Level of pesticides contamination in the major river systems: A review on South Asian countries perspective.

Pesticides are chemical compounds used worldwide for different purposes. These chemicals are well known for their long life, high toxicity, and slow degradation process. Many developed countries including South Asian countries banned the use of pesticides for their adverse effects. However, several pesticides are found incessantly in water and soil. To highlight the recent situation of pesticide contamination in South Asian river systems, we have studied 136 relevant articles published from 2015 to 2020. Articles were gathered using several commonly available search engines and organized according to information related to river systems of South Asian countries. After thoroughly examining those research articles, we summarized that most of the river systems are contaminated by pesticides, where DDTs, HCHs, endosulfan, heptachlor, and chlorpyrifos are the key recognized compounds among them. Comparing the level of pesticides with standard guidelines, we found that the Tapi River and Chilika Lake of India are considerably more contaminated than other river basins. Multivariate analyses identified the industrial discharge and agricultural run-off of chemicals as the probable sources of pesticides in these rivers. By analyzing the amount of annual pesticide production, their use, and accordingly their considerable presence in the water systems of the South Asian countries, it is evident that the banned pesticides are used regularly by these countries and thus contaminating the environment. Therefore, the formulations of appropriate rules and their enforcement to control the manufacture and solicitation of such pesticides are an urgent need to save the environment.

Introduction

It has no longer been prolonged that people started the use of chemical compounds to increase agricultural productivity [1]. These chemicals are regularly referred to as pesticides, even though they are determined underneath unique brands and names in the market. A large number of pesticides are synthetic compounds and are usually used to forestall, crush, repulse, and moderate bugs and vermin [2]. These are also extensively employed for farming, forestry, parks, industrial sites, sports fields, and educational facilities [3, 4]. Many pesticides are capable of breaking down a large range of pests or weeds, although others are devised for particular pests or pathogens [5]. Based on a report by Pimentel [6], only a few percentages (0.1 %) of the applied pesticides spend on targeted pests whereas 99.9 % remain in the environment. These chemical compounds are now a worldwide concern for their unfriendly and poisonous impacts on humans, including immunological, cancer-causing, and regenerative issues [1, 7]. Currently, the presence of Persistent Organic Pollutants (POPs) in surface water makes all environmental scientists and policymakers curious and worried [8]. The attributes of pesticides, for example, the long half-life, bioaccumulation, high lipophilicity, and the capability of long reach transport, have expanded the odds of defiling the air, water, and soil even after numerous long periods of usage [5]. Pesticides are classified depending on their chemical nature (e.g., organochlorines and carbamates), application requirements (e.g., agriculture and public health), and targeted organism (e.g., herbicides and fungicides). Some common and widely used pesticides are endosulfan, dichlorodiphenyltrichloroethane (DDT), dichlorodiphenyldichloroethylene (DDE), dichlorodiphenyldichloroethane (DDD), hexachlorocyclohexane (HCH), chlorpyrifos, methyl parathion, heptachlor, carbofuran, and pentachlorophenol [5, 9, 10, 11]. Although some studies have shown that organic compounds such as pesticides, polycyclic aromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs) have frequently been spotted in aquatic environments around the globe [8, 12, 13], however, pesticides are of major concern nowadays among these compounds due to their extensive production and use, and high toxicity. Owing to their low solubility in water, these chemicals tend to deposit on sediment beds or living beings [14, 15, 16, 17]. According to a survey of the World Health Organization (WHO), a large portion of pesticides were used by developing countries for the maximum outcome and advanced quality product [18, 19, 20]. Because of their bio-magnification of the food chain, many nations in the developed universe and some emerging countries banned these compounds for general use for the last 25-years. However, some surveys and researches by FAO [21], Calamari et al. [22], Wejuli et al. [23], Kishimba et al. [24], Chakraborty et al. [25], Ali et al. [26], Saad et al. [8], Mitra et al. [27], Nag et al. [28], Hashmi et al. [29], and Ramírez-Morales et al. [1] reported the existence of these banned chemicals in the aquatic environment, sediments, soils, flora, and fauna of the rivers, lakes, and some other small or large reservoirs of water, especially in developing countries [19, 30].

South Asia, the southern part of Asian countries, consists of Afghanistan, Bangladesh, Bhutan, India, Maldives, Nepal, Pakistan, and Sri Lanka (Figure 1). It is geographically distributed by the Indian Plate and mostly demarcated to the south by the Indian Ocean and the north by the Himalayas, and Karakoram mountains [31]. South Asia occupies about 11.71 % of the Asian landmass or 3.5 % of the global land area. The population is about 1.89 billion (~39.49 % of the Asian population) which is around a fifth (or around 24 %) of the world's population and thus making it the most heavily developed and overcrowded urban region in the world [32, 33]. Most of the South Asian nations are developing countries. With a huge population, this zone of Asia is viewed as the most water-stressed region of the globe, both in terms of quantity and water quality [34]. The geographical map of South Asian countries with major river systems is shown in Figure 1.

Figure 1

Map of South Asian countries with major river systems.

Many studies have focused on the utilization of chemical pesticides in South Asia in recent decades [19]. The annual use of pesticides in some of the South Asian countries such as India, Pakistan, Bangladesh, Nepal, and Bhutan are 56120, 27885, 15833, 454, and 12 tonnes, respectively [30]. Each South Asian country has rules and regulations for using pesticides and according to which, most of the pesticides are forbidden for general use in these countries. Despite this, several studies [1, 8, 21, 22, 23, 24, 25, 26, 27, 28, 29] proved that the use of pesticides is not being stopped, in other words, pesticides are using illegally. So, the actual scenario of water pollution and the quality of water is different as we thought it is. However, no extensive research works, as well as review articles revealing the complete scenario of the pesticide contamination of South Asian riverine systems are not reported yet, except some discrete studies for a specific river or aquatic system of specific countries [35]. Further, the extensive production and use of pesticides may affect the water and soil environment of the neighboring countries [35]. The South Asian counties have quite similar weather and aquatic system. Not only these but also they have uniformity in culture, food habits, and agro-based products. These countries have some transboundary rivers between them. According to the report of a newspaper, Impakter [36] South Asian countries share 20 major rivers among them. Although some researcher separately reported the pesticides level in the river systems of some South Asian countries [1, 8, 21, 22, 23, 24, 25, 26, 27, 28, 29, 36], the combined scenario for the pesticides contaminations in this large interconnected region is still not extensively studied, which is very important for taking appropriate mitigation control measures for a sustainable environment. Given the lack of appreciation in the previous studies, this review work makes a bridge to close the knowledge gap. Thus, in this review article, we intend to make a comprehensive structure that demonstrates the quality of the river systems in South Asia and its level of contamination by pesticides. For this purpose, we have investigated around 136 documents including journal articles, books, and other trustworthy national and international materials published from prominent organizations during 2015–2020 intending to explore the most recent status of the river systems. From those articles, we have extracted and highlighted significant information, which demonstrates the pollution level of the river systems by pesticides. In this way, this study significantly contributes to the existing literature. This review article will also be helpful for planners, policymakers, and researchers dealing with pesticide pollution in the river environment to take necessary mitigation, remediation, controlling, and managing steps for promoting a healthy environment in the universe.

