International Journal of Environmental Analytical

Chemistry

ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/geac20

Polycyclic aromatic hydrocarbons in diverse

agricultural soils of central India: occurrence,
sources, and potential risks

Dinesh Kumar Yadav, Asirvatham Ramesh Kumar, Somasundaram

Jayaraman, Sangeeta Lenka, Suyog Gurjar, Abhijit Sarkar, Jayanta Kumar
Saha & Ashok Kumar Patra

To cite this article: Dinesh Kumar Yadav, Asirvatham Ramesh Kumar, Somasundaram
Jayaraman, Sangeeta Lenka, Suyog Gurjar, Abhijit Sarkar, Jayanta Kumar Saha & Ashok
Kumar Patra (2022): Polycyclic aromatic hydrocarbons in diverse agricultural soils of central
India: occurrence, sources, and potential risks, International Journal of Environmental Analytical
Chemistry, DOI: 10.1080/03067319.2022.2125307

To link to this article: https://doi.org/10.1080/03067319.2022.2125307

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INTERNATIONAL JOURNAL OF ENVIRONMENTAL ANALYTICAL CHEMISTRY
https://doi.org/10.1080/03067319.2022.2125307

Polycyclic aromatic hydrocarbons in diverse agricultural soils

of central India: occurrence, sources, and potential risks
Dinesh Kumar Yadav a, Asirvatham Ramesh Kumar b,
Somasundaram Jayaraman a, Sangeeta Lenka a, Suyog Gurjarb, Abhijit Sarkar a
,
Jayanta Kumar Sahaa and Ashok Kumar Patraa
a
ICAR-Indian Institute of Soil Science, Bhopal, India; bCSIR-National Environmental Engineering Research
Institute, Nagpur, India

ABSTRACT ARTICLE HISTORY

Polycyclic aromatic hydrocarbons (PAHs) are a class of organic Received 5 June 2022
compounds found ubiquitously in the environment and pose ser­ Accepted 1 September 2022
ious threat to the environment and humans. PAHs have received KEYWORDS
considerable attention due to their carcinogenic properties. In this Polycyclic aromatic
study, the 16 EPA priority PAHs were determined in agricultural hydrocarbons; agricultural
surface soils (0–5 cm) from diverse agricultural land use systems in soils; toxicity equivalents;
central India. ∑16 PAHs of different agricultural soil samples ranged diagnostic ratio; soil organic
from ND–122.52 µg/kg. Total concentration of 7 carcinogenic PAHs carbon
ranged between ND and 101.64 µg/kg. The levels of PAHs in differ­
ent agricultural soil samples varied widely. In general, we observed
that ∑16 PAHs concentrations in agricultural soils have followed the
order: sewage irrigated soil ≥ fly ash treated agricultural soil sam­
ples > horticulture land use soil > conservation agriculture soil >
organic farming soil. PAH levels in fly ash-treated soils were 2–5
times higher than that in sewage-irrigated soils. PAHs containing
four to five rings were the most common. Soil organic carbon (SOC)
was positively correlated with ∑16 PAHs (p < 0.05). The principal
sources of PAHs in these samples according to diagnostic ratios
were biomass burning (grass, wood, crop residue) and coal com­
bustion. In addition, it was found that agricultural soils treated with
fly ash and sewage irrigated soils showed higher carcinogenic
potential based on B[a]Peq TEQs. PAHs with higher carcinogenic
potency in these contaminated soils indicate a higher level of
health risk for humans.

1. Introduction
Polycyclic aromatic hydrocarbons (PAHs) are one of the ubiquitous organic contaminants
in the environment which pose adverse effects on the environment and biota including
human beings [1,2]. Due to their environmental persistence, long-range atmospheric
transport, and various toxicological effects on humans and animals, PAHs draw global
attention [3]. These organic contaminants originate from both natural and anthropogenic
sources [4,5]. The major sources of PAHs in the environment are emissions from biomass

CONTACT Asirvatham Ramesh Kumar ar_kumar@neeri.res.in

Supplemental data for this article can be accessed online at https://doi.org/10.1080/03067319.2022.2125307
© 2022 Informa UK Limited, trading as Taylor & Francis Group
2 D. K. YADAV ET AL.

