Arch Environ Contam Toxicol (2014) 66:295–302 DOI 10.1007/s00244-013-9968-3
Horizontal and Vertical Distribution of Lead, Cadmium, and Zinc in Farmlands Around a Lead-Contaminated Goldmine in Zamfara, Northern Nigeria Ibrahim Mohammed • Nafiu Abdu
Received: 14 September 2013 / Accepted: 1 November 2013 / Published online: 29 November 2013 Ó Springer Science+Business Media New York 2013
Abstract This study was undertaken to evaluate the vertical and horizontal distribution of lead (Pb), cadmium (Cd), and zinc (Zn) in farmlands around a Pb-contaminated goldmine. Total concentrations of Pb and Cd recorded were at maximum values of 2,246.55 and 68.7 mg kg-1, respectively; these are greater than the threshold values for Pb (300 mg kg-1) and Cd (3 mg kg-1). However, the concentration of Zn was within acceptable limits (300 mg kg-1). Down the soil profile, concentrations of Pb, Cd, and Zn showed two peaks; these were attributed to facilitated transport and ground-water enrichment. Incubation of the soil samples with glucose indicated low microbial process(es), which could be due to the increased levels of Pb and Cd. Factor analysis showed a close association of Pb and Cd with the soil-exchange complex with a possibility of these heavy metals replacing Ca and other divalent cations in the soil-exchange site. This will, however, increase the risk of Pb and Cd leaching and uptake by plants. Although the metals were more associated with resistant soil fraction (sand), which also indicates their geogenic origin, chemical weathering under the influence of pH could release these metals into the soil-exchange site.
A major concern of the environment pertains to the fate of contaminants. Contaminants are derived geogenically or from anthropogenic activities and are either organic or
I. Mohammed (&) N. Abdu Department of Soil Science, Faculty of Agriculture, Ahmadu Bello University, P.M.B. 1044, Zaria, Nigeria e-mail: [email protected] N. Abdu e-mail: [email protected]; [email protected]
inorganic in nature. Metals are the major form of inorganic contaminants. Although the native concentration of metals in soil is not necessarily indicative of contamination (McLean and Bledsoe 1992), metals have been widely emitted into the environment through anthropogenic activities, such as agriculture, smelting, and mining operations (Abdu 2010; Ross 1994). The behavior of these metals in soil is governed largely by their sorption and desorption reactions with different soil constituents, especially clay components (Appel and Ma 2002). Chemical processes—such as cation release from contaminated materials, specific adsorption onto surfaces of minerals and soil organic matter, and precipitation of secondary minerals—dictate the extent of these reactions (Manceau et al. 2000). Reactions specific to heavy metals in soil is such that when metals are introduced into the environment, downward movement is limited unless the retention capacity of the soil is exceeded or the mobility of the metal is enhanced by its interaction with the associated soil matrix (McLean and Bledsoe 1992). This influences the distribution of metal within a soil profile and could also suggest the origin of the metal as well as indicate specific pedogenic processes taking place or that might have occurred over time. The relative mobility of a metal could also be indicated by the downward distribution of the metal in the soil profile (Abdu et al. 2011; Agbenin 2002). Illuviation of colloids was suggested to have enhanced heavy-metal mobility through facilitated transport due to the strong association of metals with colloids (McLean and Bledsoe 1992). Predicting the risk of contaminant transfer, as well as the metals’ mobility and leachability to contaminate groundwater, would therefore include a detailed observation of the distribution of the metal over the landscape as well as further down the profile because heavy metals can be associated with various soil components, such as organic matter, clays, Fe and Mn oxides, lattice of silicate, or carbonate
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minerals, all of which differ in their ability to retain or release the metals (Ure et al. 1993; Alloway 1995; Tack et al. 1996). The resultant pollution of expanse of land and water from artisanal mining of gold in Dareta, northern Nigeria, has placed many people at risk. Although Pb has been recorded as the major contaminant in these locations, Cd and Zn have also been implicated as contaminants from mining activities (Sparks 2005) and have been reported at increased concentrations in some Pb-contaminated farmlands of Zamfara, northern Nigeria (Abdu and Yusuf 2012). Similarly, mercury is commonly associated with gold mining, whereby the gold ores are extracted as gold amalgam (Newton 2010). This was not of interest in this study because mining was performed crudely by washing the ores directly in flowing water. Arsenic also was not considered because it is mostly a byproduct of mining and the purification of silver metal (Newton 2010). Therefore, this study was performed to assess the horizontal and vertical distribution of Pb, Cd, and Zn in the farmlands around a Pb-contaminated goldmine in Dareta, Zamfara state, northern Nigeria.
