BIOLOGICAL TRACE ELEMENT RESEARCH 4, 21-28 (1982)
Cadmium, Copper, and Zinc Levels in the Rice and Rice Field Soil of Houston, Texas SHOSUKE S U Z U K I
Department of Public Health, School of Medicine, Gunma University, 3-39-22 Showamachi, Maebashi 371, Japan AND
SOICHIRO IWAO*
Department of Environmental Health, Universityof Occupational and Environmental Health, Japan, 1-1 lseigaoka, Yahatanishi-ku, Kitakyushu 807, Japan Received July 8, 1981; Accepted July 29, 1981
Abstract Fifty-one pairs of hulled rice samples and the soil from which each rice sample was grown were analyzed for heavy metals in August, 1979, in order to estimate the background contamination of cadmium (Cd), copper (Cu), and zinc (Zn) in rice grown in the Houston, Texas area. Both samples were divided into three groups by soil types and colors. The cadmium concentration in Texas rice was only one-half to one-quarter lower than that of Asian rice. However, the levels of Cu and Zn in rice in Texas were similar to those reported. Soil heavy metals were lower than ever reported, but these values were consistent with the geochemical characteristics of the Texas Houston area. No particular relationship was found between the three metals in rice and the metals in soil where the sampled rice was grown.
Index Entries: Cadmium in rice, and in rice fields; copper in rice, and in rice fields; zinc in rice, and in rice fields; interrelationships of heavy metals; rice, Cd, Cu, and Zn in.
Introduction Since recent concern about heavy metal pollution has focused on cadmium (Cd) accumulation in the human body, the study of background Cd levels as well as its toxic levels has become necessary (1). The Swedish government recently an9 1982 by The Humana Press Inc. All rights of any nature whatsoever reserved. 0163~.984/82/0300~0021$2.00
21
22
SUZUKI AND IWAO
nounced a regulation banning major uses of Cd (2). This action was taken because of a gradual but steady increase of the Cd level in wheat produced near Stockholm over the past few decades (3). For people such as the Japanese, whose staple food it is, rice is a major contributor to daily Cd intake (4-6). The Japanese accumulation of Cd in the renal cortex is three to five times higher than that found among Swedish and US inhabitants (7-9). Thus in Japan, it is very important not to grow rice in Cd-polluted soil. In fact, higher levels of Cd have been detected in rice, as well as in the soil office fields, located downstream from metal mines or refining industries (10). Consequently, the monitoring of background levels of Cd and other metals in various foods is necessary for the better understanding of heavy metal accumulations in the human body. There are many reports on environmental Cd in the world, but there are few data on Cd levels in rice in the US (11, 12). This is probably because almost all rice produced in the US, mainly in California and the Texas Gulf Coast area, is exported and less than 10 g/day is consumed by the Americans themselves (13). This study was undertaken to obtain data on general background levels of Cd in the US and to assess the daily Cd intake among those who eat rice grown in the US. The environmental levels of Cd, Cu, and Zn in rice samples from Texas and the soil in which they were grown are reported and the interrelationships among these metals according to the type of soil are evaluated.
Materials and Methods Fifty-one pairs of hulled rice samples and the superficial soil just beneath the samples were collected during the harvest period on August, 10, 1979, from several representative rice fields near Houston, Texas. Soil samples were classified into three categories by color and the soil association was identified by a soil map (14). The soil type of the Texas Gulf Coast area is Usters of Vertisol according to the soil classification system of the US Department of Agriculture (14). Fourteen pairs of rice and soil samples were taken from the eastern part of Harris County, which includes Houston. The black soil samples were classified into Lake Charles-Bernard Association or Midland-Beaumont Association, which are poorly drained, slowly permeable, loamy and clayey soils (14). The remaining 37 of the 51 soil samples were lighter, gray or white, classified into Clodine-AddicksGessner Association and Katy-Aris Association, which are poorly drained but moderately permeable soil types (14). Approximately 5-15 g of the hulled rice on stalk was packed in a polyethylene bag and transferred to a filter paper bag, dried in a room with a temperature of 23-25~ and a relative humidity of 50%. Two months later, the samples were hulled by pounding. Six to seven grains of unhulled brown rice were weighed and dried in an oven at 105~ for 48 h. The ashing of the grains was completed by addition of diluted (14%) nitric acid to obtain 1.0 mL of ashed solution. Each sample was analyzed twice or three times to ensure accuracy of measurement. In addition, 50, 100, and 200 mg of powdered rice, provided by the US Na-
23
CD, C u , AND ZN IN TEXAS RICE AND RICE FIELDS
tional Bureau of Standards as Standard Reference Material No. 1568, was ashed for correction when necessary. The three metals in the solution were analyzed by atomic absorption spectrophotometer (AAS) (Perkin-Elmer 603). Concentrations of Cd and Cu were determined by flametess AAS (HGA 2100) under the suggested conditions, and Zn in the ashed rice solution was analyzed by conventional AAS after five-fold dilution with deionized water, under the conditions recommended. The soil samples were dried in an oven at 105~ for 72 h. The dried soil samples without stones and pebbles were mixed well and 3 g each of the powdered soil was processed. The acidity of soil was determined by a glass electrode pH meter (model: Seromatic SS-3) using the supernatant of 1 : 1 soil-water mixture (15). The metals were extracted from soil to solution by two methods: by adding 1N HNO3 and by adding 1N CH3COOH (16). The soil suspension was left for 3 days and the supernatant was filtered. The metals in the filtrate were analyzed by AAS. Among them Cd was determined by a flarneless method, and others were processed by the conventional method. No standard materials were used in the soil analysis.