burning, fossil fuel burning, high-temperature industrial processes, and wet and dry
atmospheric depositions [6]. Out of hundreds of knowns PAHs compounds, sixteen
unsubstituted PAHs have been identified by USEPA as priority pollutants. Among these,
seven PAHs species have been classified as carcinogenic by International Agency for
Research on Cancer (IARC) [7–9]. They are viz. Benzo(a)pyrene [carcinogenic to humans-
(Group 1)], Dibenz (a, h) anthracene [Probably carcinogenic to humans-(Group 2A)] and
Benzo(a)anthracene, Chrysene, Benzo(k)fluoranthene, Benzo(b)fluoranthene, and Indeno
[1,2,3-cd] pyrene [Possibly carcinogenic-(Group 2B)]. PAH emissions in developing coun­
tries such as India have been steadily rising [10]. PAHs are hydrophobic with moderately
higher octanol-water partition coefficients (log kow) 3.29–6.84; hence they preferentially
partition to soils and sediments. Soil is considered as the sink for most organic pollutants
[11]. Hence, PAHs are generally found at higher concentrations on the surface layer (0–5
cm) of the soil profile [12]. As agricultural soils are rich in organic matter, PAHs tend to
accumulate in these soils [13]. Several studies on PAHs distribution from urban, rural and
industrial soils of India were reported [14–16]; however, only few studies on the assess­
ment of PAHs in agricultural soils of India have been reported [11]. Currently, no standards
or guidelines for PAH levels in soil is available in India. However, various regulatory
agencies have prescribed standards for PAHs in soils. For example, Dutch (20–50 µg/kg),
Mexico (0–6000 µg/kg), and the Canadian Council of Ministers for Environment (CCME,
100–50,000 µg/kg) for agricultural landuse soils are widely accepted and used globally
[11,17].
In terms of human health risk, arable soils are the most vulnerable part of the soil
ecosystem [18,19]. Plants take up PAHs and can increase human exposure via food chains
[20]. Inhalation of contaminated air, consumption of contaminated food and water, or
coming in contact with contaminated soil are the major sources of human exposure to
PAHs [21]. Because of their persistence and toxicity, PAHs in soil may pose potential risks
to humans [22]. Wastewater irrigation is widely practiced in India and some urban areas in
Bhopal use wastewater for irrigation [23,24]. Untreated municipal waste water irrigation is
recognised as a major source of contamination of food chain and hence it is a cause of
concern to human health [25]. Therefore, assessment of PAHs in different agricultural soils
is important to identify sources, levels, composition, and toxic potential so that appro­
priate risk reduction measures can be taken. The objective of this work was to investigate
PAHs levels in diverse land use systems comprising agricultural soils, sewage irrigated soil,
horticultural soil, and to evaluate health risk assessment due to these soils. To the best of
our knowledge, this is the first study in India, assessing PAH levels in various agricultural
soils.

2. Materials & methods

2.1. Sampling
Samples were collected from the Bhopal district of Madhya Pradesh (India), which is
located at 485 metres above mean sea level (amsl) and has geographic coordinates of
23° 18′ 21.91′′ N and 77° 24′ 24.40′′ E. The soil under the study area was a Vertisol
(isohyperthermic, Typic Haplustert) with 58% clay, 22% silt, and 20% sand in the top
surface (0–5 cm) layer. A total of forty-three surface soil samples were collected from
 INTERNATIONAL JOURNAL OF ENVIRONMENTAL ANALYTICAL CHEMISTRY 3

diverse land use pattern agricultural soils during March, 2021. The different land use
pattern of samples were sewage irrigated land from Bhanpur, Malikhedi, Imliya and
Islamnagar, villages of Bhopal district (8 samples, AS1-AS8); transformer oil contaminated
soils (2 samples, AS9-AS10); fly ash treated agriculture soils (8 samples, AS11-AS18);
organic farming soil (12 samples, AS19-AS30); conservation agriculture soils (9 samples,
(AS31–39); horticulture land use soils (4 samples, AS40-AS43). Except the sewage irrigated
soils, all other land use types were the research farms of ICAR-IISS, Bhopal, Madhya
Pradesh, India. Sewage irrigated villages are located at a distance of about 10 kms from
ICAR-IISS campus, whereas the other land use areas the research farms of ICAR-IISS and
are located at a distance of about 1–2 kms from each other. Wastewater irrigation is
widely practiced in the study area and is considered as the primary source of PAHs in
sewage-irrigated land use, while biomass burning (the combustion of grass, crop residue,
wood, and coal), industrial emissions, and automotive activities also contribute to the
occurrence of PAHs. Also, some farmers use fly ash from coal powered thermal power
plants as soil amendment to increase soil characteristics. Figure 1 depicts the location of
sampling sites in the study area.

