Introduction

Serpentinites are hydrothermally altered ultramafic rocks with heavy metal enrichment compared to many other rock types, which cover about 1% of the earth’s surface (Turekian and Wedepohl 1961; Brooks 1987; Proctor 2003). It is well documented that a substantial quantity of siderophile elements including chromium (Cr), nickel (Ni) and cobalt (Co) in these rocks are exceptionally high compared to the Earth’s crust. The concentrations, fractionation and availability of metals in soils over ultramafic rocks throughout the world have been extensively studied (Turekian and Wedepohl 1961; Schwertmann and Latham 1986; Graham et al. 1990; Bonifacio et al. 1997; Dinelli et al. 1997; Shallari et al. 1998; Godard et al. 2000; Quantin et al. 2002b; Proctor 2003; Lee et al. 2004; Brearley 2005; Skordas and Kelepertsis 2005; Becquer et al. 2006; Kierczak et al. 2007; Rashmi et al. 2009; Alves et al. 2011; Tashakor et al. 2014a). The concentrations up to 125,000 mg kg−1 Cr (Adriano 2001) and more than 10,000 mg kg−1 Ni (Hseu 2006) were observed in serpentinite soils, while the concentration of these metals in other soils commonly ranges up to 100 mg kg−1 (Kabata-Pendias 2000). Heavy metals in the environment are receiving increasing attention among soil scientists and biologists (Ashraf et al. 2015) due to growing scientific and public awareness of their polluting and potential carcinogenic role, with heavy metal contamination representing a significant challenge due to their persistence in the environment. While metals from anthropogenic origins are often considered to be more of an environmental problem, lithogenic metal release should be considered in certain areas, such as those over ultramafic rocks. Soils over ultramafic rocks represent sources of natural geogenic contamination (Oze et al. 2004; Hseu et al. 2007; Kierczak et al. 2008), particularly in areas where there are significant human populations. It is not surprising that parent rock metal content is transferred to soils, though their geochemical and mineralogical characteristics have the potential to alter metal tenure and mobility, and therefore influence the environment with possible knock-on effects on human health. These soil quality issues are particularly important in South-East Asia given the presence of large area of ultramafic rocks (Repin 1998; Proctor 2003; Tashakor et al. 2014b; van der Ent et al. 2016). The natural metal sources can be traced back to geological and pedogenic processes through which metals are liberated from parent materials and may enter the wider environment. However, it is more important to understand the stocks and fluxes within and between different fractions in which such metals are held in rocks and soils if we are to gain a full understanding of the importance of ultramafic lithologies in potential geogenic contamination. Though emphasis is often placed on the role of pH in mobility and solubility of metals, the presence of soil organic matter, oxides and hydroxides plays an inhibiting role in release of metallic elements. In fact, our findings and others (Kaasalainen and Yli-Halla 2003) show the affinity of heavy metals to bind with different soil fractions, which may vary significantly, determines how strongly metals are retained in soils and how easily they might be released into soil solution and subsequently to adjacent surface waters (Vardaki and Kelepertsis 1999; Cancès et al. 2003). As soil is composed of a heterogeneous mixture of organic and inorganic constituents with varying ability to interact physically and chemically with heavy metals, metals present in soils can be associated with several reactive components. Soil metal mobility is limited largely by initial mineralogical speciation and subsequent metal binding forms associated with soil constituents and/or speciation that may partition into various soil fractions. Based on the affinity of metals to bind with different soil constituents, they are defined representing a decreasing degree of metal availability, ranging from ions in soil solution to ions in rock crystal lattices that can be assessed using sequential extraction procedures (Tessier et al. 1979; Silveira et al. 2006). Few studies have addressed potential pollutants in water resources and agricultural or natural soils resulting from the large regional extent of ultramafic rocks in the Malaysian tropical setting, particularly with methods revealing species contributions such as the selective sequential extraction (SSE) method. This is one of the first studies using a combination of mineralogical analysis (XRD), elemental content (XRF) and soil physical and chemical characteristics, in which we aim to determine the availability of metals in rocks, soils and waters in such environments. It therefore contributes to our understanding of potential health effects on populations living on or near ultramafic lithologies.

