All over the world, approximately three million people get poisoned and about 200,000 die each year from pesticide poisoning and a majority of those belong to the developing nations. The ﬁgures may be even greater due to under reporting or lack of authentic and reliable data. Residues of organochlorine pesticides (OCPs) are considered as endocrine disrupters and carcinogens. Accumulation of OCPs in human body may increase risk of various types of human cancer including breast, lung, cervix, prostate and some other health effects such as endometriosis, hypospadias and cryptorchidias. Also, associations have been reported about the effects of OCPs exposure on human growth despite ban on their production and usage. Dichlorodiphenyltrichloroethane (DDT) and hexachlorocyclohexane (HCH) are two OCPs, which have been widely used as insecticide in vector control programs and as pesticides in agriculture. These compounds are ubiquitous anthropogenic environmental contaminants and have been linked with various health-related and environmental risks and damage. In tropical Asian countries, application of OCPs such as DDTs and HCHs has been considered with concern as they constitute major part of pesticide consumption. Since OCPs are non-bio-degradable or degraded very slowly, their concentrations accumulate in the environment. Accumulation of these compounds in the food chain via agricultural ﬁelds has led to them being extremely widespread in nature with the associated environmental risks (Simonich and Hites, 1995; Pereira et al., 2010). These pollutants contaminate surrounding soils and water resources and in some cases may endanger drinking water supplies.
Agricultural soils may act as a source of ‘‘aged’’ and recent OCPs as a fraction of them may get volatilized and dispersed, making them reservoirs of these pollutants and a risk for soils and sediments. Also the differences in HCHs and DDTs composition and the ratios between a-HCH and g-HCH and pp-DDT and pp-DDE are therefore often used as indicators of relative importance of contamination sources, i.e. Technical HCH/Lindane or Recent/ Previous DDT exposure (Toan et al., 2007). Among the countries that continue to use OCPs, India is ranked one of the major producers and consumers in recent years. In India, OCPs especially DDT and HCH were used extensively from past several decades till recently for both agricultural and health purposes due to their low cost and versatility in action but there is always a tendency to use them in excess amount. Between the periods of 1958–2004, production of pesticides (especially insecticides like DDT and HCH) in India had increased from 5000 to 85,000 metric tons (Gupta, 2004).
DDT, one of the most notorious pesticides of all time, is being used even today on a larger amount to overcome malaria situation, despite of its ban in most of the countries. Since malaria is still a serious health problem in different parts of our country, the present perception of DDT use is reasonable in India. Indoor residual spray with DDT is a comparatively cheap, quick, short term and main method for vector control (Rogan and Chen, 2005). Environmental fate of OCPs residues is now getting much more attention due to contamination of food, groundwater and even drinking water supplies. Although OCPs have been applied for more than 30 years in this region, no studies have been carried out to check the contamination status of soil system of the area and the human exposure to these chemicals. Alarming levels of OCPs have been reported in the environment (air, soil and water) as well as in biological and food materials from various parts of India (Viswanathan, 1985).
The current perspective emphasizes upon incessant monitoring and observation of OCPs status in soil proﬁle of this region where these are being applied in large quantities. The present study was conducted as a preliminary survey of soil contamination to evaluate the potential risks to human health and safety of environment in the region. The approach of the present study is to provide useful information on the extent of HCHs and DDTs contamination in soils of selected districts of Assam state and their spatial distribution to identify the possible sources and pathways of pollution and to explore the factors affecting contamination to prevent further deterioration of the environment.