2.2. Sample collection

After removing surface plant materials such as twigs, residues and undecomposed
materials, soil samples from the surface (0–5 cm) were collected. At each location, four
samples were collected and a composite sample was made after thorough

Figure 1. Map of Bhopal depicting sampling sites.

4 D. K. YADAV ET AL.

homogenisation. All samples were collected in polythene zipped bags, preserved in ice-
box and transported to CSIR-NEERI, Nagpur laboratory. Soils samples (40 g) were oven
dried at 30-35� C for 24 hours. Samples were sieved through a 2 mm sieve and stored at
4� C till analysis [11,12,26].

2.3. Chemicals
Analytical standard mixture comprising 16 PAHs (USEPA 610 method) was purchased
from Sigma Aldrich (Wyoming, USA). In addition, individual PAHs were purchased
from Accustandard (USA). All solvents viz. acetone, acetonitrile (ACN), cyclohexane,
dichloromethane (DCM), and hexane were of HPLC grade and purchased from Sigma
Aldrich. Deionised water (18.2 M Ω. cm) was obtained from Millipore water purifica­
tion system. Other chemicals/reagents such as sodium sulphate, silica gel (70–230
mesh, 63–230 µm, Sigma Aldrich) were of chromatographic grade (Sigma
Aldrich, USA).

2.4. Extraction and clean-up of samples

Ten grams of soil samples were extracted with 30 mL, 1:1, hexane: acetone in Teflon vials
using a microwave extraction system (Model MARS 6, CEM Corporation, USA) with the
following parameters: power: 1800 watts, ramp time: 20–25 minutes, hold time: 20 min­
utes, and temperature of 115°C (Modified USEPA 3546 method). The extracts were
evaporated to dryness under a gentle stream of nitrogen (N2 Fast Vap @ 40°C) and the
solvent was exchanged to cyclohexane (2 ml). Silica gel column chromatography was
used to cleanup the sample extracts (USEPA method 3630C). 10 g of silica gel was
activated at 130 � C for 24 hours and stored in desiccator. Sodium sulphate was activated
at 400 � C for 6 hours and kept in a desiccator. A slurry of silica gel (10 g) was made in DCM
(40 ml) and packed into the column. The column was washed with hexane (40 mL) and the
washings were discarded. Cyclohexane extract was loaded onto the column and rinsed
with another 2 ml cyclohexane. The column was eluted with hexane (65 mL) and dis­
carded. Finally, PAHs were eluted with hexane: DCM (6:4) mixture of 60 mL, and concen­
trated to dryness using nitrogen evaporator, solvent exchanged to acetonitrile (ACN), and
transferred to an HPLC vial for further analysis.

2.5. Analysis
PAHs were determined by the modified method of Meadows [27]. using ultra high
performance liquid chromatography (UHPLC, UltiMate 3000, Thermo Fisher Scientific,
USA) equipped with photodiode array detector (DAD), quaternary pump and a RP-C18
(10 cm × 2.1 mm × 2 µm), Supelco Ascentis® Express column. Detector was set to 254 nm
and was used for analysis. The mobile phase gradient used was (Acetonitrile: Water), 40%
acetonitrile held for 30 seconds; 50% acetonitrile from 30 seconds to 1 min; 60% acetoni­
trile for 1 to 1.5 min; 70% acetonitrile from 1.5 min to 3 min; 80% acetonitrile from 3 min to
17 min; 60% acetonitrile from 17 min to 18 min; 40% acetonitrile from 18 min to 19 min,
and flow rate (0.2 mL/min) was used. The temperature of the column was kept constant at
35°C. The standards were prepared in acetonitrile and 2 µL volumes of samples were
 INTERNATIONAL JOURNAL OF ENVIRONMENTAL ANALYTICAL CHEMISTRY 5

injected. Total run-time was 20 minutes. The method was validated by spiked samples and
details of the validation parameters were given in Table S2 of supplementary information.
For quantitative analysis, external calibration method was used, and PAHs were iden­
tified by comparing their retention times (RT) with that of standards.The PAHs standard
was made up of a mixture of 16 priority pollutant PAHs (USEPA 610 method), comprised
6-ring PAHs ([Indeo(1,2,3-cd)pyrene (IP) and Benzo(g,h,i)perylene (B[ghi]P)], 5-ring PAHs
[Benzo(k)fluoranthene (B[k]F), Benzo(a)pyrene (B[a]P), Dibenz(ah)anthracene (DB[ah]A),
and Benzo(b)fluoranthene (B[b]F)], 4-ring PAHs [Chrysene (CHR), Pyrene (PYR),
Fluoranthrene (FLA) and Benzo(a)anthracene (B[a]A)], 3-ring PAHs [Anthracene (ANT),
Acenaphthene (ACE), Acenaphthylene (ACY), Fluorene (FLO), Phenanthrene (PHE)],
2-ring PAHs [Naphthalene (NAP)]. The peak area response was used to calculate the
concentration of each PAHs compounds.

