Development of a Depth-Integrated Sample Arm to Reduce Solids ...

More documents

Recommendations

Info

Selbig and Bannerman E a ES p1,500- 0 U E 51,000- m 500. 0 "H'I 1 H'I 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Event Number Figure 3-SSCs at the lower, middle, and upper DISA intake locations in the residential study area. Only samples that were collected successfully from all three intake locations are shown. On average, differences in flow between successive sub-samples were less than 2%. Differences in Organic Content. The amount of organic material in a water sample can be expressed as the ratio of TVSS to TSS. Previous studies have demonstrated TSS as an inferior metric to measure solids in stormwater when compared with suspended sediment (Clark and Siu, 2008; deRidder et al., 2002; Gray et al., 2000; Kayhanian et al., 2008; Roesner et al., 2007; Selbig et al., 2007). The degree of error is correlated positively with concentration or particle size, or both (Selbig et al., 2007). Given the bias of TSS as a method to quantify solids in stormwater, the organic content of water samples collected as part of this study was expressed as a percentage of SSC. This definition of percent organic content is based on the assumption that equal amounts of organic material were transferred from the cone splitter into the laboratory containers used for determination of TVSS and suspended sediment, and none of the organic material was excluded from the laboratory analysis. The distribution of organic material, to some extent, was uniform throughout the water column. This is likely the result of the low specific density typically associated with organic detritus when compared with sand. Kayhanian et al. (2008) reported specific gravities of organic detritus ranging from 1.6 to 1.8 g/cm 3 - much lower than that of sand (2.65 g/cm 3 ). Cristina et al. (2002) reported the specific gravity of highly organic solids as 1.1 g/cm 3 . Based on Stokes Law (1851), organic solids can remain in suspension much longer than sand because of their lower specific density and therefore may achieve a more homogenous distribution throughout the water column, despite relatively low turbulence, as often is the case in storm sewers. Figure 4 shows the range of organic content as a percentage of suspended sediment in the residential study area for the lower, 352 middle, and upper intake locations of the DISA. The residential study area was used to describe the characteristics of organic material in urban runoff because of its extensive tree canopy and other organic detritus that is transported easily during a storm event. The range of organic content generally is less than 50% for all vertical points in the water column. Median and mean percentages of organic material in samples collected from the middle intake location appear to be slightly greater than the lower or upper locations, but are not statistically significant (Kruskall- Wallis test, p = 0.05). Furthermore, although there is no statistical difference in the percentage of organic material at the lower, middle, or upper intake locations collected by the DISA, each vertical location has an organic content that is statistically greater than paired samples collected by the fixed-point sampler (Wilcoxon signed-rank test, p = 0.05). This suggests that use of a fixed-point sampler might result in overestimation of the inorganic fraction and underestimation of the organic fraction in urban runoff. Particles collected by the fixed-point sampler in the residential study area had larger median and mean specific densities than those collected by the DISA for all vertical intake locations (Table 3). The densities shown in Table 3 may differ from the actual values, as a result of compounding errors in analytical procedures. Concentrations of suspended sediment include all particles, regardless of size, but the volumetric concentrations determined by the LISST were restricted to the largest aperture of the instrument-approximately 350 gIm. Large, organic detritus, such as grass clippings, would be included in the concentration of suspended sediment, but not in the volumetric concentration. This limitation is best visualized in Figure 5, where approximately 30% of specific densities are less than 1.0 (the density of water). However, a comparison between the ranges of estimated densities Water Environment Research, Volume 83, Number 4
Lower M iddle Upper Selbig and Bannerman Figure 4-Percent organic content of water quality samples collected using the DISA at the lower, middle, and upper intake locations. does support a distinction between sampler types. This is best illustrated by plotting the cumulative. frequency distribution of estimated, average specific densities for both the DISA sample intake locations and the fixed-point sampler (Figure 5). Figure 5 shows that the frequency distributions for the lower, middle, and upper sample-intake locations for the DISA sampler were similar to 'each another. The fixed-point saimpler had a frequency distribution favoring particles with a higher specific density. Approximately 75% of all particles in the fixed-point sampler and nearly all particles collected by the DISA sampler had average specific densities less than that of sand (2.65 g/cm 3 ). Further evidence of homogeneous distribution of organic material throughout the water column is illustrated in Figure 6. As the depth of water in the storm sewer decreases, the percentage of organic material tends to increase. The rate of increase is similar for all three sub-sample locations spaced vertically throughout the water column. A considerable amount of Table 3-Comparison of average specific density estimated for samples collected from the DISA and fixed-point samplers. DISA sample Intake location Statistic Lower Middle Upper Fixed-point Number of observations 38 38 20 84 Median 1.3 1.1 • 1.3 1.6 Mean 1.4 1.2 .1.3 1.9 Standard deviation 0.8 0.7 0.5 1.0 Variation coefficient 0.6 0.6 0.4 0.6 April 2011 S50 0 04 0. 60 a 30 4 U . 20 0 o 0 oV 0 A- 4- variability exists in percent organic content when the depth of water in the storm sewer is less than approximately 0.2 m (0.5 ft). One explanation might be the inherent variability in organic detritus accompanied by the timing of each sample. A greater amount of organic material could become entrained in runoff early in the rising limb of the hydrograph but may dissipate or dilute as the volume of water increases. A similar inverse relationship is apparent between the percent organic content and concentrations of suspended sediment. As the percentage of organic material decreases with increasing water depth, the percentage of inorganic sediment would increase. Similar conclusions were made by Lenhart and Lehman (2006) when analyzing concentrations of TSS and TVSS in stormwater samples collected from several cities throughout the United States. It is this inorganic fraction that tends to have higher specific gravities and is more prone to stratification in the water column. Subsequently, concentrations of inorganic solids may show greater variability when using a fixedpoint sampler. i Differences in Particle Size Distribution. The range of particle sizes in urban runoff can vary considerably. Bent et al. (2000) reviewed studies from several countries around the world that related sediment in runoff from a variety of urban areas. The median particle size (d50) of sediments collected in these studies ranged from 0.0 13 to 1.00 mm. Other studies differ on the particle size that autosamplers are able to capture effectively (range from 250-to 2000 ptm), regardless of their location in the water column (Clark et al., 2008; Pedrick and Tellessen, .2008). Despite disagreement in particle size collection efficiency, these studies tend to agree that the location of the sample intake has no significance if the water column is well-mixed. However, Clark et al. (2008) concluded that the location of a sampler intake would 353
Page 1 and 2: Development of a Depth-Integrated S
Page 3 and 4: Thus, the motorized ram rotates the
Page 5: (a) C (b) 100,000 10,000 o 1,000 *
Page 9 and 10: 1£l] .... I IM, 70 ED 50 30 3to (a
Page 11 and 12: tions in the fixed-point samples ge

Development of a Depth-Integrated Sample Arm to Reduce Solids ...

Create successful ePaper yourself

Delete template?

Save as template?