The City of Merced is a regulated Small Municipal Separate Storm Sewer System (MS4). This means that the sewer system and storm drain system are separate. Wastewater that enters into the sewer system is directed to the local City of Merced Wastewater Treatment Facility (WWTF), treated, then released back into the environment. Stormwater is precipitation runoff that is directed to the MS4, not treated, and directly discharges to local creeks, such as, Bear Creek, Black Rascal Creek and Cottonwood Creek.
In 1990, the Environmental Protection Agency (EPA) established the Phase I National Pollutant Discharge Elimination System (NPDES) Stormwater Program, the City of Merced falls under the Phase II portion of this program. Operators of regulated Small MS4's are required to design their programs to:
- Reduce the discharge of pollutants to the "maximum extent practicable" (MEP);
- Protect water quality; and
- Satisfy the appropriate water quality requirements of the Clean Water Act.
The Construction MEP's for Small MS4 Construction Stormwater are:
- Construction Site Runoff Control
Developing, implementing, and enforcing an erosion and sediment control program for construction activities that disturb 1 or more acres of land (controls could include silt fences and temporary stormwater detention ponds).
- Post-Construction Runoff Control
Developing, implementing, and enforcing a program to address discharges of post-construction stormwater runoff from new development and redevelopment areas. Applicable controls could include preventative actions such as protecting sensitive areas (e.g., wetlands) or the use of structural BMPs such as grassed swales or porous pavement.
Uncontrolled runoff from construction sites is a water quality concern because of the devastating effects that sedimentation can have on local waterbodies, particularly small streams. Numerous studies have shown that the amount of sediment transported by stormwater runoff from construction sites with no controls is significantly greater than from sites with controls. In addition to sediment, construction activities yield pollutants such as pesticides, petroleum products, construction chemicals, solvents, asphalts, and acids that can contaminate stormwater runoff. During storms, construction sites may be the source of sediment-laden runoff, which can overwhelm a small stream channel’s capacity, resulting in streambed scour, streambank erosion, and destruction of near-stream vegetative cover. Where left uncontrolled, sediment-laden runoff has been shown to result in the loss of in-stream habitats for fish and other aquatic species, an increased difficulty in filtering drinking water, the loss of drinking water reservoir storage capacity, and negative impacts on the navigational capacity of waterways.