Material and methods
The investigation area was located in districts Nagaon and Dibrugarh of Assam state. The state is strategically placed, bounded by the hill states of Arunachal Pradesh, Manipur, Meghalaya, Mizoram, Nagaland and Tripura, and shares an international border with Bhutan on the north and Bangladesh on the south. Assam is the most populous (31.17 million in year 2011) and second largest state (78,523 km2) in north-eastern India. Assam alone, with only 2.6 percent of the country’s population, contributes more than ﬁve percent of the total malaria cases in the country. The region is highly affected by malaria transmission due to excessive (2000–3000 mm) and prolonged rainfall and high humidity (60–90 percent) and warmer climates (22–33 1C) for most part of the year, which promotes vector breeding and longevity. Anapheles minimus, A. dirus and A. ﬂuviatilis are the main vectors responsible for malaria transmission in the state. All these vectors are found to be highly susceptible to DDT. Thus, residual spraying with two rounds of DDT (1 g/m2) is the main method for vector controls. Despite the global treaty to ban the use of DDT and related organochlorines, due to lack of suitable alternative for malaria eradication in the north-eastern India, these compounds have been used in this region for more than 30 years. The study area is rich in forests and water and has vast tracts of fertile land. More than 77 percent of the population is engaged in agriculture and allied activities. Around 40 percent of the total area is cultivated and cereals like rice and wheat and plantation crops like tea are grown extensively. Due to intensive agriculture in this region, application of technical HCH and lindane is very high in both the districts.
Collection of soil samples
Surface soil samples (0–30 cm depth) were collected from different land uses including agricultural ﬁelds of paddy and vegetable ﬁelds, tea gardens, fallow land and urban land during 2009–2010. Each sample was a composite of 5–6 subsamples of surface soil that were mixed thoroughly before packing in sterile polythene bags. Each sample was divided into two portions, one for OCPs analysis and the other for soil physico-chemical analysis. A global positioning system (GPS, Garmin12 XL) was employed to precisely record each location of soil sampling. Basic properties of soil samples are given in Table 1. Soils of the study area are composed of ﬂuvial sediments, classiﬁed as new and old alluvium. According to USDA texture classiﬁcation, soil texture is mainly sandy loam to sandy clay loam for new alluvium and ﬁne to coarse loam for old alluvium. The clay (o0.002 mm) and silt (0.05–0.002 mm) content in Nagaon district varies from 8.7 to 35.3 percent (Av. 22.1
percent) and 19.2 to 46.6 percent (Av. 28.7 percent). In Dibrugarh district, the clay and silt amount varies from 6.3 to 28.4 percent (Av. 19.2 percent) and 12.5 to 34.9 percent (Av. 24.3 percent). Total organic carbon was determined by wet oxidation using method given by Walkley and Black (1965). A conversion factor of 1.724 has been used to convert organic carbon to organic matter based on the assumption that organic matter contains 58 percent organic C (i.e. organic matter¼organic C Â 1.724) (Nelson and Sommers, 1996). The organic carbon content was found to be relatively higher in these soils. Mean values for soil organic carbon are 4.8 and 4.3 percent in districts Nagaon and Dibrugarh, respectively. Soil pH was determined from 1:5 soil– water slurry by pH meter. Soils were generally slightly to moderately acidic having mean values of pH 5.9 and 6.2 for districts Nagaon and Dibrugarh, respectively. 2.3. Analytical procedure
In the laboratory, soil samples were dried and sieved through 100 meshed steel mesh. Extraction of pesticides from soil was done according to the method described by Singh et al. (1999). Air dried soil sample (50 g) was taken in 100 ml solvent of hexane:ethyl acetate (9:1 v/v) in a conical ﬂask, plugged with cotton wool and then kept on a rotary shaker at 200–250 rpm for 12 h. The mixture was ﬁltered and the remains were reextracted twice with additional 50 ml of the same solvent mixture. Final extracts were pooled and cleanup of samples was done using a glass column packed with ﬁve percent deactivated alumina and anhydrous sodium sulfate. The column was eluted with n-hexane:benzene (1:1, v/v). After cleanup, the ﬁnal extracts were concentrated to near dryness using rotary vacuum evaporator. Quantitative analysis of the OC-DDTs and OC-HCHs were undertaken using an Agilent model 6890 Gas Chromatograph (GC) equipped with Electron Capture Detector (Ni63). The column used for the analysis was HP-5 capillary column of 0.32 mm id, 0.25 mm ﬁlm thickness and 35 m length. The oven temperature started from 150 1C with hold time for 1 min, then increased to 200 1C at a rate of 5 1C min À 1 with hold time for 5 min and ﬁnally to 290 1C at a rate of 10 1C min À 1. The injection port temperature was kept at 285 1C and the detector temperature was at 300 1C. Nitrogen was used as carrier gas at a constant ﬂow of 1 ml min À 1. Nitrogen was also used as a makeup gas at 75 ml min À 1. 1 ml volume of samples was injected in split/splitless mode.