Methodology

The keywords such as South Asia, country name, river water, river sediment, and pesticides were used to identify as many relevant articles using several search engines such as PubMed, ScienceDirect, Web of Science, Scopus, Google Scholar, and Google. Around thousands of research articles were found by search, however, most of them were irrelevant to our topics. Thus, most of the documents were excluded based on the title, abstract, keywords, and irrelevance. Duplicated studies were also avoided. Further, no suitable and relevant review articles reporting the pesticide contamination level in South Asian riverine systems were found. Instead of specifying certain pesticides, we have tried collecting data as much as we found from approximately 136 full-text relevant articles retrieved after initial screening. Further eligibility of the 136 articles to be included in the review was tested based on their reference lists, important information, sampling years, relevance, and quality issues. Some studies were avoided because of their small sample number, less selection of parameters, repetitive data representations, and sampling years. Based on the eligibility test and aforementioned selection criteria, a total of 95 relatable articles were identified as more effective and accordingly included in the current review as the main studies. Studies of these databases were conducted from September to November 2020.

Afterward, basic information reported in the selected articles such as the number of pollutants and their concentration, degree of use of pollutants, geographical location of the rivers, amount of waste received per year by the rivers, and the significance of the rivers on the neighboring regions were collected and sorted. Ultimately, we assemble pertinent data from these articles on surface water and sediments of several rivers of South Asian countries. We categorized the data according to countries, rivers, and deltas. Eventually, the data were extracted and converted into a uniform unit with each other after plotting in Tables. To summarize the pollution level and understand its probable impact on the environment, we have compared all data with the critical maximum concentration, health value, and maximum acceptable value provided by the recognized organizations. Multivariate statistical approaches including Principal Component Analysis (PCA), Cluster Analysis (CA), and Pearson's Correlation Matrix (CM) were also utilized to study the interrelationships among the commonly reported pesticides and the considered South Asian river systems, and thereby to predict the probabilistic origin of the pesticides in the river systems [37, 38]. The whole methodology described above is depicted briefly in the form of a flow diagram as shown in Figure 2.

Figure 2

Flow diagram for the identification, exclusion, and inclusion (or selection) of the relevant studies considered in the review along with the processing steps of the information and data reported in the articles.

Results and discussion

India

India, the leading South Asian country with a population of ~1.3 billion is among the most drinking water-focused country across the globe [39, 40]. Since the 1970s, the Green Revolution has started in India to enhance the excess food production but at the same time, resultant in groundwater overdraft and the opportunity of the overview of point and nonpoint-sourced anthropogenic pollutants in the ecosystem [41]. To enhance agricultural efficiency, the ubiquitous use of chemical fertilizers and pesticides was initiated in the later 1980s-early 1990s, which may have augmented the pollutant load through non-point sources and urban agricultural discharge system [42]. About 50,000 tonnes of pesticides were utilized in the Indian agricultural field from 2011 to 2012 [43,44]. India is the major pesticide manufacturing country in South Asia (12th in the world). For pesticide production, India ranked second in Asia during 2005–2006 with the production of 82,000 tonnes of pesticide. After 2 years in 2008, a green peace report ranked India, the largest pesticide-producing country in Asia, with an annual production of 90,000 tonnes [45, 46].

At present, more than 800 different types of pesticides are employed for controlling insects, rodents, fungi, and unwanted plants in the process of agricultural production. These toxic compounds leave the products or degrade in air, soil, and water, and may cause potential harm to the environment. Further, the pesticide chemicals can be transported from the cultivation fields to the human body via the food chain imposing tremendous human health hazards [47]. India has 16 % of the world's population, while they have only 4 % of the freshwater reserve. The quality of freshwater in India and other related biodiversity is currently facing severe challenges due to an extensive rise in economy, increased industrialization, and the broad agricultural sector [48]. After the death of 100 people in India due to having pesticide-contaminated wheat flour, a committee was set up by the Indian Council of Agricultural Research (ICAR) to recommend potential solutions to tackle toxicity caused by pesticide and their residue in food products [49]. In 2009, WHO regional office for Southeast Asia analyzed and recorded the poisoning-related cases in India from January 2000 to December 2006 and submitted it to the Poison Information Center (PIC) at the National Institute of Occupational Health, Ahmedabad, Gujarat, India [10]. According to them in total 2395 cases, 1636 cases are accounted for pesticide. About 68.3 % of poisoning cases are due to pesticide pollution. Among those cases, 1006 were owing to agricultural pesticides and 630 cases were due to household pesticides.

Brahmaputra River

The Brahmaputra is a transboundary river. It passes in India by entering Arunachal Pradesh from the Himalayan Mountain and flows from Assam to Bangladesh. In Bangladesh territory, it is known as Jamuna. It converges with the Meghna River before falling into the Bay of Bengal forms the world's biggest delta, the Sunderbans delta. Sunderbans, the world's largest mangrove forest, outspread West Bengal in India to Bangladesh. Brahmaputra River provides a permanent source of freshwater to the plains of India's eastern, northeastern states, and Bangladesh to support agriculture, human, and industrial use [48]. The population surrounding the Brahmaputra basin is about 29.1 million, who uses 24 km3 of surface water per year [25]. A group of researchers [25] collected the surface water from different locations of the Brahmaputra River targeting to analyze organochlorine pesticides (OCPs) in their samples. From their viewpoint, this group of pesticides is used heavily around the neighboring areas of the Brahmaputra River. In the river, ∑OCPs are found in higher concentrations and varied from 0.002-0.245 μg L−1 (Mean ± SD: 0.047 ± 0.067 μg L−1). γ-HCH exhibited the maximum level of detection among HCHs which demonstrated the continuous use of lindane (γ-HCH). ∑DDTs also found with high concentration as ND-0.225 μg L−1 (Mean ± SD: 0.030 ± 0.066 μg L−1) in which o,p'-DDT showed the highest concentration and p,p'-DDD showed the second highest. Details of the concentration of the pesticides are given in Table 1.