Results The average percentage of dry weight of 51 unpolished rice samples to wet weight measurement was 89.8%, ranging from 87.5 to 91.5%. The Cd, Cu, and Zn levels of 51 rice samples are shown in Table 1. Arithmetic mean and standard deviation of each heavy metal were 23.4 -+-+19.0 ng/g for Cd, 2.49 --+ 1.01 Ixg/g for Cu, and 22.9 --+ 3.74 ~g/g for Zn. The frequency distribution of the values was so skewed Table 1 Cadmium, Copper, and Zinc Content in 51 Pairs of Rice and Soil Samples from Houston, Texas, USAa
Mean SD CV, % Skewness Minimum Maximum GM GD
Cd in rice, ng/g
Cu in rice, Ixg/g
Zn in rice, Ixg/g
Cd in soilo, ng/g
Cu in soilb, Ixg/g
Zn in soilb, t~g/g
Cd in Zn in soilC, soiF, ng/g Ixg/g
23.4 19.0 81.2 0.85 1 72 14.8 3.04
2.49 1.01 40.0 0.72 0.6 6.0 2.28 1.48
22.9 3.74 16.3 -.71 11 29 22.6 1.20
26.9 13.4 50.0 I. 10 6 71 23.8 1.65
1.79 0.87 48.6 0.48 0.14 3.7 1.55 1.84
35.4 23.9 67.5 2.24 7.9 140 29.8 1.78
21.0 21.1 100 3.66 5.4 123 16.6 1.83
21.4 15.5 72.6 3.00 5 100 17.7 1.77
aAll figures of concentrationare dry wt. basis. Abbreviationsand marks: Cu, copper;Zn, zinc; Cd, cadmium; Mean, arithmetic mean; SD, standard deviation; GM, geometric mean, GD, geometric deviation. bMetals extracted by nitric acid. CMetals extracted by acetic acid.
24
SUZUKI AND IWAO
that the geometric mean and geometric standard deviation were calculated, as shown in Table 1. The levels of the three heavy metals obtained by the two extraction methods in the 51 soil samples are shown in Table 1. Means and standard deviations of Cd, Cu, and Zn concentrations were 26.9 --- 13.4 ng/g, 1.79 --- 0.87 ixg/g, and 35.4 --- 23.9 Ixg/g, respectively, by the nitric acid extraction method. Metals extracted by acetic acid were determined as 2 1 . 0 - 21.1 ng/g for Cd and 21.4 --- 15.5 ~g/g for Zn. All metal concentrations obtained by the nitric acid extraction method showed higher than those obtained by the acetic acid extraction method. Concentrations of the metals in rice samples indicated little difference from the concentrations in soil samples in this series of analysis. Black soil samples from the eastern part of the Houston area contained more Cd, Cu, and Zn than gray and white soil samples from the other Houston when compared by the nitric acid extraction method (Table 2). The Zn level in the black soil Table 2 Difference of Cd, Cu, and Zn Contents in 51 Pairs of Rice and Soil Samples from Houston, Texas, USA, by Three Soil Types~
Number of samples
Soil type 1. White 2. Gray 3. Black Significant difference
13 24 14
Cu in soil ixg/g~ AM
Cd in rice, ng/g
Cu in rice, Ixg/g
Zn in rice, I~g/g
Cd in soil ng/gb
GM
AM
AM
GM GD
GD
SD
SD
13.3 3 . 4 1 2.22 0.78 22.7 4.1 13.5 3.49 2.66 1 . 2 2 22.9 3.4 19.1 2.04 2.44 0.66 23.2 4.3 (-) (-) (-)
Zn in soil ixg/g~
Cd in soil Ixg/gC
Zn in soil txg/gC
17.6 1.69 21.5 1.49 37.7 1.38 (+) p < 0.001
Soil pH
SD
GM
GD
GM
GD
AM
SD
AM
1.15 0.60 1.59 0.57 2.73 0.75 (+) p < 0.001
22.1 30.0 37.8
1.89 1.74 1.48
13.3 18.2 16.8
1.84 1.72 1.99
14.0 24.2 23.3
5.7 19.4 12.4
5.38 0.25 5.35 0.21 5.60 0.25 (+) p < 0.01
(+) p < 0.05
(-)
(+) p < 0.05
SD
aAll the soils sampledbelong to Usters (suborder) of Vertisol (order). "Black" of them belongs to Lake Charles-BernardAssociationor Midland-BeaumontAssociation. "Gray" and "White" of thembelongsto Clodine-Addicks-GessnerAssociationor Katy-ArisAssociation. Abbreviations and marks: Cd, cadmium; Cu, copper; Zn, zinc; GM, geometric mean; GD, geometricdeviation;AM, arithmeticmean, SD, standarddeviation;p, level of statistical significance. bSee Table 1 footnote. cSee Table 1 footnote.
