Environmental Geochemistry and Health, 1991, 13(4), page 197

Elemental concentrations in Jamaican peat E.M. Harty, G.C. Lalor and Hilary Robotham Centre for Nuclear Sciences, University of West Indies, Mona, Kingston 7, Jamaica

Abstract

The peat of the Negril Morass in Western Jamaica was sampled at depths down to 7 m in directions parallel and perpendicular to the seashore, and the samples were analysed for AI, As, B, Br, Ca, Ce, CI, Co, Cr, Cs, [3y, Eu, Fe, Hf, I, La, Mg, Mn, Na, Sb, Sc, Sm, Sr, Ti, U and V by instrumental neutron activation analysis and by spectrophotometry. The peat is high in ash content, but the concentrations of most elements are below crustal abundances and therefore provide no evidence of nearby mineralisation. The elemental concentrations indicate that no particular environmental hazards are to expected from the use of this peat in electricity generation.

Introduction Our laboratory is engaged in a multi-purpose geochemical mapping of Jamaica. This includes a study of the island's major deposits of peat which are found in coastal wetland areas known as the Negril and the Black River Morasses. The peat reserves of Negril and Black River together are in excess of 20 million tonnes, enough to generate 120 MW of electricity for 30 years (Robinson, 1983). This is a considerable portion of Jamaica's present electricity demand at present met almost entirely by imported petroleum. There is also some potential for the use of peat as a horticultural medium. Moreover, the Morass itself is a possible recipient of treated effluent from sewage treatment plants under consideration. The Negril Morass is adjacent to the famous Negril Beach, an area of major tourist activity, and proposals for the construction of peat-fuelled thermal plants have raised environmental concerns. The potential value of the Morass and environmental concerns suggest the need for a more complete knowledge of its elemental composition which might itself contribute to a better understanding of the regional geochemisuy of Jamaica. The Negrit Morass has an area of about 2,700 ha with an average peat thickness of 5.6 m (Robinson, 1983). The Morass had its origin some 6,500 years ago when an encroachment of the sea enlarged the swampy area at Negril (Hendry, 1982a, 1982b). Thereafter, peat formation kept pace with the rising sea level which has slowed markedly over the past 4,000 years. Although there are probably still some intrusions of salt water, there has been a subsequent increase in the fresh-water content of the bog from rivers originating in the surrounding mountainous limestone regions and from several small springs around its periphery. Mangrove peats are generally found in the basal layers of the deposit, overlain by sedge peat with woody non-mangrove peat interspersed throughout. Mangrove peat also occurs in the more saline areas near to the coastline. In fresh-water environments the dominant peat is

Cladium (sawgrass). Previous studies have reported on moisture and ash contents, heating value, volatiles and concentrations of A1, Ca, K, Mg, Na, N, P and S (Digerfeldt and Magnus, 1984; Robinson, 1983). Results on twenty major, minor and trace elements are presented in this paper.

Methodology Sampling and sample preparation Sampling sites were chosen as shown in Figure 1 "alongtwo lines roughly parallel and perpendicular to the sea shore respectively. Samples were obtained using a Mackintosh peat sampler at three depths in the ranges 0.5-1.0 m, 2.5-3.0 m and 6.5-7.0 m, and at two locations about 3 m apart. At site 6 the bog was too shallow to obtain the deepest sample. Each sample consisted of three cores, which were combined to give a 1 kg composite. After removal of fresh plant remains, the samples were solar-dried for 7-10 days to a moisture content below 5%, ground to as fine powder and stored in airtight polyethylene containers. Spectrophotometry The boron content was determined spectrophotometrically by its reaction as boric acid with curcumin using a Zeiss PMQ3 spectrophotometer. Neutron activation analysis Accurately weighed pellets of approximately 0.5 g of the finely ground dry material were heat sealed in acid-washed polyethylene vials and irradiated in the inner sites of the UWI SLOWPOKE-II nuclear reactor. Because of the high stability of the neutron flux of the SLOWPOKE-II reactor, simultaneous irradiation of comparator standards is unnecessary. Instead, multi-element standards were prepared using atomic absorption standard solutions (Aldrich) and high purity chemicals (SPEX) and used to determine an activation constant for each element to be used in the calculation ot

