Potential Health Risk Assessment for the Occurrence of Heavy Metals in Rice field Influenced by Landfill Activity in Can Tho City, Vietnam

— The study was conducted to assess potential risk of heavy metals in the soil and rice plants in the ricefield around the landfill in Dong Thang commune, Co Do district, Can Tho city, Vietnam. Four soil samples in which three samples were collected around the landfill and one sample was collected one km away from the landfill for the analysis of heavy metals including Mn, Zn, Cu, Cr, Ni, Pb and Cd. Rice samples were collected during ripening stage (few days before the harvest) at the same locations with the soil sampling, for the same heavy metal species analysis. The findings revealed that six out of seven heavy metals occurred in the soil.The decreasing order of theheavy metals concentrationsin the soil samples was Mn > Zn> Ni> Cr> Cu> Pb. This study found that accumulation of heavy metals in parts of rice at S1-S3 was higher that those at S4 (except for Zn and Pb at rice roots) and decreased in the order Mn> Zn> Cu> Ni> Cr (except in rice grain, Cr> Cu> Ni). Heavy metalsgenerally in the rice partswere in the magnitude order of root > stem-leave> grain. The calculated hazard index (HI) indicated that the accumulation of heavy metals in soil and rice grain is not likely to pose a threat to public health (HI <1), however, potential health and ecological risk may still exist. Measures should be taken to prevent landfill leachate leaching into the agricultural areas to minimize potential environmental and health risks.


INTRODUCTION
Vietnam has recently been facing serious environmental pollution from solid wastes as the amounts of generated wasteshave been increasing in both quantity and toxic level. According to the National Environmental Report 2011-2015 (MONRE, 2015), the total amount of urban domestic solid waste generated in the country was 32,000 tons in 2014. The amount of solid waste generated in the Mekong Delta region accounted for 5% of the generation of the whole country. Can Tho city is generatingsolid wastes of approximate 893 tons day -1 (People's Committee of Can Tho City, 2015). Solid wastes have been collected and treated at landfills. However, landfills have also been identified as a cause of soil and groundwater pollution (Fatta et al., 1999). According to MONRE (2015), only 203 out of 660 landfills across the country are sanitary landfills, and the remaining were unsanitary. However, the majority of landfills have been overloaded, exacerbating the environmental impacts, which has led to increasingly serious and complex pollution problem in the landfilling areas.
The landfill at Dong Thang Commune, Co Do District, Can Tho City, Vietnam is in a state of serious overload due to receiving a fairly large amount of waste approximate 370 tons per day -1 from several districts of Can Tho city. The untreated leachate has significantly affected water quality, soil and rice yield in the land adjacent to the landfill (Nhien and Giao, 2019). Leachate not only contains high levels of organic matter, nitrogen but also significant concentrations of heavy metals, so it may cause pollution of soil and surface water

II. MATERIALS AND METHODS 1) Soil sampling and analysis
Soil samples were collected at the depth of 0-25 cm at 4 locations, of which 3 locations in the rice field surrounding the landfill (namely S1, S2, S3) and 1 locationin the ricefield1km away from the landfill (namely S4) (Figure 1). After the collection, the soil samples were dried at room temperature, pulverized and sieved through mesh with the pore size of 0.5 mm for heavy metal analysis. The pulverized soil sample (0.5g) was digested using a microwave digester (Microwave digester, Milestone, Ethos) using the method of the United State Environmental Protection Agency (EPA3051) by adding 10 ml of 65% nitric acid and operated at 1,000 watts of power, temperature of 175 o C for 15 minutes 30 seconds. Heavy metals including Cd, Cr, Cu, Fe, Ni, Mn, Pb and Zn were determined by atomic absorption spectrometry (AAS, Agilent, AA240). All glasswares used in heavy metal analysis were cleaned washed using 0.1 M nitric acid for 24 hours and then rinsed with distilled water. Analysis of heavy metals was performed in triplicates.

