur in groundwater. This tendency was indeed found in the groundwater of the B-B’ transect. Figure 5a and 5b showed the occurrence between As concentrations and DOC, CH4 and NH4 concentrations, which are the chemical compounds representing the re- ducing environment. The concentrations of these compounds increased in the deeper wells indicated that the reduction conditions got stronger when it goes deeper. It is the favorable condition for the dissolution of Fe oxides, and therefore favorable for As releasing. This is one of the explanations for the variation of As concentrations with depth at Phuc Hoa, Xuan Phu, Van Phuc. Figure 5a. Occurrence of arsenic and other reducing condition instruction compounds in groundwater at Cam Yen - Phuc Hoa As founded in the study on Bangladesh groundwater of MacArthur et al. (2001), ar- senic will not be released from the iron min- erals without the presence of organic matter Vietnam Journal of Earth Sciences, 40(1), 70-77  76 to drive microbial reduction. This is also the case for the B-B’ transect. From the concen- trations of DOC and CH4 in Figure 5a and 5b, which are indicators for the contents of organic matters, one can also see that the or- ganic matters at Xuan Phu and Van Phuc were higher than at Cam Yen and Phuc Hoa. And the higher concentrations of DOC and CH4 corresponded with higher arsenic con-centrations. Figure 5b. Occurrence of arsenic and other reducing condition instruction compounds in groundwater at Xuan Phu - Van Phuc 4. Conclusions The arsenic concentrations of the B-B’ transect vary in a wide range: from <5 to 115 µg/L, with the lowest average concentration at Cam Yen and the highest one at Van Phuc. The As release process depends on many fac- tors such as the extent of the reducing condi- tions, the sediment age, bacteria activities, etc. From the results, one can predict the ten- dency of As that the As concentration will in- crease while the distance to river decrease. The place closer to the mountain part will Dao Viet Nga, et al./Vietnam Journal of Earth Sciences 40 (2018) 77 have lower concentration, whereas the high concentration will be found at the place closer to the river. The As concentration is also pro- portional to the depth. The deeper the well is, the higher As concentration it has. Acknowledgements This study was done at the VSL Laborato- ry (VNU-Shimadzu join laboratory for analyt- ical sciences) at the Research center for Envi- ronmental Technology and Sustainable De- velopment (CETASD), as part of the research project “Predicting the arsenic content in groundwater of the floodplains in Southeast Asia”, co-operated between VNU University of Science, Hanoi University of Mining and Geology (HUMG) and Geological Survey of Denmark and Greenland (GEUS). The authors would like to thank also the European Union (EU) for their financial support. References Anawar H.M., Akai J., Sakugawa H., Sakugawa H., 2004. 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