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SUZUKI AND IWAO
extracted by acetic acid was also higher than the Zn level in the other two types of soil. Cd, Cu, and Zn levels of rice from the black soil were higher than the other two, but the differences were not statistically significant. Table 3 indicates the coefficients of correlation of the three metals by rice, soil type, and soil acidity. Correlation coefficients between rice and soil heavy metal levels indicated no significance. In rice samples, Cd showed positive relationship to Cu (r = 0.42, p < 0.05) and Zn (r = 0.28, p < 0.05). Correlation coefficients between the metals extracted by nitric acid and acetic acid showed positive significance.
Discussion In general, Vertisol produces relatively low Cd levels in rice. For example, the East Java area of Indonesia covered by Vertisol produces lower Cd rice than the West Java area covered by Ultisol (12). The Cu and Zn levels in rice in this study were almost the same as in rice of Java, while Cd in Houston rice was about onehalf that of Javanese rice, and approximately one-quarter to one-third that of Japanese rice (6, 12). The mean Cd concentration of 51 samples from the Houston area was 23.4 -+ 19.0 ng/g, which is very consistent with that found in 11 samples from three kinds of US exported rice (23 -+ 15 ng/g) (17). However, Texas rice indicated half the Cd level, 42 +- 51 ng/g, found to be the mean of 16 rice samples exported from the US (12). The two cited reports did not record the location of rice fields. In any case, judging from these data for Cd in rice, the present Cd level falls median among those from the 14 rice-producing countries of the world (17). A great many extraction methods are used to determine Cd concentrations in soil (18, 19). Among them, extraction by strong acid seems the most reliable and widely used. Weak acids, such as acetic acid, reportedly enable Cd in soil to be extracted in a mobile form (16). Our results indicate that nitric acid binds more of the metals than acetic acid, although the difference is small. The soil Cd levels in this study are considerably lower than those previously reported: a maximum of 50 ng/g of Cd in the agricultural soils in Northwestern Indiana (20), 400 + 500 ng/g Cd from 2476 unpolluted paddy soils of Japan (5), and 300-900 ng/g in rice field soils of Japan (21). Of 15 river basins in the US, Texas belongs to one whose river and lake-water Cd levels are among the lowest (22). This could explain the soil Cd level of Texas being the lowest reported. The soil Cu level of the present study is the lowest compared to data from England and Wales, Australia, New Zealand, USSR, and USA (19, 23). The soil Zn level in this study was lower than 109 p~g/g Zn in Japanese rice fields (5, 24). The Zn-to-Cd ratio for the lithosphere is 900. The median ratio for unpolluted soil is reported to be 1400, ranging from 180 to 12,000. The Zn-to-Cd ratio of nonpolluted soil is seldom lower than 400 (5). In this study, Zn/Cd ratio was 700, suggesting no Cd pollution in this area. Little relationship was observed between heavy metal levels in rice and soil. This may be because factors such as desiccation of samples, humus content, and soil acidity predominate (25, 26). Black soil indicated relatively low acidity and
CD, C u , AND ZN IN TEXAS RICE AND RICE FIELDS
27
higher metal concentrations than the other soils investigated, which may be the result of high humus content. Humus causes a reductive soil condition, sulfate ion shifting to hydrogen sulfide, thus preventing absorption of Cd to rice (25). This may explain the lack of significance in the heavy metal contents of rice samples when compared in each of the three types of soils, even though significant differences were found in the metal concentrations of the three soil types themselves.
