Original article

Selenium source in the selenosis area of the Daba region, South Qinling Mountain,

Luo Kunli Æ Xu Lirong Æ Tan Jian’an Æ Wang Douhu Æ Xiang Lianhua

Abstract Naore Village in the Shuang’an Introduction countryside, , Province is Selenium is closely related to the well being of organic life located in the Daba region of South Qinling generally, and to human health specifically. A narrow Mountain. It is one of two selenosis or selenium range of concentration exists between toxicity and defi- poisoning areas in China. Crops grown in the area ciency (Rosenfeld and Beath 1964; Thornton and others are enriched in selenium. The soil is derived from 1983; Yang and others 1983). The selenium content in the high-selenium, pyritic, black carbonaceous slate and Earth’s crust and mantle is quite low, about 0.05 mg/kg volcanic tuff of the Lujiaping Formation, Early (Liu and others 1984). China has two known selenosis Cambrian and Late Neoproterozoic in age, which is (selenium poisoning) areas; the first is Enshi, Hubei exposed in this area. The Lujiaping Formation is Province (Yan 1991; Yan and Wu 1993); the other is the more than 40-m thick. Selenium content of the Shuang’an countryside in Ziyang County, Shaanxi Prov- volcanic tuff averages 32 mg/kg Se and the black ince, in the Daba region of South Qinling Mountain, carbonaceous slate averages 22 mg/kg Se, which Central China (Mei 1985) (Fig. 1). The most concerning represent the highest concentrations recorded area of selenosis in the Shuang’an countryside of Ziyang among all the strata and rocks sampled in the Daba County is very small, limited to Naore village (Mei 1985; region of South Qinling Mountain. This series of Edit Committee on Ziyang County Annals 1989). The selenium-enriched rocks is also found in other selenium content of corn in some fields of Naore village is places in the Daba region, South Qinling Mountain, 6.6 mg/kg (Luo and Qiu 1995), and horsebeans reach where the soil contains about 10 to 30 mg/kg Se. 48.84 mg/kg Se (Mei 1985). The selenium content of soil in These zones are latent selenosis zones in the Daba these fields is 15.74 mg/kg (Mei 1985) and 26 mg/kg (Zhao region. In addition, a selenium-enriched black and others 1993). Village people consume the selenium- carbonaceous shale, of Early Cambrian age, and enriched crops from these fields. It is clear that the sele- containing 10 to 40 mg/kg Se, occurs in several other nium in the crops came from local selenium-enriched soil provinces in the south of China. (Mei 1985; Edit Committee on Ziyang County Annals 1989; Keywords Selenosis Æ Selenium enrichment Æ Early Zhao and others 1993), but what is the source of the Cambrian Æ China Æ Shaanxi Province selenium in the soil? Mei (1985) suggested that the selenium in this selenosis area may come from a selenium-enriched coal bed and Ziyang County is cited as a selenium-rich region in his introduction and summary, but he did not explain what Received: 19 December 2002 / Accepted: 21 August 2003 kind of coal caused the selenosis or where the selenium Published online: 4 September 2003 came from in this selenium-rich region. When more recent ª Springer-Verlag 2003 papers or reports mention the selenosis in Ziyang, most of them cite Mei’s contention that the selenium is from L. Kunli (&) Æ X. Lirong Æ T. Jian’an burning Se-enriched coal. CAS, Institute of Geographical Sciences and Natural Resource In the Daba region, the biggest stone coal mine (Silurian in Research, Building 917, 3 Datun Road, age) is at Haoping (Edit Committee on Ziyang County 100101 , China E-mail: [email protected] Annals 1989; Luo and others 1995), located about 10 km Tel.: +86-010-64856503 southeast of Naore village (Fig. 1); it has large reserves and Fax: +86-010-64851844 its coal is very commonly used in the Daba region. At W. Douhu Naore village, there is a small deposit of stone coal, but it is Xi’an University of Science and Technology, not used because the quality is poor and the reserve is 710054 Xi’an, China small since it is enclosed in the volcanic tuff (Fig. 2). As X. Lianhua there is no highway between Naore village and the Ha- Ziyang Bureau of Mineral Development Supervision, oping stone coal mine, this stone coal is more expensive 725300 Ziyang, Shaanxi, China than the local cornstalks used as fuel by most villagers in

