leaching after most or all of the inorganic salts have dissolved. The presence of simple salts such as calcium oxide, anhydrite or gypsum, sodium and potassium sulfates, and magnesium and calcium carbonates is indicated by the composition of leachate samples. For example, the extreme alkalinity of leachates of the first few pore volumes (Figure la) is largely attributable to hydrolysis by calcium oxide. The relatively low concentrations of soluble sodium, potassium, and magnesium compared to calcium precludes salts of these elements as major causes of alkaline hydrolysis. Hydrolysis by most other salts likely to occur in the fresh ash would not generate such strong initial alkaline conditions. With leaching, the highly reactive calcium oxide dissolves and is largely removed by the percolating water after displacement of -20 pore volumes as evidenced by the 1-unit drop in pH values and the concomitant decline in soluble calcium levels. A portion of the calcium oxide upon contact with water may also be converted to calcium carbonate (6). The slow dissolution and hydrolytic reactions of carbonate compounds could then explain the progressive slow decline in pH values and the rise in soluble magnesium levels of leachates collected after the first 20 pore volumes. Similarly, the occurrence of sodium sulfate in ash could partially account for the initial high levels followed by a rapid decline in soluble sodium and sulfate levels in leachates of the first 200 pore volumes. The parallel trends displayed by soluble levels of sulfate and calcium with leaching from 500-2500 pore volumes indicate that gypsum (originally as anhydrite in fresh ash) is a likely common source for these two dissolved constituents. The behavior of minor and trace elements may be considered in a similar fashion. For example, the apparent high leachability of boron likely arises from its presence in ash as an admixed borate salt of moderate solubility. Its relatively rapid loss from ash with leaching (Table I) without a proportionate parallel loss of sodium or other highly ionic major matrix element strongly suggests that most of the boron does not occur within the glassy ash particles. Other minor and trace elements, such as phosphorus and lead, are not leached from ash (Table I), which suggests that they occur within the silicate matrix or possibly as adsorbed constituents. The attainment of nearly constant levels of sodium and the simultaneous absence of detectable quantities of potassium after leaching with 1000 pore volumes (Figure IC)suggest that salts of these two elements in ash have completely dissolved. The subsequent soluble sodium concentration in leachates ( 4 . 4 5 ppm) probably represents dissolution of fly ash matrix. Although other elements also display nearly constant soluble concentrations after leaching with 2500 pore volumes, their levels in solution may not be solely related to dissolution of ash matrix since a disproportionate relationship exists for
some elemental ratios between leachate and residue composition. Differences in the content of relatively mobile constituents such as calcium, boron, and strontium in top and bottom halves of fly ash cores (Table I) further suggest that complete dissolution of admixed or surface adsorbed salts has not yet been reached. Results obtained in this leaching trial cannot be easily compared to other published studies on ion release characteristics of fly ash because of differences in experimental procedures. Other studies have usually involved leaching for extremely short time periods ranging from seconds ( 1 )to a few days ( 2 , 3 )or equilibrations or limited numbers of extractions in closed containers ( 5 - 7 , l l ) . In conclusion, results of this study indicate that short-term dissolution characteristics of fly ash are largely dominated by the nature and the quantity of admixed and surface adsorbed inorganic salts. As these salts are solubilized and removed from ash, concentrations of soluble constituents released from chemical alteration of the glassy matrix are maintained a t relatively low levels. Acknowledgment I express my appreciation to Lloyd Hodgins of the Alberta Soil and Feed Testing Laboratory for ICP analysis and to George Braybrook for assistance with SEM. Literature Cited (1) Cox, J. A.; Lundquist, G. L.; Przyjazny, A.; Schmulbach, C. D. Enuiron. Sci. Technol. 1978,12,722-3. (2) Eggett, J. M.; Thorpe, T. M. J. Enuiron. Sci. Health, Part A 1978, 13,295-313. (3) James, W. D.; Janghorbani, M.; Baxter, T. Anal. Chem, 1977,49,
,.
