Trace metal deposition and mobility in the sediments of two lakes near Sudbury, Ontario

Trace metal deposition and mobility in the sediments af two lakes near Sudbwy, Ontario INRS-Eau, ~nive~t~ du Q&XC, C. P. 7500, Ste-Foy, Q.&X,

Canada, GlV 4C7

.f. 0. NltlAGU National Water Research fnstitute. 857 Lakeshore Rd., Burlington, &&xio Canada, tfR

445

Abstract-The accumulation and mobility of Fe, Mn, Ai, Cu, Ni and Pb in the sediments of two lakes @atwater, pW 4.5; and McFarlane, pH 7.5) near Sudhury, Ontario have heen inves&ated. The PII, Cu and Ni concer@atians are expect~y &&iv&y high in the overlong waters of C&x-water Lake a& much fawtt~for AI and Cu in McFariane Lake* The low trace metal concenPrations found in the anoxic porewaters of Clearwater Lake cotdd be expitined by a sharp increase in porewater pH concomitant with SO:- r~du~ioR and &S Mouton within the first l-2 cm of the sediments, which has conceivably led to the ~~~ipi~tion of mine& phases such as AL(O&, NiS, and CuS. In both lakes, Fe concentrations in anoxic porewaters appear to be controIled by FeS and/or FeCO3 fomation. Solubility calculations als~ indicate MnCOs precipitation in M&Wane Lake. In Clearwater Lake, however, both porewater and total Mn were relative&y low, a possible m&t of the continuous loss of MI@) through the acidic interface. It is suggested that upwardIy decreasittg total Ma pm&s resulting from the removaf of MR from the top sediment layers under acidic conditions may constitute a reliable symptom of recent lake acidification. The downward diffusion of Al, Cu and Ni from the over&& water to the sediments has been estimated from theif con~nt~~on ~~~n~ at the int~a~ and compared tct their total ~urn~~~~on rates in the sediments. In both lakes the dieon of Al is negligible compared to its a~urnulati~~ rate. However, diffusion accounts for 2442% of the accumulation of Cu in the sediments of Clearwater Lake, but appears negIigible in Mci?arlane Lake. The do-ward diffusive flux of Ni is important and may explain 76461% of the estimated Ni aaztmulation rate in CEeazwater Lake, and 59% in McFarlane lake. The porewater Cu and Ni pro&s suggest that the subsurface sedimentary trace met& peaks observed in Clearwater Lake (as in other acid lakes) may not ti caused by sediment leaching or by a recent reduction in ~men~tio~ but may have a diagenetic origin instead. Diffusion to the sediments thus appears to be an irn~~~t and pre~ou~ly averlooked trace metal deposition m~han~m, ~~cula~y in acid lakes. XNTRODUCI-ION IT tS WIDELY recognized that industrial activities have considerably enhanced the atmospheric emission and subsequent de~siti~n of several elements, including many trace metals, Our best evidence for this recent increase comes from chronological reconstructions of trace metal deposition in dated lake sediments (ROBBINS and EDGINGTOPJ, 1975; NRIAGU et at, 1982; GALKWAY and LIKENS, 1977; KAHL and NORTON, 1983). As dicta out by some fathom, the inte~retation of dated trace metal profiles as hi~o~cal records of deposition assumes the absence of any significant post-depositional remobi~i~~on or transport mechanism within the sediment column. However, several recent observations suggest that postdepositional mobility can occur in the su&ial sediments of acid lakes. For example, pronounced peaks of total Cu, Ni, Zn, Cd, Pb and Fe are found below the sediment surface (Z-4 cm), and within the “‘Pb defined mixed layer of some acid (pIi 4.2-4.6) fakes of Ontario (OME, 1982). Similar observations have been reported in lakes of other areas (K.&HI, and NORTON, 19831, Such profiles have either been interpreted as the result of decreased sedimentation of trace metals in progressively more acidic takewaters,

or as the result of a post-depositional leaching of tbe metals from sur%ciai sediment layers by overlying waters that have been recently acidified, Subsurface peaks in trace metal concentration conceivaMy can also result from the diffusive transom of d~~~lv~ trace metals from regions of high concentrations (acidic overlying waters) to some hypothetical sedimentary sink (adaptions p~~pi~~~~n~ located a few centimeters betow the lament-~ter interface. These three different interpretations have considerable implications in our undemanding of trace metal dynamics in lakes. Although post-depositional mohihty has been d~monstmted for Fe and Mn (Rcrss~s and CALLENDER, L975; BEKNER, L!XO), few s@_~%es have yet addressed the question for other metals. The active ~st~e~sitiona~ leaching of sediment trace metals by acidic waters should be, in principle, easily verifiable from porewater studies since any significant diff%siue ioss from the sediments to the overlying water should be apparent from the presence of marked concentration gradients at the sediment-water interface, Similarly, the presence of an important dcrwnward diffusive flux from the overlying water to a subsurface sink should be apparent from porewater profiles. We have tested these possibilities

1753

by studying

1754

R. Carignan and J. 0. Nriagu

the distribution of several metals in the porewaters and sediment solids of two lakes. one acidic (Clearwater. 46O22’N. 8 1’03W; pH 4%4.h: znllr = ? 1..i m: f = 8.4 m: area = 76.5 ha), and the other one slightly alkaline and well buffered (McFarlane. 46”2S’N, 80”57W: pH 7.5-8.0: zmeh = 20.0 m; f = 7.3 m; area = 141 ha). The two lakes are located in the Sudbury area (north-eastern Ontario), about 1.5km south of the m~~l~iferous (NiS, CuS) Sudbury Nickel frruptive. A detailed description of the bedrock and surface deposit geology of the area can be found in CARD (1978). BURWASSER(1979) and OME ( 1982). The watersheds of both lakes are characterized by the predominance of exposed bedrock (quartzite, gabbro, felsic gneisses) typicat of the Canadian Shield. with relatively rare pockets of surficial deposits and forest cover confined to local depressions. The lakes of this region are particularly suited to this type of study because of the high concentrations of pollutant metals found in both their waters and sediments (NRIAGU ef al., 1982: YAN and DILLON, 1984; DILIBN and SMITH, 1984). The lakes are located only 5 km apart and therefore receive similar atmospheric loadings of trace metals from the smelters nearby. The effects and history of trace metal pollution in this area have been extensively documented (e.g. HUTCHINSON. 1979: OME. 1982).

spectromety on diluted sub-samples and trdce metals were measured usmg a Varian GTA-95 graphite furnace. Porewater Zn could not be measured in this stud\ because of a~ important contamination (l-15 ppb) or$nating from thr: sampling vials. The porewater profiles of Cu. 4I and I-P obtained in 1982 for the 15 and 20.2 m stations showed a pronounced maximum 1-2 cm below the interface which was apparently due to the formation of Fe(OHbi and concomitant scavenging of Cu and Al within the dialyzer‘x chamber. This artifact was su~quentiy avoided tn 14XI84 by limiting the equilibration time to I5 days. AI1 other methodological detaiis concerning the handling of the dralyzers and the measurement of porewater pH. H&O,. CH,. NH: and PO:- concentrations are given elsewhere (C‘~KIGNAN.1984).