2) Rice sample collection and analysis for heavy metals
Rice samples were collected during ripening stage (few days before the harvest) at the same locations with the soil sampling ( Figure 1). Five whole rice plants were carefully removed from soil at five positions in an area of 1 m 2 for every sampling location. The collected rice plants were divided into three parts including the root, stem and leave, and grain. The separated parts of the rice plants at three locations surrounding landfill (S1, S2, and S3) were pooled to reduce the analysis cost due to limited budget. The heavy metals including Cd, Cr, Cu, Ni, Mn, Pb and Zn were analyzed in the rice parts. The procedure for analyzing heavy metals in rice samples was performed in the similar manner to that for analyzing soil samples.

3) Risk assessment
Hazard index (HI) due to heavy metals in soil and rice (grain) was assessed according to Where: Cs is concentration of pollutants in soil or rice (mg/kg); IngR is the rate of ingestion of pollutants in soil or rice (mg day -1 ); InhR is inhalation rate of suspended particles in soil (m 3 day -1 ); EF and ED are frequency of exposure (dayyear -1 ) and duration of exposure (years); CF is conversion factor = 1.00E-06 (kg mg -1 ) and , BW is average body weight (kg); AT is average time of noncarcinogenic (days); PEF is soil-to-air particulate emission factor (m 3 kg -1 ), SL is soil-to-skin adherence factor (mgcm -2 ); SA is skin surface area available for exposure (cm 2 ) and ABS is dermal absorption factor. Detail of these factorfor risk assessment wasindicated in Table 1.
The risk assessment for non-carcinogenic was calculated using Equation 5: where m and n are type and number of pollutants; RfD is reference dose (mg kg day -1 ) ( Table 1); D is daily uptakedose (mg kg -1 day -1 ); HQijis risk for the exposure path. HI <1 means there is no possibility of adverse human health effects, whereas HI> 1 means there is likely to have adverse effect on human health.  Table 3 presented the concentrations of heavy metals in the soil surrounding the landfill. Six out of seven heavy metals occurred in the two soil layers around the landfill with average concentrations ranging from 12.3 ± 2.14 -291 ± 38.85 mg kg -1 . The concentrations of Mn, Zn, Cu, Cr at the locations S1, S2 and S3 were all higher than those at S4 (1 km away from the landfill). Cd was the only metal not detected in all soil samples. Notes: Data were presented as Mean ± SD, n = 3. Diffrerent letters a, b, c indicated statistically significant at significance level 5% (p<0.05). a Error! Reference source not found., b Error! Reference source not found.; ND: Not detected.

III. RESULTS AND DISCUSSION 1) Occurrence of heavy metals in soil
Most of heavy metal concentrations in soil were in compliance with QCVN 03-MT: 2015/BTNMT, CCME (2007). Concentration of Mn was the metal with the highest concentrations in soil ranging from 240 ± 0 -321 ± 2 mg kg -1 (Table 3). Mn concentrations at S1 and S3 were always higher than that at S4 showing the negative impact of the landfill leachate on soil environment. Similar to Mn, Cr concentration at S4 was lower than those at S2 and S3 and this could be because Cr is not directly affected by the landfill leachate. The presence of Cr in soil is a major threat to plants and humans because under appropriate environmental conditions, Cr (III) is easily converted to Cr (VI) -a form always toxic to plants (Ba, 2008). At locations around the landfill sites (except S3), Ni concentration ranging from 30.5 ± 0.25 -36.3 ± 0.50 mg kg -1 . The results of Zn concentration ranged from 65.8 ± 0.35 -82.7 ± 0.70 mg kg -1 . The distribution of Ni and Zn concentration at the locations and the soil were mainly influenced by the impact of leachate, mobility of the metals and soil properties. Cu and Pb were presented in soil with relatively low concentration at 16.3 ± 2.20 -18.1 ± 2.66 mg kg -1 and 11.2 ± 0.46 -12.3 ± 2.14 mg kg -1 , respectively (Table 3). Pb concentration in the soilat S4 (13.1 ± 0.50 mg kg -1 ) was higher than those at S1-S3 (9.66 ± 0.08 -12.6 ± 0.02 mg kg -1 ). Six out of seven heavy metals occurred in the soil samples collected at the surrounding landfill and 1km away from landfill. The presence of heavy metals not only affects the quality of the soil but also threatens the groundwater and rice production.

