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Indirect Methods to Estimate Suspended Sediment Concentration: Reliability and Relationship of Turbidity and Settleable Solids
ARTICLE IN PRESS Biosystems Engineering (2005) 90 (1), 75–83 doi:10.1016/j.biosystemseng.2004.09.001 SW—Soil and Water
Indirect Methods to Estimate Suspended Sediment Concentration: Reliability and Relationship of Turbidity and Settleable Solids D. Pavanelli1; A. Bigi2 1
Department of Agricultural Economics and Engineering, University of Bologna, Italy; e-mail of corresponding author: [email protected] 2 Department of Agricultural Economics and Engineering, University of Bologna; e-mail: [email protected] (Received 21 January 2004; accepted in revised form 1 September 2004; published online 18 November 2004)
A key element in a stream-monitoring programme for sediment transport is the choice of the measuring technique for suspended sediment concentration: this can highly affect both project costs and data reliability. The gravimetric method represents the standard analysis to directly measure suspended sediment concentration in a water sample. Indirect techniques are often employed for their inexpensiveness, although they need to be calibrated on gravimetric analysis results. In this study, the reliability of settleable solids in Imhoff cones was addressed as an alternative indirect method to estimate suspended sediment concentration (SSC), verifying the results with the turbidity measures of a laboratory nephelometer. The results show a high correlation of settled solids with suspended sediment concentration. Also, the results from the turbidimetric analysis showed a good correlation with SSC, but limited to water samples that did not need dilution. Measurements were repeated after storing the samples for 1 month: results exhibit an increase in turbidity and settleable solids, probably due to algae growth and anaerobic processes with the production of gases, causing an increase in matter. From our study, the Imhoff cone was shown to be a useful instrument to estimate suspended sediment concentration for the simplicity, the reliability and the low cost of their results. Our results prove how Imhoff cones are preferable to turbidimetric analysis to estimate SSC, specifically for highly turbid samples. r 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
events, when SSC can vary by an order of magnitude. The importance of fluvial suspended sediment as a vector for the transport of nutrients and many natural and anthropogenic pollutants is well established and the concentration of many substances bound to suspended solids can be at least an order of magnitude higher than their concentration in the dissolved phase (Walling & Woodward, 2000). Sediment transport monitoring is then designed to establish the stream water quality and basin morphology dynamics; but, notwithstanding, monitoring programmes are strongly dependent on sampling protocol and sample analysis techniques. Among the latter, the choice of a valid tool to estimate suspended sediment concentration (SSC) is very important: many
1. Introduction Suspended sediment yields mainly fine materials ranging from fine sands to clay, in different quantities according to the hydraulic characteristic of the stream, geomorphological and pedological conditions of the catchment, the climatic regime of the area and the presence of vegetation. Suspended sediment concentration (SSC) and particle size are fundamental controls on the dynamics of sediment entrainment, transport and deposition, and information on the particle size characteristics of sediment is an essential requirement for investigations of the flux and storage of sediment associated substances in the channel and riparian zone. Annual sediment load is mainly carried during flood 1537-5110/$30.00
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r 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
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measurements techniques have been developed in the last decades and many of them are based on the assessment of surrogate quantities for estimating SSC; these techniques are becoming numerous and, notwithstanding their qualities, they might not lead to univocal and comparable results (Gray et al., 2002). Gravimetric analysis represents the standard methodology to physically quantify suspended sediment and consists of physically separating the liquid phase from the solid phase in a water sample. The main disadvantages of this method are its labour intensity, its timeconsuming process and the economical impact on a research project budget; however, the analytical results are the most reliable tools to estimate SSC and are essential to properly calibrate measurements of the various surrogates (Gray et al., 2002). Among many other different methodologies, the use of optical turbidimeters, acoustic turbidimeters and laser diffraction is quite common in stream monitoring programmes (Schoellhamer & Wright, 2002). Acoustic turbidimeters measure the acoustic backscattered signal as a surrogate for SSC; these turbidimeters have the great advantage of not being subjected to biological fouling like optical turbidimeters (Gartner, 2002) and they can be easily coupled to acoustic Doppler current profilers (ADCP) (e.g., Creed et al., 2001; Taylor & Vincent, 2001). However, they present still some disadvantages: they need more field-testing, they might have to be specifically designed (e.g., Froehlich, 2002) and their results are not completely reliable yet (Gartner, 2002). Laser in situ scattering and transmissiometry (LISST) gives a measure of the angle of diffraction when a laser beam hits a particle; it provides an estimate of SSC and a complete particle size distribution, but the device is expensive and the outputs need particular training to be interpreted (Wren et al., 1999). Optical turbidimeters produce an estimate for SSC through measuring either the backscattering of a light (or an infrared beam) or the attenuation of a light beam passing through a water sample. They have been used in various studies: Halfman and Scholz (1993) and Paul et al. (1982) used it in a lake environment; Mitsch and Reeder (1992) in wetlands; Brabben (1981), Gippel (1995) and Jansson (1992) in streams. Continuous monitoring of suspended sediment in streams presents a number of technical problems, possibly leading to biased measurements: growing algae over the optical sensor or plugging of the intake tubing (Jansson, 1992), entraining of air bubbles and floating debris (Wagner et al., 2000). Furthermore, the relation of SSC with nephelometric turbidity units (NTU) for high sediment concentrations range is not completely investigated yet.
Some of these failings have been overcome by adding specific features to the monitoring device, such as the presence of self-cleaning optical sensors (e.g. Buchanan & Ganju, 2003), however theoretical limitations remain and are addressed in the next paragraph. Despite these drawbacks, if the necessary data quality is met (Eads & Lewis, 2002), continuous turbidity monitoring may give a good representation of SSC (Christensen et al., 2002). The present study addresses to three possible methodologies to estimate suspended solids in stream flows: gravimetric analysis, turbidimetric analysis and settleable solids analysis. These three methodologies are currently employed in a monitoring and sampling campaign started in 1997 on three Apennine streams. The reliability and the comparability of the results obtained with these techniques have been verified with a laboratory study. In the remainder of the paper, a simple alternative procedure is presented to estimate the suspended sediment concentration in water by measuring settleable solids.
