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Published online 3 April 2006
Published in J Environ Qual 35:815-823 (2006)
DOI: 10.2134/jeq2005.0219
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Surface Water Quality

Measuring Runoff-Suspended Solids Using an Improved Turbidometer Method

Daniel Ginting and Martha Mamo*

Department of Agronomy and Horticulture, University of Nebraska, Lincoln, NE 68583

* Corresponding author (mmamo3{at}unl.edu)

Received for publication June 1, 2005.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Differences in particle size distribution between runoff standards and unknown samples affect the accuracy of estimation of total suspended solids (TSS) concentration using the nephelometric turbidity (NTU) method. The objective was to quantify the effects of a sucrose solution as suspending medium and contrasting particle size distribution on nephelometric turbidity and accuracy of TSS estimation. Nineteen benchmark soils varying in texture and color were divided into particle size distribution of <250 and <2000 µm. Soils from these two aggregate classes were then made into suspension ranging from 0.2 to 15 g L–1 using distilled deionized water. Runoff suspensions ranging from 0.2 to 21 g L–1 were also collected from different watersheds. Turbidity of soil and runoff suspensions was measured in sucrose solution and in distilled deionized water. The sucrose solution density ranged from 1.10 to 1.30 kg L–1. Increasing sucrose solution density decreased turbidity. The TSS concentration was most sensitive to changes in turbidity with the 1.30 kg L–1 sucrose solution. Using the 1.30 kg L–1 sucrose solution, particle size bias and error of TSS estimates were decreased by at least 20% compared to distilled deionized water. Reduction in refraction index differences between the suspended particles and sucrose solution combined with reduced particle settling and reduced Brownian motion resulted in dampening the effects of particle size distribution. We propose a sucrose solution of 1.30 kg L–1 as a better suspending medium to dampen the effect of particle size distribution and thus improve suspension TSS concentration estimation.

Abbreviations: CRM, coefficient of residual mass • Dgm, geometric mean diameter • ME, model efficiency • NTU, nephelometric turbidity unit • RI, refractive index • RMSE, root mean square error • RTD, relative turbidity difference • SSC, suspended sediment concentration • TDS, total dissolved solids • TSS, total suspended solids


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
TOTAL SUSPENDED SOLID (TSS) is an important parameter of water quality for human and aquatic lives (Wass and Leeks, 1999; Moog and Whiting, 2002a, 2002b). Impacts of agricultural land management on surface runoff quality are commonly measured by sediment and pollutant loading in surface runoff (Boesch et al., 2001; Moog and Whiting, 2002a, 2002b). Thus, accurate measurement of sediment concentration in runoff is critical.

Sediment concentrations in surface runoff are measured either directly or indirectly. Direct measurement of sediment concentration in surface runoff is commonly done by filtration followed by drying and weighing using two common standard methods, namely the American Public Health Association (1998) and American Society for Testing and Materials (2000). The American Public Health Association (1998) method measures TSS of a runoff subsample (usually 500 mL) by filtration and drying at 103 to 105°C. The American Society for Testing and Materials (2000) employs three test methods to measure suspended sediment concentration (SSC) on the whole runoff suspension (note the APHA uses TSS and the ASTM uses SSC). The first ASTM method entails determining SSC by only evaporation at 103 ± 2°C assuming the predominance of clay-sized particles. Dissolved solids correction is made if the concentration exceeds 10% of the SSC. The second ASTM method adds a filtration step before sample drying due to the predominance of sand-size particles. The third ASTM method substitutes wet-sieving filtration for standard filtration before drying. This method yields concentration of sand-, silt-, and clay-size particles. The APHA and ASTM methods give comparably close results when the large majority of sediments in suspension are finer than 62 µm, but results deviate between the two methods as sample particle size increases (Gray et al., 2000). However, both the APHA and ASTM standard methods are time consuming, tedious, and require large suspension volume, especially when the concentration is low (Sadar, 1998). Another nonstandard method used primarily in agricultural water quality research is without filtration, where runoff suspension is dried in two steps: evaporation of a known volume of suspension in a container at below boiling point, followed by drying at 105°C. Since the evaporate contains total dissolved solids (TDS), the concentration of TDS in suspension is estimated from electrical conductivity (EC) measurement (Tabbara, 2003). The TSS is calculated as the weight difference between the evaporates and the TDS. This procedure also needs a large runoff sample volume, especially when the runoff contains a small amount of sediment. This process, too, is tedious and slow due to the drying time.