Materials and Methods Study Area The soils for this study were collected from the farmlands in Dareta village, Anka local government area (LGA) of Zamfara State, northern Nigeria. Anka is located at 12°060 3000 N 5°560 0000 E and has an area of 2,746 km2 and a population of 142,280 at the time of the 2006 census (National Population Commission 2000). The climate of Zamfara is a Sudan savannah with a mean annual rainfall of *579 mm. The soils are formed from basement complex rocks and are generally classified as ferruginous tropical lithosols according to Food and Agricultural Organization classification. Soil-Sampling Strategy Surface soil samples were collected from the farmlands at 0–20 cm depth. Farmlands on each cardinal point of the village were sampled. The living perimeter of the village was secured (Fig. 1), and sampling proceeded along the four cardinal points (north, south, east, and west), starting from the first farmland after the last house, on a grid at distances of 10, 30, 50 150, 300, 500, and 1,000 m. Composite samples were collected by random sampling procedure in two replicates. Profile pits, one for each cardinal point of the village, were also dug. Composite soil samples were also taken from each horizon of the soil profile in duplicate up to a maximum depth of 3 m. The sampling proceeded from the bottom horizon upward to maintain horizon boundary delineations and identity. The samples were collected into polythene bags
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and labeled appropriately. The samples were air dried, crushed, and passed through a 2-mm mesh sieve before analysis. Analytical Procedures Particle size distribution was determined by the hydrometer method (Gee and Or 2002). Cation exchange capacity was determined using ammonium acetate (NH4OAc) saturation method (Rhoades 1982). Soil pH was measured in 1:2.5 soil/ water suspension. Electrical conductivity was determined using the conductivity measuring bridge, type M.C.S. model EBB/10 manufactured by Electronic Instrument Ltd, London. Organic carbon was determined by the acid-dichromate method of Walkley and Black (Nelson and Sommers 1986). Total Pb, Cd, and Zn Air-dried and finely ground soil samples were digested with a 10:3:2 mixture of HNO3, HF, and HCl, respectively, at 80 °C. The digested soil sample was set aside to cool and then filtered through Whatman no. 42 filter paper. Total concentrations of Pb, Cd, and Zn were determined using atomic absorption spectrometer (AAS). The effect of soil redox condition on solubility of Pb, Cd, and Zn was evaluated. To this end, 10 g of soil were incubated under saturated water state with 1 and 3 % glucose and left to equilibrate for 7 days. A control without glucose was also made up. The pH of the suspension was measured at the end of incubation, and the soil redox condition was measured using the combined pH/millivolt metre. The rate of metal dissolution was measured from the changes in metal concentration in the soil solution. The suspension was filtered through Whatman no. 42 filter paper, and concentrations of Pb, Cd, and Zn in the filtrate were determined using AAS. Statistical Analysis All data were subjected to one way analysis of variance, and treatment means were compared using Fisher’s least significant difference (LSD0.05) test. Factor analysis was employed to ascertain the underlying coherence in the structure of the data set to deduce the factors governing the distribution of Pb, Cd, and Zn in the soil. All statistical analyses were performed using statistical analysis system (SAS) version 9.0 (SAS, 2002).
Results and Discussion Horizontal Distribution of Pb, Cd, and Zn Statistical comparison of heavy-metal concentration across sampling intervals showed statistical independence for
Arch Environ Contam Toxicol (2014) 66:295–302
297
Fig. 1 Field sampling layout
most of the samples, especially Pb and Zn. Few statistical similarity were observed were for Cd at Dareta East and Dareta West within the first 150 m of sampling. There was no uniform trend in the distribution of the metal, but high concentrations were observed within the first 150-m distance (from the starting point), although similar or even greater concentrations were observed much farther away (Fig. 2a–d). The greatest concentration of Pb was observed 30 m away from the villages at Dareta North and Dareta South, whereas a similar observation was made at 50 m away from the villages at Dareta East and Dareta West. The greatest concentration of Cd was recorded at 10 m away from the villages at Dareta North and Dareta East (Fig. 2b, c). However, the maximum concentration for Zn at Dareta West was observed 1,000 m away from the village (Fig. 2c). The wide heterogeneity observed for the samples is characteristic of environmental investigations, in which the parameters observed are highly influenced by unquantifiable factors, such as soil-forming factors, which influenced the distribution of the metals at the study location. This is evident from the factor analysis where metals were more associated with the resistant soil fraction (sand), an indication of the predominance of geogenic factors. Vertical Distribution of Pb, Cd, and Zn The concentration of Pb decreased from the surface (615– 334.75 mg kg-1) to the subsurface (572.1–167.55 mg kg-1)
horizons, except for Dareta West, where the concentrations within the first two horizons were similar (335.5 and 348.3 mg kg-1) (Fig. 3). At Dareta North, Dareta South, and Dareta East, Pb concentrations down the profile showed a double maximum; one maximum concentration occurring in the surface horizon (473.1, 342.75, and 615.9 mg kg-1, respectively) and the other occurring in the third horizon (998.1, 316.68, and 573 mg kg-1, respectively). The concentrations of Pb in the bottom horizon at Dareta West (481.95 mg kg-1) and Dareta North (471.15 mg kg-1) likewise showed an upward trend, although the values recorded were less than those of the first and third horizons (Fig. 3). The trend of a greater concentration of heavy metals in the top soil than the subsoil can be attributed to superficial enrichment through human activities, such as mining. It could also reflect metals affinity for organic matter (OM) (Agbenin 2002). Lead fixation by OM has been pointed out by various researchers (Li and Shuman 1996) to be more important than fixation by hydrous oxides; and the surface horizons of most soils contain greater OM relative to the successive lower horizons. The increased concentration of Pb observed in the bottom horizons at Dareta West and Dareta North relative to the overlaying horizons could be attributed to enrichment through pedogenic transformation of the parent material from which the soil was derived. It could also be as a result of a capillary increase of Pb polluted groundwater. The source of pollution of the groundwater is most likely from the washing of metal ores in streams and household wells.