The linear range of the detector was determined from injection of standard mixtures. Calibration lines were performed for all the OCPs and the resulting correlation coefﬁcients (r2) for the calibration curves were all greater than 0.99. The concentrations of individual OCs were then quantiﬁed by comparing the peak area of the particular compound in sample extracts to that of the corresponding external standard after replicate analysis.
With each set of 15 samples, a procedural blank and a spiked matrix sample with known amounts of standards were run to check for contamination, peak identiﬁcation and quantiﬁcation. The limits of detection (LOD) of OCPs were determined as the concentration of analytes in a sample that gives rise to a peak with a signal to noise ratio (S/N) of 3. Method LODs for DDTs and HCHs ranged between 0.014–0.042 ng/g and 0.019–0.039 ng/g. The mean levels for each pesticide residue were calculated with the assumption of zero for undetected values. The level was designated as undetected (ND) if it was below LOD. Method precision and accuracy was tested by spiking experiments with all studied compounds at 3 different spiking levels. Pesticide recoveries were determined relative to the ratio of direct injection of extract and the working standards prepared in hexane. The mean recovery of OCPs was estimated at mean concentration levels. Recovery percentages ranged between 83.4 and 97.2. Triplicate samples were analyzed during each run to evaluate the reproducibility of the overall method. The relative standard deviations (RSD) for triplicate samples were less than ten percent. Procedural blanks, which consisted of water instead of sample, were included with each sample batch and analyte values obtained in the blanks were subtracted from values found in the soil extracts.
All statistical analysis was carried out using the SPSS Microsoft version 16.0 for Windows. Concentrations of OCPs in soils were summarized using arithmetic means, standard deviations together with minimum and maximum values. Outliers were detected with Grubb’s test also called ESD method (Extreme studentized deviate). As the distribution of soil organochlorine levels was not
normal, the non-parametric Mann–Whitney U-test and Kruskal–Wallis test was applied to detect the differences in DDTs and HCHs between districts, different soil ﬁelds, etc. The correlation of organochlorines with various soil parameters was analyzed by the Karl–Pearson correlation coefﬁcient. A P-value of less than 0.05 (two tail) was considered to be statistically signiﬁcant. Statistical analysis was conducted for all residues of DDT metabolites and HCH isomers in all investigated matrices.
Status of OCPs residues in soil
The mean, minimum and maximum values along with standard deviation and detection frequencies of HCHs and DDTs in different agricultural soils of districts Nagaon and Dibrugarh are presented in Table 2. All the soil samples were found to be contaminated by different levels of these pollutants.
High concentrations of DDTs were detected in the soils from both the districts. The total DDT (sum of o,p0 -DDE, p,p0 -DDE, p,p0 -DDD, o,p0 -DDT and p,p0 -DDT) ranged from 166 to 2288 ng/g and 75 to 2296 ng/g with a mean value of 903 ng/g and 757 ng/g for the districts Nagaon and Dibrugarh, respectively. Among the various DDT metabolites in soil, p,p0 -DDT was the predominant congeners accounting for 38.9 and 40.5 percent of the total DDTs and its concentration was between 8–1478 ng/g and 8–1199 ng/g with a mean value of 351 ng/g and 306 ng/g for the districts Nagaon and Dibrugarh, respectively. p,p0 -DDE was the second highest component accounting for 30.6 and 31.3 percent of total DDTs and was in the range of 11–1159 ng/g and 4–1187 ng/g with a mean concentration of 276 ng/g and 237 ng/g for districts Nagaon and Dibrugarh, respectively. The mean values of o,p0 -DDT and p,p0 -DDD were 150 ng/g and 73 ng/g for Nagaon while it was 122 ng/g and 56 ng/g for Dibrugarh. The soils under investigations showed variable concentrations of HCH residues in the surface layer. The total HCH content (sum of a þ b þ g þ d HCH) was between 98–1945 ng/g and 178–1701 ng/g with a mean concentration of 825 ng/g and 705 ng/g for districts Nagaon and Dibrugarh, respectively. The difference in HCH levels between the districts was signiﬁcant (Po0.05). Among the four isomers, b-HCH is the most dominant isomer with a mean concentration of 326 ng/g and 285 ng/g and 100 percent detection frequency for Nagaon and Dibrugarh. The next major component was g-HCH accounting for 27.6 percent.
