Study of Organochlorine Pesticides in Ecogeochemical Survey of Soil in Shandong Province

Qing CAI Fang LIU Chunlan LI Shixin CAO Binbin LIU Jingjing WANG

Abstract Based on a comprehensive ecogeochemical survey of Shandong Province， the differences in pesticide residue situation and contents of DDTs and HCHs in the soils of Yantai City and Southwest Shandong Province （abbreviated as SSP） were studied in this paper. The results showed that the detection rates of DDTs and HCHs in the soil of SSP were apparently higher than Yantai City. However， the mean contents of DDTs and HCHs in the soil of Yantai were 9 and 25 times of SSP， respectively， and higher than the mean contents of many other cities and areas. p，p′DDE was the main pesticide residual form in the soil of SSP. It is recognized as the degradation product of DDTs in oxidation environment in past， nevertheless， 5.52% of the soils in Yantai City still have a little of DDT input recently， which has posed a big impact on soil environment quality.HCH was the main form of the four isomers of HCHs in Yantai City， and HCH/HCH in SSP was low. These reveal that the residual time of HCHs in the soils of both the two areas has a long period. After primary analysis， the risk of DDTs and HCHs in the soil of SSP is low， but DDTs in the soil of Yantai City still has a certain risk.

Key words Soil; Organochlorine pesticides; Ecological risk; Yantai City; Southwest Shandong Province

Organochlorine pesticides （chlorobenzene and its derivatives， potassium chloride naphthalene preparations） are characterized by high efficiency， low cost， wide insecticidal spectrum and convenient use. They have been widely used in countries all over the world， and the organochlorine pesticides which have the highest yield and the largest application amount are hexachlorocyclohexanes （HCHs） and dicophanes （DDTs）[1]. DDTs and HCHs are typical persistent organic pollutants （POPs）， which are widely concerned by the environmental science community because of their environmental persistence and bioaccumulation and their easy enrichment with the food chain. China has banned the use of HCHs and DDTs since 1983， and it has been more than 20 years so far， but in some areas， the soil still contains different levels of organochlorine pesticides[2-4]. Therefore， the residue of organochlorine pesticides in the soil and its potential harm to the ecological environment and human health are issues that need attention.

In 2002， Shandong Province launched the Ministry and Province Cooperation Project "Ecological and Geochemical Survey in the Lower Reach of the Yellow River in Shandong Province"， and also invested in the "Agroecological Geochemical Survey of East Shandong Province" in 2006. The two projects systematically investigated the soil organochlorine pesticide residues of 11 000 km2 in Southwest Shandong Province and 13 000 km2 in Yantai City （Fig. 1）. Since the reform and opening up， with the rapid development of industrialization， agroagriculture and urbanization， a large number of toxic and harmful elements have entered the environment， thus affecting the growth of animals and plants and the safety of agricultural products. Many previous studies have reported geological hazards and related soil heavy metals in the two regions[5-6]， but so far， no studies have been conducted on the pollution and changes of organochlorine pesticides （DDTs， HCHs） in the soils of the two regions. In view of this， the author conducted a comparative study on the residues and composition characteristics of HCHs and DDTs in the soils of the two regions， and proposed the geochemical basis of the pollution sources， so as to provide basic information for the development of modern agriculture and the construction of agricultural product safety system in Shandong Province.

Sample Collection and Testing

Sample collection

With 1≥50 000 topographic map as the working manual， the topsoil samples were collected at a density of 1 point/36 km2 when performing positioning with a satellite positioning system （GPS）， and 399 and 299 samples were collected in Yantai City and Southwest Shandong Province area， respectively. The surface impurities were removed during sampling， and the soil from the surface to a depth of 20 cm were collected vertically uniformly. Animal and plant residues， gravel and fertilizer pellets were discarded， and the obtained soil was filled in a clean cloth bag with an original weight greater than 500 g. The soil sample was airdried， broken， and sieved with a 20mesh nylon sieve， and the final weight of the sample for testing was 200 g.

Sample testing

The sample testing was undertaken by the Central Laboratory of Shandong Academy of Agricultural Sciences. The analytical indicators were HCHs， HCH，HCH， HCH， HCH， DDTs， p，p′DDT， p，p′DDT， p，p′DDE and p，p′DDD. The instrument was an Agilent 6890 gas chromatograph， which used a capillary separation column （EPA8081A） with a detection limit of 0.001 mg/kg. During the analysis， the operation was strictly according to the national standard of the Peoples Republic of China， "Soil Quality―Determination of HCH and DDT―Gas Chromatography" （GB/T 1455093）.