Table 1

Level of pesticides in the water and sediment of the considered South Asian river systems.

Pesticides	South Asian countries∗
	India				Pakistan		India		Pakistan
	Concentrations (μg L−1) of pesticides in the river/lake water						Concentrations (ng g−1) of pesticides in the river sediment
	Brahmaputra River	Chilika Lake	Tapi River	Hooghly River	Kabul River	Indus River	Tapi River	Hooghly River	Indus River
α-HCH	0.001 ± 0.002	0.025–0.265	-	0.003 ± 0.005	-	0.018 ± 0.008	-	0.06	1.01 ± 0.10
β-HCH	0.0004 ± 0.002	-	-	0.003 ± 0.006	-	0.022 ± 0.007	-	0.01	0.89 ± 0.27
γ-HCH	0.006 ± 0.007	0.03–6.08	-	0.013 ± 0.008	-	0.0019 ± 0.0003	-	0.07	0.49 ± 0.18
δ-HCH	0.001 ± 0.003	0.05–0.26	-	0.019 ± 0.033	-	0.003 ± 0.001	-	0.06	0.89 ± 0.08
Ɛ-HCH	-	-	-	-	-	-	-	0.02	-
∑HCHs	0.008 ± 0.008	-	-	0.003 ± 0.028	-	0.045 ± 0.010	-	-	-
o,p'-DDD	-	8.99–23.4	-	-	-	0.008 ± 0.005	-	0.09	1.10 ± 0.47
p,p'-DDD	0.014 ± 0.033	-	-	-	-	0.009 ± 0.006	-	0.08	1.44 ± 0.53
o,p'-DDE	-	0.116	-	-	-	0.013 ± 0.010	-	0.29	2.10 ± 0.56
p,p'-DDE	-	0.017–0.062	-	-	-	0.017 ± 0.013	-	1.60	1.96 ± 0.97
o,p'-DDT	0.015 ± 0.034	-	-	0.003 ± 0.008	-	0.011 ± 0.009	-	0.39	1.52 ± 0.74
p,p'-DDT	0.001 ± 0.002	-	-	0.0003 ± 0.001	-	0.016 ± 0.012	-	0.17	1.54 ± 0.86
∑DDTs	0.030 ± 0.066	-	0.11 ± 0.58	0.004 ± 0.008	-	0.074 ± 0.004	0.17 ± 0.58	2.62	-
HCB	-	-	-	-	-	0.002 ± 0.001	-	0.19	0.05 ± 0.04
Cis-Chlordane	-	-	-	-	-	0.001 ± 0.000	-	-	0.03 ± 0.02
Trans-Chlordane	-	-	-	-	-	0.007 ± 0.004	-	-	1.09 ± 0.31
Heptachlor	0.001 ± 0.003	0.04–1.0	-	0.014 ± 0.006	-	0.014 ± 0.009	-	-	0.83 ± 0.54
Aldrin	0.0003 ± 0.001	-	-	0.004 ± 0.004	-	-	-	-	-
Dieldrin	0.004 ± 0.006	-	-	0.0004 ± 0.002	-	-	-	-	-
∑Aldrin	0.004 ± 0.006	-	-	0.004 ± 0.004	-	-	-	-	-
α-Endosulfan	0.001 ± 0.002	-	3.23 ± 7.08	-	-	-	2.01 ± 2.70	-	-
β-Endosulfan	0.003 ± 0.011	-	4.51 ± 8.16	0.002 ± 0.004	-	0.0007 ± 0.0005	3.05 ± 5.04	-	0.03 ± 0.01
∑Endosulfan	0.003 ± 0.013	-	-	0.002 ± 0.004	-	-	-	-
∑OCPs	0.047 ± 0.067	-	-	0.062 ± 0.035	-	0.143 ± 0.052	-	-	14.83 ± 2.95
Chlorpyrifos	-	0.019–2.730	0.09 ± 0.18	-	0.009 ± 0.003	0.007 ± 0.000	0.01 ± 0.03	-	-
Dichlorvos	-	0.647		-	-	-	-	-	-
Methyl parathion	-	-	0.09 ± 0.16	-	-	-	0.18 ± 0.27	-	-
Methomyl	-	-	-	-	5.50 ± 3.90	0.20 ± 0.03	-	-	-
Carbaryl	-	-	-	-	0.001 ± 0.000	0.001 ± 0.001	-	-	-
Carbofuran	-	-	-	-	0.003 ± 0.003	0.005 ± 0.001	-	-	-
Triclosan	-	-	-	-	0.002 ± 0.001	0.002 ± 0.001	-	-	-
Caffeine	-	-	-	-	30.2 ± 28.3	8.0 ± 0.2	-	-	-
References	[25]	[28]	[29]	[25]	[8]	[8, 26]	[29]	[27]	[26]

No reportable works were found for the pesticides level in the river system of other South Asian countries such as Nepal, Bangladesh, Afghanistan, Bhutan, Maldives, and Sri Lanka during the considered time periods. N.B.: The values written after ‘±’ symbol is the standard deviation. ‘-’ means results are not reported or not detectable.

Chilika Lake

Chilika Lake, covering an area of 1100 km2, is Asia's largest and the world's second-largest coastal lagoon. It spans over three districts of Odisha, India [28]. This lake is the first Indian wetland which is categorized as a wetland of broad international importance under the Ramsar Convention in 1981 due to its enriched biodiversity and great socioeconomic significance. Wide and diversified plant and animal supplies, including fisheries, are maintained by the ecology of the lake. However, the presence of pesticide residues in the lake from various sources degrading the overall environment of the lake ecosystem [28]. In 1993, the name of Chilika Lake is added to the Montreux Record (threatened list of Ramsar site) [50] due to the change of its ecological character caused by the declination of its ecosystem [51]. To evaluate the recent status in terms of pesticide contamination, Nag et al. [28] carried out multiple sampling from the potentially contaminated sites of Chilika Lake from 2012 to 2016. They analyzed surface water, sediment, and fish samples of the lake by targeting sixteen organochlorine pesticides, ten organophosphate pesticides, and eight synthetic pyrethroids, and reported the existence of the considered amount of HCHs, DDD, DDE, heptachlor, chlorpyrifos, and dichlorvos in the water samples. However, they observed that the level of detection of pesticides varied from season to season. Interestingly, no such pesticides were found in the sediment samples, except for fenpropathrin (81.0 ng g−1) which is one of the synthetic pyrethroids. According to their research, the aquatic animal intake pesticides resulting in the decrease of pesticides concentration in water [28]. This is supported by the fact that the research team also found a high amount of pesticides in fish flesh [28]. Table 1 shows detailed information on the concentration of pesticides in the water of Chilika Lake.