Measurement of TSS has also been made indirectly using sensors in the laboratory or in situ (Foster et al., 1992; Brett et al., 2005; Harter and Mitsch, 2003; Hayes et al., 2005; Heathwaite et al., 2005). The most common indirect measurement technique is the nephelometric turbidity method, and is based on the theory that light scattering intensifies as particle concentration increases. Since the intensity of scattered light at an angle of 90° to the beam is proportional to the total scattering (van de Hulst, 1957), turbidity can be used to estimate suspended particle concentration. The method requires establishing an empirical relationship (standard curve) between known particle concentrations (mass per unit volume) and their nephelometric turbidity units (NTU). To obtain accurate concentration estimates, the properties of the runoff suspensions must be consistent with those of the standard curve. This is often a difficult task because suspension turbidity is affected by many factors such as color and aggregate size distribution of suspended particles (Gippel, 1989; Foster et al., 1992; Gippel, 1995; Wass and Leeks, 1999). Particle size also affects light scattering. Smaller particles scatter shorter (blue) wavelengths more intensely while larger particles scatter longer (red) wavelengths more intensely (Sadar, 1998).

Runoff from different rainfall intensity and duration results in different particle size distribution of runoff suspensions (Bogen, 1992; Foster et al., 1992; Peart and Walling, 1992; American Public Health Association, 1998). If the suspension in runoff samples contains larger particles than the ones used for the standard curve, the nephelometric turbidity will underpredict TSS concentration and, thus, grossly underestimate TSS loadings in overland flow. To reduce the effect of particle size distribution, runoff suspensions, including those used for the standard curve, need to be dispersed, either by grinding or adding a dispersant agent, as an additional step. Consequently, a better method is needed that will reduce the effects of particle size distribution on sediment concentration determined indirectly from turbidity without increasing the sample processing time.

Measurement of turbidity in the laboratory or in situ uses water as the suspending media and in some cases a dispersant agent such as Calgon or sodium hexametaphosphate (Foster et al., 1992). Foster et al. (1992) used 10% Calgon suspending media and observed no difference in TSS to that of stream water suspending media. More recently, Leigh and Hyne (1999) used formaldehyde as a way to reduce flocculation or aggregation of fluvial water samples before turbidity measurement. A thorough literature search revealed that no other solution has been used as suspending media for turbidity measurement to enable more accurate measurements of TSS. In theory, a suspending media with a refractive index (RI) closer to the RI of soil particles will result in less shift in the angle of refraction, thus resulting in lower turbidity (Sadar, 1998). At the same time, a viscous solution will reduce the settling of coarser particles and Brownian movement of colloidal size particles.

The hypothesis is that using a more viscous sucrose solution (rather than water) as suspending medium for runoff water will significantly reduce the effects of particle size distribution and improve the accuracy of the measured sediment concentration. The objectives of this study were to quantify the effects of sucrose solution on nephelometric turbidity and the accuracy of TSS concentration estimation for suspensions with contrasting particle size distributions.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Simulating Runoff Suspension from Soil Aggregates
Soil Aggregate Preparation and Aggregate Size Distribution
Soil suspensions, to mimic runoff suspensions, were made from 19 benchmark soils previously ground to pass a 2-mm sieve opening. These soils varied in physical (texture and color) and organic C content (Table 1). From each soil type, an approximate 30-g sample was ground with a corundum mortar and pestle to pass 250-µm openings. Grinding to a fine particle size was performed to mimic soil dispersion by rainfall drops before transport by runoff water.


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  		Table 1. Benchmark soils and their selected properties used to prepare soil suspension.

