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Two-sample technique for determining the influence of soil organic matter on copper concentration
Journal of Geochemical Exploration, 7(1977)361--371
361
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
TWO-SAMPLE TECHNIQUE FOR DETERMINING THE INFLUENCE OF SOIL ORGANIC MATTER ON COPPER CONCENTRATION
DOUGLAS E. PRIDE
Department of Geology and Mineralogy, Ohio State University, Columbus, Ohio 43210 (U.S.A.) (Received October 6, 1976; revised version accepted February 23, 1977)
ABSTRACT Pride, D.E., 1977. Two-sample technique for determining the influence of soil organic matter on copper concentration. J. Geochem. Explor., 7 : 361--371. A sample grid was established 9.5 km northwest of Ladysmith, Wisconsin, to provide information on the potential significance of organic matter on the concentration of Cu in a grey-brown podzolic soil. Two samples were taken from each of 130 sample locations within the grid. One sample was collected from near the b o t t o m of the zone of obvious organic enrichment in the soil, and the other was taken a short distance below the organic zone. Data from individual sample holes were transformed to maximize the potential influence of organic matter on Cu enrichment. By combining the data from the entire grid, it was possible to derive empirical populations representing the variability of weight percent organic matter and Cu concentration within individual sample holes. By further combining these populations, graphically, it is possible to delineate regions of interest that may be used to identify organic influences on Cu concentration within sample holes. The study suggests that in similar sampling situations, organic contents up to 5% dry weight do not greatly influence Cu concentrations within the soil profile. T o some degree, the combined influence of differences in clay content and content of hydrous iron and manganese oxides between sample levels within individual holes can also be demonstrated.
INTRODUCTION
The present work was undertaken to provide information on the potential significance of small percentages of organic matter on the concentration of Cu within a soil sample plot. The study area is located a b o u t 9.5 km northwest of Ladysmith, Rusk County, Wisconsin (Fig. 1). The relationship of organic matter to the concentration of metals in soils, like that of the hydrous oxides of iron and manganese is n o t y e t fully understood. Certain metals are known to become concentrated in the organic layers of soils; and bogs, swamps and lake b o t t o m s may also exhibit high metal values. The degree of metal enrichment may vary according to pH and Eh, the type(s) of vegetation in which the concentration is taking place, and the metal involved. Schnitzer and Khan (1972) have reviewed the role of humic
362
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Fig. 1. L o c a t i o n a n d c o n f i g u r a t i o n o f t h e sample plot, 9.5 k m n o r t h w e s t o f L a d y s m i t h , R u s k C o u n t y , Wisconsin.
substances in the migration and accumulation of elements in soils as well as other environments, including reactions between humic substances and the hydrous oxides of iron and manganese and humic substances and clay minerals. Jenne (1968) stressed the importance of the hydrous iron and manganese oxides over organic matter in concentrating Co, Ni, Zn and Cu, suggesting however that metals fixed in the soil by the hydrous oxides may initially have been extracted from it by plant roots. The interaction that may take place between metals and humic matter in soils has been discussed by numerous other workers, among them Himes and Barber (1957), Schnitzer and Skinner (1965, 1966), Khan (1969), and Saxby (1969). The relationship of humic substances to geochemical anomalies in soils is discussed by Barakso (1969) and Chowdhury and Bose (1971). Discussions with reference to podzolic soils include Presant (1966, 1971), Horsnail and Elliott (1971) and Baker (1973). In the following sections an attempt is made to deal empirically with the potential effects of small amounts (less than 5% by weight) of organic matter
363
on the concentration of Cu in a grey-brown podzolic soil. It was felt that sampling situations in which there are small b u t noticeable changes in the amount of organic matter in a soil produce a degree of uncertainty in the interpretation of results, and the present study attempts to lend some understanding to at least that one aspect of the relationship between metal enrichment and organic content. COPPER IN THE SOIL
The sample grid (Fig. 1) covers an area 250 m by 425 m, with holes offset on 30-m centers. The grid lies in an open area consisting at the present time of brome grass, quack grass and canary grass, with minor timothy, alfalfa and clover. The field is surrounded by mixed deciduous and coniferous forest, and may represent land that has been cleared for cultivation. Thus, the soil may have been influenced by both forest and grassland vegetation, although it is n o w grassland. The soil, shown schematically in Fig. 2, is probably what is called a "mildly podzolic soil" by Buckman and Brady (1960, p. 305). Evidences of recent cultivation and fertilization were n o t noted, and the soil is free of calcium carbonate. The soil is developed on glacial drift, and presently consists of a 2- to 5-cm thick layer of organic debris, grading downward into a 15- to 20-cm thick layer of mixed organic and mineral material that in turn grades downward into tan to light-brown material containing an occasional plant rootlet. In this type of soil, organic matter has become mixed with mineral material, resulting in a thin surface organic layer and an underlying layer that is brown in color and n o t grey as in true podzols. The brown color indicates a lack of the severe leaching that characterizes podzols. Data were collected at the points designated " a " and " b " in Fig. 2, at each of the 130 sample locations shown in Fig. 1. Samples " a " were taken near the b o t t o m of the zone of obvious organic enrichment, and those thickness loose organic debris, black in color
15-35cm
--sample
"a"
--sample
"b"
mixed o r g a n i c m a t t e r and mineral material, g r e y to b l a c k in c o l o r
mineral material with occasional plant r o o t l e t s , tan to liQht brown in color
Fig. 2. Generalized soil profile and location of samples within individual sample holes.
