Isotopic effects on inorganic carbon in a tropical river caused by caustic discharges from bauxite processing

Applied Geochemistry 16 (2001) 197±206

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Isotopic e€ects on inorganic carbon in a tropical river caused by caustic discharges from bauxite processing J.E. Andrews a,*, A.M. Greenaway b, P.F. Dennis a, D.A. Barnes-Leslie b a School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK Department of Chemistry, University of the West Indies, Mona Campus, Kingston, 7, Jamaica, WI

b

Received 19 February 1999; accepted 24 January 2000 Editorial handling by R. Harmon

Abstract Stable C isotope compositions of dissolved inorganic C (DIC) and carbonate sediment in a Jamaican river (Rio Cobre), are used as natural tracers of accidental spillage of bauxite processing liquor and waste water. Bauxite processing produces highly caustic (OHÿ and CO2ÿ 3 ) liquor and wash waters. These hydroxide-rich waters absorb atmospheric CO2 that is isotopically fractionated resulting in very negative carbonate d13C and d18O values. Accidental spillage of these liquors into rivers causes rapid precipitation of CaCO3 as a ®ne-grained suspension (`whiting') and subsequent deposition as calcite sediment. At the time of DIC sampling `whiting' was not evident; however, d13CDIC values at sites with a history of contamination were about 2- more negative than ambient values. The history of bauxite processing spillages is recorded in the d13C values of carbonate riverbed sediments. At sites known to be impacted, particulate carbonate samples have d13C values between ÿ11.2 and ÿ14.2-; values that are between 1 and 4- more negative than the predicted ambient d13C value. Similarly, d18O values of carbonate sediments at impacted sites are on average 2- more negative than those from sites above and below them, supporting the interpretation that the `whiting events' form precipitates with isotopically negative values. Contamination is quite localized because carbonate sediments downstream of impacted sites show no evidence of anomalous isotope values. This suggests that the particulate carbonate is either ¯ushed or re-dissolves, and is diluted downstream. The carbonate `whitings' are thus highly visual but relatively benign, although the associated pH and dissolved Al3+ and Na+ ¯ushes might have more serious impacts on the river environment. 7 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction The speciation, concentration and stable isotopic composition of dissolved inorganic C (DIC) in river waters are usually controlled by the input of natural

* Corresponding author. Tel.: +44-1603-592-536; fax: +441603-507-719. E-mail address: [email protected] (J.E. Andrews).

sources of C to the water. Most surface waters have pH values between 7 and 8, and under these conditions bicarbonate (HCOÿ 3 ) is the dominant DIC species. In limestone regions, the 3 main C inputs are: 1. dissolved CO2 from the decay of terrestrial vegetation in soils, or `soil C' CH2 O…org† ‡ O2…g† 4 CO2…aq† ‡ H2 O…1† ; 2. dissolution of atmospheric CO2,

0883-2927/01/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 8 8 3 - 2 9 2 7 ( 0 0 ) 0 0 0 3 3 - 0

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J.E. Andrews et al. / Applied Geochemistry 16 (2001) 197±206

CO2…g† ‡ H2 O…1† $ CO2…aq† ‡ H2 O…1† ; and 3. dissolution of limestone (CaCO3) bedrock ÿ 2‡ CaCO3 ‡ CO2…g† ‡ H2 O…1† $ 2HCO…aq† ‡ Ca…aq† :

Each of these inputs have a distinctive stable C isotopic composition (d13C): (1) soil C has a strong mode around 0ÿ24- (assuming vegetation composition is dominantly C3 plants; Deines (1980)); (2) atmospheric CO2 0ÿ7- (Keeling, 1960), and (3) marine limestone carbonate typically between ÿ3 and +3- (Hudson, 1977). These end member compositions can therefore be used to identify the dominant sources of C in natural waters. For example, in many fast ¯owing British and European tufa-depositing streams, the tufa precipitates have isotopically light d13C values indicative of a major soil C contribution to DIC (Andrews et al., 1993, 1997). The present study focuses on a tropical river, the Rio Cobre, Jamaica. This river system was chosen because in its upper reaches it is impacted by hydroxide and carbonate-rich waste waters from a bauxite processing plant. The Bayer Process (Hudson, 1987) Ð used to produce alumina from bauxite Ð involves; 1. the digestion of blended bauxites in concentrated caustic (equivalent to 03.5 mol dmÿ3 NaOH) at 01408C (low temperature process, 2508C for the high temperature process) and elevated pressure; Al2 O3 :3H2 O…s† ÿ …gibbsite† ‡ 2NaOH…aq† 4 2Na‡ …aq† ‡ 2Al…OH†4…aq†