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Fig. 2 a Mean concentrations of Pb, Cd, and Zn across the sampling distance in Dareta North. b Mean concentrations of Pb, Cd, and Zn across the sampling distance in Dareta South. c Mean concentrations
of Pb, Cd, and Zn across the sampling distance in Dareta East. d Mean concentrations of Pb, Cd, and Zn across the sampling distance in Dareta West
Where the water saturating this horizon is polluted, it could result in accumulation of metals at this zone relative to other zones; hence, contamination from a receding groundwater table is suspected. In the third horizon (77–103 cm), where the concentration of Pb was also observed to increase (Fig. 3), could imply facilitated transport of colloidal particles (Mclean and Bledsoe 1992), which resulted in significant Pb movement and redeposition within the profiles. These particles include Fe and Mn oxides, clay minerals, and organic matter. Their surfaces have a high capacity for metal sorption (Mclean and Bledsoe 1992), and the majority of metals partition onto particulate matter, such as clay fraction (Eggleton and Thomas 2004). Based on the fact that the third horizon contained more clay (data not shown), it could be suggested that Pb was translocated by way of facilitated transport with the colloidal clay particles during elluviation; hence, redeposition of clay was accompanied by deposition of Pb. Furthermore, the second horizon contained more sand compared with the underlying horizon. Abdu et al. (2012) showed that heavy-metal movement can be facilitated by sandy textures; likewise, Gschwend and Reynolds (1987) reported significant mobility of colloidal particles in a sandy medium. A similar trend for Pb was observed by Agbenin (2002), although this observation was
partly attributed to a possible discontinuity in the profiles (Agbenin 2002), a similar inference cannot be made of these soils because no sign of lithologic/profile discontinuity was observed. The trend for Cd down the profile (Fig. 4) was similar to that observed for Pb at the bottom horizon (Fig. 3). Similar attribution to groundwater enrichment, as made for Pb, can be made for this heightened concentration. Except at Dareta North, where there was a sharp decrease in Cd concentration between the first (12.25 mg kg-1) and fourth (1.95 mg kg-1) horizons, Cd concentration showed a relatively uniform distribution down the profile. The fairly uniform Cd distribution from depth of 30 cm downward suggests relative immobility of Cd in this soil. The pH range of the soils of this region (6.14–7.45) also supports the immobility assertion; this was suggested earlier by Abdu et al. (2011) in some garden soils in Kano that exhibited a pH range of 6.9–7.4. The distribution of Zn showed a major increase in Zn concentration only in the third horizon (68.25–197.7 mg kg-1) (Fig. 5), similar to what was observed for Pb and Cd. This could also be ascribed to Zn partitioning onto clay fractions (Eggleton and Thomas 2004) and subsequent translocation and illuviation (as indicated by the greater clay content at the horizon). However, because the total Zn concentration recorded at the
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299
Fig. 4 Vertical distribution of Cd down the profile in Dareta, Nigeria Fig. 3 Vertical distribution of Pb down the profile in Dareta, Nigeria -1
study location (66.45–330 mg kg ) did not indicate possible pollution from Zn, it could be the reason why increased Zn concentrations (as observed for Pb and Cd) were not observed in the last horizon. Factor analysis of the correlated Pb, Cd, and Zn and soil properties using Varimax rotation to maximize the sum of the variance of the factor coefficients (Gotelli and Ellison 2004) showed three factors with eigenvalues \1, which explained 77 % of the variation in the data set (Table 1). Factor 1 is dominated by clay and silt fractions of the soil, CEC, Cd, and Zn and accounted for 35.7 % of the total variance. This factor represents the soil colloidal fraction and the dominance of the soil-exchange complex by Cd and Zn. Given the similarities between these metals and Ca, a dominant cation on the exchange complex of the soils of the Nigerian savannah, Cd and Zn can easily replace Ca and hence increase the risk of plant accumulation of heavy metals and eventual food chain contamination. Factors 2 and 3 comprise the sand fraction, pH, Pb, and Cd with high loadings probably reflecting association of Pb and Cd with resistant soil matrix, which can be released under pH manipulation.
Effect of Redox Condition on Concentrations of Pb, Cd, and Zn At the end of the 7-day incubation period, the measured redox potential (Eh) for the soils were \300, which is the borderline potential signaling the onset of decrease due to the depletion of oxidants, such as O2 and NO3 (Ponnamperuma 1972; Bohn et al. 2001). The lowest potential for the samples was 98 mV with a corresponding pH value 5.37 (Table 2), whereas the highest redox potential measured for the samples was 149 mV with a corresponding pH value of 5.19. Although a decrease reaction is a protonconsuming process, only a slight relationship was observed between pH and redox potential because observations were not uniform (Fig. 6); however, all of the measured Eh values were within the range of Fe3?–Fe2? and O2–H2O redox couples (Bohn et al. 2001; McBride 1994). The extent of metal solubility as induced by glucose addition was measured from the changes in metal concentration in the soil solution. After the 7-day incubation period, the concentrations of heavy metals in solution differed slightly from each other and from the control. In most cases, the differences in metal concentrations were not significant
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Arch Environ Contam Toxicol (2014) 66:295–302 Table 2 pH and redox potential of soil samples after 7 days incubation under two glucose rates Dareta North
Dareta South
Dareta East
Dareta West
1–5.37
4.74
5.04
5.19
3–4.65
4.45
4.79
4.94
pH (%)
Redox potential (mV) (%) 1–98
127
107
149
3–136
148
126
119
Fig. 5 Vertical distribution of Zn down the profile in Dareta, Nigeria Fig. 6 Eh–pH relationships after 7 days of incubation at 1 and 3 % glucose levels Table 1 Rotated-component matrix for Pb, Cd, and Zn and soil properties in a contaminated soil of Dareta, Nigeria
Table 3 Mean concentrations (mg kg-1) of solubilized Pb, Cd, and Zn under different levels of glucose amendment
Parameter
Factor 1
Factor 2
Factor 3
Metal Pb (%)
Clay
0.525
–0.110
–0.215
Silt
–0.417
–0.230
–0.155
Sand
–0.084
0.441
0.394
pH
–0.035
0.420
–0.561
CEC
0.373
0.272
–0.065
OC
–0.197
0.338
0.301
Pb
–0.134
–0.480
0.301
Cd
0.388
0.076
0.522
Zn
0.441
–0.323
–0.039
Eigenvalue
3.212
2.29
1.45
% of variance
35.69
25.42
16.15
Cumulative %
35.69
61.11
77.26
Dareta North
Dareta South
Dareta East
Dareta West
1
0.09b
0.22a
0.20a
0.30a
3
a
a
a
0.45a
a
2.36
0.19
a
0.19
0.08
0.18
0.18
0.32a
1
0.03b
0.68a
0.13a
0.10a
3
a
a
a
0.09a
a
0
b
Cd (%) 0.76
0.61
a
0.17
0.01
0.58
0.14
0.12a
1
0.24b
1.76a
0.29b
0.27a
3 0
2.40a 0.32b
0.98b 1.11b
1.17a 0.25b
0.27a 0.24a
0
b
Zn (%)
Loading factors [3.5 are shown in bold
a,b Rank of means. Means followed by the same letter are statistically the same at 5 % probability
(Table 3). Although the addition of glucose was to provide energy required for microbial catalysis/decrease of the soil by biotic mechanism, the absence of significant differences in heavy metals solubilized between the two levels of amendments for most of the samples suggests inability of the soil microbial population to recover after air-drying of
the samples and long-term storage. Karolien et al. (2001), Xiang et al. (2008) and Fierer and Schiemel (2002) found that drying and rewetting impose significant stress on microbial populations. Drying and rewetting cycles showed that the longer the time of drying, the more difficult it was for the microbial population to re-establish; in most cases
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they do so only sparingly. This observation could also imply decreased microbial activity due to the high levels of the heavy metals in the samples. It is now widely accepted that the presence of above-threshold concentration of metals in the environment imposes an adverse effect on microbial populations. Under such conditions, only organisms that have developed special adaptive mechanism to occlude or sequester these contaminants have been found to exist.