Materials and methods

Study area and sampling

Sabah is one of the two Malaysian states located in north-west Borneo. Ultramafic peridotite outcrops extend widely over about 3500 km2 (Repin 1998). The current study focuses on the Ranau area in north-west Sabah between the latitudes of 5°57′–6°02′N and longitudes of 116°40′–116°45′E (Fig. 1) where the ultramafic series forms a broad expanse of land covering about 41 km2 (Hing 1969). The common rock types in the area are tremolite peridotites and spinel lherzolites (Hutchison 2005). However, peridotites of Ranau are partially to totally altered through intense serpentinization and weathering and are difficult to recognize in the field. Serpentinites are characterized by their blue colour with small ill-defined greenish lenses. Their appearance is waxy, although some specimens appear to be crystalline. Associated metamorphic rocks are greenschists and amphibolites. The serpentinites are in fault contacts associated with hornblende gabbro. Diabase dikes intrude the ultramafic formations in some areas. The sedimentary rocks of the Ranau area are mainly associated with the north-west Borneo eugeosyncline of late Cretaceous to Tertiary age. These sedimentary rocks (Trusmadi or Crocker formations) consist of thick successions of sandstones and shale.

Fig. 1
figure 1

Map of Ranau area (Sabah, Malaysian Borneo) studying the geochemistry of metal transfer between rocks, soils and water. Sampling locations are noted as rock (R), soil (S) and water (W)

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Four rock and ten soil samples (<20 cm depth including A and upper B horizons) were taken from the environs of the villages of Kinaratuan, Lohan Skim 1 and 2, Lohan Ulu and Tuhan (Fig. 1). Soils were taken from upper horizons after removal of debris and vegetation. Water sampling (from c. 10 cm depth) included samples taken from rivers flowing over serpentinites, namely Bongkud, Liwagu, Lohan, Napatau and Takorek. The focus of this study is on the surface water composition, though additional seepage water from Lohan Ulu, collected by hand pumping through a plastic pipe, was also analysed for comparison. Samples of 150 mL were kept in pre-washed polyethylene containers, and 2 mL of concentrated high-purity HNO3 was added to each of them to preserve the samples and avoid precipitation of oxyanions. They were analysed elementally by ICP-MS (Perkin-Elmer ELAN 9000).

Soil physical and chemical characterization

Particle size distribution of soil samples was determined using the hydrometer method (British Standard Institution 1990). The pH was measured in a 1:2.5 (w:v) soil:distilled water suspension. The cation exchange capacity (CEC) was calculated by summation of Ca, Mg, K, Na and titratable acidity, all extracted with 1 M ammonium acetate (CH3COONH4). The solutions were filtered through cellulose nitrate membrane filters (45 µm) and were analysed elementally by ICP-MS (as above).

Mineralogical analyses

X-ray diffraction (XRD) analysis of rock and air-dried, pulverized and sieved (<2 mm) soil samples was performed on a Bruker D8 Advance diffractometer. The analysing radiation was CuKα with a wavelength of 1.5406 Å (0.15406 nm). X-ray diffractograms were collected on powder samples within the 2θ range [5°–60°], with 0.02° 0.1 s step.

Bulk elemental analyses

The bulk chemical composition of rock and soil samples was determined by X-ray fluorescence spectrometry (XRF) using a Bruker S8 Tiger X-ray with the exciting energy source of Rhodium K-α line. In order to measure ten major oxides (SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O and P2O5), the powdered soils (<30 μm) were made into 32-mm-diameter fused beads, by igniting 0.5 g of sample with 5.0 g of Johnson–Matthey Spectroflux 110. To measure trace elements (Appendix), pressed powder pellets were prepared by applying a pressure of 20 tonnes for one minute to 1 g of sample with 6 g of pure boric acid powder.

