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The effect of benthic invertebrates on the clearance of mercury from sediments
ECOTOXlCOLOGY
AND
ENVIRONMENTAL
SAFETY
3,236-244
(1979)
The Effect of Benthic Invertebrates on the Clearance of Mercury from Sediment9 M.J. Division
A. S. W. DEFREITAS,AND
BODDINGTON,
of Biological
Sciences,
Narionul
Research Received
May
Council
of Canada,
D.R. Oitawa
MILLER KIA
OR6, Canada
4, 1979
Tubificid worms caused a significant loss of radiolabeled inorganic mercury and methylmercury from contaminated sediments over a 3-week period. The fractional clearance rate of mercury from the sediments was linearly related to worm density. The most significant loss was from the first 1 cm of sediment. The presence of worms changed the appearance, and caused some redistribution, of fine sediment. Loss of mercury could be contained by a sand overlay but the upward movement of mercury into the sand overlays when worms were present suggested that their effectiveness in preventing such movement would be limited over longer periods of time. While a number of considerations prevent the direct application of the clearance coefficient to natural systems, it was possible to demonstrate, on the basis of certain simplifying assumptions, that the benthic macrofaunacould be entirely responsible for the clearance of mercury from the bed sediments of flowing water systems.
At any instant the bulk of mercury within an aquatic system is associated with the bed sediments, while water and its suspended solids, in flowing water systems, contribute most to the dynamics (e.g., Kudo et al., 1978; Ottawa River Project Group, 1979). Although laboratory experiments demonstrate that the desorption of mercury from natural sediments results in very low fractional clearance with a half-life of about 180 years (Kudo et al., 1975), natural systems often clear much more rapidly. In the Ottawa River the half-life of sediment mercury in a 3-mile section was inferred to be 1.4 years (Ottawa River Project Group, 1979), while Lake St. Clair showed a 64% reduction in sediment concentrations between 1970 and 1974 indicating a half-life of about 2.8 years (Thomas et al., 1975). There are many reasons why natural systems clear more rapidly than might be anticipated in the laboratory; one reason which has received little attention is the involvement of the benthic macrofauna. Jernelov (1970) showed that methylmercury was released from sediments contaminated with inorganic mercury. By labeling successive 0.5cm depths of sediment he found that in the presence of benthic invertebrates mercury could be released from more than just the first 0.5 cm. The magnitude of the release at any depth was no greater than the surface release with no invertebrates present and the half-life was probably 75 years (assuming 5 x 0.5 g fish and 100 g labeled sediment). These observations were confirmed by the data of Bongers and Khattak (1972) and would not lead one to suspect that benthic invertebrates play a major role in increasing the fractional clearance rate and reducing the half-life of natural systems to 1-3 years as observed in natural systems. However, these observations pertain to closed systems and we thought that in flowing waters, the fractional ’ Issued
as NRCC
17507.
0147-6513/79/030236-09$02.00/O
Copyright 0 1979 by Academic Press. Inc. All rights of reproductmn in any form reserved.
236
237
BENTHICMERCURYCLEARANCE
TABLE
1
SOME CHAR~TERISTICS OF THE SEDIMENT USED BEFORE AND AFTER WATER SEPARATION
Sediment
Dry wt (%I
Organic material (%I
Total mercury (w g-7
Radiolabel (%I
Whole Fine Coarse
100 6 94
2.5 26 1
45 30 280
100 85 15
clearance rate of contaminated sediments might be greatly enhanced by the presence of benthic invertebrates. The present paper reports on the effect of differing densities of tubificid oligochaetes on the fractional clearance rate of mercury from sediments contaminated with radiolabeled inorganic mercury and methylmercury in a flowing water apparatus. We also examine the effectiveness of sand overlays in reducing clearance since it is unclear how effective overlays can be as barriers to mercury-contaminated sediments (Bongers and Khattak, 1972; Widman and Epstein, 1972; Wolery and Walters, 1974; Jernelov et al., 1975), when benthic invertebrates can disturb the barrier (Bongers and Khattak, 1972). We wish to thank H. Akagi for his analysis of methylmercury in the present study. MATERIALS
AND METHODS
Worms. Tubificid oligochaete worms were obtained from Arbor Scientific, Toronto, and identified as Limnodrilus hoffmeisteri Claparede (after Brinkhurst and Jamieson, 1971, p. 463). Upon receipt worms were placed in fresh running water at room temperature (21°C) and maintained 2-3 days in this manner prior to experimentation. Background analysis of the tubificids for mercury by flameless atomic absorption spectrophotometry (Ottawa River Project Group, 1979) showed them to have levels of about 240 ng g-’ dry wt (ppb) total mercury. Sediments. Sediments consisting of sand clay and wood chips were obtained from the Ottawa River, Ottawa. The collection point was in the channel north of Kettle Island downstream of the Canadian International Paper Company (Ottawa River Project Group, 1979). The sediment was first washed through a standard 850qm (No. 20) sieve with as little water as possible, removing most of the visible wood chips. The sieved material was allowed to settle for 24 hr after which the supernatant water containing colloidal and slow-settling material was decanted off. The sieved sediment was water separable into coarse and fine fractions which had different background levels of mercury and which accepted different amounts of radiolabeled mercury (Table 1). The wet sediment was prepared in bulk for the experiments by adding approximately 50% by weight of water containing sufficient amounts of either inorganic mercury or methylmercury radiolabel to provide 2000 cpm g-l. The slurry was stirred vigorously and allowed * Radiolabel from New England Nuclear as ““SHgCI, and CH,‘“3HgC1 with respective specific activities of 0.30 and 0.22 mCi mg-’ on experiment Day 0. Purity of CH, ““3HgCl determined as 85% on experiment Day 0.
