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Using X-ray fluorescence core scanning to assess acid sulfate soils.

Introduction

Acid sulfate soils (ASS) are naturally occurring soils that are rich in iron sulfides (predominantly pyrite, FeS2) and/or have been affected by iron sulfide oxidation and sulfuric acid formation. Globally at least 24Mha of surface or near-surface soils are ASS (Ritsema et al. 2000). They are distributed across a broad climatic and geographical range but occur most commonly in tropical and coastal regions (Fig. 1) (Dent 19866; Dent and Pons 1995a).

Acid sulfate soils arc problematic as the associated release of acid and metals can be extremely harmful for plants, animals, drinking water, and steel and concrete structures (Dent 19866; Dent and Pons 1995a; Osterholm and Astrom 2002; Fitzpatrick and Shand 2008). To minimise this problem, potential ASS (PASS) need to remain anaerobic (waterlogged), implying that excess drainage, excavation, groundwater extraction, drought and fluctuations in sea-level need to be avoided or closely monitored.

The formation of ASS and the associated chemical reactions have been described in numerous publications, with most studies focusing on relatively homogenous, fine-grained sediment (e.g. Brinkman and Pons 1973; Van Breemen 1982; Dent 1986a; Dent and Pons 19956, 1995a; Astrom 1998a, 19986; Dear et al. 2002; Osterholm and Astrom 2002; Astrom and Deng 2003; Sohlenius and Obom 2004; Broughton 2008; Fitzpatrick and Shand 2008; Nordmyr et al. 2008; Johnston et al. 2010, 2011; Claff 2011; Groger et al. 2011; Thomas 2011; Unland et al. 2012; CSIRO 2013a; Department of the Environment 2013). These processes can be summarised as follows and in Fig. 2.

Upon aeration, pyrite in PASS is oxidised, a process that is associated with the release of sulfuric acid. If this acid is not neutralised in the soil, the soil pH drops. Soils with pH [less than or equal to] are defined as actual ASS (AASS) (Fig. 2). During the oxidisation and acidification process, a suite of metals can become mobilised and removed fully or partially from the sediment. If not fully removed, elements can precipitate as or become incorporated in authigenic minerals such as jarosite (K[Fe.sup.3+.sub.3][(S[O.sub.4]).sub.2][(OH).sub.6]) or Fe-oxyhydroxides. These precipitates commonly occur in the oxidised part of the soil or in a transition zone between the oxidised and the reduced zone (Fig. 2). After formation, most precipitates are relatively stable (Brinkman and Pons 1973; Claff 2011). The mobilisation and subsequent enrichment of elements has been observed in ASS worldwide, implying that the vertical distribution profiles of certain elements are characteristic for these soils (e.g. Astrom 1998a, 19986; Astrom and Deng 2003; Groger 2011; Groger et al. 2011). Further, as these element-specific enrichments are associated with specific soil conditions, element enrichment profiles can be used to infer the approximate depth of the oxidised and transitional zone in ASS.

[FIGURE 1 OMITTED]

To assess ASS and the potential of ASS to become AASS, various methods have been proposed (e.g. Dent 19866; Gray 1996; Fitzpatrick and Shand 2008; Groger 2011). Recognition of AASS in the field includes the identification of indicative mottling, a pH (1:1 soil: water) [less than or equal to] 4 and high electrical conductivity (EC). In addition, a quick and qualitative initial assessment of PASS can be performed by pouring hydrogen peroxide ([H.sub.2][O.sub.2]) onto a subsample. If this subsample reacts and causes the [H.sub.2][O.sub.2] pH to drop to <4, the sampled material may originate from a PASS horizon. In addition, an indirect and qualitative determination of sulfide can be undertaken with lead acetate (Neckers and Walker 1952; Vegas-Vilarrubia et al. 2008). Both field tests arc quick; however, in organic-rich sediments the test results can be inconclusive and may not indicate acid sulfate conditions (Vegas-Vilarrubia et al. 2008). All subsequent analyses are often time consuming and need to be performed in the laboratory. These tests can include detailed measurements of soil pH, EC, and calcium carbonate (CaC[O.sub.3]), incubation experiments, determination of total sulfur, as well as X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses (e.g. (Brinkman and Pons 1973; Fitzpatrick and Shand 2008; Vegas-Vilarrubia et al. 2008; Groger 2011).

