Chemical characteristics of groundwater in southeastern Hanoi

Abstract. To examine the occurrence and distribution of arsenic in groundwater, 137samples were collected from private wells in Southeastern Hanoi. According to the concentrations of major cations and anions, groundwater compositions vary from Ca-HCO3 to Na-HCO3 type. Groundwater has a very large range of chemical compositions and the spatial variability of arsenic concentrations even between samples collected within a few km of each other. Arsenic was detected in 103 out of 137 groundwater samples with high spatial variation, ranged from less than 5 to 549 µg/L (MDL = 2 µg/L, average 56.3 µg/L). Total Fe concentration varies over a wide range, from below quantity limit (0.25 mg/L) up to 49.4 mg/L (average 9.7 mg/L). Manganese concentration ranged from less than 0.1 to 8.1 mg/L with an average of 0.8 mg/L. Approximately 58%, 44% and 34% of samples exceed the Vietnam National technical regulation on groundwater quality for Fe, Mn and As, respectively. Arsenic showed poor correlations with other compositions in groundwater even with the parameters which are common in reducing conditions and relate to releasing of arsenic. These finding showed that more deep studies need to be carried out to have better understanding about arsenic occurrence in groundwater.

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44 HNUE JOURNAL OF SCIENCE DOI: 10.18173/2354-1059.2017-0053 Chemical and Biological Science 2017, Vol. 62, Issue 10, pp. 44-51 This paper is available online at CHEMICAL CHARACTERISTICS OF GROUNDWATER IN SOUTHEASTERN HANOI Tran Thi Mai 1 , Pham Thi Kim Trang 1 , Vi Thi Mai Lan 1 , Vu Thi Duyen 1 , Dao Viet Nga 1 , Dao Manh Phu 2 , Bui Van Minh 1 , Pham Hung Viet 1 and Benjamin C. Bostick 3 1 Research Center for Environmental Technology and Sustainable Development (CETASD), University of Science, Vietnam National University 2 Vietnam Environment Administration, Ministry of Natural Resources and Environment 3 Lamont-Doherty Earth Observatory (LDEO), Columbia University, Palisades, New York 10964, USA Abstract. To examine the occurrence and distribution of arsenic in groundwater, 137samples were collected from private wells in Southeastern Hanoi. According to the concentrations of major cations and anions, groundwater compositions vary from Ca-HCO3 to Na-HCO3 type. Groundwater has a very large range of chemical compositions and the spatial variability of arsenic concentrations even between samples collected within a few km of each other. Arsenic was detected in 103 out of 137 groundwater samples with high spatial variation, ranged from less than 5 to 549 µg/L (MDL = 2 µg/L, average 56.3 µg/L). Total Fe concentration varies over a wide range, from below quantity limit (0.25 mg/L) up to 49.4 mg/L (average 9.7 mg/L). Manganese concentration ranged from less than 0.1 to 8.1 mg/L with an average of 0.8 mg/L. Approximately 58%, 44% and 34% of samples exceed the Vietnam National technical regulation on groundwater quality for Fe, Mn and As, respectively. Arsenic showed poor correlations with other compositions in groundwater even with the parameters which are common in reducing conditions and relate to releasing of arsenic. These finding showed that more deep studies need to be carried out to have better understanding about arsenic occurrence in groundwater. Keywords: Arsenic, groundwater, chemical characteristic, Southeastern Hanoi. 1. Introduction The presence of trace elements in groundwater is an important issue because it affects possible use of water. Particularly, arsenic contamination of groundwater that used for drinking is major concern to human health due to its neurotoxicity and potential to cause cardiovascular diseases and different types of cancer, of which skin and bladder cancers are most common [6]. Natural As in groundwater at concentration above the World Health Organization Maximum Contaminant Level (WHO, 2004) of 10 µg/L have been Received November 22, 2017. Revised December 9, 2017. Accepted December 16, 2017. Contact Pham Hung Viet, e-mail address: phamhungviet@hus.edu.vn Chemical characteristics of