238
BODDINGTON,
DE FREITAS,
AND
MILLER
to stand overnight before being added to the various experimental tubes. The ratio of the concentrations of the label between the fine and coarse fractions (7O:l) was not the same as the ratio of the background levels (9S:l) (Table 1). Using a smaller fine fraction, 0.4% Kudo et al. (1977) reported that 44% of the label was absorbed by the fine fraction yielding a ratio of 181: 1. In the presence of wood chips this was further modified such that the concentration ratio appeared dependent upon the number and quantity of competing fractions present. Flowing wafer apparatus. A simple apparatus was assembled to allow the flow of water over the experimental sediments (Fig. 1). This consisted of a water bath maintained at 21°C from which water was pumped through a linear distributor providing a series of water outlets. The outlets from the distributor were controlled by stopcocks and the water flowed over the sediment surface and drained to a charcoal filter. The apparatus was set to deliver a flow of about 15 ml min-’ but the daily range was IO-20 ml min-‘, and regular adjustment was required.
distributor
<:>
-
StOpCOck
-
C:>
22 m m O.D. gloss tubing
drain
FIG. 1. Schematic
diagram
of the arrangement
of the long and short tubes in the flowing
water
apparatus.
BENTHIC
MERCURY
CLEARANCE
239
EXPERIMENTS Series I A known weight of about 20 g, about 4 cm wet wt, of inorganic mercury- or methylmercury-labeled sediment slurry was added to gamma-counting vials, allowed to settle for 7 days, and counted on a Bicron deep-well crystal with a Inotech multichannel analyzer. Three replicates of 1 and 0.5 g wet wt of L. hoffmeisteri representing, respectively, densities of 2600 and 1300 g wet wt m-2 or 5.72 and 2.86 x lo5 individuals mm2 were added to each of the two radiolabeled sediments. Two controls, with no worms added, were run for each labeled sediment. The tubes were placed in the flowing water apparatus as illustrated (Fig. 1) and counted at l-week intervals for 3 weeks. Series IZa Glass tubes 23 cm long with a diameter of 2.2 cm were filled with about 70 g of either inorganic mercury- or methylmercury-labeled sediment slurry to a height of 14 cm. These were allowed to settle for 7 days. Two replicates of 1 g L. hoffmeisteri, representing a density of 2400 g wet wt mm2 or 5.26 x lo5 individuals me2, were added to the two radiolabeled sediments. Replicated controls for Day 0 were fractionated (see below) immediately while replicated controls for Day 21 were run with the experimental tubes. The tubes were covered with aluminum foil to the height of the sediment, providing a unidirectional light source, placed in the flowing water apparatus as illustrated (Fig. l), and fractionated after 21 days. The sediment column was forced out of the tube by means of a rubber-tipped plunger and each centimeter was sliced into a gamma-counting vial via a filter funnel, washed, and topped to a height of 5 cm with water to retain constant counting geometry. After counting, a selection of the sections were analyzed for methylmercury using a dithizone benzene extraction with a Florisil column and silica gel thin-layer, chromatography (H. Agaki, unpublished). Series IZb A 4-cm overlay consisting of the coarse fraction which had been ashed at 600°C for 24 hr was added to a further series of 14 cm labeled sediment. The Day 0 control was taken as being the same as in IIa, the ashed sediment having no mercury or radioactivity associated with it. At the time of column fractionation every effort was made to achieve a precise separation of the two layers. Distribution was analyzed by analysis of variance using the least significant range (LSR) as an a posteriori comparator (Sokal and Rohlf, 1969). RESULTS Series I. Short Tubes During the 7-day settling period the sediment assumed two distinct layers; about 3.5 cm of predominantly coarse material overlayed with about 0.4 cm of the fine. The upper layer represented about 25% of the total fine material present. When the tubes were placed in the flowing water apparatus the upper layer compacted about 0.3 cm. When worms were added the upper layer assumed a flocculent
240
BODDINGTON,
DE FREITAS,
AND
MILLER
appearance and increased to a depth of 1 .Ocm, although the overall depth increased by only 0.2 cm. The upper layer now consisted of about 70% of the total fine material present. The control tubes had some variation associated with their counts over time but these changes were insignificant and no significant loss was recorded. There was a significant loss of mercury over the total period from all tubes containing worms. With 1 g present, 23 and 21% (range 10-35) was lost from the inorganic mercuryand methylmercury-labeled sediments, respectively. These proportions did not differ significantly (t = 0.44; ns; n = 6). The total loss with 0.5 g of worms was 13 and 7% for inorganic mercury- and methylmercury-labeled sediments, respectively. While these values did differ significantly (r = 2.8 1; P < 0.05; n = 6) they approximated one-half the l-g values. The daily fractional clearance (ng Hg lost ng Hg present-’ day-‘) of the sediments was determined between each counting period for each worm density and both mercury labels. Within any group there appeared to be no trend for the time of loss. The fractional clearance could be uniform over all weeks, or disproportionately large during any one of the three. It was considered that the loss was an event dependent upon worm activity which should, on the average, be uniform over time. The group averages of the daily fractional clearance (Table 2) between inorganic mercury and methylmercury were not significantly different but the values for 1 g worms were approximately twice those for 0.5 g worms. When these figures were adjusted for worm density (using g worm g sediment-‘) there was no significant difference in any of the resultant values. The mean density-corrected fractional clearance was 2.238 x 10-l ? 1.561 x 1O-2 day-’ [g worm g sed-‘1-l. Series II. Long Tubes: Total Mercury
As an initial comparison the counts of each centimeter segment of the fractionated long tubes were totaled and adjusted by sediment weight (Table 3). The control tubes, Day 0 and Day 21, showed no significant difference within either the inorganic mercury- or methylmercury-labeled sediments, with or without the sand overlay. When worms, but no sand overlay, were present, there was a significant loss, about 8% of the total for both inorganic mercury- and methylmercury-labeled sediments. The fractional clearance rates adjusted for worm density gave values TABLE FRACTIONAL
CLEARANCE OF SEDIMENTS CONTAINING
Inorganic Worm wet wt (8)
Sediment wet wt (8)
