1+filtration of 1 or 2 liter samples, however, produced a precision of detter than ±5% at levels of...
TRANSCRIPT
.4 " _
ASSESSMENT OF CENTRIFUGA TION AND FIL TRA TION AS METHODS FOR DETERMINING LOW CONCENTRA TIONS OF SUSPENDED SEDIMENT IN NA TURAL WATERS
by P. CAMPBELL
S. ELLIOTT
FISHERIES AND MARINE SERVICE
SERVICE DES PECHES ET DES SCIENCES DE LA MER
TECHNICAL REPORT No. RAPPORT TECHNIQUE N° 545
1975 1+ Environment Environnemenl
Canada Canada
Fisheries Service des peches and Marine et des sciences Service de la mer
Technical Reports
Technical Reports are research documents that are of sufficient importance to be preserved, but which for some reason are not appropriate for primary scientific pUblication. Inquiries concerning any particular Report should be directed to the issuing establishment.
Rapports Tecbniques
Les rapports techniques sont des documents de recherche qui revetent une assez grande importance pour etre conserves mais qui, pour une raison ou pour une autre , ne conviennent pas a une publication scientifique prioritaire. Pour toute demande de renseignements concernant un rapport particulier, il faut s'adresser au service responsable.
Department of the Environment
Fisheries and Marine Service
Research and Development Directorate
TECHNICAL REPORT NO. 545
(Numbers 1-456 in this series were issued as Technical Reports of the Fisheries
Research Board of Canada. The series name was changed with report number 457)
Minist~re de l'Environnement
Services des P~ches et des Sciences de la mer
Direction de 1a Recherche et D~veloppement
RAPPORT TECHNIQUE NO. 545
(Les num6ros 1-456 dans cette serie furent utilis~s comme Rapports Techniques de l'Office
des recherches sur les p6cheries du Canada. Le nom de 1a s~rie fut change avec Ie
rapport num~ro 457)
ASSESSMENT OF CENTRIFUGATION AND FILTRATION
AS METHODS FOR DETERMINING LOW CONCENTRATIONS
OF SUSPENDED SEDIMENT IN NATURAL WATERS
by
P. CAMPBELL
and
s. ELLIOTT
This is the sixty-second Ceci est Ie soixante-deuxi~me
Technical Report from the Rapport Technique de la Direction de la
Research and Development Directorate Recherche et D~veloppement
Freshwater Institute Institut des eaux douces
Winnipeg, Manitoba Winnipeg, Manitoba
1975
•
The data for this report were obtained as a result of investigations
carried out under the Government of Canada Environmental~Social
Program, Northern Pipelines, Task Force on Northern Oil Development .
iii
TABLE OF CONTENTS
Page
LIST OF TABLES . . . . . • . • • • . • • • • • • • . . • • . . iv
ACKNOWLEDGEMENTS • . • • • • . . .• . ' . ' • • • . . • • • • . . . v
ABSTRACT .••..•••••.•••••••••••••... vi
INTRODUCTION • • • • • • • • • • • • • • • • • • • • • • • • , 1
FI ELD METHODS. • • • • • • • • • • • • • • • • • • • • • • • . 2
LABORATORY METHODS • • • • • • • • • • • • • • • • • • • • • . 5
STATISTICAL METHODS· • • • • • • • • • • • • • • • • • • • • • 5
RESULTS. • • • • • • • • • • • • • • • • • • • • • • • • • • • 6
DISCUSSION • • • • • • • • • • • • • • • • • • • • • • • • • • 15
CONCLUSIONS. • • • • • • • • • • • • • • ' . ' • • • • • • • • • 16
REFERENCES . • • • . • • • • • • • • • • • • • • • • • • • • • 18
Table 1.
Table 2.
Table 3.
Table 4.
Table s.
Table 6.
Table 7.
Table 8.
Table 9.
iv
LIST OF TABLES
Destination and treatment of water samples collected from the Harris River on June 11, 1973
Destination and treatment of water samples collected from the Harris River on August 22, 1973
Suspended sediment concentrations, means and standard deviations derived for centrifuged samples collected from the Harris River, June 11) 1973.
Suspended sediment concentrations, means and standard deviations derived for centrifuged samples collected from the Harris River, August 22, 1973.
Suspended sediment concentrations, means and standard deviations derived for samples collected from the Harris River, June 11, 1973, and filtered in Fort Simpson.
Suspended sediment concentrations, means and standard deviations derived for samples collected from the Harris River, June 11, 1973 and filtered in Yellowknife.
Suspended sediment concentrations, means and standard deviations derived from filtrations of the entire contents of 2-liter bottles collected from the Harris River, August 22, 1973.