364 designated " b " were selected from the tan to light-brown sediment below the organic zone. Sample depth ranged to about 60 cm below the surface. Cu content in any soil depends on its abundance in the soil parent material, the degree to which Cu from any nearby mineralization reaches the soil profile, and factors that influence its migration once it has reached the profile. Cu from any source is " f i x e d " in the soil largely by adsorption onto hydrous iron and manganese oxides, and onto clay minerals and organic matter. The concentration of Cu in soil samples therefore may depend on the grain size of the material, the type and amount of organic matter in the soil, and the degree to which the hydrous oxides of iron and manganese have been retained within the soil horizons sampled. The pH of the soil is important because it may affect the cation exchange capacity of clay minerals; and along with Eh, pH may also influence the movement of the hydrous oxides of iron and manganese within the soil profile. Subsurface drainage and soil organic content in turn influence pH and Eh values in the soil. Some minor variation in the grain size of samples was noted within the sample grid. However, within individual sample holes the size fractions were quite homogeneous, and it is hoped that this fact minimizes the potential influence that grain size differences, particularly clay content, may have on the content of Cu. Because the stabilities of the hydrous oxides of iron and manganese are in part pH-Eh-dependent, it was felt that some measure of their behavior, and thus the behavior of Cu adsorbed onto them, might be obtained through study of the pH and Eh of the soil at the two sample levels within the individual sample holes. To this end, 20 representative sample holes were chosen for pH and Eh determinations. The samples were prepared according to the procedure described by Bear (1964, p. 500); and then were analyzed using a pH meter equipped with standard reference and inert platinum electrodes. Final Eh values were determined by adding the Eh of the reference electrode (0.241 volt at 25°C) to the readings obtained. Virtually no difference exists in the soil acidity for the " a " and " b " sample levels, averaging respectively 5.64 and 5.65. The presence of organic matter in the shallower samples, "a", has produced a somewhat lowered average oxidation potential (0.558 volt) than for samples from the " b " level (0.576 volt). These data suggest that differences in the content of the hydrous oxides of iron and manganese may not have been great between the " a " and " b " levels at the time the samples were collected. Thus, a sampling situation was established by which to monitor the potential influence of organic content on the concentration of Cu within the soil profile. The following section describes the derivation of empirical populations of weight percent organic matter and Cu concentration that are combined to detect significant organic influence~ on copper concentration within individual sample holes. By using these populations it is possible to control the homogeneity of data throughout a sample plot, and to detect true Cu anomalies where they may exist. False Cu anomalies related to differences
365
in clay content or in the content of the hydrous iron and manganese oxides within individual sample holes are also detectable. LABORATORY STUDY
A portion of each of the bulk samples was digested in a heated mixture of concentrated perchloric acid and nitric acid. This was done to eliminate potential organic interferences while at the same time liberating the adsorbed Cu. The residue was evaporated to dryness, leached with dilute hydrochloric acid, and analyzed by atomic absorption spectroscopy. A portion of each of the samples was digested in concentrated hydrogen peroxide and evaporated to dryness. The loss in dry weight gave a measure of the percent organic matter in the samples. In this manner, Cu and organic contents were determined for both samples from each of the sample holes. Cu values range from 1 to 30 ppm. The amount of organic matter in all cases was less than 5% by weight. The analytical data for the "a" and "b" levels of the sample locations shown in Fig. 1 have been plotted in Fig. 3. The data suggest that Cu content in general is independent of organic content. Knowing this, it was felt that the data could be manipulated in such a way as to identify those few Cu concentrations that may n o t be independent of organic content. To accomplish this, the data from the "a" and "b" levels within individual sample holes have been combined and transformed according to the following expressions:
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Fig. 3. Plot of Cu concentration versus organic content, demonstrating the lack of dependence of Cu enrichment on content of organic matter in either sample level "a" (Fig. 3A) or sample level "b" (Fig. 3B).
366 Omax Cumax Cum-~n where:
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maximum weight percent organic content for either the "a" or " b " sample levels minimum weight percent organic content for either the "a" or " b " sample levels maximum Cu content (ppm) for either the " a " or " b " sample levels minimum Cu content (ppm) for either the "a" or " b " sample levels Cu content (ppm) of the sample level with the maximum weight percent organic content Cu content (ppm) of the sample level with the minimum weight percent organic content.