Tapi River

The Tapi River, a renowned river in India, runs through the central region of the country. The river initiates from the Madhya Pradesh region of India and covered a total of 65,145 sq. km area by the basin. This spacious river covers almost 2 % geographical area of India [29]. The people of rural and tribal areas along the Tapi River cultivate various types of major crops around it. Currently, Hashmi et al. [29] analyzed water and sediment samples from this river targeting five main pesticides such as DDTs, α-endosulfan, β-endosulfan, chlorpyrifos, and methyl parathion. During three separate seasons, they have studied samples from 10 sampling sites. In the Tapi River, endosulfan is the principal chemical found in both the water and sediment samples. The level of α- and β-endosulfan in water is ranged as ND-35.21 μg L−1 and ND-37.56 μg L−1, respectively. In sediment, the values are ranged from ND-7.63 ng g−1 and ND-38.38 ng g−1, respectively. No chlorpyrifos was found in the sediment sample, however, its concentration (ND-0.86 μg L−1) in the water samples cannot be avoided. Methyl parathion has been detected in both water (ND-0.43 μg L−1) and sediment (ND-0.77 ng g−1) samples. They found that the level of pesticide residues fluctuates following every season. However, the Tapi River receives a huge amount of pesticides containing wastewater. Considering the three seasons and 10 sampling sites, the average concentrations of the determined pesticides by Hashmi et al. [29] for Tapi River water and sediment samples are given in Table 1.

Hooghly River

The Hooghly River is a 260 km long distributary of the Ganges River and it streams south through West Bengal, India. The Hooghly is a significant branch of the waterway Ganges, and it appears to be naturally touchy. The river embraces an immense load of contaminants from fishing harbors, numerous industries, and the discharge from the tributaries. Almost 493 million gallons of water are depleted daily from the Hooghly River to satisfy water needs in the neighborhood [27, 48]. The Central Pollution Control Board of India disclosed that 1573 million liters of sewage were emitted daily in the Hooghly River [52]. This is the reason for worry about the maintainability of the biological system and rebuilding of the river [27]. The increased discharges are wreaking havoc on the river ecosystem. In Hooghly River, ∑OCPs were detected ubiquitously in high concentrations (Table 1) which ranged from 0.012-0.154 μg L−1 (Mean ± SD: 0.062 ± 0.035 μg L−1) in water. Data of sediment samples collected from different reports also showed a high level of pesticides in the surface sediment samples of the Hooghly River [27]. Among HCHs, lindane (γ-HCH) is found with maximum concentration. Total mean concentrations of ∑DDTs are detected as equal to 2.62 ng g−1 in the sediment sample. Table 1 shows the current mean levels of pesticide contamination in the water and sediment of the Hooghly River.

Gomti River

The Gomti is the key tributary of the Ganges River coming from a natural reservoir system in the marshy and compactly wooded land areas, Miankot Pilibhit, Uttar Pradesh (India) with an elevation of about 200 m from the mean sea level. In Lucknow City, it passes after flowing 250 km from the starting point. The river acts as the main source of drinking water for Lucknow city, and more than 4.5 million of its population rely on this vital river [53]. Kumar et al. [53] analyzed eleven OCPs in the water and sediment samples from the Gomti River. They collected samples from 10 locations dividing them into 3 sections: upstream, midstream, and downstream. The total concentration of OCPs in the water samples was found equal to 519.9 μg L−1 while in sediments, the concentrations were found equal to 2572.5 ng g−1. The values of ∑OCPs in water samples are found in higher concentrations that exceed their country's standard limit [53].

Pakistan

Pakistan is an underdeveloped country with a very rapidly growing economy. About 212.2 million people live in Pakistan with an area of 881,913 sq. km [39, 40]. In recent studies, due to the rise of manufacturing and agricultural operations, Pakistan has recorded a higher rate of POPs related activities. In the residential, manufacturing, and agricultural industries, pesticides are widely used [26]. Without proper treatment, excess water from these sources is discharged into the river, causing these contaminants to infiltrate water bodies and waterways that leach into soil or groundwater where the chemicals can survive for an extended period due to their persistent nature in the aquatic environment [26]. The polluted water is further utilized for farming and domestic uses by which the POPs residues can enter the human food chain cycle posing a danger to human health and the environment [54]. However, research on the occurrence of such compounds in the aquatic system in Pakistan is very scarce. Some studies stated the presence of pesticides such as HCH, DDT, and cyclodienes in the water systems and agricultural products such as dairy milk, vegetables, and fruits [55, 56, 57]. All these chemical species are well-known for their toxicity in the ecosystem, and some are already banned in most of the developed countries, whereas those compounds are still utilized in Pakistan [58].

In Pakistan, three waterways such as the Kalpani stream, the Kabul River, and the Indus River are interconnected. Due to the geographical location of the Kalpani stream, many unprocessed industrial, and domestic effluents are discharged into the stream regularly. The contaminated water then falls into the Kabul River which also receives huge untreated discharges from its nearby sources and finally, in turn, this river joins with the Indus River. By studying some of the research articles published during 2015–2020, we found some data on the pesticide levels in the Kabul and Indus Rivers [59].