Elemental speciation

Chromium, Ni and Co were fractionated with the seven-step extraction procedure scheme proposed by Tessier et al. (1979) and modified by Silveira et al. (2006) (Table 1). This method is adapted for tropical Oxisols in which the mineralogy is dominated by Fe and Mn oxides and provides data on operationally defined soil fractions. Unlike many extraction procedures, the extraction scheme of Silveira et al. (2006) is capable of differentiating metals trapped in amorphous or poorly crystalline oxides from crystalline forms. Selective sequential extraction was performed using 1 g of air-dried and sieved soil samples (<2 mm) in 50-mL polycarbonate centrifuge tubes. Reagents were added in a stepwise fashion to digest the soils, and the suspensions were equilibrated as described in Table 1. The supernatants were separated from the solid phases by centrifugation for 18–24 h at 12,880 RCF. They were then filtered through 45-µm cellulose nitrate membrane filters and analysed by ICP-MS (as above) following each stage of extraction.

Table 1 Sequential extraction procedure for metals in tropical soils as proposed by Silveira et al. (2006)
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Data quality

All samples were analysed in duplicate to verify the precision of analysis; the relative percentage difference between duplicate measurements was always less than 10%. Precision of the ICP-MS was shown by analysing the international reference standard STD 125 ppb-1 five times giving a relative standard deviation of less than 1.6% in all cases. Detection limits for Cr, Ni and Co were 0.05 µg g−1 and 0.1 µg L−1 for soil and water samples, respectively. The international standard for corundum was used as a reference material for the XRD analysis, whereas several certified reference materials with varying SiO2 content (USGS-BCR-2, SO-2, W-2a, CANMET SY-2 and CRPG-GSN) were analysed by XRF. There are no reference materials certified for all steps of the SSE, so, in order to evaluate the extraction efficiency, the concentrations of the various forms of Cr, Ni and Co were summed after extraction from each sample and compared to their total concentration obtained by XRF determination of the bulk sample.

Results and discussion

Mineralogical composition of rocks and soils

The rock mineral assemblage is characteristic for ultramafics containing antigorite, chrysotile and lizardite (Fig. 2). Different types of spinel, especially chromite and magnetite, also occur in the rocks. The studied serpentinite-derived soils typically showed representative features of tropical soils with major contents of oxides and hydrous oxides dominated by hematite and including goethite (that are weathering products of magnetite) (Fig. 2). The serpentine soils typically contain chromite and minor amounts of chlorite and ilmenite and trace amounts of inherited serpentine minerals are also recognized. The dominant clay mineral is kaolinite. Allochthonous quartz is present in almost all the soil samples, probably related to their aeolian origin.

Fig. 2
figure 2

X-ray diffractograms of representative serpentinite rock (left) and soil (right) samples in the vicinity of Ranau, Sabah (Malaysian Borneo)

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Bulk composition of serpentinite rocks and soils

