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

Selbig and Bannerman
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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