or Al2 O3 :H2 O…s† …boehmite† ‡ 2NaOH…aq† ‡ 2H2 O 4 2Na‡ …aq† ÿ ‡ 2Al…OH†4…aq†

2. the separation of insoluble residues from the supersaturated aluminate liquor; and 3. the precipitation and separation of gibbsite from the clari®ed liquor ÿ ÿ 2Al…OH†4…aq† 4 Al2 O3 :3H2 O…s† ‡ 2OH…aq† :

The spent liquor is then recycled to process more bauxite. The gibbsite product is extensively water washed (the wash water is recycled) and the entire plant is surrounded by ditches designed to catch waste waters and return them to the plant. Every e€ort is made to contain the process liquor and wash waters,

but there are occasional spillages, particularly during heavy rainfall. The bauxite mining process ensures that negligible limestone enters the liquor. However, bauxites generally contain small amounts of organic matter (<1 wt% organic C) present predominantly as humic materials. A d13Corg of ÿ23.7- has been reported for a fulvic acid extracted from a blended Jamaican bauxite used as feed to a low temperature processing plant (Andrews et al., 1998). The bauxite digestion process can be expected to be very e€ective at extracting humics from the bauxite as it is similar to the method used to extract such materials from soils (e.g. Grith and Schnitzer, 1989). The recycling of the processing liquor leads to an accumulation of dissolved organic materials. Organic C concentrations in Bayer liquors of up to 25 gC dmÿ3 have been reported, 22±25% and 3±5% of which are considered to be fulvic and humic acid type materials respectively (Norman et al., 1997; Baker and Greenaway, 1998). The humic materials gradually undergo oxidative degradation to produce aromatic and aliphatic carboxylic and hydroxy carboxylic acids and eventually oxalate and carbonate ions (e.g. see Baker et al., 1995). The concentration of organic C in the processing liquor is controlled by removal of oxalate (e.g. see Lever, 1978). Since the liquors are largely exposed to the atmosphere during processing, and because of the degradation of organic matter in the liquors, carbonate concentrations reach about 0.8 mol dmÿ3. Absorption of atmospheric CO2 in these hydroxide-rich waters is probably the main carbonate source, and the reaction is likely to cause strong isotopic fractionation. The reason for isotopic disequilibrium in hydroxide-rich waters has been demonstrated by Clark et al. (1992). They showed that uptake of atmospheric CO2 by Ca(OH)2 spring waters in Oman was accompanied by isotopic fractionation to produce very negative travertine calcite d18O and d13C values. In a series of laboratory studies Clark et al. (1992) suggested that 18O depletion is caused by an enhanced reaction rate between aqueous CO2 and 16OHÿ compared with 18 OHÿ, the isotopic signature then being transferred to the precipitating calcite. Similarly, a kinetic depletion of around 15.5- for 13C also occurs during hydroxylation of aqueous CO2 where the 12CO2(aq)+OHÿ (aq) 4 H12COÿ reaction rate is faster than the 3(aq) 13 13 ÿ CO2(aq)+OHÿ (aq) 4 H CO3(aq) reaction rate. Again, the isotopically negative HCOÿ 3(aq) imparts its signature to the precipitating calcite. These highly caustic and carbonated bauxite-processing liquids, when accidentally mixed with natural waters by spillage, cause rapid precipitation of CaCO3 as a ®ne-grained suspension (`whiting') which results in a temporary cloudiness in the water. The whiting and sediments produced from whitings should be isotopi-

J.E. Andrews et al. / Applied Geochemistry 16 (2001) 197±206

cally labelled by the disequilibrium processes described above. We therefore used stable C isotopes to try and assess the impact of these anthropogenic events on the overall isotopic composition of DIC and precipitated CaCO3 in the river. 2. The Rio Cobre catchment and sample sites The 30 km long Rio Cobre drains an area of 1256 km2 (Fig. 1) rising in the Benbow Inlier (undi€erentiated Cretaceous sedimentary and volcanic rocks) at an elevation of 300±500 m. These rocks are in the centre of the island, but the river mainly ¯ows over the Eocene±Miocene aged White Limestones (Hose and Versey, 1956). This study mainly focuses on the upper part of the catchment, where small springs feeding the