2. Analysis procedures For this study, 12 water samples with a specific concentration of solids were prepared in the laboratory. Suspended sediment concentration and particle size for these samples were prepared in order to be comparable to concentrations measured during flood events of the Sillaro Torrent, one of the mountain stream currently under monitoring (Pavanelli & Pagliarani, 2002). Therefore, in order to obtain laboratory samples, filtered stream water has been used with sediments collected from the streambed next to the instrumental sampling hose of the Sillaro monitoring station. In Table 1 are presented sediment concentrations for the 12 laboratory samples. A sample of filtered stream water was used as a datum. This procedure provided 12 samples with constant mineralogy, constant particle size distribution across samples and specific suspended sediment concentrations. These sample characteristics are necessary to improve measurement reliability and comparability, since particle size and mineralogy affect instrumental readings. Consequently, stream-water samples collected during the ordinary monitoring campaign were not suitable for the purposes of this study. The particle size distribution obtained from the data collected from 82 water samples collected during flood events occurred in the Sillaro Torrent in the last 3 years is shown in Table 2 and compared to the particle size distribution of the laboratory samples. Particle size distribution of samples was determined by sedimentation
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velocity studies using Stokes’ Law, the standard employed being the Udden scale with the classification of Wentworth (G.U. n.248, 21-10-1999). All laboratory samples have been analysed through the following procedures: (1) turbidity analysis with a laboratory turbidimeter (manufacturer: Hach, model: 2100AN); (2) settleable solids in Imhoff cones; and (3) gravimetric analysis and particle size analysis with the pipette method. Turbidity measures have been repeated three times for each sample. Laboratory samples preparation, gravimetric and particle size analyses have been performed by the ARPA laboratories (Regional Agency for Health Prevention and Environmental Protection) of the Emilia-Romagna Region. In order to study the behaviour of samples during storage, the series of analysis were fully repeated 1 month after the first analysis: samples were stored at room temperature and exposed to daylight. The intent of this procedure was to recreate samples with characteristics similar to those of the ordinary stream-water samples. In fact stream-water samples are often analysed almost 1 month after sampling, because of the intrinsic Table 1 Suspended sediment concentration (SSC) in each laboratory sample SSC, g l1
Sample no. 1 2 3 4 5 6 7 8 9 10 11 12
15 25 40 55 70 85 100 125 150 200 250 300
characteristics of the automatic sampler (it holds 24 bottles) and the arrangement of the monitoring programme. 2.1. Total suspended solid concentration—gravimetric method Suspended solids represent the matter in a solid phase present in a water sample. The water sample is firstly centrifuged in order to separate the water from the suspended sediment; once the water has been clarified, the material collected is then oven-dried at 105 1C for about 10 h, until the matter reaches a constant weight. This is the procedure employed by the ARPA laboratories which performed the gravimetric and particle size analysis. 2.2. Turbidity measure Turbidity is an optical property of water and it can be used to measure relative water clarity. Since turbidity is mainly due to particles in suspension, it is also employed to estimate the concentration of suspended sediment. When a light beam passes through a water sample, it is subjected to scatter, transmission and absorption for the presence of particles in suspension: electronic photodetectors set around the water sample measure the amount of light transmitted and scattered as an indication of turbidity. It has been extensively proved that colour, shape and mostly particle size along with wavelength affect the scatterance of the sample (Gray et al., 2002; Foster et al., 1992; Gippel, 1988). Furthermore, small particles scatter light more uniformly and are more sensitive to shorter wavelengths, whereas large particles tend to forward scatter the incident light (Sadar, 1996). This leads to a greater sensitivity of the instrumentation for finer particles rather than larger ones (Lewis, 1996). For this reason, solutions of equal
Table 2 Particle size distribution averaged over 82 field samples; percent of total material and particle size distribution of the laboratory sample using the Udden standard of fraction and the Wentworth classification Category
Fraction, mm
Particle size distribution, % Stream-water samples
Clay Fine silt Coarse silt Sand SD, standard deviation.
o0002 0002–002 002–005 02–005
Laboratory samples
Average
SD
Max value
459 4175 505 355
73 59 29 25
69 60 15 13
44 37 95 95
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90˚ detector Forward scatter detector
Back scatter detector
Transmitted light detector LED source
Sample cell
Fig. 1. Schematic cross-section of a laboratory turbidimeter; LED, light-emitting diode
suspended solid concentration but different composition may not scatter the same amount of light. Also water colour (Malcolm, 1985) and temperature (Sadar & Engelhardt, year not specified) may also bias turbidity measurements, but their effect is less severe. Another failing of turbidimetric analysis is the low repeatability of diluted sample measurement results, turbidity being an apparent optical property of water. Optical instruments measuring transmitted light are named transmissiometers and are different from the optical backscatter sensors (OBS), which measure light scattered by particles at right angles. All analytical measurements require a primary standard upon which the calibration is based: one nephelometric turbidity unit (NTU) was established as the turbidity resulting from a suspension of one part per million of silica. Formazine is used as a primary turbidity standard (method 2130B, APHA, 1999) to calibrate the laboratory nephelometer and the measurement unit is the nephelometric turbidity unit. The instrument used for this study is a laboratory turbidimeter Hach 2100AN equipped with four photosensors for transmitted and scattered light and ranging from 0 to 10 000 NTU (Fig. 1). When the sample has an over-range turbidity, it is possible to have a NTU measurement by diluting the sample. In order to analyse water samples, the instrument requires a sub-sample of 30 ml extracted from the original sample after a proper agitation. This procedure might produce a non-representative sub-sample, particularly when a dilution is needed. When a sample needs dilution, turbidity is calculated using the following relationship (US EPA, 1999): TR ¼ TD
ðw þ sÞ : s
(1)
where TR is the turbidity of the undiluted sample in NTU; TD is the turbidity of the diluted sample in NTU; w is the content of clear water in the dilution in ml; and s is the content of sample water in ml. To convert NTU data records to suspended sediment concentration in samples, it is necessary to establish a
mathematical correlation between nephelometric units and SSC on a sufficiently wide sample population. 2.3. Settleable solids Settleable solids are defined as the solids that settle in an undisturbed sample of liquid after a specific time period. Water samples are thoroughly mixed and poured in Imhoff cones: settleable matter is measured volumetrically after 1 and 24 h; the readings are standardised on a 1 l sample. This technique is very common in wastewater analysis and its units are usually expressed in ml/l. Measurement precision of the graduated cone is 0.1 ml for volumes less than 2 ml; it increases to 0.5 ml for volumes of less than 10 ml and it is 1 ml for volumes ranging between 10 and 100 ml. In the remainder of the paper, settled solids are given as free settled solids (FSS) to indicate that the material settled is mostly inorganic matter and subject to free settling (method 2540F: APHA, 1999).