1994-7
(4) Phung, H. T.; Lund, L. J.; Page, A. L.; Bradford, G. R. J. Enuiron. Qual. 1979,8,171-5. ( 5 ) Shannon. D. G.: Fine. L. 0. Environ. Sci. Technol. 1974. 8. 1026-8. (6) Talbot, R. W.; Anderson, M. A.; Andren, A. W. Enuiron. Sci. Technol. 1978,12,1056-62. (7) Theis, T. L.; Wirth, J. L. Enuiron. Sci. Technol 1977, 11, 1096-100. ( 8 ) Plant, C. 0.; Martins, D. C. J. Soil Water Conseru. 1973, 4 , 177-9. (9) Linton, R. W.; Loh, A,; Natusch, D. F. S.; Evans, C. A.; Williams, P. Science 1976,191,852-4. (10) Pawluk, S. At. Absorpt. News. 1967,6,53-6. (11) Dressen, D. R.; Gladney, E. S.; Owens, J. W.; Perkins, B. L.; Wienke, C. L.; Wangen, L. E. Enuiron. Sci. Technol. 1977, 11, 1017-9. (12) Fisher, G. L.; Prentice, B. A.; Silberman, D.; Ondov, J. M.; Biermann, A. H.; Ragaini, R. C.; McFarland, A. R. Enuiron. Sci. Technol. 1978,12,447-51.
Received for review August 4,1980. Accepted March 19,1980. This project was funded by a grant from the Alberta Environmental Research Trust.
Evidence of the Unsuitability of Gravity Coring for Collecting Sediment in Pollution and Sedimentation Rate Studies Murdoch S. Baxter,* t John G. Farmer,* Ian G. McKinley,t David S. Swan,t and William Jacks University of Glasgow, Glasgow G12 8QQ, Scotland, United Kingdom
Introduction During the last decade there has been a rapid increase in the number of investigations which have examined records + Department of Chemistry. t Department of Forensic Medicine and Science. 8 Department of Natural Philosophy.
of (a) sedimentation, via radiometric dating techniques (l4), (b) metal (6-11) and organic (12, 13) pollution, and (c) reworking processes Of activity (l4-I6)9 from the measured vertical distributions of chemical species preserved in layers of coastal marine and lacustrine sediment. Of primary importance in such studies is the collection of undisturbed sediment, in particular the efficient sampling of top-
0013-936X/81/0915-0843$01.25/0 @ 1981 American Chemical Society
Volume 15, Number 7, July 1981 843
W Profiles of lead, manganese, lead-210, cesium-134, and cesium-137 are compared in sediment cores collected by two different sampling devices in the Clyde Sea area. Small-diameter gravity coring is shown to be unsuitable for the efficient collection of upper layers of unconsolidated sediment.
The extensive sediment loss and mixing process which accompanies gravity coring produces quite feasible, and thus particularly misleading, vertical profiles of chemical parameters. A soft-landing, hydraulically-damped coring device is found to be more reliable for such work.
most layers of deposit. Despite the available range of coring devices, it is commonly acknowledged (17) that problems in core collection can occur and may include the loss of highly porous upper layers of sediment, significant shortening during penetration, mixing of layers, overpenetration, and tilting. Nonetheless, the “collection of undisturbed samples” is often claimed but seldom investigated or proved. In this report, we wish to reemphasize the inherent dangers, specifically with respect to the not uncommon application of small-diameter gravity coring, in the collection of highly porous unconsolidated sediment as found in many coastal and lacuspine environments. We present evidence based on the depth distributions of stable P b and Mn, 210Pb, 134Cs,and 13’Cs in sediment cores collected at the same sites by two separate devices: (a) a small-diameter gravity corer and (b) a soft-landing hydraulically damped Craib corer (18). The sediments studied were fine-grained silty clays from the deep basins of two fjordic sea lochs, Loch Goil and Gareloch, of the Clyde Sea area. This material was relatively unconsolidated and highly porous (-90% at surface, 70-80% at 30-40-cm depth). The major sources of investigated species in these sediments are well-defined: (a) catchment area input for Mn, followed by diagenetic remobilization (19, 20), (b) pollution-related release of Pb from the nearby Clydeside conurbation (21), (c) atmospheric 222Rnand in situ 226Ra decay for unsupported and supported 210Pb,respectively (22), and (d) global fallout from nuclear weapon tests for 13’Cs plus additional inputs of both 137Csand 134Csthrough coastal water transport from the Irish Sea into which nuclear waste is discharged by the British Nuclear Fuels Ltd. reprocessing plant at Windscale, Cumbria (23,24).