The data transformations are intended to maximize differences within sample holes, while ignoring differences between sample holes. The first of the two expressions describes the degree of difference in organic content between sample levels, multiplied by the absolute differences between the levels. The expression minimizes the importance of small differences between levels regardless of the magnitude of the values, and maximizes the potentially more significant larger differences. The transformation results only in positive values. The second expression gives a measure of the degree of Cu enrichment between sample levels, and relates this enrichment, in either a positive or negative sense, to the content of organic matter in the samples. The greatest interest lies in those sample holes that demonstrate a positive relationship between organic matter and Cu enrichment. The transformed data from the sample locations shown in Fig. 1 are plotted as histograms in Fig. 4. The distribution of transformed organic values forms an inverse exponential distribution, and the transformed Cu concentrations conform to a normal distribution centered about a mean o f - - 1 . 3 3 . The shift of the mean from a theoretical mean of 0.0 suggests that the samples " b " may contain a slightly higher content of clay minerals or the hydrous oxides of iron and manganese (and thus of Cu) than samples "a". The general curves for the two distributions have been standardized to c o m m o n scales and are presented in Fig. 5. The critical regions (shaded) for the two populations are also shown in this figure. Data points falling within these regions may represent significant differences in the organic or Cu contents between samples " a " and " b " within individual sample holes. Any degree of confidence may be chosen according to the sampling program
367
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and the confidence with which summary statements are to be made. A 95% confidence level was chosen for purposes of demonstrating the procedure. This is convenient because in the case of the population of transformed Cu values it represents the area under a normal curve between plus and minus two standard deviations from the mean. Because the population of transformed organic contents does n o t approximate a normal distribution, a geometric manipulation was required to derive the 5% of the curve that demarks the critical region for that population. FIELDS OF INTEREST Fig. 6 delineates several fields of interest according to the populations and critical regions outlined in Fig. 5. By combining the two distributions, it is possible to identify sample holes in which significantly large differences in the content of organic matter in samples " a " and " b " may have resulted in an abnormally high Cu content in the organic-rich sample. Actually, the fields as shown in Fig. 6 may be used n o t only to m o n i t o r the potential influence of organic matter on Cu concentration, b u t also to some degree the combined influences of adsorption onto clay minerals and the hydrous oxides of iron
368
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d i s t r i b u t i o n s s h o w n in Fig. t h a t t h e abscissa as seen in population of transformed are s h o w n at t h e 95% level
4. T h e p o p u l a t i o n for t r a n s f o r m e d Fig. 4 c o r r e s p o n d s in p o s i t i o n a n d C u c o n t e n t s . T h e critical regions of confidence.
and manganese. If the sole interest of the test is to monitor organic influences, the population of transformed Cu contents can be converted to a positive onetailed distribution, thus increasing the precision of the test. Field I outlines the region for which a significant organic enrichment in either of samples " a " or " b " within an individual sample hole is suspected, but for which there is no corresponding Cu enrichment. While data points falling within this region seemingly do n o t represent significant Cu enrichment, caution in their interpretation is probably advisable because of the significant organic enrichment. Data points falling within field H represent a situation in which, for a given sample hole, neither " a " nor " b " is significantly enriched in either organic matter or Cu. This is perhaps the field of greatest interest to the exploration geologist, because data points falling within this field represent a homogeneity in the influences of the various factors that may affect Cu enrichment. Data points from across the sample plot may thus be compared with some assurance that (if present) anomalous Cu values are due to an outside influence, and n o t to factors within the sample holes themselves. Fields III and III' are of interest to the exploration geologist because data points falling within these regions represent significant Cu enrichment in either the " a " or " b " samples within individual sample holes. However, there is no evidence of a corresponding enrichment in organic matter, and thus the differential metal enrichment in either the " a " or " b " samples may be due to significant differences in the content of the hydrous iron and manganese oxides, or in the clay content, or both.
369
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Fig. 6. Fields of interest to the exploration geologist, outlined according to the populations and critical regions shown in Fig. 5.