RESULTS AND DISCUSSSIOR

The total metal concentration profiles m the sedlments of both lakes (Figs. 1 and 2) are essentIalI> similar to those reported for the same lakes in other studies (OME, 1982: NRIAGI~fl al., 19821, In relation to the deeper layers. the top 6- 10 cm of all profiles shows a pronounced enrichment in Cu and Vi which has been attributed to the increased anthropogenic deposition of both metals from smelting activities which began in 1888. Many of the sediment profiles show a more or less pronounced subsurface maximum in Cu and Ni within the topmost 3-3 cm for Clearwater Lake, and 6-8 cm for McFarlane Lake. .:“I METHODS notable exception to this trend is the 20.2 m C’u Sediment cores were collected with a lightweight piston corer (WILLIAMSand PASHLEY, 1979) in June 1982 at 10.0, profile of Clearwater Lake, where the subsurface maximum was absent. As stated above, several factors 15.0, and 20.2 m in Clearwater Lake. and at 19.8 m in McFarlane Lake. The cores were sectioned every cm within (decreased sedimentation due to the leaching of susone hour of collection. Freezedeed subsamptes were digested pended solids or surficial sediments. diffusion below with a HClO,-HF mixture. and analyzed for total Al, Fe. the interface) may result in the establishment 01 Mn, Cu and Ni by atomic absorption spectrometry (VARIAN, subsurface peaks. An additional complication also 1979) corrected for background absorption. Porewater samples were obtained by dialysis at the same arises from the fact that trace metal deposition ma> stations (within 3 m of the coring sites) in September 1982. have recently decreased in this area since the con1983 and in June 1984. The samplers used in the study struction of the 381 m superstack at Copper Cliff in were similar in design to those described by HEUBLEIN (1976a), and contained two vertical rows of 6.5 cm Iong 1972. However, the absence of a subsurface maximum in total Cu at the Cleanvater 20.1 m station suggests X 6.5 mm wide X 5 mm deep horizontal chambers spaced 1 cm (center to center) and covered by a Gelman HT-450 that factors other than decreased deposition may or HT-200 membrane. The empirical equilibration time (at contribute to the shape of the observed profiles (see 23°C) for trace metals was found to be approximately one below). week in similar sediments. and porewater trace metal conTotal Fe shows a marked enrichment in the upper centrations obtained by this technique (Pb. Zn, Ni, Co. Cu. 10 cm of a11profiles. This increase cannot be att~buted Cd. Cr) were comparable lo those found by careful centrifugation/filtration techniques performed under anoxic conentireIy to the upward post-depositional migration of ditions (CARIGNAN Ed al.. unpublished). The samplers were Fe2+ (BERNER, 1980) since the Fe enriched layer left in the sediments for 30 days in 1982, 15 days in 1983usually extends well below the oxidized surhcial layer 1984, and all the samples were removed from the chambers of the sediments, as indicated by the presence of H$ within 5 minutes following retrieval from the sediments. The first vertical row of chambers was used for pH, ZH*S. and CH, below 2 cm in most porewater profiles ZC02 and CH, analyses. Samples for ZH2S measu~ments (Tables 1-2). Other causes must then be rnvoked to were collected in 1 ml glass syringes purged with NI and explain the observed Fe enrichment. The retativelq the calorimetric reagent KLINE, 19691 iniected within 15 bulk Fe deposition i 100-400 minutes of collection. Samples from the second row were high atmospheric mg . me2 *y-l) observed in the vicinity of Sudbur) kept for NH;, SO:-. PO:-, H,SiO,, Fe. Mn, Al. Ca, Mg. Pb. Cu and Ni analyses. These latter samples were injected (OME, 1982) probably accounts for much of the into pre-washed (soaked in 5% HNOz for 48 hours) pkystyobserved Fe enrichment. In addition. the possibility rene vials (Falcon tubes) pre-acidified with 30 ~1 of 1 N of increased weathering and erosional transport of Fe Ultrex HCI. The SO$ was measured by injection of a 0.4 within the watershed, as a result of the nearly complete ml subsample into a Dionex ion chromatograph. Fe, Mn, Ca and Mg were measured by flame atomic absorption deforestation of this area cannot be ruled MI;.

