Original article
Selenium source in the selenosis
area of the Daba region, South
Qinling Mountain, China
Luo Kunli Æ Xu Lirong Æ Tan Jian’an Æ Wang Douhu Æ Xiang Lianhua
Abstract Naore Village in the Shuang’an
countryside, Ziyang County, Shaanxi Province is
located in the Daba region of South Qinling
Mountain. It is one of two selenosis or selenium
poisoning areas in China. Crops grown in the area
are enriched in selenium. The soil is derived from
high-selenium, pyritic, black carbonaceous slate and
volcanic tuff of the Lujiaping Formation, Early
Cambrian and Late Neoproterozoic in age, which is
exposed in this area. The Lujiaping Formation is
more than 40-m thick. Selenium content of the
volcanic tuff averages 32 mg/kg Se and the black
carbonaceous slate averages 22 mg/kg Se, which
represent the highest concentrations recorded
among all the strata and rocks sampled in the Daba
region of South Qinling Mountain. This series of
selenium-enriched rocks is also found in other
places in the Daba region, South Qinling Mountain,
where the soil contains about 10 to 30 mg/kg Se.
These zones are latent selenosis zones in the Daba
region. In addition, a selenium-enriched black
carbonaceous shale, of Early Cambrian age, and
containing 10 to 40 mg/kg Se, occurs in several other
provinces in the south of China.
Keywords Selenosis Æ Selenium enrichment Æ Early
Cambrian Æ China Æ Shaanxi Province
Received: 19 December 2002 / Accepted: 21 August 2003
Published online: 4 September 2003
ª Springer-Verlag 2003
L. Kunli (&) Æ X. Lirong Æ T. Jian’an
CAS, Institute of Geographical Sciences and Natural Resource
Research, Building 917, 3 Datun Road,
100101 Beijing, China
E-mail: luokl@igsnrr.ac.cn
Tel.: +86-010-64856503
Fax: +86-010-64851844
W. Douhu
Xi’an University of Science and Technology,
710054 Xi’an, China
X. Lianhua
Ziyang Bureau of Mineral Development Supervision,
725300 Ziyang, Shaanxi, China
426
Environmental Geology (2004) 45:426–432
Introduction
Selenium is closely related to the well being of organic life
generally, and to human health specifically. A narrow
range of concentration exists between toxicity and deficiency (Rosenfeld and Beath 1964; Thornton and others
1983; Yang and others 1983). The selenium content in the
Earth’s crust and mantle is quite low, about 0.05 mg/kg
(Liu and others 1984). China has two known selenosis
(selenium poisoning) areas; the first is Enshi, Hubei
Province (Yan 1991; Yan and Wu 1993); the other is the
Shuang’an countryside in Ziyang County, Shaanxi Province, in the Daba region of South Qinling Mountain,
Central China (Mei 1985) (Fig. 1). The most concerning
area of selenosis in the Shuang’an countryside of Ziyang
County is very small, limited to Naore village (Mei 1985;
Edit Committee on Ziyang County Annals 1989). The
selenium content of corn in some fields of Naore village is
6.6 mg/kg (Luo and Qiu 1995), and horsebeans reach
48.84 mg/kg Se (Mei 1985). The selenium content of soil in
these fields is 15.74 mg/kg (Mei 1985) and 26 mg/kg (Zhao
and others 1993). Village people consume the seleniumenriched crops from these fields. It is clear that the selenium in the crops came from local selenium-enriched soil
(Mei 1985; Edit Committee on Ziyang County Annals 1989;
Zhao and others 1993), but what is the source of the
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
kind of coal caused the selenosis or where the selenium
came from in this selenium-rich region. When more recent
papers or reports mention the selenosis in Ziyang, most of
them cite Mei’s contention that the selenium is from
burning Se-enriched coal.