2.6. Analysis of soil organic carbon (SOC)

A TOC analyser (Shimadzu, Solid Sample Module; SSM-5000A) was used to determine the
total organic carbon content of soils using orthophosphoric acid.
SOC ¼ TC IC
(Where SOC, TC, and IC are stands for Soil Organic Carbon, Total Carbon, and Inorganic
Carbon, respectively.)

2.7. Quality control

Stringent quality control and quality assurance measures were followed during sample
collection, transport and analysis. Sample preparations were done in clean fume hood to
avoid external contamination. Accuracy of the analysis was checked by spiking with
known concentrations of all PAHs and the spike recovery was in the range of 85–120%.
Repeatability (% RSD) of standards and samples were below 10% (three replicates). Field
blank and method blanks were prepared and analysed between every 10 samples. A five
point calibration was developed with regression coefficient of >0.99 for all analytes. Limit
of detection (LOD) and limit of quantitation (LOQ) were calculated as 3 and 10 times the
standard deviation (σ) of blank signals (Table S2, Supplementary information).

3. Results & discussion

3.1. PAHs concentrations in soil
Table 1 summarises the concentrations of ∑16 PAHs and ∑7 carcinogenic PAHs in forty-
three different agricultural soil samples. The concentrations of the ∑16 PAHs in all samples
ranged between ND-122.52 µg/kg (AS17). The concentration of ∑7 PAHs (out of seven,
some are possible and some are probable carcinogenic) varied from ND-101.64 µg/kg
(AS17). The ∑16 PAHs concentrations showed wide variation among different agricultural
soil samples. In general, the observed ∑16 PAHs concentrations followed the order: sewage
irrigated soil (AS1–8)> fly ash treated agricultural soil (AS11–18) > horticulture plot soil
(AS40–43)> conservation agriculture soil (AS31–39)> organic farming soil (AS19–30). Out
6 D. K. YADAV ET AL.

Table 1. Pahs concentrations in agricultural soil samples from

various sites (µg/kg, dry wt.).
Sample No. ∑16 PAH ∑7 PAH SOC (%)
AS1 15.31 3.68 1.42
AS2 19.32 6.25 1.31
AS3 24.86 5.66 1.82
AS4 14.96 4.09 1.39
AS5 27.62 3.85 1.65
AS6 26.72 3.39 1.36
AS7 22.99 22.55 1.78
AS8 17.65 17.65 1.60
AS9 23.07 0.66 1.65
AS10 15.38 0.51 1.28
AS11 24.79 17.36 1.32
AS12 12.86 3.58 1.24
AS13 15.68 4.47 1.27
AS14 10.88 2.25 1.19
AS15 16.94 4.41 1.10
AS16 31.88 22.19 1.18
AS17 122.52 101.64 1.89
AS18 7.28 ND 1.01
AS19 10.43 ND 1.20
AS20 13.55 ND 1.18
AS21 28.22 12.05 0.97
AS22 9.65 1.77 0.92
AS23 26.71 16.22 0.83
AS24 12.41 2.52 0.81
AS25 7.74 ND 1.08
AS26 9.25 ND 0.98
AS27 10.64 ND 0.90
AS28 ND ND 1.08
AS29 9.03 1.45 0.80
AS30 13.9 3.55 0.76
AS31 11.37 1.12 1.20
AS32 6.22 ND 1.10
AS33 6.63 3.85 0.70
AS34 6.39 1.98 0.90
AS35 5.98 2.48 0.80
AS36 3.82 1.68 0.78
AS37 11.21 1.55 1.10
AS38 13.35 ND 1.20
AS39 8.78 ND 0.90
AS40 2.21 ND 0.84
AS41 17.95 8.75 1.20
AS42 13.33 2.66 1.08
AS43 6.45 ND 0.98
ND: Not detected; SOC: Soil organic carbon.

of eight fly ash treated soils, only two samples (AS17, AS18) had higher PAHs levels. These
two soils were amended with 18% of fly ash and remaining soils (AS11–16) were treated
with 1–10% of fly ash. Our results were similar to previous studies that showed higher
levels of PAHs [28]. Other than the two higher fly ash treated soils (AS17, AS18), PAHs
content of the remaining fly ash treated samples were lower than that of sewage-irrigated
soils.
PAHs concentrations observed in organic farming and conservation agriculture soils
were much lower than that of sewage irrigated and fly ash treated soils. Among individual
PAHs, naphthalene (NAP), and fluoranthene (FLA) were not detected in any soil samples
 INTERNATIONAL JOURNAL OF ENVIRONMENTAL ANALYTICAL CHEMISTRY 7