Summary The Cd level in rice harvested near Houston, Texas, indicated a concentration of one-half that of Javanese rice, and one-quarter to one-third that of the level in Japan. However, Cu and Zn levels of rice in Houston were the same as in Java and Japan. The Cd level in Houston rice falls median among 14 rice-producing countries in the world. Soil Cd, Cu, and Zn levels of the rice fields in the Houston area showed lower values than reports from other areas. This could be explained when the geochemical characteristics of the area were taken into consideration. No significant relationship was found between rice and rice field soil from the correlation coefficient tables of the heavy metals at this level of exposure.
Acknowledgments We wish to thank Mr. Houston Horder of the Texas Agricultural Extension Service, Harris County, for giving us permission to sample rice and soil in the Houston area. Also much gratitude to the late Professor Frederick Sargent II of the School of Public Health, the University of Texas Health Science Center at Houston. His continuous encouragement made possible our work in Houston.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
B. A. Fowler, Environ. Health Perspect. 28, 297 (1979). Environmental Health News Letter 18, 21 (1979), Washington DC, EPA. T. Kjellstrom, L. Linnman, and C-G. Elinder, Arch. Environ. Health 30, 321 (1975). S. Suzuki, lgaku-no Ayumi (Progress in Medicine) 99, 150 (1976) (in Japanese). N. Yamagata, in Cadmium Studies in Japan--A Review, K. Tsuchiya, ed., Kodansha, Tokyo, 1978, pp. 19-37. S. Iwao, M. Sugita, and K. Tsuchiya, Keio J. Med. 31, 17 (1981). K. Tsuchiya and S. Iwao, Kankyo Hoken Report (Environmental Health Report) 38, 36 (1976) (in Japanese). K. Tsuchiya, in Cadmium Studies in Japan--A Review, K. Tsuchiya, ed., Kodansha, Tokyo, 1978, pp. 37 44. T. Kjellstrom, Environ. Health Perspect. 28, 169 (1979). K. Tsuchiya, in Cadmium Studies in Japan--A Review, K. Tsuchiya, ed., Kodansha, Tokyo, 1978, pp. 144-253.
28
SUZUKI AND IWAO
11. P. J. Peterson and B. J. Alloway, in The Chemistry, Biochemistry, and Biology of Cadmium, M. Webb, ed., Elsevier/North-Holland Biomedical Press, 1979, pp. 45-92. 12. S. Suzuki, N. Djuangsih, K. Hyodo, and Soemarwoto, Arch. Environ. Contam. Toxicol. 9, 437 (1980). 13. Organization for Economic Co-operation and Development, Food Consumption Statistics, 1955-1973, Paris, OECD 1975. 14. US Department of Agriculture, Soil Conservation Service, Map of the Soil Types of Texas, USDA, 1975. 15. M. K. John, C. T. Van Vaerhoven, and H. H. Chuah, Environ. Sci. Technol. 5, 1105 (1972). 16. G. W. Leeper, in Pollution Engineering and Technology, vol. 6, Dekker, New York, 1978, p. 40. 17. R. Masironi, S. R. Koirtyohann, and J. O. Pierce, Sci. Total Environ, 7, 27 (1977). 18. T. Thornton, in Proc. First Internat. Cadmium Conference, San Francisco, Metal Bulletin, London, 1978, pp. 109-110. 19. T. Thornton, in Copper in the Environment, J. O. Nriagu, ed., Wiley, New York, 1979, pp. 171-216. 20. R. I. Pietz, R. J. Vetter, D. Masarik, and W. W. McFee, J. Environ. Quality 7, 381 (1978). 21. T. Azami, and K. Kato, J. Sci. Soil Manure, Japan 48, 335 (1977) (in Japanese). 22. J. F. Kopp and R. C. Knoner, A Five-Year Summary of Trace Metals in Rivers and Lakes of the U.S. (1962-1967), US Department of Interior, Federal Water Pollution Control Administration, 1969. 23. D. J. Swaine, J. Soil Sci. 11,347 (1960). 24. Institute of Agricultural Technology, A Study on the Specific Pollutants in Soil and Clops, Japan Ministry of Agriculture, Forestry and Fishery, 1976 (in Japanese). 25. H. Itoh and K. Ihimura, J. Sci. Soil Manure, Japan 46, 82 (1975) (in Japanese). 26. F. T. Bingham, Environ. Health Perspect. 28, 39 (1979).