In the Daba region, the biggest stone coal mine (Silurian in
age) is at Haoping (Edit Committee on Ziyang County
Annals 1989; Luo and others 1995), located about 10 km
southeast of Naore village (Fig. 1); it has large reserves and
its coal is very commonly used in the Daba region. At
Naore village, there is a small deposit of stone coal, but it is
not used because the quality is poor and the reserve is
small since it is enclosed in the volcanic tuff (Fig. 2). As
there is no highway between Naore village and the Haoping stone coal mine, this stone coal is more expensive
than the local cornstalks used as fuel by most villagers in
DOI 10.1007/s00254-003-0893-z
Original article
Fig. 1
Map showing the study area and the geologic setting of Naore village
in Ziyang County. Please note: 1 stratigraphic boundary, 2 fault,
3 river, 4 Early Paleozoic volcanic rock, 5 Upper Neoproterozoic,
6-Early Paleozoic syenite porphyry, 7 Daguiping Formation (Silurian),
8 Lower Neoproterozoic, 9 Meiziya Formation (Silurian), 10 Lujiaping
Formation (Cambrian), 11 Donghe Group, 12 Jianzhuba Formation
(Lower Cambrian), 13 siliceous escarpment, 14 Naore village
Naore. Only a few families use the Haoping stone coal as
fuel.
If the selenium in this selenosis area comes from burning
selenium-enriched coal, why are there no selenosis problems reported in the town of Haoping and other places
where the citizens use more stone coal for warmth and
cooking than in Naore village, and why is the only selenosis occurrence in a small area of the Naore valley? Why
is there no selenosis in other valleys adjacent to the Naore
village where the geographical conditions and living habits
are the same?
Li and others (1982) first reported that there were big differences in selenium content in water, soil, crops, and human hair in different localities of the Daba region, but they
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
0.324 mg/kg to 23.53 mg/kg. Luo and others (1994, 2001)
analyzed the selenium content of the bedrock strata in the
Daba region and demonstrated that there were great differences in selenium content of different types of rocks.
Previously, the researchers studying the selenosis problems in China were mainly physicians, ecologists or
agriculturalists; few geologists have paid attention to the
selenosis problem. Field geological work in the Daba
region is complicated by poor communication since most
places have no highway access. In addition, the stratigraphic sequence in the Daba region is complicated by
faulting. Previously, no detailed geological fieldwork has
been done in the Naore village.
In 1982, Luo began to study the strata sequences and
geochemical character of Cambrian and Silurian rocks in
the Daba region. She and her research group (Luo and
others 1994, Luo and Jiang1995, Luo and others1995,
2001, 2002) studied the selenium content and its distribution pattern in early Paleozoic rocks and stone coal,
and the relationship between selenium content of the
rocks and their mineral and chemical composition.
Recently, they studied the elemental activity of different
early Paleozoic rocks in the supergene environment of
the Daba region, South Qinling Mountain. In this paper,
the relation of the selenium content of the soil and
bedrock in Ziyang County, especially at Naore village is
discussed. The selenium content in soil and rock in other
parts of the Daba region are reported, particularly at
Tieluba-Zuolong (Langao County, Fig. 3), at LujiapingJianzhuba (Ziyang County, (Fig. 3) and at Suang’anXinglong (Fig. 3), where geological conditions are similar
to Naore village. In addition to the known selenosis
areas, latent selenosis regions in the Daba area, South
Qinling Mountain are outlined in this paper, which is the
first such report.
Materials and Methods
The selenosis area at Naore village, where the selenium
content of the farmland soil is the highest in Ziyang
County, was the first selected for surveying and sampling
of the strata. When surveying and sampling in Naore or
other areas, we took ten rock samples from each type of
rock bed, such as black shale, carbonaceous slate, limestone, etc. At the same time, samples of the natural or
cultivated soil (at a depth of 0–30 cm) were collected at
the points where bedrock is exposed. The soil at the
bottom of the Naore village valley and on the sloping
fields on both sides is mainly weathered from local
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 residential area; 9 Se-enriched area, l0 location of
soil test pit
about 0.3 m, while the soil on the sloping fields is thin,
varying from 0.1 m to 0.3 m.