Quality control and quality evaluation

Blank sample： One blank sample was analyzed per batch of samples to confirm the cleanliness of the reagents and containers.

Samples selected randomly in the laboratory： 22 samples were randomly sampled for repeated analysis to confirm the reproducibility of the test results. Referring to "Soil quality―Determination of BHC and DDT―Gas chromatography" （GB/T 145501993） and "Agricultural Environmental Monitoring Quality Assurance and Quality Control Implementing Regulations"， specifically， the requirement of repeated analysis pass rate >90%， the repeated analysis pass rates of DDTs and HCHs were both 100%.

Spiked samples： A total of seven quality control samples were inserted in the test， and the recoveries of the recovery indicators in the soil were in the range of 80%-120%. By repeated standard addition into the substrates， the detection limits of HCH，HCH， HCH， HCH， p，p′DDE， p，p′DDD， p，p′DDT and o，p′DDT were 0.017， 0.016， 0.024， 0.020， 0.019， 0.018， 0.023， and 0.029 ng/g， respectively.

The gas chromatograph was subjected to the necessary calibration， verification and conditioning before sample testing. The standard sample of the same concentration was repeatedly determined for 5-7 times until the relative standard deviation （RSD） was lower than 5%， and then， the sample testing was started to ensure the accuracy of the instrument. Qualitative analysis was performed with retention time and standard samples， and quantitative analysis was performed by the external standard method and fivepoint calibration curve.

Results and Discussion

Content characteristics of soil organochlorine pesticides

There were significant differences in the detection rates of HCHs and DDTs between the soils in Yantai City and Southwest Shandong Province （Table 1）. The detection rate of DDTs in the soil of Yantai City was 34.59%， and the concentration ranged from 10 to 2 660 ng/g， with a mean value of 160 ng/g. The detection rate of DDTs in the soil of Southwest Shandong Province was 82.27%， which was 2.38 times of that of Yantai， but the concentration only ranged from 1 to 260 ng/g， with an average of 16.88 ng/g， which was about 10.55% of the mean value of Yantai. p，p′DDE and p，p′DDT were the main forms of soil DDTs in the two regions. From the average and maximum values， the residual amount of p，p′DDT in Yantai was the largest， and its average and maximum values were 185 and 1 780 ng/g， respectively， reflecting the recent pollution input. However， the residual amount of p，p′DDE was the largest in the Southwest Shandong Province， and its average and maximum values were 10.75 and 182 ng/g， respectively. It is the main longterm degradation product of DDTs.

The detection rate of soil HCHs in Yantai City was only 2.00%， and the content ranged from 26 to 561 ng/g， with an average content of 144 ng/g. The average residue of HCH was 235 ng/g which was the largest， andHCH was the main residue. The detection rate of soil HCHs in Southwest Shandong Province was 17.39%， which was 8.70 times of Yantai City， and the concentration ranged from 1 to 70 ng/g， with an average value of 5.65 ng/g， which was only 3.93% of the average value of Yantai City. HCH was the main residue.

It can be seen from the above analysis that the average contents of soil DDTs and HCHs in Yantai City were significantly higher than those in Southwest Shandong Province， but the detection rates were lower. The main reason for this difference is the widespread and local use of organochlorine pesticides. The fruit industry is the pillar industry of Yantai City， especially apples and pears. Various counties （districts） in Yantai City began to use a large number of pesticides such as DDTs and HCHs to control orchard pests from the 1950s or the 1960s in the 20th century， and the usage amount of orchards was significantly greater than those of other agricultural areas in Shandong Province. This study shows that some areas of Yantai City still use HCH and DDT pesticides， resulting in obviously higher content of organochlorine pesticides in the soil.

The comparison of DDTs and HCHs in the soils of the two regions and the residual organochlorine in the soils of other regions of the country or in individual countries （regions） in the world is shown in Table 2. Compared with the countries and regions where organochlorine pesticides are still used， the residues of HCHs and DDTs in the soils of the two regions were relatively lower. However， compared with the countries that completely ban organochlorine pesticides， the residues of HCHs and DDTs in Southwest Shandong Province were relatively lower， while the residues of HCHs and DDTs in Yantai City were relatively higher. For instance， the residual amount of HCHs was 12.5-27.6 times of Germany， 6.6 times of Austria and 57.6 times of Mexico， and the amount of DDTs in the soil was 7.3 times of Austria. Compared with the global high background points―the high mountains and polar regions， the residues of HCHs and DDTs in the soils of the two regions were obviously higher. For instance， the residual amount of HCHs in Yantai City was 369 times of the maximum residue in the Andean mountainous area of South America and 294 times of the high mountains in Europe， and the residual amount of soil DDTs in Southwest Shandong Province was 43.6 times of the Andean mountainous area and 13.9 times of the Antarctic region. Compared with the surface soils of the domestic Beijing suburbs， Taihu Lake and Tianjin， the residual DDTs in the soil of Yantai City was also significantly higher， second only to the Tianjin sewage irrigation area， equivalent to the DDTs residue in the farmland soil in southern Jiangsu， and the DDT residue in the Southwest Shandong Province was lower， only higher than the Taihu area.