Cadmium, Copper, and Zinc Levels in the Rice and Rice Field Soil of Houston, Texas SHOSUKE S U Z U K I
Department of Public Health, School of Medicine, Gunma University, 3-39-22 Showamachi, Maebashi 371, Japan AND
SOICHIRO IWAO*
Department of Environmental Health, Universityof Occupational and Environmental Health, Japan, 1-1 lseigaoka, Yahatanishi-ku, Kitakyushu 807, Japan Received July 8, 1981; Accepted July 29, 1981
Abstract Fifty-one pairs of hulled rice samples and the soil from which each rice sample was grown were analyzed for heavy metals in August, 1979, in order to estimate the background contamination of cadmium (Cd), copper (Cu), and zinc (Zn) in rice grown in the Houston, Texas area. Both samples were divided into three groups by soil types and colors. The cadmium concentration in Texas rice was only one-half to one-quarter lower than that of Asian rice. However, the levels of Cu and Zn in rice in Texas were similar to those reported. Soil heavy metals were lower than ever reported, but these values were consistent with the geochemical characteristics of the Texas Houston area. No particular relationship was found between the three metals in rice and the metals in soil where the sampled rice was grown.
Index Entries: Cadmium in rice, and in rice fields; copper in rice, and in rice fields; zinc in rice, and in rice fields; interrelationships of heavy metals; rice, Cd, Cu, and Zn in.
Introduction Since recent concern about heavy metal pollution has focused on cadmium (Cd) accumulation in the human body, the study of background Cd levels as well as its toxic levels has become necessary (1). The Swedish government recently an9 1982 by The Humana Press Inc. All rights of any nature whatsoever reserved. 0163~.984/82/0300~0021$2.00
21
22
SUZUKI AND IWAO
nounced a regulation banning major uses of Cd (2). This action was taken because of a gradual but steady increase of the Cd level in wheat produced near Stockholm over the past few decades (3). For people such as the Japanese, whose staple food it is, rice is a major contributor to daily Cd intake (4-6). The Japanese accumulation of Cd in the renal cortex is three to five times higher than that found among Swedish and US inhabitants (7-9). Thus in Japan, it is very important not to grow rice in Cd-polluted soil. In fact, higher levels of Cd have been detected in rice, as well as in the soil office fields, located downstream from metal mines or refining industries (10). Consequently, the monitoring of background levels of Cd and other metals in various foods is necessary for the better understanding of heavy metal accumulations in the human body. There are many reports on environmental Cd in the world, but there are few data on Cd levels in rice in the US (11, 12). This is probably because almost all rice produced in the US, mainly in California and the Texas Gulf Coast area, is exported and less than 10 g/day is consumed by the Americans themselves (13). This study was undertaken to obtain data on general background levels of Cd in the US and to assess the daily Cd intake among those who eat rice grown in the US. The environmental levels of Cd, Cu, and Zn in rice samples from Texas and the soil in which they were grown are reported and the interrelationships among these metals according to the type of soil are evaluated.
Materials and Methods Fifty-one pairs of hulled rice samples and the superficial soil just beneath the samples were collected during the harvest period on August, 10, 1979, from several representative rice fields near Houston, Texas. Soil samples were classified into three categories by color and the soil association was identified by a soil map (14). The soil type of the Texas Gulf Coast area is Usters of Vertisol according to the soil classification system of the US Department of Agriculture (14). Fourteen pairs of rice and soil samples were taken from the eastern part of Harris County, which includes Houston. The black soil samples were classified into Lake Charles-Bernard Association or Midland-Beaumont Association, which are poorly drained, slowly permeable, loamy and clayey soils (14). The remaining 37 of the 51 soil samples were lighter, gray or white, classified into Clodine-AddicksGessner Association and Katy-Aris Association, which are poorly drained but moderately permeable soil types (14). Approximately 5-15 g of the hulled rice on stalk was packed in a polyethylene bag and transferred to a filter paper bag, dried in a room with a temperature of 23-25~ and a relative humidity of 50%. Two months later, the samples were hulled by pounding. Six to seven grains of unhulled brown rice were weighed and dried in an oven at 105~ for 48 h. The ashing of the grains was completed by addition of diluted (14%) nitric acid to obtain 1.0 mL of ashed solution. Each sample was analyzed twice or three times to ensure accuracy of measurement. In addition, 50, 100, and 200 mg of powdered rice, provided by the US Na-
23
CD, C u , AND ZN IN TEXAS RICE AND RICE FIELDS
tional Bureau of Standards as Standard Reference Material No. 1568, was ashed for correction when necessary. The three metals in the solution were analyzed by atomic absorption spectrophotometer (AAS) (Perkin-Elmer 603). Concentrations of Cd and Cu were determined by flametess AAS (HGA 2100) under the suggested conditions, and Zn in the ashed rice solution was analyzed by conventional AAS after five-fold dilution with deionized water, under the conditions recommended. The soil samples were dried in an oven at 105~ for 72 h. The dried soil samples without stones and pebbles were mixed well and 3 g each of the powdered soil was processed. The acidity of soil was determined by a glass electrode pH meter (model: Seromatic SS-3) using the supernatant of 1 : 1 soil-water mixture (15). The metals were extracted from soil to solution by two methods: by adding 1N HNO3 and by adding 1N CH3COOH (16). The soil suspension was left for 3 days and the supernatant was filtered. The metals in the filtrate were analyzed by AAS. Among them Cd was determined by a flarneless method, and others were processed by the conventional method. No standard materials were used in the soil analysis.