Conclusion Total heavy-metal concentration as determined by mixedacid digestion (HCl ? HNO3 ? HF) yielded an extreme concentration for Pb (2,246.55 mg kg-1); likewise, the concentration of Cd was very high at *70 g of kg-1; and the concentration of Zn was within the acceptable limit even although an extreme concentration was recorded at one location (374 g kg-1). Down the soil profile, the concentration of metals was observed to decrease relative to the top horizon; this is due to the greater concentration of OM in the top horizon, and heavy metals have a high affinity for organic matter. Down the profile, the metals all showed a double peak, which could be due to facilitated transport by colloids, as suggested by the soil texture change down the profile as well as groundwater enrichment. No discernible trend was observed in metal concentrations with distance. Although high concentrations of Pb, Cd, and Zn were found within 150 m of the sampling points, even greater concentrations were recorded farther away. At the end of the incubation experiments at two rates of glucose application, concentration of metals in solution showed little significant difference in soluble concentrations of Pb, Cd, and Zn. Although this could imply inactivation of microbial catalysis due to air-drying of the samples and long-term storage, the few significant differences in soluble Pb, Cd, and Zn under different glucose treatment suggest that the addition of glucose exerted an indirect role in the increased rate of heavy-metal decrease by promoting the growth of microorganisms as well as an abiotic decrease of oxide components of the soil.
References Abdu N (2010) Availability, transfer and balances of heavy metals in urban agriculture of West Africa. Ph D. Dissertation, Faculty of Organic Agricultural Sciences, University of Kassel, Germany. Abdu N, Abdulkadir A, Agbenin JO, Buerkert A (2011) Vertical distribution of heavy metals in wastewater-irrigated vegetable garden soils of three West African cities. Nutr Cycl Agroecosyst 89:387–397
301 Abdu N, Agbenin JO, Buerkert A (2012) Fractionation and mobility of cadmium and zinc in urban vegetable gardens of Kano, Northern Nigeria. Environ Monit Assess 184:2057–2066 Abdu N, Yusuf AA (2012) Heavy metal contamination of soil and crop, and human health impacts in a contaminated village around gold mine in Zamfara state. In: Proceedings of the 46th annual conference of the Agricultural Society of Nigeria, Kano 2012, pp. 961–965 Agbenin JO (2002) Lead in Nigerian savanna soils under long-term cultivation. Sci Total Environ 286:1–14 Alloway BJ (1995) Heavy metals in soils. Blackie and Academic Professional, London, pp 11–17 Appel C, Ma L (2002) Concentration, pH, and surface charge effects on cadmium and lead sorption in three tropical soils. J Environ Qual 31:581–589 Bohn HL, McNeal BL, O’Conner GA (2001) Soil chemistry, 3rd edn. John Wiley and Sons, NY Eggleton J, Thomas KV (2004) A review of factors affecting the release and bioavailability of contaminants during sediment disturbance events. Environ Int 30:973–980 Fierer N, Schimel JP (2002) Effects of drying–rewetting frequency on soil carbon and nitrogen transformations. Soil Biol Biochem 34: 777–787 Gee GW, Or D (2002) Particle-size analysis. In: Dane JH, Topp GC (eds) Methods of soil analysis, part 4. SSSA Book Series. No. 5. Soil Science Society of America, Madison, WI, pp 255–293 Gotelli NJ, Ellison AM (2004) A primer of ecological statistics, 1st edn. Sinauer Associates, Sunderland, p 492 Gschwend PM, Reynolds MD (1987) Monodisperse ferrous phosphate colloids in an anoxic groundwater plume. J Contam Hydrol 1:309–327 Karolien D, Johan S, Heleen B, Serita DF, Edward TE, Roel M, Keith P (2001) Influence of dry-wet cycles on interrelationship between aggregate, particulate organic matter, and microbial community dynamics. Soil Biol Biochem 33:1599–1611 Li Z, Shuman LM (1996) Heavy metals movement in metal contaminated soil profiles. Soil Sci 161:656–666 Manceau A, Lanson B, Schlegel ML, Harge JC, Musso M, EybertBerard L et al (2000) Quantitative Zn speciation in smeltercontaminated soils by EXAFS spectroscopy. Am J Sci 300: 289–343 McBride MB (1994) Environmental Chemistry of Soils. Oxford University Press, NY McLean JE, Bledsoe BE (1992) Behavior of materials in soils. Groundwater Issue. EPA/540/S-92/018, October 1992 National Population Commission (2000) National population census. National Population Commission, Calverton Nelson DW, Sommers LM (1986) Total carbon, organic carbon and organic matter. In: Sparks DL (ed) Methods of soil analysis. Part 2. Chemical methods. American Society of Agronomy, Madison, pp 961–1010 Newton DE (2010) Chemical elements, 2nd edn. Gale Cengage Learning, Detroit Ponnamperuma FN (1972) The chemistry of submerged soils. Adv Agron 24:2996 Rhoades JD (1982) Cation exchange capacity. In: Page AL, Miller RH, Keeney DR (eds) Methods of soil analysis. Part 2. Chemical and microbiological properties. Soil Science Society of America, Madison, pp 167–179 Ross S (1994) Toxic metals in soil–plant systems. Wiley, Chichester Sparks DL (2005) Toxic metals in the environment: the role of surfaces. Elements 1:193–197 Statistical Analysis System (2002) Statistical Analysis System (SAS) version 9.0. Users guide. SAS, Cary
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302 Tack FM, Callewaert OWJJ, Verloo MG (1996) Metal solubility as a function of pH in a contaminated, dredged sediment affected by oxidation. Environ Pollut 91(2):199–208 Ure AM, Quevauviller P, Muntau H, Griepink B (1993) Speciation of heavy metal in soils and sediments. An account of the improvement and harmonization of extraction techniques undertaken
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Horizontal and Vertical Distribution of Lead, Cadmium, and Zinc in Farmlands Around a Lead-Contaminated Goldmine in Zamfara, Northern Nigeria Ibrahim Mohammed • Nafiu Abdu
Received: 14 September 2013 / Accepted: 1 November 2013 / Published online: 29 November 2013 Ó Springer Science+Business Media New York 2013
Abstract This study was undertaken to evaluate the vertical and horizontal distribution of lead (Pb), cadmium (Cd), and zinc (Zn) in farmlands around a Pb-contaminated goldmine. Total concentrations of Pb and Cd recorded were at maximum values of 2,246.55 and 68.7 mg kg-1, respectively; these are greater than the threshold values for Pb (300 mg kg-1) and Cd (3 mg kg-1). However, the concentration of Zn was within acceptable limits (300 mg kg-1). Down the soil profile, concentrations of Pb, Cd, and Zn showed two peaks; these were attributed to facilitated transport and ground-water enrichment. Incubation of the soil samples with glucose indicated low microbial process(es), which could be due to the increased levels of Pb and Cd. Factor analysis showed a close association of Pb and Cd with the soil-exchange complex with a possibility of these heavy metals replacing Ca and other divalent cations in the soil-exchange site. This will, however, increase the risk of Pb and Cd leaching and uptake by plants. Although the metals were more associated with resistant soil fraction (sand), which also indicates their geogenic origin, chemical weathering under the influence of pH could release these metals into the soil-exchange site.