 
The size distribution of aggregates in the ground soils that passed 2-mm openings and those that passed 250 µm was determined by dry sieving according to the procedure described by Kemper and Rosenau (1986). Five to ten grams of soil aggregates were sieved through 53-, 106-, 250-, 500-, and 1000-µm openings. The sieving resulted in aggregate categories 0 to 53 µm (silt and clay size), 53 to 106 µm (very fine sand), 106 to 250 µm (fine sand), 250 to 500 µm (medium sand), 500 to 1000 µm (coarse sand), and 1000 to 2000 µm (very coarse sand). Each aggregate class size for each soil was weighed and reported in terms of weight percentage of the sample. The mean diameter of the soil aggregates mixture is approximated with the geometric mean diameter of the aggregate sizes as follows (Haan et al., 1994):

Formula[1]
where Dgm is geometric mean diameter, {Delta}i represents fraction of class size i, and Di represents the average of the upper and lower values of each class size. The particles size distribution and the geometric mean for <250 and <2000 µm are presented in Tables 2 and 3, respectively.


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  		Table 2. Size distribution of two contrasting soil aggregates used for making soil suspension.

 

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  		Table 3. Improvement in estimation of total suspended solids (TSS) concentration of 19 benchmark soil suspensions using sucrose solution over distilled deionized water as suspending media. The standard curves were generated with suspension of particle size distribution of <250 µm for estimation of the TSS concentration of particle size distribution of <2000 µm.

 
Soil Suspension Preparation
Into 220-mL polyethylene cups (60-mm-i.d. base, 80-mm-i.d. top, and 70-mm height), 0.02, 0.04, 0.06, 0.08, 0.1, 0.25, 0.5, 0.8, 1.0, and 1.5 g of soils were added. Into each cup, 100 mL of distilled deionized water were added and then capped. This made soil suspensions with TSS concentrations of 0.2, 0.4, 0.6, 0.8, 1, 2.5, 5, 8, 10, and 15 g L–1. In this case, the volume of soil aggregates is negligible compared to the volume of distilled deionized water. The suspensions were made from each benchmark soil for both the aggregate size groups (<2000 and <250 µm). The soil suspensions were stored at 4 ± 2°C for turbidity measurements.

Collection of Surface Runoff from Watersheds
Sixty runoff samples were collected from three 1- to 6-ha subwatersheds nested in a 4000-ha watershed in eastern Nebraska from November 2003 to November 2004. Runoff samples ranged from 0.2 to 21 g L–1 in TSS concentration. The TSS concentrations of these samples were determined by filtration followed by drying (American Public Health Association, 1998). A volume (approximately 50 mL) of runoff was weighed and then filtered with a preweighed Whatman (Maidstone, UK) 42 filter paper (a process that takes approximately 12 h). The filter paper weight was determined after drying at 75 ± 2°C and cooling to 23°C in a desiccator. The runoff filtrate was then dried at 75 ± 2°C, cooled down to 23 ± 2°C in a desiccator, and then weighed. The weight of solids was determined by weight difference from the weight of filter paper. The volume of water (mL) was determined as the weight difference (in unit g) between the suspension weight and solid weight (measured water density at 23 ± 2°C was 0.98–0.99 Mg m–3). The TSS concentration was determined as the ratio between solid weight and volume of water assuming that the volume of particles is negligible compared to the volume of water.

Suspending Media
Two suspending media were selected for estimating TSS concentration from nephelometric turbidity: 1.30 kg L–1 sucrose solution and distilled deionized water. The advantage of sucrose solution is that it is nontoxic and relatively inexpensive (grocery store–grade sugar).

The 1.30 kg L–1 sucrose media was chosen because a preliminary study indicated that the TSS concentration is most sensitive to changes in turbidity compared to using sucrose media of 1.20 and 1.10 kg L–1. Saturated sucrose solution (1.32 kg L–1) was not used because it contained crystal sugar that affected turbidity. The selection of distilled deionized water and sucrose solution concentrations is intended to make a final density of the mixture of runoff and suspending media to be 1.0 and 1.25 kg L–1, respectively. The final densities of mixture were based on 2:10 volume ratio of soil or runoff suspension and suspending medium. The 2:10 ratio is selected based on operational and sampling accuracy. A preliminary test indicated that the ratio is appropriate for a 14-mL turbidity meter cuvet for several reasons: to avoid bubbles during end-over-end mixing, to obtain a mixture with a workable viscosity, and to increase sampling volume.