Results The average percentage of dry weight of 51 unpolished rice samples to wet weight measurement was 89.8%, ranging from 87.5 to 91.5%. The Cd, Cu, and Zn levels of 51 rice samples are shown in Table 1. Arithmetic mean and standard deviation of each heavy metal were 23.4 -+-+19.0 ng/g for Cd, 2.49 --+ 1.01 Ixg/g for Cu, and 22.9 --+ 3.74 ~g/g for Zn. The frequency distribution of the values was so skewed Table 1 Cadmium, Copper, and Zinc Content in 51 Pairs of Rice and Soil Samples from Houston, Texas, USAa
Mean SD CV, % Skewness Minimum Maximum GM GD
Cd in rice, ng/g
Cu in rice, Ixg/g
Zn in rice, Ixg/g
Cd in soilo, ng/g
Cu in soilb, Ixg/g
Zn in soilb, t~g/g
Cd in Zn in soilC, soiF, ng/g Ixg/g
23.4 19.0 81.2 0.85 1 72 14.8 3.04
2.49 1.01 40.0 0.72 0.6 6.0 2.28 1.48
22.9 3.74 16.3 -.71 11 29 22.6 1.20
26.9 13.4 50.0 I. 10 6 71 23.8 1.65
1.79 0.87 48.6 0.48 0.14 3.7 1.55 1.84
35.4 23.9 67.5 2.24 7.9 140 29.8 1.78
21.0 21.1 100 3.66 5.4 123 16.6 1.83
21.4 15.5 72.6 3.00 5 100 17.7 1.77
aAll figures of concentrationare dry wt. basis. Abbreviationsand marks: Cu, copper;Zn, zinc; Cd, cadmium; Mean, arithmetic mean; SD, standard deviation; GM, geometric mean, GD, geometric deviation. bMetals extracted by nitric acid. CMetals extracted by acetic acid.
24
SUZUKI AND IWAO
that the geometric mean and geometric standard deviation were calculated, as shown in Table 1. The levels of the three heavy metals obtained by the two extraction methods in the 51 soil samples are shown in Table 1. Means and standard deviations of Cd, Cu, and Zn concentrations were 26.9 --- 13.4 ng/g, 1.79 --- 0.87 ixg/g, and 35.4 --- 23.9 Ixg/g, respectively, by the nitric acid extraction method. Metals extracted by acetic acid were determined as 2 1 . 0 - 21.1 ng/g for Cd and 21.4 --- 15.5 ~g/g for Zn. All metal concentrations obtained by the nitric acid extraction method showed higher than those obtained by the acetic acid extraction method. Concentrations of the metals in rice samples indicated little difference from the concentrations in soil samples in this series of analysis. Black soil samples from the eastern part of the Houston area contained more Cd, Cu, and Zn than gray and white soil samples from the other Houston when compared by the nitric acid extraction method (Table 2). The Zn level in the black soil Table 2 Difference of Cd, Cu, and Zn Contents in 51 Pairs of Rice and Soil Samples from Houston, Texas, USA, by Three Soil Types~
Number of samples
Soil type 1. White 2. Gray 3. Black Significant difference
13 24 14
Cu in soil ixg/g~ AM
Cd in rice, ng/g
Cu in rice, Ixg/g
Zn in rice, I~g/g
Cd in soil ng/gb
GM
AM
AM
GM GD
GD
SD
SD
13.3 3 . 4 1 2.22 0.78 22.7 4.1 13.5 3.49 2.66 1 . 2 2 22.9 3.4 19.1 2.04 2.44 0.66 23.2 4.3 (-) (-) (-)
Zn in soil ixg/g~
Cd in soil Ixg/gC
Zn in soil txg/gC
17.6 1.69 21.5 1.49 37.7 1.38 (+) p < 0.001
Soil pH
SD
GM
GD
GM
GD
AM
SD
AM
1.15 0.60 1.59 0.57 2.73 0.75 (+) p < 0.001
22.1 30.0 37.8
1.89 1.74 1.48
13.3 18.2 16.8
1.84 1.72 1.99
14.0 24.2 23.3
5.7 19.4 12.4
5.38 0.25 5.35 0.21 5.60 0.25 (+) p < 0.01
(+) p < 0.05
(-)
(+) p < 0.05
SD
aAll the soils sampledbelong to Usters (suborder) of Vertisol (order). "Black" of them belongs to Lake Charles-BernardAssociationor Midland-BeaumontAssociation. "Gray" and "White" of thembelongsto Clodine-Addicks-GessnerAssociationor Katy-ArisAssociation. Abbreviations and marks: Cd, cadmium; Cu, copper; Zn, zinc; GM, geometric mean; GD, geometricdeviation;AM, arithmeticmean, SD, standarddeviation;p, level of statistical significance. bSee Table 1 footnote. cSee Table 1 footnote.