Distribution of DDT and HCH under three different types of agricultural ﬁelds
Distribution pattern and composition of HCH isomers and DDT metabolites in soils from different agricultural ﬁelds were also investigated. The distribution of total DDT and total HCH in different types of agricultural ﬁelds is given in Fig. 2. The predominant components of DDT and HCHs were found to be p,p0 -DDT and b-HCH, respectively, accounting for about 38.9 and 40.5 percent of total DDTs while 39.5 and 40.4 percent of total HCHs in Nagaon and Dibrugarh, respectively. On comparison of three different types of agriculture ﬁelds, it was observed that the percent contribution of g-HCH was more in tea garden while the b-HCH was found highest in paddy ﬁelds for both the districts. The a-HCH isomer showed no deﬁnite pattern for different agriculture ﬁelds. Among DDT metabolites, p,p0 -DDT, the parental compound was found in higher percentage for tea garden (high organic carbon and high acidic soils) whereas the p,p0 -DDE accounted for higher percentage in paddy soil (comparatively low organic carbon and high clay content). The soils from tea gardens had higher OC content and lower pH value and thus they could have an effect on the fate of OCPs in soil system.
Spatial distribution of DDT and HCH
A spatial distribution analysis of OCPs (DDT and HCHs) in soils was investigated using ArcGIS 9.3, a GIS (Geographical Information System) mapping software. The higher levels of total HCH in soils were observed in the middle, upper and western parts of the district Dibrugarh whereas in the upper and the middle parts of the district Nagaon. In case of DDT residues, the higher soil levels were observed in the north, south east and west and in few small patches in middle part of the district.
Dibrugarh. In the district Nagaon, the north, middle and south eastern parts showed higher DDT levels in the investigated soils. As the lower part of the district Nagaon (the area around Lanka and Hojai) is almost semi-desert type, there are less agricultural activities and also less vegetation cover. As a result, low concentrations of both the pesticides in soils of this area. Moreover, it has been observed that along the stretches of river Brahmaputra and major tributaries especially in their lower reaches, the soil contains higher pesticide load.
The present study is the ﬁrst attempt to characterize the DDT and HCH residue levels in the soils from different agricultural ﬁelds in two districts of Assam state, North-east region of India. Among various residues, b-HCH and p,p0 -DDT were the two predominant components in terms of their levels and detection frequencies. These results are comparable with other studies on agricultural soils in Yixing city and Beijing, China, which also reported that p,p0 -DDT was most dominant metabolite. b-HCH is the most persistent and least reactive among the four HCH isomers because of its most stable structure thermodynamically. Therefore it is highly stable with low solubility and vapor pressure. Hence the decomposition of b-HCH is slow in comparison to g-HCH.