120

∑16 PAH

100 ∑7 PAH
B[a]Peq

80
Concentrations, µg/kg, dw

60

40

20

0
AS1
AS2
AS3
AS4
AS5
AS6
AS7
AS8
AS9
AS10
AS11
AS12
AS13
AS14
AS15
AS16
AS17
AS18
AS19
AS20
AS21
AS22
AS23
AS24
AS25
AS26
AS27
AS28
AS29
AS30
AS31
AS32
AS33
AS34
AS35
AS36
AS37
AS38
AS39
AS40
AS41
AS42
AS43
Figure 2. Comparison of ∑16 PAHs, ∑7 PAHs and B[a]Peq concentrations in different agricultural soil
samples (µg/kg, dw).

(Table S1 , Supplementary information). The most prominent PAHs found among various
samples were benzo(b)fluoranthene, benzo(a)pyrene, pyrene, benzo(a)anthracene, and
benzo(k)fluoranthene. The highest ∑16 PAHs concentration 122.52 µg/kg was found in the
fly ash treated agricultural soil sample (AS17) compared to other samples (Figure 2).
However, the observed concentrations were less than the Canadian soil quality criteria
for agricultural land use (CCME 2010); i.e. individual PAH concentration in soil is allowed to
be up to 100 µg/kg.
Singh et al [12]. reported a higher concentration of total PAHs up to 1550 µg/kg in
agricultural soils of Delhi (India) which is 12 times greater than the present study. This
might be due to biomass burning, industrial and vehicular emissions, which are several
times higher in Delhi compared to the small city such as Bhopal. Similarly, another
study carried out in urban agricultural soils reported a higher concentration of total
PAHs ranged from 830–3880 µg/kg [11]. The greater concentration could be attributed
to emissions from nearby power plants and automobile activities. Masih et al [29].
reported a higher concentration of total PAHs up to 6730 µg/kg in agricultural soils of
Agra city (India). This higher concentration could be attributed to stubble and biomass
burning activities prevalent in the area. The lower concentration of PAHs observed in
this study could be due to the absence of any major emission sources nearby the
study area.
The results of present work of PAHs concentrations were compared with global studies
(Table 2). Wilcke et al [30]. reported slightly lower concentration of total PAHs in agricul­
tural soils in Brazil than the present study. PAHs concentration from agricultural soils in UK
8 D. K. YADAV ET AL.

Table 2. Comparison of PAHs concentrations in various countries agricultural soil samples.

Sample site Conc. (µg/kg, dw; dry weight) Number of PAH analysed Reference
Brazil 96 20 [30]
UK 190 12 [31]
UK 450 16 [32]
USA 220 19 [33]
Japan (Sapporo) 46-2000 21 [34]
Korea (Nationwide) 38-1057 16 [35]
India (New Delhi) 1550 16 [12]
Shanghai (China) 92.2–2062.7 16 [36]
Huanghuai, China 15.7–1247.6 16 [37]
India (Agra) 6730 14 [29]
China 77.3-1188 16 [10]
India (New Delhi) 830-3880 16 [11]
India (Bhopal) 122 16 Present study

and USA reported by Wild and Jones [31], and Wagrowski and Hites [33] were 1.5 and 1.75
times higher than the present study, respectively. The higher concentration may be due
to more industrial activities in the immediate vicinity. The total PAHs concentration of
agricultural soils from UK was 4 times greater than the present study [32]. In agricultural
soils of Shanghai and Huanghuai, China have reported higher ∑16 PAHs concentrations
92.2–2062.7 µg/kg and 15.7–1247.6 µg/kg respectively. The higher concentration of PAHs
in these agricultural soils may be due local anthropogenic activities [36,37].
Few reports are available on the PAHs contamination in agricultural soils due to
sewage water irrigation and PAHs uptake and translocation by plants [38]. Kumar et al
[39]. reported the concentrations of ∑16 PAHs in wastewater, sediments up to 35.22 µg/L,
and 19,321 µg/kg (dry wt.), respectively. Zhang et al [20]. investigated the uptake of 16
PAHs in a wastewater-irrigated maize crop. The concentrations of ∑16 PAHs in surface soil,
roots, leaves, and grains were found to be up to 479, 1414, 1016 and 336 ng/g, respec­
tively [20]. Hence, assessment is very essential for PAHs levels in wastewaters irrigated
soils.