The sucrose solution was made after determining sucrose crystal density and water density at room temperature (23 ± 2°C). Water density was 1 (±0.01) kg L–1. The sucrose was pure granulated sugar commonly available at grocery stores. A known weight of sucrose crystal (approximately 160 g) was added to a 500-mL volumetric flask of known weight. Distilled deionized water was added to fill about 75% of the flask and allow the sugar to dissolve. Then distilled deionized water was added to the 500-mL mark on the flask and then weighed. The volume of distilled deionized water is determined as weight difference between total weight and the flask plus sucrose weight. The volume of sucrose is 500 mL minus the volume of distilled deionized water and the sucrose density is determined as sucrose weight (g) divided by sucrose volume (mL). The procedure was repeated three times and the average sucrose crystal density was 1.578 kg L–1. All steps of density determination were made at room temperature (23 ± 2°C).

The amount of sucrose crystal needed to make the 1.30 kg L–1 was based on the formula:

Formula 2[2]
where V, M, and D were volume, mass, and density, respectively. The subscript terms sc, ss, and w indicate sucrose crystal, sucrose solution, and distilled deionized water, respectively. For example, to make Dss (sucrose solution density) of 1.30 kg L–1, 1000 g of sucrose crystal is mixed with 587.24 mL (or g) of distilled deionized water. For rapid sucrose dissolution, the sucrose crystal and distilled deionized water was combined in a tight screw-cap glass container and heated to 75 to 90°C while mixing with a magnetic stirrer on a hot plate or in a water bath. After complete dissolution, sucrose solution is then cooled to room temperature (23 ± 2°C) for use as suspending medium. Sucrose solution is stored at 4 ± 2°C to avoid microbial growth.

Measurements with Nephelometric Turbidity Meter
Because the runoff suspension and sucrose solution were stored at 4 ± 2°C, they were allowed to equilibrate to room temperature (23 ± 2°C) before use. Into a 14-mL cuvet, 10 mL of suspending medium and 2 mL of the prepared runoff suspension were dispensed using pipets. For consistency, pipeting was assisted with a pipet helper (Drummond Scientific, Broomall, PA). At all times during pipeting, the particles were kept in suspension with a magnetic stirrer (Corning Inc., Corning, NY) set at maximum mixing (scale 5) and the same magnetic bar was used for all the runoff suspensions. The turbidity cuvet was then capped and cleaned with lint free paper (Kimberly-Clark, Roswell, GA). The soil or runoff suspension and the suspending medium were then shaken gently end-over-end 15 to 20 times and immediately inserted into a bench-top nephelometric instrument (LaMotte, Chestertown, MD) for turbidity reading. All turbidity measurements were made in triplicate.

Data Analysis for Turbidity and Total Suspended Solids Concentration
Soil Suspension
Relative turbidity difference (RTD) between water (NTUw) and sucrose solution (NTUss) is expressed as:

Formula 3[3]

The RTD for each particle size distribution (<2000 or <250 µm) of each soil indicates the intrinsic effects of sucrose solution (relative to water) on turbidity.

The benefits of using sucrose solution rather than water for estimating TSS concentration of each soil with different aggregate size distribution (<2000 and <250 µm) was approximated as:

Formula 4[4]
where i, 2000, and 250 are various TSS concentrations and the aggregate sizes of <2000 and <250 µm for each soil, respectively. The particle size bias indicates the relative closeness of turbidity (thus estimates of TSS concentration) for the two distinct aggregate sizes. The average bias of zero indicates perfect fit of turbidity measured with sucrose and water. Particle bias higher than zero indicates that sucrose solution is better than water for TSS concentration determination.

