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

Development of a Depth-Integrated Sample 

Arm to Reduce Solids Stratification Bias in 

Stormwater Sampling 

ABSTRACT: A new depth-integrated sample arm (DISA) was developed 

to improve the representation of solids in stormwater, both organic 

and inorganic, by collecting a water quality sample from multiple points in 

the water column. Data from this study demonstrate the idea of vertical 

stratification of solids in storm sewer runoff. Concentrations of suspended 

sediment in runoff were statistically greater using a fixed rather than multipoint 

collection system. Median suspended sediment concentrations 

measured at the fixed location (near the pipe invert) were approximately 

double those collected using the DISA. In general, concentrations and size 

distributions of suspended sediment decreased with increasing vertical 

distance from the storm sewer invert. Coarser particles tended to dominate 

the distribution of solids near the storm sewer invert as discharge 

increased. In contrast to concentration and particle size, organic material, 

to some extent, was distributed homogenously throughout the water 

column, likely the result of its low specific density, which allows' for 

thorough mixing in less turbulent water. Water Environ. Res., 83, 347 

(2011). 

KEYWORDS: suspended solids, suspended sediment, stormwater, 

runoff, autosampler, organic, specific density. 

doi: 10.2175/106143010X 12851009156006 

Introduction 

Collection of representative stormwater quality samples in urban 

runoff can be difficult, as a result of the large sources of variability 

that are both temporal and spatial, such as the flashy nature of urban 

runoff events or sediment stored in storm sewer systems between 

events (Horowitz, 1995; Selbig and Bannerman, 2007). The use of 

automated water quality samplers has vastly improved the way 

water resources professionals collect samples in these environments, 

but an increasing body of evidence suggests that some of the 

concentration data collected from urban conveyances, both in the 

past and presently, may be biased (Fowler et al., 2009; Smith, 

2002). One source of potential bias is the stratification of solids by 

particle size in a flowing water column. Previous studies have 

suggested that sediment accumulated in pipes may be mobilized as 

bed load or suspended load, depending on the concentration and 

size distribution of the sediment and energy of flow (Bent et al., 

2000, DeGroot et al., 2009; Edwards and Glysson, 1999). The 

U.S. Geological Survey, Wisconsin Water Science Center, Middleton, 

Wisconsin. 

2 Wisconsin Department of Natural Resouices, Madison, Wisconsin. 

*U.S. Geological Survey, Wisconsin Water Science Center, 8505 

Research Way,' Middleton, WI 53562; e-mail: wrselbig@usgs.gov. 

William R. Selbigl*, Roger T. Bannerman 2 

location of the sampler intake can be critical, depending on the 

degree of stratification in the storm sewer pipe. 

Typically, the intake orifice of an autosampler is located near the 

pipe invert to capture low-flow conditions. The recommended intake 

orifice diameter for automated samplers is approximately 0.95-cm 

(0.38-in.) (ISCO, 2008). Therefore, in large-diameter pipes, the 

stormwater quality sample collected' by the autosampler represents 

only the bottom 0.95 cm (0.38 in.) of the pipe. During higher flows, 

the majority of the water column is not sampled. If the water column 

is mixed fully, creating a homogenous distribution of sediment, this 

sampling scheme may be adequate; however, Smith (2002) has 

shown that the concentration and. distribution of sediment in a 

circular pipe with a diameter as small as 0.3 m (1 ft) can be 

concentrated near the bottom of the pipe over a range of flow 

conditions. Similarly, in a controlled laboratory setting, DeGroot et 

al. (2009) reported that sediment concentrations collected by an 

autosampler fixed near the bottom of a 46-cm (1 8-in.)-diameter pipe 

were much greater than known concentrations of particle sizes 

ranging from silts and clays (

Selbig and Bannerman 

1.25 x 0.375 Inch elliptical 

Stainless steel shaft \ 

Mounting bracket/ 

Polycarbonate end cap 

with 0.25 Inch I.D. Intake orifices 

Figure 1-Schematic drawing of the DISA. 

et al. (2005) suggested designing a new autosampler system that 

uses a float system to place the intake at the midpoint of a flow 

path. DeGroot et al. (2009) designed an intake manifold that 

adjusts itself to the depth of flow in the pipe by use of a fin. 

