Md.Saiful ISLAM,Md.Kawser AHMED,Md.Habibullah Al-MAMUN and Dennis Wayne EATONDepartment of Soil Science,Patuakhali Science and Technology University,Dumki,Patuakhali 8602(Bangladesh)

2Graduate School of Environment and Information Sciences,Yokohama National University,79-7 Tokiwadai,Hodogaya-ku,Yokohama,Kanagawa 240-8501(Japan)

3Department of Oceanography,University of Dhaka,Dhaka 1000(Bangladesh)

4Department of Fisheries,University of Dhaka,Dhaka 1000(Bangladesh)

5Contract Faculty,Rutgers University,New Brunswick,NJ 08901-8554(USA)

ABSTRACTTrace metal contamination in soil is of great concern owing to its long persistence in the environment and toxicity to humans and other organisms.Concentrations of six potentially toxic trace metals,Cr,Ni,Cu,As,Cd,and Pb,in urban soils were measured in Dhaka City,Bangladesh.Soils from different land-use types,namely,agricultural field,park,playground,petrol station,metal workshop,brick field,burning sites,disposal sites of household waste,garment waste,electronic waste,and tannery wast,and construction waste demolishing sites,were investigated.The concentration ranges of Cr,Ni,Cu,As,Pb,and Cd in soils were 2.4-1 258,8.3-1 044,9.7-823,8.7-277,1.8-80,and 13-842 mg kg-1,respectively.The concentrations of metals were subsequently used to establish hazard quotients(HQs)for the adult population.The metal HQs decreased in the order of As ＞Cr ＞Pb ＞Cd ＞Ni ＞Cu.Ingestion was the most vital exposure pathway of studied metals from soils followed by dermal contact and inhalation.The range of pollution load index(PLI)was 0.96-17,indicating severe contamination of soil by trace metals.Considering the comprehensive potential ecological risk(PER),soils from all land-use types showed considerable to very high ecological risks.The findings of this study revealed that in the urban area studied,soils of some land-use types were severely contaminated with trace metals.Thus,it is suggested that more attention should be paid to the potential health risks to the local inhabitants and ecological risk to the surrounding ecosystems.

Key Words: hazard quotient,health risk,land use,potential ecological risk,soil pollution,trace metals,urban soil

INTRODUCTION

Trace metal contamination in soil is of great concern owing to its long persistence in the environment and toxicity to humans and other organisms (Raghunath et al., 1999).Naturally, metals can exist in the environment as trace elements in rocks and soils.However,they can be released into the environment as a result of anthropogenic activities(Kurt-Karakus, 2012). In urbanized areas, trace metals may originate from various activities such as emissions from vehicular exhaust and application of sewage,sludge,and wastewater to arable lands (Thornton, 1991). In the last few decades, urban environmental pollution has been accelerated owing to unplanned urbanization and extensive industrialization. Consequently, urban soils are generally receiving excessive amounts of trace metals and other pollutants(Wei and Yang,2010;Xia et al.,2011),which lead to the deterioration of the soil biology,functions,and physicochemical properties, and also create other environmental problems(Papa et al.,2010).Recently,lands of Dhaka City of Bangladesh have been changed to meet public demand for commercial,residential,and industrial interests.Depending on their nature, the non-conforming land uses (open storage sites of different wastes,construction of gas and petrol stations,metal and car workshops,etc.)are potentially hazardous to the surrounding environment and may jeopardize human health(Man et al.,2010).Cities have been considered as regional sinks for resource consumption and sources of chemical emissions.Because of the proximity to large human populations in the urban areas, urban soils contaminated with trace metals can pose significant human health risks due to soil ingestion(Luo X S et al.,2011;Luo et al.,2012b;Okorie et al., 2011), inhalation of volatiles and fugitive soil particulates(Laidlaw and Filippelli,2008),and dermal contact (DEFRA, 2002; Li et al., 2011). Routes of trace metal exposure to humans from soil are shown in Fig.1.

Fig.1 Routes of trace metal exposure to humans from soil(DEFRA,2002).

Although a number of studies have reported human health impacts due to metal contamination in urban soils in some regions of the world (Luo et al., 2007; Man et al.,2010,2013),to our knowledge,no such study has yet been conducted in Bangladesh. This is the primary study reporting the scenario of metal contamination in urban soils of different land-use types.The objectives of this study were to examine the concentrations of trace metals(Cr,Ni,Cu,As,Cd,and Pb)in soils of different land-use types,to assess possible non-carcinogenic and carcinogenic risks through three exposure pathways (ingestion, dermal contact, and inhalation),and to evaluate the potential ecological risk of soil contamination by trace metals in Dhaka City.