3. Results 3.1. Turbidity-suspended sediment concentration relationship Three to four turbidity measurements were made on each laboratory sample. Readings might result within or outside the instrumental NTU range: in the former case, readings had high repeatability and small deviations; in the latter case it has been verified as lower reliability of the results according to the increase in dilution. Table 3 presents the results of the turbidimetric analysis for samples 1–6: samples 1–4 did not need any dilution, whereas samples 5–6 required dilution because the turbidity was slightly outside the instrumental range. Samples 7–12 required high dilution rates and this manipulation affected the quality of the results. To further test the existence of a difference in consistency between the diluted and undiluted samples,
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Table 3 Summary statistics of three repeated nephelometric turbidity unit (NTU) measurements on undiluted samples 1–4, diluted samples 5 and 6 Sample no.
0 1 2 3 4 5 6
Turbidity, NTU TU1
TU2
TU3
10 835 1915 4163 5425 11 204 9470
10 910 1908 3918 5475 9928 10 517
10 786 1890 3890 5931 8652 11 564
Average 10 844 1904 3990 5610 9928 10 517
For the stored samples, NTU reading averages have been employed also for the diluted samples, these measurements being very consistent and reliable compared to those for fresh samples. The existing relationships between SSC and NTU were also investigated using the complete dataset of measurements on fresh samples and 1-month-old samples (Fig. 3). For both groups, correlation coefficients are extremely high and the value for r is less than 001. Comparing the two series of analyses, it is evident that there is an increase in NTUs after 1 month, mathematically shown by a regression curve in Fig. 4. This behaviour might be explained by the development of microorganisms and algae, clays aggregation and flocculation, besides other biological reactions and gas production. Although aggregation increases sedimentation velocity, the gaseous product of the biological reactions aggregates to flocs and supports solids suspension. Moreover, algal growth increases turbidity and changes water colour. Since this can lead to an overestimation of SSC, it is therefore necessary either to use a specific regression curve to calculate SSC from stored samples, or to calculate the corresponding NTU value of the fresh sample, or to store samples in a refrigerated and dark container.
Turbidity, NTU
the Pearson correlation coefficient r for each pair of readings was calculated (Table 4). The coefficient measures the strength of the linear relationship between the considered variables and r indicates the statistical significance of the estimated correlations: only NTU readings from 1 to 4 are correlated with a confidence level higher than 95%. The results from the eight diluted samples indicate that there is a decrease in NTU readings according to a power law when the dilution increases. Also, a linear regression (Fig. 2) has a comparable coefficient of determination and it has been employed to re-calculate NTU values for the diluted samples (Table 5): clear differences existed for NTU values calculated through a linear regression and through NTU averaging. The linear regression results are denoted as the ‘correct NTU values’ and are employed in all the following analysis. It has been noted that suspended sediment concentration within the instrumental range (0–10 000 NTU) can be measured with a high reliability; beyond this concentration, it was necessary to dilute the sample with a decrease in the quality of the measurement. However, the lower was the dilution, the higher was the measurement accuracy. Besides the intrinsic effect of dilution, this is also due to the combined effect of an increase in the coarser fraction for samples with more suspended solids and the lower sensitivity of the instrument to sands and coarse silt.
40 000 35 000 30 000 25 000 20 000 15 000 10 000 5000 0 0
2
4
6 8 10 Dilution factor
12
14
Fig. 2. Turbidity results after dilution from the relationship in Eqn (1): , sample no. 7; J, sample no. 8; K, sample no. 10; }, sample no. 12
Table 4 Pearson’s product–moment correlation coefficient q for the statistical significance of the estimated correlations: values below 0.05 indicate that the hypothesis for a correlation coefficient of zero can be rejected with a 95% confidence level Sample
Variables
Number of samples
Correlation coefficient (r)
Pearson’s coefficient (r)
Undiluted Diluted
TU1/TU2 TD1/TD2
4 8
0997 0673
00023 0067
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Table 5 Nephelometric turbidity unit (NTU) final values for diluted samples calculated using a linear regression and averaging NTU readings Turbidity, NTU Average 5 6 7 8 9 10 11 12
SSC, g/l
Regression based
9928 10 517 8083 10 760 22 640 18 665 38 496 31 200
9933 10 976 11 151 14 188 19 032 23 888 32 939 44 481
70 90 100 125 150 200 250 300
SSC, suspended solid concentration.