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SUZUKI AND IWAO
extracted by acetic acid was also higher than the Zn level in the other two types of soil. Cd, Cu, and Zn levels of rice from the black soil were higher than the other two, but the differences were not statistically significant. Table 3 indicates the coefficients of correlation of the three metals by rice, soil type, and soil acidity. Correlation coefficients between rice and soil heavy metal levels indicated no significance. In rice samples, Cd showed positive relationship to Cu (r = 0.42, p < 0.05) and Zn (r = 0.28, p < 0.05). Correlation coefficients between the metals extracted by nitric acid and acetic acid showed positive significance.
Discussion In general, Vertisol produces relatively low Cd levels in rice. For example, the East Java area of Indonesia covered by Vertisol produces lower Cd rice than the West Java area covered by Ultisol (12). The Cu and Zn levels in rice in this study were almost the same as in rice of Java, while Cd in Houston rice was about onehalf that of Javanese rice, and approximately one-quarter to one-third that of Japanese rice (6, 12). The mean Cd concentration of 51 samples from the Houston area was 23.4 -+ 19.0 ng/g, which is very consistent with that found in 11 samples from three kinds of US exported rice (23 -+ 15 ng/g) (17). However, Texas rice indicated half the Cd level, 42 +- 51 ng/g, found to be the mean of 16 rice samples exported from the US (12). The two cited reports did not record the location of rice fields. In any case, judging from these data for Cd in rice, the present Cd level falls median among those from the 14 rice-producing countries of the world (17). A great many extraction methods are used to determine Cd concentrations in soil (18, 19). Among them, extraction by strong acid seems the most reliable and widely used. Weak acids, such as acetic acid, reportedly enable Cd in soil to be extracted in a mobile form (16). Our results indicate that nitric acid binds more of the metals than acetic acid, although the difference is small. The soil Cd levels in this study are considerably lower than those previously reported: a maximum of 50 ng/g of Cd in the agricultural soils in Northwestern Indiana (20), 400 + 500 ng/g Cd from 2476 unpolluted paddy soils of Japan (5), and 300-900 ng/g in rice field soils of Japan (21). Of 15 river basins in the US, Texas belongs to one whose river and lake-water Cd levels are among the lowest (22). This could explain the soil Cd level of Texas being the lowest reported. The soil Cu level of the present study is the lowest compared to data from England and Wales, Australia, New Zealand, USSR, and USA (19, 23). The soil Zn level in this study was lower than 109 p~g/g Zn in Japanese rice fields (5, 24). The Zn-to-Cd ratio for the lithosphere is 900. The median ratio for unpolluted soil is reported to be 1400, ranging from 180 to 12,000. The Zn-to-Cd ratio of nonpolluted soil is seldom lower than 400 (5). In this study, Zn/Cd ratio was 700, suggesting no Cd pollution in this area. Little relationship was observed between heavy metal levels in rice and soil. This may be because factors such as desiccation of samples, humus content, and soil acidity predominate (25, 26). Black soil indicated relatively low acidity and
CD, C u , AND ZN IN TEXAS RICE AND RICE FIELDS
27
higher metal concentrations than the other soils investigated, which may be the result of high humus content. Humus causes a reductive soil condition, sulfate ion shifting to hydrogen sulfide, thus preventing absorption of Cd to rice (25). This may explain the lack of significance in the heavy metal contents of rice samples when compared in each of the three types of soils, even though significant differences were found in the metal concentrations of the three soil types themselves.