199

tributaries (e.g. the Black River, Jericho and Byndloss Gullies) come from soil and shallow groundwaters in the White Limestone. The exposed limestones in this part of the catchment have low Mg±calcite compositions (<2 mol% MgCO3). These upper catchment tributaries respond quickly to storm events, suggesting that direct runo€ and shallow soil-zone through-¯ow provide a strong component of total ¯ow. Pleistocene± Holocene alluvium in the upper catchment is present only in the upper part of Byndloss Gully (above the sample site). Valley ¯oor alluvium is more common in the central (Linstead±Bogwalk) and lower (Spanish Town±Portmore) parts of the catchment. The underlying bedrock in this area is again mainly the White Limestone, which toward the base becomes more magnesian, and in places dolomitic (Hose and Versey, 1956). These

Fig. 1. Maps showing (A) the Rio Cobre, the bauxite processing plant marked `B and the downstream sample sites (site numbers in Tables) and (B) the details of the main sample sites (1±5) in the vicinity of the bauxite processing plant. Box B is approximately 3 km wide. The position of the Rio Cobre is shown in the inset map of Jamaica.

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J.E. Andrews et al. / Applied Geochemistry 16 (2001) 197±206

dolomitic horizons may interact with groundwater in the middle and lower catchment, and certainly a large spring provides considerable ¯ow to the Rio Cobre at Wake®eld near Bogwalk (Fincham and Ashton, 1967). Potential organic matter inputs to the river water include natural vegetation, soil organic matter and agricultural sources, augmented in the lower part of the catchment by citrus and milk product processing wastes (around Bogwalk, Fig. 1), land®ll leachate (particularly below Spanish Town), and residential and industrial sources (particularly in the Linstead, Bogwalk, Spanish Town and Portmore areas, Fig. 1). The Rio Cobre becomes estuarine as it drains into Hunts Bay in the NW corner of Kingston Harbour (Fig. 1). The sample sites from the upper part of the catchment (the main focus of this study) were as follows (see also Fig. 1). Site 1 on the Black River about 500 m upstream of its con¯uence with the Jericho Pumping Station (JPS) stream and 20 m upstream of a weir. The river here is about 10 m wide and 0.5 m deep. Site 2 is located approximately 500 m below the con¯uence with the Byndloss Gully, 300 m below a 50 m long rapid where the water ¯ows at a depth of about 0.3 m over large stones. This e€ects ecient mixing of the component waters. The river at the site is about 1 m deep and 10 m wide. Site 4 (JPS) is a fast ¯owing 1± 2 m wide, 0.3 m deep stream, white site 5 is 20 m upstream of the con¯uence of the Byndloss Gulley with the Rio Cobre. The gully is a 0.1±0.2 m deep 10 m wide slow ¯owing stream that occasionally stops ¯owing. The sample sites from the middle and lower part of the catchment (Fig. 1) include site 3, a shallow fast ¯owing section (0.3 m deep, 15 m wide) of the Rio Cobre in the vicinity of the Linstead township. Site 6 is within 500 m of the main road bridge where the river enters a gorge that extends from Bogwalk to the outer reaches of Spanish Town. The river here is about 10±20 m wide and about 1±2 m deep. Samples were taken from the eastern bank. Site 7 is at a fast ¯owing shallow section of the river well below any entering gullies/drains and about 5 km from Hunts Bay. All samples were taken from a depth of 15 cm midstream unless otherwise mentioned.

3. Methodology Standard physical and chemical water parameters (i.e. temperature, conductivity and pH in the ®eld and alkalinity (Gran titration method), suspended solids, and Na+, Mg2+ and Ca2+ ion concentrations (AAS) upon returning to the laboratory) were measured at sites 1±7 (February 1993±November 1994) to constrain the carbonate chemistry of the river system and to