y = 0.00065 x + 2.78 r2 = 0.79
3 2 1 0 0
40
25 20 15 y = 0.00060 x + 1.42 r2 = 0.98
10 5 0
0
1 2 3 4 Turbidity after one month, NTU
y = 0.43 x + 0.65 r2 = 0.99
35
5 × 10 000
10 000 20 000 30 000 40 000 50 000 60 000 Turbidity, NTU
y = 0.0005 x2 + 0.25 x + 1.47 r2 = 0.99
30 SSC, g/l
SSC, g/l
30
y = 0.88 x − 326.34 r2 = 0.99
Fig. 4. Regression analysis for the nephelometric turbidity unit (NTU) readings on fresh samples and on 1-month-old samples; note that the angular coefficient is less than one: , 99% confidence bands: , 95% confidence bands; r2, coefficient of determination
40 35
4 Turbidity, NTU
Sample
× 10 000 5
25 y = 0.12 x + 4.50 r2 = 0.90
20 15 10
Fig. 3. Regression analysis between suspended sediment concentration (SSC) g l1 and nephelometric turbidity units (NTU) with the complete set of measurements performed soon after sample preparation and after 1 month: K, fresh samples; &, month-old samples; , linear regression (fresh sample); , linear regression (1-month-old sample); r2, coefficient of determination
3.2. Imhoff cones—suspended sediment relationship Imhoff cones, usually used to estimate settleable matter for sewage, appear to be a very interesting tool for estimating suspended sediment concentration in stream water for their characteristics of simplicity, cost competitiveness and reliability. Readings were taken 1 and 24 h after the samples were poured into the cones. Besides, the analyses of the samples were completed soon after their preparation and after storing them for 1 month. In Figs 5 and 6, the regression analyses for SSC on FSS readings are presented: the 24 h readings present a linear relationship with SSC, whereas the 1 h readings are well represented by a polynomial relationship. It is worth noting that, for all four cases, the value for r is less than 001 (indicating a statistical significance at the
5 0
0
25
50
75 100 125 150 175 200 225 250 275 FSS, ml/l
Fig. 5. Regression curves between suspended solids concentration (SSC) and free settled solids (FSS) for 1 and 24 h readings for fresh samples: J, cone 1 h; K, cone 24 h; , linear (1 h); , linear (24 h); , polynomial (24 h); r2, coefficient of determination
99% confidence level) and that the coefficient of determination r2 is higher than 090. First evidences from a qualitative analysis of Figs 5 and 6 are: the 1 h readings are always higher than the 24 h readings, due to compaction of solids through time; the 1 and 24 h readings from the analysis performed after 1 month are always higher than those from the analysis after sample preparation; and the 24 h readings of fresh samples compared with those of the stored samples show a lower difference than 1 h readings, probably due to the compaction of particles.
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45 40
y = 0.32 x + 1.60 r2 = 0.97
35
y = 0.16 x + 2.69 r2 = 0.99
80
60 FSS, ml/l
30 SSC, g/l
81
25 20 15
y = 0.74 x + 1.99 r2 = 0.99
40
20
10 5 0
0 0
25
50
75 100 125 150 175 200 225 250 275 FSS, ml/l
Fig. 6. Regression curves between suspended solids concentration (SSC) and free settled solids (FSS) for 1 and 24 h readings 1-month-old samples: K, cone 24 h after 1 month; J, cone 1 h after 1 month; , linear 1 h after 1 month; , linear 24 h after 1 month; r2, coefficient of determination
0
20
40 60 80 FSS after one month, ml/l
100
Fig. 7. Regression analysis between the 24 h free settled solids (FSS) readings for the fresh and for the stored sample: , 99% confidence bands; , 95% confidence bands; r2, coefficient of determination
4. Conclusions Table 6 Summary statistics of Imhoff cone readings on 12 laboratory samples Settleable solids, ml l1 After 1 h Average Minimum Maximum
590 18 2412
After 1 h After 24 h After 24 h, 1 month later 1 month later 789 16 2471
259 31 724
321 30 988
The 1 h readings seem less reliable and less precise, since finer particles are still in suspension and the reading on the cone scale of settled solids is affected by uncertainty. Moreover, readings lower than 1 ml/l are not possibly measurable with this method, because of the definition of the graduated scale; in this study, the corresponding SSC is around 1 g l1. The differences between the two reading series (Table 6) can be explained by an increase in the volume of matter; however, not due to an increase in solids but to an increase in microorganisms and biological matter, with gas development and other anaerobic phenomena. In Fig. 7, a clear linear relationship is shown between the two series of 24 h FSS readings: as expected, values after 1 month are consistently higher. For the high significance and repeatability of the FSS technique, Imhoff cones can be used to estimate SSC in water samples, especially employing the 24 h readings for their reliability and their solid linear relationship with SSC.
This study resolved a few problems with the analytical methodologies employed in a monitoring campaign, in particular: reliability of the surrogate measure for suspended sediment concentration (SSC) on stored samples and on samples with high suspended sediment concentration, and consistency of measurements of settleable solids. Results confirm the high reliability of the nephelometer as an instrument to estimate for SSC, for nephelometric turbidity unit (NTU) values within the instrumental range, whereas, for higher levels of suspended sediment concentration turbidity, the technique is less reliable. In the latter case, the turbidity measurement requires sample dilution: this operation adds uncertainty due to the influence of the dilution factor and the skill of the operator. In addition, the lower is the dilution, the higher the measurement correctness. Another factor to be taken into account for turbid samples is a possible high component of coarser fraction which might affect the reliability of the measure. The SSC-NTU regression analysis both for fresh samples and 1-month-old samples show a very good correlation (coefficient of determination r2 of 099); however, an increase in NTU for the analysis is evident 1 month later. This phenomenon might be explained with the flocs of clays and the development of algae and gases, which increase the suspended material and modified the optical behaviour. Imhoff cones maintained good performance on samples with high SSC, especially compared to the nephelometric device. It is therefore preferable for high SSC, since the sample is not subjected to manipulation to be analysed and because Imhoff cone measurements
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are not significantly influenced by the variations of particle size distribution among different samples. This method is not suitable instead for low SSC, corresponding to free settled solids (FSS) around 1 ml l1. Settled solids of fresh samples and 1-month-old samples, both for the 1 and 24 h reading, showed a very good correlation with SSC. However, the 24 h readings are considered more reliable because the boundary between the liquid phase and the settled solids is easier to see, since there is no sediment in suspension. Also for Imhoff cones, as for the nephelometer, FSS measurements after 1 month are higher: algae development and anaerobic reaction with production of gases probably led to an apparent increase in sediment volume. In this study, the FSS technique resulted a reliable tool to estimate SSC in water samples for the repeatability and the reliability of the measurement, the simplicity, the low cost and the minimal exposure to measurement errors.