MATERIALS AND METHODS

Study area and sampling

This study was conducted in the urban area of Dhaka City, Bangladesh, and targeted twelve different land-use types:agricultural farm(AF)-traditional farming systems with different types of vegetables and cereal crops; park(PA)-area for exercise and other recreational activities for the people; playground (PG)-playing field for residents;petrol station(PS)-petrol filling station;metal workshop(ME)-recycling of toxic metals with some machinery activities,metal smelting,and preparation of new products;brick field(BF)-burning of coal and wood for brick kiln;household waste disposal site(HO)-disposal site for household waste;garment waste disposal site(GA)-disposal site of garment waste with dyeing materials;electric waste disposal site(EL)-breaking down of electronic components;tannery waste disposal site(TA)-disposal site of tannery waste with leather products;construction waste site(CO)-field for demolishing building materials;and burning site(BU)-sites for the burning of household,farm,and industrial wastes.The metropolitan area of Dhaka City is about 815.8 km2and located at the center of Bangladesh.Dhaka City is one of the most densely populated cities in the world,home to approximately twelve million people(Islam et al.,2014).Numerous industries (leather, textile, metal processing, paper mill,electronic goods, power plant, fertilizer, pharmaceutical,dyeing, battery manufacturing, ink manufacturing, Pb-Zn smelting, brick field industries, etc.) are situated near the study area(Rahman et al.,2012).The basic information of the study area is presented in Table SI(see Supplementary Material). About 70 composite soil samples were collected during February-March,2012 and August-September,2013 from Dhaka City(Fig.2).Seventy sampling sites were selected depending on the current land-use types.In each land use category,we collected 4 to 5 samples.Due to the similar land use activity,the samples collected in the present study will represent the selected land-use type for the vast area in Dhaka City.At each sampling site,fine sub-samples from up to 10 cm depth were collected. The sub-samples were thoroughly mixed to form a composite sample. Soil samples were air-dried (ISO, 2006) at room temperature(24-28◦C)for two weeks,then ground and homogenized.The dried samples were crumbled and pulverized with a porcelain mortar and pestle,sieved through a 2-mm nylon sieve,and stored in airtight clean Ziploc bags until chemical analysis.The processed samples were brought to Yokohama National University,Japan,for chemical analysis.

Sample preparation

All chemicals were analytical grade reagents and Milli-Q(Elix UV5 and MilliQ,Millipore,USA)water was used for preparation of all solutions.The Teflon vessel and polypropylene containers used in the experiments were soaked in 5%HNO3for more than 24 h,then rinsed with Milli-Q water and dried.For total metal analysis,0.3 g of soil sample was treated with 1.5 mL 69%HNO3(Kanto Chemical Co,Tokyo,Japan)and 4.5 mL 35%HCl(Kanto Chemical Co,Tokyo,Japan)in a closed Teflon vessel and was digested in a Microwave Digestion System(Berghof Speedwave®,Eningen,Germany).The digested soil samples were transferred into a Teflon beaker and total volume was made up to 50 mL with Milli-Q water.The digested solution was then filtered using a syringe filter (DISMIC®- 25HP PTFE, pore size=0.45µm)(Toyo Roshi Kaisha,Ltd.,Japan)and stored in 50 mL polypropylene tubes(Nalgene,New York).

Fig.2 Map of the study area in Dhaka City,Bangladesh.

Instrumental analysis and quality control

For trace metals,samples were analyzed using an inductively coupled plasma-mass spectrometer(ICP-MS,Agilent 7700 series,USA).Multi-element standard XSTC-13(Spex Certi Prep®Metuchen, USA) solutions were used to prepare calibration curves.The calibration curves with R2＞0.999 were accepted for concentration calculation.Internal calibration standard solutions containing 1.0 mg L-1of indium,yttrium,beryllium,tellurium,cobalt,and thallium were purchased from Spex Certi Prep®Metuchen, USA.Multi-element solution(Agilent Technologies,USA),1.0µg L-1,was used as a tuning solution covering a wide range of masses of elements.All test batches were evaluated using an internal quality approach and validated if they satisfied the defined internal quality controls(IQCs).Before starting the analysis sequence,relative standard deviation(RSD ＜5%)was checked using a tuning solution purchased from Agilent Technologies.