augment the data from an earlier study (Greenaway and Parkin, 1993; Parkin, 1992). Water samples for DIC were collected on 14 January 1994 from a depth of about 15 cm in 200-ml glass medical bottles with screw caps at sample sites 2, 4, 6 and 7 in the Rio Cobre (Fig. 1). Dissolved inorganic C was precipitated from water samples as SrCO3 following the method of Bishop (1990). In addition, ®negrained river sediments were collected to augment samples already collected by Greenaway and Parkin (1993). Representative limestone rock samples (sites 4, 8 and 9, Fig. 1) from the area were sampled to constrain inputs from limestone weathering, and a sample of bauxite processing liquor was intended to constrain the bauxite processing input (but see results). Stable C and O isotope analyses from CaCO3 and SrCO3 solids were made exactly as described in Andrews et al. (1993). Organic matter was removed from sediment samples prior to isotopic analysis by low temperature plasma ashing. Isotope ratios were measured with a VG Sira Series II mass spectrometer. The machine was calibrated using the NBS 19 and NBS 18 standards and results are expressed relative to the VPDB scale. Replicate analyses of a Carrara marble laboratory standard gave a 2s precision of 20.16-. Yields of CO2 measured during isotope preparation were used to calculate weight percentage of CaCO3 in the sediment samples. 4. Geochemical background The routine water sampling and chemical analyses of Greenaway and Parkin (sites 1±5, 1993; Tables 1 and 2) showed that bauxite process liquor spillages reach the Rio Cobre through the Jericho Pumping Station (JPS) stream and the Byndloss Gully (BG; sites 4 and 5, Fig. 1). Over the 2 a period (1987±1989) of that study the JPS had consistently high Na (Tables 1 and 2) and Al (7.5±80 mmol dmÿ3) concentrations with 2 Na (12,400 and 11,400 mmol dmÿ3) and 4 Al (914, 455, 185 and 199 mmol dmÿ3) extreme episodes. By comparison the Na concentrations at site 1 (upstream of the bauxite processing plant) averaged 243 mmol dmÿ3 and Al was generally lower than the 2 mmol dmÿ3 detection limit. The pHs and alkalinities were generally similar to other Rio Cobre sites although the extreme Na concentrations were accompanies by correspondingly high pHs and alkalinities and very high calcite supersaturations (SI data in Tables 1 and 2). The JPS stream averaged 27.98C and was consistently at least 28C warmer than the waters at other sampling sites suggesting that spillage reaching there had a warm liquor origin. Byndloss Gully behaved di€erently (Tables 1 and 2); alkalinities, Al (average 43.9, range 4±115 mol dmÿ3)

J.E. Andrews et al. / Applied Geochemistry 16 (2001) 197±206

and Na were consistently high and extreme events less apparent. Occasionally at this site the gully bed was covered with a white precipitate (shown to be CaCO3 by Ca2+ and CO2ÿ analyses) and the water was 3 cloudy. Samples from BG were always dicult to ®lter suggesting abundant ®ne-grained suspensions (a feature not experienced with samples from JPS). Data collected from sites 1±7 during the present study (February 1993±November 1994, Table 2 for the monitoring data and Table 3 for the DIC sampling data) indicate that the contamination from the processing plant was not as marked, particularly at site 5. The water chemistry data from the present study (Tables 2 and 3) are consistent with the earlier data, with all data showing that the water at all sites was close to saturation or supersaturated with respect to calcite unless the pH dropped below 7.3 (9 of 152 observations). 5. Stable isotope results and interpretation The average non-impacted Rio Cobre d13CDIC value is ÿ11.7- (Table 3). This value is slightly more negative than the usual range of riverine d13CDIC values (about ÿ6.0 to ÿ9.0-, according to Anderson and Arthur (1983), table 1±6), and implies that a large component of isotopically light C contributes to the Rio Cobre DIC. If all the C in these waters came from a C3 soil C source (0ÿ24-, based on sediment and fulvic acid data from the Rio Cobre in Andrews et al. (1998)), d13CDIC in equilibrium with d13C soil CO2 at ®eld water temperatures (25±308C) would be about