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Indirect Methods to Estimate Suspended Sediment Concentration: Reliability and Relationship of Turbidity and Settleable Solids D. Pavanelli1; A. Bigi2 1
Department of Agricultural Economics and Engineering, University of Bologna, Italy; e-mail of corresponding author: [email protected] 2 Department of Agricultural Economics and Engineering, University of Bologna; e-mail: [email protected] (Received 21 January 2004; accepted in revised form 1 September 2004; published online 18 November 2004)
A key element in a stream-monitoring programme for sediment transport is the choice of the measuring technique for suspended sediment concentration: this can highly affect both project costs and data reliability. The gravimetric method represents the standard analysis to directly measure suspended sediment concentration in a water sample. Indirect techniques are often employed for their inexpensiveness, although they need to be calibrated on gravimetric analysis results. In this study, the reliability of settleable solids in Imhoff cones was addressed as an alternative indirect method to estimate suspended sediment concentration (SSC), verifying the results with the turbidity measures of a laboratory nephelometer. The results show a high correlation of settled solids with suspended sediment concentration. Also, the results from the turbidimetric analysis showed a good correlation with SSC, but limited to water samples that did not need dilution. Measurements were repeated after storing the samples for 1 month: results exhibit an increase in turbidity and settleable solids, probably due to algae growth and anaerobic processes with the production of gases, causing an increase in matter. From our study, the Imhoff cone was shown to be a useful instrument to estimate suspended sediment concentration for the simplicity, the reliability and the low cost of their results. Our results prove how Imhoff cones are preferable to turbidimetric analysis to estimate SSC, specifically for highly turbid samples. r 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
events, when SSC can vary by an order of magnitude. The importance of fluvial suspended sediment as a vector for the transport of nutrients and many natural and anthropogenic pollutants is well established and the concentration of many substances bound to suspended solids can be at least an order of magnitude higher than their concentration in the dissolved phase (Walling & Woodward, 2000). Sediment transport monitoring is then designed to establish the stream water quality and basin morphology dynamics; but, notwithstanding, monitoring programmes are strongly dependent on sampling protocol and sample analysis techniques. Among the latter, the choice of a valid tool to estimate suspended sediment concentration (SSC) is very important: many
1. Introduction Suspended sediment yields mainly fine materials ranging from fine sands to clay, in different quantities according to the hydraulic characteristic of the stream, geomorphological and pedological conditions of the catchment, the climatic regime of the area and the presence of vegetation. Suspended sediment concentration (SSC) and particle size are fundamental controls on the dynamics of sediment entrainment, transport and deposition, and information on the particle size characteristics of sediment is an essential requirement for investigations of the flux and storage of sediment associated substances in the channel and riparian zone. Annual sediment load is mainly carried during flood 1537-5110/$30.00
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r 2004 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd
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measurements techniques have been developed in the last decades and many of them are based on the assessment of surrogate quantities for estimating SSC; these techniques are becoming numerous and, notwithstanding their qualities, they might not lead to univocal and comparable results (Gray et al., 2002). Gravimetric analysis represents the standard methodology to physically quantify suspended sediment and consists of physically separating the liquid phase from the solid phase in a water sample. The main disadvantages of this method are its labour intensity, its timeconsuming process and the economical impact on a research project budget; however, the analytical results are the most reliable tools to estimate SSC and are essential to properly calibrate measurements of the various surrogates (Gray et al., 2002). Among many other different methodologies, the use of optical turbidimeters, acoustic turbidimeters and laser diffraction is quite common in stream monitoring programmes (Schoellhamer & Wright, 2002). Acoustic turbidimeters measure the acoustic backscattered signal as a surrogate for SSC; these turbidimeters have the great advantage of not being subjected to biological fouling like optical turbidimeters (Gartner, 2002) and they can be easily coupled to acoustic Doppler current profilers (ADCP) (e.g., Creed et al., 2001; Taylor & Vincent, 2001). However, they present still some disadvantages: they need more field-testing, they might have to be specifically designed (e.g., Froehlich, 2002) and their results are not completely reliable yet (Gartner, 2002). Laser in situ scattering and transmissiometry (LISST) gives a measure of the angle of diffraction when a laser beam hits a particle; it provides an estimate of SSC and a complete particle size distribution, but the device is expensive and the outputs need particular training to be interpreted (Wren et al., 1999). Optical turbidimeters produce an estimate for SSC through measuring either the backscattering of a light (or an infrared beam) or the attenuation of a light beam passing through a water sample. They have been used in various studies: Halfman and Scholz (1993) and Paul et al. (1982) used it in a lake environment; Mitsch and Reeder (1992) in wetlands; Brabben (1981), Gippel (1995) and Jansson (1992) in streams. Continuous monitoring of suspended sediment in streams presents a number of technical problems, possibly leading to biased measurements: growing algae over the optical sensor or plugging of the intake tubing (Jansson, 1992), entraining of air bubbles and floating debris (Wagner et al., 2000). Furthermore, the relation of SSC with nephelometric turbidity units (NTU) for high sediment concentrations range is not completely investigated yet.
Some of these failings have been overcome by adding specific features to the monitoring device, such as the presence of self-cleaning optical sensors (e.g. Buchanan & Ganju, 2003), however theoretical limitations remain and are addressed in the next paragraph. Despite these drawbacks, if the necessary data quality is met (Eads & Lewis, 2002), continuous turbidity monitoring may give a good representation of SSC (Christensen et al., 2002). The present study addresses to three possible methodologies to estimate suspended solids in stream flows: gravimetric analysis, turbidimetric analysis and settleable solids analysis. These three methodologies are currently employed in a monitoring and sampling campaign started in 1997 on three Apennine streams. The reliability and the comparability of the results obtained with these techniques have been verified with a laboratory study. In the remainder of the paper, a simple alternative procedure is presented to estimate the suspended sediment concentration in water by measuring settleable solids.