Health risk assessment

Risk assessment is a multi-step procedure comprising data collection,evaluation,exposure assessment,and risk characterization(USEPA,1989; USDOE,2011).For performing non-carcinogenic and carcinogenic risk assessment,the mathematical expressions were taken from“Exposure Factors Handbook”(USEPA,1997).The exposure to Cr,Ni,Cu,As,Cd,and Pb in different land-use types of urban soils was calculated following some assumptions concerning the exposure frequency and time of metal exposure(based on daily life activities)in different land-use types in Dhaka City(Table I).

In this study,six trace metals,i.e.,Cr,Ni,Cu,As,Cd,and Pb, were identified as toxic metals to human health.Human exposure to trace metals in the urban soils can occur via three main pathways (De Miguel et al., 2007;Luo et al.,2012a):i)direct ingestion of substrate particles;ii)inhalation of suspended particulates emitted from soil,through the mouth and nose;and iii)dermal absorption of trace metals in particles adhered to exposed skin.Both noncarcinogenic and carcinogenic risks of these exposure routes were considered.In the step of risk assessment,a specific approach characteristic for human exposure was applied.In this study,we considered the critical population groups that may be affected by trace metals from soils under different land-use types in Dhaka City during their daily activities.

The carcinogenic risk was calculated for the lifetime exposure, estimated as the incremental probability of an individual developing cancer over a lifetime as a result of total exposure to the potential carcinogens. The noncarcinogenic and carcinogenic risks through each of the three exposure routes were considered and calculated using Eqs.1-6 adapted from USEPA(1989,1997)and USDOE(2011). Non-carcinogenic risk was assessed based on the hazard quotients(HQs)as follows(Eqs.1-4):

where HQing,HQder,and HQinhare the HQ values associated with exposure to trace metals from soil by the local inhabitants through ingestion (ing), dermal contact (der),and inhalation exposure (inh), respectively; CRing, CRder,and CRinhare the carcinogenic risk values of trace metals through ingestion(ing),dermal contact(der),and inhalation exposure (inh), respectively. The definitions of the other symbols,used values of Bangladesh-specific variables,and explanation of parameters are shown in Tables I and II.For carcinogens, the doses for As and Pb were multiplied by the corresponding slope factors(Table II)to produce a level of excess lifetime cancer risk (Eqs. 5 and 6). Though the effect from interactions between some metals might occur in a synergistic manner(Xu et al.,2011;Luo et al.,2012a),it was assumed that all metal risks were additive.Thus,it is possible to calculate the cumulative non-carcinogenic risk for individual exposure pathways.

The reference dose (RfD) is an estimation of maximum permissible risk on human population through daily exposure. The particle emission factor (PEF) relates the concentration of a chemical constituent in soil to the concentration in dust particles in air (USEPA, 1996), which has been considered for two aspects-presence of vegetative cover(AF,PA,and PG)or absence of vegetative cover(PS,ME,BF,HO,GA,EL,TA,CO,and BU).If the HQ is equal to or higher than 1,there is a potential health risk,and related interventions and protective measurements should be taken.

TABLE IVariables for the estimation of human exposure to trace metals in soils of different land-use types in the urban area of Dhaka City,Bangladesh

TABLE IISome toxicological parameters of the investigated trace metals used for health risk assessment in the present study

Calculation of ecological risk indices

Enrichment factor. Enrichment factor(EF)for each metal was calculated to evaluate the anthropogenic influences on trace metals in soil using the following formula(Islam et al.,2015a,b):

where(CM/CAl)sampleis the ratio of concentration of trace metals (CM) to that of Al (CAl) in the soil sample and(CM/CAl)backgroundis the same reference ratio in the preindustrial soil sample. In general, an EF value of about 1 suggests that a given metal may be entirely from crustal materials or natural weathering processes(Rashed,2010).Sample enrichment factor ＞1.5 was considered indicative of human influence and,arbitrarily,EFs of 1.5-3,3-5,5-10,and ＞10 are considered indicative of minor, moderate,severe,and very severe modification,respectively(Islam et al.,2015a).