The mean SiO2 content in the rocks was 43.2% (by weight) (Table 2). The mean content of MgO was 35.5% and Fe2O3 showed a mean content of 8.8% (Table 2). The mean CaO content was very low (2.5%) as were Na2O (0.18%), K2O (0.09%) and P2O5 (0.01%). Analyses of major oxides of rocks from Ranau accord with the chemical compositions of numerous ultramafic massifs throughout the world listed by Brooks (1987). Among the 20 trace metals analysed, Cr, Ni and Co are considered further (Table 2). The total concentrations of Cr and Ni ranged from 2622 to 2781 mg kg−1 and 1602 to 1840 mg kg−1, respectively, and the observed range of Co was between 61 and 64 mg kg−1. The former two elements are notably higher compared to the average Earth’s crust (Cr 100 mg kg−1 and Ni 25 mg kg−1) (Mason and Moore 1982). The most striking result with regard to the soils was, again, notably high concentrations of Cr, Ni and Co (Table 2). Chromium in Ranau soils had a high mean value of 14,208 mg kg−1 (range 2427–27,863 mg kg−1). The global content of Cr in soils range up to 200 mg kg−1 (Bourrelier and Berthelin 1998); however, in the presence of ultramafic bedrocks, such as serpentinites, the Cr concentration is up to 10,000 mg kg−1 (Stueber and Goles 1967; Schwertmann and Latham 1986). This study showed a mean Ni concentration of 1647 mg kg−1 (range 850–4753 mg kg−1). The common background average for Ni is less than 100 mg kg−1 (Kabata-Pendias 2000), while numerous studies have reported Ni concentrations of more than 10,000 mg kg−1 in ultramafic soils (Brooks 1987; Hseu 2006). Cobalt had a mean concentration of 112 mg kg−1 and ranged from 35 to 167 mg kg−1. According to Kabata-Pendias and Mukherjee (2007), surface soil Co ranges from 4.5 to 12 mg kg−1. Chromium, Ni and Co concentrations in serpentinite rocks and soils of Sabah were also in agreement with the reported values from serpentinites of India (Rashmi et al. 2009), Oman (Godard et al. 2000), Greece (Skordas and Kelepertsis 2005) and Iran (Ghaderian and Baker 2007). Nevertheless, serpentinite soils of New Caledonia contained much higher concentrations of Cr (31,000 mg kg−1) and Ni (12,000 mg kg−1) (Massoura et al. 2006). SiO2 shows strong positive correlation with CaO (0.81), Na2O (0.82) and K2O (0.87) (Appendix) and these alkali and alkali earth elements also indicate the same correlation with each other, whereas Fe2O3 forms another very strongly correlated pair with MnO having a correlation coefficient of 0.80. Cobalt and Cr correlate with this group showing strong coefficients: Fe2O3–Co (0.66), Fe2O3–Cr (0.80), MnO–Co (0.61) and MnO–Cr (0.67). There is a strong positive correlation between Cr and Co (0.96). However, Ni presents a different behaviour and shows moderate association with MgO with a correlation coefficient of 0.60 due to its calchophile nature.

Table 2 The concentration of major oxides and trace elements in serpentinite rocks and soils in the vicinity of Ranau, Sabah (Malaysian Borneo)
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Physico-chemical characteristics of soils

The soils are slightly acidic, showing a mean pH value of 5.8 ± 0.6 (Table 3). The pH ranges from 5.2 to 7.2 in line with soils derived from mafic and ultramafic rocks in lowland tropical regions (Brearley 2005; Massoura et al. 2006; Garnier et al. 2009). Silt is the predominant fraction in all of the soils except for sample S12, which is mainly composed of clay particles (Table 3). The soils are thus classified as silty loam to silty clay loam and clay (Soil Survey Division Staff 1993). Cation exchange capacity (CEC) shows a broad range with a mean of 12.0 ± 10.4 cmol(+) kg−1 (Table 3); the CEC was dominated by exchangeable Mg2+ with the mean value of 11.3 ± 10.2 cmol(+) kg−1, over 90% of the mean CEC. The highest CEC was shown by sample S3 (33.9 cmol(+) kg−1), followed by clay sample S12 (23.7 cmol(+) kg−1). Sample S4 showed the lowest CEC (1.2 cmol(+) kg−1), near a regional contact inflexion, and possible faulting, approximately midway between the highest S3 and S12 CEC samples.