The TSS concentrations of unknown suspensions are estimated with a predetermined standard curve using sucrose or distilled deionized water. The standard curve relating turbidity and TSS concentration for each soil is generated from the aggregate size of <250 µm (Table 4) using sucrose solution and distilled deionized water. The standard curve parameters were derived with nonlinear regression analysis of SAS Version 8.0 (SAS Institute, 1999). Test of normality at probability level of 0.05 with Shapiro–Wilk, Kolmogorov–Smirnov, Cramer–von Mises, and Anderson–Darling analyses was made using SAS PROC UNIVARIATE. The standard curves were then used to estimate the TSS concentrations for the aggregate size of <2000 µm. For each suspending medium, error analysis was made to determine goodness of fit between measured and estimated TSS concentrations. Root mean square error (RMSE), model efficiency (ME), and coefficient of residual mass (CRM) were calculated for error analysis as follows:

Formula 5[5]

Formula 6[6]

Formula 7[7]
where Pi and Oi are predicted and observed concentration, O is the average of the observed concentration, and i is the number of observation ranging from 1 to n.


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  		Table 4. Power function parameters of empirical relationships between turbidity and total suspended sediment (TSS) concentration of 19 benchmark soils with aggregate size of <250 µm.{dagger}

 
The value of RMSE would be equal to zero if the fit is perfect between measured and estimated TSS concentration. The ME is a measure of deviation between measured and estimated concentration relative to the scattering of the observed data (Nash and Sutcliffe, 1970). The ME value would be one for a perfect fit. The CRM indicates the difference between measured and estimated relative to the measured data. The CRM value would be zero for a perfect fit between the measured and estimated values.

The improvement in TSS estimation with sucrose solution over distilled deionized water is calculated as the difference in RMSE and CRM between distilled deionized water and sucrose solution. Improvement in ME is expressed as the difference in ME value between the sucrose solution and distilled deionized water relative to the perfect fit value. Positive, zero, or negative values of the differences indicates that sucrose solution made improvement, no improvement, or reduced the accuracy of TSS concentration estimation, respectively.

Watershed Runoff Suspension
Because aggregate size distributions of each runoff samples are unknown (and likely are varied from one sample to the other), the efficacy of using 1.30 kg L–1 sucrose solution over distilled deionized water was also measured using error analysis. Runoff samples were split into low concentration (<1 g L–1) and high concentration (≥1 g L–1). For each group, 10 samples (from lowest to highest TSS concentration) were chosen randomly using survey select procedure of SAS Version 8.0 (SAS Institute, 1999). The 10 selected samples were used to generate standard curves using sucrose solution and distilled deionized water. The improvement in accuracy of estimating TSS concentration was determined using the error analysis as described above.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Effects of Suspending Media on Nephelometric Turbidity
Increasing the density of sucrose solutions reduced NTUs for both soils tested (Fig. 1). The RI of sucrose solution increases as the density increases. The RI of water, 1.1, 1.2, 1.25, and 1.3 kg L–1 sucrose solutions at 25°C are 1.33, 1.37, 1.41, 1.43, and 1.45, respectively (Piramoon Technologies, 2005). Eshel et al. (2004) reported that average soil RI was 1.5. The RI of soil particles is closer to RI of sucrose solution than distilled deionized water. Reduction in RI differences resulted in lower turbidity in sucrose solution than in water for the same TSS concentration and particle size distribution. This suggests that using sucrose solution minimizes the effects of particle RI, thus allowing greater sensitivity of turbidity to changes in TSS concentration. Increasing density of sucrose did not affect light transmission because our measurements indicated that visible light transmission without soil suspension was the same in 1.3 kg L–1 sucrose solution and distilled deionized water.


Figure 1
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  		Fig. 1. Effects of sucrose and water as suspending media on total suspended solids (TSS) concentration and nephelometric turbidity (NTU) of Sharpsburg silt clay loam and Valentine sand. Error bar represents standard error.

 
Increasing density of sucrose solution also increases viscosity. The viscosity of 1.25 kg L–1 sucrose solution and distilled deionized water is 0.026 and 0.00086 kg m–1 s–1, respectively. Based on Stokes law, the terminal falling velocity of particles in sucrose solution was 30 times slower compared to water. Therefore, sucrose solution reduced coarser-particle settling out during the 2 s of turbidity measurement. Sucrose solution will also reduce velocity of random Brownian motion of colloidal-size particles, thus reducing scattering of light. In most cases, the reduction in turbidity with increasing sucrose viscosity was more pronounced for the suspension containing more fine particles (Fig. 1).