Contamination factor. Contamination factor of a single trace metal i(Cif)is the ratio obtained by dividing the concentration of each metal(CM)in the soil sample by the baseline or background value(Cbackground,metal concentration in the pre-industrial soil sample):

The contamination levels may be classified based on their intensities on a scale ranging from 1 to 6:low degree(Cif＜1),moderate degree(1 ≤Cif＜3),considerable degree(3≤Cif＜6), and very high degree(Cif≥6). Thus, the Cifvalues can monitor the enrichment of one given metal in soils over a period of time.

Pollution load index. To assess the quality of soil in terms of trace metal contamination,an integrated approach of pollution load index(PLI)of the six trace metals is calculated according to Rashed(2010).The PLI is defined as the nth root of the multiplications of Cifof metals(Bhuiyan et al.,2010;Islam et al.,2015a):

The PLI gave an assessment of the overall toxicity status of the sample,and also it is a result of the contribution of six trace metals.

Geoaccumulation index. The degree of contamination from trace metals could be assessed by determining the geoaccumulation index(Igeo).The Igeowas used to assess the accumulation of trace metals in soil(Muller,1969).In order to characterize the level of pollution,Igeovalues were calculated using the following equation:

The factor 1.5 is introduced to minimize the possible variations in the background values,which may be attributed to lithogenic effects.The Igeovalues were interpreted as:practically uncontaminated(Igeo≤0);uncontaminated to moderately contaminated(0 ＜Igeo≤1);moderately contaminated(1 ＜Igeo≤2);moderately to heavily contaminated(2＜Igeo≤3);heavily contaminated(3 ＜Igeo≤4);heavily to extremely contaminated(4 ＜Igeo≤5); and extremely contaminated(5 ＜Igeo).

Potential ecological risk. Comprehensive potential ecological risk index(PER)was used to assess the contamination degree of trace metals in soils.The equations used for calculation of PER were proposed by Guo et al.(2010)as follows:

where the sum of Ciffor all metals represents the integrated pollution degree(Cd)of the environment,Eiris the potential ecological risk index of a single trace metal i, and Tiris the biological toxic factor of an individual metal i. The toxic-response factors for Cr,Ni,Cu,As,Cd,and Pb were 2,6,5,10,30,and 5,respectively(Guo et al.,2010).

Statistical analysis

Data were statistically analyzed using the statistical package SPSS 22.0(SPSS,USA).The means and standard deviations of the trace elements concentrations in soils were calculated.

RESULTS AND DISCUSSION

Metal contamination in soil

Fig.3 Trace metal concentrations in soils of different land-use types in the urban area of Dhaka City,Bangladesh.Vertical bars represent the standard errors of means(n=6).Horizontal dotted lines indicate background values of the study area.AF=agricultural field;PA=park;PG=playground;PS=petrol station;ME=metal workshop;BF=brick field;HO=household waste;GA=garment waste;EL=electronic waste;TA=tannery waste;CO=construction waste;BU=burning site.