Distribution characteristics and environmental quality assessment of organochlorine pesticides in soil

It can be seen from Table 3 that the soil in Southwest Shandong Province had good environmental quality in DDTs and HCHs. The soils of Grade I for HCHs soils accounted for 99.67%， and the soils of Grade I for DDTs accounted for 93.31%. There were no soils of grade III and worse than grade III. The soil in Yantai City had poor environmental quality in DDTs and HCHs. In the soils of grade II where the content of organochlorine pesticides （HCHs， DDTs） was greater than 50 ng/g and less than or equal to 500 ng/g in Yantai City， there were five points for HCHs， mainly distributed in southern Longkou， and 53 points for DDTs， distributed in other areas except Haiyang City， most wide in Longkou and Qixia; in the soils of grade III with a content greater than 500 ng/g and less than or equal to 1 000 ng/g， there was one point for HCHs， distributed in northeastern Laizhou， and 10 points for DDTs， four of which were distributed in the southern part of Muping， and other were scattered in Penglai， Haiyang， Qixia and other places. There were three points with DDT content greater than 1 000 ng/g， distributed in southern Longkou and northern Zhaoyuan， where the contents exceeded the national grade III standards for soil environmental quality， belonging to the soil worse than grade III， as shown in Fig. 2.

Study on the change characteristics of organochlorine pesticides

The residual amounts of organochlorine pesticide isomers in soil are different， which represents the environments of different soils and the geochemical behaviors of organochlorine pesticides such as degradation， migration and transformation. Therefore， the contents and distribution characteristics of isomers can reveal the sources of organochlorine pesticides and differences in the effects of soil environment on the geochemical effects of organochlorine pesticides.

Change characteristics of HCH isomers

In China， HCHs were once used in two forms， one of which is a mixture of four HCH isomers which have the contents in hexachlorocyclohexane as HCH （65%-70%），HCH （5%-6%）， HCH （12%-14%） and HCH （6%）， of which HCH is the main insecticidal ingredient. Among the four isomers，HCH is low in water solubility and difficult to biodegrade， and the most stable in nature. The other form is lindane， in which the content of HCH can reach more than 99%[10-11]. As a mixture of pesticides， the HCHs have a ratio of HCH to HCH of about 4-7， and a value greater than or less than this range indicates that HCHs have undergone a longterm degradation process.

Among the eight samples of HCHs detected in Yantai City， HCH was not detected in seven samples， and the content ofHCH was the highest in five samples. Although the content ofHCH in HCHs is lower， its stability is the highest of the HCH compounds， and the high content ofHCH isomer in the soil samples in the region indicates that HCHs are not recently used in these samples. Among the 52 samples of HCHs detected in Southwest Shandong Province， HCH was not detected in 47 samples; and the HCH/HCH values of four samples were between 0.88-2.50， and one sample had the value of 7.32. It indicates that HCHs had been used for a long time， and their components have undergone great changes through physical and chemical effects. For each of the soils in Chenpo Township， Yuncheng County and Fushan East Orchard， there was a sampling point with a ratio of HCH to HCH less than 4， and the content of HCH was larger than that ofHCH， indicating that the residue of HCHs at the two sample points might be caused by the use of lindane.

Change characteristics of DDT derivatives

DDTs contain approximately 80% p，p′DDT， 15% o，p′DDT， 5% p，p′DDE and less than 5% p，p′DDD and other substances， and it can be seen that p，p′DDT is the main component of DDTs. p，p′DDE and p，pDDD are products of p，p′DDT metabolism. p，pDDT is mainly oxidatively degraded to p，p′DDE under aerobic conditions， and reduced by microbial degradation to p，pDDD under anaerobic conditions， and the ratio of DDD to DDE can reflect its redox condition. After longterm degradation of soil contaminated by DDTs， the ratio of （p，p′DDD + p，p′DDE）/p，p′DDTs is generally greater than 1. If the value is less than 1， it indicates that there is new pollution[12].