groundwater in Southeastern Hanoi 45 reported in many countries such as Bangladesh, India, Vietnam, China, Argentina, Chile, Mexico, the US and Spain [1] with at least 150 million people are directly affected by As contamination all over the world [6]. There are several hypotheses to explain how arsenic released from sediment into the groundwater. Most research suggests that arsenic initially is present as Arsenate (As V) within the Fe-oxide crystal lattice and during microbial or/and chemical reductive dissolution of the Fe-oxide, arsenic are released in arsenite (As III) ion states [3]. An alternative, but somewhat complementary release mechanism suggested by some researchers is that arsenic released by competition the absorbed sites on the Fe-oxide with other ions such as HCO3 – , PO4 3 – [8], many of which are created as byproducts of the reductive dissolution of iron oxides by microorganisms. In reality, both of these mechanisms could be important but separated in time and space, because groundwater travels through sediments in aquifers, and thus separates dissolved arsenic, phosphate and bicarbonate from their sources. The composition and environmental condition of sediment and groundwater is heterogeneous, and, combination with the mobility of dissolved As, this leads to the complicated distribution of arsenic in groundwater. Within the Red river delta in Vietnam the spatial variability of arsenic concentrations is considerable over distances of a few km, next to the location has low arsenic concentration (less than 5 µg/L) is the location has very high arsenic level (~700 µg/L) [5]. Therefore, it is necessary to have detail studies in specific areas to access groundwater quality, especially the arsenic distribution in groundwater, as a basis to evaluate and avoid the impact of contaminated water on human health. The main objectives of this study were to characterized the chemistry of groundwater and investigated the extent of natural As distribution and the correlation between As and other compositions in groundwater in Southeastern Hanoi, and to use this information to understand in greater detail what controls the spatial distribution of arsenic, and its evolution. 2. Content 2.1. Materials and methods 2.1.1. Sample collection A total of 137 samples private wells were sampled during 2013 - 2014 over an area of about 200 km 2 in Southeastern Hanoi, included some districts: Dong Da, Thanh Xuan, Hai Ba Trung, Hoang Mai, Thanh Tri, Thanh Oai, Long Bien, Gia Lam - Hanoi city and Van Giang district - Hung Yen province. Samples were collected from random private wells with the density of one sample per about 2 km 2 . The private wells screened at various depths from 7 to 80 m, but most were screened at 30 – 40 m. Samples were taken after 5 - 15 minutes pumping, when the dissolved oxygen concentration (DO) and electrical conductivity (EC) in the water reached stable values. Water temperature (T), pH and redox potential were also recorded on-site by HACH and Mettler-Toledo equipment. T. T. Mai, P. T. K. Trang, V. T. M. Lan, V. T. Duyen, D. V. Nga, D. M. Phu, B. V. Minh, P. H. Viet and B. C. Bostick 46 Figure 1. The investigated area in Southeastern Hanoi (The circles show the well locations and As concentration depicted by color and size) Groundwater was filtered (0.45 µm cellulose acetate filters) on-site in two polypropylene bottles. One bottle for the analysis of metals, ammonium and phosphate was acidified with concentrated nitric acid to reach a pH < 2. Dissolved organic carbon (DOC) samples were filtered with nylon 0.2 µm filters 24 ml glass vials (washed and baked at 500 o C in 6 hours before sampling) and acidified with concentrated hydrochloric acid to pH < 2. All samples were stored in a refrigerator at 4 o C until laboratory analyses. 