The concentrations of six trace metals(Cr,Ni,Cu,As,Cd, and Pb) in soil samples of different land-use types are presented in Fig. 3 and Table SII (see Supplementary Material for Table SII).The mean concentrations of trace metals in soil were in the descending order of Cu(267 mg kg-1) ＞Cr (239 mg kg-1) ＞Ni (206 mg kg-1) ＞Pb(195 mg kg-1) ＞As(58 mg kg-1) ＞Cd(16 mg kg-1).The levels of trace metals varied among the sampling sites of different land-use types and followed the descending order of TA ＞ME ＞EL ＞AF ＞GA ＞BU ＞CO ＞BF ＞HO ＞PS ＞PG ＞PA. Metal concentrations were found statistically significant(P ＜0.05)among the studied sites(Table SII,see Supplementary Material for Table SII).The highest mean concentration of Cr was obtained at TA site (1 112 mg kg-1), which can be due to utilization of chromium salt for leather treatment (Krishna and Govil,2007;Gowd et al.,2010).A considerable amount of Cr was also observed in soil collected from AF site,which might be due to the use of tannery waste for the supplement of organic matter for crop production.Similar to agricultural field,the other land-use types may receive Cr from the unplanned activities of tannery industries in Dhaka City. During our field sampling,we observed some tannery waste at BU site,which might be the reason for higher Cr in soils of those sites.Elevated mean concentrations of Ni were observed at ME and EL sites,which can be attributed to emissions from metal smelters. Such extremely high levels of Ni in soils can be found in metal industries and waste disposal sites,which result from localized additions or accidental spillages of Ni containing materials(Govil et al.,1998;Krishna and Govil,2007).Elevated concentrations of Cu were observed in soils from waste disposal sites,which can be due to the emission of Cu from the uncontrolled industrial and waste burning activities(Kashem and Singh,1999;Gowd et al.,2010;Luo C L et al.,2011).Among the sites,TA,BF,BU,and AF sites showed considerable levels of As.Interestingly,TA site showed the highest As concentration(112 mg kg-1).In general,a huge amount of ground water containing As(Neumann et al.,2010)is being used for tanning in relation to some chemicals,especially arsenic sulfide(Asaduzzaman et al., 2002; Bhuiyan et al., 2011). Moreover, emissions and waste from brick fields and incineration activities might have contributed to the high concentration of As(Olawoyin et al., 2012). Arsenic in agricultural soils can be derived from both natural and anthropogenic sources, especially use of ground water for irrigation(Neumann et al.,2011)and uncontrolled application of As-enriched fertilizers and pesticides (Renner, 2004). Elevated mean concentration of Cd was obtained from the EL, ME, TA, and BF sites,which can be attributed to the metal processing, battery manufacturing,and smelting industries.Severe Cd pollution has been reported from areas surrounding smelters in many countries(Martley et al.,2004;Rawlins et al.,2006).The ME,EL,and BU sites showed elevated concentrations of Pb,which can be due to the emission of Pb contaminated waste from these sites(Gowd et al.,2010;Luo C L et al.,2011).During our sampling,we observed leachates from defused Ni-Cd batteries,Cd plated items,casting Pb,and Pb products manufactured at the sampling sites. From the data, it was observed that some trace metals showed higher standard deviation,which could be indicative of the lack of uniformity of the elemental distribution among the sites. This was probably due to various land-use activities and disturbances such as digging,excavation,and construction,as well as other natural processes such as weathering and erosion, which could alter stabilization of the soil environment (Amuno,2013).

In the present study,we used some international guidelines for trace metals in soil to understand the current situation of metal contamination,since there were no locally derived environmental standards for trace metal contamination in soils of Bangladesh. Hence, soil quality guidelines from other countries were used to compare the levels of trace metals. The following guidelines were applied: The New Dutch List(VROM,2000),Canadian Environmental Quality Guidelines (CCME, 2003), China Environmental Quality Standard(CSEPA,1995),Australian Guidelines on the Investigation Levels for Soil(NEPC,2013;DEP,2003),and Sweden Soil Remediation Goals(SEPA,2002).The mean concentrations of Ni,Cu,As,and Cd in soils from the twelve different land-use types were higher than the aforementioned recommended guideline values(see Supplementary Material for Table SII).The site-specific concentrations of studied metals mostly exceeded the Dutch soil quality target and intervention values(Fig.3).The functional properties of these soils, such as nutrients and contaminant storage,transformation,and biomass production,may be seriously affected due to continuous crop production, which could potentially cause adverse effects on humans, plants, and animals(Man et al.,2010).The present study mainly used the Dutch soil quality standard,which is regarded as the most comprehensive guideline considering all possible exposure pathways for protecting humans,plants,and animals.If all metal concentrations in soil are below their respective Dutch target values,the soil is considered clean.If the concentration level lies between the target and intervention values,the soil is regarded as slight to moderately contaminated.In contrast,if the value is above the Dutch Intervention Value,the soil is considered detrimental to humans,plants,and animals.

Health risk assessment

Trace metals in the contaminated soils might have a serious impact on human health (Okorie et al., 2011). In urban areas,soils of different land-use types are considered as important exposure media of trace metals through ingestion,dermal contact, and inhalation (Bright et al., 2006; De Miguel et al., 2007; Zheng et al., 2010; Luo X S et al.,2011).According to the risk assessment approach,both noncarcinogenic and carcinogenic risks of trace metals through three exposure pathways were characterized in this study.