To detect ASS over a large region, airborne electromagnetic surveying and electrical resistivity mapping both from airborne and ground-based instruments have been used (Astrom and Astrom 1997; Sohlenius et al. 2009). Together with geological maps and descriptions of the sampled material, these data can be used to locate the occurrence of sulfidic sediments. However, these approaches do not identify ASS themselves (Astrom and Astrom 1997; Sohlenius et al. 2009). Other approaches, such as hyperspectral sensing of indicator minerals and mapping of soil EC together with high-resolution topographical data, have been successfully used for large-scale ASS mapping (Huang et al. 2014; Shi et al. 2014).

X-Ray fluorescence core scanning is a fast and nondestructive method to analyse the chemical element composition of unconsolidated sediments and rocks at millimetre resolution in a cost-effective manner (Croudacc et al. 2006; Rothwell and Rack 2006; Weltje and Tjallingii 2008; Croudace and Rothwell 2010; Hennekam and De Rick 2012). XRF-scanners use incident X-radiation to excite electrons from inner atomic shells of chemical elements in the sediment sample. This creates vacancies that are filled by electrons from the outer shells--a process that is associated with the release of secondary X-radiation. The character and wavelength spectra of this secondary radiation are diagnostic for the chemical element from which it has been emitted, which allows for the estimation of clement abundance (Croudacc et al. 2006; Rothwell and Rack 2006). The main advantage of XRF-scanning over conventional XRF-powder analysis is the possibility to produce a near-continuous record directly on the surface of the sampled material with minimal preparation. As such, large sample sets can be measured in an automated way, making it a time- and cost-effective method. XRF-scanning has been shown to be a powerful tool for the detection and reconstruction of various environmental parameters such as sedimentary, biogenic and diagenetic processes, as well as pollution (Croudace et al. 2006; Weltje and Tjallingii 2008). The application of XRF scanning techniques to soil studies, however, has been limited to the quantification of gypsum (Weindorf et al. 2009), and we have found no publications on the use of XRF-scanning techniques to assess ASS.

[FIGURE 2 OMITTED]

The aim of this study is to demonstrate how XRF scanning data from sediment cores can be used as an analytical step during the identification and characterisation of ASS. We demonstrate that by using specific element signatures, it is possible to roughly estimate the depth of the oxidised and transition zone in ASS. We suggest that results from such an analysis can be used to guide a more detailed chemical analysis or to target specific geographical regions of interest.

Material and methods

Acid sulfate soils cover at least 215 000 [km.sup.2] or 21.5 Mha of the Australian continent and are particularly common along the coast (Fitzpatrick and Shand 2008; CS1RO 2013a). Sediments for this study were collected in a coastal, tropical location where ASS occurrence is highly likely. The study area is near the town of Wyndham (15.4872[degrees]S, 128.1247[degrees]E; 11 m a.m.s.l.), ~100km north-west of Kununurra in the northern Kimberley (Fig. 3). In this region, pyrite-rich material is known to occur in depths of up to 6 m below modem land surface and the probability of ASS occurrence is high (Thom et al. 1975; Lawrie et al. 2010; CS1RO 20136; Proske et al. 2014). However, as no detailed soil survey has been carried out yet, confidence in this probability is low (CSIRO 2013a). Plans for future development of the region include increased activity in Wyndham's port and the establishment of sugar-handling facilities associated with the Ord River Irrigation Area Stage 2 expansion (Ministry for Planning--Kimberley Development Commission 2000). These developments will require the expansion of infrastructure across regions with suspected ASS, which could cause substantial problems for water management and the agricultural industry.

[FIGURE 3 OMITTED]

Wyndham's climate is classified as semi-arid and monsoonal with an average annual rainfall of -820 mm (Bureau of Meteorology 2011). Mean temperatures vary between 39[degrees]C (November) and 17[degrees]C (July) (Bureau of Meteorology 2011). High temperatures coinciding with precipitation minima lead to extreme evaporation during the early summer months. The semi diurnal tides in Wyndham can range up to 8.3 m above the Lowest Astronomical Tide ('king tides'; Bureau of Meteorology 2011).

Cores KR01 (15[degrees]29'22.272"S, 128[degrees]06'15.732"E; length 350cm) and OR01 (15[degrees]26'10.716"S, 128[degrees]07'6.384"E; length 300 cm) were collected in April 2011 with a 50-cm-long D-section or Russian corer at the edges of small tidal creeks dissecting the tidal flats. Both sites are subject to inundation by seawater during king tides and are in areas of suspected ASS occurrence. At site KR01, the water level was recorded at 96 cm below the land surface in the borehole. All cores were described in the field (Table 1). Core sections were wrapped in cling film and stored in air-tight containers.