Summary The Cd level in rice harvested near Houston, Texas, indicated a concentration of one-half that of Javanese rice, and one-quarter to one-third that of the level in Japan. However, Cu and Zn levels of rice in Houston were the same as in Java and Japan. The Cd level in Houston rice falls median among 14 rice-producing countries in the world. Soil Cd, Cu, and Zn levels of the rice fields in the Houston area showed lower values than reports from other areas. This could be explained when the geochemical characteristics of the area were taken into consideration. No significant relationship was found between rice and rice field soil from the correlation coefficient tables of the heavy metals at this level of exposure.
Acknowledgments We wish to thank Mr. Houston Horder of the Texas Agricultural Extension Service, Harris County, for giving us permission to sample rice and soil in the Houston area. Also much gratitude to the late Professor Frederick Sargent II of the School of Public Health, the University of Texas Health Science Center at Houston. His continuous encouragement made possible our work in Houston.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
B. A. Fowler, Environ. Health Perspect. 28, 297 (1979). Environmental Health News Letter 18, 21 (1979), Washington DC, EPA. T. Kjellstrom, L. Linnman, and C-G. Elinder, Arch. Environ. Health 30, 321 (1975). S. Suzuki, lgaku-no Ayumi (Progress in Medicine) 99, 150 (1976) (in Japanese). N. Yamagata, in Cadmium Studies in Japan--A Review, K. Tsuchiya, ed., Kodansha, Tokyo, 1978, pp. 19-37. S. Iwao, M. Sugita, and K. Tsuchiya, Keio J. Med. 31, 17 (1981). K. Tsuchiya and S. Iwao, Kankyo Hoken Report (Environmental Health Report) 38, 36 (1976) (in Japanese). K. Tsuchiya, in Cadmium Studies in Japan--A Review, K. Tsuchiya, ed., Kodansha, Tokyo, 1978, pp. 37 44. T. Kjellstrom, Environ. Health Perspect. 28, 169 (1979). K. Tsuchiya, in Cadmium Studies in Japan--A Review, K. Tsuchiya, ed., Kodansha, Tokyo, 1978, pp. 144-253.
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11. P. J. Peterson and B. J. Alloway, in The Chemistry, Biochemistry, and Biology of Cadmium, M. Webb, ed., Elsevier/North-Holland Biomedical Press, 1979, pp. 45-92. 12. S. Suzuki, N. Djuangsih, K. Hyodo, and Soemarwoto, Arch. Environ. Contam. Toxicol. 9, 437 (1980). 13. Organization for Economic Co-operation and Development, Food Consumption Statistics, 1955-1973, Paris, OECD 1975. 14. US Department of Agriculture, Soil Conservation Service, Map of the Soil Types of Texas, USDA, 1975. 15. M. K. John, C. T. Van Vaerhoven, and H. H. Chuah, Environ. Sci. Technol. 5, 1105 (1972). 16. G. W. Leeper, in Pollution Engineering and Technology, vol. 6, Dekker, New York, 1978, p. 40. 17. R. Masironi, S. R. Koirtyohann, and J. O. Pierce, Sci. Total Environ, 7, 27 (1977). 18. T. Thornton, in Proc. First Internat. Cadmium Conference, San Francisco, Metal Bulletin, London, 1978, pp. 109-110. 19. T. Thornton, in Copper in the Environment, J. O. Nriagu, ed., Wiley, New York, 1979, pp. 171-216. 20. R. I. Pietz, R. J. Vetter, D. Masarik, and W. W. McFee, J. Environ. Quality 7, 381 (1978). 21. T. Azami, and K. Kato, J. Sci. Soil Manure, Japan 48, 335 (1977) (in Japanese). 22. J. F. Kopp and R. C. Knoner, A Five-Year Summary of Trace Metals in Rivers and Lakes of the U.S. (1962-1967), US Department of Interior, Federal Water Pollution Control Administration, 1969. 23. D. J. Swaine, J. Soil Sci. 11,347 (1960). 24. Institute of Agricultural Technology, A Study on the Specific Pollutants in Soil and Clops, Japan Ministry of Agriculture, Forestry and Fishery, 1976 (in Japanese). 25. H. Itoh and K. Ihimura, J. Sci. Soil Manure, Japan 46, 82 (1975) (in Japanese). 26. F. T. Bingham, Environ. Health Perspect. 28, 39 (1979).