198

Elemental concentrations in Jamaican peat Table 1 Nuclear data and conditions for radionuclide analysis and

detection limits. Product nuclide

Gamma energy (keV)

Detection limits (rag kg-1)

Product nuclide

Gamma energy (keY)

Detection limits (rag kg-1)

~A1 a 7~As c 82Br c 41Ca a 141Ced

779 559 776 3,084 145 1,643 1,333 320 796 95 122 1,291 482 443

100 2.4 3.2 1,300 0.9 100 0.1 2.0 0.1 0.05 0.03 200 0.2 2.5

14OLaa

1.597 208 1,014 1,811 1,368 564 889

2,500 0.2 0.2 500 50 100 0.03 0;02 5.0 0.i 0.1 0A 2.4

3SCI b

6OCoa 51Crd 134Csd 165~ b t~y 152Eud 59Fed 18Hfd 1281 b

177Lu c

25Mg a 5~Mn b UNa c iZZSbc ~Sc d 153Sm e 87Srb 233Thd 5tTi ~ Z~gU ~ 52V a

102

388 312 320 228 1,434

"irradiation: 4 rain at Z5 • 1011 n era-2 s-l; decay I0 rain; counting: lOmm. b irradiation: 4 rain at 2.5 • 1011 n cm -2 s-l; decay 30 rain; counting: 10 rain. c irradiation: 2 h at 1 x 1012 n cm -2 s-l; decay 5 days; counting: 1 h. dirradiation: 2h at 1 • 1012n cm-2s-1; decay 21 days; counting: 1 h.

package from EG&G Ortec was used in peak search and photopeak area determinations. The radionuclides and photopeaks used, and the detection limits under the conditions used in this study are shown in Table 1. Nuclear interferences were found to be insignificant for all isotopes excel~t for 27Mg which was corrected for interference from the~TAl(n,p)27-Mg reaction' The precision and accuracy of the method were evaluated by replicate analysis of the following NBS Standard Reference Materials: SRM 1571 Orchard Leaves, SRM 1575 Pine Needles, SRM 1633 Coal Fly Ash and SRM 1635 Coal. The results show that for most elements the concentrations fall within the 95% confidence limits of certified and information values, and that even in the worst cases, where the concentrations are close to the detection limits, e.g. for Lu, concentrations can be determined to better than • 16%.

N

A - Canal B - Nor~ Negrll R i w r C - South Negrll R l ~ r

9

[s.mpn~S 1 ~ l

Results and Discussion Duplicate analyses were done on one fifth of the samples and these agreed within • 10%. The results on samples taken about 3 m apart also agree within _+ 10%, showing that site variance is low.

NEGR Ik

O I

l~m J

Figure 1 Sampling sites in the Negril Morass.

its concentration (Robotham et al., 1987). The g a m m a spectrometry system consisted of a 72 cm 3 active v o l u m e C a n b e r r a R e v e r s e Electrode Hyperpure Germanium detector (relative efficiency of 15% and 1.9 keV full width at half maximum at the 1,332 keV photpeak of 6~ and associated electronics, coupled to a Series 40 muitichannei analyser. The GELIGAM software

Ash content The ash content on ignition at 550~ varied from 9% to 23%, but the average of 15.1% was close to the value of 16% found in previous work (Digerfeldt and Magnus, 1984). There is no significant increase with depth in ash content in these samples. There is virtually complete loss of As, Dy, I, Sb and U and significant loss of Br and C1 on ignition. Comparison with crustal abundances Elemental concentrations in peat are primarily derived by plant uptake from underlying soils during the early development of the swamp, but when the root systems are

E.M. Harty, G.C. Lalor and H. Robotham

199

100

10

0.1

0.01

B NaMgAICICaScTIV CrMnFeCoAsBrSrSbl ScLaCeSmEuDyLuHfThU Figure 2 Elemental concentrations in the peat of the Negril Morass normalised to average crustal abundances.