The technical grade DDT generally constitutes 75 percent p,p0 DDT, 15 percent o,p0 -DDT, 5 percent p,p0 -DDE and rest of the metabolites (WHO, 1979). As various DDT metabolites persist long time in the environment, their gradual degradation occurs under aerobic conditions as DDE and as DDD under anaerobic conditions. In the environment, DDT is effectively transformed by anaerobic processes, which include reductive dechlorination to DDD and then dehydrochlorination of DDD to DDE. As the parental compounds (p,p0 DDT) degrades with time and the major products are DDD and DDE, the ratio of p,p0 -DDT/(p,p0 -DDE þp,p0 -DDD) is used as an indicator of the resident time of p,p0 -DDT in the environment (Li et al., 2008). In the present study, ratio of p,p0 -DDT/(p,p0 -DDEþ p,p0 -DDD) varied from 0.03–39.96 and 0.05–14.76 with a mean value of 1.25 and 1.82 for the districts Dibrugarh and Nagaon, respectively. These ratios are higher than those reported in agricultural soils from Guangzhou (0.86), Jiangsu Province (0.93), Shanghai (0–1.21), China and Hong Kong. However, the ratios were similar to those observed by Yang et al. (2008) for the vegetable soils in North Jiangsu province, which have a mean ratio of 1.83 for the top soil. A small DDT: ratio generally indicates aged DDT mixtures while a value greater than 1 indicates clearly fresh application of DDT (Jaga and Dharmani, 2003) or the slow decomposition process. Besides, the ratio was 41 in 73 percent of the samples, which revealed new application of DDT in most areas. However, about 27 percent of the samples had the ratio less than 1, denoting that soil contamination occurred in the past by technical DDT. In the decomposition process of a contaminant instead of its high stability, soil pH may be a major intervening factor. The soil in the area is of slightly to moderately acidic nature while the degradation of p,p0 -DDT to p,p0 -DDE preferentially occurs in basic media (WHO, 1989).
In India, mainly two categories of commercial HCH formulations have been used, technical mixture of HCH and Lindane. Technical grade HCH is a mixture composed of eight isomers out of which a, b, g and d-HCH are generally identiﬁed, while pesticide lindane is made up entirely of g-HCH insecticidal form. Commercial HCH contains around 60–70 percent a-HCH, 5–12 percent b-HCH, 10–12 percent g-HCH, 6–10 percent d-HCH and 3–4 percent of e-HCH (Ramesh et al., 1990). Technical HCH has been banned in agriculture since 1997. Government of India is now encouraging the use of lindane (g-HCH), which comprises all the insecticidal properties of technical HCH (Gupta, 2004). If the same criteria have been followed strictly, then soil samples should have more g-HCH levels, which have not been observed in the present study. Instead, a higher concentration of b-HCH, the most persistent isomer of technical HCH, was found in all the samples indicating a continuous application of technical HCH in the study area (Kannan et al., 1992; Wang et al., 2008). The ratio of a/g HCH is relatively a constant value in technical grade HCH, which ranges from 4 to 7 and nearly zero for lindane (Zhang et al., 2004). Thus, this ratio is frequently used to determine the possible source of HCHs in any region whether it is technical HCH or lindane. The ratio of a/g HCH ranged between 0.00–29.73 (with a mean value of 2.78) and 0.00–21.9 (with a mean value of 2.51) suggesting existence of potentially mixed technical HCH and lindane emission sources. Our results are consistent with the HCH sources in particulate matter (Xu et al., 2005) and soils from Beijing.
As India is by far the largest consumer and producer of DDT worldwide, the annual production of DDT for vector control is estimated at 4100 tons in 2003 and 4250 tons in 2005, which increased to 6334 tons in 2007 (Henk, 2008). In several Asian and African countries, a major problem reported is the suspicion of illegal trafﬁcking and use of DDT in sectors other than health, particularly in the agricultural and domestic environment (UNEP, 2008). Presence of such higher levels of DDT residues in soil samples of these districts is attributed to its current use permitted for malaria control as well as its illegal usage in agricultural ﬁelds (Imphal Free Press, 2008).
India is reported to be one of the largest consumers of HCHs and is one of the most contaminated nations in the world (Li et al., 2003). Cumulative use of HCHs in India till 1985 was 575,000 tons and since then about 45,000 tons of HCHs have been used annually (Voldner and Li 1995). Resulting from such intensive application, comparatively higher HCH levels were observed in the human and natural environment of India. Much higher concentrations of HCHs in our environmental and biological media than in other countries in the world has been shown by previous reports on OCPs in air, water, sediments, ﬁsh, birds and food stuffs.
In addition, the results of present study were compared with those of monitoring OCP residues in soils from India as well as other countries. Compared with the previous studies in India, DDT levels observed in the present study was found to be 2.6 to 26.5 times higher than agricultural soils from Farrukhabad (Agnihotri et al., 1996), Haryana (Kumari et al., 1996), Dehradun (Babu et al., 2003), Aligarh and soils from malarious DDT-sprayed area, Haridwar (Dua et al., 1996). In comparison with other countries, mean total DDT value was 1.3–42 times higher than the soils from China, Poland, Vietnam and USA (Bidleman and Leone, 2004). However, these levels were much low as compared to soils contaminated with DDT sprayed for malaria control from Chipas, Mexico (Herrera-Portugal et al., 2005), and in surface soils near residential areas from south-east China (Zhang et al., 2009). HCH concentrations in the present study were found to be about 2–5 times higher than in agricultural soils from Delhi, Haryana, rice ﬁelds from Dehradun, Farrukhabad, India, and NW Spain. The levels were several magnitude higher than those in Aligarh (Nawab et al., 2003), Beijing, Shanghai (Jiang et al., 2009), Taihu Lake, Pearl River Delta, NE and SE China, Vietnam (Thao et al., 1993), Hong Kong and Poland (Falandysz et al., 2001). However, the levels were considerably low as compared to alarming levels reported in soils from surrounding areas of lindane manufacturing plant (LPL) Lucknow, India (Prakash et al., 2004). It has been observed that the soils from paddy ﬁelds contain substantially and signiﬁcantly (Po0.05) higher amount of HCH and DDT levels in comparison to soil levels from tea gardens, other agricultural ﬁelds and fallow land. These results indicate their persistence in paddy grown areas, which subsequently act.
Similar results of higher OCPs levels from paddy ﬁelds have been observed in Thiruvallur, South India (Jayashree and Vasudevan, 2005) and Vietnam (Thao et al., 1993). The higher HCH levels in paddy ﬁelds may be attributed to relatively high consumption but higher DDT values may probably be due to ignorance of farmers or illegal application in paddy ﬁelds as these are cheap as well as effective chemicals. Illegal use of DDT in agricultural activities has also been reported by other researchers (Thao et al., 1993; Agnihotri et al., 1996; Kannan et al., 1997; Imphal Free Press, 2008; Sarkar et al., 2008). Rice is the second largest crop after cotton, which grows on 24 percent of the cultivated area and consumes about 25 percent of the total pesticides used in the country (Abhilash and Singh, 2009). In Dibrugarh and Nagaon districts, a major part around 81,553 ha (24.09 percent) and 160,035 ha (38.94 percent) area is under paddy cultivation (summer, winter and autumn paddy) with 166,309 ha (49.12 percent) and 234,633 ha (61.25 percent) total sown area (NIC, 2004-05a, 2004-05b) that is subjected to high pesticide application, which may be a possible explanation for elevated HCH and DDT levels in these districts.
The water table in the region is normally shallow due to high rainfall for most part of the year as well as presence of number of large perennial rivers, lakes and water bodies. Depth to water level is varying between 0.43 and 3.76 m (average 2.89 m) for Dibrugarh and 0.8 and 7.39 m (average 2.97 m) for Nagaon (CGWB, 2010). In those areas where water table is shallow and/ or downward movement is facilitated by low organic matter and clay content of soil, such elevated DDT and HCH levels in soil system deﬁnitely posing potential risk of groundwater contamination for long time and thus affecting drinking water supplies of the region.
Fate of pesticide residues (retention, mobility and degradation) in soil system depends on various environmental factors, soil properties like soil type, pH, soil moisture, soil organic carbon content and properties of compounds itself such as solubility, vapor pressure Henry’s law constant, etc. (Miglioranza et al., 2003; Kumar et al., 2006; Pereira et al., 2010). These parameters are responsible for OCP levels in soil system as well as for possible adsorption on various soil components such as organic matter, sometimes clay and other minerals. Thus adsorption of OCPs to soil components, especially soil organic matter prohibits the further microbial degradation of these in the soil (Wang et al., 2006; Skrbic and Mladenovic, 2007).