Lake sediment trace met&

1755

matter will influence the capacity of a given sediment to resist acidification. Trace rncrals. The concentrations of Cu. Ni and Al in the porewaters of Clearwater Lake vary inversely with pH. The relativeiy high metal concentrations in the acidic overlying water reflect the combined infiuence of low pH and high rate of supply of fu and Ni from the smelters. The sign of the concentration gradients of Cu. Ni and Al shows that these metals are not presently being leached from but are rather diffusing to the sediments. fn Clearwater Lake. the three Af prohfes show a steep decrease from a relatively high concentration in the overlying water to a minimum concentration located a few cm below the interface. Below this The porewater profiles of pH, ZH& ZCOz , CW4, level, porewater Al increases gradually to 5-10 SOL-, PQi-+ Si. NH:. Fe, Mn. Al. Cu and Ni are &mot * 1-l at 35 cm. The behavior of dissolved Al in presented for both lakes either in Figs. I and 2, or in relation to the pH gradient is consistent with the well known pH dependence of AI solubihty in natural Tables 1 and 2. Detectable (>O.OOS pmot m 1-l) Pb waters ~STUMM and MORGAN, 198 I). Low AI conwas found only in three Clearwater Lake porewater samples: 0.010 bmol - I-’ at the interface of the 15 centrations in the porewaters could be due to an increased adsorption at higher pH or to the authigenic and 20.2 m stations, and 0.015 prnol. I-’ at 4 em precipitatian of Al-bearing mineral phases. In order below the interface of a single 10 m pro&e. Conseto test this fast possibifity. saturation indexes were quently. no Pb profifes are presented here. caicutated For common Ai sohd phases using the L%(#&eand pff. In tfeanvater Lake. porewater pIf profiles show an inverse relationship with SOi- at chemical speciation model WATEQ2 (BALL et al.. 1980) and the chemical matrixes of Tables I and 2, the three stations. At the two deeper stations, the and Figs. 1 and 2. Figure 3 shows that the overlying porewater acidity is rapidly neutralized in the top l2 cm of the sediments. It should be noted that waters are undersaturated with respect to several clay porewater pH maxima are observed at the levels of minerals (kaolinite, hahoysite. montmo~~~onite) and gibbsite whereas the porewaters are supersaturated. most intense sulfate reduction, In these sediments, suggesting possible precipitation. Gibbsite formation sulfate reduction. coupled with Fe(OHb reduction seems particularly likely here since the calculated and FeS precipitation according to the reaction: saturation index with respect to this mineral (log 4Fe(OH),(s) -1-4SC$ -+ 9CHzQ * lAP/I&,) at all sites is relatively close to 0 for this mineraf. Moreover, gibbsite pre~ipjtat~on can be 4FeS(s) + CUz -i- 8HCQ; + 1LH@ f 1) readity induced under laboratory conditions by neuprobably accounts for much of the alkalinity genertralizing a dilute Al solution at normal temperature ation or H* consumption. This mechanism is sup and pressure (MAY et al., 1979). The slight supersatported by (a) the almost complete disappearance of uration observed here with respect to gibbsite could porewater SOa- within the top 5 cm of the sediments, be due to an overestimation of monomeric porewater (b) the absence of equivalent quantities of H2S but Al con~entmt~ons. Between QH 6 and 7. polymeric an abundance of porewater Fe (presumabty Fe’+) At species and organic complexes are expected to suggesting the formation of FeS (see betow), and (c) comprise a significant fraction (PuLFER and KRAMER, the presence of a large pool of hydroxylamine extract1983) of total Al measured by atomic absorption. able Fe (presumably iron oxyhydroxides) within the These same Al species are probably responsible for top l-2 cm of the sediments (TESSIER ef al., 1984). the observed increase in porewater Al with depth in However. other reactions, such as the weathering of the protiles of both lakes. silicates and oxyhydroxides ~STUMM and MORGAN, AtI profiles show a pronounced decrease in levels I981 IT as we11 as the ion exchange of H* for other of porewater Ni and Cu in the zone of active sulfate adsorbed sedimentary cations, may contribute to reduction. Solubility calculations indicate that the buffer the PM of the porewaters. The near-neutral porewaters are all supersaturated (Cu) or close to pH observed in the first 1-2 cm of the 15 and 20.2 saturation (Ni) with respect to Ni and Cu sulfides m stations of Clearwater Lake do not support the (Fig. 3). The extreme supersaturation observed for common belief (e.g. TutoNEN and ~AAKK~_A, i 983; Cu could be due to its high affinity for organic HAYASet ai’., 1984) that take acidification also results hgands. Sulfide minerat formation is thus a probable in sediment acidification, or that sediment pH is a porewater Ni and Cu sink in these sediments. The good indicatar of the past acidity of lakes. According saturation indexes calculated for Ni consistently show to the above reaction, other factors such as the a slight supersaturation (Sl = 0.5 to 2.0) a few cm availability of reducible Fe, SO:- and labile organic below the sediment-water interface, and reach satuIn contrast to the Fe profiles, the Al profifes show a general decrease in the upper IO-20 cm of the sediments. In Cleatwater Lake this decrease in the recent sediments has been interpreted as the result of sediment acidification and consequent Al teaching @ME, 1982). However, the absence of positive porewater Al concentration gradients across the sedimentwater interface of this lake does not support this hypothesis, Rather, the fact that a similar decrease is also observed in neutral McFarlane Lake suggests that pH may not be the primary cause. The increased erosionat transport of Al-poor material following deforestation appars to be a more likety explanation.

R. Cangnan and J. 0. Nriagu

1756

1

‘

I--. 1000

0

;I-

0

,

a5

2000

1.0

0

1000

0

1000

2000

1.5

0

E

E

T 15

iI40.'

,PH,’

4

0

5

IO0

, 6

200

7

300

A

0 0

03

1.0

_-I---i-._.

2QOO

1.5

I

e

Ei

0

20.2

401' 4

5

I

6

7

0

1000

2000

FIG. la. Porewater SOio:*,Cu and Ni (a), pH (0). and total sediment Cu and NI ((0:) at the three stations of Clearwater Lake. All dissolved components concentrations expressed in pmol -I ’ (upper axes), and all total metal content expressed in fig* g-’ dry weight (lower axes). The profiles were obtained in September 1982, unless otherwise indicated on the figures.

ration, or slight undersaturation (SI = -0.3 to -1.0) 20-40 cm below the interface. In the case of sulfide minerals. deviations of the order of 0.3 to OS units

in saturation indexes are probably not stgnificant since the measured porewater sulfide concentrations are close to, or at the analvtical detection limit. T‘hc

1757

Lake sediment trace metals

Mn

Fe 200

400

-----o-/0-0 .B /O 9 8 '$. pi 90 \.

\?

0

i

! I

i i

I

t L 0

50 000

100000

0

200

400

0

OV

401 0

10000

20000

200

-c----->

t

1984

t

i

* 0

30000

400

IO

30000

30

20

5

---*-

8,o”“80

0.

.; fd ; d

10

t a

o=z;p

?