Field I V encompasses data points that represent the extreme of the case described for fields III and III'. In this situation significant organic enrichment is notable, yet Cu is significantly enriched in the sample level with the lowest organic content. Thus, the influence of some factor or factors other than organic content is quite strong within the sample hole. Field IV' is also of interest to the exploration geologist. Like field IV, the region of field IV' is outlined by the critical regions of the populations of transformed organic and Cu contents. However, field IV' represents the situation in which significant Cu enrichment in either of samples "a" or "b" may be due directly to significant organic enrichment in the sample bearing the maximum Cu enrichment. COMMENTS
The sampling technique described in the foregoing paragraphs involves the collection of two samples of varying organic content from each sample hole. In practice, however, the two-sample procedure is probably necessary only in situations where an abnormal organic influence is suspected, or when an occasional check on potential organic contributions is desirable. In fact, the results of the present study suggest that, in similar sampling situations, organic contents of up to 5% dry weight may not greatly influence Cu concentrations within the soil profile. The results of the study are strictly applicable only to sampling situations where the parent material and the geologic and vegetative processes are the
370
same as those of the study area. However, the results can probably be applied with caution to similar though not directly analogous sampling situations. Samples from organic-rich lakes, swamps, and bogs, as well as from soils, provide special problems in the interpretation of metal values. Whereas the intent of the study was to provide information that might aid in the interpretation of samples in which there is a small but noticeable component of organic matter, it may be possible by analogy to monitor variations in Cu content over an area in which the samples contain appreciable organic matter, as long as the organic influences are homogeneous for all sample locations. Additional work of the type described in the preceding paragraphs should be considered for Cu, Zn and related metals in other soil types, as well as in organic-rich environments such as lakes, swamps and bogs. Also, studies of the influences of clay content and the content of hydrous iron and manganese oxides upon metal concentrations might yield valuable information. ACKNOWLEDGMENTS
Permission to collect samples on land controlled by Bear Creek Mining Company is gratefully acknowledged. The help of Mr. Edwarde R. May in locating the sample grid and in interpreting some aspects of the underlying geology is very much appreciated. Mr. Byron Pride helped to characterize the flora in and around the sample grid, and his help is also appreciated. The manuscript was critically read by Mr. Edward J. Mahaffey of Kennecott Exploration, Inc., and by Professors R.L. Bates and G. Faure of the Department of Geology and Mineralogy of the Ohio State University, and their comments and suggestions for improvement deserve thanks.
REFERENCES Baker, W.E., 1973. The role of humic acids from Tasmanian podzolic soils in mineral degradation and metal mobilization. Geochim. Cosmoehim. Acta, 37: 269--281. Barasko, J.J., 1969. Soil and plant relationships to bedrock at some mineralized areas in British Columbia. Q. Colo. School Mines, 64: 507=-508. Bear, F.E., 1964. Chemistry of the Soil. Am. Chem. Soc., Monogr. Ser., 160, Reinhold, N e w York, N.Y. Buckman, H.O. and Brady, N.C., 1960. The Nature and Properties of Soils.Macmillan, N e w York, N.Y. Chowdhury, A.N. and Bose, B.B., 1971. Role of "humus matter" in the formation of geochemical anomalies. Can. Inst. Min. Metall., Spec. Vol., 11: 410--413. Himes, F.L. and Barber, S.A., 1957. Chelating ability of soil organic matter. Proc. Soil Sci. Soc. Am., 21: 368--373. Horsnall, R.F. and Elliott, I.L., 1971. Some environmental influences on the secondary dispersion of molybdenum and copper in western Canada. Can. Inst. Min. Metall., Spec. Vol., 11: 166--175. Jenne, E.A., 1968. Controls on Mn, Fe, Co, Ni, Cu and Zn concentrations in soils and water: the significant role of hydrous Mn and Fe oxides. Am. Chem. Soc., Adv. Chem. Ser., 73: 337--387.
371
Khan, S.U., 1969. Interactions between the humic acid fraction of soils and certain metallic cations. Proc. Soil Sci. Soc. Am., 33: 851--854. Presant, E.W., 1966. A trace element study of podzol soils, Bathurst district, New Brunswick. Geol. Surv. Can. Paper, 66-54: 222--232. Presant, E.W., 1971. Geochemistry of iron, manganese, lead, copper, zinc, arsenic, antimony, silver, tin, and cadmium in the soils of the Bathurst area, New Brunswick. Geol. Surv. Can. Bull., 1 7 4 : 9 3 pp. Saxby, J.D., 1969. Metal-organic chemistry of the geochemical cycle. Rev. Pure Apph Chem., 19: 131--150. Schnitzer, M. and Khan, S.U., 1972. Humic Substances in the Environment. Marcel Dekker, New York, N.Y. Schnitzer, M. and Skinner, S.I.M., 1965. Organo-metallic interactions in soils, 4. Carboxyl and hydroxyl groups in organic matter and metal retention. Soil Sci., 99: 278--284. Schnitzer, M. and Skinner, S.I.M., 1966. Organo-metallic interactions in soils, 5. Stability constants of Cu 2 ÷-, Fe 2 +-, and Zn 2 +-fulvic acid complexes. Soil Sci., 102: 361--365.
361
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
TWO-SAMPLE TECHNIQUE FOR DETERMINING THE INFLUENCE OF SOIL ORGANIC MATTER ON COPPER CONCENTRATION
DOUGLAS E. PRIDE
Department of Geology and Mineralogy, Ohio State University, Columbus, Ohio 43210 (U.S.A.) (Received October 6, 1976; revised version accepted February 23, 1977)
ABSTRACT Pride, D.E., 1977. Two-sample technique for determining the influence of soil organic matter on copper concentration. J. Geochem. Explor., 7 : 361--371. A sample grid was established 9.5 km northwest of Ladysmith, Wisconsin, to provide information on the potential significance of organic matter on the concentration of Cu in a grey-brown podzolic soil. Two samples were taken from each of 130 sample locations within the grid. One sample was collected from near the b o t t o m of the zone of obvious organic enrichment in the soil, and the other was taken a short distance below the organic zone. Data from individual sample holes were transformed to maximize the potential influence of organic matter on Cu enrichment. By combining the data from the entire grid, it was possible to derive empirical populations representing the variability of weight percent organic matter and Cu concentration within individual sample holes. By further combining these populations, graphically, it is possible to delineate regions of interest that may be used to identify organic influences on Cu concentration within sample holes. The study suggests that in similar sampling situations, organic contents up to 5% dry weight do not greatly influence Cu concentrations within the soil profile. T o some degree, the combined influence of differences in clay content and content of hydrous iron and manganese oxides between sample levels within individual holes can also be demonstrated.