2) Heavy metals in rice plant
It was found that six out of seven heavy metals occurred in the rice plant parts including root, stem-leave, and rice grain ( Table 4). The Cd concentration was below the detection limit, and below the FAO/WHO regulatory standard (0.2 mg kg -1 ).Heavy metals were found highly accumulated in the rice roots in this study ( Table 4). The concentrations of Mn, Zn, Cu, Pb, Ni and Cr in the rice root at S1-S3 were 674 ± 12.53 mg kg -1 , 87.6 ± 0.93 mg  Table  4). The results indicated that heavy metal concentrations in rice roots in the area influenced by the landfill leachate were higher than those without influenced by the landfill activity.
Among the heavy metals, Mn was highly accumulated in rice plants that could be due to its higher mobility compared to the others (Prechthai et al., 2008). This study found that accumulation of heavy metals in parts of rice at S1-S3 was higher that those at S4 (except for Zn and Pb at rice roots) and decreased in the order Mn> Zn> Cu> Ni> Cr (except in rice grain, Cr> Cu> Ni). Heavy metals accumulated in rice parts with decreasing order root> stem -leave>grain (except for Mn at S4 and Cr at S1 -S3).

3) Health risk assessment
The mean concentration of heavy metals found in soils and rice grainswere used to calculate health risk and the results were showed in Table 5.
Health risk assessment was performed for heavy metals contaminated in soil and rice grains. The result indicated that there is no health risk for children and adults since all the hazard indexes (HI) were less than 1. Children were likely to suffer more risk than adults because the HI values for children were higher than adults (Table 5). Previous studies also indicated that there was no possible risk for human when exposed to soil and rice grains contaminated with heavy metals surrounding the landfill [13,20]. It was clearly showed that HI values calculated for the heavy metals in the area surrounding the landfill (S1-S3) were higher than those calculated for heavy metals at the location S4 (Table 5). This could mean that higher health risk was expected for the area around the landfill. In the soil sample, the level of health risk (for adult) gradually decreases via HQing>HQinh>HQder routes; However, the potential health risk for children via ingestion (HQder) was higher that that via inhalation (HQinh). Among the heavy metals, Pb was the metal could pose the highest health risk, although this is the metal present with the lowest concentration in the soil. The health risk levels of the examined heavy metals were arranged as decreasing order Pb> Mn> Cr> Ni> Cu > Zn. In the rice grains, the estimated potential health risk of the heavy metals were in the order of Mn> Cr> Ni> Zn> Cu. Comparing the values of HI between soil and rice grains in both child and adult, it could be seen that the risk for rice grain consumption was higher than the human exposed to the soil contaminated heavy metals. This study suggested that the agricultural activity, especially rice cultivation in the area surrounding the landfill is no longer suitable because the soil was contaminated with heavy metals and the heavy metals started to be accumulated in the rice parts. Long-term consumption of agricultural products produced in the study area could lead to potential health risk.

IV. CONCLUSION
Six out of seven heavy metals including Mn, Zn, Ni, Cr, Cu and Pd were detected and were lower than the permitted limits of QCVN 03-MT: 2015/BTNMT and CCME. The concentration of the detected heavy metals in the topsoil (0-25cm) around the landfill (S1-S3) were higher than those at the location 1 km from the landfill (S4) with the exception for Ni, Pb. The concentration of the heavy metals in the rice parts in the surrounding landfill sites decreased from Mn> Zn> Cu> Ni> Cr (except for the heavy metals in the rice grains with the order of Cr> Cu> Ni). Cd was not detected in the rice and Pb only appeared in the rice roots. The calculation of the hazard index (HI) shows that the health risk due to heavy metals contamination in soil and rice grain for children was higher than for adult, however, all HI values fell into safe level. Health risk for rice consumption was higher than that for exposure to soil contaminating heavy metals. In addition, health risk for due to exposure to heavy metals by all routes in the area surrounding the landfill were higher than that at 1km away from the landfill. Measures should be taken to minimize the leakage of leachate into rice fields.