Kabul River

Mardan is a city of the Mardan district of Khyber Pakhtunkhwa province in Pakistan, located in the valley of Peshawar (Wikipedia). Wastewater from Mardan city discharges into the Kalapani stream, which then falls into the Kabul River. Some research was performed to determine the water quality of the Kalapani stream and the Kabul River [8]. Kabul is a very significant river for the inhabitants of that region. Saad et al. [8] worked on the water samples of the Kabul River by targeting six pesticide compounds viz., chlorpyrifos, methomyl, carbaryl, carbofuran, triclosan, and caffeine, and reported a considerable concentration of most of these compounds in the river water. In the domestic wastewater system, both caffeine and triclosan were identified with a higher caffeine content of 92 ± 51 μg L−1 [8]. Caffeine in sewage water could be mainly derived from tea consumption. Based on the concentrations, the order of the other four pesticides is found as methomyl > chlorpyrifos > carbofuran > carbaryl. This justified the severe application of pesticides in the farmland of the riverbank area and established the load received from local tributary streams. Methomyl was found in high concentration (max. 8.3 μg L−1) which is described to trigger high toxicity to water fleas [8].

Indus River

In the agro-farming sector of Pakistan, the Indus River plays a vital role. It has distinct ecological significance in this area. This river has a length of 3180 km with a total drainage basin of 1,165,000 km2 and a mean discharge of 207 billion cubic meters [60, 61]. Zhang et al. [62] collected and analyzed 74 water and sediment samples from the four sampling stations of the Indus River and reported considerable pesticide contamination in the river. The escalating anthropogenic activities along the Indus River may seriously contaminate the area with various types of pesticide compounds. Continuous use of banned pesticides in the nearby regions and untreated domestic and industrial effluents could be plausible sources of pesticides in this river. Another major source could be the agricultural fields near the river where banned pesticides are used frequently. Subsequently, Ali et al. [26] also conducted a study to appraise the current contamination status of the area in terms of OCPs. They collected 37 water and sediment samples separately, dividing the sampling zone into 4 stations. Station-1 denoted for Taunsa from where 7 water and sediment samples were collected, station-2 denoted Kot Mithan and 6 samples were obtained from there. Station-3 and 4 indicated Guddu and Sukkur from where 13 and 11 samples were collected, respectively. All these stations lie on the Indus River of Pakistan covering an area of approximately 500 km. According to this work, the concentration of ∑OCPs in water ranged from 0.052-0.285 μg L−1. The mean concentrations of different OCPs are found to follow the order: ∑DDTs > ∑HCHs > heptachlor > chlordane > HCB > β-endosulfan. Among DDT metabolites, the content of p,p'-DDE was the maximum of total DDTs. The concentration of ∑HCHs ranged from ND-0.088 μg L−1. Among HCHs isomers, β-HCH was the most predominant metabolite. In sediment samples, the range of ∑OCPs is found as 5.6–29.2 ng g−1 with the highest recorded mean concentration of ∑HCHs equal to 8.81 ± 5.33 ng g−1, and the mean concentrations of OCPs follow the order: ∑HCHs > ∑DDTs > chlordane > heptachlor > HCB > β-endosulfan [26]. Among HCHs isomers as reported for the sediment samples, α-HCH was found as the most dominating metabolite with the mean value of 1.01 ng g−1 while regarding the DDTs metabolites, the highest concentration was reported for o,p'-DDE (2.10 ng g−1). In fact, most of the analyzed pesticides in the water and sediment samples of the Indus River were found in the reportable levels. According to Ali et al. [26], the total average concentration of pesticides considering all four stations in the Indus River along with the mean concentration of some other pesticides reported by Saad et al. [8] are shown in Table 1, which demonstrated an alarming situation.

Nepal

Nepal, the landlocked area, is a self-governing country in the South Asian region. It is mostly located in the Himalayas but contains parts of the Indo-Gangetic Plain as well. The agricultural sector is highly vulnerable because it is heavily reliant on monsoon rainfall, with just 28 % of arable land being irrigated since 2014 [63,64]. Nepal is one of the fastest-growing economies on the globe. Despite this, it is one of the principal contributors to economic development in agriculture [65]. The pesticide was introduced in Nepal first for removing the malaria parasite during the 1950s; DDT was used for the malaria eradication program, which was later imported by the government of Nepal. Other pesticides such as gammexane and nicotine sulfates were later introduced for the same reason. New forms of pesticides such as organochlorines, organophosphates, and carbamates were progressively added. However, the domestic consumption of pesticides (almost 0.142 kg/ha) in Nepal is low compared with the other countries [66]. Fungicide is the most used form of pesticide in Nepal. During 2011–2012, 48 % of pesticide was used in the form of fungicide [35]. In the same periods, 345 tonnes of pesticides were used in which a very little amount was issued for public health [2]. The plant protection division of the agriculture department in Nepal reported the use of approximately 55.8 tonnes of pesticides per year [67]. By now, 67 registered firms are working for pesticide importation, and 5 companies are working for pesticide formulation. Also, ten associations have granted a license to spray industrial pesticides. A total of 108 pesticides were registered under various generic names, while 1098 were registered under separate trade names [2]. However, to the best of our knowledge, no report was found determining the level of pesticides in the river systems of Nepal.

Bangladesh

Pesticides are used regularly in Bangladesh. But, it was in negligible use until the 1960s. However, the rate of using pesticides increased dramatically in the country over the past four decades [68] due to a partial reason for the preference of the government to implement chemical protection steps to improve crops yield, as well as, to reduce crops losses before and after harvesting [69, 70, 71]. A rapid rise in pesticide consumption, however, raises concerns about its possible impact on the health of farmers as well as on the environment. Pesticides related studies in Bangladesh are relatively very limited although news/magazines frequently reported the extensive use of prohibited pesticides including pesticide poisoning incidences [70, 72]. No significant data were found which can determine the level of contamination in the river systems of Bangladesh by pesticides. However, surveys prove that farmers are routinely used pesticides for farming and other purposes. There is no solid evidence on annual damage by the pest, though a study by Teng et al. [73] showed that during pest outbreak, Bangladesh losses 30–80 % of crops, contrariwise on non-outbreak time the percentage of losses is 3–20 %. The two main export products including tea and jute also got affected by the pest [72].