2.1.2. Chemical analysis Metal elements were analyzed by flame atomic absorption spectrophotometer on a Shimadzu AAS 6800 instrument. Total arsenic was determined on the same instrument using HVG hydride generator connected to AAS 6800. Determination of total arsenic requires that all inorganic arsenic compounds be in the As (III) state. Therefore, As (V) in groundwater was reduced to As (III) with sodium iodide before reaction with sodium borohydride to produce arsenic hydride AsH3 by HVG. The minimum detection limit for theses analyses is 2 µg/L for As, and similar values for other elements. Accuracy was tested by analyzing ICP Multi element standard IV (1.11355 - Merck), ICP Multi element standard IX (1.09494 - Merck), IC multi element standard I (1.70398 - Merck). Replicate analyses were carried out to check precision of the results; accuracy and precision were within the range of ±10% and relative standard deviation (RSD) less than 5%. Major anions like chloride (Cl – ), sulfate (SO4 2– ), nitrate (NO3 – ) and other anions were determined by ion chromatography using a Shimadzu LC20AD/HIC-20ASuper. Groundwater samples were PTFE 0.2 µm filtered before analyzed by instrument. While NH4 + , SiO3 – and PO4 3- , were determined by UV-VIs (Shimadzu UV-3101) using respectively the nitroprusside, the ammonium molybdate and ascorbic acid methods. The minimum detection limit for theses analyses is 5 - 10 ppb for most anions. Chemical characteristics of groundwater in Southeastern Hanoi 47 DOC concentration was analyzed on Total organic carbon analyzer (Shimadzu, TOC-VCSH). The principle of DOC analysis is determined carbon dioxide generated by 680 o C combustion catalytic oxidation after acidified and sparged to remove inorganic carbon and purge-able organic carbon. This method required blank subtraction, but resulted in stable and reproducible DOC concentrations when DOC exceeded ~0.1 mg/L (as it did for all samples). 2.2. Results and discussion 2.2.1. Chemical characteristic of groundwater Groundwater in studied area is about neutral; with pH ranging from 6.1 to 7.6. Arsenic levels were usually higher in groundwater with pH of 7 or greater. Temperature is around 26 o C and specific conductivity varies from 147 to 2040 µS/cm (average 569.5 µS/cm). Field measured Eh values ranged from -169.5 to +160.8 mV. Almost 70% investigated wells have reduced Eh values, indicated reducing conditions that is favorable for process of release arsenic from sediment into groundwater and 30% studied wells have oxidizing conditions. According to the concentrations of major cations and anions, the groundwater composition varies from Ca-HCO3 to Na-HCO3 type (Fig.2), as a results of alumino- silicate weathering with carbonic acid, as well as dissolution of carbonate minerals, followed by Ca and/or Mg ion exchange with Na on clay minerals [8]. The SiO3 - concentration ranges from 6 to 32.1 mg/L, but most were 25 - 30 mg/L, near the solubility limit for most SiO2 polymorphs. These concentrations reflect dissolution of silicate minerals in the aquifers, followed by re-precipitation of secondary phases. Figure 2. Piper diagram consisting of major ions in groundwater in Southeastern Hanoi SO4 2– , NO3 – and Cl – may trace anthropogenic input to groundwater in agriculture areas, or be derived from sewage. However, in most of the wells, SO4 2– , NO3 – and Cl – exhibit relatively low concentrations. 53% and 94% of the samples contained less than 0.2 mg/L of SO4 2– and NO3 – , respectively, while Cl – concentrations range from 1.4 to 180.4 mg/L (average 36.4 mg/L) with a medium value of 19.9 mg/L. These low levels suggest T. T. Mai, P. T. K. Trang, V. T. M. Lan, V. T. Duyen, D. V. Nga, D. M. Phu, B. V. Minh, P. H. Viet and B. C. Bostick 48 that anthropogenic inputs are not significant in the area. Alternatively, the SO4 2– , NO3 – could be removed by reduction in reducing waters, but the salty solutions would like persist, resulting in variable Na and Cl - concentrations. Given the occasionally higher Cl - levels found in the environment, it is possible that some of these components have entered groundwater but not been preserved. Total Fe concentration varies over a wide range, from below quantity limit (0.25 mg/L) up to 49.4 mg/L (average 9.7 mg/L). The generally high concentrations of Fe found in most wells indicate that most of the waters are reducing. Manganese concentration ranged from less than 0.1 to 8.1 mg/L with an average of 0.8 mg/L. Almost 58% and 44% of samples exceed 5 mg/L Fe and 0.5 mg/L Mn, respectively, which are the Vietnam National technical regulation on groundwater quality (QCVN 09-MT:2015/BTNMT). These regulations are based on the suitability of water for use rather than health impacts, but indicate that the water is often sufficiently metallic as to have problems with metallic taste and will change the color of food such as rice prepared in untreated groundwater. 