3.2. PAHs profiles

Figure 3 depicts PAH profiles in relation to the number of benzene rings in the agricultural
soil samples. Almost all of the samples were totally devoid of the two-ring PAHs
(Naphthalene). The proportion of 4 and 5 ringed PAHs in sewage-irrigated agricultural
soil was higher than the proportion of 3-ringed PAHs. Additionally, it was reported that
the sewage-irrigated agricultural soil lacked two-ring PAHs [40]. Likewise, fly ash treated
agricultural soil showed the higher proportion of 4 and 5-ringed PAHs than 3- ringed
PAHs and 2-ringed PAHs were absent. This shows the application of fly ash in soil result in
the highest percentage of 4 and 5-ringed PAHs in samples (AS13-AS18). According to
Sahu et al [41], fly ash from coal-fired thermal power plants contained up to 936 µg/kg of
∑16 PAHs. The feed coal properties such as an aromatic content, chemical composition, C/
H ratio, trace metals and particle size of ash have an impact on PAHs formation during
combustion [41]. PAH emissions from power plants are influenced by the quality of the
feed coal. Indian coal, which is largely utilised in thermal power plants, is of poor quality
[41]. The majority of the 4-ringed PAHs (40–100%) were found in soil samples from
 INTERNATIONAL JOURNAL OF ENVIRONMENTAL ANALYTICAL CHEMISTRY 9

2 rings 3 rings 4 rings 5 rings 6 rings

100%

90%

80%

70%

60%
Contribution

50%

40%

30%

20%

10%

0%
AS1
AS2
AS3
AS4
AS5
AS6
AS7
AS8
AS9
AS10
AS11
AS12
AS13
AS14
AS15
AS16
AS17
AS18
AS19
AS20
AS21
AS22
AS23
AS24
AS25
AS26
AS27
AS28
AS29
AS30
AS31
AS32
AS33
AS34
AS35
AS36
AS37
AS38
AS39
AS40
AS41
AS42
AS43
Samples

Figure 3. Average contribution of PAHs with different rings in different land use soils [6-ring (IP and
B[ghi]P); 5-ring PAHs (B[a]p, B[k]F, DB [ah]a and B[b]F; 4-ring (CHR, PYR, FLA and B[a]A; 3-ring (ANT,
ACE, ACY, FLO, and PHE); 2-ring (NAP)] in diverse agricultural soil samples of central India, Bhopal.

organic farming, however, some samples also contained 5-ringed PAHs. Conservation
agriculture soil sample showed the higher proportion of 4 -ringed PAHs followed by
5-ringed and 3-ringed PAHs. The horticulture land use soil showed the higher proportion
of 4 -ringed PAHs followed by 5-ringed PAHs. Combustion of agricultural residue and
biomass at low-temperatures and low oxygen conditions promotes the formation of low
molecular weight (LMW) PAHs [42]. Furthermore, agricultural soils are prone to the
unintentional introduction of fossil fuels derived PAHs, due to the use of farm vehicles.
The usage of generators, tractors, and other agricultural equipment could contribute to
higher molecular weight (HMW) PAHs in agricultural soils [11].

3.3. Correlation of PAHs and with soil organic carbon (SOC)

Soil total organic carbon (SOC) is a key indicator of PAH sorption, sequestration, and fate
[43]. PAHs have a significant interaction with soil organic matter (organic carbon) [44,45].
The majority of the 3,4,5-ring PAHs were highly correlated with each other (p < 0.05)
(Table 3). Similar results were also reported by Agarwal et al [11]. However, the authors did
not find a significant positive connection with 6-ring PAHs. Also, Singh et al [12]. reported
similar findings. Among individuals PAHs, 2-ring PAH (Naphthalene, NAP) was found in
none of the agricultural soil samples. SOC showed significant correlations with ∑16 PAHs, 7
carcinogenic PAHs, and 3-ring PAHs (except ANT), 4-ring PAHs (except B[a]A and CHR),
and 5-ring (excluding B[k]F) PAHs in this study. In comparison to 3,4 and 5-ringed PAHs,
6-ring PAHs showed poor correlation with SOC.
 10

Table 3. Pahs concentrations and SOC (%) in agricultural soils: correlation coefficient matrix.
D. K. YADAV ET AL.