Suspended sediment concentrations, means and standard deviations derived from I-liter filtrations of partially (1.5 liters) filled 2-liter bottles collected from the Harris River, August 22, 1973.
Intercepts, slopes and correlation coefficients from linear regression analyses carried out on "aliquot number" versus "weight of suspended sediment obtained per aliquot" for 20-liter filtered samples collected from the Harris River, June 11, 1973.
PAGE
3
4
7
8
9
10
11
12
14
v
ACKNOWLEDGMENTS
Many useful comments and crltlClsms were provided by G. Brunskill, M. Capel, B. Graham, M. Healey, G. Morden, G. Vascotto and R. Wagemann. D. Rosenberg and summer assistants were largely responsible for the collection of samples. Similarly, most of the laboratory determinations were carried out by L. Dory, S. Michaelis and G. Morden.
The manuscript was typed by G. Porth and C. Plumridge.
vi
ABSTRACT
Campbell, P. and S. Elliott. 1975. Assessment of Centrifugation and Filtration As Methods For Determining Low Concentrations of Suspended Sediment in Natural Waters. Fish. Mar. Servo Res. Dev. Tech. Rep. 545: 18 pp.
An investigation was carried out to determine if concentrations of less than 10 mg i - l sus pended sediment could be reliably measured in natural waters. Centrif uga t ion of even relatively large (20 liter) volumes of wa t er resu lted in h i ghly variable suspended sediment concentrations. Poor precision also resulted when concentrations of suspended sediment were determined after filtering 1 liter aliquots taken from a 20-liter sample. It was shown that in large samples a stratification of suspended sediment resulted wi thin hours of collection. Vigorous shaking Defore removing an aliquot was not sufficient to homogenize the sample. Filtration of 1 or 2 liter samples, however, produced a precision of Detter than ±5% at levels of 2-3 mg £-1 suspended sediment. This high precision was attained irrespective of whether filters were dried to constant weignt oy desiccat ion or in an oven at 90-950 C.
~ ~
RESUME
Campbell, P. and S. Elliott. 1975. Assessment of Centrifugation and Filtration As Methods For Determining Low Concentrations of Suspended Sediment in Natural Waters. Fish. Mar. Servo Res. Dev. Tech. Rep. 545: 18 pp.
On a mene une enqu~te pour determiner s'il etait possible de mesurer d'une facon fiable, dans les eaux naturelles, des concentrations de moins de 10 mg f~l de sediments en suspension. A la suite de la centrifugation de volumes d'eau relativement importants (20 litres), on a remarque des concentrations tr~s variables de sediment en suspension. Apr~s avoir ffltre un litre d'aliquotes prises d'un echantillon de 20 litres, on a obtenu des concentrations de sediments en suspension d'une pietre precision. 11 est apparu qu'une stratification de sediments en suspension se forme en quelques heures apres le prel~vement, sur les echantillons plus importants. Le fait de secouer vigoureusement avant de retirer une aliquote, ne suffit pas h homogeneiser l'echantillon. La filtration de I a 2 litres d'ecnantillons, a toutefois donne une precision de plus de ±5% a des niveaux de 2-3 mg i-l de sediments en suspension. Cette haute precision a ete obtenue que1 que soit 1e moyen utilise pour Becher les filtres afin de leur donner un poids constant, c'est-a-dire 1a dessiccation ou 1e four a 90-950 C.
1
* INTRODUCTION
In previous studies of the aquatic ecology of the Mackenzie and Porcupine watersheds, we have found that zoobenthic organisms are very sensItIve to increases in suspended sediment and sedimentation on stream bottoms (Brunskill et al., 1973). This appeared to be especially true of streams, rive rs andlakes that have small concentrations of suspended sediments « 10 mg t- l ). In i nitiating a series of experiments on the ecological effects of adding known amounts of riverbank sediments to a clear stream (Harris River), it became apparent that we required better precision and sensitivity in measur i ng our experimental variable, suspended sediment. In addition, we required a better estimate of suspended sediment concentrations from clear rivers for our attempts to estimate and compare annual rates of transport of suspended sediments (and their contained minerals and elements) in turbid and clear rivers of the Mackenzie Valley. In this case, we required a precise estimate of suspended sediment mass per unit volume, as well as a sample of large mass for elemental and mineralogical determinations. Previous methods research (Wagemann, 1973, Appendix VIII B in Brunskill et al., 1973) on precision of this measurement was done on river waters wit~high suspended sediment concentrations . (~6S0 mg i-I).
The field or lab separation of the suspended phase from the solution is operationally dependent - for example, on filter pore size. It seems unlikely that any method can truly separate all natural particulate material from the solution without greatly disturbing the physical and chemical equilibrium between these phases. It is necessary, therefore, to establish quantitatively the comparability and precision of different methods used to achieve such separations.
The lower the concentration of suspended sediment, the more difficult it is, of course, to reliably measure such concentrations. For relatively dilute waters «10 mg i-I suspended sediment), the following experiments were carried out in order to compare the suspended sediment concentrations obtained by two common methods, namely centrifugation and filtration, and to determine the precisions of these methods.
The experiments were designed such that if poor comparability of preCision did result, the source(s) of discrepancy might be identified. A possible source of discrepancy conSidered, for both methods, was use of different sized samples. For filtration only, considerations included the effect of lag time between sample collect jon and filtratjon (sample storage time), the effect of drying filters using different methods (oven drying versus drying by desiccation) and the significance of subsampling (whether or not from a larger sample, the choice of a particular aliquot
* The first in a series of 13 technical reports on ecological studies of aquatic systems in the Mackenzie and Porcupine drainages in relation to proposed pipeline and highway developments.
2
over any other could influence the magnitude of the resultant suspended sediment concentratiun). And, for centrifugation only, the significance of carrying out ti,e operation using different flow rates was investigated.
FIELD METHODS
On June 11, 1973, water containing approximately 2-4 mg l ~l
suspended sediment was collected from the Harris River 600 meters above the mouth. The Harris River is an eastern tributary of the Mackenzie River and is located opposite the town of Fort Simpson, N.W.T. (Lat. 61° 52'N, Long. 121 0 19'W). Detailed physical and chemical descriptions of the river and its watershed may be found in Brunskill et al., (1973) and Rosenberg and Snow (1975). Samples were obtained from-rhe-center of the stream (depth = 0.7 meters) about 10 meters below a riffle. Twenty-liter carboys and 2-liter bottles were alternately filled until a total of 9 each was obtained. Filling was achieved by partial submergence (without disturbing the stream bottom) so that about 1/2 of the container mouth was below the surface of the water. Total sample collection time was 18 minutes. As the samples were collected, each carboy-bottle pair was labelled in chronological order numbers 1 through 9. Containers numbered 1, 4, 7 were set aside for centrifugation in Yellowknife and 2, 5, 8 for filtration in Yellowknife; 3, 6, 9 were filtered in the Fort Simpson laboratory immediately after collection (within 5 hours). In Yellowknife, filtrations were carried out 62-68 hours after sample collection; centrifugations took place 64-97 hours following sample collection. It is approximately 300 miles by road from Fort Simpson to Yellowknife. Table 1 indicates the destination and treatment of the individual water samples collected on June 11.
A second set of water samples was collected approximately 100 meters upstream of the mouth of the Harris River on August 22, 1973. Filling procedure was as above and samples were again obtained from the center of the stream abollt 10 meters below a riffle and depth approximately 0.7 meters. On this occasion, pairs of 2-liter bottles coupled with a single 20-liter carboy were filled until a total of 9 pairs of 2-liter bottles, 9 carboys and one additional pair of 2-liter bottles was obtained. One 2-liter bottle of each pair was partially (about 1.5 liters) filled whereas the second 2-liter bottle as well as the carboy were completely filled. As the samples were collected, the 3-container sets were labelled in chronological order numbers 1 through 9; the final, partial set of only 2-liter bottles was labelled number 10. Total sample collection time was 18 minutes. All 20-liter carboys were centrifuged in Yellowknife 7-14 days after collection. All 2-liter bottles were filtered in Fort Simpson 13-15 hours after sample collection. Table 2 indicates the destination and treatment of the individual water samples collected on August 22.
Table 1. Destination and treatment of water samples collected from the Harris River on June 11, 1973.
Sample Type A B C D E F
Container volume (liters) 20 20 20 2 2 2
Volume of sample collected (liters) 20 20 20 2 2 2
Container numbers 1,4,7 3,6,9 2,5,8 1,4,7 3,6,9 2,5,8
Sample treatment cent 1 fi1t2 filt cent fil t filt
Laboratory YK3 FS4 YK YK FS YK
Theoretical flow rate -1 Vol (ml min ) 111 111
Volume of aliquots centrifuged or filtered (liters) 20 1 1 2 1 1
Total volume centrifuged or fil tered (liters) 20 20 20 2 2 2
Filter drying method des 5 des des des
1. Centrifuged 2. Filtered 3. Yellowknife 4. Fort Simpson S. Desiccated
Table 2. Destination and treatment of water samples collected from the Harris River on August 22, 1973.
Sample Type G tI J K L M N
Container volume (liters) 20 20 20 2 2 2 2
Volume of sample collected (liters) 20 20 20 2 2 1.5 1.5
Container numbers 1,4,7 2,S,8 3,6,9 1,3, S, 2,4,6, 1,3,5, 2,4,6, 7,9 8,10 7,9 8,10
Sample treatment cent 1 cent cent filt2 filt filt filt
Laboratory YK 3 YK YK FS4 FS FS FS
Theoretical flow rate -1 (ml min ) 111 56 37 -I'>
Volume of aliquots centrifuged or fi ltered (liters) 20 20 20 2 2 1 1
Total volume centrifuged or filtered (liters) 20 20 20 2 2 1 1
Filter drying method des S 6 des oven oven
1. Centrifuged 2. Filtered 3. Yellowknife 4. Fort Simpson S. Desiccated 6. Oven dried
LABOAAl'ORY METHODS
Filtration: Whatman GF/C 47.5 mm diameter glass fiber filters were ignited at 525°C for a minimum of 8 hours and weighed to the nearest 0.01 mg. Filtration was carried out in the same sequence as sample collection using glass Millipore filter units and a vacuum of 5-10 psi. Volumes of aliquots filtered (a 1000 ml graduated cylinder was used) are indicated in Tables 1 and 2. Prior to removing each aliquot for filtration, the sample container was shaken vigorously for at least 30 seconds.
All filters from the June 11 experiment (8, C, E and F - Table 1) were dried by desiccating for not less than 24 hours, then weighed to the nearest 0.01 mg. Included among the filtrations of the August 22 experiment '''ere six 1000 ml deionized distilled water (DDH20) "control" fll trat ions. Three of these "control" filters as well as filters K & M (Table 2) were dried to constant weight by desiccation. Filters from L & N (Table 2) as well as the three remaining "control" filters were ovendried to constant weight at 90-95°C. In both experiments, weighing sequence was as for sample collection and filtration.
Centrifugation: "Flow-through" centrifugations of entire samples @15,000 rpm were carried out on a Sorvall centrifuge model RC2-8 after tho,rough mixing by shaking the sample container. 1000 ml volumetric flasks were used in transferring the sample to the centrifuge intake vessel. All containers were rinsed well with distilled water to ensure complete transfer of the sediment to the centrifuge. Samples designated A, D, and G (Table 1 and 2) were supplied to the centrifuge, theoretically, at a rate of III ml min-I, samples H at a rate of 56 ml min- l and sample J at 37 ml min-I. (Flow rates calculated for individual samples are recorded in Tables 3 and 4.) Centrifugation was also carried out in the same sequence as sample collection. Sediment collected was dried and weighed to the nearest 0.01 mg.
STATISTICAL METHODS
Means and standard deviations for replicate determinations of suspended sediment concentrations were calculated for each sample type.
The basic approach taken in the analysis of variation among sample types was to initially compare concentrations of suspended sediment obtained for centrifuged sample types . only, then filtered sample types only, and finally to compare centrifuged with filtered sample types. No statistical comparisons were attempted between samples collected on June 11 and those collected on August 22.
In order to determine if parametric statistics could be employed in
6
the comparisons outlined above, Pearson and tlartlcy's (1970) modification of Bartlett's test lVas used to check for homogeneity of variance when mor(; than two sample types were to be compared. On the one occasion when only the two sample types A and 0 (20 and 2 liter centrifuged samples) were to be compared, an F-test for equality of two variances was carried out (Snedecor and Cochran, 1971).
When homogeneity of variance was demonstrated, analysis of variance was used to test for equality of mean concentrations of suspended sediment obtained by the different treatments (Snedecor and Cochran, 1971). The Mann-Whitney U-t es t (Siegel, 1956) was chos en for those cases where nonparametric statistics was required .
Twenty-liter carboys Band C were analyzed in order to determine if sample stratification had taken place between time of sample collection and fi ltration. For each container, linear regressions were done on "al iquot number" versus "weight of suspended sediment collected per aliquot". The slope (b), intercept (a) and correlation coefficient (r) were calculated. Subsequently, covariance analysis was performed to test if the slopes were significantly different from one another (Snedecor and Cochran, 1971).
RESULTS
For each of the centrifuged sample types A, D, G, Hand J, volumes of samples centrifuged, corresponding weights of suspended sediment recovered, expression of the results as concentrations as well as means and standard deviations may be found in Tables 3 and 4. Similarly, results for filtered sample types B, C, E, F, K, L, M and N are reported in Tables 5, 6,7 and 8. For all filtered sample types, concentrations of suspended sediment determined exhibited a standard deviation from the mean of less than 10-11%. However, for the 20-liter centrifuged sample types, standard deviations ranged from 13-27% and for the 2-liter centrifuged sample type 0 the standard deviation was 51%.
Comparisons of sample types collected on June 11: Pearson and Hartley's test indicated no significant difference among the variances of filtered samples B, C, E and F (M = 1.99; d.f. = 3; a = 0.05). Furthermore, analysis of variance demonstrated the concentrations obtained for these samples did not differ significantly (F - 0.61; d.f. - 3,8; a = 0.05).
The centrifuged sample types A and 0 were found to have unequal variances (F = 42.84; d.f. = 2.2; a = 0.05). However, the Mann-Whitney Utest indicated that there was insufficient evidence to state that the concentrations of suspended sediment obtained from these two sample types were significantly different (U = 0; nl = 3, n2 = 3; P = 0.1). But, it was noted that all concentrations derived from sample type A were lower than those from sample type 0 intimating that, perhaps if a larger sample size
Table 3. Suspended sediment concentrations, means and standard deviations derived for centrifuged samples collected from the Harris River, June 11, 1973.
Sample Type A
I 0
Cont ainer No. 1 4 7 1 4
-1 Flow rate (ml min ) 99 110 41 55 105
Actual volume centrifuged (liters) 19.40 19.79 19.04 1. 65 2.10
Weight of suspended sediment collected (mg) 31.92 33.50 42.48 10.61 4.76
Concentration of suspended sediment (mg l-l) l. 65 l.69 2.23 6.43 2.27
Mean concentration of suspended sediment (mg l-l) 1. 86 4.12
Standard deviation ± 0.32 ± 2.12 (±17.2%) (±Sl.2%)
7
68
2. OS .
7.S3
--.J
3.67
Table 4. Suspended sediment concentrations, means and standard deviations derived for centrifuged samples collected from the Harris River, August 22, 1973.
- -- .- -
Sample Type G H J
Container No. 1 4 7 2 5 8 3 6
Flow rate -1 (ml min ) 115 100 92 47 47 47 35 34
Actual volume centri-fuged (liters) 18.56 18.81 17.95 19.16 18.46 17.16 18.33 18.79
Weight of suspended sediment collected (mg) 64.47 53.89 161.00* 37.91 42.44 56.84 49.77 54.00
Concentration of sus- 1 pended sediment (mg 1..-- ) 3.47 2.86 - 1. 98 2.30 3.31 2.72 2.87
Mean concentration of suspended sediment (mg .e.-I) 3.17 2.53 2.59
Standard deviation ± 0.43 ± 0.69 ± 0.36 C±13.6%) (±27 . 3%) (±13.9%)
* Sample spoiled (by burning?); therefore, omitted from statistical analyses.
9
35
18.55
40.51
2.18 (Xl
9
Table 5. Suspended sediment concentrations, means and standard deviations derived for samples collected from the Harris River, June 11, 1973 and filtered in Fort Simpson.
Sample Type B E
Container Number 3 6 9 3 6 9
Actual Total volume filtered (liters) 19 18 19 2 2 2
Aliquot Volume filtered Weight of suspended Weight of suspended Number (liters) sediment collected (mg) . sediment collected
1 1.00 3.08 1. 87 1.85 1.88 2.04 1. 70
2 1.00 2.28 1. 93 1.85 2.03 2.14 2.15
3 1.00 3.74 1. 80 1. 93
4 1.00 2.08 1.97 1.83
5 LOO 2.10 1.92 2.04
6 1.00 2.61 2.08 2.02
7 1.00 2.18 2.12 2.18
8 1.00 1.91 2.05 2.04
9 1.00 2.10 1. 98 1.49
10 1.00 3.16 2.04 2.25
11 1.00 2.06 2.05 2.14
12 1.00 2.29 2.04 2.17
13 1.00 2.08 2.04 2.27
14 1.00 2.03 2.03 2.32
15 1.00 2.28 2.05 2.45
16 1.00 2.11 2.22 2.42
17 1.00 1. 97 3.12 2.47
18 1.00 2.03 2.36 2.41
19 1.00 2.19 2.40
Total suspended sediment collected (mg) 44.28 37.67 40.53 3.91 4.18 3.85
Concentration of suspended sediment (mg l-l) 2.33 2.09 2.13 1.96 2.09 1.93
Mean concentration of suspended sediment (mg i-l) 2.18 1. 99
Standard deviation ±0.13 ±0.09 (±5.9%) (±4.3%)
(mg)
10
Table 6. Suspended sediment concentrations, means and standard deviations derived for samples collected from the Harris River, June 11, 1973 and filtered in Yellowknife.
Sample Type
Container number
Actual total volume filtered (liters)
2
19
C
5 8
18 18
Aliquot Number
Volume filtered (liters)
Weight of suspended sediment collected (mg)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
1. 00
1. 00
1. 00
1. 00
1. 00
1. 00
1. 00
1. 00
1. 00
1.00
1. 00
1. 00
1. 00
1. 00
1. 00
l. 00
1. 00
1.00
1.00
Total suspended sediment
1. 81
l. 84
2.16
1.84
2.01
2.01
2.21
2.14
2.18
2.13
2.31
2.53
2.54
2.49
2.36
2.70
2.64
2.45
2.58
collected (mg) 42.93
Concentration of sus- -1 pended sediment (mg ~ ) 2.26
Mean concentration of -1 suspended sediment (mg .f )
Standard deviation
2.04
1. 70
1.72
1. 88
1. 57
1. 80
1. 85
1. 78
2.06
1. 99
2.30
2 . 31
2.32
2.71
2.67
2 . 95
2.89
2.96
39.50
2.19
2.11
± 0.21 (± 10. 0%)
1. 29
1. 52
1. 20
1. 38
1. 64
1. 55
1. 49
1. 59
2.00
1. 83
2.06
2.16
2.16
2.18
2.46
2.30
2.35
2.56
33.72
1.87
F
2 5 8
2 2 2
Weight of suspended sediment collected (mg)
2.26
2.40
4.66
2.33
1. 75
2.24
3.99
2.00
2.07
± 0.23 (± 11. 1 %)
1. 52
2.24
3.76
1. 88
Table 7. Suspended sediment concentrations, means and standard deviations derived from filtrations of the entire contents of 2-liter bottles collected from the Harris River, August 22, 1973.
Sample Type K (desiccated) L (oven dried)
Container number 1 3 5 7 9 2 4 6 8
Actual volume filtered (liters) 2.000 2.000 2.075 2.070 2 . 115 1. 925 2.055 2.095 2.110
Weight of suspended sediment collected (mg) 15.36 5.19 5.78 5.77 5.48 I 5.28 5.37 5.69 5.99
Concentration of suspended sediment
12.68 I 2.74 (mg t -1) 2.79 2.61 2.72 2.84 2.60 2.79 2.59
Mean concentration of suspended sediment (mg t-I) 2.69 2.75
Standard deviation ± 0.09 ± 0.10 (±3.3%) (±3.6%)
10
2.115
6.04
,.....
2.86 ,.....
Table 8. Suspended sediment concentrations, means and standard deviations derived from I-liter filtrations of partially (1.5 liters) filled 2-liter bottles collected from the Harris River, August 22, 1973.
Sample Type M (desiccated) N (oven.:.dried)
Container number 1 3 5 7 9 2 4 6 8 10
Actual volume filtered (liters) 11. 000 1.000 1. 000 1. 000 1. 000 1 1. 000 1.000 1.000 1.000 1.000
Weight of suspended sediment collected (mg) 12.82 2.60 2.67 2.74 2.65 12.72 2.81 2.63 2.83 2.88
Concentration of suspended sediment (mg i-I) 12.82 2.60 2.67 2.74 2.65 12.72 2.81 2.63 2.83 2.88
Mean concentration of suspended sediment (mg i-I) 2.70 2.77
Standard deviation ± 0.09 ± 0.10 (±3.3%) (±3.6%)
..... N
13
(more replicates) had been obtained, a real difference might have been shown to exist.
Because ·of the above-noted ambiguity, A and 0 were treated separately in further tests. Pearson and Hartley's test showed no significant differences between the variances of A and the filtered sample types B, C, E, and F (M = 3.50; d.f. = 4; a = 0.05). Nor was there a significant difference in the concentrations of suspended sediment obtained (F = 3.29; d.f. = 1,3; a = 0~05). However homogeneity of variance did not exist hctween 0 and the filtered group (M = 26.11; d.f. = 4; a = 0.05). In addi t ion, the ~1ann-Whi tney U-test indicated that centrifuged 2-1 iter samples (D) produced significantly different concentrations of suspended sediment from the filtered samples (U = 2; n l = 3; n2 = 12; a = 0.05). The results of these tests lend further support to the contention that concentrations of suspended sediment derived from A may truly have been different from those for D.
Linear regression analyses for individual carboys of the filtered sample types Band C are summarized in Table 9. All correlation coefficients (r) and slopes (b) were significant at a = 0.05. It should be noted that the correlation coefficients increased with time passed between sample collection and filtration. Covariance analysis indicated that the six slopes were significantly different from one another (F ~ 28.56; d.f. = 5,99; ~ = 0.05).
Comparisons of sample types collected on August 22: Centrifuged sample types G, Hand J had homogeneous variances eM ; 0.88; d.f. = 2; a = 0.05). Analysis of variance indicated no significant difference in the mean concentrations of suspended sediment obtained from these three sample t~·pes (F = 0.98; d.f. = 2,5; a = 0.05).
Both oven dried and desiccated filters of sample types K, L, M and N, ~ollt'cted o~ August 22, as well as all DDH20 "control" filters achieved constant weIght less than 16 hours after drying had begun. Average gain or loss in weight of either the oven dried or desiccated "control" filters was 0.08 mg, that is less than +3% of the weight of suspended sediment collected by filtration of one liter of Harris River water. Therefore, no attempt was made to calculate a correction factor for filtered sample types K, L, M and N based on the "control" filter results.
No significant difference existed among the variances of K, L, M and N (M = 0.09; d.f. = 3; a = 0.05). In addition, by analysis of variance, the mean concentrations of suspended sediment for these sample types were equal (F = 1.03; d.£. = 3,16; a = 0.05).
It was necessary to use non-parametric statistics to compare concentrations obtained by centrifugation and by filtration (G, H, J versus K, L, M, N) due to heterogeneity of variance (M = 27.77; d.£. = 6; a = 0.05). The Mann-Mlitney U-test, however, demonstrated no significant difference between the magnitudes of the concentrations of suspended sediment obtained (U = 73.5; III = S, 112 = 20; a = 0.05).
• .. • ~ •
Table 9. Intercepts, slopes and correlation coefficients from linear regression analyses carried out on "aliquot number" versus "weight of suspended sediment obtained per aliquot" for 20-liter filtered samples collected from the Harris River, June 11, 1973.
Sample Type B C
Number of hours between sample 1 > 5 62 >68 collection and filtration
Container number 3 6 9 2 5 8
Intercept (a) 2.738 1.770 1.762 1. 805 1.451 1.148
Slope (b) -0.041 0.034 0.037 0.045 0.078 0.076
Correlation coefficient (r) -0.474 0.636 0.793 0.909 0.901 0.956
Degrees of freedom 17 16 17 17 16 16
...... ~
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DISCUSSION
Precision and magnitude of suspended sediment concentrations obtained by centrifugation were found to be dependent on sample size. In every instance, concentrations of suspended sediment obtained from 2-liter bottles were greater than those from 20-liter carboys (Table 3). Although the Mann-Whitney U-test gave insufficient evidence to say that 20 liter centrifugations (A) were not different from 2 liter centrifugations (D) it should, however, be noted that A was shown to be equivalent to 20 and 2 liter filtered sample types B, C, E and F and D was not. Therefore, from these results, it is possible to deduce that 20 liter centrifugations were not equivalent to 2 liter centrifugations.
In way of a possible explanation of higher, more variable concentrations obtained from centrifuged 2-liter bottles, it should be noted that 5-20 ml of sample water was retained with the suspended sediment collected for drying and weighing. That is, after drying, the dissolved ions and organic matter in this sample water were included as a portion of the weight of suspended sediment obtained. During the June 11 sam~ling,
total dissolved solid in the Harris River was approximately 200 mg L- . Therefore, in 5-20 ml of sample water, from 1-4 mg of dissolved solids would have been present. For 2-liter samples, this could have resulted in an apparent doubling in concentration of suspended sediment, i.e. in an increase of up to 2 mg [-1. In the case of 20-liter samples, such an additive effect would, of course, be considerably less (0.05-0.20 mg L-l).
No optimum rate of supply of sample to the centrifuge was discovered. Flow rates varying from 37 to III ml min- l resulted in the determination of the same mean concentrations of suspended sediment (G-H-J).
Unlike centrifuged samples, equivalent mean suspended sediment concentrations were achieved for filtered samples independent of sample size (B-E and C-F).
Storage for 3 days before filtering did not signifj.cantly alter the gross recovery of suspended sediment from either 2-liter or 20-liter swnplcs (E-F and B-C). However, precisions did decrease slightly with time (standard deviations ranged from 4-6% for samples filtered 1-5 hours after collection and from 10-11% for samples filtered 62-68 hours after collection) and stratification was evidenced in these same samples within 6 hours of collection even though they were mixed prior to the removal of each I-liter aliquot filtered. Evidence for stratification in the 20-liter carboys was reported in Table 9; due to the fact that the slopes as well as correlation coefficients increased with time and coupled with the exhibition, by covariance analysis, of a significant difference between slopes, there is a positive indication of increased stratification with time. In the case of the 2-liter bottles E and F, it should be noted that in every instance the second aliquot filtered resulted in the collection of more sediment than from the first (Tables 5 and 6). Therefore, if concentration of suspended sediment were based on the filtration of a single I-liter subsample from either of these container types, the degree to which such a result would be representatjve of the entire sample would be questionable. This problem was subsequently overcome by showing that partially filling (1.5 liters)
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a 2-liter bottle in the field and mIxIng well immediately before filtering 1 liter of sample was equivalent to fj ltering the entire contents of a completely filled 2-liter bottle (M-K and N- L).