1
20.67
0.5
22.76
(1 Mean
2 SE. n = 9.
INORGANIC DIFFERENT
Fractional clearancea (day-‘) x x x x
MERCURY DENSITIES
AND METHYLMERCURY OF TUBIFICID WORMS
FROM
Methylmercury
mercury
1.139 a3.464 6.000 k1.700
2
Sediment wet wt w 10-Z 10-S 1O-3 10-S
20.87 22.93
Fractional clearancea (day-‘) 1.078 k2.574 4.311 k1.108
x x x x
t 10-Z 1O-2 10-S 10-3
0.141 (ns) 0.832 (ns)
241
BENTHICMERCURYCLEARANCE
(Table 3) closely approximating those found in the short-tube experiments. When worms and a sand overlay were present, there was no significant loss of mercury from the system. In the tubes labeled with inorganic mercury, analysis ‘showed that about 7% of the label was in the methyl form. There was no difference between tubes with or without worms. In the methylmercury-labeled sediments the Day 0 controls had about 85% of their label as methylmercury. By Day 21 there had been a decline in the methylmercury fraction to about 60%, although the content of the first centimeter remained high at about 85%. There was no difference due to the presence of worms. Distribution (a) Without overlay. The depth distributions of both inorganic mercury and methylmercury were similar (Fig. 2A). The controls showed a significantly higher concentration in the first centimeter of sediment than was present in the remaining 13 cm. This was due to the relatively slow settling of the fine material. The values of methylmercury were lower than those for inorganic mercury. This was probably an artifact of column or slurry preparatioh for the difference was consistent within replicates. The effect of the worms was the same in both cases: The label was lost from the first centimeter such that its concentration was indistinguishable from the remaining depths. Although not evident from the uniform distribution, in the case of methylmercury there must have also been some upward redistribution. Thus the change in the first centimeter of methylmercury from 63.9 to 23.8 ng g-l is insufTABLE3 COMPARISON
OF TOTAL RADIOACTIVITY LONG-TUBE EXPERIMENTS, lnorgamc
WIthout overlay Day 0 (control) Day
21 (control)
Pooled
mean
1 g wet wt worms Meall With overlay Day 21 (control) 1 g wet
Mean
wt worms
FRACTIONAL SERIES IIa
CLEARANCE AND
Total wet wt
66.19 70.88 64.17 65.64
1.346 1.376 1.360 1.390
70.89 68.69 67.88
1.236 1.276
1.361 ~0.0210 1.373” kO.0185 1.367 1-0.0175 1.256’ kO.0283
90.32 88.60 87.00 87.33 88.3 1
1.399 1.388 I.436 1.408
1.394 kO.0078 1.422” i-o.0199
Treatment (mean *SD)
IN THE
IIb
mercury
Weightadjusted total cpm (X IO”)
r SD
AND
Methylmercury Densltycorrected fractional clearance”
2.620
x 10 ’
Total wet wt
Weightadjusted total cpm (X10”)
64.17 68.87 67.23 65.00
I.814 1.738 1.761 1.743
69.60 66.62 66.91
1.633 1.684
I .776 -co.0537 1.752” kO.0122 I.764 1-0.0347 1.659” 20.0359
96.29 93.62 93.23 95.09 94.47
I.732 1.674 1.705 1.712
1.703 kO.0408 1.708” to.0051
Treatment (mean *SD)
o See text for detads. Fractmnal clearance expressed as [day-‘.@ worm/g sediment-‘) ‘I. ’ Control Day 0 vs Day 21: morgamc mercury, I = 0.606 (ns): methylmercury. r = 0.612 (ns). ’ Pooled control vs worms: morgamc mercury. I = 5.671 (P c 0.011: methylmercury, t = 3.428 (t’ < 0.01). ” Sand control vs sand worms: Inorganic mercury, I = 1.824 (ns); methylmercury, I = 0.183 (ns).
Densitycorrected fractional clearance”
-
1.902
x 10~’
-
242
BODDINGTON,
DE FREITAS,
AND
MILLER
DEPTH
I
12 13 14
FIG. 2. Schematic representation of changing inorganic mercury (I) and methylmercury (M) distribution in sediments between control (C) tubes and those containing worms (W) in both the absence (A) and the presence (B) of sand overlays. Insert values are ng Hg g dry wt-I; ppb, grouped and pooled after ANOVA and LSR testing.