2.2.2. Arsenic in the groundwater and the correlation between As and other chemical parameters The spatial distribution of arsenic in groundwater in Southeastern Hanoi is illustrated in Fig.1. High arsenic concentration wells are mainly in Thanh Tri and Hoang Mai district (center of the map in Fig. 1 and Fig. 3a), those areas could be contaminated due to arsenic mobilization caused by extensive pumping in the water supply plants. Arsenic was detected in 103 out of 137 samples from study area with concentration ranging from less than 5 to 549 µg/L (average 56.3 µg/L). Approximately 34% of the examined wells exhibited arsenic concentration higher than 50 µg/L, which is the Vietnam National technical regulation on groundwater quality. More than half of wells that contained measurable As exceeded 10 µg/L (about 43% of all wells), which is WHO guideline. Several wells have high As concentrations (on the right side of Red river in Fig.3a, b), showed relatively high values of Fe concentrations. However, in most of the wells showed As concentrations are not well correlated with the concentration of Fe (r = 0.13, p = 0.129), this is not the same with what is generally expected in case of As release from reductive dissolution of Fe oxide in such reducing environments [9, 10], which implies additional mechanisms affect either Fe levels, or As levels. The wells had lowest Fe concentrations; also showed the highest pH values and contrast in other wells. These observations may be relate to an increase of the net Fe precipitate rate, when pH increases to the neutral range, resulting in lower Fe concentration in solution [8], even when reduction can release As into solution. From the redox perspective if conditions are reducing enough to release As, they are certainly reducing enough to release Mn, therefore when As concentration has high value, Mn also has high value of concentration. However, the groundwater data in this study reveals that sites where As concentrations is high (from 100 to 549 µg/L), also have relatively low Mn concentrations (less than 0.1 to 1.6 mg/L) (Fig. 3a, c). This contrast can be explained that Mn re-precipitated with dissolved carbonate which is created from biodegradation organic matter, resulting decrease Mn concentration in groundwater [7]. Indeed, HCO3 - concentrations showed relatively high values, ranged from 65 to 1310 mg/L (average 327 mg/L). Chemical characteristics of groundwater in Southeastern Hanoi 49 Figure 3. Spatial distribution of As (a), Fe (b), Mn (c), NH4-N (d) in groundwater in Southeastern Hanoi (the well depths range from 7 to 60m) The high NH4-N concentration of some studied wells (Fig. 3d) is explained to be derived as the product of microbial decomposition of organic matter or the ammonification of organic nitrogen in peat therefore NH4-N is the factor express reducing condition and normally has a positive correlation with As concentration in groundwater [4]. However, in this study, the NH4-N concentration was not significantly correlated with As level (r = 0.142, p = 0.09), which is not similar with some previous researches in Bangladesh, Nepal and Vietnam [4]. The detail role of NH4-N concentration in increasing As concentration cannot be clarified from the chemical parameters in this study, and must be examined further. We tentatively suggest that the ammonium is retained in sediments through exchange, or that most of the waters have traveled from their sources, resulting in the redistribution of arsenic and ammonium during transport, and the apparent decoupling of As levels. For example, at the source, arsenic and ammonium levels are correlated in solution because of reductive dissolution, but the arsenic