"s'" -00

b

0

50000

15

a---

;,,,,

i ': %

Y

40000

> '0 '0 0' 6

“b b I

10000

20000

30000

200

400

30000

40000

50000

FIG. 1b. Porewater (0) and total sediment (0) Fe, Mn and Al at the three stations of Clearwater Lake. Units as in Fig. la.

influence of sulfur pollution on the geochemistry of trace metals is thus particularly well illustrated in these sediments. Above the zone of sulfate reduction (the first l-2 cm of the sediments), porewater trace metal concentrations appear to be controlled by

adsorption onto solid substrates such as Fe oxyhydroxides (TESSIER et al., 1984). Trace metal diffusion versus sedimentation. Figure 1 shows the presence of steep concentration gradients in porewater Cu, Ni and Al close to the sediment-

R. Car&an and J. 0. Nriagu

1758

,

PH

6

, 7

8

7

6

PH 40

6

Ftci. 2a. Porewater SO: , Cu and Ni (0). pH (O), and total sediment Cu and Ni 10) m McFartan~

Lake. Units as in Fig. la

interfiice of the two deeper stations d CLearwater Lake (pH 4.6). Because of their lower solubility or higher adsorption at higher pH, Cu and Al gradients are much less pronounced in McFarlane Lake. Cu concentration gradients measured in September I982 were also reLatively low at tbe ~y~~irnuetj~ station of Clearwater, where the over&ing water pH bad risen to 5.4 at that time of the year. In contrast to Cu and Al, Ni is much more soluble in oxic waters of pH 6-9 but appears to form a relatively insoluble sulfide (millerite) in anoxic sediments which may account for the pronounced concentration gradients in the top few cm of the sediments of both fakes. The transfer of trace metais from the water column to the sediment has, in the past, generally been irn~ljcit~y assumed to result exclusively from the settling of particutate matter. However, the pronounced trace metal concentration gradients observed across the ~iment-water interface ofCleanvater and McFariane Lake suggest that diffusion from overlying waters to the sediments may be an important mechwater

anism of trace metal transfer to some fake sediments. Diffusive fluxes across the sediment-water interface are usually calculated from Fick’s first law, as applied to the sedimentary environment:

where d, is the sediment porosity, D, is a composite diffusion coefficient corrected for sediment tortuosity and including any random transport mechanism such as biodi&rsion. gas ebullition, wave mixing. etc., and &Y/&x is the concentration gradient (approximated here as bC{rlx, where &X = i cm) of the species of interest at the interface. Because of the high water content (94-9&&o) of the surficial sediments of the fakes studied, we have assumed that porosity elects are negligible (#J = If. We have also assumed that U, equals I&, the molecular ~l~~iffusion &~~Gient calculated at in situ temperature by the StokesEinstein relation (LI and GREGQRS. 1974). Equating

Lake sediment

1759

trace metals

Al

Fe

1.

O__--_-_

“,. IO -

20-

‘0

40

40000

50000

60000

0

40000

50000

60000

-3 % - -

l\

8

*\

O0

PI

z l, *o”o-o

,o** ‘s ‘t ‘,

“\, 30 -

0

6

‘., ‘\ \

0

40000

80000

FIG. 2b. Porewater Ia) and total (0) Fe, Mn and Al in McFarlane Lake sediments. Units as in Fig. la.

D, to Db may lead to an underestimation of the true fluxes at the interface. Recent in situ steady-state tracer diffusion experiments in a similar lake (CARIGNAN, unpublished) show that De is about 1.3 to 2.5 times larger than & in the top 2-5 cm of the sediments. depending on benthic populations. Nevertheless, as will be seen later, the possible flux underestimation resulting from this assumption will only strengthen our final conclusions. The coefficients of molecular diffusion (o”,“) used here for Cu2+, Ni’+ and A13+were respectively 7.33 X 10w6,6.79 X 10V6, and 5.59 x 10m6cm2 .s-’ and were taken from LI and GREGORY (1974). Since chemical speciation calculations using WATEQ2 showed that the free ion was rarely the main chemical form expected in these porewaters (Table 3) the above diffusion coefficients could not be used directly in the flux calculations. Instead, we arbitrarily set the diffusion coefficient (at 2.5’C) of all neutral and monovalent predicted species [Cu(HS);. Cu+, NiCO!, NiHCO;, Al(OH):, A&OH):] as equal to 1 X lo+ cm2 - s-j. For trivalent

species, Di’ was assigned the value of 5 X 10-O cm2*s-‘. These crude assumptions were necessary in view of the absence of diffusivity measurements for these complexes: they should not, however. affect the final results by more than 30% for Cu and Al, and 10% for Ni in Clearwater Lake. Finally. composite diffusion coefficients were calculated as: D = ZD,.F,

(3)

where D, is the diffusion coefficient of the ith species and F,is its fraction of the total metal concentration. In both lakes. fluxes were calculated at 10°C. This temperature is representative within 2°C of the mean annuai temperatures for the three Clearwater Lake and the McFariane Lake 8.8 m stations, and within 4°C of the in situ temperatures at sampling time for all stations in June or September. Table 4 shows the calculated diffusive fluxes of Cu, Ni and Al at the sediment-water interface of the 15 and 20.2 m stations of Ciearwater Lake, and at

R. Carignan and J. 0. Nriagu

1760 TABLE end

4 3

-

la.

Concentrations

llg In the

If 0


(20 (20

il0

(20

(10

(20 (20

30

1

50

2 3

20 50

4

(10

5 7


9

30

t20

100

(20

I, 13

280

15 11 19

510 650 770

21

860

25 29 33 37 41

930 1020 II20 1220 1320

TABLE

Depth

C20 (20 t20

of


- 4.5

- 1.5 - 0.5 0.5 1.5 2.5 3.5 4.5 5.5 7.5 9.5 11.5 13.5 15.5 17.5 19.5 21.5 25.5 29.5 33.5 37.5

Depth

4.5 3.5 2.5 1.5 0.5 0.5 1.5

2.5 3.5 4.5 5.5 6.5 7.5 9.5 11.5 13.5 15.5 17.5

19.5 21.5 23.5 27.5 31.5 35.5 39.5

I).!

co.1

<,

to.

<

CO. I 0.3 to.,

31 ?i

0.2

3!

to.1

32

0.1 0.6

33 35

.

31

I

t20 :

poI-P.

at

the

51.

i49 141 147

56 53

55

I42 :53

54

ro / 10. I

151

56 56

i57

55

cc / to. i co. 1

i53

O.?

149 153

< 6

0.1 0.3

49 i32

I

170 169

0.4

233

0.4

144

I70

22 27

0.5 :.ll

297

co.: co. i

139 142

33 42 51 59 65

1.1 1.8 1.8

CO.

143 146 160

510 650 760

cli4

;.2

2.4 2.6

71

Lake.

I5 m station.

NHq’

w4-P

to.

345 382 421 451 479 491 504

58 53

14*

I5

200 280 340 440

La

146

120

60

HtS.