Non-carcinogenic risk. The HQs of individual metals from different land-use types are presented in Table III and Fig. 4. The HQs for the studied metals through soil ingestion were higher than those of the other two exposure pathways,indicating that the target populations of the study area were exposed through ingestion. Similar trends have been reported for urban soils around the industrial clusters in Ghaziabad,India(Chabukdhara and Nema,2013).Among the land-use types,higher HQ values were observed for AF,ME,and TA sites in each of the exposure pathways,implying a potential health risk to the workers and inhabitants living close to these sites. Hazard quotients of each metal were decreased in the order of As ＞Cr ＞Pb ＞Cd ＞Ni ＞Cu.Metal-specific HQ values were less than 1,indicating that intake of a single metal through different pathways of soil does not pose a considerable non-carcinogenic risk to the exposed people(Fig.4).Exposure from food items harvested in the contaminated agricultural fields should be considered in future assessments.

Carcinogenic risks of ASand Pb. The carcinogenic risks of As and Pb for adult inhabitants were estimated and presented in Table IV.Considering the exposure pathways,carcinogenic risk followed the descending order of ingestion＞dermal contact ＞inhalation. Site specific assessment of carcinogenic risk showed a considerable risk of As for TA site and Pb for ME site (Table IV). The carcinogenic risk of As ranged from 3.0 × 10-6to 6.6 × 10-5, 2.8 ×10-9to 6.2×10-8,and 5.5×10-10to 4.2×10-8,while that of Pb ranged from 6.4 × 10-8to 1.2 × 10-6, 1.5 ×10-10to 2.7 × 10-9, and 4.3 × 10-12to 3.6 × 10-10for ingestion,dermal contact,and inhalation,respectively(Table IV). In general, cancer risks lower than 10-6are considered to be negligible, cancer risks above 10-4are considered unacceptable(USEPA,1989;Guney et al.,2010),and cancer risks lying between 10-6and 10-4are generally considered acceptable(Fryer et al.,2006;Hu et al.,2012).The sums of carcinogenic risks for Pb from three exposure pathways for all the sites were less than 10-6and regarded as negligible,while the sums of risks for As were within the acceptable range of 10-6to 10-4for all sites(Table IV).Therefore, based on the results of the present study, the potential health risks to the humans due to metal exposure through soil should not be overlooked considering the fact that dietary exposure is not taken into account in this study.