201

ÿ15- (using the Emrich et al. (1970) relationship). If it is assumed that limestone rock weathering and/or atmospheric CO2 are the other main sources of C, with equilibrium d13CDIC values around 0- and +1- respectively, then the average measured value of ÿ11.7suggests that roughly 80% of the Rio Cobre d13CDIC has a soil CO2 source (see also Fig. 2). This calculated percentage would change with weathering of certain types of limestone rock (typical d13C values for this catchment in Table 3) or if C4 plants (e.g. sugar cane), were important C sources. However, isotopic analysis of Rio Cobre suspended particulate organic matter and sediment particulate organic matter, con®rm that C3 organics are dominant (Andrews et al., 1998, table 1); the calculated percentages would be relatively insensitive to the envisaged small inputs from these other sources. In short, the Rio Cobre d13CDIC has a major soil CO2 source. The d13CDIC of the spent processing liquor (liquor left after gibbsite precipitation) could not be accurately determined, probably because the method could not precipitate all of the DIC in the liquor. (The measured value was ÿ7.2-, but reasoning below shows that the liquor value should be much more negative.) Atmospheric CO2 is a major C source in the liquor because most of the processing is done under very caustic conditions in tanks open to the atmosphere. These conditions almost certainly cause disequilibrium isotope e€ects during CO2 uptake (Clark et al., 1992). Some C will also come from organic matter in the bauxite. Fulvic acid extracted from a Jamaican bauxite has a d13C of ÿ23.7- (Andrews et al., 1998, table 3), typical of degraded terrestrial C3 organic matter.

Table 1 Rio Cobre water chemistry data collected between December 1987±December 1989a. From Greenaway and Parkin (1993) and Parkin (1992) Site pH

Alkalinity (meq dmÿ3) Na+ (mmol dmÿ3)

Hardness (meq dmÿ3)

SIb

1

4.0 (0.5, 13) (2.9±4.7) 5.3 (0.8, 12) (3.4±6.2) 5.3 (0.6, 19) (3.6±6.0) 6.0 (1.8, 20) (4.2±11.8) 8.3 (1.0, 18) (6.4±10.1)

2.1 (0.2, 13) (1.5±2.3) 2.1 (0.2, 12) (1.6±2.4) 2.2 (0.3, 19) (1.6±2.5) 2.1 (0.8, 19) (0.8±4.8) 1.7 (0.5, 18) (0.4±2.5)

4.9 (1.5, 12:1)c (2.0±10.0) 2.7 (1.2, 12) (0.7±4.6) 2.4 (0.8, 19) (0.7±3.5) 4.4 (1.9, 16:4)c (1.8±10.8) 4.9 (2.5, 11:7)c (0.7±21.2)

8.2 (0.1, 13) (7.9±8.4) 7.7 (0.3, 12) (7.2±8.0) 7.6 (0.2, 19) (7.1±8.0) 8.3 (0.7, 20) (7.6±10.0) 8.1 (0.5, 20) (7.0±8.9)

2 3 4 5

240 (32, 13) (180±300) 1780 (1000, 12) (380±3410) 1340 (570, 19) (400±3100) 3930 (2940, 20) (1530±12,400) 5060 (1860, 18) (1310±8800)

Data in brackets are arranged in the following order: X (s, n ) (min±max). 2+ 2+ 2+ Saturation Index=(0.7  (0.8[Ca +Mg ])  0.7[CO2ÿ +Mg2+] from hardness titrations, 0.8 Ð the fraction of 3 ])/Ksp: [Ca hardness present as Ca2+, see footnote to Table 2. 0.7 Ð activity coecient for cations and anions; Ksp=4.5  10ÿ9. (See e.g. Freeze and Cherry, 1979; Krauskopf and Bird, 1995.). c SI values > 8.0 have not been included in the averages, the number of observations excluded from the averages is given after the colon. a

b

3.7 (0.4,7) (3.4±4.3) 4.4 (0.6, 7) (3.4±5.1) 4.6 (0.6, 7) (3.6±5.5) 5.2 (1.7, 7) (3.8±8.6) 6.6 (1.3, 7) (5.3±9.2) 4.4 (0.3, 6) (3.8±4.7) 4.2 (0.4, 6) (3.6±4.5)

Alkalinity (meq dmÿ3) 320 (100, 7) (140±460) 1770 (1050, 7) (570±3570) 1380 (710, 7) (520±2610) 4240 (1240, 7) (2450±6220) 2550 (530, 7) (1840±3180) 570 (100, 6) (450±740) 630 (100, 6) (500±760)

Na+ (mmol dmÿ3) 1.5 (0.9, 6) (0.2±2.4) 1.8 (0.7, 6) (0.5±2.4) 2.0 (0.8, 6) (0.6±2.6) 1.4 (0.8, 6) (0.4±2.4) 2.1 (0.8, 6) (0.8±3.0) 1.7 (0.8, 6) (0.5±2.5) 1.8 (0.8, 6) (0.7±2.6)

