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

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
Selbig and Bannerman
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 (