ficient to account for the change in the mean from 27.0 to 23.8 ng g-l. Such is not the case for inorganic mercury. Furthermore while the fractional clearance rates for both labels were similar (Table 3) the concentration of methylmercury in the first centimeter was proportionately much lower than for the inorganically labeled sediment. (b) With sand overlay. When sand overlays were used (Fig. 2B) the concentration in the first centimeter of the controls was considerably reduced. Since these controls did not differ in total from those without the overlay (Table 3) the reduction was an artifact associated with the addition, and inadequate separation, of coarse uncontaminated sediments to the fine surface. Even so, when worms were present the distribution did change. Separation was more difficult as only 3 cm of what was clearly sand remained. While there was no overall loss from the system (Table 3) there was an upward movement of radiolabel into the proximal layer (D) of the overlay. In the case of methylmercury this was sufficient to render the first centimeter of sediment indistinguishable from the remaining 13 cm. In the case of inorganic mercury while there was a significant reduction in the concentration of the first centimeter and an upward movement into the proximal layer (D) of overlay, the first centimeter was still distinguishable from the remaining 13 cm. DISCUSSION The results show that tubificid worms caused a significant, density-dependent loss of mercury from sediments. This loss occurred mainly from the first centimeter of sediments although some redistribution occurred. Sand overlays formed an effective barrier within the experimental period, but some upward transport was observed. The fractional clearance rate, when adjusted in terms of worm density, was consistent for all experimental situations. Its reciprocal when multipled by In 2 gives
BENTHIC
MERCURY
243
CLEARANCE
the half-life of the system. Thus the coefficient of 2.328 x 10-l represents a half-life of 2.98 days when 1 g of worms is present in 1 g of sediment. In the present experiments where the short tubes had 1 and 0.5 g of worms in 20 g of sediment and the long tubes had 1 g of worms in about 70 g, the respective half-lives would be 60, 120, and 200 days. The latter density is close to that used by Jernelov (1970) and it is quite evident that the present clearance rates are far in excess of Jernelov’s methylation rates which would yield a half-life of about 75 years. A worm density of less than 4 x 1O-5 g g sediment-’ would yield a half-life of 180 years, a result indiscernible from situations when no worms were present (cf. Kudo et al., 1975). Such would be the case in the Ottawa River where tubificid worms can be calculated as having a standing stock of 4 x lop6 g g sediment-’ (after Trudel et al., 1977). Other situations, such as Toronto Harbour, where densities approach 500 g wet wt rnp2 (Brinkhurst, 1970), one-fifth those presently used, could be approximated by 1 g worms in 100 g sediment at a depth of 4 cm. The resultant half-life would be only 300 days. Furthermore, if other organisms, such as molluscks, are equally effective in clearance as they are in methylation (Jernelov, 1970), then broader application of the clearance coefficient may be possible. For example, while the Ottawa River has a low standing stock of tubificids, it is high in bivalve mollusks giving 5.5 x 10e3 g organism g sediment-’ (wet wt, after Trudel et al., 1977). On this basis the half-life could be as low as 1.6 years, close to that inferred from sediment analysis (Ottawa River Project Group, 1979). There are, of course, many reservations to such extrapolations. For instance sediments vary tremendously in their characteristics such that they may not be amenable to the same process. Equally the seasonal changes in temperature will affect the activity of the organisms. The concept that overlays can act as effective barriers is questionable in the long term. While they contained the loss in the present circumstances there was an upward distribution. On the basis of the present results it would take about 30 weeks for all of the overlay to achieve the mean concentration of the contaminated sediments. Again one must temper this statement with considerations of the density of tubificid employed but be equally aware that other invertebrates could aid in the process. REFERENCES BONGERS, Sediments. BRINKHURST,
Harbour, BRINKHURST,
L.
L.,
AND
KHATTAK,
M.
N.
(1972).
Sand
and Gravel
Overlay
for
Control
of Mercury
in
E.P.A.-16080-HVA-01/72. R. 0. (1970). Distribution and abundance of tubificid Lake Ontario. J. Fish. Res. Bd. Canad. 27, 1961-1969. R. O.,
Edinburgh. JERNELOV, A. (1970). mercury at different
AND
JAMIESON,
B. G. M. (1971)
Aquaric
(Oligochaeta)
Oligochaera
species
of the World.
in Toronto Oliver
and
Boyd,
Release depths.
of methylmercury from sediments with layers containing inorganic Limmol. Oceanogr. 15, 958-960. JERNELOV, A., LANDER, L., AND LARSSON, T. (1975). Swedish perspectives on mercury pollution. J. Water Polk. Contr. Fed. 47, 810-822. JERNELOV, A., AND LANN, H. (1973). Studies in Sweden on feasibility of some methods for restoration of mercury-contaminated bodies of water. Environ. Sci. Technol. 7, 712-718. KUDO, A., MILLER, D. R., AKAGI, H., MORTIMER, D. C., NAGASE, H., TOWNSEND, D. R., AND WARNOCK, R. G. (1978). The role of sediments on mercury transport (total- and methyl-) in a river system. Prog. Water. Technol. 10, 329-339.
244
BODDINGTON,
DE FREITAS,
AND
MILLER
KUDO, A., MORTIMER, D. C., AND HART, J. S. (1975). Factors influencing desorption of mercury from bed sediments. Canad. J. Earth Sci. 12, 1036- 1040. KUDO, A., TOWNSEND, D. R., AND MILLER, D. R. (1977). Prediction of mercury distribution in river sediments. J. Amer. Sot. Civil Eng. EE, 606-614. Ottawa River Project Group (1979). Mercury in the Ottawa River. Environ. Res. 19, 231-243. SOKAL, R. R., AND ROHLF, F. J. (1969). Biometry. Freeman, San Francisco. THOMAS, R. L., JAQUET, J.-M., AND MURDOCH, A. (1975). Sedimentation processes and associated changes in surface sediment trace metal concentrations in Lake St. Clair, 1970- 1974. Proc. Int. Conf. Heavy
Met.