For each soil and particle size distribution, the difference in NTUs between distilled deionized water and 1.30 kg L–1 sucrose solution (as also applied to the 1.10 and 1.20 kg L–1 sucrose solutions) was larger as the TSS concentration increased. However, for each soil of the same particle size distribution, the relative NTU difference is similar regardless of increasing TSS concentration. Unlike this result, Foster et al. (1992) found no difference in sediment concentration between stream-water suspending media and 10% Calgon suspending media, suggesting that the role of Calgon was to disperse aggregates rather than dampen the effects of fine particles.

Turbidity and Total Suspended Solids Concentration Relationships
The empirical relationship between TSS concentration and turbidity is best described by the power function:

Formula 8[8]
where Y, X, a, and b are TSS concentration, NTUs, and constants, respectively. In all soil suspensions, the turbidity correlates well with TSS concentration for both sucrose solution or distilled deionized water. The constants a and b for suspensions containing soil particles smaller than 250 µm (fine sand size) are presented in Table 4. The slope of the relationship:

Formula 9[9]
suggests that the change in TSS concentration per unit change in NTU is larger for low NTUs (low sediment concentration) than for high NTUs (high sediment concentration). This is a significant improvement for TSS concentration determination with the nephelometric method, because at low TSS concentration the analytical accuracy of gravimetric method is low. The constant a for 1.30 kg L–1 sucrose medium is nearly twice or more compared to those of distilled deionized water; thus, the rate of change of TSS concentration is larger in sucrose medium than water medium. One may speculate that the increase in sensitivity also means greater uncertainty (error) of TSS prediction. However, this was not the case, as discussed in the next section.

Effects of Particle Size Distribution on Error of Total Suspended Solids Concentration Estimates
Since the sensitivity (perhaps also the uncertainty) of TSS estimation increases with increasing density, 1.30 kg L–1 sucrose solution was selected for further discussions in this study. The selection represents the worst case scenario of sensitivity and uncertainty for the contrasting particle size distributions between the standard curve and the runoff samples. For all soils studied, except the KoleKole and Wahiawa soils, the relative NTU difference was larger for smaller geometric mean diameter (Table 3). For example, the RTD for dark colored soils (10YR 2/1) decreased as the geometric mean diameter increased (Fig. 2). The logarithmic increase of RTD with decrease in particle diameter indicated that small particles are the main source of turbidity. Distilled deionized water as suspending medium resulted in lower NTU for smaller particle size distribution (<250 µm) than the larger one (<2000 µm) (Fig. 3a). The turbidity of suspensions of contrasting particle size distributions were closer by using 1.30 kg L–1 sucrose solution compared to using distilled deionized water; that is, the differences in turbidity between aggregate size of <250 and <2000 µm were smaller in sucrose solution than in distilled deionized water. For most soils, the particle size bias was 25% higher in distilled deionized water compared to sucrose solution (Table 3).


Figure 2
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  		Fig. 2. The effects of geometric mean diameter of aggregates on the turbidity difference between distilled deionized water and sucrose solution as a fraction of the turbidity using distilled deionized water for dark 10YR 2/1 soils.

 

Figure 3
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  		Fig. 3. The effects of suspending media on turbidity of two contrasting aggregate sizes of (a) Sharpsburg 10YR 2/1, (b) KoleKole 7.5YR 2.5/3, and (c) Wahiawa 7.5YR 2.5/3 soils. Error bar represents standard error.

 
There are three possible interrelated mechanisms that govern the converging turbidity despite contrasting differences in particle size distribution. First, the RI of soil particles is closer to the RI of sucrose solution. As the difference between the refractive indices of suspended particle and suspending fluid decreases, light scattering becomes less intense, and vice versa. Second, sucrose solution dampens the effects of fine particles, thereby increasing the contribution of larger particles (in relative terms compared to the fine particles). Dampening the effects of fine-size particles enhances contribution of coarser particles to the TSS concentration (i.e., assumption of negligible mass of fine particles). Third, the viscous sucrose solution retains settleable particles in suspension more than distilled deionized water.

