Heavy metals contamination in groundwater resource in Dak Nong province, Viet Nam

Abstract. This study aims to understand the concentrations of selected heavy metals including arsenic, iron, manganese, cadmium and mercury in groundwater used for domestic purposes in Dak Nong Province of Viet Nam. A total number of 572 groundwater samples including 61 duplicates were collected during October, 2018 – April, 2019. The collected samples were measured onsite for pH and preserved with 2 % HNO3 until pH < 2 before being analyzed in the laboratory for heavy metals by inductively coupled plasma mass spectrometry. The relative percent differences of all duplicate samples were less than 30 %, indicating that the analytical results were within acceptable ranges. The average concentrations of As, Fe, Mn, Cd, Hg and Pb in groundwater were 9.53 ± 46.36, 307.1 ± 1,178, 166 ± 478, 4.66 ± 6.2, 1.0 ± 1.8 µg/L and not detected, respectively. Within the analytical results, however, some samples had heavy metals concentrations exceeding the QCVN 09-MT:2015/BTNMT Vietnamese regulation values for groundwater quality and for domestic use requirements. The research results therefore suggest that groundwater containing rather high concentrations of heavy metals at the studied area should not readily be used for domestic purposes without proper treatments.

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Vietnam Journal of Science and Technology 58 (3A) (2020) 42-49 doi:10.15625/2525-2518/58/3A/14247 42 HEAVY METALS CONTAMINATION IN GROUNDWATER RESOURCE IN DAK NONG PROVINCE, VIET NAM Tran Tuan Viet 1, * , Trinh Thi Anh Nguyet 1 , Tran Ai Quoc 1, 3 , Ta Duc Trung 1 , Ho Nguyen Huy 1 , H Wien Nie 1, 3 , Tran Quoc Minh 2 , Pham Hong Nhat 3 1 Institute for Tropical Technology and Environmental Protection, 57A Truong Quoc Dung St., Phu Nhuan Dist., Hochiminh City, Viet Nam 2 Industrial University of Hochiminh City, 12 Nguyen Van Bao St., Go Vap Dist., Hochiminh City, Viet Nam 3 Hochiminh City University of Food Industry, 140 Le Trong Tan St., Tan Phu Dist., Hochiminh City, Viet Nam * Email: viet.vittep@gmail.com Received: 18 August 2019; Accepted for publication: 15 April 2020 Abstract. This study aims to understand the concentrations of selected heavy metals including arsenic, iron, manganese, cadmium and mercury in groundwater used for domestic purposes in Dak Nong Province of Viet Nam. A total number of 572 groundwater samples including 61 duplicates were collected during October, 2018 – April, 2019. The collected samples were measured onsite for pH and preserved with 2 % HNO3 until pH < 2 before being analyzed in the laboratory for heavy metals by inductively coupled plasma mass spectrometry. The relative percent differences of all duplicate samples were less than 30 %, indicating that the analytical results were within acceptable ranges. The average concentrations of As, Fe, Mn, Cd, Hg and Pb in groundwater were 9.53 ± 46.36, 307.1 ± 1,178, 166 ± 478, 4.66 ± 6.2, 1.0 ± 1.8 µg/L and not detected, respectively. Within the analytical results, however, some samples had heavy metals concentrations exceeding the QCVN 09-MT:2015/BTNMT Vietnamese regulation values for groundwater quality and for domestic use requirements. The research results therefore suggest that groundwater containing rather high concentrations of heavy metals at the studied area should not readily be used for domestic purposes without proper treatments. Keywords: heavy metals, ground water, Dak Nong. Classification numbers: 3.3.1, 3.4.2, 3.6.2. 1. INTRODUCTION Heavy metals are toxic and considered to cause many problems for human health and environment [1]. Among them, arsenic (As) is a toxic metal present in environment and biological systems. As is known to be carcinogenic to human (of class A), causing cancer to Trace metals analysis of marine benthos certified reference materials 43 skin, lung and bladder [2]. The World Health Organization (WHO) suggests that As concentration in drinking water should not exceed 10 µg/L [3]. In India, China, Myanmar, Pakistan, Nepal, Cambodia and Viet Nam, about 100 million people are estimated to be exposed to unsafe level of As in drinking water [4]. In the recent years, high concentrations of As in groundwater have been found in some areas in Viet Nam [5, 6]. As reported, about 13 million Vietnamese people are using groundwater directly from tube-wells for domestic purposes [7]. In Dak Nong province of Viet Nam, As contamination in groundwater was reported in 2008, that 374 samples were determined for As using quick test kits (Merck) which was found to range between 0.25 and 85 µg/L [8]. After this