Statistics showed that （Table 4）， among the 138 samples detected in Yantai City， 133 samples had a p，p′DDD/P， P′DDE ratio less than 1， accounting for 96.38%; and among the 245 samples detected in Southwest Shandong Province， 243 samples had a ratio less than 1， accounting for 99.18%， indicating that the DDTs in the soils of the two regions mainly depend on aerobic degradation， and the anaerobic degradation in waterlogged environment is rare. Through field investigation， the seven samples with p，p′DDD/p，p′DDE ratio greater than 1 in the two regions were distributed around Nansihu Lake， including the waterlogged anaerobic environments of the wash in the south part of Yanshui City， the south part of Shewopo Town and the north part of Facheng Town. The conclusion is consistent with the natural conditions of the study area.

In Southwest Shandong Province， the number of soil samples with （p，p′DDE+p，p′DDD）/p，p′DDT greater than 1 was 241， accounting for 80.60% of the total sample， and the （p，p′DDE+p，p′DDD）/p，p′DDT values of 18.06% of the soils were absent （DDTs were not detected in the soil， and DDTs in the soil were completely degraded）， indicating that DDTs in 98.66% of the soils in Southwest Shandong Province were mainly DDTs input in the past. Only 1.34% of the soils （four sample points） showed a （p，p′DDE+p，p′DDD）/p，p′DDT value less than 1， and the absolute content of DDTs at the four points was small （10-69 ng/g）， indicating that the DDTs in this region are mainly in the past tense， and the amount of DDTs input recently is very small. The number of soil samples in Yantai City with a （p，p′DDE+p，p′DDD）/p，p′DDT value greater than 1 accounted for 29.07%， and the number of samples with missing （p，p′DDE+p，p′DDD）/p，p′DDT values accounted for 65.41%. Although DDTs in a total of 94.48% of the soils were imported in the past， and 5.52% of the soils （22 sampling points） had a （p，p′DDE+p，p′DDD）/p，p′DDT value less than 1 and a relatively large absolute content （72-2 660 ng/g）， indicating that there might be new sources of DDT pollution input in a certain range in Yantai City. And， 6.51% of the soils （26 sampling points） having a p，p′DDE/p，p′DDT value less than 1 also fully demonstrates this. It can be seen from Fig. 2 that the input points of these new exogenous DDTs were mainly sampling points of soils of grade III or worse than Grade III， indicating that the recently input DDTs have a great impact on soil environmental quality. In industrial DDTs， o，p′DDT/p，p′DDT is about 1≥5.6， while the ratio of dicofol is about 7.0[11]. For the 22 points judged to have new external input， the ratios of o，p′DDT/p，p′DDT were all less than 1， indicating that the main source of new input of soil DDTs is industrial DDTs， and should not be dicofol pesticides. The sources of pollution need further research.

Conclusions

（1） The HCHs and DDTs that have been deactivated for more than 20 years still have residues in the surface soil of Shandong Province. The land use is different， and the characteristics of soil organochlorine pesticides are greatly different. The contents of DDTs and HCHs in Yantai City taking forestry and fruit growing as the pillar industry were mainly higher than those in the general agricultural areas in Southwest Shandong Province， and the average contents of DDTs and HCHs in soil were， respectively， 160 and 144 ng/g， which were 9 and 25 times higher than that in Southwest Shandong Province， but the detection rate was low. The local pollution of DDTs in Yantai City is prominent. For DDTs， a total of 3.26% of the soils belonged to the soil of grade III and the soil worse than grade III， mainly distributed in southern Juping and Longkou， with high ecological risk. DDTs and HCHs in Southwest Shandong Province satisfied the national grade II standards for soil environmental quality， and the environmental quality was good. The large historical use amount and the recent new external input are important reasons for the high content of organochlorine pesticides in Yantai City.

（2） p，p′DDE was the main form of DDTs in the soils of Yantai City and Southwest Shandong Province. By studying the characteristics of （p，p′DDE+p，p′DDD）/p，p′DDT values， the DDTs in the soils of the two regions were mainly the products of the degradation of DDTs input in the past in the oxidation environment. However， there were still a small number of DDT input （22 samples） in Yantai City， and the soil environmental quality has been greatly affected. It is considered that the input source should not be dicofol， and the specific source of pollution needs further research and confirmation. The isomerHCH in the soil of Yantai City was the main residue of HCHs， and the content was the highest; and the HCH/HCH values at all sample points in Southwest Shandong Province were not in the range of 4-7， indicating the use time of HCHs in the soils of the two regions is longer， but the possibility of lindane use cannot be ruled out for HCHs at local points.

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