Unlike the other 17 benchmark soils, for the volcanic ash soil of KoleKole containing high clay (5.8 g kg–1) and high organic matter (1.0 g kg–1) and the highly weathered (high iron oxide) and very high clay (9.0 g kg–1) Wahiawa soils, larger aggregates scattered more light than the finer aggregates in distilled deionized water as suspending media (Fig. 3b and 3c). The mechanism that governs this anomaly in distilled deionized water is not clear. It was unlikely due to settling of larger particles in water because particle size distribution of <250 µm had lower turbidity. The soils are red because soil particles reflect long wavelength (red) of visible light. Since larger particles scatter long wavelengths more intensely compared to smaller particles (Sadar, 1998), the particle size distribution of <2000 µm resulted in higher turbidity compared to the particle size distribution of <2000 µm.

Dampening the light scattering by viscous sucrose solution causes the TSS estimation to be less responsive to differences in particle size distribution between the standard curve and the unknown suspension, thus improving the accuracy of indirect measurements of TSS. For 18 out of the 19 benchmark soils, sucrose solution improved the accuracy of TSS concentration estimation compared to distilled deionized water. In most cases, sucrose solution improved RMSE, ME, and CRM values by 20% or more compared to distilled deionized water (Table 3). These error analyses further indicate that sucrose solution has less uncertainty in TSS concentration estimation even though TSS concentration is more sensitive toward NTU change in sucrose solution than in distilled deionized water. Therefore sucrose solution is a better suspending medium than distilled deionized water in estimating TSS concentration, particularly when there is an extreme difference in aggregate size distribution between the standard curve and the unknown samples. In real runoff, particle size sorting can result in extreme distributions between the standard curve and the unknown samples.

Error Analysis of Runoff Total Suspended Solids Concentration Estimates
There are two separate relationships occurring between TSS concentration and turbidity. One is for TSS concentration of <1 g L–1 and the other for TSS concentration of ≥1 g L–1. The relationship for TSS concentration of <1 g L–1 fits best with simple linear function and for TSS concentration of ≥1 g L–1 fits best with the power function, as described in Table 5. Foster et al. (1992) have also shown a linear relationship for NTU and suspended sediments when the concentration is below <1 g L–1 for a series of particle size fractions ranging from less than 4 to 63 µm. For both the low and high runoff TSS concentrations, using sucrose solution improved the accuracy of TSS concentration estimates. The RMSE and ME showed significant improvement in accuracy of TSS concentration in runoff for both low and high TSS concentrations; however, there was no improvement in CRM values (Table 5).


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  		Table 5. Error statistics of estimation of total suspended solids (TSS) concentration using sucrose solution over the distilled deionized water as suspending media.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Indirect measurement of TSS using nephelometric turbidity is simple and fast. However, differences in particle size distribution between the standard curve and the unknown samples resulted in erroneous TSS concentration estimation by using water as suspending medium. The use of sucrose solution as suspending medium improved sensitivity of turbidity to detect changes in TSS concentration by reducing the RI differences between the suspending media and the suspended particle, and by retaining settleable particles in suspension. This is a significant benefit of sucrose medium (over water medium) in estimating unknown TSS concentration from a standard curve that has different particulate size distribution from the unknown sample. Accuracy of TSS concentration can be improved by as much as 20% or more using sucrose solution, even when the standard curve and the unknown samples have contrasting particle size distribution. Sucrose solution of 1.30 g L–1 is proposed as suspending medium to dampen the effect of particle size distribution and improve TSS concentration estimation in runoff.


   ACKNOWLEDGMENTS
 
The authors wish to thank the United States Department of Agriculture Cooperative State Research, Education, and Extension Service (USDA-CSREES Contract Number 2003-51130-02072) and the University of Nebraska-Lincoln Agricultural Research Division for the financial support on this project. The authors also wish to thank Mr. Roger Renken and Mr. Sonny Fankhouser for their laboratory assistance, and the USDA-ARS SWCRU-Lincoln for providing the benchmark soils.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Contribution of the University of Nebraska Agricultural Research Division, Lincoln, NE 68583. Journal Series no. 14551.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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The SCI Journals Agronomy Journal Crop Science
Vadose Zone Journal Journal of Plant Registrations
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal