A key requirement for an NPDES (Phase I) stormwater permit is a program to detect and eliminate illicit discharges. Field screening outfalls during dry weather can be an effective way to find illicit discharges. For this reason, many Municipal Separate Storm Sewer Systems (MS4s) institute a Dry Weather Monitoring Program as part of their individual Storm Water Management Plans (SWMP).
Cities with a population greater than 100,000 are designated as Large and Medium MS4s. These MS4s are required to apply for an individual permit with specific Dry Weather Monitoring Program requirements. As a Medium MS4, the City of Lincoln is required to screen 250 outfall sites each permit term (or 50 outfall sites each permit year).
Dry Weather Monitoring Programs use field screening methods for inspecting stormwater drainage areas to help locate and identify harmful and illegal discharges to a municipal stormwater system. Field screening methods can include observations of physical characteristics and basic tests of chemical properties.
Outfall selection is based on an Illicit Discharge Potential Risk Assessment. The assessment uses several screening factors to calculate the potential risk of illicit discharges within each of Lincoln’s 275 subbasins. Factors include age of infrastructure, outfall density, proximity to industrial sites, history of illicit discharges, and whether an outfall discharges to a wetland or sensitive waterbody.
The City of Lincoln follows EPA guidelines for detecting illicit discharges. Samples are collected at outfalls with flowing water. Observations are made about the physical characteristics of each sample including color, odor, and clarity, as well as the presence of any unnatural foams, films, or sheens. The samples are also tested in the field for basic parameters that may be indicative of an illicit discharge. These parameters include:
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