IO m statlon.

to.. co !
1i 31


to.1

161 167 186

0.3 0.i

Legend

a5

ii

:b$s

28 28 28 28 29 32 4c 55 !05

to..

for

55 56 56 61 61

58 53 59 63 64 73 77 7' 84

Table

ca

io.

‘lY

iy~l.i-‘i



210 440 690 890 II20 I.250 1240 1240 ,230 II30

(20 (20 t20 (20 t20 t20 20 50 130 300 380

1200 1330 1720

440 490 500 520 510 610 5BO

1320 1360

650 710

1410 1410 1500

700 730 830

'CO2

CH4

cu.1 CO.1 CO.1

0.4 0.i 9 IS 24

65 71 75 76 73 31 33 87 90 93 96 102 109

116 123

NHqA

0.8 0.2 0.2 0.2 0.) 0.) C0.i 0.1 0.3 0.2 0.4 I.1 0.5 0.9 0.5 0.5

140 192 233 233 322 353 386

0.6 0.7

413 436 457 474 501 516 530

w4-P

51

CO.! (0 i CO. / (O.! ‘O.!

0.. 0.4

18.‘ 1.4 2.8 0.3 to.1

ro. / __ co.: CO.! ._

150 146

5: 55

!48 i48

56 57 56

143

144 142 129 119 131 143 151

55 57

146 152 152 158 161 164 165

50 62

49 4R 5! 55 58

60 63 65 64 69 69 72 77 78

164 175 191 135

co., 0.1

CH2"

CS

&

~"rml.l-':

(cm)

-

NHd+.

Lake

0.2 0.1

i!

(cm)

- 3.5 - 2.5

CH4.

Clearwater

(20 (20

lb. Clearwater

x0*

of JC02,

wxewater

600 560 600 620 750 310 II30 ,510

Ii00 1270 1340 1210 1200 1130 ,030 IO80 III0 ,190

Iis0

1240 1340 1310 ,440 1480 1610

(20 (20 (20 (20 30 20 50 80

200 270 340 320 300 310 330 370 400

450 540 560 650 750 340 910 1020

13 14 I'

23 33 55

122 146 109 84 66 61 57 59 63 68 77 85 91 96 104 115 123 140 147

0. I 0.1 0.1 0.1 0.2 0.9 i.3 1.7

0.4 0.3 0.3 0.4 0.3 0.7 0.9 0.9 0.9 I.3 I.4 1.9 2.0 2.5 2.5 2.4 4.0

35

co.;

35 35 36 4! 44 55 67 32 100 126

to., CO.! CO. i 0.6 1.9 0 5 0.2 0.1 _~ 0.8

151 173 234 289 334 365 395 420 436 454

473 494 507 52!

0.i .._ 0.7 0.2 0.7 0.3 0.7 1.1 0.i 0.1 0.1 .. CO., _-.

135 I40 137 13.3 139 149 152 165 159 150 141 151 149 148 140 I42 150 163 160 161 167 172 lea 195 197

-._____.

56 57 56 56 53 58 58 65 65 60 59 63 59 62 60 63 64 70 69 71 74 79 32 86 86

the 19.8 m station of McFarlane Lake. i he resul:~ for the Clearwater 10 m station are omitted hen! because the concentration profiles of all porewater constituents suggest the presence of an unportant downward advective or convectrve Hus of overlyrng waters, as evidenced by the absence of measurable concentration gradients between the overly rng water and the first 9 cm of the sedtment. The convectiv< exchange of rapidly cooling surface waters at rhr5 time of the year with surfrcial porewaters 1s the mosr probable cause (HESSLEIN. i976b) for the okscr\,cd profiles at this depth. Clearly. diffusive fluz calculations based on Eqn. 2 cannot be applied in such J situation since D, is expected to be much iarger than II,. Also shown in Table 4 arc the estimated current total accumulation rates of (‘u. Ni and ,Ai based on ““Pb bulk sedimentation rates and on rhe tota! sediment metal profiles of Figs. I and 7. I o mrmmir< the possibility of underestimation of current metai accumulation rates. maximum sediment metal contents have been used when subsurface peaks are present. The bulk sediment accumulation rates used are 6.0 and 15.7 mg * cm-’ +> ’ respectively for Clear. water and McFarlane lakes (OME. 1982: i+jRIAC;l: t’i ul., 1982); these values correspond approximately t<\ a sedimentation rate of I mm.) -! for both lakes. The Clearwater Lake value corresponds kJ an average of three values (5.6. 6.6. and 5.9 mg.cm’. y I: obtained below the surficial btoturbated layer at l_S.Z. 15.5, and 19 m respectively. whereas the value used for McFarlane Lake was ohtamed from a single con: at maximum depth. A comparison between the estimated diffusive fluxes and total accumulatron rate:+ by the sediments reveals that diffusion contributes a significant fraction (52%) of the total c‘u f&md m the sediments of Clearwater Lake at the I.5 m station. At the 20.2 m station. diffusion accounts for 24% rG the observed Cu accumulation m June 1984, hur appears to be negligible in September 1982 (data not shown) when the overlying water Cu concentrations were reduced to 0. I rmol - 1 ’ as a result of Increased overlying water pH and/or losses of hypolimnetic c‘u to the sediments after a few months of stratification. At this station, the annual diffusive loss of Cu to the sediments is therefore expected to be substantially lower than the June estimate of 24?&. In McFarlane Lake the calculated downward ditfusive fh~x of Cu to the sediments was negligible ( 1.1%). In both lakes a major portion (59-16 I%) tt not all the Ni appears to reach the sediments by downward diffusion, The fact that the estimated present Ni diffusion at the 15 m station is higher than its estimated accumulation rate may be indicative of nonsteady-state conditions. In McFarlane Lake. the Ni concentration profile suggests an intense mobilization/ desorption at the interface since Ni appears to tx simultaneously diffusing from the interface to the sediments, and from the interface to the overlying water. Such a profile is probably not representative of steady-state conditions. Anox~c conditions were

Lake sediment trace metals TABLE and

Mg

Depth

2a.

Conccntratlons

in the

X02

Of X02.

porewater

CH4

of

RcFarlane

NH4+

NH4+.

Lake

w4-P

et

5,

m4-P, the

8.8

ZH2S

5,.

H25.

Ca

m station.