INTRODUCTION
The present work was undertaken to provide information on the potential significance of small percentages of organic matter on the concentration of Cu within a soil sample plot. The study area is located a b o u t 9.5 km northwest of Ladysmith, Rusk County, Wisconsin (Fig. 1). The relationship of organic matter to the concentration of metals in soils, like that of the hydrous oxides of iron and manganese is n o t y e t fully understood. Certain metals are known to become concentrated in the organic layers of soils; and bogs, swamps and lake b o t t o m s may also exhibit high metal values. The degree of metal enrichment may vary according to pH and Eh, the type(s) of vegetation in which the concentration is taking place, and the metal involved. Schnitzer and Khan (1972) have reviewed the role of humic
362
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Fig. 1. L o c a t i o n a n d c o n f i g u r a t i o n o f t h e sample plot, 9.5 k m n o r t h w e s t o f L a d y s m i t h , R u s k C o u n t y , Wisconsin.
substances in the migration and accumulation of elements in soils as well as other environments, including reactions between humic substances and the hydrous oxides of iron and manganese and humic substances and clay minerals. Jenne (1968) stressed the importance of the hydrous iron and manganese oxides over organic matter in concentrating Co, Ni, Zn and Cu, suggesting however that metals fixed in the soil by the hydrous oxides may initially have been extracted from it by plant roots. The interaction that may take place between metals and humic matter in soils has been discussed by numerous other workers, among them Himes and Barber (1957), Schnitzer and Skinner (1965, 1966), Khan (1969), and Saxby (1969). The relationship of humic substances to geochemical anomalies in soils is discussed by Barakso (1969) and Chowdhury and Bose (1971). Discussions with reference to podzolic soils include Presant (1966, 1971), Horsnail and Elliott (1971) and Baker (1973). In the following sections an attempt is made to deal empirically with the potential effects of small amounts (less than 5% by weight) of organic matter
363
on the concentration of Cu in a grey-brown podzolic soil. It was felt that sampling situations in which there are small b u t noticeable changes in the amount of organic matter in a soil produce a degree of uncertainty in the interpretation of results, and the present study attempts to lend some understanding to at least that one aspect of the relationship between metal enrichment and organic content. COPPER IN THE SOIL
The sample grid (Fig. 1) covers an area 250 m by 425 m, with holes offset on 30-m centers. The grid lies in an open area consisting at the present time of brome grass, quack grass and canary grass, with minor timothy, alfalfa and clover. The field is surrounded by mixed deciduous and coniferous forest, and may represent land that has been cleared for cultivation. Thus, the soil may have been influenced by both forest and grassland vegetation, although it is n o w grassland. The soil, shown schematically in Fig. 2, is probably what is called a "mildly podzolic soil" by Buckman and Brady (1960, p. 305). Evidences of recent cultivation and fertilization were n o t noted, and the soil is free of calcium carbonate. The soil is developed on glacial drift, and presently consists of a 2- to 5-cm thick layer of organic debris, grading downward into a 15- to 20-cm thick layer of mixed organic and mineral material that in turn grades downward into tan to light-brown material containing an occasional plant rootlet. In this type of soil, organic matter has become mixed with mineral material, resulting in a thin surface organic layer and an underlying layer that is brown in color and n o t grey as in true podzols. The brown color indicates a lack of the severe leaching that characterizes podzols. Data were collected at the points designated " a " and " b " in Fig. 2, at each of the 130 sample locations shown in Fig. 1. Samples " a " were taken near the b o t t o m of the zone of obvious organic enrichment, and those thickness loose organic debris, black in color
15-35cm
--sample
"a"
--sample
"b"
mixed o r g a n i c m a t t e r and mineral material, g r e y to b l a c k in c o l o r
mineral material with occasional plant r o o t l e t s , tan to liQht brown in color
Fig. 2. Generalized soil profile and location of samples within individual sample holes.