Table 3 Physical and chemical characteristics of serpentine soils in the vicinity of Ranau, Sabah (Malaysian Borneo)
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Enrichment and depletion of elements from rocks to soils

During chemical weathering and pedogeological processes, the concentrations of certain elements from the parent rocks may significantly increase or decrease in the resultant soils (Osama 2007). Al2O3 and MnO were minor components of bulk rock composition, while the amounts of these elements showed a threefold to fivefold increase in derived soils along with Fe2O3 (Table 2). The concentrations of Cr and Co were also enriched five and two times, respectively, in the soils compared to parent materials. The concentration of Ni in the soils was equal to that in the parent rocks whereas MgO showed a tenfold depletion. The observed enrichment and depletion pattern in this study is similar to that proposed by Kostić et al. (1998) for wet environments. In general, tropical climates, by inducing intense and complete hydrolysis, lead to the immobilization of poorly leachable metals in the secondary minerals and oxides and accumulation in soil profiles, especially towards the surface. During the pedogenesis of serpentinite, highly mobile elements such as Mg and Si leach from the soil profile at the early stages of weathering, while newly formed clay minerals become enriched in less mobile elements such as Fe and Al.

Distribution of Cr, Ni and Co in various soil fractions

The difference between the total metal concentrations (by XRF) and the sum of the concentration of metals extracted from each step of the SSE analysis was less than 20% in all cases indicating a reliable extraction (Tessier et al. 1979). The SSE indicated the main incorporation of Cr and Ni was within the residual fractions of soils (Fig. 3); this was 12,256 mg kg−1 for Cr and 1233 mg kg−1 representing 92.3 and 89.8% of the total extracted metals (assuming the XRF analyses represent the total concentration of the metals). Combining this result with mineralogical observation, we conclude that Cr and Ni are bound in serpentine minerals and chromite spinels that are highly resistant to weathering, extracted only at the last stage using strong acidic reagents. This is in agreement with other research in New Caledonia (Quantin et al. 2002a), Brazil (Garnier et al. 2009), Poland (Kierczak et al. 2008) and Taiwan (Cheng et al. 2011). The use of microscopic (e.g. synchrotron, EDX, electron microprobe) techniques would aid further research to determine the co-location of these metals with certain primary or secondary minerals. Amorphous (poorly crystalline) and crystalline Fe-oxide and Mn-oxide fractions were the most abundant metal pools among the non-residual fractions, as predicted for ‘lateritic’ tropical soils. About 3.3% of the total extracted Cr (432 mg kg−1) was ‘trapped’ in the crystalline Fe-oxide fraction, and the concentration of Cr in the Mn-oxide fraction (371 mg kg−1; 2.8%) was higher than that in the poorly crystalline Fe-oxide fraction (135 mg kg−1; 1.0%). With respect to Ni, a preferential association with crystalline Fe oxide (73 mg kg−1; 5.0%) rather than with poorly crystalline Fe oxide (25 mg kg−1; 1.6%) or Mn oxide (50 mg kg−1; 3.3%) was observed. This result is in agreement with the work of Garnier et al. (2009) who found Ni to be preferentially associated with crystalline Fe oxide over poorly crystalline Fe oxide and Alves et al. (2011) who found comparable amounts of Ni in crystalline Fe oxides compared with poorly crystalline Fe oxide plus Mn oxides. However, it contrasts with the work of Massoura et al. (2006) who suggested that in highly weathered tropical soils, a higher proportion of the Ni should be found in the poorly crystalline Fe-oxide fraction. van der Ent et al. (2016) found higher proportions of Ni in the amorphous Fe oxides (<30%) and crystalline Fe oxides (<10%). However, these soils were covered by Ni hyperaccumulator plants with >0.01% Ni in plant parts that may return Ni to the surface soil and increase the soil Ni content. In comparison with Cr and Ni, Co showed a much higher affinity for the Mn-oxide fraction of soils (65.6 mg g−1; 58%) in addition to its abundance in the residual fraction (24.8 mg g−1; 23%). The non-resistant components including the first three fractions of soluble–exchangeable, surface adsorbed and organic matter contained very small quantities (less than 1.5% in total) of Cr, Ni and Co extracted from all the samples (Fig. 3). This suggests that the metals of concern are not easily leachable and thus not readily transferable within the environment under ambient conditions. This is also influenced by the nature of these soils being low in organic matter as soils with higher organic matter content are likely to have more Ni in organic fractions (Hseu et al. 2016).