All samples were analyzed by the Northwest Geological
Testing Center which is a first class analytical laboratory
of CNNC (China National Non-Ferrous Metals Industry
Corporation), accredited by China National Metrology
Bureau, and which provides high quality analytical results. The soil and rock samples were dried at a low
˚
temperature of 35 C for 6-12 h to avoid loss of Se
through volatilization. The samples were disaggregated to
pass a 2-mm-nylon sieve mesh and were ground in an
agate vibrating-cup mill to <150 microns. The soil and
rock samples were treated by HNO3+H2SO4 dissolution
and the selenium content was determined by the hydride
generation atomic fluorescence spectrometric (HG-AFS)
technique. For quality control in chemical analysis, the
standard reference materials [GBW 07401–07408 (GSS
1–8, soil, China), GBW 07103 (GSR 1, rock, China),
Chinese Standard Sample Study Center, Chinese Academy
of Measurement Sciences] were randomly analyzed with
each batch of soil and rock samples. The relative standard deviation was 10% and the detection limit was 10)9.
428
Environmental Geology (2004) 45:426–432
Results
The study area
The Daba region of South Qinling Mountain is located in
the southern part of Shaanxi Province, in Central China
(Fig. 1). The Daba region is mountainous with steep slopes
and deep ravines. It is heavily dissected by streams and
rivers fed by seasonal rains.
Naore village is very small with about 30 people and
located at the mouth of a deep, narrow valley (trending
N300º), 3 km northeast of Shuang’an town (Fig. 1). A
small fault (trending N190º) is crossed by a regional fault
(trending N300º) at the bottom of the valley, an area 500-m
long and 60-m wide, with 15º gradients, where the people
of Naore village dwell and farm. To both sides of the Naore
village valley are steep hill slopes. There is no usable stone
coal near Naore village, so the local people use corn and
rice stalks and some wood as fuel. A few use stone coal
from the mine at Haoping, but this is limited since there is
no highway to Naore village, and the distance from
Haoping is 40 km over several great mountains.
Original article
Fig. 3
The selenosis and latent selenosis zones in
Daba region, South Qinling Mountain
Naore village are in the gentle valley floor. The valley is
underlain by black pyritic carbonaceous slate and pyritic
volcanic tuff of the Lujiaping Formation, which are rich in
selenium and sulfur. The selenium content of the volcanic
tuff is 26.6–48 mg/kg and of the pyritic carbonaceous slate
is about 13–42 mg/kg, mostly averaging about 20 mg/kg.
On the south side of valley occurs gray-green silty shale
and limestone of the Jianzhuba Formation (Early Cambrian), the selenium content here is not high (Table 1,
bed 6).
Naore is situated on rocks that are faulted. Fig. 2 shows the
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 deGeology and selenium distribution rived from the bedrock. As a result, the pyritic carbonaceous slate and volcanic tuff of the Lujiaping Formation
at Naore Village
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 Lujiathe selenosis in the Naore village.
ping Formation of Early Cambrian and Late Neoproterozoic age (Fig. 2).
Selenium content of soil and bedrock
North of Naore village, resistant Lower Cambrian siliceous
in other parts of the Daba region
rocks form a steep hillside, which is not tillable. South of
the village Neoproterozoic rocks form even steeper slopes. Table 1 shows the selenium content of natural soil above
different types of bedrock of the Lujiaping Formation in
The main residential area and most of the fields of the
Selenosis only occurs in the villagers living in the Naore
valley; no selenosis has been detected in adjacent valleys.
In 1980, the local epidemic prevention and health station
found the selenium content of human hair at Naore to be
very high. Investigators also analyzed the selenium content
of local grain and soil in Naore village. Corn in the some
fields is high in selenium and the soil in those fields is
15.74 mg/kg Se (Mei 1985). The reason the local people
lost their hair and nails were clear, they were suffering
from selenosis induced by consuming selenium-enriched
corps from the fields in the valley (Edit Committee on
Ziyang County Annals 1989).