The practice of using pesticides mainly depends on the aggression of pests and the types of harvests. It was occasionally used in past days. Some data revealed that the use of pesticides is correlated with the use of fertilizer [74]. According to a survey in 2009, the amount of pesticide use is 3.13 kg/ha, which is indicated heavy use of pesticides compared to India (0.57 kg/ha), and some other countries [75]. In Bangladesh, the main approach for pest management has been chemical control. The government encouraged the use of pesticides until 1974 by distributing them to farmers free of charge (100 % subsidy). The subsidy was then lowered in 1974 to 50 % and was finally eliminated in 1979 [70]. Pesticides must be licensed for sale on the market by statute. In 1971, the Pesticide Ordinance was adopted to govern the importation, manufacture, formulation, and sale of pesticides which was then subsequently revised in 1980, primarily to meet the licensing requirements [69]. To enforce the terms of the Pesticide Ordinance, a series of regulations governing the use of pesticides is framed in 1985. The decree includes all pesticides related compounds, whether they are used for agriculture, public health, or other uses [69]. A sum of 94 active ingredients was used for agricultural applications, which were advertised by 628 brands in 2005. Another 21 kinds of active ingredients were promoted by an extra 170 brands, which were implied for public health purposes [69]. From 1989 to 2005, the number of brands of pesticides is enlarged from 158 to 628 [69]. Additionally, the number of organizations occupied with the pesticide business is expanded from 31 to 111 within these 16 years [71]. Over the last few years, the rise in the number of organizations, brands, and use of pesticides has exploded dramatically [71, 76].

Other South Asian countries

For the lack of research, we could not gather enough information about pesticides related pollution in Afghanistan, Bhutan, and the Maldives, which we can relate to our topic. However, Bhutan has started several campaigns encouraging local farmers to adopt organic farming that could decrease the excessive use of pesticides [77]. Sri Lanka has started the use of pesticides mainly in the agricultural sector and DDT was the first employed pesticide in the country for malaria demolition after Second World War [19]. However, there is little but not updated information available on the Kelani River basin of Sri Lanka. The study was conducted in 2016 by Mahagamage et al. [78] considering seven pesticides such as chlorothalonil, profenofose, oxyflurefen, penthoate, chlophyriphose, and diazinon. All of these pesticides were detected at high concentrations. It is considered that the Kelani River is one of the most polluted rivers in Sri Lanka. However, no further research has been done on this topic [78].

Overall comparison of the river's pesticides level with standard guidelines

Health organizations and governments of some countries introduced the allowable residue levels and required guidelines for the pesticides in the water while implementing the policies to lessen the contamination level in the surface water and groundwater. The prior attention is on drinking water. Those organizations provide the limits by considering health hazards, and the effect on the environment and aquatic life. Although the river sediment acts as the final sink for pollutants, however, the river water is the primary recipient of all pollutants influencing the aquatic life in the river environment depending on the level of pollutants. Thus, to identify only the water quality of South Asian rivers based on the pesticide contamination, we compared the obtained data from the South Asian riverine systems in this investigation with the standard guidelines provided by five organizations as shown in Table 2. For this comparison, we consider the maximum concentrations (taken directly from the specific article) of some reported pesticides in the Brahmaputra River, Chilika Lake, and Hoogly River water; and the mean concentrations (depending on the data reported patterns of the specific article) of some reported pesticides in the Tapi, Indus, and Kabul River water of South Asian riverine systems for which adequate standard guidelines are also available (Table 2). We utilized the acute value given by the United State Environmental Protection Agency (USEPA), guideline value of the World Health Organization (WHO), Maximum Contaminanted Level (MCL) provided by the United State, health value provided by Australia, and Maximum Accepted Value (MAV) provided by the New Zealand Ministry of Health [79]. After comparing, we observed that the maximum numbers of pesticides are reported for Brahmaputra and Hoogly River of India, and Indus River of Pakistan. However, the concentrations of all the pesticides (as shown in Table 2) for these rivers are found below the standard guidelines. Considering the highest concentrations of the reported pesticides, Chilika Lake and Tapi River of India are found as highly contaminated (Figure 3). Chilika Lake is found to be contaminated in terms of chlorpyrifos, ∑DDTs, heptachlor, and ∑HCHs while Tapi River is contaminated in terms of chlorpyrifos, ∑DDTs, ∑endosulfan; and the concentrations of all of these mentioned pesticides in the water of the respective rivers were exceeded most of the standard limits (Table 2). Concomitantly, a very high concentration of ∑endosulfan (14.18 μg L−1) is reported in Tapi River water (Figure 3) which is highly alarming. Kabul River of Pakistan contains a considerable level of methomyl (5.50 ± 3.90 μg L−1). However, the pesticide level in the remaining river systems is found below the standard limits and still, is in acceptable condition.

Table 2

Comparison of the concentrations (μg L−1) of some common pesticides in the water of the considered South Asian river systems with those of some standard guidelines.

Pesticides	Rivers of India				Rivers of Pakistan		Standard Guidelines
	Brahmaputra River	Chilika Lake	Tapi River	Hoogly River	Indus River	Kabul River	USEPA, AV	WHO, GV	USA, MCL	AUS, HV	NZ, MAV
Aldrin	0.005	-	-	0.009	-	-	0.36	0.03	-	0.03	-
Dieldrin	0.019	-	-	0.007	-	-	0.36	0.03	-	0.03	-
Carbaryl	-	-	-	-	0.001	0.001	-	-	-	30.0	-
Carbofuran	-	-	-	-	0.005	0.003	-	7.0	40	10.0	8.0
Chlorpyrifos	-	2.73	0.27	-	0.007	0.009	0.083	-	2.0	-	70.0
∑Chlordane	-	-	-	-	0.008	-	2.40	0.20	-	1.0	-
Dichlorvos	-	0.65	-	-	-	-	-	-	-	1.0	-
∑DDTs	0.225	23.58	3.15	0.026	0.074	-	1.10	2.0	-	20.0	2.0
∑Endosulfan	0.053	-	14.18	0.010	0.001	-	0.22	-	0.20	30.0	-
Heptachlor	0.010	1.00	-	0.026	0.014	-	0.52	0.03	0.40	0.30	0.04
∑HCHs	0.022	6.60	-	0.114	0.045	-	2.00	-	0.20	20.0	2.0
HCB	-	-	-	-	0.002	-	6.00	1.0	1.0	-	1.0
Methomyl	-	-	-	-	0.200	5.50	-	-	-	-	-

N.B.: AV = Acute Value, GV = Guideline Value, MCL = Maximum Contaminant Level, MAV = Maximum Acceptable Value, HV = Health Value. USEPA (United State Environmental Protection Agency) [79, 80], WHO (World Health Organization) [79, 81], USA (United States of America) [79], AUS (Australia) [79, 82], NZ (New Zealand) [79, 83].