Environ.
2 (2), 691-708.
TRUDEL, B. K., DEFREITAS, A. S. W., AND MILLER, D. R. (1977). Mercury dynamics of the aquatic invertebrate community in the Ottawa River study area. In Ottawa River Project Final Report. NRCC, Ottawa. WIDMAN, M. U., AND EPSTEIN, M. M. (1972). PolymerFilm Overlay SystemforMercury Contaminated Sludge-Phase
I. E.P.A.
36080-HT2-OY72.
WOLERY, T. J., AND WALTERS, L. J., JR. (1974). Pollutant mercury and sedimentation in the western basin of Lake Erie. Proc. Conf. Great Lakes Res. 17, 235-249.
AND
ENVIRONMENTAL
SAFETY
3,236-244
(1979)
The Effect of Benthic Invertebrates on the Clearance of Mercury from Sediment9 M.J. Division
A. S. W. DEFREITAS,AND
BODDINGTON,
of Biological
Sciences,
Narionul
Research Received
May
Council
of Canada,
D.R. Oitawa
MILLER KIA
OR6, Canada
4, 1979
Tubificid worms caused a significant loss of radiolabeled inorganic mercury and methylmercury from contaminated sediments over a 3-week period. The fractional clearance rate of mercury from the sediments was linearly related to worm density. The most significant loss was from the first 1 cm of sediment. The presence of worms changed the appearance, and caused some redistribution, of fine sediment. Loss of mercury could be contained by a sand overlay but the upward movement of mercury into the sand overlays when worms were present suggested that their effectiveness in preventing such movement would be limited over longer periods of time. While a number of considerations prevent the direct application of the clearance coefficient to natural systems, it was possible to demonstrate, on the basis of certain simplifying assumptions, that the benthic macrofaunacould be entirely responsible for the clearance of mercury from the bed sediments of flowing water systems.
At any instant the bulk of mercury within an aquatic system is associated with the bed sediments, while water and its suspended solids, in flowing water systems, contribute most to the dynamics (e.g., Kudo et al., 1978; Ottawa River Project Group, 1979). Although laboratory experiments demonstrate that the desorption of mercury from natural sediments results in very low fractional clearance with a half-life of about 180 years (Kudo et al., 1975), natural systems often clear much more rapidly. In the Ottawa River the half-life of sediment mercury in a 3-mile section was inferred to be 1.4 years (Ottawa River Project Group, 1979), while Lake St. Clair showed a 64% reduction in sediment concentrations between 1970 and 1974 indicating a half-life of about 2.8 years (Thomas et al., 1975). There are many reasons why natural systems clear more rapidly than might be anticipated in the laboratory; one reason which has received little attention is the involvement of the benthic macrofauna. Jernelov (1970) showed that methylmercury was released from sediments contaminated with inorganic mercury. By labeling successive 0.5cm depths of sediment he found that in the presence of benthic invertebrates mercury could be released from more than just the first 0.5 cm. The magnitude of the release at any depth was no greater than the surface release with no invertebrates present and the half-life was probably 75 years (assuming 5 x 0.5 g fish and 100 g labeled sediment). These observations were confirmed by the data of Bongers and Khattak (1972) and would not lead one to suspect that benthic invertebrates play a major role in increasing the fractional clearance rate and reducing the half-life of natural systems to 1-3 years as observed in natural systems. However, these observations pertain to closed systems and we thought that in flowing waters, the fractional ’ Issued
as NRCC
17507.
0147-6513/79/030236-09$02.00/O
Copyright 0 1979 by Academic Press. Inc. All rights of reproductmn in any form reserved.
236
237
BENTHICMERCURYCLEARANCE
TABLE
1
SOME CHAR~TERISTICS OF THE SEDIMENT USED BEFORE AND AFTER WATER SEPARATION
Sediment
Dry wt (%I
Organic material (%I
Total mercury (w g-7
Radiolabel (%I
Whole Fine Coarse
100 6 94
2.5 26 1
45 30 280
100 85 15
clearance rate of contaminated sediments might be greatly enhanced by the presence of benthic invertebrates. The present paper reports on the effect of differing densities of tubificid oligochaetes on the fractional clearance rate of mercury from sediments contaminated with radiolabeled inorganic mercury and methylmercury in a flowing water apparatus. We also examine the effectiveness of sand overlays in reducing clearance since it is unclear how effective overlays can be as barriers to mercury-contaminated sediments (Bongers and Khattak, 1972; Widman and Epstein, 1972; Wolery and Walters, 1974; Jernelov et al., 1975), when benthic invertebrates can disturb the barrier (Bongers and Khattak, 1972). We wish to thank H. Akagi for his analysis of methylmercury in the present study. MATERIALS
AND METHODS
Worms. Tubificid oligochaete worms were obtained from Arbor Scientific, Toronto, and identified as Limnodrilus hoffmeisteri Claparede (after Brinkhurst and Jamieson, 1971, p. 463). Upon receipt worms were placed in fresh running water at room temperature (21°C) and maintained 2-3 days in this manner prior to experimentation. Background analysis of the tubificids for mercury by flameless atomic absorption spectrophotometry (Ottawa River Project Group, 1979) showed them to have levels of about 240 ng g-’ dry wt (ppb) total mercury. Sediments. Sediments consisting of sand clay and wood chips were obtained from the Ottawa River, Ottawa. The collection point was in the channel north of Kettle Island downstream of the Canadian International Paper Company (Ottawa River Project Group, 1979). The sediment was first washed through a standard 850qm (No. 20) sieve with as little water as possible, removing most of the visible wood chips. The sieved material was allowed to settle for 24 hr after which the supernatant water containing colloidal and slow-settling material was decanted off. The sieved sediment was water separable into coarse and fine fractions which had different background levels of mercury and which accepted different amounts of radiolabeled mercury (Table 1). The wet sediment was prepared in bulk for the experiments by adding approximately 50% by weight of water containing sufficient amounts of either inorganic mercury or methylmercury radiolabel to provide 2000 cpm g-l. The slurry was stirred vigorously and allowed * Radiolabel from New England Nuclear as ““SHgCI, and CH,‘“3HgC1 with respective specific activities of 0.30 and 0.22 mCi mg-’ on experiment Day 0. Purity of CH, ““3HgCl determined as 85% on experiment Day 0.