Both cores were cleaned and scanned for their elemental distribution (elements Al to Pb) using the ITRAX- scanner at the Institute for Environmental Research at ANSTO in Sydney. Each core was scanned using a molybdenum tube set at 55 kV and 30 mA and a dwell time of 10s. A step size of 1000 pm was chosen to capture the elemental variations in the core. X-Radiograph images were also obtained, which aided in further identifying the changes in distinct horizons throughout the core. Analysis time for a 1-m core was ~3.5 h. The ITRAX-scanner measures all elements relevant to this study with good accuracy (Croudacc et al. 2006). XRF scanning results are given in counts per second; however, no calibration procedure was available at the time, and so element ratios rather than proportions are used (Croudace et al. 2006).

The sediment in the region is primarily of clay and silt size, with sand being found only in the Cambridge Gulf s outer estuary north of the study area (Thom et al. 1975). To test the homogeneity of the sediment, selected samples were measured for their grain-size distribution by using laser diffraction. All samples were treated with 30% [H.sub.2][O.sub.2] and 10% HC1 and transferred into calgon ([Na.sub.6][O.sub.18][P.sub.6]). In addition, every sample was physically dispersed using ultrasound for 2 min (Ryzak and Bieganowski 2011). All samples were measured five times with the Malvern Mastersizer 2000 (Malvern Instruments Ltd, Malvern, UK) with Hydro MU attachment at the The Fenner School of Environment and Society at ANU. The average spectrum of these five repeat measurements for each sample was used to represent the grain-size distribution. Characteristic statistics were calculated based on the geometric method of moments using the GRAD1STAT package (Blott and Pye 2001).

Results

Both cores consist of relatively homogenous, poorly sorted, silt-sized sediment with small portions of clay and sand. In core KR01, a change of sediment type was observed between 245 and 246 cm (Table 1). Mottling was recorded in both cores at various depths (Table 1, Appendix 1, Fig. 4). In some parts of core KR01, this mottling manifested as sand-sized nodules. Further, gypsum was observed between ~95 and 170 cm in core KR01.

Titanium has been chosen as the denominator for the element ratios because it remains relatively immobile during ASS formation processes in homogenous, fine-grained sediments (Astrom 19986; Osterholm and Astrom 2002; Astrom and Deng 2003).

[FIGURE 4 OMITTED]

The results of all element ratios are shown in Fig. 4. In core KR01 above -200 cm, Ca/Ti and Sr/Ti showed a profile that indicates depletion of Ca and Sr relative to Ti, with values steadily increasing below this depth. The Mn/Ti profile showed a distinct peak between ~40 and 50 cm and numerous peaks below 200 cm. The profiles of Fe/Ti and Pb/Ti were similar in shape and showed synchronous peaks at 30-40, 60-70, 85-125 and 140-150 cm. The K/Ti profile mirrored the peaks in the Fe/Ti and Pb/Ti profiles between 85 and 125 cm but otherwise showed little variation. The Zn/Ti, Ni/Ti and Y/Ti-profiles show large internal variation, with a distinct peak between 180 and 200 cm.

In core OR01, the Ca/Ti profile showed a general increase in values with depth. The Sr/Ti profile showed several peaks, with the most prominent peak between 25 and 80 cm. Values of Mn/ Ti showed a sharp decrease at ~20cm and a gradual increase from ~80 cm downwards. Synchronous peaks of K/Ti, Fe/Ti and Pb/Ti were observed between 20 and 70 cm. As in KR01, the Zn/ Ti, Ni/Ti and Y/Ti profiles varied substantially; in particular, Ni/ Ti and Y/Ti showed a synchronous peak between 90 and 120 cm.

Discussion

Acid sulfate soils are known to exhibit coloured mottling and nodules that are indicative of secondary Fe-minerals (Fitzpatrick and Shand 2008). In both cores, pale straw yellow and orange-reddish mottling was observed. Pale straw yellow is a diagnostic colour for minerals of the jarosite group, which form under oxidising conditions at soil pH between 2.5 and 3.5 (Table 2, Fig. 4) (Fitzpatrick and Shand 2008). Coinciding with this mottling, several synchronous peaks in K/Ti, Fe/Ti, Sr/Ti and Pb/Ti are evident (Fig. 4). Minerals of the jarosite group, with jarosite K[Fe.sub.3][(S[O.sub.4]).sub.2][(OH).sub.6] being the most common form, are known to incorporate several trace metals such as Sr and Pb (Groger et al. 2011; Welch et al. 2007). Thus, synchronous peaks in K/Ti, Fe/Ti, Sr/Ti and Pb/Ti coinciding with pale straw yellow mottling can be used as indicator for minerals of the jarosite group.