within the peat itself, the elemental influx from the surroundings becomes dominant. Inflows of substances in solution, detrital deposition from surface waters, deposition of atmospheric dust and the reworking of bedrock and underlying clays are important inputs into morasses. Metal enrichment, particularly of Zn, U, V, Fe, Cu and Mn, in peat in bogs which are over or downstream from even very weak mineralisations is well known (Cannon, 1955; Horsnail and Elliot, 1971) and the observed anomalies are usually higher than those found in soils or stream sediments related to the same source. Therefore, although the uptake of elements by peat is complex - depending on both biotic and abiotic processes - the metal concentrations in peat often reflect the geochemistry of the surroundings, and may provide clues to occurrences of mineralisation or local pollution. Digerfeldt and Magnus (1984) reported on the concentration of AI, Ca, Mg, Mn and Fe in the Negril Morass over several sites and depths. Except for Na, where the results fall in a tight band, there is a greater spread in

the ranges of their results at depths similar to those used in the present study. Nevertheless, in all cases the ranges overlap. The analytical results from this study, normalised to crustal abundances (Clarke and Washington, 1924; Taylor, 1964) are summarised in Figure 2. All elements, except As, B, Br, CI, I and Sb, are consistently below average crustal abundances. Lu, Mn, Na and U are above average crustal abundances in a few samples only. The halides Br, CI and I exceed crustal abundances by factors >100, 100 and >10 respectively. Except for U, the concentrations of the elements vary by no more than an order of magnitude. The occurrence of anomalous concentrations of, for example, As, Mn, and Mo, in the stream sediments of rivers draining a cretaceous inlier to the north-east and east of the morass has been reported (CIDA, 1988). However, although water inflows into the morass originating in this inlier are possible, no significant As and Mn concentration is apparent.

200

Elemental concentrations in Jamaican peat

Table 2 Concentrations of Na, Ca, Mg, Cl, Br and I as a function of

Table 4 Concentrations of Al, Ti, V, Cr~ Mn, Fe and Co.

depth at selected locations in the Negril Morass. Sample Depth number (m) 5

6

Na (%)

Ca (%)

Mg (%)

Cl (%)

Sample Depth AI 17 V Cr Mn Fe Co number (ra) (%) (rag kg-1) (rag kg-J)(mg kg-l)(mg kg-1)(%) (rag kg-t)

Br I (rag kg-1)

