We are an industry of abbreviations and acronyms. The terms we use on a daily basis can sometimes hold a general or broad meaning in our minds, but the actual definition of these terms may leave our thumbs hovering over the game-show buzzer. To help ease the furrowed brows, we have collected and defined the top 11 terms every Stormwater Engineer should know:

1. NPDES – National Pollutant Discharge Elimination System
As a result of the Clean Water Act, the NPDES program was developed to control the discharge of polluted runoff to the nation’s waterways from point and non-point sources, such as pipes or wells and stormwater runoff.  The program was developed by the EPA, but for the majority of the country, the program is administered at the state level.

2. DMR – Discharge Monitoring Report
NPDES permit holders are required to submit a DMR to the EPA at the issuance of a new permit or a modification of an existing permit. The report details the effluent pollutant concentrations of discharges to surface waters.

3. BMP – Best Management Practice
Generically, a BMP is considered a best practice or method of addressing a problem such as pollution. The term is widely used to describe stormwater management practices involving structural or engineered control devices and pollution prevention techniques. The term encompasses proprietary devices, as well as more traditional designs such as above-ground retention ponds or riparian areas.

4. LID – Low Impact Development
This site design practice employs stormwater management approaches that help to mimic pre-development hydrology by reducing or eliminating stormwater runoff. Effort is made to manage runoff near the source using smaller integrated stormwater management practices. Examples of these solutions include porous paving systems, biofiltration, rainwater harvesting, infiltrations systems, green roofs, and filtration systems. Effort is made during site design to minimize the amount of impervious cover created, conserve important natural areas, and retain runoff on site.

5. LEED – Leadership in Energy & Environmental Design
Developed by the U.S. Green Building Council, LEED is a program that certifies and recognizes new building projects for their innovative design practices. Certification is based on a point system for a particular category, including building design and construction, interior design and construction, building operations and management, neighborhood development, and homes. Projects can be certified under four categories, ranging from the minimum required point value to the highest: certified, silver, gold, or platinum.

6. CSO – Combined Sewer Overflow
Older infrastructure across the nation combined sanitary waste and stormwater runoff in a single conveyance line. Typical operation of the system would carry collected flows to a municipal wastewater treatment facility. During peak storm events, the combined system overflows, or discharges, directly to a body of water. CSO overflows are typically triggered when the volume of runoff exceeds the capacity of the applicable wastewater treatment facility. These systems are no longer standard practice, and the EPA is slowly working with NPDES permit holders to have them removed to eliminate pollutants released to receiving waters during overflow events.

7. SSO – Sanitary Sewer Overflow
An SSO describes an event causing collected sewage to discharge into surrounding water bodies or groundwater sources. Factors contributing to these overflows include line defects, blockages, overload due to stormwater inflow and infiltration, lack of maintenance, or vandalism.

8. 303(d) List
The 303(d) List refers to a section of the Clean Water Act that comprises of impaired waterways. Pollution in these bodies of water impacts the applicable water quality standard for its designated use, such as drinking, aquatic habitat, or recreation. Bodies of water can eventually be removed from the list after a TMDL is developed, or changes are implemented to positively affect the water quality.

9. TMDL – Total Maximum Daily Load
The TMDL program was specifically developed to restore impaired waterways. It is used to describe the maximum amount of a pollutant a waterbody can receive and still meet water quality standards. TMDL’s are developed for a number of water quality impairments including fecal/bacteria, sediment, nutrients (such as Phosphorus and Nitrogen), metals (such as Copper and Zinc), temperature, and pH. A growing number of TMDL’s are being implemented, and stormwater discharges to impaired waters must be managed accordingly.

10. TSS – Total Suspended Solids
Used to describe the amount of solids suspended/transported in runoff as well as being a specific analytic method used to measure the concentration of solids in water. TSS is currently the most commonly regulated stormwater pollutant. Particle sizes and densities vary widely. BMP effectiveness also varies from one location to the next and designing BMP’s for very specific performance objectives can be difficult.

11. MS4 – Municipal Separate Storm Sewer System
An MS4 is a conveyance system or a system of conveyances, including roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, or storm drains owned by a state, city, town or other public entity that discharges to waterways of the state. These systems are designed to collect and convey stormwater only, and are not a combined sewer nor are they part of a publicly owned treatment works facility (sewage treatment plant).