ment enters the barrel through the flexible metal fingers of the core catcher, positioned at the base of the plastic liner. On withdrawal from the sediment, the one-way valve closes and the fingers of the core retainer shut. The Craib corer (internal diameter, 5.7 cm; core length, typically 18 cm; manufactured in the Scottish Marine Biological Association workshops, Dunstaffnage, Oban, Scotland) is described fully by Craib (18)and is specifically designed to soft-land on the sea bed and retain intact the light superficial layer of sediment largely by means of a hydraulic damper which ensures a slow approach to and penetration of the sediment. In both cases, supernatant water was immediately removed by syringe, and the cores were frozen before being returned to the laboratory where they were sliced into 1-cm thick sections with a band saw. Sediment sections were weighed wet and oven-dried at 90 “C to constant weight. Material for analysis was selectively removed from the central portions of sections to prevent possible contamination of sediment from different depths, due to frictional smearing of the deposit along the corer tube as the liner penetrates the sediment. Metals were measured by flame atomic absorption spectrometry on solutions derived from initial leaching of 0.5-2 g of dried, ground sediment at 60 ‘C with 100 mL of 25% CH3C02H solution, 0.25 M in NH20H.HC1 (21). Lead-210 (half-life was 22.3 yr) was determined by CY spectrometric counting of granddaughter zlOPo(half-life = 138.4 days) released from ca. 5 g of dried, homogenized sediment by successive digestion with concentrated acids and then spontaneously deposited on silver plating disks; cesium-134 and cesium-137 were measured by direct y-ray spectrometry using a GeLi detector (25).
Experimental Section Sediment samples were collected at the sites listed in Table I by using a conventional small-diameter gravity corer and a Craib corer. The conventional small-diameter gravity corer (internal diameter, 6 cm; core barrel length, 90 cm; manufactured by Underwater and Marine Equipment Ltd., Farnborough, Hampshire, England) consists essentially of a lead-weighted barrel enclosing an exchangeable plastic liner. In operation, the corer is allowed to free-fall from a few meters above the sediment surface, a one-way valve at the top of the barrel opening as the corer descends to allow escape of water. Sedi-
Results Depth profiles of Pb, Mn, total 210Pb,134Cs,and 137Csare shown for Craib (C) and gravity (G) core pairs LGCS and LGG5 (Figure l),LGC4 and LGG7 (Figure 2), and GLC and GLG (Figure 3). LGCS and LGG5. There are obvious differences between the cores in the profiles of all five entities: Mn, 210Pb,134Cs, and 137Csare significantly elevated in the upper sections of LGCS relative to LGG5, Pb concentrations are >350 pglg through the length of LGCS but only down to 7 cm in LGG5; the irregular structure of the Mn profile in LGCS (related both
Table 1. Core Collection Sites in Loch Goil and Gareloch loch
Loch Goil
Loch Goil
Gareloch
a
core
*
LGG5
4 Feb, 1976
LGCS
10 Aug, 1976
LGG7
4 Feb, 1976
LGC4
3 Mar, 1976
GLG
7 Nov, 1975
GLC
7 Nov, 1975
Final-letter key: C, Craib, G, gravity.
844
date of collectlon
Environmental Science 8, Technology
location
56’8’20’’N 4’53‘21 NW 56’8’20NN 4’53‘21”W 56’6’48” N 4°5’6NW 56’6’48”N 4’5’6“W 56’2’42”N 4O49’36”W 56’ 2‘42NN 4’49’36”W
water depth, m
core length, cm
82
39
82
17
40
38
40
17
48
33
48
15
LGC4
l
m a
m
l
I
1
. .
0
e l ’
a
a c a t
0
a
8
b
200
c
a
a
5-
c
m
a
m . a
IO-Pb
Em
I.
a
a
a
.
a
a a
a
a a a m
B b
a
B
b b I 1
I
a* a
.:
Mn 010
m
1
a
a
a
a
+
fE
1
a
a P
a
t
a
a
a a
a
om
a m
1
a
a
a
I
15-
a a
a
a . a
I
a a m
a
a
I
a
a a a
a
I..
. . .
I I m I
I.
1
o
1
10 .
a
3
m
.
..
0.5
400
.
b
a
a a
a
a
I
LC ;5
.
a
a
b
30
I;L
LGG 7
Figure 1. Concentration/sediment depth profiles of Pb, Mn, 210Pb, 13%s, and 137Cs in Loch Goil sediment cores LGCS (Craib) and LGG5 (gravity).