Ca

mg

(rlllOl.l-')

(cm1

-4 -3 -2 -

CH4,

8 II

to.1 0.1

409 407 409 404

192 192 195 193

30.6 28.5

I8 64

0.4 0.3

414 396

193 196

17.8 17.4

268 334

0.1 --

384 404

I88 185

I 1

550 550 630 550

t20 (20 t20 t20


(0.1 to., (0.1 4.3

630 830

t20 t20


2 3

860 980

26 38

0

t20 30

8

8

CO.1

(0.1

4 6

IO00 1280

20 50

45 52

15.2 2n.e

368 3.89

to.1 --

409 417

192 193

8 IO I2

1350 1400 1370

50 50 80

55 61 66

20.5 25.6 24.6

389 414 422

0.1 0.1 to.,

409 399 394

189 197 IT4

14

1380

80

70

23.1

428

(0.1

399

191

I6 18 20

1390 1360 ,400

80 130 150

76 81 86

25.5 28.2 30.9

437 445 449

(0.1 to.1 to.1

377 379 384

191 187 I83

471 501 522 541 543

to.1 to.1 to.1 CO.1 CO.1

392

187

402 419 399 399

187 201 191 194

24

1480

220

95

35.9

28 32 36 40

1680 1710 1760 1979

290 350 410 540

109 I20 130 136

21.1 35.9 36.4 33.2

TABLE

DePth

2b.

ncFerlane

ZC02

CH4

Late,

19.8 m station.

NH4+

(cm)

-4 -3 -2 -I

0 4 5 6 8 10 I2 14

51

as

ZH2S

for

Table

Za.

ca

mg

434 439 444 439 476 464 479 474 479 404 513 514 494 546 524 519 479 477 509 464 454 462 434 419

209 216 210 210 211 210 222 229 222 218 229 227 234 231 236 242 241 230 229 225 211 205 204 197

(~frol.1-')

I280 __

50

I280 1360 1760 2000 1990 2040 2010 2020 2080 2230 2320 2420

70 80 II0 150 270 370 480 520 710 890 I050 1140 1340 ,470

2490 I8 20 22 26 30 34 36 40

po4-P

Legend

2550 2620 2730 2860 2900 3080 3220 3290

1480 1510 1760 2090 2250 2520 2360

37 41 45 51 61 TO II2 I23 134 149 163

189 210 241 256 272 280 283 297 312 321 332 344 360

0.3 0.4 0.4 0.4 1.7 8.7 35.9 58.9 56.0 51.0 48.1 45.2 41.1 41.4 42.9 39.7 35.9 32.9 31.7 29.8 25.9 31.1 29.7 28.9

109 II2 II.5 I23 146 247 362 428 463 477 485 492 485 493 493 499 481 484 485 486 404 493 500

to.1
just appearing in the lower hypolimnion at the time of sampling and the Ni liberation from the interface may have been associated with the reduction of Mn(lV) apparent at this station from the porewater Mn profiles. In contrast to Cu and Ni, the diffusion of Al to the sediments appears to be unimportant in Al accumulation. The lack of information on true diffusivities at the interface and on annual variations in temperature and porewater chemistry may lead to errors as large as +50% on these flux estimates. Nevertheless, even such an error would not weaken the conclusion that diffusive processes across the sediment-water interface must be taken into consideration in the study of trace metal dynamics in polluted lakes. Because of the recent nature of trace metal pollution in these lakes. steady-state diagenetic equations (BERNER. 1980) cannot be applied to model the distribution of trace metals in these sediments. However, given

1761

the importance of diffusive processes in trace metal accumulation by lake sediments, a comparison of the porewater and solid phase Ni profiles at the 15 m station in Cleatwater (Fig. 1) reveals an important feature of Ni diagenesis in this type of sediment. The presence of an almost linear porewater Ni concentration gradient from 0 to 3 cm suggests that Ni is diffusively transported at a constant rate (K/d\= k) down to 2-3 cm below the interface. Consequently. there appears to be little loss of porewater Ni to the solid phase during its diffusive transport to 2-3 cm. where it is apparently precipitated as NiS. Such conditions, if representative of other times of the year, would lead to the build-up of a peak in sedimentary Ni concentration at approximately 2.5 cm. This is precisely what is observed in the sedimentary Ni profile of this station. Moreover. both the 15 m and 20.2 m porewater Ni profiles show a region located between 5 and 9 cm from the interface where the concentration gradient is positive, suggesting that an upward diffusive flux of Ni is also contributing to the formation of the subsurface peak. For the 20.2 m profile, this positive Ni concentration gradient reaches a maximum value of 0.00025 pmol . cmm3.cm-’ at 6.5 cm. At the same depth. the calculated Ni speciation is Ni*+ (69%), NiHCO; (26%), and NiCOl (5%) which corresponds to a 0:” of approximately 5.0 X l0-h cm’ - SC’and to an annual upward flux of -2.3 pg. cm-*. y-’ across the 6.5 cm plane. On the other hand. we can estimate the present rate of solid phase Ni burial between 6-7 cm to be about 2. I pg. cm-’ . y- ’ from a total Ni content of 340 pg *g-r at 6.5 cm (Fig. I) and a bulk sedimentation rate of 6 mgecm-*a y-’ obtained from a nearby location (OME, 1982). It therefore appears that the present net rate of Ni burial across the 6.5 cm plane is approximately zero, or slightly negative. We interpret this result as a strong indication of significant upward post-depositional mobility in the case of Ni m Clearwater Lake sediments. The porewater Ni profiles obtained in late spring or late summer were very similar and the above mechanism appears representative of summer conditions at least. Wintertime porewater Ni profiles may be different due to possibly lower rates of sulfate reduction at lower temperatures. Moreover the NiS formed at 2-3 cm during summertime may be partially oxidized and remobilized during wintertime as a result of a seasonal deepening of the oxidized layer. For instance, we have observed in Clearwater Lake that the thickness of the oxidized layer can change from g-10 mm in early June, when the sediment temperature is still relatively cold, to O-2 mm later in the summer. At the I5 m station. the porewater Cu profile indicates that an important fraction of the total sediment Cu may reach the 2-3 cm horizon by diffusion to produce a subsurface peak in total Cu. This mechanism is consistent with the fact that such a peak is absent from the 20.2 m station. where the diffusive accumulation of Cu below the interface is