A major concern of the environment pertains to the fate of contaminants. Contaminants are derived geogenically or from anthropogenic activities and are either organic or
I. Mohammed (&) N. Abdu Department of Soil Science, Faculty of Agriculture, Ahmadu Bello University, P.M.B. 1044, Zaria, Nigeria e-mail: [email protected] N. Abdu e-mail: [email protected]; [email protected]
inorganic in nature. Metals are the major form of inorganic contaminants. Although the native concentration of metals in soil is not necessarily indicative of contamination (McLean and Bledsoe 1992), metals have been widely emitted into the environment through anthropogenic activities, such as agriculture, smelting, and mining operations (Abdu 2010; Ross 1994). The behavior of these metals in soil is governed largely by their sorption and desorption reactions with different soil constituents, especially clay components (Appel and Ma 2002). Chemical processes—such as cation release from contaminated materials, specific adsorption onto surfaces of minerals and soil organic matter, and precipitation of secondary minerals—dictate the extent of these reactions (Manceau et al. 2000). Reactions specific to heavy metals in soil is such that when metals are introduced into the environment, downward movement is limited unless the retention capacity of the soil is exceeded or the mobility of the metal is enhanced by its interaction with the associated soil matrix (McLean and Bledsoe 1992). This influences the distribution of metal within a soil profile and could also suggest the origin of the metal as well as indicate specific pedogenic processes taking place or that might have occurred over time. The relative mobility of a metal could also be indicated by the downward distribution of the metal in the soil profile (Abdu et al. 2011; Agbenin 2002). Illuviation of colloids was suggested to have enhanced heavy-metal mobility through facilitated transport due to the strong association of metals with colloids (McLean and Bledsoe 1992). Predicting the risk of contaminant transfer, as well as the metals’ mobility and leachability to contaminate groundwater, would therefore include a detailed observation of the distribution of the metal over the landscape as well as further down the profile because heavy metals can be associated with various soil components, such as organic matter, clays, Fe and Mn oxides, lattice of silicate, or carbonate
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minerals, all of which differ in their ability to retain or release the metals (Ure et al. 1993; Alloway 1995; Tack et al. 1996). The resultant pollution of expanse of land and water from artisanal mining of gold in Dareta, northern Nigeria, has placed many people at risk. Although Pb has been recorded as the major contaminant in these locations, Cd and Zn have also been implicated as contaminants from mining activities (Sparks 2005) and have been reported at increased concentrations in some Pb-contaminated farmlands of Zamfara, northern Nigeria (Abdu and Yusuf 2012). Similarly, mercury is commonly associated with gold mining, whereby the gold ores are extracted as gold amalgam (Newton 2010). This was not of interest in this study because mining was performed crudely by washing the ores directly in flowing water. Arsenic also was not considered because it is mostly a byproduct of mining and the purification of silver metal (Newton 2010). Therefore, this study was performed to assess the horizontal and vertical distribution of Pb, Cd, and Zn in the farmlands around a Pb-contaminated goldmine in Dareta, Zamfara state, northern Nigeria.
Materials and Methods Study Area The soils for this study were collected from the farmlands in Dareta village, Anka local government area (LGA) of Zamfara State, northern Nigeria. Anka is located at 12°060 3000 N 5°560 0000 E and has an area of 2,746 km2 and a population of 142,280 at the time of the 2006 census (National Population Commission 2000). The climate of Zamfara is a Sudan savannah with a mean annual rainfall of *579 mm. The soils are formed from basement complex rocks and are generally classified as ferruginous tropical lithosols according to Food and Agricultural Organization classification. Soil-Sampling Strategy Surface soil samples were collected from the farmlands at 0–20 cm depth. Farmlands on each cardinal point of the village were sampled. The living perimeter of the village was secured (Fig. 1), and sampling proceeded along the four cardinal points (north, south, east, and west), starting from the first farmland after the last house, on a grid at distances of 10, 30, 50 150, 300, 500, and 1,000 m. Composite samples were collected by random sampling procedure in two replicates. Profile pits, one for each cardinal point of the village, were also dug. Composite soil samples were also taken from each horizon of the soil profile in duplicate up to a maximum depth of 3 m. The sampling proceeded from the bottom horizon upward to maintain horizon boundary delineations and identity. The samples were collected into polythene bags
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and labeled appropriately. The samples were air dried, crushed, and passed through a 2-mm mesh sieve before analysis. Analytical Procedures Particle size distribution was determined by the hydrometer method (Gee and Or 2002). Cation exchange capacity was determined using ammonium acetate (NH4OAc) saturation method (Rhoades 1982). Soil pH was measured in 1:2.5 soil/ water suspension. Electrical conductivity was determined using the conductivity measuring bridge, type M.C.S. model EBB/10 manufactured by Electronic Instrument Ltd, London. Organic carbon was determined by the acid-dichromate method of Walkley and Black (Nelson and Sommers 1986). Total Pb, Cd, and Zn Air-dried and finely ground soil samples were digested with a 10:3:2 mixture of HNO3, HF, and HCl, respectively, at 80 °C. The digested soil sample was set aside to cool and then filtered through Whatman no. 42 filter paper. Total concentrations of Pb, Cd, and Zn were determined using atomic absorption spectrometer (AAS). The effect of soil redox condition on solubility of Pb, Cd, and Zn was evaluated. To this end, 10 g of soil were incubated under saturated water state with 1 and 3 % glucose and left to equilibrate for 7 days. A control without glucose was also made up. The pH of the suspension was measured at the end of incubation, and the soil redox condition was measured using the combined pH/millivolt metre. The rate of metal dissolution was measured from the changes in metal concentration in the soil solution. The suspension was filtered through Whatman no. 42 filter paper, and concentrations of Pb, Cd, and Zn in the filtrate were determined using AAS. Statistical Analysis All data were subjected to one way analysis of variance, and treatment means were compared using Fisher’s least significant difference (LSD0.05) test. Factor analysis was employed to ascertain the underlying coherence in the structure of the data set to deduce the factors governing the distribution of Pb, Cd, and Zn in the soil. All statistical analyses were performed using statistical analysis system (SAS) version 9.0 (SAS, 2002).