Figure 2. Concentration/sedimentdepth profiles of Pb, Mn, 210Pb, 134Cs, and 137Csin Loch Goil sediment cores LGC4 (Craib)and LGG7 (gravity) (LGG7 was not analyzed for 210Pb).
to the upward migration/surface reoxidation of Mn ions and variations in Ca and carbonate contents of the sediment) is markedly different from the gradual downward decline in LGG5; the persistence of higher levels of total 210Pb in the Craib core is reflected in a more gradual rate of decline of “unsupported” 210Pbfrom 6.4 to 4.1 dpm/g (10-17 cm) compared to the rapid decrease from 8.3 to 2.4 dpm/g in “unsupported” zloPb from 2-8 cm in LGG5; the levels of 137Cs,a marker of recent origin, are greater by a factor 4-6 in the top 4 cm of LGCS relative to LGG5 and fall only to the levels of LGG5 below 8-cm depth; 134Cs,the shorter-lived (half-life = 2.1 yr) isotope of Cs, is observed down to 9 cm in LGCS but is beneath detection limits (-1 dpm/g) in LGG5. LGC4 and LGG7. Similar differences in species’ profiles are observed: (a) Pb concentrations exceed 190 pg/g throughout LGC4 but only down to 3 cm in LGG7; (b) Mn in the upper 4 cm of LGC4 is significantly greater than in the surface of LGG7; and (c) 134Csis absent in LGG7 but measurable to 8 cm in LGC4 where 137Cslevels are also much enhanced. GLC and GLG. The significant discrepancies include (a) the “bulge” in Pb content from 3 to 9 cm in GLG, (b) conaistently higher total 210Pblevels in GLC and a more rapid decline in “unsupported” 2loPb for GLG, and (c) for 134Cs, presence to 13 cm in GLC but absence in GLG and, for 137Cs, major differences in concentration.
elevated surface levels of Pb, from anthropogenic activities, declining to background concentrations at depth (LGG5; LGG7); enhanced surface levels of Mn, as a consequence of natural diagenetic/mobilization mechanisms (LGG5; LGG7); the 210Pb data, once corrected for the “supported” 2loPb contribution from in situ 226Radecay, displaying an exponential decline in activity which can be used in conjunction with the 22.3-yr half-life to calculate sedimentation rates and to provide a time scale for the sediment columns (LGG5; GLG); and the presence of the artificial radionuclide 137Cs, released to the environment during only the last 25 yr, in surface sediment (LGG5; GLG). The observed disparities between Craib and gravity core profiles at all three stations (Figures 1-3), however, point to the substantial (215 cm) loss of surface sediment using the small-diameter gravity corer. The single piece of evidence most damaging to the validity of the small-diameter gravity coring technique is the absence of 134Cs,the Windscale-released short-lived isotope of Cs, from LGG5, LGG7, and GLG. It follows that not even the presence of 137Csin gravity cores necessarily guarantees the retention of undisturbed topmost layers of sediment. Indeed, the slight elevation of 137Cs in the upper sections of LGG5, LGG7, and GLG relative to the lower sections of LGC9, LGC4, and GLC appears to reflect, as do the profiles of Pb, Mn, and 210Pb, some distortion of profiles probably through partial retention and mixing of some particles of surface sediment during the coring operation. It must be concluded, therefore, that vertical overlapping of Craib and gravity core profiles is not valid, despite the apparently reasonable matching patterns which can emerge on occasion (e.g., GLC and GLG).
Discussion It must be emphasized that the gravity core profiles shown here, considered in isolation, seem perfectly plausible: e.g.,
Volume 15, Number 7, July 1981 845
-
5E
400
800
0.05
GLC 0.1
5
10
40
80
5-
Acknowledgment
I-
pu
We thank the staff of the Clyde River Purification Board for provision of sampling equipment, ship time, and associated expertise.
10-
.-c 15
-
20
-
25
-
30
-
Literature Cited
GLG
Figure 3. Concentration/sedimentdepth profiles of Pb, Mn, *I0Pb, 134Cs, and I3’Cs in Gareloch sediment cores GLC (Craib)and GLG (gravity).
In view of the demonstrated loss of sedimentary material and the possible distorting effects of small-diameter gravity coring operations in the collection of unconsolidated finegrained sediment, the consequence of misguidedly accepting data from such cores, without independent corroboration, could be serious. For example, it could lead to significant underestimation of the extent of metal pollution or of the inventory of an artificial radionuclide; e.g., the inventory of “excess” lead in LGC4 is 3-4 times that from 0-20 cm in LGG7. In addition, there could be implications for the quantification of exchange and bioturbation processes across and within surface sediment layers if erroneous values were used for concentrations of species in the surface layer. The distortions of 21oPb profiles could result in large errors in the estimation of sedimentation rates and the dating of sediment columns; e.g., the sedimentation rate based on “unsupported” 210Pb data from 0-15 cm in GLC is -7-8 times higher than that based on the corresponding data from 4-14 cm in GLG, at least partially explaining the occurrence of apparently very high natural background levels of P b (500-600 pg/g) at depth in GLG. Probably the only really useful evidence from the smalldiameter gravity cores obtained in this study is estimates of the natural background concentrations of trace elements (e.g., Pb) at depth, prior to the onset of the Industrial Revolution, and of the activity of 210Pb supported by 226Radecay. In contrast, there is good evidence from a series of additional cores collected a t Loch Goil Station 1that Craib coring is a
846
comparatively reliable technique for the recovery of undisturbed sediment cores (26). In summary, small-diameter gravity coring, and possibly other techniques using heavy and high-velocity devices, in unconsolidated surface sediments can induce extensive loss of material while simultaneously distorting the vertical distribution of sedimentary species of interest. The resultant profiles, considered independently, are surprisingly feasible, resembling many published elsewhere. We consider that the large volume of published data on sediments collected by small-diameter gravity coring should be treated with caution and that, in general, considerable attention should be paid to ensuring the complete retention, and preservation of vertical integrity, of sediments.