Figure 3

Graphical representation of the concentrations (μg L−1) of some common pesticides in water of the considered South Asian river systems.

Inter parameters associations and probable sources of pesticides in the river systems

Although a large number of pesticides are reported for the considered South Asian river systems, all pesticides are not equally important based on their detection levels in the river samples, general uses, and chemical behaviors. Only those pesticides which are very common in use and reported at a considerable level rather than a very lower level are important to consider for evaluating the effective contamination levels along with their associated probable environmental impact in the studied rivers. For this purpose, the study of the significant associations among the pesticides and the river systems based on their concentration in the samples and also the identification of the probable sources of these chemicals in the rivers are found to be effective and helpful [37, 84]. The interrelationships among the reported pesticides and their probabilistic sources in the considered river systems were identified using multivariate statistical analyses such as Principal Component Analysis (PCA), Cluster Analysis (CA), and Pearson's Correlation Matrix (CM) [37, 38]. While performing these analyses using SPSS software (version 20, IBM Corporation, Armonk, NY, USA), the pesticides and their concentrations as summarized in Table 2 for the considered river water systems were utilized to ensure the statistically valid and significant outcomes from the analyses. Before conducting the multivariate analyses, the assumptions of the normal distribution pattern of the datasets were verified and satisfied by Kolmogorov-Smirnov and Shapiro-Wilk's test of normality (p < 0.05) [38,84]. The results of the normality test (Table S1) showed that all datasets of the considered pesticides for the river water are normally distributed.

The PCA of the datasets of 13 pesticides reduced the whole data dimension without their considerable loss into four controlling factors/components with Eigenvalues >1 (Figure 4a) which explained 97.34 % of the total variance (Table 3). The PC1, PC2, PC3, and PC4 accounted for 40.89, 67.85, 83.90, and 97.34 % of the total variance, respectively. The PC1 showed strong positive loadings for chlorpyrifos, dichlorvos, ∑DDTs, heptachlor, and ∑HCHs. Carbofuran, ∑chlordane, and HCB loaded strongly and positively on PC2 while methomyl showed strong positive loading on PC3. Carbaryl and carbofuran loaded positively on both PC2 and PC3. The PC4 showed strong positive loading for ∑endosulfan, however, aldrin and dieldrin loaded negatively on it. Based on the reported concentrations of pesticides, the PCA for the six considered rivers of South Asia extracted three components with Eigenvalues >1 (Figure 4b) explaining 87.22 % of the total variance (Table 3). The PC1, PC2, and PC3 explained 36.93, 69.18, and 87.22 % of the total variance, respectively. Among the six rivers, Brahmaputra River and Chilika Lake loaded strongly and positively on PC1while Indus and Kabul River revealed strong positive loadings on PC2. The PC3 showed strong positive loading for Tapi River, however, Hoogly River loaded negatively on PC3.

Figure 4

Principal component analysis by scree plot of the characteristic roots (eigen values) and component plot in rotated space for (a) the reported pesticides in water and (b) the considered river systems of South Asia.

Table 3

Varimax rotated factor loadings and communalities of the common pesticides in water and the considered sampling rivers in South Asian countries (strong loadings are in bold face).

Pesticides (R-mode)	PC1	PC2	PC3	PC4	Communalities
Aldrin	-0.332	-0.339	-0.363	-0.707	0.856
Dieldrin	-0.313	-0.316	-0.348	-0.695	0.802
Carbaryl	-0.224	0.669	0.704	0.082	1.000
Carbofuran	-0.204	0.853	0.473	0.075	1.000
Chlorpyrifos	0.984	-0.121	-0.081	0.105	1.000
∑Chlordane	-0.115	0.989	-0.083	0.043	1.000
Dichlorvos	0.994	-0.091	-0.048	0.019	1.000
∑DDTs	0.977	-0.133	-0.100	0.130	1.000
∑Endosulfan	-0.263	-0.300	-0.350	0.846	0.998
Heptachlor	0.995	-0.086	-0.058	0.003	1.000
∑HCHs	0.994	-0.090	-0.054	0.010	1.000
HCB	-0.115	0.989	-0.083	0.043	1.000
Methomyl	-0.174	-0.108	0.977	0.063	1.000
Eigenvalue	5.756	3.765	1.691	1.444
% of total variance	40.893	26.951	16.058	13.441
Cumulative % of variance	40.893	67.845	83.902	97.343

Based on the knowledge from PCA, hierarchical cluster analysis (CA) for the pesticides and the river systems were performed with Ward's method and the Euclidean distance as a measure of similarity to identify their spatial similarities [38] and to classify the pesticides based on their chemical similarities [84]. In the dendrogram derived from the CA of the pesticides (Figure 5a), three main clusters are differentiated. The Cluster-1 included ∑chlordane, HCB, carbaryl, carbofuran, and methomyl which are loaded combinedly on PC2 and PC3; Cluster-2 contains aldrin, dieldrin, and ∑endosulfan where aldrin and dieldrin are loaded negatively on PC4 while ∑endosulfan loaded positively on PC4; Cluster-3 grouped heptachlor, ∑HCHs, dichlorvos, chlorpyrifos, and ∑DDTs together which are loaded positively on PC1. The CA of the six rivers identified three Clusters (Figure 5b) based on the level of pesticides where Brahmaputra River and Chilika Lake of India constitutes Cluster-1 (loaded positively on PC1); Tapi and Hooghly River grouped in Cluster-2 (loaded positively and negatively, respectively on PC3); and Indus and Kabul rivers of Pakistan classified in Cluster-3 (loaded positively on PC2). Thus, the outcomes of the CA are well agreed with the PCA. The sampling rivers belonging to the same Cluster showed similar characteristics based on the considered pesticide levels and might receive similar kinds of chemicals from the surrounding environments (e.g., from industrial and agricultural activities).

Figure 5

Dendrogram obtained by hierarchical cluster analysis for (a) the reported common pesticides in water and (b) the considered river systems of South Asia.