Fig. 3
figure 3

Percentage of chromium, nickel and cobalt found in seven soil fractions following selective sequential extraction (see Table 1) for serpentinite soils in the vicinity of Ranau, Sabah (Malaysian Borneo). Percentage recovery (in relation to total determination by XRF) is noted on the right-hand side of the figure

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The ‘quality’ of serpentinite soils

The mean concentrations of Cr, Ni and Co in the serpentine soils are about 130, 40 and 10 times higher than average global soil composition (Table 4). Using the geoaccumulation index (Igeo) of Müller (1969) and the background values for relevant soil Fe concentrations from Hamon et al. (2004), we determined that the soils were moderately to strongly contaminated (Igeo class 3 on a six-point scale) by Ni and strongly contaminated (Igeo class 4) by Cr. Based on the concentration of Cr and Ni, all the soils were attributed to group C in the Dutchlist (2009) standard (indicating significant pollution and a serious toxic threat to the environment), while the Co concentration corresponds to group B (referring to polluted soils that may lead to possible harmful effects) (Table 4). Comparing with soil quality guidelines developed by Australian and New Zealand Environment and Conservation Council (ANZECC/NHMRC 1992), the concentration of Cr in the studied soils is about two orders of magnitude higher than the maximum background value for Cr (110 mg kg−1). Similarly, Ni in the investigated soils exceeds the highest background concentration level (400 mg kg−1) given by ANZECC/NHMRC (1992). Thus, the metals in the surveyed serpentinite soils are also considered polluted by this standard. Using the Greater London Council (2001) guidelines, with regard to Ni and Cr, the soils are categorized as ‘unusually heavily contaminated’, which is point 5 on a five-point scale; these guidelines do not furnish any classification for Co contamination. It should, of course, be noted that these standards do not necessarily relate to the specifics of tropical soil characteristics as they were largely and obscurely adopted from European standards (McLaughlin et al. 2000). In a comprehensive study, Zarcinas et al. (2004) suggested the 95th percentile values of the 241 randomly selected soils sampled from Peninsular Malaysia as ‘Investigation Level’ (Table 4); this would apply to all soils sampled from Ranau in this study. Nevertheless, the 95th percentile via random sampling does not reconcile differing parent geology, mineralogy and climate or the interactions between them to either mitigate or accelerate potential bio-toxicity.

Table 4 The mean concentration of chromium, nickel and cobalt in serpentine soils in the vicinity of Ranau, Sabah (Malaysian Borneo), and their geoaccumulation index enrichment (Igeo) in comparison with the average global composition, the 95% ‘Investigation Levels’ determined for Malaysia, ANZECC/NHMRC and Greater London Council guideline (all values are mg kg−1)
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Metal concentrations in waters