NAP ACY ACE FLO PHE ANT FLA PYR B[a]A CHR B[b]F B[k]F B[a]P B[ghi]P IP DB [ah]A ∑16 PAHs ∑7 PAHs SOC %
NAP 1.00
ACY - 1.00
ACE - −0.07 1.00
FLO - −0.04 −0.05 1.00
PHE - −0.06 0.04 0.77*** 1.00
ANT - −0.04 0.80*** −0.03 −0.03 1.00
FLA - - - - - - 1.00
PYR - 0.20 0.23 0.28* 0.43* 0.04 - 1.00
B[a]A - 0.17 0.17 −0.09 −0.12 0.05 - 0.18 1.00
CHR - −0.07 −0.07 0.48*** 0.35* −0.04 - 0.16 −0.14 1.00
B[b]F - 0.07 0.41** −0.09 0.02 0.25 - 0.41** 0.55*** −0.14 1.00
B[k]F - - - - - - - - - - - 1.00
B[a]P - −0.08 −0.06 0.92*** 0.70*** −0.03 - 0.18 −0.16 0.49*** −0.15 - 1.00
B[ghi]P - 0.37* −0.04 −0.03 −0.03 −0.02 - 0.06 −0.08 −0.04 −0.08 - −0.05 1.00
IP - 0.17 −0.05 −0.03 −0.04 −0.03 - 0.00 −0.01 −0.05 −0.04 - 0.01 −0.03 1.00
DB [ah]A - −0.04 −0.04 −0.03 −0.03 −0.02 - −0.22 −0.08 −0.04 −0.08 - −0.05 −0.02 −0.03 1.00
∑16 PAHs - 0.03 0.13 0.90*** 0.75*** 0.09 - 0.48** −0.02** 0.47** 0.08 - 0.93*** −0.03 0.09 −0.09 1.00
∑7 PAHs - −0.05 −0.05 0.92*** 0.69*** −0.03 - 0.21 −0.09 0.49*** −0.08 - 0.99*** −0.06 0.10 −0.03 0.95*** 1.00
SOC (%) - 0.31* 0.29* 0.37* 0.46** 0.11 - 0.39* 0.13 0.17 0.40** - 0.45** 0.02 0.17 −0.24 0.57*** 0.47* 1.00
*Significant at 10% level of significance (LOS); ** Significant at 5% LOS; *** Significant at 1% LO.
 INTERNATIONAL JOURNAL OF ENVIRONMENTAL ANALYTICAL CHEMISTRY 11

3.4. Isomer pair ratios for possible source of PAHs

For the source identification of PAHs, the determination of the diagnostic ratios is widely
used [49]. Table 4 shows some of the most common diagnostic ratios used by previous
researchers [46–48]. There are two types of PAHs, a) Low molecular weight (LMW) PAHs,
which contain 2,3-aromatic rings, and b) high molecular weight (HMW), which contains
four or more rings. Previous investigations have shown that, LMW/HMW <1 suggests
pyrogenic sources, such as incomplete burning of fossil fuels or wood, whereas LMW/
HMW >1 indicates petrogenic sources, such as spilled oil or petroleum [46]. Based on the
average contribution of PAHs with various rings in soils of different land uses, it was found
that HMW(4- and 5-rings) PAHs were observed in most samples, indicating, biomass
burning (grass, wood, and coal combustion), emissions from high temperature industrial
process could be the major source of PAHs (Figure 3). The intrasource heterogeneity is
evident in PAH diagnostic ratios, although intersource similarity is also evident. Petroleum
appears to have a FLO/(FLO + PYR) ratio closer to 0.4, with values ranging from 0.4 to 0.5
typical of liquid fossil fuel combustion and the values greater than 0.5 more typical of grass,
wood, or coal combustion [47]. Based on the ratio value described in Table 4, the (B[a]A/
B[a]A+CHR ratio implies petroleum, either petroleum or combustion, and combustion [48].
A variety of anthropogenic activities release PAHs, which can be traced back to
petrogenic and pyrogenic sources by isomer pair ratios [48]. In order to identify the
most probable source, PAH isomer pair ratios have also been reported Ping et al [50].
and Pies et al. 2008 [51]. In this study, 3-ring PAHs were predominant in sewage irrigated
and transformer oil contaminated soils, suggesting the petrogenic (fossil fuel) sources of
origin. The ratio of benz[a]anthracene to benz[a]anthracene plus chrysene (B[a]A/B[a]A
+CHR) has been used to identify possible sources of PAHs [46,48]. In this investigation, the
ratio of B[a]A/B[a]A+CHR was less than 0.35. According to Yunker et al [48], petroleum and
combustion sources were indicated by values of <0.2 and >0.35, respectively, while values
in the middle indicate mixed origin, such as petroleum combustion. Hence, burning of
biomass could be the major source of PAHs in the agricultural soil of the area.

