Municipal Separate Storm Sewer System General Permit (MS4)

Program Perspective

Larimer County has a long history of protecting our natural resources, including stormwater. When dealing with stormwater issues, as a land developer or an individual homeowner, water quality should be viewed as a valued resource. When we protect this resource from pollution, we become good stewards of our waterways.

Under provisions of the Colorado Water Quality Control Act and the Federal Water Pollution Control Act, Larimer County was designated by the Water Quality Division of the Colorado Department of Public Health and Environment (CDPHE) as a Phase II Municipal Separate Storm Sewer System (MS4) Community. With this designation, Larimer County was required to apply for certification under the Colorado Discharge Permit System (CDPS) General Permit for Stormwater Discharges Associated with MS4s. Under the regulation of this permit, Larimer County is required to have a Stormwater Quality Management Plan that has the goal to protect stormwater quality by controlling and/or eliminating pollutant discharges into stormwater sewer systems.

Stormwater General Permit

As an operator of a Phase II Regulated Small Municipal Separate Storm Sewer System (MS4), Larimer County is required by the State of Colorado to be certified to discharge stormwater under the general permit (COR090000). A Program Description Document (PDD) that meets the requirements found in the MS4 permit is available to the public for review and comment.  As part of the certification, the following control measures need to be implemented within the County's MS4 permit coverage area.  For a more detailed map of the coverage area, use the Flood Indicator Map (FIL).  Once there, click on the "Layer List" to show the layers you can click on and off.  One layer to click on is the "MS4 Boundary Layer" .  Then zoom the map into your area of interest to see whether your area is within the County MS4 permit area.  Ensure that the "City Limits" layer is also clicked on as the County's MS4 permit areas are regularly removed through annexation.

Control Measures

1. Public Education and Outreach:
   Educate residents and members of the business community to help reduce water quality impacts associated with pollutants in stormwater runoff.
    
2. Illicit Discharge Detection and Elimination:
   Develop, implement and enforce a program to detect and eliminate spills, illegal dumping and other non-stormwater discharges into the County's storm sewer system.
    
3. Construction Site Pollutant Control:
   Develop, implement and enforce a program to reduce discharge of pollutants from construction activities.
    
4. Post-Construction Stormwater Management:
   Prevent or minimize impacts to stormwater from new development or redevelopment by ensuring that stormwater quality structures operate as designed and are maintained.
    
5. Pollution Prevention/Good Housekeeping for Municipal Operations:
   Develop and implement an operation and maintenance program to prevent or reduce pollutants from municipal operations.

Larimer County Stormwater Quality Ordinance serves as one of the County’s regulatory mechanisms to implement and enforce requirements to prohibit non-stormwater discharges into the storm drainage systems.