Environmental Geology (2004) 45:426–432
429
Original article
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
2.1
2.3
42.3
36.2
14.2
22.6
0.9
2.1
–
–
13
22
1.2
1.6
–
–
15
27
1.2
2.5
–
–
13
35
±0.1
±1.0
±0.2
±1.0
±0.3
±1.0
26.1
36.4
0.8
4.0
13
28
–
–
21
37
0.5
3.0
17
15
0.8
5.0
±0.3
±2.0
±0.1
±1.0
Phyllite in contact Natural soil
zone (bed 6)
Bed rock
Pyritic volcanic
Natural soil
tuff (bed 4)
Volcanic tuff
in contact zone
Natural soil
with pyritic
Black slate
black
carbonaceous
slate (bed 5)
Pyritic volcanic
Natural soil
tuff (bed 4)
Pyritic tuff
Dolomite (bed 1) Natural soil
Bedrock
Table 2
Selenium content in natural soil above some bedrocks of Early
Paleozoic strata in Ziyang County, Daba region, South Qinling
Mountain
Sampling spot
Wafangdian
South of Shuang’an,
Haoping
Gaotan
North of Maobaguan
Maobaguan
Age of bedrock
–
S2m
–
S1+2d
–
S2m
–
S1+2d
–
O
–
C3
–
C2
Selenium content (mg/kg)
Natural soil
Grey shale
Natural soil
Shale
Natural soil
Grey shale
Natural soil
Stone coal
Natural soil
Marlite and
shale
Natural soil
Limestone
Natural soil
Marlite
0.71
0.52
0.63
0.54
0.44
0.40
4.20
6.51
0.20
0.12
0.16
0.14
0.23
0.26
four different localities in the Daba region, South Qinling
Mountain. It is clear that the bedrock controls the selenium content of soils. The pyritic carbonaceous slate and
volcanic tuff of the Lujiaping Formation are the seleniumrich units within the formation (Luo and others 2001).
The selenium content of soil and bedrock of other Late
Neoproterozoic and Early Paleozoic formations of the
Daba region of Ziyang County is shown in Table 2. These
rocks and their derived soils are all below 1 mg/kg Se (Luo
and others 2001), except for the stone coal of Haoping. The
stone coal in the Daba region is enriched in selenium, and
the weathered soil above the coals is enriched also. However, the only stone coal that exceeds 10 mg/kg Se is the
very limited occurrence in the Lujiaping Formation (Luo
and others 1995, 2001). Most of the stone coal in the Daba
region is enveloped in trachitic pyroclastic rock, not as
continuous beds but as lenticular, irregular bodies. Even at
the top-quality stone coal deposit in Haoping, Ziyang
County, which has reserves exceeding a million tons, a
single coal body is generally only 3 to 4-m wide, 1 to 5-m
high, and from several meters to tens of meters in length.
430
Environmental Geology (2004) 45:426–432
While the stone coal in this area contains some selenium,
which also appears in soil, the stone coal has little influence on the broad environment since it occurs in discontinuous layers in country rock with normal selenium
content. Furthermore, there is no selenosis reported in the
region in which the stone coal of Haoping is commonly
used.
Distribution of the selenium-rich strata
of the Lujiaping Formation
Since the selenium in the soil at Naore village comes from
selenium-rich bedrock, the pyritic carbonaceous slate and
pyritic volcanic tuff of the Lujiaping Formation, what is
the distribution of these selenium-rich units elsewhere in
the Daba region?
In the Daba region, Late Neoproterozoic and Early
Paleozoic formations composed of siliciclastic and carbonate 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
in belts from Jianzhuba to Lujiaping on the border of
Sichuan and Shaanxi Province, in a narrow area from
Tieluba to Zuolong, Langao County, and extending to the
east, in addition to an outcrop belt extending from
Shuang’an to Xinglong including Naore village (Figs. 1,3).