3.5. Soil toxicity assessment based on ∑7 PAHs concentrations

Out of the seven carcinogenic PAHs, only benzo[a]pyrene has enough toxicological data
to derive a carcinogenic potency factor [52]. Toxicity equivalence factors (TEFs) were used
to compare the carcinogenicity of PAHs to benzo[a]pyrene equivalent concentrations
(B[a]Peq) [53]. According to the USEPA, the calculated TEFs for CHR, B[k]F, B[a]A, B[b]F, IP,

Table 4. Diagnostic ratios used for source identification.

Ratio Value range Source Reference
∑ Low/High MW < Pyrogenic [46]
>1 Petrogenic
FLO/(FLO+PYR) <0.5 Petrol emissions [47]
>0.5 Diesel emissions
B[a]A/B[a]A+CHR <0.2 Petrogenic sources [48]
0.2–0.35 Coal combustion
>0.35 Vehicular emissions/combustion
12 D. K. YADAV ET AL.

B[a]P, and DB [ah]A are 0.001, 0.01, 0.1, 0.1, 0.1, 1 and 1, respectively [54]. In this
investigation, only seven PAH concentrations were used to calculate the corresponding
TEFs into benzo[a]pyrene equivalent concentrations (B[a]Peq) for each sample [54]. The
following formula was used to calculate total (B[a]Peq) concentrations.
X
B½a�Peq Conc: ¼ Ci � TEFi

Where, individual PAH concentration and toxic equivalency factor of individuals PAH are
expressed by Ci and TEFi, respectively. The B[a]Peq concentrations in different agricultural
soils samples ranged between 0.00–101.66 µg/kg (dry wt.). Fly ash treated agricultural soil
sample (AS17) reported the highest total B[a]Peq concentration indicating a higher
carcinogenic potency (Figure 2). Sahu et al [41]. reported total B[a]Peq concentration up
to 18 µg/kg in fly ash from coal fired power plants of India. When compared to other land
use soil samples such as AS7, AS8, AS11, AS16, AS23, and AS41, B[a]Peq concentration of
fly ash treated soils were 4–10 times higher. This shows fly ash amendment of agricultural
soils increase the carcinogenic potential of soils and may pose risk via food chain.

4. Conclusions
In agricultural soil samples of various land-use patterns, 14 PAHs were reported, out of the
16 EPA priority PAHs.Total PAHs concentrations from different land use agricultural soils
varied from ND-122.52 µg/kg. The observed ∑16 PAHs concentrations followed the order
of sewage irrigated soil ≥ fly ash treated soil >horticulture plot soil> conservation
agriculture soil> organic farming soil. However, B[a]Peq concentrations 4–10 times higher
in fly ash treated agricultural soil samples compared to other land use areas. PAHs with
higher carcinogenic potency in soils indicate a significant level of health risk. Fly ash
amendment should be used with caution, considering the higher BaP concentration.
Similarly, wastewater irrigation should be used with caution due to the higher PAHs
concentration. Further studies are required in the area to identify the possible uptake
by plants and accumulation pattern.

Acknowledgment
The authors thank the Director, ICAR-IISS, Bhopal for his continuous encouragement. DKY thanks
the Director, CSIR-NEERI, Nagpur for making available research facilities at Stockholm Convention
Regional Center (SCRC) on Persistent Organic Pollutants (POPs), CSIR-NEERI, Nagpur.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
This work does not receive specific funding.
 INTERNATIONAL JOURNAL OF ENVIRONMENTAL ANALYTICAL CHEMISTRY 13

ORCID
Dinesh Kumar Yadav http://orcid.org/0000-0001-6615-1002
Asirvatham Ramesh Kumar http://orcid.org/0000-0002-6974-9955
Somasundaram Jayaraman http://orcid.org/0000-0003-3486-4109
Sangeeta Lenka http://orcid.org/0000-0002-8933-5506
Abhijit Sarkar http://orcid.org/0000-0003-3284-2571

Authors contributions
ARK: Designing, conceptualisation, draft editing, and supervision. DKY, SG: Methodology, data
creation, analysis, and original draft writing. DKY, SL and AS: Statistical analysis. SJ, JKS and AKP:
Draft editing and guidance. All authors read and approved the final version of the manuscript.

Consent to publish
All authors consent for publication.

Data Availability statement

All data generated for this study are included in the manuscript and supplementary data sheet.

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