Ca2+ (mmol dmÿ3) 0.4 (0.2, 6) (0.2±0.6) 0.4 (0.1, 6) (0.2±0.5) 0.4 (0.1, 6) (0.2±0.5) 0.3 (0.1, 6) (0.2±0.4) 0.3 (0.1, 6) (0.2±0.4) 0.4 (0.1, 6) (0.2±0.5) 0.4 (0.1, 6) (0.2±0.5)

Mg2+ (mmol dmÿ3)

b

Data in brackets are arranged in the following order: X (s, n ) (min±max). 2+ 2ÿ Saturation Index=0.7[Ca ]  0.7[CO3 ]/Ksp. [Ca2+]/([Ca2+]+[Mg2+]): average=0.76 (0.09, 66), range 0.45±0.92; 10 ratios < 0.7. c SI values > 8.0 have not been included in the averages, the number of observations excluded from the averages is given after the colon.

7

6

5

4

3

a

8.2 (0.2, 7) (7.8±8.4) 7.8 (0.2, 7) (7.5±8.0) 8.0 (0.4, 7) (7.6±8.8) 8.7 (0.4, 7) (8.2±9.2) 7.5 (0.2, 7) (7.2±7.8) 7.8 (0.5, 6) (7.3±8.8) 8.2 (0.2, 6) (7.9±8.4)

1

2

pH

Site

Table 2 Rio Cobre water chemistry data collected between February 1993±November 1994a

3.5 (2.8, 6) (0.7±8.0) 2.1 (1.4, 6) (0.8±4.4) 3.0 (1.2, 5:1)c (1.5±8.1) 5.3 (0.4, 3:3)c (4.8±23.7) 1.8 (0.8, 6) (0.5±2.7) 2.9 (2.3, 6) (0.3±7.1) 4.4 (3.4, 4:2)c (1.2±10.4)

SIb

202 J.E. Andrews et al. / Applied Geochemistry 16 (2001) 197±206

Sample site

Description

b

a

pH in the Gully itself was 7.1. (WE) denotes major whiting event.

Dissolved inorganic carbon Ð Rio Cobre Impacted sites 14194-1 2 Immediately downstream of Byndloss Gully 14194-2 4 Jericho Pumping Station Mean of impacted sites Non impacted sites 14194-7 6 Bog Walk 14194-10 7 Crum Ewing Bridge Mean of non impacted sites Carbonate river sediment Ð Rio Cobre catchment Impacted sites 18588 4 Jericho Pumping Station 16189 4 Jericho Pumping Station 18588 5 Byndloss Gully 16189 5 Byndloss Gullyb (WE) (duplicate) Mean of impacted sites Non impacted sites 14194-4 1 Black river (above bauxite plant) 16288 3 Linstead 27788 3 Linstead 281288 3 Linstead 14194-11 7 Crum Ewing Bridge 9394 7 Crum Ewing Bridge Mean of non impacted sites Carbonate rocks in Rio Cobre catchment 14194-3 4 Jericho Pumping Station 14194-8 8 Limestone Gorge below Bog Walk 14194-9 9 Winsor Heights, St Catherine (Ferry Inn)

Sample No. (date)

ÿ5.8 ÿ8 ÿ8.6 ÿ8.4 ÿ8.2 ÿ7.8 ÿ6.1 ÿ5.2 ÿ6.8 ÿ5.6 ÿ5.5 ÿ4.7 ÿ5.6 ÿ3.7 ÿ2.5 ÿ4.9

ÿ11.2 ÿ14.2 ÿ11.5 ÿ13.7 ÿ13.6 ÿ12.8 ÿ9.3 ÿ8.1 ÿ9.2 ÿ9.7 ÿ9 ÿ9.8 ÿ9.2 ÿ7.5 ÿ0.1 ÿ4.4

2.08

4.00 3.33 2.58 5.67

7.70 8.20

pH

ÿ12.5 ÿ10.8 ÿ11.7 13.83 43.50 0.42 79.08

Wt% CaCO3

7.2a 8.40

d18O (- VPDB)

ÿ13.8 ÿ14.4 ÿ14.1

d13C (- VPDB)

Table 3 Isotopic and associated data for Rio Cobre carbonate sediments, DIC and catchment rocks

5.00 4.00

4.80 4.60

alkalinity (meq dmÿ3)

0.74 0.82

1.42 0.42

Ca2+ (mmol dmÿ3)