238
BODDINGTON,
DE FREITAS,
AND
MILLER
to stand overnight before being added to the various experimental tubes. The ratio of the concentrations of the label between the fine and coarse fractions (7O:l) was not the same as the ratio of the background levels (9S:l) (Table 1). Using a smaller fine fraction, 0.4% Kudo et al. (1977) reported that 44% of the label was absorbed by the fine fraction yielding a ratio of 181: 1. In the presence of wood chips this was further modified such that the concentration ratio appeared dependent upon the number and quantity of competing fractions present. Flowing wafer apparatus. A simple apparatus was assembled to allow the flow of water over the experimental sediments (Fig. 1). This consisted of a water bath maintained at 21°C from which water was pumped through a linear distributor providing a series of water outlets. The outlets from the distributor were controlled by stopcocks and the water flowed over the sediment surface and drained to a charcoal filter. The apparatus was set to deliver a flow of about 15 ml min-’ but the daily range was IO-20 ml min-‘, and regular adjustment was required.
distributor
<:>
-
StOpCOck
-
C:>
22 m m O.D. gloss tubing
drain
FIG. 1. Schematic
diagram
of the arrangement
of the long and short tubes in the flowing
water
apparatus.
BENTHIC
MERCURY
CLEARANCE
239
EXPERIMENTS Series I A known weight of about 20 g, about 4 cm wet wt, of inorganic mercury- or methylmercury-labeled sediment slurry was added to gamma-counting vials, allowed to settle for 7 days, and counted on a Bicron deep-well crystal with a Inotech multichannel analyzer. Three replicates of 1 and 0.5 g wet wt of L. hoffmeisteri representing, respectively, densities of 2600 and 1300 g wet wt m-2 or 5.72 and 2.86 x lo5 individuals mm2 were added to each of the two radiolabeled sediments. Two controls, with no worms added, were run for each labeled sediment. The tubes were placed in the flowing water apparatus as illustrated (Fig. 1) and counted at l-week intervals for 3 weeks. Series IZa Glass tubes 23 cm long with a diameter of 2.2 cm were filled with about 70 g of either inorganic mercury- or methylmercury-labeled sediment slurry to a height of 14 cm. These were allowed to settle for 7 days. Two replicates of 1 g L. hoffmeisteri, representing a density of 2400 g wet wt mm2 or 5.26 x lo5 individuals me2, were added to the two radiolabeled sediments. Replicated controls for Day 0 were fractionated (see below) immediately while replicated controls for Day 21 were run with the experimental tubes. The tubes were covered with aluminum foil to the height of the sediment, providing a unidirectional light source, placed in the flowing water apparatus as illustrated (Fig. l), and fractionated after 21 days. The sediment column was forced out of the tube by means of a rubber-tipped plunger and each centimeter was sliced into a gamma-counting vial via a filter funnel, washed, and topped to a height of 5 cm with water to retain constant counting geometry. After counting, a selection of the sections were analyzed for methylmercury using a dithizone benzene extraction with a Florisil column and silica gel thin-layer, chromatography (H. Agaki, unpublished). Series IZb A 4-cm overlay consisting of the coarse fraction which had been ashed at 600°C for 24 hr was added to a further series of 14 cm labeled sediment. The Day 0 control was taken as being the same as in IIa, the ashed sediment having no mercury or radioactivity associated with it. At the time of column fractionation every effort was made to achieve a precise separation of the two layers. Distribution was analyzed by analysis of variance using the least significant range (LSR) as an a posteriori comparator (Sokal and Rohlf, 1969). RESULTS Series I. Short Tubes During the 7-day settling period the sediment assumed two distinct layers; about 3.5 cm of predominantly coarse material overlayed with about 0.4 cm of the fine. The upper layer represented about 25% of the total fine material present. When the tubes were placed in the flowing water apparatus the upper layer compacted about 0.3 cm. When worms were added the upper layer assumed a flocculent
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appearance and increased to a depth of 1 .Ocm, although the overall depth increased by only 0.2 cm. The upper layer now consisted of about 70% of the total fine material present. The control tubes had some variation associated with their counts over time but these changes were insignificant and no significant loss was recorded. There was a significant loss of mercury over the total period from all tubes containing worms. With 1 g present, 23 and 21% (range 10-35) was lost from the inorganic mercuryand methylmercury-labeled sediments, respectively. These proportions did not differ significantly (t = 0.44; ns; n = 6). The total loss with 0.5 g of worms was 13 and 7% for inorganic mercury- and methylmercury-labeled sediments, respectively. While these values did differ significantly (r = 2.8 1; P < 0.05; n = 6) they approximated one-half the l-g values. The daily fractional clearance (ng Hg lost ng Hg present-’ day-‘) of the sediments was determined between each counting period for each worm density and both mercury labels. Within any group there appeared to be no trend for the time of loss. The fractional clearance could be uniform over all weeks, or disproportionately large during any one of the three. It was considered that the loss was an event dependent upon worm activity which should, on the average, be uniform over time. The group averages of the daily fractional clearance (Table 2) between inorganic mercury and methylmercury were not significantly different but the values for 1 g worms were approximately twice those for 0.5 g worms. When these figures were adjusted for worm density (using g worm g sediment-‘) there was no significant difference in any of the resultant values. The mean density-corrected fractional clearance was 2.238 x 10-l ? 1.561 x 1O-2 day-’ [g worm g sed-‘1-l. Series II. Long Tubes: Total Mercury
As an initial comparison the counts of each centimeter segment of the fractionated long tubes were totaled and adjusted by sediment weight (Table 3). The control tubes, Day 0 and Day 21, showed no significant difference within either the inorganic mercury- or methylmercury-labeled sediments, with or without the sand overlay. When worms, but no sand overlay, were present, there was a significant loss, about 8% of the total for both inorganic mercury- and methylmercury-labeled sediments. The fractional clearance rates adjusted for worm density gave values TABLE FRACTIONAL
CLEARANCE OF SEDIMENTS CONTAINING
Inorganic Worm wet wt (8)
Sediment wet wt (8)