2. Analysis procedures For this study, 12 water samples with a specific concentration of solids were prepared in the laboratory. Suspended sediment concentration and particle size for these samples were prepared in order to be comparable to concentrations measured during flood events of the Sillaro Torrent, one of the mountain stream currently under monitoring (Pavanelli & Pagliarani, 2002). Therefore, in order to obtain laboratory samples, filtered stream water has been used with sediments collected from the streambed next to the instrumental sampling hose of the Sillaro monitoring station. In Table 1 are presented sediment concentrations for the 12 laboratory samples. A sample of filtered stream water was used as a datum. This procedure provided 12 samples with constant mineralogy, constant particle size distribution across samples and specific suspended sediment concentrations. These sample characteristics are necessary to improve measurement reliability and comparability, since particle size and mineralogy affect instrumental readings. Consequently, stream-water samples collected during the ordinary monitoring campaign were not suitable for the purposes of this study. The particle size distribution obtained from the data collected from 82 water samples collected during flood events occurred in the Sillaro Torrent in the last 3 years is shown in Table 2 and compared to the particle size distribution of the laboratory samples. Particle size distribution of samples was determined by sedimentation
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velocity studies using Stokes’ Law, the standard employed being the Udden scale with the classification of Wentworth (G.U. n.248, 21-10-1999). All laboratory samples have been analysed through the following procedures: (1) turbidity analysis with a laboratory turbidimeter (manufacturer: Hach, model: 2100AN); (2) settleable solids in Imhoff cones; and (3) gravimetric analysis and particle size analysis with the pipette method. Turbidity measures have been repeated three times for each sample. Laboratory samples preparation, gravimetric and particle size analyses have been performed by the ARPA laboratories (Regional Agency for Health Prevention and Environmental Protection) of the Emilia-Romagna Region. In order to study the behaviour of samples during storage, the series of analysis were fully repeated 1 month after the first analysis: samples were stored at room temperature and exposed to daylight. The intent of this procedure was to recreate samples with characteristics similar to those of the ordinary stream-water samples. In fact stream-water samples are often analysed almost 1 month after sampling, because of the intrinsic Table 1 Suspended sediment concentration (SSC) in each laboratory sample SSC, g l1
Sample no. 1 2 3 4 5 6 7 8 9 10 11 12
15 25 40 55 70 85 100 125 150 200 250 300
characteristics of the automatic sampler (it holds 24 bottles) and the arrangement of the monitoring programme. 2.1. Total suspended solid concentration—gravimetric method Suspended solids represent the matter in a solid phase present in a water sample. The water sample is firstly centrifuged in order to separate the water from the suspended sediment; once the water has been clarified, the material collected is then oven-dried at 105 1C for about 10 h, until the matter reaches a constant weight. This is the procedure employed by the ARPA laboratories which performed the gravimetric and particle size analysis. 2.2. Turbidity measure Turbidity is an optical property of water and it can be used to measure relative water clarity. Since turbidity is mainly due to particles in suspension, it is also employed to estimate the concentration of suspended sediment. When a light beam passes through a water sample, it is subjected to scatter, transmission and absorption for the presence of particles in suspension: electronic photodetectors set around the water sample measure the amount of light transmitted and scattered as an indication of turbidity. It has been extensively proved that colour, shape and mostly particle size along with wavelength affect the scatterance of the sample (Gray et al., 2002; Foster et al., 1992; Gippel, 1988). Furthermore, small particles scatter light more uniformly and are more sensitive to shorter wavelengths, whereas large particles tend to forward scatter the incident light (Sadar, 1996). This leads to a greater sensitivity of the instrumentation for finer particles rather than larger ones (Lewis, 1996). For this reason, solutions of equal
Table 2 Particle size distribution averaged over 82 field samples; percent of total material and particle size distribution of the laboratory sample using the Udden standard of fraction and the Wentworth classification Category
Fraction, mm
Particle size distribution, % Stream-water samples
Clay Fine silt Coarse silt Sand SD, standard deviation.
o0002 0002–002 002–005 02–005
Laboratory samples
Average
SD
Max value
459 4175 505 355
73 59 29 25
69 60 15 13
44 37 95 95
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90˚ detector Forward scatter detector
Back scatter detector
Transmitted light detector LED source
Sample cell
Fig. 1. Schematic cross-section of a laboratory turbidimeter; LED, light-emitting diode
suspended solid concentration but different composition may not scatter the same amount of light. Also water colour (Malcolm, 1985) and temperature (Sadar & Engelhardt, year not specified) may also bias turbidity measurements, but their effect is less severe. Another failing of turbidimetric analysis is the low repeatability of diluted sample measurement results, turbidity being an apparent optical property of water. Optical instruments measuring transmitted light are named transmissiometers and are different from the optical backscatter sensors (OBS), which measure light scattered by particles at right angles. All analytical measurements require a primary standard upon which the calibration is based: one nephelometric turbidity unit (NTU) was established as the turbidity resulting from a suspension of one part per million of silica. Formazine is used as a primary turbidity standard (method 2130B, APHA, 1999) to calibrate the laboratory nephelometer and the measurement unit is the nephelometric turbidity unit. The instrument used for this study is a laboratory turbidimeter Hach 2100AN equipped with four photosensors for transmitted and scattered light and ranging from 0 to 10 000 NTU (Fig. 1). When the sample has an over-range turbidity, it is possible to have a NTU measurement by diluting the sample. In order to analyse water samples, the instrument requires a sub-sample of 30 ml extracted from the original sample after a proper agitation. This procedure might produce a non-representative sub-sample, particularly when a dilution is needed. When a sample needs dilution, turbidity is calculated using the following relationship (US EPA, 1999): TR ¼ TD
ðw þ sÞ : s
(1)
where TR is the turbidity of the undiluted sample in NTU; TD is the turbidity of the diluted sample in NTU; w is the content of clear water in the dilution in ml; and s is the content of sample water in ml. To convert NTU data records to suspended sediment concentration in samples, it is necessary to establish a
mathematical correlation between nephelometric units and SSC on a sufficiently wide sample population. 2.3. Settleable solids Settleable solids are defined as the solids that settle in an undisturbed sample of liquid after a specific time period. Water samples are thoroughly mixed and poured in Imhoff cones: settleable matter is measured volumetrically after 1 and 24 h; the readings are standardised on a 1 l sample. This technique is very common in wastewater analysis and its units are usually expressed in ml/l. Measurement precision of the graduated cone is 0.1 ml for volumes less than 2 ml; it increases to 0.5 ml for volumes of less than 10 ml and it is 1 ml for volumes ranging between 10 and 100 ml. In the remainder of the paper, settled solids are given as free settled solids (FSS) to indicate that the material settled is mostly inorganic matter and subject to free settling (method 2540F: APHA, 1999).