We analyzed rocks and soils in these areas (Table 1),
finding selenium contents of about 10 to 30 mg/kg in
weathered soil. However no selenosis has been reported
from these additional areas.
The selenium-enriched units of pyritic carbonaceous slate
and volcanic tuff in the Lujiaping Formation are only 40meters thick in aggregate. The rivers and valleys of the
Daba region generally trend perpendicular to the strike of
the strata. Few localities have the valley aligned with the
strike of the strata as at Naore village, where this series of
pyritic carbonaceous slate and volcanic tuff are partially
repeated by faulting creating a wide, flat area of seleniumrich soils in the valley floor. Elsewhere these selenium-rich
units are relatively thin and closely associated with siliceous rock that forms very steep mountain slopes. Few
people inhabit such areas, so there is no widespread
Original article
selenosis in the Daba region as noted by Mei (1985) who
stated that
‘‘Therefore, the outcrop belts of this 40-meter-thick
package of pyritic carbonaceous slate and volcanic tuff
of Lujiaping Formation could be considered latent
selenosis zones within the Daba region (Fig. 3).’’
Discussion
The variations in the selenium content of the soils in the
Daba region are due to variations in the bedrock from
which the soil is derived. The selenosis problem at Naore
village of Suang’an in Ziyang County is due to the selenium-rich pyritic carbonaceous slate and volcanic tuff of
the Lujiaping Formation which crop out in the floor of the
valley and weather to selenium-rich soils. Crops grown on
these soils are selenium-enriched and consumed locally.
The problem is exacerbated at Naore village because the
valley floor parallels the strike of the selenium-rich units.
These units, which together are 40-meters thick, crop out
in several belts in the Daba region, but no widespread
selenosis has been reported since most valleys are perpendicular to the strike of the strata and the area underlain
by the thin, selenium-rich units is small. These areas are
mountainous and steep with few inhabitants. These outcrop belts should be considered to be latent selenosis
areas.
In south China, in the provinces of Guizhou, Yunnan,
Sichuan, and Hunan, Early Cambrian black shale ranging
from 1 to 6 meters in thickness has polymetallic enrichment of Fe-Ni-Mo near the base of the black shale of the
Niutitang Formation in Guizhou, and the lowermost part
of the Qiongzhusi Formation in Yunnan (Fan and others
1983; Luo and others 1999; Xiang and others 1999; Steiner
and Erdtmann 2001; Steiner and others 2001), but selenium is not mentioned. Recently, we collected samples of
the Fe-Ni-Mo layers of the lowermost Cambrian Niutitang
Formation in Guizhou, Yunnan, Sichuan, and Hunan, and
analyzed the trace element content. The selenium values
are about 10 to 40 mg/kg.
In other parts of the World, black shales of Early
Cambrian age are present in the U.S.A., South Australia,
Canada, Britain, and other areas (Berry and Wilde 1978;
Leggett 1980; Jenkins and Hasenhohr 1989; Horan and
others 1994; Pasava and others 1996; Martens and Suarez
1997), but the selenium contents of the shales have
apparently not been widely investigated.
Acknowledgements The authors express their heartfelt thanks to
Dr. J. Stewart Hollingsworth, for detailed editing of the English in
the text and for suggestions and amendments. Data in this paper
came from the authors’ fieldwork in the study area. The authors
express their heartfelt thanks to students from the 1989 to 1994
classes of the Geology Department in Xi’an University of Science
& Technology and to some local people in Ziyang County for
their valuable help with the fieldwork. The Chinese National Key
Project supported this work for Basic Research (Grant NO.
G1999022212–02), other support came from the Knowledge
Innovation Foundation of Institute of Geographical Sciences and
Natural Resource, Chinese Academy of Sciences (Grant
NO.SJ10G-A01–03), and the Subject Leader Foundation of the
Ministry of Coal Industry (Grant NO. 2300213).
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