Results and Discussion Horizontal Distribution of Pb, Cd, and Zn Statistical comparison of heavy-metal concentration across sampling intervals showed statistical independence for
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297
Fig. 1 Field sampling layout
most of the samples, especially Pb and Zn. Few statistical similarity were observed were for Cd at Dareta East and Dareta West within the first 150 m of sampling. There was no uniform trend in the distribution of the metal, but high concentrations were observed within the first 150-m distance (from the starting point), although similar or even greater concentrations were observed much farther away (Fig. 2a–d). The greatest concentration of Pb was observed 30 m away from the villages at Dareta North and Dareta South, whereas a similar observation was made at 50 m away from the villages at Dareta East and Dareta West. The greatest concentration of Cd was recorded at 10 m away from the villages at Dareta North and Dareta East (Fig. 2b, c). However, the maximum concentration for Zn at Dareta West was observed 1,000 m away from the village (Fig. 2c). The wide heterogeneity observed for the samples is characteristic of environmental investigations, in which the parameters observed are highly influenced by unquantifiable factors, such as soil-forming factors, which influenced the distribution of the metals at the study location. This is evident from the factor analysis where metals were more associated with the resistant soil fraction (sand), an indication of the predominance of geogenic factors. Vertical Distribution of Pb, Cd, and Zn The concentration of Pb decreased from the surface (615– 334.75 mg kg-1) to the subsurface (572.1–167.55 mg kg-1)
horizons, except for Dareta West, where the concentrations within the first two horizons were similar (335.5 and 348.3 mg kg-1) (Fig. 3). At Dareta North, Dareta South, and Dareta East, Pb concentrations down the profile showed a double maximum; one maximum concentration occurring in the surface horizon (473.1, 342.75, and 615.9 mg kg-1, respectively) and the other occurring in the third horizon (998.1, 316.68, and 573 mg kg-1, respectively). The concentrations of Pb in the bottom horizon at Dareta West (481.95 mg kg-1) and Dareta North (471.15 mg kg-1) likewise showed an upward trend, although the values recorded were less than those of the first and third horizons (Fig. 3). The trend of a greater concentration of heavy metals in the top soil than the subsoil can be attributed to superficial enrichment through human activities, such as mining. It could also reflect metals affinity for organic matter (OM) (Agbenin 2002). Lead fixation by OM has been pointed out by various researchers (Li and Shuman 1996) to be more important than fixation by hydrous oxides; and the surface horizons of most soils contain greater OM relative to the successive lower horizons. The increased concentration of Pb observed in the bottom horizons at Dareta West and Dareta North relative to the overlaying horizons could be attributed to enrichment through pedogenic transformation of the parent material from which the soil was derived. It could also be as a result of a capillary increase of Pb polluted groundwater. The source of pollution of the groundwater is most likely from the washing of metal ores in streams and household wells.
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Fig. 2 a Mean concentrations of Pb, Cd, and Zn across the sampling distance in Dareta North. b Mean concentrations of Pb, Cd, and Zn across the sampling distance in Dareta South. c Mean concentrations
of Pb, Cd, and Zn across the sampling distance in Dareta East. d Mean concentrations of Pb, Cd, and Zn across the sampling distance in Dareta West
Where the water saturating this horizon is polluted, it could result in accumulation of metals at this zone relative to other zones; hence, contamination from a receding groundwater table is suspected. In the third horizon (77–103 cm), where the concentration of Pb was also observed to increase (Fig. 3), could imply facilitated transport of colloidal particles (Mclean and Bledsoe 1992), which resulted in significant Pb movement and redeposition within the profiles. These particles include Fe and Mn oxides, clay minerals, and organic matter. Their surfaces have a high capacity for metal sorption (Mclean and Bledsoe 1992), and the majority of metals partition onto particulate matter, such as clay fraction (Eggleton and Thomas 2004). Based on the fact that the third horizon contained more clay (data not shown), it could be suggested that Pb was translocated by way of facilitated transport with the colloidal clay particles during elluviation; hence, redeposition of clay was accompanied by deposition of Pb. Furthermore, the second horizon contained more sand compared with the underlying horizon. Abdu et al. (2012) showed that heavy-metal movement can be facilitated by sandy textures; likewise, Gschwend and Reynolds (1987) reported significant mobility of colloidal particles in a sandy medium. A similar trend for Pb was observed by Agbenin (2002), although this observation was
partly attributed to a possible discontinuity in the profiles (Agbenin 2002), a similar inference cannot be made of these soils because no sign of lithologic/profile discontinuity was observed. The trend for Cd down the profile (Fig. 4) was similar to that observed for Pb at the bottom horizon (Fig. 3). Similar attribution to groundwater enrichment, as made for Pb, can be made for this heightened concentration. Except at Dareta North, where there was a sharp decrease in Cd concentration between the first (12.25 mg kg-1) and fourth (1.95 mg kg-1) horizons, Cd concentration showed a relatively uniform distribution down the profile. The fairly uniform Cd distribution from depth of 30 cm downward suggests relative immobility of Cd in this soil. The pH range of the soils of this region (6.14–7.45) also supports the immobility assertion; this was suggested earlier by Abdu et al. (2011) in some garden soils in Kano that exhibited a pH range of 6.9–7.4. The distribution of Zn showed a major increase in Zn concentration only in the third horizon (68.25–197.7 mg kg-1) (Fig. 5), similar to what was observed for Pb and Cd. This could also be ascribed to Zn partitioning onto clay fractions (Eggleton and Thomas 2004) and subsequent translocation and illuviation (as indicated by the greater clay content at the horizon). However, because the total Zn concentration recorded at the