0.9 2.9

0.6 2.4

SI

J.E. Andrews et al. / Applied Geochemistry 16 (2001) 197±206 203

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J.E. Andrews et al. / Applied Geochemistry 16 (2001) 197±206

The DIC of river water precipitating carbonate `whitings' after a liquor spillage, would presumably have a d13CDIC value from 3 sources (see also Fig. 2); (1) from the bauxite processing liquor (value unknown); (2) from the ambient Rio Cobre Ð including rock weathering and soil organic matter sources Ð (value around ÿ11.7-); and (3) from atmospheric CO2 that is absorbed into liquor contaminated waters (equilibrium value probably around +1-). Typical alkalinities for uncontaminated waters in the area are about 4 meq dmÿ3, whereas contaminated waters have alkalinities of 9 meq dmÿ3. Calculations based on the Na+ and CO2ÿ ion concentrations during a `whiting 3 event', and known processing liquor Na+ and CO2ÿ 3 ion concentrations (Greenaway and Parkin, 1993), show that about 20±30% of the alkalinity is supplied by the spilt liquor itself (approximately 2.5±3 meq dmÿ3). This implies that the additional 2.5 meq dmÿ3 of alkalinity (required to total 9 meq dmÿ3, as above) is supplied by atmospheric CO2 absorbed into the impacted stream water, with the possibility of limited fractionation due to the initial high pH. On this basis, an isotope mass balance calculation, i.e.:

liquor spillage (Tables 1 and 2). In January 1994 the Rio Cobre d13CDIC values (Table 3) immediately downstream (site 2, Table 3) of JPS and BG (Fig. 1) were about 2- more negative than ambient values, con®rming that bauxite processing contaminant was impacting the Rio Cobre DIC at that time. The progressively higher DIC values further downstream at Bogwalk and Crum Ewing Bridge (Fig. 1 and Table 3) suggest that those sites were apparently not impacted. These values are probably caused by downstream equilibration between atmospheric CO2 and Rio Cobre DIC. It is also possible that contributions from tributaries and regional groundwater with di€erent d13CDIC characteristics were also mixed in below Linstead (Fig. 1) as Fincham and Ashton (1967) note the presence of large springs in this vicinity. The possible longer-term e€ects of bauxite processing liquor spillages on the river may be recorded in carbonate sediments. When CaCO3 precipitation occurs it is well known that the solid should record values close to the DIC isotopic composition. Data from sediment samples collected between 1987 and 1994 (Table 3) yielded the following information.

4 2:5 2:5 …ÿ11:7-† ‡ …x-† ‡ …‡1:0-† ˆ ÿ149 9 9

1. The Black River site (above the JPS and BG impacted sites), and all sites downstream of these impacted sites have sediment d13C values between ÿ8.1 and ÿ9.8- (mean ÿ9.2-, Table 3). Carbonate precipitated in isotopic equilibrium with mean non-impacted Rio Cobre DIC (d13CDIC=ÿ11.7-) at ®eld temperatures should have a d13C around 1.5- less negative (Emrich et al., 1970) i.e. 0ÿ10.2-. The measured values around ÿ9-, therefore suggest that the river sediments have a small amount of an isotopically heavier carbonate mixed with them. This is probably ®ne-grained carbonate rock (d13C values between 0 to ÿ7-, Table 3). 2. At the sites known to be a€ected by processing liquor (sites 4 and 5; Fig. 1) particulate carbonate samples taken on 18 May 1988 (Table 3) have d13C

(where fractions are based on alkalinity data above; ÿ11.7-=d13CDIC ambient Rio Cobre; +1.0=d13CDIC equilibrated with atmospheric CO2; ÿ14=d13CDIC impacted sites. Solving for x yields a value of ÿ33- for d13CDIC in the Bayer process liquor) suggests that the Bayer process liquor has a d13CDIC of about ÿ33- (24-), consistent with marked disequilibrium isotope e€ects. As the time of DIC sampling (January 1994) the JPS and BG streams (sites 4 and 5, Fig. 1) had pHs of 8.4 and 7.1 and alkalinities of 4.6 and 4.8 meq dmÿ3 (Table 3). These values are similar to, or slightly lower than longer-term average data (Tables 1 and 2), and much lower than the very high pHs caused by plant

Fig. 2. Representation of the ranges in d13C values encountered in this study. The pecked arrow represents probable equilibrium between soil CO2 gas and Rio Cobre DIC. The estimate of d13C disequilibrium in the Bayer process liquor (about 10-) is consistent with the experimental open system values reported by Clark et al. (1992).