Although this device showed promising results for accurately 

collecting sand-sized particles in a small-diameter pipe, an 

alternate mechanism would be necessary for pipes larger than 

61 cm (24 in.) in diameter (DeGroot et al., 2009). These ideas 

mark the genesis of new technologies to improve the way a 

stormwater quality sample is acquired by use of autosamplers. 

This study discusses the development of a new prototype depthintegrated 

sample ann (DISA) designed to integrate with existing 

autosampler configurations for collection of stormwater quality 

samples of urban runoff in a storm sewer. Use of the DISA 

facilitates collection of stormwater quality samples from a single 

or multiple point(s) in the water column. Integrating samples from 

the entire water column, rather than from a single, fixed point, can 

result in a more accurate representation of stormwater-borne 

solids. In this study, the DISA and a fixed-point sampling method 

were used to collect samples of urban runoff. Results from the two 

methods are compared on the basis of concentrations of suspended 

sediment, organic content, and PSDs using field-collected samples 

of urban runoff. Deficiencies of the DISA and suggested 

modifications to improve the device also are discussed. 

348 

Ram piston 

Rear mounting hole 

12v wMre 

'F -12 VDC Motorized: ram 

Aluminum support frame 

Materials and Methods 

The general field of application of the DISA described herein is 

intended to be in closed conveyances, such as storm sewers, that 

are used to route stormwater runoff in an urban environment. 

However, the DISA also may be used in any water quality 

sampling environment where the distribution of sediment in flow 

can be shown to be heterogeneous and not easily corrected using 

established manual sampling techniques, such as equal-width or 

equal-depth increment sampling. 

Use of the Depth-Integrated Sample Arm to Collect a Water 

Quality Sample. The DISA has a support frame, a motorized 

piston (also referred to as a linear actuator), a rotary 

potentiometer, and a sample arm assembly (Figure 1). As the 

piston extends or retracts, it pivots the sample arm assembly 

around the potentiometer axle. The rotation of the DISA, and thus 

the position of the intake orifice, is controlled by an external data 

logger or other programmable logic control (PLC) device that 

activates a relay to energize the piston. The PLC may be 

programmed to set the intake orifice to a percentage of the water 

depth. The depth of water is measured by an acoustic-velocity 

sensor or similar device. For example, if the target sample position 

is set to be 50% of a water depth of approximately 0.2 m (0.5 ft), 

then the PLC will activate the motorized ram until the 

potentiometer reads the voltage representing 0.08 m (0.25 ft). 

Water Environment Research, Volume 83, Number 4

Thus, the motorized ram rotates the sample arm assembly to any 

vertical depth within the storm sewer. Once the PLC determines 

that the potentiometer has reached the target voltage, it triggers 

the autosampler to operate with normal purge/withdraw cycles to 

collect a stormwater quality sample and deposit it in one or more 

storage containers. Once the sample has been acquired, the DISA 

either moves to a new position for collection of another sample or 

fully retracts to the horizontal position, whi6h removes the DISA 

from the flow path with any debris that may have accumulated on 

the sample arm assembly while acquiring a sample. 

Method to Compare the Depth-Integrated Sample Arm with 

a Fixed-Point Autosampler Configuration. This study involved 

collection and chaiacterization of data derived from 

stormwater quality samples collected in two urban drainage areas 

located in Madison, Wisconsin. The first was a 2.4-ha (6-ac) 

commercial parking area adjacent to a shopping center complex; 

the second was a 21-ha (53-ac) single-family residential area.. 

Runoff was collected in multiple storm sewer inlets at each site 

and then was conveyed through a single, circular, concrete pipe 

with diameters of approximately 91 and 107 cm (36 and 42 in.) at 

the parking lot and residential drainage areas, respectively. 