Orange, red and brownish mottling is typical for secondary iron oxides and hydroxides that accumulate in aerobic and seasonally anaerobic zones of ASS (Fitzpatrick and Shand 2008). Iron oxides, in particular Fe(III)-oxides, can adsorb heavy metals such as Pb (Claff et al. 2011). Therefore, in the absence of a K/Ti peak, synchronous peaks in Fe/Ti and Pb/Ti together with reddish mottling are likely to indicate Fe-oxides.

The occurrence of jarosite and other secondary Fc-minerals is characteristic for the oxidised zone of ASS; therefore, we can estimate that at least the uppermost ~145 cm in KR01 and the uppermost ~75cm in OR01 have been oxidised. The Fe-oxides between -215 and 250 cm in KR01 are most likely a product of an earlier and probably less pronounced soil formation process, because this mottling occurs below the postulated transition zone into reduced sediment (see below).

In core KR01, gypsum crystals and coinciding peaks in Ca/Ti are observed between -95 and 170 cm (Fig. 4). Gypsum is a secondary mineral in saline ASS of low-rainfall regions and precipitates in those soil depths that alternate between saturated, anaerobic (wet season) conditions and drier, more aerobic (dry season) conditions (Fitzpatrick and Shand 2008; Jennings and Driese 2014). All suspected ASS in the study area are in a coastal, semi-arid setting, implying that gypsum is probably a common secondary mineral.

The transition zone between the oxidised and reduced zone in ASS is characterised by a sharp increase in pH and is often marked by distinct enrichments of Zn, Ni, Y and Mn (Astrom 19986; Groger et al. 2011) (see Table 2). In both sediment cores, ratios of all four elements show a general depletion pattern in the oxidised zone, implying that these elements have been mobilised. Several synchronous peaks of Ni/Ti, Mn/Ti and Zn/ Ti together with peaks in Fe/Ti in the oxidised zone most likely indicate that these heavy metals are adsorbed to secondary Fe-minerals (Johnston et al. 2010; Claff et al. 2011). In the absence of a pronounced Fe/Ti peak, however, synchronous peaks in Zn/Ti, Ni/Ti, Y/Ti and Mn/Ti probably indicate the enrichment of the elements in the transition zone. Therefore, we can estimate that the transition between oxidised and reduced sediment occurs between -180 and 210 cm in core KR01 and ~90 and 120 cm in core OROl. In support of this estimate, ratios of elements that are highly mobile during ASS formation and that show a depletion profile in the oxidised zone (e.g. Ca, Mn) increase below the transition zone. Therefore, we can assume that reducing conditions prevail below -210 cm in KR01 and 120cm in ORO1.

Conclusion

We demonstrate how combining observations from sediment core descriptions with XRF scanning data can be used to roughly estimate the depth of the oxidised and transition zones in ASS in a time- and cost-effective manner. This information can then be used to guide more detailed analyses or to map the extent of ASS over large areas. Detecting jarosite and other secondary indicator minerals is often an important step to identify ASS in the field. In sandy soils, however, jarosite is often not visible and the occurrence of gypsum can be inconclusive (Thomas 2011). Thus, use of XRF data can be beneficial in cases where macroscopic indicator minerals have not been recognised.

Further, our study provides insights into metal transport within ASS. For example, our data document the behaviour of Zn, Ni, Y and Mn in the transition zone and thus contribute to the understanding of chemical reactions within ASS.

We have used XRF core scanning to detect the element composition of the sediments. For an XRF analysis in the field, handheld or portable XRF analysers are available; however, these often show a more limited performance (Ellis et al. 2008; Migliori et al. 2011; Zhu et al. 2011).

With regard to use of the method outlined here, a few cautionary notes need to be made. If information on the actual acidity of the oxidised zone is required, pH measurements need to be carried out in addition to the measurements proposed here. Although jarosite forms under acidic conditions, its presence docs not imply that the oxidised zone is actually acidic (Brinkman and Pons 1973). Further, our method is based on observations on clayey and silty sediments. Before applying this approach to coarser grained soils, testing on sandy and inhomogeneous material is necessary. In soils that are not homogeneous, it may be more powerful to use Si as denominator for the element ratios as Ti is grain-size dependent. Also, in some locations, elements show a different behaviour than outlined here. Such deviations are mostly due to agricultural activity; which emphasises that understanding the regional context is paramount for the application of this method.

Appendix 1. Stratigraphic column and element ratios of all relevant elements in the Two sediment cores. Black icons in the stratigraphic column mark depths in which Mottling has been observed (see also Table 1).