TABLE III Hazard quotient(HQ)values of trace metals due to ingestion,dermal contact,and inhalation on adults in the urban area of Dhaka City,Bangladesh Metal Land-use typea)AF PA PG PS ME BF HO GA EL TA CO BU CrIngestion 1.1×10-1 1.1×10-2 3.1×10-3 1.3×10-2 2.8×10-2 1.7×10-2 2.7×10-2 2.5×10-2 5.1×10-3 3.4×10-1 1.3×10-3 4.4×10-3 Dermal 2.4×10-4 2.6×10-5 7.1×10-6 2.9×10-5 6.4×10-5 3.9×10-5 6.3×10-5 5.8×10-5 1.2×10-5 7.8×10-4 2.9×10-6 1.0×10-5 Inhalation 4.6×10-4 4.9×10-5 1.4×10-5 1.1×10-4 2.4×10-4 1.5×10-4 2.4×10-4 2.2×10-4 4.5×10-5 3.0×10-3 1.1×10-5 3.9×10-5 NiIngestion 7.9×10-3 1.2×10-3 2.7×10-3 4.8×10-3 3.4×10-2 3.4×10-3 2.8×10-3 2.9×10-3 2.9×10-3 6.6×10-3 9.9×10-4 6.6×10-4 Dermal 1.8×10-5 2.7×10-6 6.2×10-6 1.1×10-5 7.9×10-5 7.8×10-6 6.4×10-6 6.7×10-6 6.6×10-6 1.5×10-5 2.3×10-6 1.5×10-6 Inhalation 2.6×10-4 3.8×10-5 8.7×10-5 3.1×10-4 2.2×10-3 2.2×10-4 1.8×10-4 1.9×10-4 1.9×10-4 4.3×10-4 6.5×10-5 4.3×10-5 CuIngestion 6.4×10-3 1.0×10-3 9.4×10-4 3.1×10-3 1.0×10-2 2.2×10-3 2.8×10-3 3.2×10-3 1.5×10-3 6.9×10-3 7.6×10-4 1.0×10-3 Dermal 1.5×10-5 2.4×10-6 2.2×10-6 7.1×10-6 2.4×10-5 5.2×10-6 6.5×10-6 7.4×10-6 3.5×10-6 1.6×10-5 1.8×10-6 2.3×10-6 Inhalation 7.5×10-6 1.2×10-6 1.1×10-6 7.2×10-6 2.5×10-5 5.3×10-6 6.6×10-6 7.6×10-6 3.6×10-6 1.6×10-5 1.8×10-6 2.4×10-6 AsIngestion 1.7×10-1 5.8×10-2 2.7×10-2 6.0×10-2 1.8×10-1 1.8×10-1 1.4×10-1 8.6×10-2 2.4×10-2 3.4×10-1 1.5×10-2 2.5×10-2 Dermal 4.0×10-4 1.3×10-4 6.3×10-5 1.4×10-4 4.1×10-4 4.1×10-4 3.2×10-4 2.0×10-4 5.6×10-5 7.9×10-4 3.6×10-5 5.7×10-5 Inhalation 5.1×10-4 1.7×10-4 8.0×10-5 3.5×10-4 1.0×10-3 1.0×10-3 8.0×10-4 5.0×10-4 1.4×10-4 2.0×10-3 9.1×10-5 1.5×10-4 CdIngestion 5.8×10-3 6.3×10-3 1.6×10-3 9.1×10-3 2.5×10-2 1.4×10-2 1.2×10-2 5.9×10-3 3.7×10-3 2.5×10-2 9.9×10-4 1.6×10-3 Dermal 1.3×10-5 1.5×10-5 3.7×10-6 2.1×10-5 5.7×10-5 3.1×10-5 2.8×10-5 1.4×10-5 8.5×10-6 5.7×10-5 2.3×10-6 3.6×10-6 Inhalation 8.6×10-5 9.3×10-5 2.3×10-6 2.7×10-4 7.3×10-4 4.0×10-4 3.6×10-4 1.7×10-4 1.1×10-4 7.3×10-4 2.9×10-5 4.6×10-5 PbIngestion 4.1×10-2 2.1×10-2 5.6×10-3 1.4×10-2 7.9×10-2 3.0×10-2 4.0×10-2 2.9×10-2 9.6×10-3 2.0×10-2 4.4×10-3 1.0×10-2 Dermal 9.4×10-5 4.9×10-5 1.3×10-5 3.3×10-5 1.8×10-4 6.9×10-5 9.2×10-5 6.6×10-5 2.2×10-5 4.6×10-5 1.0×10-5 2.3×10-5 Inhalation 4.5×10-5 2.3×10-5 6.3×10-6 3.2×10-5 1.8×10-4 6.6×10-5 8.8×10-5 6.3×10-5 2.1×10-5 4.4×10-5 9.8×10-6 2.2×10-5 a)AF=agricultural field;PA=park;PG=playground;PS=petrol station;ME=metal workshop;BF=brick field;HO=household waste;GA=garment waste;EL=electronic waste;TA=tannery waste;CO=construction waste;BU=burning site.

Fig.4 Box and whisker plots for hazard quotient(HQ)values of trace metals in soils of different land-use types in the urban area of Dhaka City,Bangladesh,as sum of ingestion,dermal contact,and inhalation exposure.Circle symbol(o)represents outlier.AF=agricultural field;PA=park;PG=playground;PS=petrol station;ME=metal workshop;BF=brick field;HO=household waste;GA=garment waste;EL=electronic waste;TA=tannery waste;CO=construction waste;BU=burning site.

Ecological risk

In this study, EF, Cif, PLI, Igeo, and Cdhave been applied to assess the contamination of trace metals in soils of different land-use types in Bangladesh.For most sites,EFs of the studied metals were higher than 1.5(Fig.5),indicating strong human influence from various industries in Dhaka City(Rashed,2010).Generally,studies have observed that low enrichment values indicate a great contribution from crusted source to the soil, while high EFs indicate a considerable contribution from anthropogenic sources(Rashed, 2010).As a whole,the mean EF values of all studied metals suggest their enrichment in soils of different land-use types in Dhaka City,Bangladesh.

Fig.5 Enrichment factor(EF)values of trace metals in soils of different land-use types in the urban area of Dhaka City,Bangladesh.The horizontal dotted line indicates human influence of trace metals,considered to have EF value of 1.5.AF=agricultural field;PA=park;PG=playground;PS=petrol station;ME=metal workshop;BF=brick field;HO=household waste;GA=garment waste;EL=electronic waste;TA=tannery waste;CO=construction waste;BU=burning site.