1762

R. Carignan and J. 0. Ntiagu SATURATION

INDEX

FIG. 3. Saturation indexes for common Fe, Al. Mn, Si. Cu and Ni minerals in the porewaters ~,i Clearwater and McFarlane Lake sediments: Ckxuwater IS m (0 8); Cleawater 20.2 m {Clnt: McFartane 19.8 m (A A). Values greater than 0 indicate su~~tumtion. All speciation calculations are from WATEQ2 (at 10°C) using the chemical matrix given Tables 1-2 and Figs. l-2.

expected to he less important because of the seasonal reduction of dissolved hypolimnetic Cu concentrations observed in this lake. Iron and mangunex. The formation of several mineral phases of Fe and Mn (amorphous FeS, mackinawite, siderite. rhodochrosite) has been invoked

TABLE 3. lakes,

Average concentration

and

obtained

calculated

to account for the concentrations of these metals II) the porewaters of several anoxic sediments (MATISOFF ef al.. 1980: EMERSON. 1976: COOK, 1984). Saturation index calculations (Fig. 3) suggest that tron sulfide may be forming at the level of sulfate reduction at the Clearwater 15 m. 20.7 m. and McFarlane 19.X

gradients of Co, Ni and Al at the sediment-water

cheatftat spefiation

at each station every year

and

ccnrpcsite diffusion

it was sampled.

interface in Ciearwater and McFar
coefficients

at

IO'C.

Values for 1982-1983 are an average

Duplicate

profiles

were

of 4 measurementsinstead o+

2 for 1983 or 1984. STATION

CONCENTRATION

GRADIENT

lwol.cm-3.cm-1) -.-~CLEARWATER

15 no

cu 4.0x10-4*

Ni ----_A

2.0x10-"+

CDWJTED

- _ Ni

cu+ (85)

NiZ+ (97)

Cu(ttS),-i97)

NiL+ (771

~-- Al A13+ 140)

cu:+ill) CLEARWATER 20.2 m

2.7x10-"*

1.1x10-'

4.4x10-3**

NiKO,)" 19.8 m

6.0x10-'

4.3x10-J

6.0x10-y

CuiS,f2'w&,s-

c_-

t

1983 Data

*

Average 1982-83

** 1984 Data

(63) ilS1

Al(OH),+

(3ti;

AllOH)'+

(19)

Al(OI4)," iltl Al(OH),+

(2%

t(ilCO,)O (951

Al(OH1:"

(921

Ni2+ (3)

Al(OH!,-2

Ni(HCO,l+ MCFARLANE

COMPOSITf iJ

$x

cu

2.9x10-'*

SPECIAlION

r"sOF TOTAL)

(181

iO-"cm-.5,er.“! ~____~_. .. C.;

N

h.3

.1:1

1

4-I 4. /

b.t,

3.i

9.‘. j,

; Y

?I.$

i..i

(41 l8! __.~~I ._I_-~

_ _..

~_

1763

Lake sediment trace metals TABLE 4. Estimated trace metal diffusion across the sediment-water interface and total acc~ul~tion rates by the sediments; negative values indicate net losses by the sediments. STATION

DIFFUSIVE FLUX

ACCWJLATION

Yg.cm-~.y-l

+5.1 t3.5 +0.5

DIFFUSIVE CONTRIBIJTION I

“g.Cd.y-’

cu&cuLYi

CLEARWATER 15 a CLEARUATER 20.2 m McFARLANE 19.8 m

RATE

ElK&

t16.3 +10.0 +51

+11.2 +23.4 - 3.2

149.: 47;

10.1 13.2 86

270 252 785

52 24 1.1

161 76 59

4.1 9.3 -0.4

* Bulk sediment accumulation rates taken frownCCIE (1982). t Bulk sediment accumulation rates taken from Nriagu et al. (1982).

m stations. This interpretation is consistent with the observation of an acid soluble black deposit on the outside of the diaiyzer’s membrane in contact with the sediments l-2 cm below the oxidized surticial layer. Also, acid-volatile sulfide and pyrite have been shown to be important S components in McFariane Lake sediments (NRIAGUand SOON,1984). In both lakes. the ion activity product {Fe”“] * {CO:-] is very close, or within the reported range of soiubility for siderite (FeCO,). Saturation with respect to rhodochrosite (MnCOs) is also probable in McFarlane Lake while the porewaters of Cleat-water Lake are clearly undersaturated. Porewater Fe concentrations are relatively high (100-300 gmol - I-‘) in both lakes. The oxidation of upwardly diffusing Fe’+ to insoluble Fe3+ within the oxidized surface layer or in the water column may at least partly explain the observed Fe enrichment (up to 10%) in the surficiai sediments of both lakes (BERNER, 1980). In McFarlane Lake, a similar mechanism could also explain the Mn enrichment (l2.5%) observed in the upper 2 cm of the sediments. As noted above, the high porewater Mn concentration observed close to the interface of the deeper station of McFariane Lake is probably due to the rapid reduction of Mn(IV) oxyhydroxides following the onset of hypohmnetic anoxia at that time of the year. At the shallower station of this lake, where Oz is always present in the water column, the vanishing Mn concentration gradient at 2 cm suggests that Mn2+ is precipitated before reaching the interface and is thus effectively retained by the sediments. In Clear-water Lake, the behavior of sedimentary Mn is totally different. Both total (ZOO-400 rg -g-i) and porewater (5-20 rcrnol. I-‘) Mn are much lower. The Eh-pH conditions prevailing in the acidic overlying waters as well as in the anoxic porewaters are always within the stability field of soluble Mn(II) (STUMM and MORGAN, 1981). Thus, the higher soiubiiity of Mn in this lake also explains its presence in measurable quantities in the overlying waters (34 pmol + I-‘), a characteristic attribute of acid lakes. Under such conditions, the porewater Mn derived from the dissolution of various Mn bearing sedimentary phases can freely diffuse out of the sediments instead of accumulating near the interface as insoluble Mn(IV) oxyhydroxides, or in the anoxic sediment as MnCOJ or adsorbed Mn2+. The presence of a small and constant (within the analytical precision of +-2