is retarded by interacting with the sediment (retardation factor of 10-15, based on a partition coefficient of 10 - 20, van Geen et al., 2013), while ammonium is both taken up as a nutrient by plants through geological window, and travels with a different retardation factor. Thus, at a given distance away, the two parameters can be either positively or negatively correlated even though they are mechanistically related. T. T. Mai, P. T. K. Trang, V. T. M. Lan, V. T. Duyen, D. V. Nga, D. M. Phu, B. V. Minh, P. H. Viet and B. C. Bostick 50 Table 1. Pearson correlation coefficients AsT FeT Mn 2+ NH4-N HCO3 - NO2 - SO4 2- NO3 - AsT 1 FeT 0.130 1 Mn 2+ -0.137 -0.207 1 NH4-N 0.142 0.075 -0.176 1 HCO3 - 0.240 0.215 -0.127 0.685 1 NO2 - -0.103 0.091 -0.023 -0.074 -0.033 1 SO4 2- -0.208 0.017 0.257 -0.143 0.097 0.198 1 NO3 - -0.085 -0.135 -0.054 -0.055 0.026 0.438 0.309 1 Pearson correlation coefficients have been calculated to examine possible relationships among the measured parameters (Table 1). Arsenic exhibited weak correlation with HCO3 – (r = 0.24). Although, the relationship between arsenic and bicarbonate are well known that bicarbonate can play an important role in the mobilization of arsenic through the competition for adsorption sites [1, 2]. Weak negative correlations of arsenic with NO3 - , SO4 2- and NO2 – are showed by minus correlation coefficients. One caveat in examining correlations is that the correlations assume linear forms, something limiting. In the case of sulfate, this is extreme. Sulfate concentrations are nearly universally low (a small dynamic range) but variable in high As wells, and when sulfate is high the As is universally low. Thus, this inverse relationship is obvious, but it is not well fit by a line, and thus results in a low quality fit with negative correlation when regressed onto a line. 3. Conclusion The groundwater composition varies from Ca-HCO3 to Na-HCO3 type. Groundwater has a very large range of chemical compositions and the spatial variability of arsenic concentrations is considerable over distances of a few km. Arsenic was ranged from less than 5 to 549 µg/L (average 56.3 µg/L) with spatial distribution are possible affected by extensive pumping by water supply plants. Approximately 58%, 44% and 34% of samples exceed the Vietnam National technical regulation on groundwater quality for Fe, Mn and As, respectively. Arsenic concentration showed weak correlations with other compositions in groundwater even with the parameters which are common in reducing condition, reflecting that secondary processes are occurring that also affect the levels of As, Fe and other elements, and also suggesting that the arsenic may have traveled from its source, which weakens or changes correlations spatially during travel. Need to be examined further on sediment in the studied area to understand these correlations. Acknowledgements. The authors acknowledge the valuable support through the PEER-based Research Project Grant 544 funded by US-AID/NSF (2013 - 2015) named "Evaluating the Sustainability of Ground Water Resources: Academic and Scientific Gaps". Chemical characteristics of groundwater in Southeastern Hanoi 51 REFERENCES [1] A. Kouras, I. Katsoyiannis, D. Voutsa, 2007. Distribution of arsenic in groundwater in the area of Chalkidiki, Northern Greece. Journal of Hazardous Materials, 147, pp. 890-899. [2] D. Postma, F. Larsen, N.T.M Hue, M.T Duc, P.H Viet, P.Q Nhan, S. Jessen, 2007. Arsenic in groundwater of the Red River floodplain, Vietnam: Controlling geochemical processes and reactive transport modeling. Geochimica et Cosmochimica Acta, 71, pp. 5054-5071. [3] D. Postma S. Jessen, N.T.M. Hue, M.T. Duc, C.B. Koch, P.H. Viet, P.Q. Nhan, F. Larsen, 2010. Mobilization of arsenic and iron from Red River floodplain sediments, Vietnam. Geochimica et Cosmochimica Acta, 74, pp. 3367-3381. [4] K. Kurosawa, K. Egashira, M. Tani, 2013. Relationship of arsenic concentration with ammonium–nitrogen concentration, oxidation reduction potential and pH of groundwater in arsenic-contaminated areas in Asia. Physics and Ch
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