TABLE IVCarcinogenic risk values of As and Pb due to ingestion,dermal contact,and inhalation of adults in the urban area of Dhaka City,Bangladesh

Fig.6 Pollution load index(PLI)values of trace metals in soils of different land-use types in the urban area of Dhaka City,Bangladesh.Vertical bars represent standard errors of means(n=6).AF=agricultural field;PA=park;PG=playground;PS=petrol station;ME=metal workshop;BF=brick field;HO=household waste;GA=garment waste;EL=electronic waste;TA=tannery waste;CO=construction waste;BU=burning site.

The PLI value equal to zero indicates perfection;value of 1 indicates the presence of only baseline level of pollutants and values above 1 indicate contamination of soil by trace metals(Rashed,2010;Islam et al.2015a).Extent of pollution increases with the increase of numerical PLI value.As per above grade,present soils were polluted considerably,since PLI of all sites were higher than 1(Fig.6).Elevated levels of PLI were observed for AF,ME,EL,and TA sites,suggesting that the waste from various industries in the agricultural soil,lead-acid battery factory,tannery,and dyeing industry had caused some extents of risks of the studied area(Bhuiyan et al., 2010; Islam et al., 2014). The Igeovalues showed the following decreasing order: Cd ＞Pb ＞Cu ＞As ＞Ni ＞Cr. The highest Igeovalues obtained for Cd, 0.34 to 3.81, indicated heavy contamination in soils of the study area (Fig. 7). Higher Igeovalues of Cd might be due to the contribution from atmospheric emission,leachates from defused batteries,and Cd plated items.The Igeovalues for Pb(-1.65 to 4.38)indicated uncontaminated to moderately contaminated soils. Lead showed outliers for Igeovalue,because of the extreme concentration of Pb at EL,ME,and BU sites.

Fig.7 Box and whisker plot for geoaccumulation index(Igeo)values of trace metals in soils of different land-use types in the urban area of Dhaka City,Bangladesh.Circle symbols represent outlier.

Håkanson (1980) defines four categories offour categories of Cd,five categories ofand four categories of PER,as shown in Table V.Theand Cdvalues are presented in Table SIII(see Supplementary Material for Table SIII).Sites of AF and TA showed very high contamination for Cr in soil,whereas ME and EL sites showed high contamination＞6) for Ni. Among the sampling sites, about 50%of samples showed very high contamination for Cu, As,and Pb in soil,whereas most of the sites showed very high contamination＞6)for Cd(see Supplementary Material for Table SIII),indicating that these four metals might pose potential risks to the surrounding ecosystems(Rashed,2010),which ultimately would degrade the ecological balance.The assessment of the overall contamination of soil based on Cdshowed considerable to very high degrees of metal contamination(Table SIII).

CONCLUSIONS

In this study, the soils from TA, ME, EL, and BUsites were heavily contaminated by trace metals.Also,both non-cancer and cancer risks to local residents and workers through ingestion,dermal contact,and inhalation exposures to contaminated soils were still within the acceptable range.However, when we consider the dietary exposure of trace metals, the public health risk will be more severe. Trace metals in soils of different land-use types showed considerable to very high degree of contamination.Among the six metals studied,only Cd had very high ecological risk,whereas the potential ecological risk indices of the other metals in the study area were very high.Finally,this study recommends further research on different exposure media and epidemiological investigations with a large number of participants in order to evaluate the actual health effects of trace metals.

TABLE VIndicesa)and grades of potential ecological risk of trace metals(Håkanson,1980;Luo et al.,2007)

TABLE VIPotential ecological risk factor,risk index,and pollution degree of trace metals in soils of different land-use types in the urban area of Dhaka City,Bangladesh

ACKNOWLEDGEMENTS

The samples collected in Bangladesh were brought into Japan based on the permission issued by the Yokohama Plant Protection Station(Import permit No.25Y324 and 25Y1009). The authors are grateful to the authority of Patuakhali Science and Technology University(PSTU),Bangladesh,and Yokohama National University,Japan,for providing laboratory facilities.The authors are also delighted to express their gratefulness and sincerest thanks to Professor Dr. Md. Shams-Ud-Din(Vice Chancellor, PSTU),for his valuable suggestions and cooperation to carry out this research.Furthermore,we are thankful for the kind help from the members of the department of Soil Resources Development Institute(SRDI)and Dhaka University,Bangladesh,during the field sampling.

SUPPLEMENTARY MATERIAL

Supplementary material for this article can be found in the online version.