Each monitoring station was equipped with two automated 

water quality samplers and instruments to measure water level and 

velocity. Measurement, control, and storage of data were 

performed by electronic data loggers. Sample collection was 

activated by a rise in water level in the pipe during'a storm. Once 

the water level threshold was exceeded, typically a depth of 

approximately 0.06 m (0.2 ft) from the pipe floor, each automated 

water quality sampler collected a discrete water sample into I-L 

plastic containers. This process was repeated for every approximately 

0.03-m (0.1-ft) increase and approximately 0.06-m (0.2-ft) 

,decrease in water level over the duration of the storm hydrograph 

until the water in the pipe receded below the water level threshold. 

Each autosampler was located side-by-side in an aluminum 

shelter directly over the storm sewer. ISCO 3700 pump heads with 

a 24-bottle configuration were used to provide consistent intake 

velocities. Pump heads were programmed with similar purge/ 

withdraw cycles to flush the sample tubing before sample 

collection. One autosampler collected a water quality sample 

from a fixed point located approximately 2.5 cm (1 in.) off the 

pipe invert (herein called the fixed-point sampler), and the other 

collected a sample from multiple points using the DISA. Upon 

sample initiation, each sampler would collect a series of three 

successive sub-samples separated 'by 1-minute increments. 

Because the fixed-point sampler's intake nozzle was secured to 

the pipe floor, each sub-sample was taken from the same location. 

The DISA was programmed to collect each sub-sample from 

different vertical locations in the water column. The first subsample 

was collected approximately 2.5 cm (I in.) from the.pipe 

invert to coincide with the intake location of the fixed-point 

sampler. The DISA then would relocate the intake nozzle to a 

vertical point approximating 30 and 60% of the water level for the 

second and third sub-samples, respectively. In doing so, subsamples 

were collected from the lower, middle, and upper onethird 

of the full water column. For example, if the water level was 

0.3 m.(1.0 ft), the first, second, and third sub-samples (herein 

called the lower, middle, and upper sub-samples) would be 

collected at a depth of 0.02, 0.09, and 0.2 m (0.08, 0.3, and 0.6 ft) 

"from the pipe invert, respectively. Although the vertical spacing of 

each sub-sample remained consistent for this study, the location of 

April 2011 


the nozzle intake can be programmed to accommodate any 

vertical position in the pipe, from completely vertical to 

completely horizontal. 

Upon runoff cessation, water quality samples were retrieved 

and transported to the U.S Geological Survey Middleton Field 

Office in Middleton, Wisconsin. Each discrete sample was split 

equally into two laboratory-prepared containers by passing the 1 L 

sample through a cone splitter. Approximately 180 mL also was 

transferred from the cone splitter into a separatory funnel for 

determination of PSD. Except for the separatory funnel, processed 

samples were chilled on ice and delivered to the Wisconsin State 

Laboratory of Hygiene (Madison, Wisconsin) for analysis of total 

suspended solids (TSS), total volatile suspended solids (TVSS), 

and suspended sediment. 

Particle Size Distribution Measurement. The PSD.of runoff 

samples was measured using a Laser In-Situ Scattering and 

Transmissometry (LISST) particle size analyzer (Sequoia Scientific, 

Bellevue, Washington). The LISST-Portable uses laser 

diffraction to measure PSD as particle volume concentration 

(microliters per liter) in 32 logarithmically spaced bins ranging in 

size from approximately 2 to 350 gtm. Additional details are 

described in Agrawal and Pottsmith (2000). Contents of the 

separatory funnel were transferred into the holding chamber of the 

LISST-Portable. Use of a separatory funnel allowed for complete 

transfer of the sample without the potential to bias coarse particle 

concentration by pouring or pipetting. 

The upper particle size fraction reported by the LISST-Portable 

was approximately 350 gim. Although the particle size fraction in 

the majority of samples collected as part of this study was below 

this reporting limit, ,some samples contained particles exceeding 

that size (


Table 1-Summary of solids concentrations at the parking lot and residential study areas for samples collected using 

the fixed-point and DISA samplers. 

TSS (mg/L) VSS (mgIL) Suspended sediment (mg1L) 

Statistic Fixed-point DISA Fixed-point DISA Fixed-point DISA 

Parking lot 

Number of observations 117 118 117 118 116 117 

Minimum 3

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Suspended Sediment Concentration (fixed-point sampler), In mg[L 

* LOWERof equal concentration 

£ MIDDLE 

* UPPER 

1 10 100 1,000 10,000 

Suspended Sediment Concentration (fixed-point sampler), In mg/L 

Figure 2-Paired concentrations of suspended sediment at the fixed-point and lower, middle, and upper DISA sample 

locations in the (ay parking lot and (b) residential study sites. 