rmo1 - 1-l) Mn concentration gradient between 0 and 40 cm at the 15 and 20.2 m stations suggests that Mn is indeed slowly lost from the sediments. The continuous diffusive loss of sedimentary Mn to the overlying water probably explains why total sediment Mn is much lower in Clearwater Lake than in McFariane Lake which has maintained a neutral to alkaline pH during the last decades. Such a m~hanism is consistent with the depletion of Mn in the top sediment layers resulting from the hypothesized recent acidification of Clearwater Lake (OME, 1982). It follows from these considerations that sediment total Mn depletion in the most recent layers may be a reliable geochemicai evidence for recent lake acidification. CONCLUSIONS Our results show that diffusion to the sediments can be an important trace metal accumulation mechanism in the sediments of acid lakes (Cu. Ni) and circumneutrai lakes (Ni). Post-depositional diffusion may also be important for other trace metals that are relatively soluble in the overlying water. For example, recent results (CARIGNANand TESSIER, 1985) indicate that diffusion is the principal mode of Zn accumulation by the sediments of polluted (Clearwater), as well as remote acid (pH 4.5-5.5) lakes. In Clearwater lake, subsurface maxima in total Cu and Ni may not necessarily be indicative of a recent post-depositionai leaching, or of decreased trace metal accumulation by the sediments as a result of recent lake acidifi~tion. Instead, they may be a diagenetic feature of sediments overlain by acidic waters having high dissolved trace metal concentrations relative to the levels in the porewaters. It should be noted that such an inte~re~tion is not contmdi~to~ to the hypothesis according to which lake acidification can result in decreased trace metal retention by the lake when expressed as a fraction of the incoming trace metal load. In the context of simultaneously increasing acidification and trace metal loading, as is usually the case, the absolute retention by the sediments may increase even if the retention relative to the incoming load is decreasing. thank Henk Don, Gary Bruce, Francois Rapin and Roger Beauchemin for assistance in the field, and Lise Hamel for assistance with sediment and

Acknowledgemennu-We

porewater analyses. Andre Tessier and three anonymous

R. Carignan and J. 0. Nriagu

1764

reviewers contributed to improve the original manuscript. This study was partially funded by a Natural Science and Engineering Research Council of Canada University Research Fellowship to R.C. Editorial handling: E. R. Sholkovitz

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HUTCHINSONT. C. (1979) The etfects of acid ramfall and heavy metals particulates on a boreal forest ecosystem near the Sudbury smelting region of Canada Mizr 1!8, Sail Pollut. 7, 42 l-438. KAHL J. S. and NORTON S. A. (1983) Metal input and mobilization in two acid-stressed lake watersheds in Maine. Completion Report. Project A-053-ME. Land and Water Resources Center. University of Maine at Orono. 70 p LI Y.-H. and GREGORY S. (1974) Diffusion of rons in sea water and in deep-sea sediments. Geochrm i rwnrwirin? Acfa 38, 703-7 14. MATISOFFCl.. LINDSAYA. H., MATIS S. and SWTER f-. M (1980) Trace metal mineral equilibria in Lake Frie se& ments. J. Great Lakes Res. 6, 353-366. MAY H. M.. HELMKE P. A. and JACKSONM. i (19’791 Gibbsite solubility and thermodynamic properties of h!.. droxy-aluminium ions in aqueous solutions at 25°C‘ Geochim. Cosmochim. Acra 43, 86 l-868.

NRIAGUJ. 0. and SOWNY. K. (1984). Arylsulfatase activit\ in polluted lake sediments. Environ. Pa/ha Srries B 8. 143-153. NRIAGU J. 0.. WONG H. K. 1. and COKER R. D. (1982) Deposition and chemistry of pollutant metals in lakes around the smelters at Sudbuv. Ontario. Emiran Sci Technol. 16, 55 I-560. OME (I 982) Studies of lakes and watersheds near Sudbun Ontario: Final limnological report. Report SES 009/82. Ontario Ministry of the Environment. Water Resources Branch, Rexdale. Ont. PULFER K. and KRAMER J. R. ( 1983) Alummium hydrolysis and organic speciation review. Internal report. Dept. of Geology, McMaster University, 5 I p. 454-458. COOK R. B. (1984) Distributions of ferrous iron and sulfide ROBBINSJ. A. and CALLENDERE. (1975) Dtagenesis of in an anoxic hypolimnion. Can. J. Fish Aquaf. Sci. 41, manganese in Lake Michigan sediments. ilnrcr .I .Sci 175, 512-533. 286-293. ROBBINSJ. A. and EDGINGTOND. N. i 1975) Stable lead DILLON P. J. and SMITH P. J. (1984) Trace metal and geochronology of fine-grained sediments in southern Lake nutrient accumulation in the sediments of lakes near Michigan. Argonne National Lab Report -1Vl -‘(i. i. p Sudbury, Ontario. In Environmental Impacts afSmelters 32-38. (ed. J. 0. NRIAGU) pp. 375-416. Wiley, N.Y. STUMM W. and MORGAN J. J. (198 I) Aquatrt (‘hemrsrw EMERSONS. P. (1976) Early diagenesis in anaerobic lake 2”d Edition. Wiley and Sons. New York, 780 p sediments: chemical equilibria in interstitial waters. GeoTESSIER A.. RAPIN F. and CARICNAN R. (1984) Trace chim. Cosmochim. Acta 40, 925-934. metals in oxic lake sediments: Possible adsorption onto GALLOWAYJ. N. and LIKENS G. E. (1977) Atmospheric iron oxyhydroxides. Geochim. C’wmochirn. Acra 49, 18% enhancement of metal deposition in Adirondak lake 194. sediments. Project No. A-067-NY. U.S. Dept. of the TOLONEN K. and JAAKKOLA T (1983) History of lake Interior, Washington D.C. 40 p. acidification and air pollution studied on sediments in HAVAS M., HUTCHINSONT. Cand LIKENS G. E. (1984) Red herrings in acid rain research. Environ. Sci. TecImol South Finland. Ann. Bar. Fennici 20, 57-58. 18, l76A-l86A. VARIAN( 1979) Anai_vticaIMethods far Flame Specrroscapv HESSLEINR. H. (1976a) An in suu sampler for close interval Varian Techtron, Springvale. Australia, I23 p. pore water studies. Limno/. Oceanogr. 21, 912-914. WILLIAMSJ. D. H. and PASHLEYA. E. (1979) Lightweight corer designed for sampling very soft sediments. .I Fislt HEUBLEINR. H. (1976b) The fluxes of CH,, X0*, and Res. Board Can. 36, 242-246. NH,-N from sediments and their consequent distribution in a small lake. Ph.D. thesis, Columbia University, New YAN N. D. and DILLON P. J. (lY84) in Envrronmen~u! York. I86 p. Impacts c$SmeIlers. (ed. J. 0. NRI4Cit’) Wilev. N ‘t