426 Environmental Geology (2004) 45:426–432 DOI 10.1007/s00254-003-0893-z Original article

Fig. 1 In 1982, Luo began to study the strata sequences and Map showing the study area and the geologic setting of Naore village geochemical character of Cambrian and Silurian rocks in in Ziyang County. Please note: 1 stratigraphic boundary, 2 fault, 3 river, 4 Early Paleozoic volcanic rock, 5 Upper Neoproterozoic, the Daba region. She and her research group (Luo and 6-Early Paleozoic syenite porphyry, 7 Daguiping Formation (Silurian), others 1994, Luo and Jiang1995, Luo and others1995, 8 Lower Neoproterozoic, 9 Meiziya Formation (Silurian), 10 Lujiaping 2001, 2002) studied the selenium content and its distri- Formation (Cambrian), 11 Donghe Group, 12 Jianzhuba Formation bution pattern in early Paleozoic rocks and stone coal, (Lower Cambrian), 13 siliceous escarpment, 14 Naore village and the relationship between selenium content of the rocks and their mineral and chemical composition. Naore. Only a few families use the Haoping stone coal as Recently, they studied the elemental activity of different fuel. early Paleozoic rocks in the supergene environment of If the selenium in this selenosis area comes from burning the Daba region, South Qinling Mountain. In this paper, selenium-enriched coal, why are there no selenosis prob- the relation of the selenium content of the soil and lems reported in the town of Haoping and other places bedrock in Ziyang County, especially at Naore village is where the citizens use more stone coal for warmth and discussed. The selenium content in soil and rock in other cooking than in Naore village, and why is the only sele- parts of the Daba region are reported, particularly at nosis occurrence in a small area of the Naore valley? Why Tieluba-Zuolong (, Fig. 3), at Lujiaping- is there no selenosis in other valleys adjacent to the Naore Jianzhuba (Ziyang County, (Fig. 3) and at Suang’an- village where the geographical conditions and living habits Xinglong (Fig. 3), where geological conditions are similar are the same? to Naore village. In addition to the known selenosis Li and others (1982) first reported that there were big dif- areas, latent selenosis regions in the Daba area, South ferences in selenium content in water, soil, crops, and hu- Qinling Mountain are outlined in this paper, which is the man hair in different localities of the Daba region, but they first such report. offer no explanation for the wide differences in selenium content encountered. Zhao and others (1993) made a systematic study of soil in Ziyang County and found great differences in selenium content, which varied from Materials and Methods 0.324 mg/kg to 23.53 mg/kg. Luo and others (1994, 2001) analyzed the selenium content of the bedrock strata in the The selenosis area at Naore village, where the selenium Daba region and demonstrated that there were great dif- content of the farmland soil is the highest in Ziyang ferences in selenium content of different types of rocks. County, was the first selected for surveying and sampling Previously, the researchers studying the selenosis prob- of the strata. When surveying and sampling in Naore or lems in China were mainly physicians, ecologists or other areas, we took ten rock samples from each type of agriculturalists; few geologists have paid attention to the rock bed, such as black shale, carbonaceous slate, lime- selenosis problem. Field geological work in the Daba stone, etc. At the same time, samples of the natural or region is complicated by poor communication since most cultivated soil (at a depth of 0–30 cm) were collected at places have no highway access. In addition, the strati- the points where bedrock is exposed. The soil at the graphic sequence in the Daba region is complicated by bottom of the Naore village valley and on the sloping faulting. Previously, no detailed geological fieldwork has fields on both sides is mainly weathered from local been done in the Naore village. bedrock. The soil at the bottom of the valley is thick,

Environmental Geology (2004) 45:426–432 427 Original article

Fig. 2 The geology of the selenosis area at Naore village, Ziyang County: M1 plan geological map, M2 measured cross section, M3 soil profiles. The measured section started at the top of the mountain to the north of Naore village, and the strike of the section is N190, nearly parallel to the direction of dip. Please note: 1 Neoproterozoic dolomite, 2 siliceous rock, 3 stone coal, 4 volcanic tuff, 5 black carbonaceous slate, 6 gray-green silty shale, 7 limestone of Jianzhuba Formation, 8 resi- dential area; 9 Se-enriched area, l0 location of soil test pit

about 0.3 m, while the soil on the sloping fields is thin, Results varying from 0.1 m to 0.3 m. All samples were analyzed by the Northwest Geological The study area Testing Center which is a first class analytical laboratory The Daba region of South Qinling Mountain is located in of CNNC (China National Non-Ferrous Metals Industry the southern part of Shaanxi Province, in Central China Corporation), accredited by China National Metrology (Fig. 1). The Daba region is mountainous with steep slopes Bureau, and which provides high quality analytical re- and deep ravines. It is heavily dissected by streams and sults. The soil and rock samples were dried at a low ˚ rivers fed by seasonal rains. temperature of 35 C for 6-12 h to avoid loss of Se Naore village is very small with about 30 people and through volatilization. The samples were disaggregated to located at the mouth of a deep, narrow valley (trending pass a 2-mm-nylon sieve mesh and were ground in an N300º), 3 km northeast of Shuang’an town (Fig. 1). A agate vibrating-cup mill to <150 microns. The soil and small fault (trending N190º) is crossed by a regional fault rock samples were treated by HNO3+H2SO4 dissolution (trending N300º) at the bottom of the valley, an area 500-m and the selenium content was determined by the hydride long and 60-m wide, with 15º gradients, where the people generation atomic fluorescence spectrometric (HG-AFS) of Naore village dwell and farm. To both sides of the Naore technique. For quality control in chemical analysis, the village valley are steep hill slopes. There is no usable stone standard reference materials [GBW 07401–07408 (GSS coal near Naore village, so the local people use corn and 1–8, soil, China), GBW 07103 (GSR 1, rock, China), rice stalks and some wood as fuel. A few use stone coal Chinese Standard Sample Study Center, Chinese Academy from the mine at Haoping, but this is limited since there is of Measurement Sciences] were randomly analyzed with no highway to Naore village, and the distance from each batch of soil and rock samples. The relative stan- ) Haoping is 40 km over several great mountains. dard deviation was 10% and the detection limit was 10 9.