No difference in results was found between filters which were oven dried at 9S DC and those which were desiccated (K- L and M- N). Although oven drying was perhaps quicker than desiccation by 2-4 hours, this offers no obvious advantage since the time taken for both methods was considerably less than 24 hours.
CONC LUSIONS
Since, for sample type 0 (Table 3), concentrations of suspended sediment were found to be unduly high as well as extremely variable (standard deviation = 51%), centrifugation of 2 liters of sample should not be considered for use on dilute waters. Further, although it has been shown that 20-liter centrifugations resulted in mean concentrations of suspended sediment equivalent to those for filtration (A- B, C, E, F, and G, H, J - K, L, M, N), this method should be avoided, as well, since precisions on triplicate sample sets were also found to be poor (14- 27%). At the moment, however, the only way sufficient quantities of suspended sediment for the purpose of elemental analyses may be obtained is to centrifuge large volumes of water. We suggest, therefore, that as much water as possible be separated from the sediment prior to drying and subsequent analysis even if some fraction of the sediment is lost in the process. Otherwise, a proportion (up to 10% in the case of the Harris River) of the material to be analyzed would not be suspended sediment at all but rather solids which were in solution.
Suspended sediment concentrations were obtained with a precision of better than 5% (based on sample sets of 5 replicates) from either desiccated or oven dried filters independent of whether the entire contents of a completely filled 2-liter bottle were filtered or, if immediately after mixing, only 1 liter of a partially filled 2-liter bottle was filtered. The procedure for anyone of these filtration variations is simple and consumes a fraction of the time required for centrifugation. ~foreover, the equipment needed is relatively inexpensive and sui table for operation in a moderately well serviced field laboratory, thereby allowing the field scientist "instant results". We believe it advisable, however, that the following precautions be taken. To ensure that filter and sediment are as dryas possible before weighing, desiccation or oven drying should be carried out for a minimum of 24 hours. It is also recommended, for .each set of suspended sediment determinations carried out, that a number of parallel DDH20 "control" filters also be processed. Although in this experiment it was not found necessary to use the DDH20 "control" filters to calculate a correction factor, it is conceivable that under certain conditions such a correction would be necessary. For example, filtration could cause the washing away of particles of the filter paper, resulting, in absence of a control, in an uncorrected weight loss; an increase in humidity could result in a weight gain not attributable to the sediment; or, under certain conditions electrostatic forces might affect the weighings
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(see Banse et al., 1963 or Eaton et al., 1969). Strickland and Parsons (1965) claim-no-complications from-electrostatic effects with use of glass filters such as in this experiment .
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REFERENCES
Banse, K., C. P. Falls, and L. A. Hobson. 1963. A gravimetric method for determining suspended matter in sea water using Millipore filters. Deep Sea Res. 10:639-642.
Brunskill, G.J., D. M. Rosenberg, N. B. Snow, G. L. Vascotto, and R. Wagemann. 1973. Ecological studies of aquatic systems in the Mackenzie-Porcupine drainages in relation to proposed pipeline and highway developments. Canada Task Force N. Oil Dev. Env. Soc. Comm. Vol. I. Report 73-40. Information Canada Cat. No. R72-l0073/l-1. QS-1533-010-EE-Al. 131 p. Vol. II. Appendices. Report 73-41. Information Canada Cat. No. R72-l0073/l-2. QS-1533-020-EE-Al. 345 p.
Eaton, J. S., G. E. Likens, and F. H. Bormann. 1969. Use of membrane filters in gravimentric analyses of particulate matter in natural waters. Water Resour. Res. 5:ll51-li56.
Pearson, E. S., and H. O. Hartley. for statisticians. Vol. I. 270 p.
Rosenberg, D. M., and N. B. Snow. organisms in the Mackenzie relation to sedimentation. Rep. 547:86 pp.
(eds.) 1970. Biometrika tables Cambridge University Press, Cambridge.
1975. Ecological studies of aquatic and Porcupine River drainages in Fish. Mar. Servo Res. Dev. Tech.
Siegel, S. 1956. Nonparametric statistics for the behavioural sciences. McGraw-Hill Book Company. New York. 312 p.
Snedecor, G. W., and W. G. Cochran. 1971. Iowa State University Press, Ames.
Statistical methods. 593 p.
The
Strickland, J.D.H., and T. R. Parsons. 1968. A practical handbook of sea water analysis. Fish. Res. Board Can. Bull. 167:311 p.