report, however, the monitoring program of Dak Nong showed that no ground water samples in 31 monitoring wells had been reported to be contaminated with As. In 2017, the Preventive Medicine Center of Dak Nong discovered that the high As concentrations in not only treated groundwater from a water supply station but also in untreated groundwater samples around Krong No area. As mainly comes from minerals, soil, sediment and rocks. It has been found as a main constituent in over 200 minerals, including elemental arsenic, arsenides, sulphides, oxides, arsenates and arsenites. Normally, As can easily be released into ground water from those sources with other metals [9]. In many previous studies, other metals considered to be studied with As included Mn, Ba, Se, Fe, V, Cr, Co, Cu, Zn, Ga, Rb, Sr, Mo, Ag, Cd, In, Sn, Sb, Cs, Tl, Pb, Hg and Bi [10, 11]. This study aims to understand the As and other heavy metals including Fe, Mn, Cd, Pb and Hg contamination in groundwater in Dak Nong province, Viet Nam. The risk of health for local consumers, who have been using Arsenic contaminated groundwater as drinking water, were also estimated by hazard quotient. 2. MATERIALS AND METHODS 2.1. Methods for sampling and analysis A total of 572 ground water samples and 61 duplicate samples were collected randomly in all seven districts and one town of Dak Nong province following the guidance on sampling groundwater TCVN 6663-11:2011. The general information including well depth and capacity of each well was supported by well’s owner. The samples were measured of pH using pH meter (Horiba U52) on-site and then preserved by 2% HNO3 (Merck, Suprapur®) until pH<2 before transferring to laboratory. The samples were then analyzed for As, Fe, Mn, Cd, Pb and Hg using an inductively coupled plasma mass spectrometry (Agilent ICP-MS) following method 3125B (SMEWW) [12]. The method detection limits for As Fe, Mn, Cd, Hg, and Pb were 0.07, 1.5, 2.0, 1.0, 0.2 and 1.5 µg/L, respectively. Results of the 61 duplicate samples were used to calculate relative percent difference (RPD) values as below: | | where LD1 is sample result and LD2 is duplicate result. The average of RPD for As Fe, Mn and Hg were 8.4 ± 7.6 %, and 12.9 ± 7.6 %, 9.6 ± 7.3, and 6.8 ± 9.2 %, respectively (Cd and Pb results of all replicate samples were not detected). Moreover, all of RPD values were controlled under 30 % indicating high precision. Tran Tuan Viet, et al. 44 2.2. Human health risk assessment Hazard quotient (HQ) was used to estimate the local people’s health risk for exposure to As contaminated water. The HQ was suggested by U.S.EPA [13] and calculated by the equation as follows: where: bw is body weight (kg). The average body weight of local people was used in this study was 60 kg, which is similar with Asian body weight 57.7 kg [14]; EDI is the estimated daily intake (µg/kg_bw/day); C is As concentration in drinking water (µg/L); dc is daily water consumption = 2 L/capita/day; EF is exposure frequency (day/year) (assuming that local people use only one source of ground water for drinking during their life time); ED is exposure duration (year) (HQ was calculated for only adults which are from 16 year-old); AT is average time (day), Life time = 27,959 days (76.6 years), the average life time of Vietnamese people [15]; RfD = 0.3 µg/kg_bw/day is reference dose for arsenic [16]. U.S.EPA. suggests that a HQ value greater than 1 indicates potential health risk for human and greater than 10 indicates significant chronic risk. 3. RESULTS AND DISCUSSION Analytical results of all studied metals are showed in Table 1 and Fig. 1. According to the results, only Pb has not been detected in all samples. Cd content ranged from ND to 14 µg/L, and all values met the limit for drinking water (30 µg/L) and for ground water (50 µg/L). Figure 1. Metals concentrations in the studied ground water samples at Dak Nong. Mercury was detected in 79 ground samples with concentrations ranging between 0.2 µg/L and 12.3 µg/L. Of these, 17 samples were found to have Hg concentrations exceeding the limit for drinking water (1 µg/L). The majority of high Hg concentration samples were found in Krong No (15 samples), while the remaining in Gia Nghia and Dak Glong. The concentrations of Fe and Mn ranged between ND-14,511 µg/L and ND-4,124 µg/L, respectively. The number of samples having Mn concentrations higher than the drinking water limit (100 µg/L) was 104 while 72 samples had exceeding concentrations in comparison with ground water quality limit (300 µg/L). Moreover, about 50 % of those samples were in Krong No district. According to the results, high As concentrations