5

0.5-1.0 2.5-3.0 6.5-7.0

0.1 nd 0.16 170 1.13 820

2.2 6.9 4,4

3.1 4.4 58

160 270 300

0.25 0.79 4.9

0.38 0,27 10.6

1

0.5-1.0 2.5-3.0 6.5-7.0

0.29 300 0.09 nd 0.22 nd

6.4 nd 8.4

3.4 2.3 9.7

210 300 1,050

0.16 0.16 4.9

0,52 0.27 3.9

30 8 16

2

0.5-1.0 2.5-3.0 6.5-7.0

0.53 760 0.21 nd 0.14 nd

15 12 34

11.3 10.2 8.6

560 750 350

! .5 3.7 3.5

0.96 0.26 0.83

7 6.6 6.5

3

0.5-1.0 2.5-3.0 6.5-7.0

0.26 160 0.32 nd 0,63 730

6.3 4.5 40

0.7 5.5 30

1,300 480 300

0.37 0.26 2.5

0.51 0.50 3.6

570 610 700

17 13 13

4

0.5-1.0 2.5-3.0 6.5-7.0

0.35 0.17 5.9 0.32 170 t0 0.63 200 20

5.3 6.9 17

1,980 790 400

0.76 0.56 2.04

0.83 0.67 2.2

690 530

9 3

6

0,5-1.0 2.5-3.0

0.40 230 0.91 400

12 14

1,720 390

1.0 0.6

1.04 0.66

0.5-1.0 2.5-3.0 6.5-7.0

0.65 1.1 1.9

2.8 2.2 1.6

0.46 0.45 0,60

0.79 1.5 2.7

790 540 800

11 13 14

0.5-1.0 2.5-3.0 6.5-7.0

0.65 1.1 1.1

3.4 2.2 1.8

0.69 0.67 0.70

0.88 1.3 1.7

810 490 800

10 15 15

0.5-1.0 2.5-3.0 6.5-7.0

0.75 2.0 3.4

3.4 2.9 1,5

0.61 0.91 0.90

0.87 2.3 2.5

160 790 2,200

0.5-1.0 2.5-3.0 6.5-7.0

0.79 1.2 1.8

4.6 2.9 1.2

0.63 0.94 0.80

0.98 1.2 2.4

640 970 1,200

0.5-1.0 2.5-3.0 6.5-7.0

0.49 0.62 1.2

4.6 2.9 1.2

0.38 0.27 0.30

0.57 0.85 1.3

0.5-1.0 2.5-3.0

0.87 0.53

3,6 2.9

0.47 0.89

1.43 0.65

22 21

n d = no dam available.

Table 3 Linear correlations between concentrations of the elements in Table 2.

I

Br

Cl

Ca

-0.1 --0.26 -0.02 -0,15 0.36

0.7 0.48 -0.26 0.41

0.9 0.56 -0.71

-0.6 -0.13

Mg

Table 5 Linear correlations between concentrations of some metallic

elements. Na

Mg Ca CI

Br

0.6

Distribution of selected elements Table 2 contains more detailed results on the concentrations of some elements which may have originated largely from inflows of sea water. Except for Ca and I, the concentrations in Table 2 increase with depth and decrease with distance from the sea as might be expected from the geography and history of the morass (Hendry, 1982a, 1982b). However, as shown in Table 3, many of the concentrations are poorly correlated. Sodium and chlorine are very strongly correlated, and there are also significant correlations between the cation concentrations and bromide and chloride, and between sodium and magnesium. All the correlations with Ca in Table 3 are negative. Indeed, this is so for every element measured in this study except for the similar elements strontium and manganese, where correlation coefficients of only 0.3 and 0.5 respectively are observed. The concentrations of Ca substantially exceeds that of Na in almost every sample, although the concentration of Na in sea water is twenty times greater. Calcium in the morass is probably derived from the underlying and surrounding limestone terrain. The strong negative correlation of Ca with Na, and to a lesser extent with Mg, may indicate cationic competition. The values for iodine are curious; they are some ten times above crustal abundance, not necessarily unusual for

Al U Th Co Fe Mn Cr V Ti

-0.30 0.92 0.80 0.44 -1.0 0.91 0.71 0.78

Ti

V

Cr

Mn

0,32 0,47 --0.08 0,33 0.83 0.71 0.94 0.11 0.64 0.67 0.91 -0.16 0.79 0.65 0.63 -0.01 -0.45 -0.34 -0.22 0.78 0.85 0.83

Fe

Co

Th

0.19 0.51 0.66

0.14 0.90

0.24

seaside locations, but the only positive correlation relates to the other halide bromine and is low. Since iodine in the environment is derived essentially from the sea, it seems surprising to find so many negative correlations for this anion. Work now in progress on the iodine content of Jamaican soils will provide more information,

Arsenic and antimony The concentration of As in the Negril peat lies in the range of 2.2-25.7 mg kg-1, with a median value of 8 mg kg"1, and increases with depth. Arsenic is potentially phytotoxic at levels varying between 1-20 mg kg-1 in different plants. Since the bioavailability of As may be high because of the slightly acidic and reducing environment in which the peat was formed (Hess and Blanchar, 1976), further study may be advisable on the use of this peat as a general horticultural medium. High As concentrations have been reported in many peat bogs, for example, in Finland (Minkkinen and Yliroukanen, 1978). Arsenic is strongly associated with gold in practically all types of gold deposits and is often

E.M. Harty, G.C. Lalor and H. Robotham

used as a pathfinder. Though there have been reports of anomalous concentrations of Ag and As in soils and stream sediments associated with cretaceous inliers (CIDA, 1988; Simpson et al., 1988), the concentrations within this Morass are well within the range typically found in Jamaican soils, i.e. 3-76 mg kg -I (Robotham et al., 1987). The concentration of antimony also increases with depth and is in the range of 0.1-0.8 mg kg-1. The good correlation between antimony and arsenic suggests that the arsenic is not anthropogenic. Bovo~2