Pearson's correlation matrices (CM) are also demonstrated significant associations among the pesticides (Table S2) and the rivers (Table S3), and the inter-parameter relationship of similar patterns strongly support results obtained from PCA and CA. For instance, carbaryl, carbofuran, ∑chlordane, HCB, and methomyl grouped in Cluster-1 are also showed association among themselves in the correlation analysis. Aldrin and dieldrin grouped in Cluster-2 and loaded negatively on PC4 showed a positive relationship with each other, however, associated negatively with ∑endosulfan. Chlorpyrifos, dichlorvos, ∑DDTs, heptachlor, and ∑HCHs of Cluster-3 loading positively on PC1 revealed significantly strong positive relations with themselves. The strong and significant correlations among the pesticides indicate their common sources of origin. Since rivers are the most attractive sinks of pollutants, most of the industries are usually established on the bank of the rivers or at a nearby suitable distance. In the rural area, however, riverside areas embrace a major portion of agricultural lands. Therefore, the presence of the aforementioned pesticides in the considered South Asian river systems may be corroborated by the industrial discharge and agricultural run-off of chemicals into the rivers [38, 85, 86].

Limitations and future works scope

For yielding the maximum outcome and expanding food production with the lesser farming field, the pesticide is the farmer's only secret. Evidently, the importance of pesticides in the agricultural sector cannot be denied. It helps to grow quality food in huge quantities. These chemicals also save us from diseases spreading by insects. However, the main concerning issue lies is in their uncontrolled and unauthorized dealings which brings not anything good for the future. The amount of using pesticides increases every year and in most cases, their used amount in one year exceeds the previous year. Most of the used pesticides are synthetic. Almost instant outcomes along with large agricultural production have driven the extreme uses of pesticides without concerning their negative impact. Further, most pesticide users have very limited knowledge about the aftermath of its impact on humans and the environment [3, 76]. To know the extent of environmental contamination by the extensive use of these pesticides, regular and continuous monitoring of their level in the environmental samples is very important. Scientifically to make good remediation, it is also important in the first place to identify the root cause of the pesticide contamination. Most of the works in the South Asian region so far we studied for this review investigated the occurrence, distribution, and sources of some pesticides in the river systems to highlight the current status of the respective aquatic systems and some works evaluated the associated ecotoxicological risks by pesticides. These studies are mostly discrete and limited to selective regions.

This implies that there is still have a much opportunity for researchers to work with this issue. The South Asian countries embrace many river systems prone to pesticide contamination, however, reports are found only for a few river systems. Considering this lack of research works, it is also very difficult to construct a comprehensive scenario of the riverine system of South Asian countries based on pesticide contamination. To well explore the level of pesticide contamination in South Asian riverine systems, many comprehensive works have to be carried out to estimate the contamination level in the remaining river systems which at least the most susceptible to pesticide contamination and this work should be continued periodically.

After that, the research works have to be extended to develop suitable and effective treatment methods for the removal of pesticides from the industrial effluents discharging directly to the river environment. Although many researchers [87] works for the removal of pesticides from contaminated water, more appropriate and effective research could be conducted for the remediation, mitigation, and other control measures for removing the residual amount of pesticides in bulk water systems. The use of nano-adsorbent materials could be the right solution for the removal of pesticides as employed by many researchers [88, 89, 90]. The application of bio-fertilizers in agricultural lands might be a good alternative solution for the remediation of pesticide contamination in the river systems besides the cultivatable landmass [91, 92]. Further, naturally, environmental fading or biodegradation of the pesticides chemicals in water and soil environment may be another important research topic in this regard [93, 94]. Natural bioaccumulation of these chemicals by aquatic biota or plant species without affecting their normal life and growth may also be an important method of pesticide minimization in the river ecosystem [95]. All the probabilistic approaches will be helpful for the overall river environment in the South Asian countries as well as other river systems in the world. This study did not consider the health effect of pesticides that deserves further investigation.

Conclusions

This analysis revealed that most of the considered river systems of South Asian countries are contaminated by pesticides, and the presence of HCHs, DDTs, endosulfans, heptachlor, and chlorpyrifos in the rivers are very common. The concentration levels of these common pesticides are found higher than the recommended standard limits in some water systems that demonstrate the unenthusiastic situation of the river systems and impose danger to the water's ecosystem. The maximum number of pesticides are reported for the Brahmaputra and Hoogly River of India, and the Indus River of Pakistan. However, Chilika Lake and Tapi River of India seem most contaminated based on the concentrations of chlorpyrifos, ∑DDTs, ∑endosulfan, heptachlor, and ∑HCHs which could be linked to the industrial discharge and agricultural run-off of chemicals in the rivers. Based on the levels of the selected pesticides in the water, the results of multivariate analyses demonstrated similar characteristics for the Brahmaputra River and Chilika Lake of India, Tapi and Hooghly River of India, and Indus and Kabul Rivers of Pakistan. The rivers showing similarities in characteristics might embrace similar kinds of pollutants from the surrounding anthropogenic sources such as industrial and agricultural activities. The presence of a considerable level of pesticides in the water systems indicates that most of the South Asian countries are regularly using the banned pesticides and thus deteriorating the overall water environment. Most of the countries in this region have strict rules and regulations for using pesticides for agricultural or other purposes, but people mostly do not obey those rules properly due to a lack of enforcement. Thus, after studying approximately 136 relevant articles, we can summarize some points regarding this critical issue as below.

Countries with a huge number of industries have the most polluted water resources. Rivers in India and Pakistan are considerably more contaminated. It also showed from previous research done by others. The probabilistic sources of river contamination by pesticides in these countries are the industrial and agricultural activities around the riversides.

Most of the South Asian countries are concerned about pollution, but due to lack of enforcement, they do not obey the rules and regulations properly.

These countries have many transboundary rivers and all are connected. Heavy use of pesticides may affect the natural resources of neighboring countries. Thus, regular monitoring is needed to reveal the exact level of using chemical compounds near the riverine area. Further, a proactive approach should be ensured by the concerned authority or policymaker of each country to stop the unauthorized use and discharge of pesticides in the river systems.

After analyzing the reported data and comparing them with the standard guidelines, we conclude that natural resources, especially rivers are not ruined yet; so it is high time to take proper actions and establish a fruitful process for removing pesticides especially before household and agricultural uses.

The ultimate aim of this review is to figure out the level of various types of pesticide residues in the river systems of South Asian countries. To build a removal process or taking action on the use of pesticide chemicals in any sector, it is crucial to have an overall view on that critical issue. Furthermore, the collective information of this study could also be useful for scientists, policymakers, environmentalists, and academia dealing with the contamination of river systems by pesticides.

Declarations

Author contribution statement

All authors listed have significantly contributed to the development and the writing of this article.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement

Data included in article/supplementary material/referenced in article.

Declaration of interests statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.