which exceeded the drinking water limit of 10 µg/L were found in Krong No, Cu Jut, Dak Glong, Dak Mil and Gia Nghia (Figs. 2 and 3). Among them, nine samples had As concentrations over 50 µg/L (Vietnamese limit for ground Trace metals analysis of marine benthos certified reference materials 45 water quality). Two highest concentrations, 504.0 µg/L and 592.5 µg/L, were found in two nearby tube-wells at Krong No. Figure 2. As concentrations in ground water samples at Dak Nong. Table 1. The metals concentrations in ground water at Dak Nong. District Metal As (LOD = 0.07) µg/L Fe (LOD = 1.5) µg/L Mn (LOD = 2.0) µg/L Cd (LOD = 1.0) µg/L Hg (LOD = 0.2) µg/L Pb (LOD = 1.5) µg/L Dak R’lap (n = 32) Mean±SD 0.93 ± 0.98 38.7 ± 45.2 24 ± 28 ND 0.3 ND Range ND-2.05 ND-141.8 ND-132 ND ND-0.3 ND Tuy Duc (n = 29) Mean ± SD 0.56 47.7 ± 50.6 55 ± 77 2.0 0.4 ND Range ND-0.56 ND-169.1 ND-275 ND-2.0 ND-0.4 ND Dak Song (n = 57) Mean ± SD 0.73 ± 1.02 60.5 ± 102.2 49 ± 136 ND ND ND Range ND-2.79 ND-647.9 ND-844 ND ND ND Dak Mil (n = 49) Mean ± SD 5.02 ± 8.48 304.7 ± 1257 30 ± 56 ND 0.5 ± 0.3 ND Range ND-31.01 ND-7984 ND-219 ND ND-0.8 ND Cu Jut (n = 113) Mean ± SD 4.22 ± 7.15 170.0 ± 358.5 60 ± 142 14 0.4 ± 0.2 ND Range ND-37.78 ND-2090 ND-1131 ND-14 ND-0.6 ND Krong No (n = 149) Mean ± SD 15.59 ± 72.99 367.8 ± 1110 413 ± 766 1.0 1.3 ± 2.1 ND Range ND-592.53 ND-11042 ND-4124 ND-1.0 ND-12.3 ND Dak G’Long (n = 92) Mean ± SD 9.33 ± 25.63 666.6 ± 2091 85 ± 301 ND 0.4 ± 0.4 ND Range ND-157.82 ND-14511 ND-2710 ND ND-1.5 ND Gia Nghia (n = 51) Mean ± SD 7.85 ± 16.58 260.4 ± 1234 46 ± 69 ND 1.7 ± 1.3 ND Range ND-66.64 ND-8661 ND-386 ND ND-2.6 ND Note: ND = Not detected; SD = Standard Deviation; LOD = Limit of Detection. Tran Tuan Viet, et al. 46 According to the result of this study, the highest concentrations of heavy metals in groundwater samples were found at Krong No. Those metals might be dissolved into ground water from soil, mineral, sediment and/or rock [9]. Similar results from other studies also showed that high concentrations of other metals such as Fe and Mn were found with high As concentrations [17–19]. pH values in ground water at Dak Nong ranged from 3.9 to 9.1, with a mean value of 6.6 ± 0.9. This showed that most of ground water samples in the studied area were in neutral condition, while some in light oxidizing and some in reducing conditions. The results in Fig. 4a show that, most of high As concentration samples were in neutral condition. Thus, the species of As in the studied ground water samples might include As(V) in H2AsO4 - or HAsO4 2- and As(III) in H3AsO3 0 [9]. Figure 3. As concentrations in ground water samples at Dak Nong. The high As concentrations presented in both dug-wells (normally less than 10 m depth, some are deeper but not exceeding 30 m) and tube-wells (normally greater than 10 m) at Dak Nong. Most samples with As concentrations greater than 10 µg/L were found in tube-wells with the depth ranging from 40 to 100 m (Fig. 4b). Similar results were also reported in Lam Dong province of Viet Nam [20] and Pakistan [21]. Figure 4. The relationship between As and pH in ground water (a) and As and well depth (b). The As was detected in total 306 samples in Dak Nong. In case the local people use ground water directly for drinking or without proper treatment, they could be at significant chronic risks Trace metals analysis of marine benthos certified reference materials 47 if they use 10 wells with high As concentration. In details, if the local people drink water from those wells for over 16 years, they will face to the health risk with HQ results greater than 1. For older people or longer exposure time, the HQ values will be greater. In case of 76 years exposure, there are 43 HQs greater than 1 (Fig. 5) indicating that the local people drink water from 43 wells in Dak Nong might be affected by their heath. Likewise, some other areas in Viet Nam also face similar situation including, for instance, the Red River delta [5, 6], Mekong Delta [17], Cat Tien [22] and Đạ Teh [23] in Lam Dong province. Figure 5. The hazard quotients for local As contamination ground water consumers. 4. CONCLUSIONS As and some other heavy metals were found with high concentrations in Dak Nong province which exceed the Vietnamese regulation values for groundwater quality and for domestic use requirements. Within the province, Krong No is the district where the majority of wells with high metal concentrations. Most of high As concentration samples were in 40 m-100 m depth tube-wells and some in 10 m-30 m dug-wells and all of them in neutral condition. 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