Different agricultural activities may change the soil properties like organic carbon (OC) content and pH (Yang et al., 2005; Wang et al., 2006). In other words, soil OC content was responsible for the adsorption of pesticides, while the lower pH value favors the accumulation of these and leads to reduction in microbial activity and thus lowers degradation in soil (Wenzel et al., 2002). In addition, as the soil pH dropped and the level of organic matter rose, levels of p,p0 -DDT increased while p,p0 -DDE concentration decreased. This presumption is supported by earlier studies by Carter and Suffet (1982), Skrbic and Mladenovic (2007) and Wang et al. (2006) who reported that association of DDT to organic matter increased as pH level decreased. In addition, the higher loading of g-HCH in tea gardens can be explained by high OC content while b-HCH in paddy ﬁelds due to low pH or isomerization of former one to b-HCH, which is energetically more favorable (Manz et al., 2001). In the present study, soil organic carbon was found to be positively associated with total HCH and DDT levels. Soil organic matter is an important factor affecting the OCPs behavior in soils.
Organochlorine pesticides tend to bind with soil organic matter due to their hydrophobic nature. Increase in soil organic matter content may supply more carbon to facilitate microbial degradation. On the other hand, a large amount of organic compounds can be adsorbed on the organic matter of soils (Zhang et al., 2006) and thus could make an impact on the soil residue levels (Gong et al., 2004; Mirian et al., 2008). In addition, soil pH can affect the concentrations of OCPs through modiﬁcation of structure of humus (Wenzel et al., 2002). However, a poor correlation was observed between TOC and other individual OCPs, which may be related to land use, particle size and chemical-speciﬁc properties that may also affect their retention in the soil. The role of clay minerals as pesticide sorbents is of minor importance where SOM contents are relatively high (Baskaran et al., 1996).
The areas with higher HCH loadings are fertile alluvial regions formed by ﬂuvial processes of major river system of Brahmaputra with its major tributaries Kalong, Kopili and Jamuna in Nagaon while Dibru and Buri Dihing in Dibrugarh. These areas are fertile zones of rice cultivation, and vegetable and cereals also grow extensively together with number of tea gardens. Due to intense agricultural practices throughout the year, these pesticides are being applied in greater quantities. Higher DDT containing areas are either near to dense vegetation and forest cover (major forest reserves such as Dibru, Joypur, Dihingmukh and Jokai in Dibrugarh; Hojai, Lumding, Diju and Bagser in Nagaon), urban areas (Dibrugarh, Tenghakhat and Joypur in Dibrugarh district while Nagaon, Koliabor, Dhing, Kampur and Lumding in Nagaon district) with relatively more DDT spray to control malaria vectors or under intense paddy cultivation and tea plantation showing the presence of high DDT values as compared to the other areas.
The present study was conducted to assess the potential risk posed by OCPs (DDTs and HCHs) in different agricultural soils of NE India. The investigations provide useful data on contamination status of soils in Dibrugarh and Nagaon districts of Assam. The results showed substantially higher concentrations of these pesticides in the surface soils of the area. In addition, the levels were found highest among the soil levels from various parts in India reported so far. Such higher levels are due to intense application of DDT and HCH for control of vector borne diseases in this malaria endemic region and for agricultural purposes to increase crop yield. The ratio of a/g-HCH and DDT/(DDEþDDD) indicated that these pesticides were applied in large quantities from the past decades till recently due to low cost, high effectiveness and lack of suitable alternatives. Highest HCH and DDT concentrations in soils of paddy ﬁelds indicate high HCH consumption in rice crop but illegal use of DDT in paddy ﬁelds cannot be denied. In addition, the higher pesticide levels in soil system in high water table areas pose a risk of groundwater contamination and subsequently the drinking water supplies. Thus, there is an urgent need of regular monitoring, assessment and reporting of contamination proﬁle of these pollutants in various environmental components, and also development and execution of alternative malaria and pest control measures.