J.E. Andrews et al. / Applied Geochemistry 16 (2001) 197±206

values of ÿ11.2 and ÿ11.5-, while samples taken at the same sites on 16 January 1989 (Tables 1 and 2) have d13C values of ÿ14.2 and ÿ13.7- (see also Fig. 2). These values are all more negative than the predicted ambient d13C carbonate value of 0ÿ10.2-. This con®rms that the sediments at the impacted sites contain isotopically light carbonate, presumably resulting from isotopic disequilibrium in the process liquor, and/or during uptake of atmospheric CO2 by the hydroxide-enriched river waters immediately after spillage. `Whiting precipitates' have been observed at BG (site 5, Greenaway and Parkin, 1993) and at JPS (site 4) although sometimes masked by red clay sediment at the latter site. Note the very high wt% CaCO3 content of the sediments in samples from JPS and BG collected on 16 January 1989 (Table 3) Ð the samples that have the most negative d13C values. The authors strongly suspect that disequilibrium precipitates are responsible for these d13C values, in part because the values are more negative than those of the ambient Rio Cobre DIC. Also because calculation of the CO2ÿ 3 ion concentration in the Byndloss Gully during a `whiting event' show that about 20±30% of the DIC is supplied by the spilt liquor itself (as discussed above). Finally, the data in Table 3 also show that these sites do not always contain high amounts of isotopically negative CaCO3 (see e.g. site 5 data 18 May 1998, Table 3). This suggests that the CaCO3 deposit is prone to resuspension and transport, implying that although the amount of deposited sediment changes temporally, the d13C values remain robust indicators of a contamination event. 3. The d18O values of CaCO3 from the liquor impacted sites (sites 4 and 5) are on average 2- more negative than the sites above and well below them (Table 3). This supports the interpretation that the `whiting carbonates' are disequilibrated precipitates because isotopically negative values are known from other high pH waters (see Clark et al., 1992).

6. Discussion and wider implications The isotopic data presented above show that riverine carbonate contamination from bauxite processing is recorded in the immediate vicinity of the impact, by anomalous isotopic compositions of the DIC and carbonate sediment in the river bed. Although at the time of DIC sampling an obvious `whiting-event' Ð caused by spillage of process liquor or wash water Ð was not occurring, the d13CDIC at sites with a history of contamination was still isotopically anomalous. At these impacted sites, carbonate sediment precipitated from a

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major `whiting-event', and the carbonate bed sediment at other times, both show clear isotopic e€ects of spillages. Further downstream, carbonate sediments contain no evidence of impact, showing that contamination is quite localized (as already shown for sediment Al, Cr and As in the Rio Cobre by Greenaway and Parkin (1993)). This suggests that after `whiting-events' most of the particulate carbonate is either ¯ushed and diluted downstream (springs are known to contribute ¯ow to the river in the Bogwalk area (Fincham and Ashton, 1967), or that it re-dissolves to contribute to the DIC (SI < 0.7 when pHs drop below 7.3; Tables 1 and 2). In either event, it no longer has a clear impact on the isotopic composition of the river's sediment carbonate or DIC. The carbonate `whitings' are thus highly visual but relatively benign in terms of impact, although the associated pH and dissolved Al3+ and Na+ ¯ushes (Greenaway and Parkin, 1993) might be much more serious for disrupting river ecology. The ®ndings presented here can be applied to other systems that are suspected to have been impacted by caustic, carbonated discharges or spillages. Preliminary work might centre on the analysis of carbonate river bed sediments, to establish the presence of historical contamination, as recommended by Greenaway and Parkin (1993) based on metal data. Once this is established, a more extensive programme of water chemistry monitoring and sampling should identify low level contamination, while major events will be obvious (through `whiting' formation) although short lived.

Acknowledgements This work stems from a British Council funded link in Environmental Chemistry between the Chemistry Department (UWI) and the School of Environmental Sciences (UEA). The School of Environmental Sciences (UEA) provided funds for isotopic analyses. AMG and DAB-L thank Alcan Jamaica Ltd, Nestles Jamaica Ltd and Alkali Ltd for ®nancial support. Bill Corbett (UEA) did some of the wt% CaCO3 determinations, and Phil Judge and Sheila Davies (UEA) assisted with the diagrams.

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