3. Results 3.1. Turbidity-suspended sediment concentration relationship Three to four turbidity measurements were made on each laboratory sample. Readings might result within or outside the instrumental NTU range: in the former case, readings had high repeatability and small deviations; in the latter case it has been verified as lower reliability of the results according to the increase in dilution. Table 3 presents the results of the turbidimetric analysis for samples 1–6: samples 1–4 did not need any dilution, whereas samples 5–6 required dilution because the turbidity was slightly outside the instrumental range. Samples 7–12 required high dilution rates and this manipulation affected the quality of the results. To further test the existence of a difference in consistency between the diluted and undiluted samples,
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Table 3 Summary statistics of three repeated nephelometric turbidity unit (NTU) measurements on undiluted samples 1–4, diluted samples 5 and 6 Sample no.
0 1 2 3 4 5 6
Turbidity, NTU TU1
TU2
TU3
10 835 1915 4163 5425 11 204 9470
10 910 1908 3918 5475 9928 10 517
10 786 1890 3890 5931 8652 11 564
Average 10 844 1904 3990 5610 9928 10 517
For the stored samples, NTU reading averages have been employed also for the diluted samples, these measurements being very consistent and reliable compared to those for fresh samples. The existing relationships between SSC and NTU were also investigated using the complete dataset of measurements on fresh samples and 1-month-old samples (Fig. 3). For both groups, correlation coefficients are extremely high and the value for r is less than 001. Comparing the two series of analyses, it is evident that there is an increase in NTUs after 1 month, mathematically shown by a regression curve in Fig. 4. This behaviour might be explained by the development of microorganisms and algae, clays aggregation and flocculation, besides other biological reactions and gas production. Although aggregation increases sedimentation velocity, the gaseous product of the biological reactions aggregates to flocs and supports solids suspension. Moreover, algal growth increases turbidity and changes water colour. Since this can lead to an overestimation of SSC, it is therefore necessary either to use a specific regression curve to calculate SSC from stored samples, or to calculate the corresponding NTU value of the fresh sample, or to store samples in a refrigerated and dark container.
Turbidity, NTU
the Pearson correlation coefficient r for each pair of readings was calculated (Table 4). The coefficient measures the strength of the linear relationship between the considered variables and r indicates the statistical significance of the estimated correlations: only NTU readings from 1 to 4 are correlated with a confidence level higher than 95%. The results from the eight diluted samples indicate that there is a decrease in NTU readings according to a power law when the dilution increases. Also, a linear regression (Fig. 2) has a comparable coefficient of determination and it has been employed to re-calculate NTU values for the diluted samples (Table 5): clear differences existed for NTU values calculated through a linear regression and through NTU averaging. The linear regression results are denoted as the ‘correct NTU values’ and are employed in all the following analysis. It has been noted that suspended sediment concentration within the instrumental range (0–10 000 NTU) can be measured with a high reliability; beyond this concentration, it was necessary to dilute the sample with a decrease in the quality of the measurement. However, the lower was the dilution, the higher was the measurement accuracy. Besides the intrinsic effect of dilution, this is also due to the combined effect of an increase in the coarser fraction for samples with more suspended solids and the lower sensitivity of the instrument to sands and coarse silt.
40 000 35 000 30 000 25 000 20 000 15 000 10 000 5000 0 0
2
4
6 8 10 Dilution factor
12
14
Fig. 2. Turbidity results after dilution from the relationship in Eqn (1): , sample no. 7; J, sample no. 8; K, sample no. 10; }, sample no. 12
Table 4 Pearson’s product–moment correlation coefficient q for the statistical significance of the estimated correlations: values below 0.05 indicate that the hypothesis for a correlation coefficient of zero can be rejected with a 95% confidence level Sample
Variables
Number of samples
Correlation coefficient (r)
Pearson’s coefficient (r)
Undiluted Diluted
TU1/TU2 TD1/TD2
4 8
0997 0673
00023 0067
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Table 5 Nephelometric turbidity unit (NTU) final values for diluted samples calculated using a linear regression and averaging NTU readings Turbidity, NTU Average 5 6 7 8 9 10 11 12
SSC, g/l
Regression based
9928 10 517 8083 10 760 22 640 18 665 38 496 31 200
9933 10 976 11 151 14 188 19 032 23 888 32 939 44 481
70 90 100 125 150 200 250 300
SSC, suspended solid concentration.