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299
Fig. 4 Vertical distribution of Cd down the profile in Dareta, Nigeria Fig. 3 Vertical distribution of Pb down the profile in Dareta, Nigeria -1
study location (66.45–330 mg kg ) did not indicate possible pollution from Zn, it could be the reason why increased Zn concentrations (as observed for Pb and Cd) were not observed in the last horizon. Factor analysis of the correlated Pb, Cd, and Zn and soil properties using Varimax rotation to maximize the sum of the variance of the factor coefficients (Gotelli and Ellison 2004) showed three factors with eigenvalues \1, which explained 77 % of the variation in the data set (Table 1). Factor 1 is dominated by clay and silt fractions of the soil, CEC, Cd, and Zn and accounted for 35.7 % of the total variance. This factor represents the soil colloidal fraction and the dominance of the soil-exchange complex by Cd and Zn. Given the similarities between these metals and Ca, a dominant cation on the exchange complex of the soils of the Nigerian savannah, Cd and Zn can easily replace Ca and hence increase the risk of plant accumulation of heavy metals and eventual food chain contamination. Factors 2 and 3 comprise the sand fraction, pH, Pb, and Cd with high loadings probably reflecting association of Pb and Cd with resistant soil matrix, which can be released under pH manipulation.
Effect of Redox Condition on Concentrations of Pb, Cd, and Zn At the end of the 7-day incubation period, the measured redox potential (Eh) for the soils were \300, which is the borderline potential signaling the onset of decrease due to the depletion of oxidants, such as O2 and NO3 (Ponnamperuma 1972; Bohn et al. 2001). The lowest potential for the samples was 98 mV with a corresponding pH value 5.37 (Table 2), whereas the highest redox potential measured for the samples was 149 mV with a corresponding pH value of 5.19. Although a decrease reaction is a protonconsuming process, only a slight relationship was observed between pH and redox potential because observations were not uniform (Fig. 6); however, all of the measured Eh values were within the range of Fe3?–Fe2? and O2–H2O redox couples (Bohn et al. 2001; McBride 1994). The extent of metal solubility as induced by glucose addition was measured from the changes in metal concentration in the soil solution. After the 7-day incubation period, the concentrations of heavy metals in solution differed slightly from each other and from the control. In most cases, the differences in metal concentrations were not significant
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Arch Environ Contam Toxicol (2014) 66:295–302 Table 2 pH and redox potential of soil samples after 7 days incubation under two glucose rates Dareta North
Dareta South
Dareta East
Dareta West
1–5.37
4.74
5.04
5.19
3–4.65
4.45
4.79
4.94
pH (%)
Redox potential (mV) (%) 1–98
127
107
149
3–136
148
126
119
Fig. 5 Vertical distribution of Zn down the profile in Dareta, Nigeria Fig. 6 Eh–pH relationships after 7 days of incubation at 1 and 3 % glucose levels Table 1 Rotated-component matrix for Pb, Cd, and Zn and soil properties in a contaminated soil of Dareta, Nigeria
Table 3 Mean concentrations (mg kg-1) of solubilized Pb, Cd, and Zn under different levels of glucose amendment
Parameter
Factor 1
Factor 2
Factor 3
Metal Pb (%)
Clay
0.525
–0.110
–0.215
Silt
–0.417
–0.230
–0.155
Sand
–0.084
0.441
0.394
pH
–0.035
0.420
–0.561
CEC
0.373
0.272
–0.065
OC
–0.197
0.338
0.301
Pb
–0.134
–0.480
0.301
Cd
0.388
0.076
0.522
Zn
0.441
–0.323
–0.039
Eigenvalue
3.212
2.29
1.45
% of variance
35.69
25.42
16.15
Cumulative %
35.69
61.11
77.26
Dareta North
Dareta South
Dareta East
Dareta West
1
0.09b
0.22a
0.20a
0.30a
3
a
a
a
0.45a
a
2.36
0.19
a
0.19
0.08
0.18
0.18
0.32a
1
0.03b
0.68a
0.13a
0.10a
3
a
a
a
0.09a
a
0
b
Cd (%) 0.76
0.61
a
0.17
0.01
0.58
0.14
0.12a
1
0.24b
1.76a
0.29b
0.27a
3 0
2.40a 0.32b
0.98b 1.11b
1.17a 0.25b
0.27a 0.24a
0
b
Zn (%)
Loading factors [3.5 are shown in bold
a,b Rank of means. Means followed by the same letter are statistically the same at 5 % probability
(Table 3). Although the addition of glucose was to provide energy required for microbial catalysis/decrease of the soil by biotic mechanism, the absence of significant differences in heavy metals solubilized between the two levels of amendments for most of the samples suggests inability of the soil microbial population to recover after air-drying of
the samples and long-term storage. Karolien et al. (2001), Xiang et al. (2008) and Fierer and Schiemel (2002) found that drying and rewetting impose significant stress on microbial populations. Drying and rewetting cycles showed that the longer the time of drying, the more difficult it was for the microbial population to re-establish; in most cases
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they do so only sparingly. This observation could also imply decreased microbial activity due to the high levels of the heavy metals in the samples. It is now widely accepted that the presence of above-threshold concentration of metals in the environment imposes an adverse effect on microbial populations. Under such conditions, only organisms that have developed special adaptive mechanism to occlude or sequester these contaminants have been found to exist.