364 designated " b " were selected from the tan to light-brown sediment below the organic zone. Sample depth ranged to about 60 cm below the surface. Cu content in any soil depends on its abundance in the soil parent material, the degree to which Cu from any nearby mineralization reaches the soil profile, and factors that influence its migration once it has reached the profile. Cu from any source is " f i x e d " in the soil largely by adsorption onto hydrous iron and manganese oxides, and onto clay minerals and organic matter. The concentration of Cu in soil samples therefore may depend on the grain size of the material, the type and amount of organic matter in the soil, and the degree to which the hydrous oxides of iron and manganese have been retained within the soil horizons sampled. The pH of the soil is important because it may affect the cation exchange capacity of clay minerals; and along with Eh, pH may also influence the movement of the hydrous oxides of iron and manganese within the soil profile. Subsurface drainage and soil organic content in turn influence pH and Eh values in the soil. Some minor variation in the grain size of samples was noted within the sample grid. However, within individual sample holes the size fractions were quite homogeneous, and it is hoped that this fact minimizes the potential influence that grain size differences, particularly clay content, may have on the content of Cu. Because the stabilities of the hydrous oxides of iron and manganese are in part pH-Eh-dependent, it was felt that some measure of their behavior, and thus the behavior of Cu adsorbed onto them, might be obtained through study of the pH and Eh of the soil at the two sample levels within the individual sample holes. To this end, 20 representative sample holes were chosen for pH and Eh determinations. The samples were prepared according to the procedure described by Bear (1964, p. 500); and then were analyzed using a pH meter equipped with standard reference and inert platinum electrodes. Final Eh values were determined by adding the Eh of the reference electrode (0.241 volt at 25°C) to the readings obtained. Virtually no difference exists in the soil acidity for the " a " and " b " sample levels, averaging respectively 5.64 and 5.65. The presence of organic matter in the shallower samples, "a", has produced a somewhat lowered average oxidation potential (0.558 volt) than for samples from the " b " level (0.576 volt). These data suggest that differences in the content of the hydrous oxides of iron and manganese may not have been great between the " a " and " b " levels at the time the samples were collected. Thus, a sampling situation was established by which to monitor the potential influence of organic content on the concentration of Cu within the soil profile. The following section describes the derivation of empirical populations of weight percent organic matter and Cu concentration that are combined to detect significant organic influence~ on copper concentration within individual sample holes. By using these populations it is possible to control the homogeneity of data throughout a sample plot, and to detect true Cu anomalies where they may exist. False Cu anomalies related to differences
365
in clay content or in the content of the hydrous iron and manganese oxides within individual sample holes are also detectable. LABORATORY STUDY
A portion of each of the bulk samples was digested in a heated mixture of concentrated perchloric acid and nitric acid. This was done to eliminate potential organic interferences while at the same time liberating the adsorbed Cu. The residue was evaporated to dryness, leached with dilute hydrochloric acid, and analyzed by atomic absorption spectroscopy. A portion of each of the samples was digested in concentrated hydrogen peroxide and evaporated to dryness. The loss in dry weight gave a measure of the percent organic matter in the samples. In this manner, Cu and organic contents were determined for both samples from each of the sample holes. Cu values range from 1 to 30 ppm. The amount of organic matter in all cases was less than 5% by weight. The analytical data for the "a" and "b" levels of the sample locations shown in Fig. 1 have been plotted in Fig. 3. The data suggest that Cu content in general is independent of organic content. Knowing this, it was felt that the data could be manipulated in such a way as to identify those few Cu concentrations that may n o t be independent of organic content. To accomplish this, the data from the "a" and "b" levels within individual sample holes have been combined and transformed according to the following expressions:
30
30
E c~
E ?0
g
20
• oo• oe
u
g
o 10
o
0;
CL O
c o u
o e•
I
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o
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•
o • 0o ~ 0
•
g
-. 0
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2 . 0t wt.
"
" -..'t. % Organic
ooo ooo-
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I
41.0
0
• •
e,..,%"
-
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;
Z.0 wt.
~
~
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I
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matter
;"
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Organic
"
"
I
I
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510
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B
Fig. 3. Plot of Cu concentration versus organic content, demonstrating the lack of dependence of Cu enrichment on content of organic matter in either sample level "a" (Fig. 3A) or sample level "b" (Fig. 3B).
366 Omax Cumax Cum-~n where:
Oma x -- O min!
I Cu (max) -- CU(min)i' i
Oma x
=
Omi n
=
Cuma x
=
Cumi n
=
CU(max )
=
Cu(min )
=
maximum weight percent organic content for either the "a" or " b " sample levels minimum weight percent organic content for either the "a" or " b " sample levels maximum Cu content (ppm) for either the " a " or " b " sample levels minimum Cu content (ppm) for either the "a" or " b " sample levels Cu content (ppm) of the sample level with the maximum weight percent organic content Cu content (ppm) of the sample level with the minimum weight percent organic content.