Environmental Science & Technology
(1) Koide, M.; Bruland, K. W.; Goldberg, E. D. Geochim. Cosmochim. Acta 1973.37.1171-87. (2) Robbins,J. A,;Edgington, D. N. Geochim. Cosmochim. Acta 1975, 39,285-304. (3) Nittrouer, C. A,; Sternberg, R. W.; Carpenter, R.; Bennett, J. T. Mar. Geol. 1979,31,297-316. (4) Smith. J. N.: Walton. A. Geochim. Cosmochim. Acta 1980.44. . , 225-40. (5) Krishnaswami, S.; Benninger, L. K.; Aller, R. C.; Von Damm, K. L. Earth Planet. Sci. Lett. 1980,47,307-18. (6) Bruland, K. W.; Bertine, K.; Koide, M.; Goldberg, E. D. Environ. Sci. Technol. 1974,8,425-32. (7) Matsumoto, E.; Wong, C. S. J. Geophys. Res. 1978,82, 547782. ( 8 ) Clifton, R. J.; Hamilton, E. I. Estuarine Coastal Mar. Sci. 1979, 8,259-69. (9) Goldberg, E. D.; Griffin, J. J.; Hodge, V.; Koide, M.; Windom, H. Enuiron. Sci. Technol. 1979,13,588-94. (10) Nriagu, J. 0.; Kemp, A. L. W.; Wong, H. K. T.; Harper, N. Geochim. Cosmochim. Acta 1979,43,247-58. (11) Farmer, J. G.; Swan, D. S.; Baxter, M. S. Sci. Total Enuiron. 1980,16,131-47. (12) Farrington, J. W.; Henrichs, S. M.; Anderson, R. Geochin. Cosmochim. Acta 1977,41,289-96. (13) Muller, G.; Grimmer, G.; Bohnke, H. Naturwissenschaften 1977, 64,427-31. (14) Robbins. J. A,: Krezoski. J. R.: Mozlev. S. C. Earth Planet. Sci. ’ Lett. 1977,36,325-33. (15) Bennineer. L. K.: Aller, R. C.: Cochran, J. K.; Turekian, K. K. Earth Planet: Sci. Lett. 1979,43, 241-59. (16) Cochran, J. K.; Aller, R. C. Estuarine Coastal Mar. Sci. 1979, 9,739-47. (17) McIntyre, A. D. Nature (London) 1971,231,260. (18) Craib, J. S. J . Cons., Cons. Int. Explor. Mer. 1965,30,34-9. (19) Murray, J.; Irvine, R. Trans.-R. SOC.Edinburgh 1894, 37, 721-42. (20) Mackereth, F. J. H. Proc. R. SOC.London, Ser. B 1966, 250, 165-213. (21) Farmer, J. G. University of Glasgow, unpublished data, 1981. (22) Robbins, J. A. In “The Biogeochemistry of Lead in the Environment”; Nriagu, J. 0. Ed.; Elsevier/North Holland Biomedical Press: Amsterdam. 1978 Vol. 1A. (23) Baxter, M. S.; McKinley, I. G.; MacKenzie, A. B.; Jack, W. Mar. Pollut. Bull. 1979910,116-20. (24) McKinlev. “ , I. G.: Baxter. M. S. N A T O Conf. Ser.. lSer.14 . . 1980 4,539-45. (25) MacKenzie, A. B.; Baxter, M. S.; McKinley, I. G.; Swan, D. S., Jack, W. J.Radioanal. Chem. 1979,48,29-47. (26) McKinley, I. G. Ph.D. Thesis, University of Glasgow, 1979. I
,
“ I
I
Received for reuiew October 28,1980. Accepted March 16,1981. The studentship support of the Natural Environment Research Council (I.G.McK.)and of the Science Research Council (D.S.S.)isgratefully acknowledged.