Boron concentrations are in the range of 17-91 mg kg-1, well above average crustal abundance. The higher values are found at lower depths at sites 2 and 5. Its origin is likely to be the sea where the median concentration is reported as 4.4 mg kg-1 (Bowen, 1979), but some input from sources such as agricultural burning and fertilisers is possible. Boron is an essential element for plants and the amounts found in the peat are adequate, especially since boron is usually bioavailable. Uranium and thorium Peat is known to accumulate uranium (Manskaya and Drosdova, 1968; Lopatkina, 1067), and the resulting concentrations may be orders of magnitude higher than those in the surrounding areas. A radiometric study of Jamaica has identified several areas, mainly associated with bauxite deposits and terra rossa with gamma activities up to 40 pR h-'. Activities are generally low (2 jaR h-1) in the Negril region, with one small area at about 11 gR h-1 (Lalor et al., 1989). Uranium concentrations range between 0.5-t7 mg kg-I, except for the high value of 56 mg kg-1 found at one site, with a median of 1.3 mg kg-1. Although the values are consistently higher in the southern sections of the Morass, there is no consistent evidence of uranium enrichment. Like uranium, thorium may be accumulated by peat, but the normal content in peat bogs is below 2 mg kg-1 (Boyle, 1982). There is no evidence of thorium enrichment in the Negril Morass as the concentrations fall in the range 0.2-1.9 mg kg-1. There is no clear gradation with depth and no significant correlation with uranium. The region around the Black River Morass has higher gamma activities, typically 11-15 gR h-1 (Lalor et al., 1989). The Morass itself is prone to flooding, and a heavy sediment load is brought in by the river system and surface-water run-off. Data for a few samples collected in areas of the highest gamma activities recorded in that Morass surprisingly show no higher concentrations of K, U and Th than that found in the Negril Morass. Interestingly, molybdenum was found at concentrations over seven times crustal abundance.

201

elements, and for Mn for which there are only two positive coefficients, neither of them high. The negative correlation with AI is interesting since U is closely associated with the island's bauxite deposits. The Mn correlations stand out. The highest value is 0.33, and all others, except for the Th value, are negative. It is clear that further study on the chemistry, including the redox conditions, of the morass and surrounding regions would be interesting. Conclusions Variation of concentrations with depth Many bogs are ombrotrophic, and in these there is often a characteristic concentration/depth relationship with higher concentrations of elements near the surface. For most of the elements reported here, the results indicate an increase in concentrations with depth which may indicate that the main sources were well below the recent surface. There also seems to be a general trend towards higher levels of elements in the southern section of the morass depths, although there is no consistent pattern of variation with depth, ash content or with position within the Morass for all elements. It appears unlikely that there is any area of mineralisation which interacts with the morass, and that sea-water incursions were probably the main source of elements in the morass. Possible environmental hazards

Peat from the Negril Morass has a large ash content and on ignition, most of the elements are retained in the ash. The concentrations of these and the volatiles examined do not appear to constitute any particular hazards in the operation of thermal electricity generation. The sulphur content (Robinson, 1983) at 1.6% of dry weight is significantly higher than that found for fuel peats in temperate countries. It is estimated that some 0.024 tonnes of sulphur dioxide would be released per MW h of electrical output. Peat has been suggested for use in Jamaican horticulture as a soil conditioner. It retains water and inorganic ions, and provides organic food for soil fertility, enhancing micro-organisms. The elemental concentrations seem generally suitable for plant growth with the possible exception of arsenic which may be high enough to affect some plants. Acknowledgements

We thank Mr John Preston for assistance with the computer work, Mr Michael Blackwood and the Petroleum Corporation of Jamaica for the use of a swamp vehicle and for assistance with field work, and the European Economic Community and the Scientific Research Council of Jamaica for support.