428 Environmental Geology (2004) 45:426–432 Original article

Fig. 3 The selenosis and latent selenosis zones in Daba region, South Qinling Mountain

Selenosis only occurs in the villagers living in the Naore Naore village are in the gentle valley floor. The valley is valley; no selenosis has been detected in adjacent valleys. underlain by black pyritic carbonaceous slate and pyritic In 1980, the local epidemic prevention and health station volcanic tuff of the Lujiaping Formation, which are rich in found the selenium content of human hair at Naore to be selenium and sulfur. The selenium content of the volcanic very high. Investigators also analyzed the selenium content tuff is 26.6–48 mg/kg and of the pyritic carbonaceous slate of local grain and soil in Naore village. Corn in the some is about 13–42 mg/kg, mostly averaging about 20 mg/kg. fields is high in selenium and the soil in those fields is On the south side of valley occurs gray-green silty shale 15.74 mg/kg Se (Mei 1985). The reason the local people and limestone of the Jianzhuba Formation (Early Cam- lost their hair and nails were clear, they were suffering brian), the selenium content here is not high (Table 1, from selenosis induced by consuming selenium-enriched bed 6). corps from the fields in the valley (Edit Committee on Naore is situated on rocks that are faulted. Fig. 2 shows the Ziyang County Annals 1989). geological structure in the vicinity of Naore village. A small fault (trending N190º) is crossed by a larger fault (trending N300º) at the bottom of the valley. There is a shattered zone at the intersection of the faults with some clay de- Geology and selenium distribution rived from the bedrock. As a result, the pyritic carbona- at Naore Village ceous slate and volcanic tuff of the Lujiaping Formation are present in a relatively flat area 60-m wide and 150-m long, where the selenium content of the soil is the highest In the Daba region, Neoproterozoic and Early Paleozoic (Fig. 2). strata, including carbonates, siliceous-carbonaceous shales The soil in the fields at Naore village is very thin, about 10 and siliciclastics, are interbedded with and intruded by to 30 cm, mainly weathered from bedrock. The highest volcanic rocks (diabase, trachy-diabase, trachyte, and selenium content in the field is in the soil just above beds volcanic tuff). Faulting and folding complicate the geologic four and five (Fig. 2). These soils range from 16.6 to structure. 48.3 mg/kg Se, averaging 26 mg/kg. The crops grown on After six field visits by Luo in 1984,1992,1994,1995,2001, this soil take up this excess selenium; corn contains and 2002, the stratigraphic sequence at Naore village is 6.6 mg/kg Se (Luo and Qiu 1995) and horsebean contains clear. The rocks are mainly dolomite, limestone, black 48.84 mg/kg Se (Mei 1985). This is apparently the cause of pyritic carbonaceous slate and volcanic tuff of the Lujia- the selenosis in the Naore village. ping Formation of Early Cambrian and Late Neoprotero- zoic age (Fig. 2). North of Naore village, resistant Lower Cambrian siliceous Selenium content of soil and bedrock rocks form a steep hillside, which is not tillable. South of in other parts of the Daba region the village Neoproterozoic rocks form even steeper slopes. Table 1 shows the selenium content of natural soil above The main residential area and most of the fields of the different types of bedrock of the Lujiaping Formation in

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Table 1 Selenium content in natural soil above different types of bedrock of Lujiaping Formation in Daba region, South Qinling Mountain (mg/kg)

Bedrock type Sampling spot Sampling depth (m) Shuang’an, Ziyang Lujiaping, Ziyang Zuolong, Langao Qingjing, Ziyang