Surprisingly, few studies have examined metal concentrations in water samples over ultramafic areas; those found here were comparable to those observed by Vardaki and Kelepertsis (1999) in Greece and Migon et al. (2011) in New Caledonia although Cr was, in general, lower. The maximum concentrations of Cr (14 µg L−1), Ni (94 µg L−1) and Co (7 µg L−1) in river samples (Table 5) were lower than the maximum permissible values (Cr 50 µg L−1, Ni 200 µg L−1, Co 50 µg L−1) proposed by Interim National Water Quality Standards of Malaysia (INWQS 2006). The metal concentrations in Ranau rivers also compared reasonably, with the exception of a single value for Ni, with the World Health Organization (WHO 2006) standard of 50 µg L−1 Cr and 70 µg L−1 Ni and the US drinking water quality guideline (EPA 2009) of 100 µg L−1 Cr. It was found that Cr, Ni and Co in all of the rivers flowing over serpentine soils fell within these ‘permitted’ INWQS thresholds and the majority were within the WHO threshold. Hence, according to these published standards, these rivers, limited by sampling constraints presented, appear safe both for drinking water and crop production. It must be noted, however, this does not address ionic complexion, speciation factors, for Cr in particular, and the potential bio-toxicity thereof. The seepage water sample analysed, however, had a Ni concentration considerably higher than in surface waters (Table 5) and clearly in excess of standards for Ni. It is conceivable that more extensive seep water sampling and more local fluvial waters will reveal analyses in excess of threshold standards. Possible reasons include low volume of seepage water relative to larger rivers subject to high flow surface run-off, regional outlier geological provenance dilution and local to regional metal precipitation through oxyanion species. While sampling was limited, evidently local seepage waters that ingress and flow through serpentine soil profiles do become enriched in these elements. Lottermoser (1997) believed almost all seepage and groundwater associated with ultramafic formations have elements beyond environmental quality guidelines in accordance with findings of this study showing, concentration of Ni up to nine times higher than INWQS standards, though Cr and Co in the seepage water are still within the ‘safe’ limits, albeit with the potential caveats aforementioned.

Table 5 Concentrations of chromium, nickel and cobalt in river and seeping water traversing serpentinites in the vicinity of Ranau, Sabah (Malaysian Borneo)
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The relationship between Cr, Ni and Co in soil and surface water

This study showed that the geochemistry of the ultramafic parent materials has a minor effect on the chemical composition of regional flowing surface waters. This may be due to various factors, as noted above, though this study reveals the low contribution of the metals in the easily leachable and exchangeable fractions of soils is the main factor responsible. The metals concerned are held mainly in the residual fraction of soils as refractory latticed minerals and secondly sorbed in the oxide and hydroxides of Fe and Mn with relatively high affinity as liganded species. Therefore, in accordance with previous studies (Becquer et al. 2003; Massoura et al. 2006), Cr, Ni and Co in the studied soils have low mobility with little tendency to readily migrate into the environment. On the other hand, the solubility of Cr and Ni in surface water is pH dependent to some degree. Chromium is soluble at pH values of less than 4 and the solubility of Ni occurs only at pH value of less than 5.5 (Rahim et al. 1996). The pH values of the investigated soils ranged from slightly acidic to circumneutral preventing significant metal solubility. In addition, the soils displayed a fine granular structure and a silty loam to silty clay loam texture. Since silt and clay grain size fractions are capable of adsorbing more metals because of their larger specific surface area, it is assumed that these soils bound higher concentrations of metals and have low tendency to release them into the environment. In addition, the aerobic status of soils may affect metal release as waterlogged conditions may lead to the release of metals by reductive dissolution of Fe and Mn oxides (Rinklebe et al. 2016).

Conclusions and prospects

Malaysian serpentinites studied here showed a ferromagnesian mineralogy with high concentrations of Cr, Ni and Co, suggesting they are capable of inducing serious natural pollution. However, surface waters flowing over serpentinite soils revealed minimal notable concentration of Cr, Ni and Co and are considered ‘safe’ as defined by relevant threshold standards. Consequently, for the most part, chemical composition of surface water is concluded to ineffectively reflect bedrock and soil geochemistry of regional expanses of serpentinites. The main reason for this is concluded to be the very low proportion of metals associated with the easily leachable and ion exchangeable fractions of the soils. Being largely immobile, ingress of these metals into surface waters via leaching is minimized and subsequent dilution may be expected to further minimize wider fluvial metal transport and contamination. Therefore, despite their anomalously high natural concentrations of certain metals, these serpentine soils are not considered to pose a health or environmental risk. However, further work should examine metal speciation and soil Cr is of particular interest, given Cr3+ is considered an essential nutrient and Cr6+ carcinogenic.