At, 7~, V, Cr, Mn, Fe, Co

The observed concentrations of these elements by site and depth are shown in Table 4. Although there are exceptions, the tendency for an increase in concentration with depth is clear, bnt there is little pattern with respect to distance from the sea. Some linear correlations are shown in Table 5. Most of the concentrations are rather well correlated; the exceptions are U which correlates poorly with all the

References Bowen, H.M.J. 1979. Environmental Chemistry of the Elements. Academic Press, New York. Boyle, R.W. 1982. Geochemical Prospecting for Thorium and Uranium Deposits. Elsevier Scientific Publishing Co., Amsterdam.

202

E l e m e n t a l concentrations in J a m a i c a n p e a t

CIDA (Canadian Intemational Development Agency). 1988. Jamaica Metallic Mineral Survey - Phase 1. Geological Survey. Report on Project No:504/0012280. Bondar-Clegg and Co. Ltd., Canada. Cannon, H.L. 1955. Geochemical relations of zinc-bearing peat to the Lockport Dolomite, Orleans County, New York. US Geol. Survey Bulletin IO00-D, pp.119-185. Clarke, F.W. and Washington, H.S. 1924. The composition of the Earth's crust. US Geological Survey, Profess. Paper 127. Digerfeldt, G. and Magnus, E. 1984. Paleoecologieal studies of the past development of the Negrft and Black River Morasses, Jamaica. In: Bjrrk, S. (ed.), Environmental Feasibility Study of Peat Mining in Jamaica. Petroleum Corporation of Jamaica, Kingston, Jamaica. Hendry, M. 1982a. The Structure, Evolution and Sedimentology of the Reef, Beach and Morass Complex at Negril, Western Jamaica. Petroleum Corporation of Jamaica, Kingston, Jamaica. Hendry, M. 1982b. In: Colquholm, D.J. (ed.), Holocene Sea-Level Fluctuations, Magnitude and Causes. University of South Carolina, Columbia, SC, USA. Hess, R.E, and Blanchar, W.R. 1976. Arsenic chemistry in Missouri soils. Soil Sci. Soc. Am. J., 40, 847-852. Horsnail, R.F. and Elliot, L.L. 1971. Some environmental influences on the secondary dispersion of molybdenum and copper in westem Canada. Geochem. Exploration, CIM Special Volume, 11, 166-175.

Lalor, G.C., Miller, J., Robotham, H. and Simpson, P.R. 1989, Gamma radiometric survey of Jamaica. Trans. lnstn Min. Metall. Section B: Applied Earth Science, 98, 34-37. Lopatkina, A.P. 1967. Conditions of accumulation of uranium in peat, Geokhimiya, 6, 708-719. Manskaya, S.M. and Drosdova, T.P. 1968. Geochemistry of Organic Substances. Pergammon Press, Oxford, UK. Minkkinen, P. and Yliroukanen, I. 1978. The arsenic distribution in Finnish peat bogs. Kemia.Kemi, 7--8, 331-335. Robinson, E. 1983. The Peat Resources of Jamaica and. Their Potential for Fuel Supply. Petroleum Corporation of Jamaica, Kingston, Jamaica. Robotham, H., Lalor, G.C., Mattis, A., Rattray, V. and Thompson, C. 1987. Trace elements in Jamaican softs. Part 1. Radioanal. Chem., 116, 27-34. Simpson, P.R., Lalor, G.C., Robotham, H., Hurdley, J., Mflodowski, A.E., Plant, J. and Smith, T.K. 1988. New evidence of epitherrnal gold potential in Andesific volcanics of the Central Inlier, Jamaica. Trans. lnstn Min. Metalt. Section B: Applied Earth Science, 97, B88-B91. Taylor, S.R. 1964. Abundance of chemical elements in the continental crust: a new table. Geochim. Cosmochim. Acta, 28, 1273-1285. (Manuscript No.244: received January 16, 1991, and accepted after revision December 2, 1991).

Elemental concentrations in Jamaican peat.

The peat of the Negril Morass in Western Jamaica was sampled at depths down to 7 m in directions parallel and perpendicular to the seashore, and the s...
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