April 2011 351 

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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 


Lower M iddle Upper 


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 

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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


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Specific Density, in g/CM( 

Figure 5---Cumulative distribution of average specific density estimated in the fixed-point and lower, middle, and 

upper DISA samples [(n) = number of samples used in analysis]. 

become important in poorly mixed flows. Therefore, proper Horowitz (1995) reported increasing concentrations of sediment 

characterization of the distribution of particles in urban runoff with increasing depth in fluvial systems. The data presented in 

requires the collection of water quality samples from multiple Figures 6 and 7 illustrate a similar trend in a storm sewer. The 

points, rather than from a single fixed point in the water column. increase in concentration is the result, in part, of an increase in 

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organic Content, In percent 

Figure 6-Relationship of the percent organic material in samples collected from the lower, middle, and upper DISA 

sample locations with the depth of water in the storm sewer at the residential study site. 

354 


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Figure 7--(a) Distribution of particles in discrete sub-samples collected using the DISA sampler. (b) Each sequence of 

sample points grouped on the rising limb and peak of the hydrograph represents the chronological progression of 

the lower, middle, and upper intake locations (Note: the upper DISA sample in sub-sample 1 did not have enough 

water for particle size analysis). 

April 2011 

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355


sand-sized material. In the absence of turbulent eddies, coarse 

particles with high specific density become stratified and tend to 

be transported along the bottom of the pipe floor. As the energy of 

flow in the pipe increases, sediment concentration and particle 

size also increase. However, the distribution of coarse particles 

may remain stratified. Figures 7a and 7b show the distribution of 

particles in water samples collected during the rising limb and 

peak of a storm hydrograph at the parking lot study area. Plots of 

the lower, middle, and upper sub-samples are shown with 

corresponding fixed-point samples to highlight changing distributions 

with water depth. Mean concentrations of suspended 

sediment for the DISA and fixed-point samplers also are shown to 

provide a correlation and comparison of sediment concentration 

and particle size with discharge (energy). At the onset of flow in 

the pipe, when discharge is low, particles are relatively small in 

diameter (d50

tions in the fixed-point samples generally stayed the same. 

Incorrect assumptions of PSDs in urban stormwater could have 

detrimental consequences when sizing treatment devices, such as 

wet detention ponds. 

Relationships between sediment concentration, organic content, 

and PSD in stormwater quality samples collected in the fixedpoint 

and DISA were consistent across a range of hydraulic 

conditions. These relationships were duplicated at two geograph- 

• ical locations with different land7use characteristics, giving 

confidence in the transferability of the results of this study to 

other urban runoff conveyance systems. Because all samples 

evaluated as part of this study were collected in a field setting, it 

was not possible to compare the results collected from each 

sampler with a known quantity. Additional research in a 

controlled laboratory environment could provide the information 

necessary to verify the ability of the DISA to collect a solids 

concentration representative of the entire water column. 

Credits 

The authors thank Becky Carvin of the U.S.. Geological Survey 

(Middleton, Wisconsin) for her tireless efforts in the field. Any 

use of trade, product, or firm names is for descriptive purposes 

only and does not imply endorsement by the U.S. Government. 

Submitted for publication May 3, 2010; revised manuscript 

submitted September 7, 2010; accepted for publication September 

28, 2010. 

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April 2011 • 357

COPYRIGHT INFORMATION 

Author: 

Title: Development of a Depth-Integrated Sample Arm to Reduce Solids 

Stratification Bias in Stormwater Sampling 

Source: Water Environ Res 83 no4 Ap 2011 p. 347-57 

ISSN: 1061-4303 

DOI: 10.2175/106143010X12851009156006 

Publisher: 

Selbig, William R.; Bannerman, Roger T. 

Water Environment Federation 

601 Wythe Street, Alexandria, Va 22314-1994 

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