The data transformations are intended to maximize differences within sample holes, while ignoring differences between sample holes. The first of the two expressions describes the degree of difference in organic content between sample levels, multiplied by the absolute differences between the levels. The expression minimizes the importance of small differences between levels regardless of the magnitude of the values, and maximizes the potentially more significant larger differences. The transformation results only in positive values. The second expression gives a measure of the degree of Cu enrichment between sample levels, and relates this enrichment, in either a positive or negative sense, to the content of organic matter in the samples. The greatest interest lies in those sample holes that demonstrate a positive relationship between organic matter and Cu enrichment. The transformed data from the sample locations shown in Fig. 1 are plotted as histograms in Fig. 4. The distribution of transformed organic values forms an inverse exponential distribution, and the transformed Cu concentrations conform to a normal distribution centered about a mean o f - - 1 . 3 3 . The shift of the mean from a theoretical mean of 0.0 suggests that the samples " b " may contain a slightly higher content of clay minerals or the hydrous oxides of iron and manganese (and thus of Cu) than samples "a". The general curves for the two distributions have been standardized to c o m m o n scales and are presented in Fig. 5. The critical regions (shaded) for the two populations are also shown in this figure. Data points falling within these regions may represent significant differences in the organic or Cu contents between samples " a " and " b " within individual sample holes. Any degree of confidence may be chosen according to the sampling program
367
30'
u.
lO
0
l
2
3
4
5
6
7
8
g
lO
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4o
so
OMax[OMax-OMin] OMln 30"
20-
110-
-5o
-4o
-3o
-2o
-io
o
lo
20
cu M~x [Cu(M.~)-Cu(Min>] ~u Mln
Fig. 4. Frequency histograms for the transformed organic contents (Fig. 4A) and Cu concentrations (Fig. 4B) from the sample locations shown in Fig. 1.
and the confidence with which summary statements are to be made. A 95% confidence level was chosen for purposes of demonstrating the procedure. This is convenient because in the case of the population of transformed Cu values it represents the area under a normal curve between plus and minus two standard deviations from the mean. Because the population of transformed organic contents does n o t approximate a normal distribution, a geometric manipulation was required to derive the 5% of the curve that demarks the critical region for that population. FIELDS OF INTEREST Fig. 6 delineates several fields of interest according to the populations and critical regions outlined in Fig. 5. By combining the two distributions, it is possible to identify sample holes in which significantly large differences in the content of organic matter in samples " a " and " b " may have resulted in an abnormally high Cu content in the organic-rich sample. Actually, the fields as shown in Fig. 6 may be used n o t only to m o n i t o r the potential influence of organic matter on Cu concentration, b u t also to some degree the combined influences of adsorption onto clay minerals and the hydrous oxides of iron
368
iF
3o!
; !
-5'0
-4'0
-3'0
-2'0
-I0
10
20
3'0
4'0
5'0
CuMax [C.(Max)-C°(M~n)J CuMin Fig. 5. G e n e r a l curves for t h e organic c o n t e n t s is r o t a t e d so scale w i t h t h e o r d i n a t e o f t h e ( s h a d e d ) for t h e d i s t r i b u t i o n s
d i s t r i b u t i o n s s h o w n in Fig. t h a t t h e abscissa as seen in population of transformed are s h o w n at t h e 95% level
4. T h e p o p u l a t i o n for t r a n s f o r m e d Fig. 4 c o r r e s p o n d s in p o s i t i o n a n d C u c o n t e n t s . T h e critical regions of confidence.
and manganese. If the sole interest of the test is to monitor organic influences, the population of transformed Cu contents can be converted to a positive onetailed distribution, thus increasing the precision of the test. Field I outlines the region for which a significant organic enrichment in either of samples " a " or " b " within an individual sample hole is suspected, but for which there is no corresponding Cu enrichment. While data points falling within this region seemingly do n o t represent significant Cu enrichment, caution in their interpretation is probably advisable because of the significant organic enrichment. Data points falling within field H represent a situation in which, for a given sample hole, neither " a " nor " b " is significantly enriched in either organic matter or Cu. This is perhaps the field of greatest interest to the exploration geologist, because data points falling within this field represent a homogeneity in the influences of the various factors that may affect Cu enrichment. Data points from across the sample plot may thus be compared with some assurance that (if present) anomalous Cu values are due to an outside influence, and n o t to factors within the sample holes themselves. Fields III and III' are of interest to the exploration geologist because data points falling within these regions represent significant Cu enrichment in either the " a " or " b " samples within individual sample holes. However, there is no evidence of a corresponding enrichment in organic matter, and thus the differential metal enrichment in either the " a " or " b " samples may be due to significant differences in the content of the hydrous iron and manganese oxides, or in the clay content, or both.
369
4O ,< - 3O
~i.~
~E :E
20"
10"
V
-80
-60
-40
,
-2i0
i
--0
i
2i0
L
CuMax [C ] CuMin u(Max)-Cu(Min')
Fig. 6. Fields of interest to the exploration geologist, outlined according to the populations and critical regions shown in Fig. 5.