1
20.67
0.5
22.76
(1 Mean
2 SE. n = 9.
INORGANIC DIFFERENT
Fractional clearancea (day-‘) x x x x
MERCURY DENSITIES
AND METHYLMERCURY OF TUBIFICID WORMS
FROM
Methylmercury
mercury
1.139 a3.464 6.000 k1.700
2
Sediment wet wt w 10-Z 10-S 1O-3 10-S
20.87 22.93
Fractional clearancea (day-‘) 1.078 k2.574 4.311 k1.108
x x x x
t 10-Z 1O-2 10-S 10-3
0.141 (ns) 0.832 (ns)
241
BENTHICMERCURYCLEARANCE
(Table 3) closely approximating those found in the short-tube experiments. When worms and a sand overlay were present, there was no significant loss of mercury from the system. In the tubes labeled with inorganic mercury, analysis ‘showed that about 7% of the label was in the methyl form. There was no difference between tubes with or without worms. In the methylmercury-labeled sediments the Day 0 controls had about 85% of their label as methylmercury. By Day 21 there had been a decline in the methylmercury fraction to about 60%, although the content of the first centimeter remained high at about 85%. There was no difference due to the presence of worms. Distribution (a) Without overlay. The depth distributions of both inorganic mercury and methylmercury were similar (Fig. 2A). The controls showed a significantly higher concentration in the first centimeter of sediment than was present in the remaining 13 cm. This was due to the relatively slow settling of the fine material. The values of methylmercury were lower than those for inorganic mercury. This was probably an artifact of column or slurry preparatioh for the difference was consistent within replicates. The effect of the worms was the same in both cases: The label was lost from the first centimeter such that its concentration was indistinguishable from the remaining depths. Although not evident from the uniform distribution, in the case of methylmercury there must have also been some upward redistribution. Thus the change in the first centimeter of methylmercury from 63.9 to 23.8 ng g-l is insufTABLE3 COMPARISON
OF TOTAL RADIOACTIVITY LONG-TUBE EXPERIMENTS, lnorgamc
WIthout overlay Day 0 (control) Day
21 (control)
Pooled
mean
1 g wet wt worms Meall With overlay Day 21 (control) 1 g wet
Mean
wt worms
FRACTIONAL SERIES IIa
CLEARANCE AND
Total wet wt
66.19 70.88 64.17 65.64
1.346 1.376 1.360 1.390
70.89 68.69 67.88
1.236 1.276
1.361 ~0.0210 1.373” kO.0185 1.367 1-0.0175 1.256’ kO.0283
90.32 88.60 87.00 87.33 88.3 1
1.399 1.388 I.436 1.408
1.394 kO.0078 1.422” i-o.0199
Treatment (mean *SD)
IN THE
IIb
mercury
Weightadjusted total cpm (X IO”)
r SD
AND
Methylmercury Densltycorrected fractional clearance”
2.620
x 10 ’
Total wet wt
Weightadjusted total cpm (X10”)
64.17 68.87 67.23 65.00
I.814 1.738 1.761 1.743
69.60 66.62 66.91
1.633 1.684
I .776 -co.0537 1.752” kO.0122 I.764 1-0.0347 1.659” 20.0359
96.29 93.62 93.23 95.09 94.47
I.732 1.674 1.705 1.712
1.703 kO.0408 1.708” to.0051
Treatment (mean *SD)
o See text for detads. Fractmnal clearance expressed as [day-‘.@ worm/g sediment-‘) ‘I. ’ Control Day 0 vs Day 21: morgamc mercury, I = 0.606 (ns): methylmercury. r = 0.612 (ns). ’ Pooled control vs worms: morgamc mercury. I = 5.671 (P c 0.011: methylmercury, t = 3.428 (t’ < 0.01). ” Sand control vs sand worms: Inorganic mercury, I = 1.824 (ns); methylmercury, I = 0.183 (ns).