y = 0.00065 x + 2.78 r2 = 0.79
3 2 1 0 0
40
25 20 15 y = 0.00060 x + 1.42 r2 = 0.98
10 5 0
0
1 2 3 4 Turbidity after one month, NTU
y = 0.43 x + 0.65 r2 = 0.99
35
5 × 10 000
10 000 20 000 30 000 40 000 50 000 60 000 Turbidity, NTU
y = 0.0005 x2 + 0.25 x + 1.47 r2 = 0.99
30 SSC, g/l
SSC, g/l
30
y = 0.88 x − 326.34 r2 = 0.99
Fig. 4. Regression analysis for the nephelometric turbidity unit (NTU) readings on fresh samples and on 1-month-old samples; note that the angular coefficient is less than one: , 99% confidence bands: , 95% confidence bands; r2, coefficient of determination
40 35
4 Turbidity, NTU
Sample
× 10 000 5
25 y = 0.12 x + 4.50 r2 = 0.90
20 15 10
Fig. 3. Regression analysis between suspended sediment concentration (SSC) g l1 and nephelometric turbidity units (NTU) with the complete set of measurements performed soon after sample preparation and after 1 month: K, fresh samples; &, month-old samples; , linear regression (fresh sample); , linear regression (1-month-old sample); r2, coefficient of determination
3.2. Imhoff cones—suspended sediment relationship Imhoff cones, usually used to estimate settleable matter for sewage, appear to be a very interesting tool for estimating suspended sediment concentration in stream water for their characteristics of simplicity, cost competitiveness and reliability. Readings were taken 1 and 24 h after the samples were poured into the cones. Besides, the analyses of the samples were completed soon after their preparation and after storing them for 1 month. In Figs 5 and 6, the regression analyses for SSC on FSS readings are presented: the 24 h readings present a linear relationship with SSC, whereas the 1 h readings are well represented by a polynomial relationship. It is worth noting that, for all four cases, the value for r is less than 001 (indicating a statistical significance at the
5 0
0
25
50
75 100 125 150 175 200 225 250 275 FSS, ml/l
Fig. 5. Regression curves between suspended solids concentration (SSC) and free settled solids (FSS) for 1 and 24 h readings for fresh samples: J, cone 1 h; K, cone 24 h; , linear (1 h); , linear (24 h); , polynomial (24 h); r2, coefficient of determination
99% confidence level) and that the coefficient of determination r2 is higher than 090. First evidences from a qualitative analysis of Figs 5 and 6 are: the 1 h readings are always higher than the 24 h readings, due to compaction of solids through time; the 1 and 24 h readings from the analysis performed after 1 month are always higher than those from the analysis after sample preparation; and the 24 h readings of fresh samples compared with those of the stored samples show a lower difference than 1 h readings, probably due to the compaction of particles.
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45 40
y = 0.32 x + 1.60 r2 = 0.97
35
y = 0.16 x + 2.69 r2 = 0.99
80
60 FSS, ml/l
30 SSC, g/l
81
25 20 15
y = 0.74 x + 1.99 r2 = 0.99
40
20
10 5 0
0 0
25
50
75 100 125 150 175 200 225 250 275 FSS, ml/l
Fig. 6. Regression curves between suspended solids concentration (SSC) and free settled solids (FSS) for 1 and 24 h readings 1-month-old samples: K, cone 24 h after 1 month; J, cone 1 h after 1 month; , linear 1 h after 1 month; , linear 24 h after 1 month; r2, coefficient of determination
0
20
40 60 80 FSS after one month, ml/l
100
Fig. 7. Regression analysis between the 24 h free settled solids (FSS) readings for the fresh and for the stored sample: , 99% confidence bands; , 95% confidence bands; r2, coefficient of determination
4. Conclusions Table 6 Summary statistics of Imhoff cone readings on 12 laboratory samples Settleable solids, ml l1 After 1 h Average Minimum Maximum
590 18 2412
After 1 h After 24 h After 24 h, 1 month later 1 month later 789 16 2471
259 31 724
321 30 988
The 1 h readings seem less reliable and less precise, since finer particles are still in suspension and the reading on the cone scale of settled solids is affected by uncertainty. Moreover, readings lower than 1 ml/l are not possibly measurable with this method, because of the definition of the graduated scale; in this study, the corresponding SSC is around 1 g l1. The differences between the two reading series (Table 6) can be explained by an increase in the volume of matter; however, not due to an increase in solids but to an increase in microorganisms and biological matter, with gas development and other anaerobic phenomena. In Fig. 7, a clear linear relationship is shown between the two series of 24 h FSS readings: as expected, values after 1 month are consistently higher. For the high significance and repeatability of the FSS technique, Imhoff cones can be used to estimate SSC in water samples, especially employing the 24 h readings for their reliability and their solid linear relationship with SSC.
This study resolved a few problems with the analytical methodologies employed in a monitoring campaign, in particular: reliability of the surrogate measure for suspended sediment concentration (SSC) on stored samples and on samples with high suspended sediment concentration, and consistency of measurements of settleable solids. Results confirm the high reliability of the nephelometer as an instrument to estimate for SSC, for nephelometric turbidity unit (NTU) values within the instrumental range, whereas, for higher levels of suspended sediment concentration turbidity, the technique is less reliable. In the latter case, the turbidity measurement requires sample dilution: this operation adds uncertainty due to the influence of the dilution factor and the skill of the operator. In addition, the lower is the dilution, the higher the measurement correctness. Another factor to be taken into account for turbid samples is a possible high component of coarser fraction which might affect the reliability of the measure. The SSC-NTU regression analysis both for fresh samples and 1-month-old samples show a very good correlation (coefficient of determination r2 of 099); however, an increase in NTU for the analysis is evident 1 month later. This phenomenon might be explained with the flocs of clays and the development of algae and gases, which increase the suspended material and modified the optical behaviour. Imhoff cones maintained good performance on samples with high SSC, especially compared to the nephelometric device. It is therefore preferable for high SSC, since the sample is not subjected to manipulation to be analysed and because Imhoff cone measurements
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are not significantly influenced by the variations of particle size distribution among different samples. This method is not suitable instead for low SSC, corresponding to free settled solids (FSS) around 1 ml l1. Settled solids of fresh samples and 1-month-old samples, both for the 1 and 24 h reading, showed a very good correlation with SSC. However, the 24 h readings are considered more reliable because the boundary between the liquid phase and the settled solids is easier to see, since there is no sediment in suspension. Also for Imhoff cones, as for the nephelometer, FSS measurements after 1 month are higher: algae development and anaerobic reaction with production of gases probably led to an apparent increase in sediment volume. In this study, the FSS technique resulted a reliable tool to estimate SSC in water samples for the repeatability and the reliability of the measurement, the simplicity, the low cost and the minimal exposure to measurement errors.
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