Stormwater Management Standards Manual - Toledo Metropolitan ...

6.4.2 Dry Weather Field Screening
The field screening technique recommended for an IDDE program is the Outfall Reconnaissance
Inventory or ORI. The ORI is a stream walk designed to inventory and measure storm drain outfalls, and
find and correct continuous and intermittent discharges without in-depth laboratory analysis. There are
four recognized steps to the ORI. As communities acquire more monitoring data, they should consider
creating a chemical fingerprint library, which is a database of the chemical make-up of the many different
flow types in the community. Over time, chemical library data can help a community better understand
the potential pollutant loads delivered to receiving waters from various generating activities.
Acquire necessary mapping, equipment and staff: Street maps may be use for the initial assessment
and minimal field equipment is required.
Determine when to conduct field screening: The best time to conduct field screening is during dry
season and leaf-off conditions. It is best to conduct field screening after a dry period of at least 48 hours
while the ground water levels are low.
Identify where to conduct field screening: The MS4 requires that all outfalls be screened in the permit
period. Communities should begin in areas with a high illicit discharge potential.
Conduct field screening: An illicit discharge detection and elimination program will be more effective and
efficient in the long run if all outfalls are marked and photographed. During the ORI communities should
collect data on the characteristics of every outfall.
There are four basic sensory indicators that field crews should look for in conjunction with a flowing
outfall. Sensory indicators can be detected by smell or sight and do not require measurement
equipment.
Odor: Field crews should record any odor coming from the outfall and give the outfall a severity rating.
A severity rating of one means that the odor is faint or the crew cannot agree on its presence or origin.
A score of tow indicates a moderate odor within the pipe. A score of three is assigned if the odor is so
strong that the crew smells it a considerable distance away from the outfall. Field crews should take
care to make sure that the odor originates from the outfall. Sometimes shrubs, trash, carrion or even the
spray paint used to mark the outfall can confuse the nose.
Color: The color of a discharge can be classified as clear, slightly tinted or intense. The best way to
measure color is to collect the discharge in a clear sample bottle and hold it up to the light. Field crews
should also look for downstream plumes of color that appear to be associated with the outfall. Color can
also help identify industrial discharges.
Turbidity: Turbidity is the measure of the cloudiness of the water. Like color, turbidity is best observed
in a clear sample bottle. Field crews should also look for turbidity in the plunge pool and note any
downstream turbidity plumes that appear to be related to the outfall.
Floatables: The presence of floatable materials is determined visually. Sewage, oil sheen, and suds are
all examples of floatable indicators. Sewage should automatically be assigned a severity score of three
since no other source looks quite like it. Surface oil sheens are ranked based on their thickness and
coverage. In some cases, surface sheens may not be related to oil discharges, but instead are created
by in-stream processes. A thick or swirling sheen associated with a petroleum-like odor may be
diagnostic of an oil discharge. Suds are rated based on their foaminess and staying power. Thick foam
that travels many feet before it breaks up is given a severity rating of three. Suds that break up quickly
may simply reflect water turbulence, and do not necessarily have an illicit origin. Decaying organic
matter creates naturally occurring foams in some streams. Suds that are accompanied by a strong
Third Edition Stormwater Standards Manual September 2008 Page 112