Field I V encompasses data points that represent the extreme of the case described for fields III and III'. In this situation significant organic enrichment is notable, yet Cu is significantly enriched in the sample level with the lowest organic content. Thus, the influence of some factor or factors other than organic content is quite strong within the sample hole. Field IV' is also of interest to the exploration geologist. Like field IV, the region of field IV' is outlined by the critical regions of the populations of transformed organic and Cu contents. However, field IV' represents the situation in which significant Cu enrichment in either of samples "a" or "b" may be due directly to significant organic enrichment in the sample bearing the maximum Cu enrichment. COMMENTS
The sampling technique described in the foregoing paragraphs involves the collection of two samples of varying organic content from each sample hole. In practice, however, the two-sample procedure is probably necessary only in situations where an abnormal organic influence is suspected, or when an occasional check on potential organic contributions is desirable. In fact, the results of the present study suggest that, in similar sampling situations, organic contents of up to 5% dry weight may not greatly influence Cu concentrations within the soil profile. The results of the study are strictly applicable only to sampling situations where the parent material and the geologic and vegetative processes are the
370
same as those of the study area. However, the results can probably be applied with caution to similar though not directly analogous sampling situations. Samples from organic-rich lakes, swamps, and bogs, as well as from soils, provide special problems in the interpretation of metal values. Whereas the intent of the study was to provide information that might aid in the interpretation of samples in which there is a small but noticeable component of organic matter, it may be possible by analogy to monitor variations in Cu content over an area in which the samples contain appreciable organic matter, as long as the organic influences are homogeneous for all sample locations. Additional work of the type described in the preceding paragraphs should be considered for Cu, Zn and related metals in other soil types, as well as in organic-rich environments such as lakes, swamps and bogs. Also, studies of the influences of clay content and the content of hydrous iron and manganese oxides upon metal concentrations might yield valuable information. ACKNOWLEDGMENTS
Permission to collect samples on land controlled by Bear Creek Mining Company is gratefully acknowledged. The help of Mr. Edwarde R. May in locating the sample grid and in interpreting some aspects of the underlying geology is very much appreciated. Mr. Byron Pride helped to characterize the flora in and around the sample grid, and his help is also appreciated. The manuscript was critically read by Mr. Edward J. Mahaffey of Kennecott Exploration, Inc., and by Professors R.L. Bates and G. Faure of the Department of Geology and Mineralogy of the Ohio State University, and their comments and suggestions for improvement deserve thanks.
REFERENCES Baker, W.E., 1973. The role of humic acids from Tasmanian podzolic soils in mineral degradation and metal mobilization. Geochim. Cosmoehim. Acta, 37: 269--281. Barasko, J.J., 1969. Soil and plant relationships to bedrock at some mineralized areas in British Columbia. Q. Colo. School Mines, 64: 507=-508. Bear, F.E., 1964. Chemistry of the Soil. Am. Chem. Soc., Monogr. Ser., 160, Reinhold, N e w York, N.Y. Buckman, H.O. and Brady, N.C., 1960. The Nature and Properties of Soils.Macmillan, N e w York, N.Y. Chowdhury, A.N. and Bose, B.B., 1971. Role of "humus matter" in the formation of geochemical anomalies. Can. Inst. Min. Metall., Spec. Vol., 11: 410--413. Himes, F.L. and Barber, S.A., 1957. Chelating ability of soil organic matter. Proc. Soil Sci. Soc. Am., 21: 368--373. Horsnall, R.F. and Elliott, I.L., 1971. Some environmental influences on the secondary dispersion of molybdenum and copper in western Canada. Can. Inst. Min. Metall., Spec. Vol., 11: 166--175. Jenne, E.A., 1968. Controls on Mn, Fe, Co, Ni, Cu and Zn concentrations in soils and water: the significant role of hydrous Mn and Fe oxides. Am. Chem. Soc., Adv. Chem. Ser., 73: 337--387.
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Khan, S.U., 1969. Interactions between the humic acid fraction of soils and certain metallic cations. Proc. Soil Sci. Soc. Am., 33: 851--854. Presant, E.W., 1966. A trace element study of podzol soils, Bathurst district, New Brunswick. Geol. Surv. Can. Paper, 66-54: 222--232. Presant, E.W., 1971. Geochemistry of iron, manganese, lead, copper, zinc, arsenic, antimony, silver, tin, and cadmium in the soils of the Bathurst area, New Brunswick. Geol. Surv. Can. Bull., 1 7 4 : 9 3 pp. Saxby, J.D., 1969. Metal-organic chemistry of the geochemical cycle. Rev. Pure Apph Chem., 19: 131--150. Schnitzer, M. and Khan, S.U., 1972. Humic Substances in the Environment. Marcel Dekker, New York, N.Y. Schnitzer, M. and Skinner, S.I.M., 1965. Organo-metallic interactions in soils, 4. Carboxyl and hydroxyl groups in organic matter and metal retention. Soil Sci., 99: 278--284. Schnitzer, M. and Skinner, S.I.M., 1966. Organo-metallic interactions in soils, 5. Stability constants of Cu 2 ÷-, Fe 2 +-, and Zn 2 +-fulvic acid complexes. Soil Sci., 102: 361--365.