Phyllite in contact Natural soil 2.1 0.9 1.2 1.2 ±0.1 zone (bed 6) Bed rock 2.3 2.1 1.6 2.5 ±1.0 Pyritic volcanic Natural soil 42.3 – – – ±0.2 tuff (bed 4) Volcanic tuff 36.2 – – – ±1.0 in contact zone Natural soil 14.2 13 15 13 ±0.3 with pyritic Black slate 22.6 22 27 35 ±1.0 black carbonaceous slate (bed 5) Pyritic volcanic Natural soil 26.1 13 21 17 ±0.3 tuff (bed 4) Pyritic tuff 36.4 28 37 15 ±2.0 Dolomite (bed 1) Natural soil 0.8 – 0.5 0.8 ±0.1 Bedrock 4.0 – 3.0 5.0 ±1.0

Table 2 While the stone coal in this area contains some selenium, Selenium content in natural soil above some bedrocks of Early which also appears in soil, the stone coal has little influ- Paleozoic strata in Ziyang County, Daba region, South Qinling Mountain ence on the broad environment since it occurs in dis- continuous layers in country rock with normal selenium Sampling spot Age of bedrock Selenium content (mg/kg) content. Furthermore, there is no selenosis reported in the region in which the stone coal of Haoping is commonly Wafangdian – Natural soil 0.71 used. S2m Grey shale 0.52 South of - – Natural soil 0.63 ang’an, S1+2d Shale 0.54 Distribution of the selenium-rich strata Haoping – Natural soil 0.44 of the Lujiaping Formation S2m Grey shale 0.40 Since the selenium in the soil at Naore village comes from – Natural soil 4.20 selenium-rich bedrock, the pyritic carbonaceous slate and S1+2d Stone coal 6.51 Gaotan – Natural soil 0.20 pyritic volcanic tuff of the Lujiaping Formation, what is O Marlite and 0.12 the distribution of these selenium-rich units elsewhere in shale the Daba region? North of Mao- – Natural soil 0.16 In the Daba region, Late Neoproterozoic and Early baguan C3 Limestone 0.14 Maobaguan – Natural soil 0.23 Paleozoic formations composed of siliciclastic and car- C2 Marlite 0.26 bonate rocks with some volcanics, are variably repeated in a series of northwest-trending outcrop belts as a result of folds and faults. Outcrops of the pyritic carbonaceous slate and pyritic volcanic tuff of the Lujiaping Formation occurs four different localities in the Daba region, South Qinling in belts from Jianzhuba to Lujiaping on the border of Mountain. It is clear that the bedrock controls the sele- and Shaanxi Province, in a narrow area from nium content of soils. The pyritic carbonaceous slate and Tieluba to Zuolong, Langao County, and extending to the volcanic tuff of the Lujiaping Formation are the selenium- east, in addition to an outcrop belt extending from rich units within the formation (Luo and others 2001). Shuang’an to Xinglong including Naore village (Figs. 1,3). The selenium content of soil and bedrock of other Late We analyzed rocks and soils in these areas (Table 1), Neoproterozoic and Early Paleozoic formations of the finding selenium contents of about 10 to 30 mg/kg in Daba region of Ziyang County is shown in Table 2. These weathered soil. However no selenosis has been reported rocks and their derived soils are all below 1 mg/kg Se (Luo from these additional areas. and others 2001), except for the stone coal of Haoping. The The selenium-enriched units of pyritic carbonaceous slate stone coal in the Daba region is enriched in selenium, and and volcanic tuff in the Lujiaping Formation are only 40- the weathered soil above the coals is enriched also. How- meters thick in aggregate. The rivers and valleys of the ever, the only stone coal that exceeds 10 mg/kg Se is the Daba region generally trend perpendicular to the strike of very limited occurrence in the Lujiaping Formation (Luo the strata. Few localities have the valley aligned with the and others 1995, 2001). Most of the stone coal in the Daba strike of the strata as at Naore village, where this series of region is enveloped in trachitic pyroclastic rock, not as pyritic carbonaceous slate and volcanic tuff are partially continuous beds but as lenticular, irregular bodies. Even at repeated by faulting creating a wide, flat area of selenium- the top-quality stone coal deposit in Haoping, Ziyang rich soils in the valley floor. Elsewhere these selenium-rich County, which has reserves exceeding a million tons, a units are relatively thin and closely associated with sili- single coal body is generally only 3 to 4-m wide, 1 to 5-m ceous rock that forms very steep mountain slopes. Few high, and from several meters to tens of meters in length. people inhabit such areas, so there is no widespread

430 Environmental Geology (2004) 45:426–432 Original article

selenosis in the Daba region as noted by Mei (1985) who Natural Resource, Chinese Academy of Sciences (Grant stated that NO.SJ10G-A01–03), and the Subject Leader Foundation of the ‘‘Therefore, the outcrop belts of this 40-meter-thick Ministry of Coal Industry (Grant NO. 2300213). package of pyritic carbonaceous slate and volcanic tuff of Lujiaping Formation could be considered latent selenosis zones within the Daba region (Fig. 3).’’ References

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