Densitycorrected fractional clearance”
-
1.902
x 10~’
-
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DEPTH
I
12 13 14
FIG. 2. Schematic representation of changing inorganic mercury (I) and methylmercury (M) distribution in sediments between control (C) tubes and those containing worms (W) in both the absence (A) and the presence (B) of sand overlays. Insert values are ng Hg g dry wt-I; ppb, grouped and pooled after ANOVA and LSR testing.
ficient to account for the change in the mean from 27.0 to 23.8 ng g-l. Such is not the case for inorganic mercury. Furthermore while the fractional clearance rates for both labels were similar (Table 3) the concentration of methylmercury in the first centimeter was proportionately much lower than for the inorganically labeled sediment. (b) With sand overlay. When sand overlays were used (Fig. 2B) the concentration in the first centimeter of the controls was considerably reduced. Since these controls did not differ in total from those without the overlay (Table 3) the reduction was an artifact associated with the addition, and inadequate separation, of coarse uncontaminated sediments to the fine surface. Even so, when worms were present the distribution did change. Separation was more difficult as only 3 cm of what was clearly sand remained. While there was no overall loss from the system (Table 3) there was an upward movement of radiolabel into the proximal layer (D) of the overlay. In the case of methylmercury this was sufficient to render the first centimeter of sediment indistinguishable from the remaining 13 cm. In the case of inorganic mercury while there was a significant reduction in the concentration of the first centimeter and an upward movement into the proximal layer (D) of overlay, the first centimeter was still distinguishable from the remaining 13 cm. DISCUSSION The results show that tubificid worms caused a significant, density-dependent loss of mercury from sediments. This loss occurred mainly from the first centimeter of sediments although some redistribution occurred. Sand overlays formed an effective barrier within the experimental period, but some upward transport was observed. The fractional clearance rate, when adjusted in terms of worm density, was consistent for all experimental situations. Its reciprocal when multipled by In 2 gives
BENTHIC
MERCURY
243
CLEARANCE
the half-life of the system. Thus the coefficient of 2.328 x 10-l represents a half-life of 2.98 days when 1 g of worms is present in 1 g of sediment. In the present experiments where the short tubes had 1 and 0.5 g of worms in 20 g of sediment and the long tubes had 1 g of worms in about 70 g, the respective half-lives would be 60, 120, and 200 days. The latter density is close to that used by Jernelov (1970) and it is quite evident that the present clearance rates are far in excess of Jernelov’s methylation rates which would yield a half-life of about 75 years. A worm density of less than 4 x 1O-5 g g sediment-’ would yield a half-life of 180 years, a result indiscernible from situations when no worms were present (cf. Kudo et al., 1975). Such would be the case in the Ottawa River where tubificid worms can be calculated as having a standing stock of 4 x lop6 g g sediment-’ (after Trudel et al., 1977). Other situations, such as Toronto Harbour, where densities approach 500 g wet wt rnp2 (Brinkhurst, 1970), one-fifth those presently used, could be approximated by 1 g worms in 100 g sediment at a depth of 4 cm. The resultant half-life would be only 300 days. Furthermore, if other organisms, such as molluscks, are equally effective in clearance as they are in methylation (Jernelov, 1970), then broader application of the clearance coefficient may be possible. For example, while the Ottawa River has a low standing stock of tubificids, it is high in bivalve mollusks giving 5.5 x 10e3 g organism g sediment-’ (wet wt, after Trudel et al., 1977). On this basis the half-life could be as low as 1.6 years, close to that inferred from sediment analysis (Ottawa River Project Group, 1979). There are, of course, many reservations to such extrapolations. For instance sediments vary tremendously in their characteristics such that they may not be amenable to the same process. Equally the seasonal changes in temperature will affect the activity of the organisms. The concept that overlays can act as effective barriers is questionable in the long term. While they contained the loss in the present circumstances there was an upward distribution. On the basis of the present results it would take about 30 weeks for all of the overlay to achieve the mean concentration of the contaminated sediments. Again one must temper this statement with considerations of the density of tubificid employed but be equally aware that other invertebrates could aid in the process. REFERENCES BONGERS, Sediments. BRINKHURST,
Harbour, BRINKHURST,
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L.,
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KHATTAK,
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(1972).
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E.P.A.-16080-HVA-01/72. R. 0. (1970). Distribution and abundance of tubificid Lake Ontario. J. Fish. Res. Bd. Canad. 27, 1961-1969. R. O.,
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KUDO, A., MORTIMER, D. C., AND HART, J. S. (1975). Factors influencing desorption of mercury from bed sediments. Canad. J. Earth Sci. 12, 1036- 1040. KUDO, A., TOWNSEND, D. R., AND MILLER, D. R. (1977). Prediction of mercury distribution in river sediments. J. Amer. Sot. Civil Eng. EE, 606-614. Ottawa River Project Group (1979). Mercury in the Ottawa River. Environ. Res. 19, 231-243. SOKAL, R. R., AND ROHLF, F. J. (1969). Biometry. Freeman, San Francisco. THOMAS, R. L., JAQUET, J.-M., AND MURDOCH, A. (1975). Sedimentation processes and associated changes in surface sediment trace metal concentrations in Lake St. Clair, 1970- 1974. Proc. Int. Conf. Heavy
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TRUDEL, B. K., DEFREITAS, A. S. W., AND MILLER, D. R. (1977). Mercury dynamics of the aquatic invertebrate community in the Ottawa River study area. In Ottawa River Project Final Report. NRCC, Ottawa. WIDMAN, M. U., AND EPSTEIN, M. M. (1972). PolymerFilm Overlay SystemforMercury Contaminated Sludge-Phase
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36080-HT2-OY72.
WOLERY, T. J., AND WALTERS, L. J., JR. (1974). Pollutant mercury and sedimentation in the western basin of Lake Erie. Proc. Conf. Great Lakes Res. 17, 235-249.