Conclusion Total heavy-metal concentration as determined by mixedacid digestion (HCl ? HNO3 ? HF) yielded an extreme concentration for Pb (2,246.55 mg kg-1); likewise, the concentration of Cd was very high at *70 g of kg-1; and the concentration of Zn was within the acceptable limit even although an extreme concentration was recorded at one location (374 g kg-1). Down the soil profile, the concentration of metals was observed to decrease relative to the top horizon; this is due to the greater concentration of OM in the top horizon, and heavy metals have a high affinity for organic matter. Down the profile, the metals all showed a double peak, which could be due to facilitated transport by colloids, as suggested by the soil texture change down the profile as well as groundwater enrichment. No discernible trend was observed in metal concentrations with distance. Although high concentrations of Pb, Cd, and Zn were found within 150 m of the sampling points, even greater concentrations were recorded farther away. At the end of the incubation experiments at two rates of glucose application, concentration of metals in solution showed little significant difference in soluble concentrations of Pb, Cd, and Zn. Although this could imply inactivation of microbial catalysis due to air-drying of the samples and long-term storage, the few significant differences in soluble Pb, Cd, and Zn under different glucose treatment suggest that the addition of glucose exerted an indirect role in the increased rate of heavy-metal decrease by promoting the growth of microorganisms as well as an abiotic decrease of oxide components of the soil.
References Abdu N (2010) Availability, transfer and balances of heavy metals in urban agriculture of West Africa. Ph D. Dissertation, Faculty of Organic Agricultural Sciences, University of Kassel, Germany. Abdu N, Abdulkadir A, Agbenin JO, Buerkert A (2011) Vertical distribution of heavy metals in wastewater-irrigated vegetable garden soils of three West African cities. Nutr Cycl Agroecosyst 89:387–397
301 Abdu N, Agbenin JO, Buerkert A (2012) Fractionation and mobility of cadmium and zinc in urban vegetable gardens of Kano, Northern Nigeria. Environ Monit Assess 184:2057–2066 Abdu N, Yusuf AA (2012) Heavy metal contamination of soil and crop, and human health impacts in a contaminated village around gold mine in Zamfara state. In: Proceedings of the 46th annual conference of the Agricultural Society of Nigeria, Kano 2012, pp. 961–965 Agbenin JO (2002) Lead in Nigerian savanna soils under long-term cultivation. Sci Total Environ 286:1–14 Alloway BJ (1995) Heavy metals in soils. Blackie and Academic Professional, London, pp 11–17 Appel C, Ma L (2002) Concentration, pH, and surface charge effects on cadmium and lead sorption in three tropical soils. J Environ Qual 31:581–589 Bohn HL, McNeal BL, O’Conner GA (2001) Soil chemistry, 3rd edn. John Wiley and Sons, NY Eggleton J, Thomas KV (2004) A review of factors affecting the release and bioavailability of contaminants during sediment disturbance events. Environ Int 30:973–980 Fierer N, Schimel JP (2002) Effects of drying–rewetting frequency on soil carbon and nitrogen transformations. Soil Biol Biochem 34: 777–787 Gee GW, Or D (2002) Particle-size analysis. In: Dane JH, Topp GC (eds) Methods of soil analysis, part 4. SSSA Book Series. No. 5. Soil Science Society of America, Madison, WI, pp 255–293 Gotelli NJ, Ellison AM (2004) A primer of ecological statistics, 1st edn. Sinauer Associates, Sunderland, p 492 Gschwend PM, Reynolds MD (1987) Monodisperse ferrous phosphate colloids in an anoxic groundwater plume. J Contam Hydrol 1:309–327 Karolien D, Johan S, Heleen B, Serita DF, Edward TE, Roel M, Keith P (2001) Influence of dry-wet cycles on interrelationship between aggregate, particulate organic matter, and microbial community dynamics. Soil Biol Biochem 33:1599–1611 Li Z, Shuman LM (1996) Heavy metals movement in metal contaminated soil profiles. Soil Sci 161:656–666 Manceau A, Lanson B, Schlegel ML, Harge JC, Musso M, EybertBerard L et al (2000) Quantitative Zn speciation in smeltercontaminated soils by EXAFS spectroscopy. Am J Sci 300: 289–343 McBride MB (1994) Environmental Chemistry of Soils. Oxford University Press, NY McLean JE, Bledsoe BE (1992) Behavior of materials in soils. Groundwater Issue. EPA/540/S-92/018, October 1992 National Population Commission (2000) National population census. National Population Commission, Calverton Nelson DW, Sommers LM (1986) Total carbon, organic carbon and organic matter. In: Sparks DL (ed) Methods of soil analysis. Part 2. Chemical methods. American Society of Agronomy, Madison, pp 961–1010 Newton DE (2010) Chemical elements, 2nd edn. Gale Cengage Learning, Detroit Ponnamperuma FN (1972) The chemistry of submerged soils. Adv Agron 24:2996 Rhoades JD (1982) Cation exchange capacity. In: Page AL, Miller RH, Keeney DR (eds) Methods of soil analysis. Part 2. Chemical and microbiological properties. Soil Science Society of America, Madison, pp 167–179 Ross S (1994) Toxic metals in soil–plant systems. Wiley, Chichester Sparks DL (2005) Toxic metals in the environment: the role of surfaces. Elements 1:193–197 Statistical Analysis System (2002) Statistical Analysis System (SAS) version 9.0. Users guide. SAS, Cary
123
302 Tack FM, Callewaert OWJJ, Verloo MG (1996) Metal solubility as a function of pH in a contaminated, dredged sediment affected by oxidation. Environ Pollut 91(2):199–208 Ure AM, Quevauviller P, Muntau H, Griepink B (1993) Speciation of heavy metal in soils and sediments. An account of the improvement and harmonization of extraction techniques undertaken
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Arch Environ Contam Toxicol (2014) 66:295–302 under the auspices of the BCR of the Commission of the European Communities. Int J Environ Anal Chem 51:135–151 Xiang S-R, Doyle A, Holden PA, Schimel, JP (2008) Drying and rewetting effects on C and N mineralization and microbial activity in surface and subsurface California grassland soils. Soil Biol Biochem 40:281–2289