[ILLUSTRATION OMITTED]

http://dx.doi.org/10.1071/SR14103

Received 28 April 2014, accepted 25 August 2014, published online 12 November 2014

Acknowledgements

UP acknowledges the German Academic Exchange Service (DAAD) for funding her Postdoctoral Fellowship at the ANU. Measurements at ANSTO were supported by an AINSE-grant to UP (grant number 12P095). Luke Bentley (DEC Kununurra), Simon Haberle (ANU) and Jens Groger (LBEG) are thanked for their help during this project. Fieldwork was carried out with the DEC Regulation 4 Authority Permit CE003064 and with financial support from the Kimberley Foundation of Australia. We thank Mats Astrom and an anonymous reviewer for their constructive comments, which have improved the manuscript greatly.

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Ulrike Proske (A,C), Henk Heijnis (B), and Patricia Gadd (B)

(A) Department of Archaeology and Natural History, Australian National University, Acton, ACT 2601, Australia,

(B) Institute for Environmental Research, Australian Nuclear Science and Technology Organisation, New Illawarra Road, Lucas Heights Campus, NSW 2234, Australia.

(C) Corresponding author. Email: ulrike.proske@anu.edu.au
Table 1. Sediment characteristics of both cores, KR01 and OR01

Depth     Mottling                       Sediment colour
(cm)

KR01

0-50      Orange mottles from 25 cm      Chocolate brown
            onwards
50-76     Pale straw yellow mottles      Chocolate brown grading
            between 72 and 144 cm,         into dark grey
            continuing orange mottles
76-150                                   Dark grey

150-160   Few orange-red mottles
160-200   No mottling observed
200-218                                  Brown
218-235   Few red mottles
235-245   Few orange-brown mottles       Dark grey
245-246   No mottling observed           Beige-brown
246-350                                  Dark grey

ORO1

0-19      No mottling observed           Chocolate brown
19-30                                    Grey
30-74     Pale straw yellow mottles      Greyish brown
            between 30 and 74 cm
74-100    No mottling observed           Dark grey
100-300                                  Dark grey grading into
                                           bluish

Depth     Grain size                     Other observations
(cm)

KR01

0-50      Silty clay to clayey silt

50-76

76-150                                   Gypsum crystals between
                                           96 and 168 cm
150-160
160-200
200-218
218-235
235-245
245-246                                  Dry, diatomite-like sediment
246-350

ORO1

0-19      Silty clay to clayey silt
19-30
30-74

74-100
100-300                                  Numerous larger and smaller
                                           plant remains

Table 2. Overview of the most commonly observed, relative abundance
of clement in soil profiles assuming a uniform distribution in parent
material, i.e. no or very little fluctuations in input

Summary based on the most common observations and assuming a uniform
distribution in parent material with no or very little fluctuations
in input. Sources: Astrom (1998a, 19986); Osterholm and Astrom
(2002); Astrom and Deng (2003); Sohlenius and Obom (2004); Broughton
(2008); Nordmyr elal. (2008); Claff (2011); Groger el al. (2011);
Thomas (2011); Unland et al. (2012)

Element     Oxidised, acidic zone

Sr, Ca      Depleted

Mn          Depleted (occasionally enriched
              in distinct peaks)
Ka          Generally depicted over whole
              zone but often enriched in
              distinct peaks when jarosite
              and/or Fe-(hydr)oxides present
Fe          Same as K

Pb          Either enriched when associated
              with secondary Fe-minerals or
              neither enriched nor depleted
Zn, Ni, Y   Depleted

Zr, Ti      Immobile (B)

Element     Transitional zone                    Reduced, alkaline or
                                                 pH neutral zone

Sr, Ca      Steep gradient between AASS and      Neither enriched nor
              PASS                                 depleted
Mn          Enriched (often towards the lower    Neither enriched nor
              portions of this zone)               depleted
Ka          Neither enriched nor depleted        Neither enriched nor
                                                   depleted

Fe          Depleted or neither enriched nor     Neither enriched nor
              depleted                             depleted
Pb          Neither enriched nor depleted        Neither enriched nor
                                                   depleted

Zn, Ni, Y   Enriched (often in the depth of      Neither enriched nor
              the pH minimum)                     depleted
Zr, Ti      Immobile (B)                         Immobile (B)

(A) Abundance of K is strongly linked to grain size and thus may
exhibit a different pattern. It is diagnostically most indicative
when co-occurring with Fe in jarosite.

(B) This has been shown for short periods of time (decades and
centuries). Both elements may become mobile when subjected to
strongly acidic conditions over longer time periods.
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Author:Proske, Ulrike; Heijnis, Henk; Gadd, Patricia
Publication:Soil Research
Article Type:Report
Geographic Code:8AUST
Date:Nov 1, 2014
Words:5359
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